electrochemistry of organophosphorus compounds

1070-3632/01/7109-1393$25.00C2001 MAIK [Nauka/Interperiodica]

Russian Journal of General Chemistry,Vol. 71, No. 9,2001, pp. 139331421. Translated from Zhurnal ObshcheiKhimii, Vol. 71, No. 9,2001,pp. 147231502.Original Russian Text CopyrightC 2001 by Kargin, Budnikova.

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Electrochemistry of Organophosphorus CompoundsYu. M. Kargin and Yu. G. Budnikova

Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,Russian Academy of Sciences, Kazan, Tatarstan, Russia

Received August 30, 2000

Abstract-Progress in electrochemistry of organophosphorus compounds is rewieved. Mechanisms ofoxidation and reduction reactions under the conditions of homogenous electrocatalysis and possibility of usethe primary compounds for the synthesis of various organic and organoelement compounds are considered.

In the recent decades, increased theoretical interestto organophosphorus compounds was combined withpermanent growing of their industrial production andextension of number of these practically significantcompounds. By the variety of structures, the numberof known compounds and by their significance inbiological processes, phosporus in many aspects co-mpetes with carbon. In the recent years, growing con-tribution to the progress of organophosphorus che-mistry was introduced by electrochemistry. Severalreviews on the electrochemistry of organophosphoruscompounds has been published, however, most ofthem either considered experimental results publishedbefore 1985 [1, 2], or concerned uncommon problemsof synthesis [3]. In the electrochemistry of organophos-phorus compounds two directions of investigation areclearly seen. The first one considers generation ofoxidized (or reduced) forms of an organophosphoruscompound just on the electrode followed by thereac-tion of this form in solution. The second one assumesmediate oxidation (reduction) of substrate under theaction of ion-radicals, metal salts and complexes,halogens, etc., which capable of cyclic regenerationon electrode. Application of electrochemical methodsto the chemistry of organophosphorus compoundsallowed to increase the level of understanding reactivi-ty of cation- and anion-radicals with reaction centeron the phosphorus atom, elementary act of the elec-tron transfer reaction, and the role of chemical structureof substrate, medium and nature of the electrode. Re-actions of various substrates with electrochemicallygenerated nucleophilic, electrophilic or radical organo-phosphorus intermediates were actively studied. Pre-ference of electrochemical methods consists also inthe possibility of following up the rate of reagentgeneration and studying the mechanism of transforma-tion of a substrate induced by electron transfer andobtaining quantitative information of the process (rateconstants for certain steps and thermodynamic charac-

teristics). Use of mediatoric electrosynthesis allowedto extend significantly the number of organophos-phorus compounds capable of oxidation (reduction)and to propose principally new and conventionalmethods for the synthesis on the ground of whitephosphorus, phosphorus chlorides and phosphorusacids derivatives, including electrochemical metallo-complex catalysis, in the chemistry of organophos-phorus compounds.

The significance of collected new experimentaldata requires their consideration and theoreticalanalysis. In this review we attempted to consider basicdirections and development of the organophosphoruselectrochemistry in the recent decades and to predicttrends and prospects of future research.

Electrochemically Generated OrganophosphorusCation-Radicals.

Electrochemical oxidation of different organophos-phorus compounds, first of all phosphines, and sta-bility of the process primary products was studied indetails by classical voltammetry, electrolysis withcontrolled potential and EPR [436].

All tertiary phosphines (excluding trimesitylphos-phine) are oxidized irreversibly and consume less thanone electron per a molecule (Table 1). Only trimesityl-phosphine cation-radicals (n = 1) are enough stablefor registration both by voltammetry and by means ofEPR spectroscopy. The EPR spectra of Mes3P

.+ cationradicals show that the paramagnetic particles are themonomeric cation radicals with spin density localizedpredominantly on the phosphoruss-orbital. Thesespectra consist of two bands due to interaction of theunpaired electron with31P nucleus with a significanthyperfine splitting on the element nucleus (dH 9.0e,giso 2.00,g|| 2.008,gZ 2.016). Similarly, the tetraaryl-diphosphine cation radicals [Ar2PPAr2]

+. haves-con-figuration [8].

RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 9 2001

1394 KARGIN, BUDNIKOVA

Table 1. Electrochemical characteristics of oxidationpolarization curves for primary and secondary phosphinesand phosphorus acids esters (reference electrodeAg/AgNO3, c 0.1 M) [4]ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ

Compound ³ ð1/2, V ³ n, 3eÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄ

R1R2R3P ³ ³ÄÄÄÄÄÄÄ ³ ³Ph3P ³ 0.90 ³ 0.64(2,4,6-Me3C6H2)3P ³ 0.37 ³ 1.00(4-MeOC6H4)3P ³ 0.60 ³ 0.41(4-MeOC6H4)2PPh ³ 0.64 ³ 0.41(4-MeC6H4)3P ³ 0.74 ³ 0.65(3-ClC6H4)PPh2 ³ 0.94 ³ 0.63(4-ClC6H4)3P ³ 1.14 ³ 0.66Et2PPh ³ 0.84 ³ 0.63i-Pr2PPh ³ 0.80 ³ 0.66Bu2PPh ³ 0.84 ³ 0.64Et3P ³ 0.92 ³ 0.62Bu3P ³ 0.82 ³ 0.66(C5H11)3P ³ 0.76 ³ 0.65(NCCH2CH2)3P ³ 1.27 ³ 0.60R1R2PH ³ ³ÄÄÄÄÄÄÄ ³ ³Et2PH ³ 0.94 ³ 0.62Pr2PH ³ 0.91 ³ 0.65Bu2PH ³ 0.88 ³ 0.63(C5H11)2PH ³ 0.85 ³ 0.66(C6H13)2PH ³ 0.83 ³ 0.65EtPhPH ³ 0.92 ³ 0.60Ph2PH ³ 0.86 ³ 0.98(4-MeC6H4)2PH ³ 0.83 ³ 0.96(4-MeOC6H4)2PH ³ 0.79 ³ 0.92(4-ClC6H4)2PH ³ 0.95 ³ 0.94(RO)nR33nP ³ ³ÄÄÄÄÄÄÄÄÄ ³ ³(MeO)3P ³ 1.66 ³ 0.64(EtO)3P ³ 1.60 ³ 0.66(BuO)3P ³ 1.55 ³ 0.61(i-PrO)3P ³ 1.52 ³ 0.61(PhO)3P ³ 1.64 ³ 0.62(EtO)2PPh ³ 1.19 ³ 0.65EtOPPh2 ³ 1.05 ³ 0.64(i-BuO)3P ³ 1.52 ³ 0.62(s-BuO)3P ³ 1.53 ³ 0.64

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ

The first step in electrochemical oxidation is one-electron transfer resulted in formation of cation radicalwhich then enter to chemical reactions with the solu-tion nucleophilic components.

R3P + e3 764.... R3P+

..

The underestimation of the number of electrons inthe most cases (n < 1) shows a possibility of reaction

between the process primary product, the cation ra-dical, and the parent phosphine, which results inreduced effective concentration of the latter on theelectrode.

R3P+. + R3P 76 R3.P3

+PR3,

R3P+. + Nu 76 R3.P3

+Nu.

Electronic structure of diphosphoric particles R3P.3

R3P+ featured in a significant contribution of atomic

2s orbitals in P3P s,s* bond [9]. This increasesconsiderably hyperfine31P splitting in EPR spectra ascompared, for example, with that of R2P3R2P

.3. Thisis a result of the fact that the structural unity R3Premains in these dimers strictly pyramidal, both thegroups are absolutely symmetrical, and all the sixsubstituents R are equivalent.

The principal final products found at the anodeoxidation of Ph3P in acetonitrile were detected Ph3POand Ph3PH+. They appear due to the interaction ofPh3P

+. with residual water and, possibly, with thesolvent [5]. With glass carbon anode two irreversiblepeaks were observed, the first was assigned to oxida-tion of Ph3P into Ph3PO with involvement of water,and the second to oxidation of Ph3PH+ (its heightdepends on the H2O concentration) [10].

Coulombometric measuring of the number of elec-trons consumed at the oxidation of Ph3P in acetonitrileon the background of NaClO4 gives n = 0.931.0 [5].It is interesting that then value for secondary aromaticphosphines determined by voltammetry also close tounity, which points to slow reaction with the parentphosphine. Adding of water (10% v/v) to the solutionof Ph3P increases the wave up to two-electronlimitand leads to quantitative formation of Ph3PO [5].

Ph3P + H2O 3 23e 76 Ph3PO + 2H+.

In recent years certain understanding of influenceof a substituent chemical structure on the reactivityhas been achieved [4]. Oxidation potentials of triaryl-phosphines correlate with sum of HammetSsXconstants of X substituents in aromatic ring [134].

Ep = (0.34+ 0.06)SsX + (0.87+ 0.05); r 0.987,n 7.

Substitution H for a R group in the phosphinemolecule does not affect substantiallyEp value.

Esters of P(III) acids are oxidized on platinum elec-trode in acetonitrile at much more positive potentialsgiving one clear anodic wave. One can assume prin-cipal analogy of the oxidation mechanisms for tertiaryamines and phosphorus(III) esters. This is supported


ELECTROCHEMISTRY OF ORGANOPHOSPHORUS COMPOUNDS 1395

by the fact that oxidation half-wave potentials oftriphenylphosphine, ethyl phenylphosphinite and di-ethyl phenylphosphonite correlate satisfactorily withthe sum of Kabachnik’s induction constants.

Ep = (0.07+ 0.02)SsIf + (0.84+ 0.06); r 0.9998,n 3.

Substitution of alkoxy groups in trialkyl phos-phates by dialkylamino groups shifts markedly half-wave potentials to the less positive values (Table 1),therewith, additivity of contributions of each intro-duced amino group to the valueE1/2 (oxidation)occurs in a first approximation. The observed shift inthe oxidation potential probably results from theincrease in the stabilizing influence of the substituenton the electron-deficient reaction center in the cationradical due to resonance conjugation between nitrogenor oxygen lone electron pair and half-vacant one-elec-tron orbital of the phosphorus atom.

Bulky ligands at phosphorus and blocking the pos-sible reaction centers in benzene rings decrease reac-tivity of the primary cation radicals [4, 11] so thatpreparative synthesis of cation-radical salt, the tri-methylphosphinylium perchlorate, and its investiga-tion by means of EPR spectroscopy becomes possible[12, 13]. Thus, at the anodic oxidation of Mes3P on aplatinum electrode in a CH3CN3THF mixture (2 :1)on NaClO4 background the radical cation perchlorateis formed and drops down as dark-violet crystals. TheEPR spectrum of the polycrystalline sample has thefollowing parameters:A

P|| 412, A

PZ 162 Oe,g|| 2.0092,

gZ 2.0044.

Population of the phosphorusp and s orbitals ps~0.07 andpp ~0.4 indicates that the cation-radical ismonomeric and possess its unpaired electron on thephosphoruss orbital.

Electronic structure of primary cation radicalsdefines their reactivity. These particles can possessdual reactivity, reacting with different substrates eitheras a radical or as an electrophile and accepting elec-trons on their emptyd orbitals. Below we illustratesuch possibilities, although exact data on the reactionmechanism are not always available.

Investigation of interaction of Mes3P+. cation

radical with solvents (CHCl3, CH3CN, and THF) [14]showed that the radical-type abstraction of hydrogenatom from the solvent occurs:

Mes3P+. + HSolv 76 Mes3+PH + Solv.

6 further transformation products.

It was also found that in time the dark-violet colorof the salt solution transforms into light-yellow,

therewith, dissolved oxygen accelerates this process.

Only one another case is known when a stableorganophosphorus cation radical is formed: fromphosphaalkene RP=C=PR [15], and its stabilization isachieved by introduction of bulky sterically hinderingtert-butyl substituents into aromatic ring R. Due tothe progress in the chemistry of lowcoordinationphosphorus compounds the realizing of red-ox pro-perties of such compounds becomes significant. Elec-trochemical oxidation of diphosphaalkenes in THFresults in formation of cation radicals at +1.5 V(hereinafter relatively to a saturated calomel electrode)[15]. These radicals are presumed to consist of tworotamers with distortion of their structures coincidingwith the Jan-Teller distortions in allene. However,such sterically hindered cation radicals are little re-active and not interesting for synthesis.

Thus, cation radicals generated electrochemicallyfrom phosphorus(III) compounds possess both posi-tive charge and unpaired electron localized at thephosphorus atom. Such cation radical as a ruleexhibits strong electrophilic properties and can reactwith various nucleophiles. This property was used fortheir application to many chemical processes con-sidered below.

Oxidation of organic phosphorus(III) derivativesgives a way to the synthesis of compounds possessingan element3carbon bond in aromatic or heteroaromaticring, the element is halogen, nitrogen, sulfur and other.The cation-radicals of P(III) derivatives generated byanodic oxidation react with aromatic compounds,olefins and with compounds bearing a labile hydrogenin substrate molecule (water, alcohols, phenols, thiolsand other). These processes follows to the generalscheme:

X3P+. + HY 76 [X3P/YH]+.76 X3P3Y + H+,

+

3e3

Y = RO, RNH, RSH, ArO.

Cation-Radical Phosphorylationof Aromatic Compounds

Among the directions of development electrosyn-thesis of organophosphorus compounds based on elec-trochemical generation of cation radicals, one includesreactions with aromatic substrates of low nucleophi-licity. In some earlier publications formation of tri-phenylphosphonium salt in the reaction of triphenyl-phosphine cation radical in the presence of benzenewas mentioned [6, 16], but due to low yield ofproducts this method probably cannot be practicallyapplied.



Anodic oxidation of tetraalkyl pyrophosphites inthe presence of aromatic compounds (benzene,toluene) leads to formation of dialkyl arylphosphona-tes [17]:

(RO)2POP(OR)2 + ArH 76 (RO)2OP(O)Ar + H+.

3ne3

One target in the electrochemistry of organiccompound is studying the reactivity of electroche-mically generated intermediates, including cationradicals. Knowing characteristics of their reactivityallows the prognosis of direction of electrochemicalsynthesis and selectivity control. However, obtainingsuch characteristics meets experimental problemscaused by high reactivity of the electrode processintermediates so that even registering of their forma-tion with modern physicochemical methods com-monly failed. Therefore only in selected cases directmeasurement of absolute rate constants can beachieved. One can overcome these problems in a greatextent using the method of competition reaction inrespect of electrophilic substitution in aromatic ring.

This method was first applied to the electroche-mical reactions in the study of reactivity of the trialkylphosphite cation radicals generated by anodic oxida-tion of complete phosphorous esters [18]. Applicationof this method allowed a conclusion on the radicalnature of the particles generated at the anodic oxida-tion of sodium dialkyl phosphites [19]. Previouslywas suggested that similarly to the cation radicals oftertiary phosphines (and complete esters of phosphorusacids) the following reactions of anodic substitutionare electrophilic reactions with participation of phos-phonic cations (RO)2PO+, the products of furthertransformation of primary dialkylphosphite cationradicals:

76 (RO)2PO. 76 (RO)2PO+.3Na+ 3e3

(RO)2P(O)Na3 e3 76 [(RO)2P(O)Na]+.

The competitive phosphorylation of toluene andbenzonitrile with anode generated trialkyl phosphitecation radicals and diethyl phosphonic radicalsshowed [19] that reaction of the trialkyl phosphitecation radicals obeys regularities of electrophilicsubstitution: the character in change of relative rateconstantkPhR/kPhH in the substitution reaction and theselectivity factorSf = log (fn/fm) (wherefi is the factorof partial substitution rate at theipso position oftoluene) in going from toluene to benzonitrile is inline with change of such characteristics for the mostknown electrophilic reagents. In the case of particlesgenerated by electrochemical oxidation of sodium di-

alkyl phosphates the data obtained clearly indicateradical character of the reaction,kPhCN/kPhH > 1, Sf > 0[19]. The authors succeeded in corfirming the ob-tained results by comparison of reactivity of the di-ethyl phosphonic radicals generated by either electro-chemical oxidation of sodium dialkyl phosphites, orphotolysis of tert-butylperoxide in diethyl hydrogenphosphite. Competitive phosphorylation of a series ofalkylbenzenes with these particles showed closenessin isomeric composition of arenephosphonates inthese two studied reactions [19].

In [20] was shown that anodic oxidation of dialkyltrimethylsilyl phosphates in the presence of benzeneresults in formation of dialkyl benzenephosphonate.

(RO)2POSiMe3 e3 76 (RO)2POSiMe+.

3e3, SiMe3+ , 3H+777776 (RO)2P(O)Ph.

PhH

It was assumed that like the case of electrochemicaloxidation of trialkyl phosphates, the considered pro-cess passes through formation of reactive cationradical that attacks aromatic ring electrophilically. Themore detailed proof of the reaction mechanism andreactivity of the intermediates in the anodic oxidationof phosphite (RO)2POSiMe was conducted by themethod of competitive reactions [21]. By the study ofinteraction of the particles formed in electrochemicaloxidation of diethyl Ia and dibutyl Ib trimethylsilylphosphates in the presence of benzene or its alkylderivatives were obtained experimentally the valuescharacterizing the intermediate substrate selectivity(KPhR/KPhH) and position selectivity (the selectivityfactor Sf = lgfn/fm) and values of partial replacementfactors inortho, metaandpara positions of alkylben-zenes. It was found that logarithm of the factor ofpartial replacement rate in themetaandpara positionsin alkylbenzenes correlates with the Brown-Okamotoelectrophilic s+ constants. The regression relationslog fn,m = F(s+) for the reactions of the intermediatesof electrophilic oxidation of phosphatesIa and Ib aregiven below:

Ia: log fn,m = 3(1.40 + 0.03)s+; r3 0.998, n 10.

Ib : log fn,m = 3(1.46 + 0.08)s+; r 0.985, n 10.

Another significant parameter is the ratiolog fn(PhMe)/Sf (PhMe), which equals to 1.23 and 1.44for the phosphatesIa and Ib , respectively, whichin each case is close to the slope of logfn on Sf plotobtained in the study of 47 electrophilic substitutionreactions in the toluene molecule in the form oflog fn(PhMe) = (1.316+ 0.102)Sf(PhMe). These dataclearly attest electrophilic character of interaction of



the intermediates formed in anodic oxidation of phos-phates with aromatic compounds.

In this respect, two schemes (1, 2) can be conside-red for the oxidation of silyl phosphates in the pre-sence of ArH.

(RO)2POSiMe3 e3 76 (RO)2POSiMe+.

3e3, SiMe3+ , 3H+777776 (RO)2P(O)Ar,

ArH

(RO)2POSiMe3 2e3 776 (RO)2PO+3SiMe3

+

3H+76 (RO)2P(O)Ar.ArH

(1)

(2)

According to Scheme 1, the aromatic compound isattacked by silyl phosphite cation radical and tri-methylsilyl cation eliminates after formation of a bondbetween phosphorus and aromatic ring carbon.Scheme 2 assumes that dialkyl phosphonic cationformed at elimination of trimethylsilyl cation fromthe product of disproportion of silyl phosphite cationradical, reacts with ArH. The Scheme 1 is supportedby the character of variation offo values in the seriestoluene3tert-butylbenzene. As far as increase in thesize of alkyl substituent the rate of substitutuion inortho position decreases, anodic oxidation of silylphosphates probably lead to formation of the particleswith steric requirements higher than that of trialkylphosphite cation radicals [18], probably the cationradicals with the groups larger than trimethylsilyl inthe (RO)2

+POSiMe3 molecule.

It was shown [22] that anodic oxidation of trialkylphosphite in the presence of benzene derivatives ArHafter treatment of the reaction mixture with a dealkylat-ing reagent gives dialkyl arylphosphonates. Thesereaction where applied to the compounds of generalformula (RO)nPR33n (n = 133) in the presence of ArH,E1/2(ArH) > E1/2 (organophosphorus compounds).Dealkylation of the formed intermediate leads toa compound wit a new P3Ar bond in the molecule,arylphosphinates or arylphosphine oxides [23].

P3OR 3 2e3 76ei ArH

P3Areig+

OR

76 P(O)3Ar.ei3R+

At the electrochemical oxidation of Pr3P in thepresence of toluene and water the anode generatedtertiary phosphine cation radicals were shown to reactwith the aromatic compound and the parent phosphine,but not with water [24]. The first reaction leads toarylphosphonium cation, the second one to diphos-phonium dication which further reacts with water.

Such reactivity is probably can be explained by

R3POH2 76 R3POH + H+,. +

3e3

R3PAr + H+,+

R3P3PR3 76 R3POH + R3PH,

+

+ + + +H2O

77763e3

3e3

ArH

7776H2O

R3P

9

9

997776

R3P+.7

assumption that in the studied reactions the cationradicals formed behave as radicals, despite the factthat non-compensated positive charge endow theradicals with electrophilic properties. On the groundof the obtained results a method for the electrosyn-thesis of arylphosphonium salts in 85393% yield wasdeveloped, in a non-diaphragm electrolyser in thepresence of water solutions of strong acids (HClO4,HBF4).

R3+PH + ArH + 23e 76 R3

+PAr + H2.

It was assumed that synthesis of quasiphosphoniumsalts at the electrochemical oxidation of tertiary phos-phines in the presence of alcohols, amines, phenols,thiols and disulfides proceeds via intermediate forma-tion of bisphosphonium salts while the synthesis ofarylphosphonium cations proceeds by the mechanismof free radical aromaticsubstitution.

Reactions with Organophosphorus Compounds

Whereas reactions of electrochemically generatedorganophosphorus cation radicals with various nucleo-philes were studied for a wide range of compounds,the routes of die of the organophosphorus cationradicals in the absence of a nucleophile, e.g., in aninert solvent in the presence of an indifferent electro-lyte remained unstudied for a long period. Theseroutes obviously compete with the main electrosyn-thesis process and can behave as more or less depend-ing on the substrate nature and reactivity and reactionconditions. Knowing of these reaction pathways andability of controlling them would promote develop-ment of existing synthetic methods and elaboration ofnew approaches.

In this connection, we studied electrochemicaloxidation of hexaethylphosphoric triamide in aceto-nitrile solution of sodium perchlorate [25]. The choiceof this compound as a model one is defined by thereduced reactivity of the cation radical in phosphorictriamide toward nucleophilic reagents due to strongelectron-donor effect of amodo group. In this case asa product of preparative oxidation was isolated acyclicbiphosphonium dication with a

+P3

+P in its molecule.

Its formation mechanism is probably as follows:

(Et2N)3P 33e 76 (Et2N)3P+.,



(Et2N)3P+. + (Et2N)3P 76 (Et2N)3P+.3P(NEt2)3

76 (Et2N)3+P3

+P(NEt2)3.

33e

In this case we succeeded in registration of bothprimary and secondary cation radicals formed at theattack of the primary particle on the molecule ofparent compound. It is assumed that this later reactionis a main route of die away of the cation radical(Et2N)3P

+., which is attested by many fixations ofdimeric cation radicals at the oxidation of many typesof phosphorus(III) derivatives [26, 27]. An alternativepathway for the dication formation by dimerization oftwo cation radicals looks like the following:

2(Et2N)3P+.76 (Et2N)3

+P3

+P(NEt2)3.

This pathway is obviously less probable, due tostrong Coulomb repulsion of the reacting particles.Such explanation is confirmed by experimental EPRdata on formation of dimeric cation radicals.

Synthesized hexaalkylbisphosphonium salts reacteffectively with compounds containing labile hydro-gen and with dialkyldisulfides but do not react underordinary conditions with aromatic and unsaturatedcompounds [28].

Besides substituted phosphines, anodic oxidationreactions were performed with phosphrus(III) esters.Electrochemical oxidation of sodium dialkyl phos-phite in the presence of trialkyl phosphite leads toformation of tetraalkyl pyrophosphates in amostquantitative yield [29].

(RO)3P + (RO)2P(O)Na76 (RO)3P3OP(O)(OR)232e3,3Na+

+

77776 (RO)2P(O)OP(O)(OR)2 + (R`O)2P(O)(OR).(R`O)2P(O)Na

3Na+

Anodic oxidation of sodium dialkyl phosphite(RO)2P(O)Na in acetonitrile with NaClO4 backgroundproceeds with average transfer of 0.730.75 electronper a molecule of initial phosphorus-containing com-pound and forms a mixture of compound amongwhich were detected [30] tetraalkyl pyrophosphite(RO)2POP(OR)2 (45%), sodium dialkylphosphate(RO)2POONa (25%), subphosphate (RO)2POP(O).(OR)2 (20%), tetraalkyl pyrophosphate (RO)2P(O).OP(O)(OR)2 (5%), and dialkyl phosphorous acid(RO)2PHO (5%).

The following scheme was proposed for thisprocess:

(RO)2P(O)Na7776 [(RO)2PO]+32e3, 3Na+

7776 (RO)2POP(O)(OR)2 + Na+(RO)2PONa

299

(RO)2PONa

(RO)2P(O)P(OR)2 + (RO)2PO2Na

It was shown later that whereas electrooxidation ofsodium dialkyl phosphites withn-alkyl groups leadsto the synthesis of tetraalkyl pyrophosphites (RO)2 .POP(OR)2 mainly, while lithium dialkyl phosphatesand the sodium salts with branched alkyl groups formtetraalkyl hypophosphates (RO)2P(O)P(O)(OR)2 [31].Earlier was established [32,33] that at the electro-chemical oxidation of (RO)2POM in the first stepphosphonyl radicals are formed, and it was assumedthat the following dimerization results in formation ofeither subphosphates or hypophosphates, dependingon the structure of substitutents.

9

97

7776

7776

R = Alk

R = i-Alk

(RO)2P(O)OP(OR)2,

(RO)2P(O)P(O)(OR)2.2(RO)2PO

.

Intermediate subphosphoric ester reacts withanother (RO)2PONa molecule affording pyrophosphite.Effect of the metal nature is supposingly assigned tothe difference in absorption properties of the salts onthe anode, but this was not confirmed unequivocally.Reaction of dialkyl phosphorous satls with halogensgave similar results.

Among the works considering oxidation of diaza-phosphorinane compound, the results on sixmemberedring formation with two nitrogen and one phosphorusatoms should also be mentioned [34, 35]. However, inthis case the process includes one-electron oxidationof the ring nitrogen rather than phosphorus atom toform unstable cation radical stabilized by furtherproton elimination from the diaminomethyl fragmentfollowed by anodic oxidation of the formed radicalinto carbocation. Yield of the formed 1,3-di-para-tolyl-5-toluidinomethyl-1,3-diaza-5-phosphorynan-2-ylium perchlorate was 52%.

Reactions with Amines,Amides and Urea Derivatives

Triphenylphosphine cation radical is enough strongelectrophile and it reacts with different nucleophilesat room temperature. Among the early studied wereits reactions with amines. Thus, in the presence ofprimary amines it forms monoalkylammonium saltsin 50360% yield [36].



Ph3P76 Ph3P+.776 Ph3P3NH2R

. +NH2R

776 Ph3P3NHR.+

3e3, 3H+

The Ph3P votammogram with NH2R excess showsapproximately tree-fold increase in the phosphineoxidation wave with coulombometric electron numberhigher than two, due probably to the side oxidation ofa part of the amine at the applied potential.

Addition of Ph3P+. cation radical to the amides

RCONH2 (R = Ph, Me, Et, Pr) andN,N-disubstitutedureas CO(NHR)2 (R = Et, i-Pr, t-Bu, Ph,cyclo-C6H11)leading to formation of the corresponding nitrilesRCN and carbodiimides RN=C=NR as final com-pounds in 60390% yield was studied under the condi-tions of preparative galvanostatic electrolysis in a dia-phragmless electrolyser in CH2Cl2 (electrolyte 2,6-lutidinium perchlorate or tetrafluoroborate) [37]. Itwas assumed that formation of such compounds canbe described by the schemes with metastable phos-phonium ion intermediate:

Ph3P 3 e3 76 Ph3P+.7776 Ph3P3OC(R)=NHRCONH2

+

3e3, 3H+

76 RCN + Ph3PO,3H+

Ph3P+.7776 Ph3P3OC(NHR)=NR3e3, 3H+

CO(NHR)2 +

76 RN=C=NR + Ph3PO.3H+

Addition of Ph3P+. ion radical to PhCSNH2 andN,N-dicyclohexylthiourea leads to formation ofPh3PS and corresponding dicyclohexylcarbodiimide[37].

Electrooxidation of tertiary phosphines in MeCNin the presence of C5H11NH3F in a Teflon cell withEt4NBF4 background leads to difluorophosphoranesby addition reaction [38], yield 69385%.

R3P + 2C5H11NH3F 76 R3PF2 + 2C5H11NH3+.

323

e

Similarly, from secondary phosphines RR`PH (R,R` = Bu, C5H11, Ph, Et) in the presence of C5H11NH3Fand pyridine, trifluorophosphoranes RR`PF3 wereobtained (yield 54 to 74%) [39].

Anodic oxidation of sodium dialkyl phosphite(RO)2PONa in the presence of secondary aliphaticamines R2NH allows obtaining of compounds with aP3N bond [40]. Characteristics of the oxidation waveof the salt (RO)2PONa in acetonitrile on a platinumelectrode with sodium perchlorate background showedthat the oxidation under the conditions of the experi-

ment occurs at the halfwave potential practically in-depending on the nature of R group [E1/2 0.63 Vreferred to Ag/AgNO3 (c 0.1 mol l31) in MeCN]with average transfer of 0.45 electron per a moleculeand the oxidation wave slope 250 mV. Thelimitoxidation current of Mes3P is known to correspond toone electron transfer per a molecule [4]. Certainunderestimation of the limit oxidation current ofsodium dialkyl phosphite as compared with that fortrimesitylphosphine is a result of the above mentionedreactions of the oxidation products with the parentcompound. In the presence of dialkylamine, the oxida-tion wave of (RO)2PONa slightly decrease, probablydue to the amine interaction with the phosphoniccation radical. Preparative electrooxidation of(RO)2PONa in the presence of secondary aliphaticamines leads to formation of amidophosphites in~30% yield [40]. O,O-Dialkyl-N,N-dialkylamido-phosphates are formed in 5310% yield and the yieldrises with rise in the amine concentration. The follow-ing scheme of the process was proposed:

(RO)2P(O)Na 3 23e 76 [(RO)2PO]+ + Na+

I

[(RO)2PO]+ + I 76 (RO)2POP(O)P(OR)2 + Na+

II

II + R2NH 76 (RO)2PNR2 + (RO)2P(O)OH

(RO)2P(O)OH + I 76 (RO)2P(O)ONa + (RO)2P(O)HÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

3(RO)2PONa + R2NH 3 23e 76 (RO)2PNR2

+ (RO)2PHO + (RO)2P(O)ONa + 2Na+

In the correspondence with the scheme, the averageelectron number per a molecule is 0.66. The growingamine concentration rises contribution of the follow-ing reaction:

I 3 23e 76 [(RO)2PO]+ + Na+,

[(RO)2PO]+ + R2NH 76 (RO)2PNR2 + H+,

I + H+76 (RO)2PHO + Na+.

A feature of this process is high purity of thereac-tion product due to the absence of side formation ofamine hydrochlorides.

Reactions with Alcohols, Thiols and Disulfides

Anodic oxidation of Ph3P in the presence of di-sulfides (or thiols) leads to formation of alkylthio- andarylthiotriphenylphosphonium salts in a good yieldwhen HClO4 is used as a background electrolyte[37, 41].



2Ph3P + RSSR 76 2Ph3+PSR.

323

e

Voltammetric study of oxidation of (RO)3P andRSSR mixtures on the NaClO4 background in CH3CNgives grounds for the assumption that formation of(RO)3P

+.SR has catalytic character with primary di-sulfide oxidation [42].

Electrochemical oxidation of Ph3P in CH3CN3ROH(R = Me, Et, Pr) mixtures with benzoic or succinicacid additives allows obtaining of alkoxytriphenyl-phosphonium salts (Ph3POR)+ClO4

3 in 40354% yieldin a diaphragmless electrolyzer [43]. Allylic andbenzylic alcohols convert under the same conditionsinto the corresponding alkoxy- or alkylphosphoniumions [44]. By this method, alkoxyphosphonium saltswere synthesized froma- and b-cholestenols withvarious nucleophiles Bu4NX (X = Br, Cl, F, N3, SCN).Separation of the electrolyzer spaces reduces yield to20330%.

A route to electrochemical desoxygenation ofprimary, secondary and tertiary benzilic and unsatu-rated alcohols was proposed, based on the electrolysisof a mixture of the corresponding alcohol, phosphineand Et4NBr in MeCN [45]. In the anode space of theelectrolyzer at the direct current the alcohols transformto alklanes.

N-Hydroxy compounds of keto- and aldoxime typeRR`C=NOH (R, R = Me, Me; cycloC6H10; Ph, Me;Pr, H; Ph, H) and hydroxamic acids RCONHOH(R = Me, Ph) react with anode generated cationradical Ph3P to form, in the case of ketoximes, cor-responding amides R`CONHR, in the case of aldoxi-mes corresponding nitriles RCN, and in thecase ofacids the substituted ureas (RNH)2CO [46]. Thefollowing mechanism was proposed for the process:

Ph3P 76 Ph3P+.

Ph3P+. + RRC=NOH776 Ph3PON=CRR3e3,3H+

+

or RCONHOPPh3.+

The initially forming phosphonium compounds arenot stable enough to be isolated and during the elec-trolyte teratmernt exert the Beckmann rearrangementto form the above final compounds.

Thermal decomposition of electrochemicallygenerated alkoxytriphenylphosphonium tetrafluoro-borates forms a basis for the method of fluorine sub-stitution for hydroxy group in primary and secondaryalcohols [47]. The substitution process consists of twosteps: electrolysis with direct current of a mixture of

an alcohol, Ph3P and Ph3PH+BF43

in CH2Cl2, remov-ing of the solvent in a vacuum and refluxing of theresidue in dioxane or THF. The secondary alcohols of3b-hydroxysteroid or 2-adamantanol groups transforminto the corresponding fluorine derivatives in a satis-factory yield when elimination is hindered. Thesimilar reaction with primary alcohols requires rigidconditions: boiling in a solvent.

Dehydroxysubstitution reactions of anomerichydroxy group in some of sugars also proceed underthese conditions and are initiated by anodic oxidationof triphenylphosphine [48]. Electrolysis of 2,3:5,6-di-O-isopropylidene-a-D-mannofuranose and 2,3,4,6-tetra-O-benzyl-D-glucopyranose in the CH2Cl231,2-dimethoxyethane/PPh3/PPh3HBF4 and CH2Cl2/PPh3/Et4NCl systems leads to desoxygenation at theanomeric position in the shugar with simultaneousfluorination or chlorination via intermediate alkoxy-phosphonium ions.

Anodic Phosphorylation of Olefins

Electrochemical generation of organophosphoruscation radicals in the presence of unsaturated com-pounds allows obtaining of various compounds withP3C bonds under mild conditions.

1-Alkenyltriphenylphosphonium salts are knownas useful intermediates in some of synthetic reactions,but their preparation is commonly a labor consumingmultistep procedure. Electrochemical oxidation ofPh3P in the presence of an olefin is a useful methodfor preparation of phosphonium salts [49].

O9O=99R9O===FFcPiec+ (65%), HC==CH3P

(CH2)n

iec+ [n = 336(50360%)],

Ph3P + C =CH3 76 C=C3PPh3,ei

3e3ei+

7DhcPiec+ (40%),3cqP

iec+ (80%),

3cqPiec+ (100%),cMe 8EI<P

iec+ (90%);

9

QS393

The process was performed in CH2Cl2 with back-ground electrolyte luthidinium perchlorate (LutClO4)and K2CO3 as a proton acceptor, in a diaphragmlesselectrolyzer [49].

In the presence of allylsilanes (the electron-richolefins) the anodic oxidation of Ph3P leads to allyl-triphenylphosphonium salts [49].

Ph3P + CH2=CH3CH23SiMe3 76

76 Ph3+P3CH23CH=CH2 (71%),

33

e



Ph3P + CH2=C(Me)3CH23SiMe3

76 Ph3+P3CH23C(Me)=CH2 (68%).

33e

The electrochemical method is advantageous dueto the absence of side products, the 1-alkenyltriphenyl-phosphonium salts formed in traditional methodswhich ordinary difficult to separate from the mixture.The higher yields were achieved with the cathodeswith high hydrogen overvoltage, e.g., with lead.

Interesting that reaction of Ph3P+. cation radicals

with electron-deficient olefins, e.g., witha,b-unsatura-ted carbonyl compounds under the similar conditionsare formed saturated 3-oxoalkyltriphenylphospho-nium salts [49, 50].

Ph3P + RCOCH=CHR 76 RCOCH2CH(R)+PPh3.

33

e

R, R (yield, %): Me, H (72); Et, H (84); H, H (79); EtO,H (83); H2N, H (55); (CH2)2 (63); (CH2)3 (67); O(CH2)2(61).

Electrolysis of Ph3P in a galvanostatic regime indichloromethane in the presence of a cycloalkeneleads to formation of the corresponding 1-cycloal-kenylphosphonium salt. Such are merely stable [51].1-Cycloalkenyltriphenylphosphonium perchlorate canbe prepared in one step under mild conditions. Theproduct yields are 53366%.

Ph3P+. +�

�999R 7776

LutClO4 �

�99RhjHPPh3

+.

7763e3, H+

�

�999RjPPh3ClO4

3+

Electrochemical oxidation of Ph3P in the presenceof a cyclic enol silyl or enol ether in a diaphragmlesscell leads to 2-oxocycloalkyltriphenylphosphoniumsalt in 20396% yield [49, 52].

Y = SiMe3, P(O)(OEt)2, COMe; R = H, Me;n = 133.

Advantage of this procedure is the one-step processand milder conditions compared to the classical pro-cedures. In the most cases the reaction product is anisomeric mixture with predominance oftrans isomer.However, with R = Me andn = 2 only one isomer

was obtained, but the stereochemistry remains in-definite. From these salts and aldehydes by the Wittigreaction were obtained (E)-2-alkylidenecycloalkan-1-ones in a good yield, used in pharmaceutical industry.

Allylsilanes RCHRSiMe3 under the similar condi-tions (lead anode, carbon glass cathode, diaphragmlesscell, CH2Cl2 solvent, LutBF4 background) form allyl-triphenylphosphonium salts R2PPh3BF4 in 24372%yield [53, 54]. The Ph3P addition is regioselective atthe g position.

Electrochemical phosphorylation of an olefin (e.g,cyclohexene and 1-heptene) with tetraalkylpyrophos-phites leads to a mixture of saturated and unsaturatedphosphonates with a new P3C bond in the molecule[17].

(RO)2POP(OR)2 +8A76 (RO)2P(O)7c

+ (RO)2P(O)7cB+ (RO)2P(O)7cDTheoretical and experimental investigations in the

recent years showed that the anode generated organo-phosphorus cation radicals can show both electrophilicand radical properties. In the last case the furtherreaction is homolytic abstraction of hydrogen atomfrom a compound where the hydrogen is labile.

R+. + YH 76 RH+ + Y..

Here YH is a donor of hydrogen atom.

This feature of cation radicals in some P(III) deri-vatives with hydrophosphoryl group >P(O)H wassuccessfully used for involvement of electrochemical-ly inactive organic (including organophopshorus)compounds to the reactions with olefins initiated byelectrochemical oxidation of organophosphorus com-pounds of (i-PrO)3P, (EtO)2POSiMe3, and Bu3P type[55]. Their E1/2 values are 1.45, 1.65 and 0.80 V,respectively [referred to Ag/AgNO3 c 0.01 mol l31 inMeCN]. At the electrochemical oxidation of theseorganophosphorus compounds in the presence ofexcess diethyl phosphorous acid cyclohexene additioncompounds were detected in the reaction mixture in aratio depending on the nature of the organophosphoruscompound, the initiator of the radical reaction. On thebasis of the experimental data the following schemewas proposed for the process:



7D(i-PrO)2PO +.

299

(i-PrO)2PHO 3H.

2993H+

7c.

76 (i-PrO)2P(O)7c+

3e376 (i-PrO)2P(O)

8

99H+

7c(i-PrO)2P(O) + (i-PrO)2PO. 7c(i-PrO)2P(O)

Chain mechanism

ÄÄÄÄÄÄÄÄÄÄÄÄ

The phosphonic radical formed reacts with cyclo-hexene to form in the first step the radical adduct.

The catalyst is regenerated not completely: a partof the cation radicals is involved to the reaction withcyclohexene and is consumed.

777763e3 , 3i-Pr+

(i-PrO)2P(O)7c+

76 Products offollowing

transformations

[(i-PrO)3P]+ +7@


Carbenium intermediate formed in the reaction isdeprotonated affording a mixture of unsaturated phos-phonates. The general scheme of electrocatalyzedolefin phosphorylation is given below.

Thus, the cation radicals generated by electroche-mical oxidation of phosphorus(III) derivatives underappropriate conditions can abstract hydrogen atomfrom the dialkyl hydrogen phosphite molecule and achain free radical reaction of hydrophosphorylation ofalkene double bond is initiated.


Further investigations showed that the trialkyl-phosphine cation radicals in a similar way initiateaddition of dialkylthiophosphites (RO)2P(S)H toalkenes resulted in formation ofO,O-dialkyl-S-alkyl-thiophosphonates. Coulombometric measurementsindicated average consumption of 3.3 F electricity forthe oxidation of 1 mol of R3P under these conditions.This fact indicates regeneration of the oxidized phos-phine in the process of electrolysis. It can be assumed


that the electrochemically generated cation radical ab-stracts hydrogen atom from the (RO)2P(S)H molecule,and the formed therewith protonated parent phosphineR3PH+ transforms into electrochemically activephosphine under the action of the base B (triso-dium phosphate) added to the reaction mixture, whilethiophosphoryl radical (RO)2PS. initiates chainprocess of (RO)2P(S)H addition at the alkene doublebond.

R = Et, Bu, i-Pr; R = Et, Pr.



The P(III) cation radicals were found to react withthiols also by the route of homolytic hydrogen abstrac-tion from the sulfhydryl SH group [56]. Oxidation oforganophosphorus compound with the general formulaX3P (X = R, Et2N, EtO) initiates thiol adddition at theolefin double bond forming nonsymmetrical sulfide.Simultaneously with this reaction, formation of di-sulfide R2S2 occurs. It is assumed that the reactionproducts are formed by the following scheme:

RShch.

R`

776RSH

3RS. RShch

R`

fH776

khR`

776

9

97

1/2R2S2

X3P76 X3P+.76 X3PH + RS.

3e3RSH

The relatively high yield of sulfides and disulfideson the consumed electricity (in the case of tributyl-phosphine oxidation extends 100%) and the fact thatcertain amount of electricity is consumed for theformation of the alkene phosphorylation compoundsindicate that in the process of electrolysis occursradical-chain reaction of thiol addition to the alkene,initiated probably by the thioalkyl radical. The mostprobable way of the radical RS. formation is reactionof the thiol with the electrogenerated X3P cationradicals.

A weighty argument in favor of the proposed me-chanism of reaction of X3P+. cation radicals with thiolsis the experimental observation of the absence ofalkylthiophosphonium salts X3PSR+ among the pro-ducts of electrosynthesis expected in the case of elec-trophilic mechanism. Besides, complete phosphineoxidation consumes much more electricity than wouldbe required in the absence of thiol at the electrosyn-thesis of alkenylphosphonium salts (2 F).

In the process of electrochemical phosphorylationof arens with the phosphonic radicals formed on theanode at the oxidation of sodium dialkylphosphites[19], benzene and its derivatives with a substituentdefining direction of the attack in the aromatic ringwere used as the aromatic substrates. At the oxidationof (RO)2PONa in the presence of phenylacetylenewhich possess several reaction centers in its moleculeobtaining of small amounts (9%) of diethyl styryl-phosphonate (EtO)2P(O)CH=CHPh (I ), diethyl2-phenylethylphosphonate (EtO)2P(O)CH2CH2Ph (II ),and diphosphonate (EtO)2P(O)CH2CH(Ph)P(O)(OEt)2(III ) has been gained [57]. The proposed mechanismof radical addition however does not accounts for theprocesses with involvement of PhC=CH as an acidand ammetric investigations are not described, andtherefore this scheme is not a reliable one.

(EtO)2PONa 76 (EtO)2PO. + Na+,33

e

(EtO)2PO. + PhC=CH 76 (EtO)2P(O)CH=.CPh,

(EtO)2P(O)CH=CPh + (EtO)2PHO76 I + (EtO)2.PO,

I + (EtO)2.PO 76 (EtO)2P(O).CHCH(Ph)P(O)(OEt)2,

(EtO)2P(O).CHCH(Ph)P(O)(OEt)2 + (EtO)2PHO

76 III + (EtO)2.PO.

CompoundII is probably formed by cathode re-duction of styrylphosphonateI .

Electrochemical oxidation of ferrocene in the pre-sence ofO,O-diethyl thiophosphorous acid [58] leadsto formation of O,O-diethyl ferrocenylthiophos-phonate C5H5FeC5H4P(S)(OEt)2 with 11% yield onthe ferrocene. The routes for the formation of substi-tuted ferrocene are unknown. However, among theproducts occur [(EtO)2P(S)]2S and [(EtO)2P(S)]2which indicate probably intermediate generation ofthe free radical moiety (EtO)PS. capable of interactionwith the ferrocenium cation with formation of thio-phosphorylated ferrocene.

Use of anode generated triarylamine cation radicalas a mediator for the oxidation of organophosphoruscompounds allows to reduce anode potential and thusto extend the scope of phosphorylated aromatic sub-strates [59]. In the presence (AlkO)3P or(AlkO)2POSiMe the current for reduction of the cationradical falls to zero while oxidation current rises totwo-electron level (at 0.7 V). This effect can probablybe assigned to theECE mechanism. By preparativeelectrolysis a mixture of diethyl thiophosphonateisomers was isolated with isomeric distributionsimilar to those observed at the direct anodic oxida-tion of organophosphorus compounds in the presenceof alkylbenzenes.

Electrochemical oxidation of dialkyl thiophosphitesand their sodium and lithium salts in the presence ofolefins [60] proceeds via the step of formation of di-alkylthiophosphoryl radical (RO)2PS. on the anode.Addition of this radical to the alkene double bond

affords corresponding radical adduct(RO)2PSicei

R`..

Further transformations of the adduct branches intotwo paths. First one is splitting off a hydrogen atomfrom the dialkylthiophosphite existing in the reactionmixture or from the solvent (MeCN) which is resultedin formation of saturated compound, alkyl(cycloalkyl)-thiophosphonate. Second one is oxidation on the

anode into carbocation which is de-(RO)2PSicei+

R`,



protonated affording alkene-2-yl(cycloalkene-2-yl)-thiophosphonate. It is assumed that prevalence of thefirst or the second reaction path is defined by com-petitive adsorption on the anode of the initial com-pound and intermediates formed at the electrolysis,the olefin and radical adduct mainly.

Electrochemical oxidation of trialkyl and dialkyltrimethylsilyl phosphites, (RO)3P and (RO)2POSiMe3,in the presence of an olefin in a cell with a diaphragmproceeds with formation of a mixture of isomericdialkyl alkenyl(cycloalkenyl)phosphonates, thealkene(cycloalkene)-2-ylphosphonate predominates(yield 41386%) [61]. The scheme proposed for thisprocess is similar to the scheme of anodic phosphona-tion of aromatic compounds [18,20,23,33]. Probablythe active moiety attacking the alkene double bond isthe cation radical formed at one-electron oxidationof (RO)3P.

(RO)3P 33e 76 (RO)3P+..

This is followed by a step of second electron trans-fer, deprotonation and elemination of alkyl cation.

(RO)3P +kiR 3 2e3 76 (RO)3P(CH2)nCH=CHR`3H+

776 (RO)2P(O)(CH2)nCH=CHR`,3NuR+

Nu

(RO)3P+. +keeii76 (RO)3P7+ ieg.

76 (RO)3P7+ ieg.3e3

n = 0, 1; Nu = I3, RO3.

Formation of isomeric alkenylphosphonates isexplained by migration of the carbocationic center inthe substituent carbon chain by the mechanism of theproton transfer in the step of deprotonation of thephosphoryl intermediate.

In the case of dialkyl trimethylsilyl phosphite(RO)2POSiMe3 the trimethylsilyl cation eliminates.Note that the described reactions of the cation radicals(RO)3P

+. and other with olefins can be assigned toelectrophilic substitution reactions only due to theformal correspondence of the reaction products to thisscheme. Actually, the first step is radical addition ofthe cation radical to the olefin double bond withformation of radical adduct. Transfer of the secondelectron affords phosphorus-substituted carbocation.Another feature of these reactions is the fact that inthis case does not take place the common route of thecarbocation stabilization, namely, addition of[outside] nucleophile.

Similar reactions were considered on an example

of electrochemical oxidation of lithium and sodiumsalts of dialkyl phosphrous acids (RO)2POM (M = Na,Li) in the presence of olefins, proceeding via the stepof generation on anode of dialkylphosphonyl radicals(RO)2PO. that initiate a chain free radical process ofdialkyl phosphite addition to the alkene double bondwith formation of saturated alkyl(cycloalkenyl)phos-phonates (yield up to 80%). This is explained byoxidation of adsorbed on the anode primary radicaladduct of the phosphonyl radical and the alkenemolecule into carbocation, followed by deprotonationof the latter [62, 63].

Electrochemical oxidation of tertiary aliphaticphosphine, like (RO)2POM, in the presence of a cyclo-alkene leads to isomeric cycloalkenylphosphoniumsalts differ by the position of multiple bond in thecycloalkenyl substituent relatively to the phosphoniumgroup, in 58375% yield [64]. The nonselective phos-phorylation can probably be explained by the protontransfer in the intermediate phosphonium structuresand the double bond migration.

The electrochemically generated amidophosphitecation radicals [(EtO)2PNEt2, (Et2N)2POEt] phos-phorylate alkenes (cyclohexene and 1-hexene) in thepresence of trisodium phosphate as a proton acceptorto form unsaturated amidoalkenylphosphonates intotal yield 42355% [65].

(Et2N)2P(O)R (Et2N)(EtO)P(O)R

R =7@, 7cB, CHCH=CHPr.

Under the same conditions, (Et2N)3P transformsinto dodekaethylhexaaminodiphosphonium [(Et2N)3

+P3+

P(NEt2)3](ClO43)2, that is, reaction with the parent

amidophosphite is preferred. Comparison of theseresults with the data on addition of dialkyl phosphitesto olefins initiated by trialkyl phosphite or dialkyltri-methylsilyl phosphite cation radicals [55] points tothe trend of diminishing the yield of alkylphosphonate,hence of decrease in initiating ability of organophos-phorus cation radical in the series (RO)3P

+.,(RO)2POSiMe+

.> (RO)2PNR2

+. > (R2N)2POR+.

>> (R2N)3P+. [65]. Effect of amino groups in the

cation radical consists probably in the conjugation ofthe unpaired electron with involvement of the nitrogenlone electron pair leading to certain delocalization ofthe free electron across the P3N bond system. Besides,probably the amino group donor effect leads also todecrease in positive charge on the phosphorus atom.These two effects obviously decrease reactivity of theamidophosphite cation radicals.

Application of acyl phosphite (EtO)2POC(O)CH3



possessing an acyl group prone to be splitted off wasnot successful in the case of anodic cyclohexenephosphorylation: a mixture of saturated and unsatu-rated reaction products was formed, the mechanismand yields remained indefinite [66].

It is noteworthy that in the most cases of the cationradical alkene phosphorylation in an electrolyzer witha diaphragm is nonselective and can doubtfullyregarded as a prospective pathway to organophos-phorus compounds.

Reactions with carboxylic acids and othercarbonyl compounds

In the development of electrochemically generatedphosphonium ions a special attention has been paid tothe acyloxitriphenylphosphonium ions Ph3P

+3OCORformed in the reaction of cation radical Ph3P

+. withcarboxylic acids [67369]. These ions are the keyintermediates in many useful processes of synthesis,but ordinary with a strong nucleophile such as halideions or RS3 as a counter ion, which can promoteundesirable side reactions [67]. Electrochemicalmethods commonly use counter-ions ClO4

3or BF4

3

with low nuceophilicity.

We choose phenylacetic acis as a model compound.It was found that electrolysis at a permanent currentvoltage of a solution of Ph3P and PhCO2H in a dia-phragmless electrolyzer at room temperature on the2,6-luthidinium perchlorate background leads toformation of (PhCO)2O, while use of Ph3P

+HClO43

instead of LutClO4 gives PhCHO.

PhCO2H 7777776 (PhCO)2O + PhCHOPPh3, CH2Cl2, 20oC

3e3

6jdN+

gH

ClO43 80% 0%

PhP+HClO43 62% 30%

backgroundelectrolyte:

The last process has been used for the synthesis ofoptically active N-Cbz-1-a-aminoaldehydes fromN-Cbz-1-a-aminoacids at330oC [68]. Mechanism andregularities of this unique one-step reduction ofcarboxylic acids RCO2H (1) to aldehydes (2) werestudied in details [69]. Formation of the aldehydeswith R = Ph,o-ClPh,n-MeOPh, Ph(CH2)2, Me(CH2)8,Ph2CH proceeds smoothly at330oC with good yield(51376%). The case of R = cyclo-C6H11 and i-Prrequires heating anda,b-unsaturated acid does notreact at all. Cyclic voltammetry of the anolyte afterelectrolysis of the solution Ph3P, Ph3P

+HClO43

inCH2Cl2 in a cell with a separator gave rise to the fol-

lowing scheme of the process: formation on the anodeof acyloxitriphenylphosphonium ion (3), reaction ofthe latter with Ph3P leading to formation of acyltri-phenylphosphonium ion (4), its reduction on thecathode into a-hydroxyalkyltriphenylphosphoniumion (7) whch in turn decomposes into the correspond-ing aldehyde (2) and Ph3P after treatment the electro-lyte with water.

The voltammetric study allowed to conclude thatthe conversion (3)76 (4) step defines yield of al-dehyde and electrolisys conditions. Thus, prior to theelectrolysis in a cell with a diaphragm the anolytesolution components are not reduced at the potentialin the range from 0 to31.7 V (Ag/AgNO3). After theelectrolysis, at least two cathode-active intermediatescan be registered on the transformation (1)6 (2) route,and it was found that the moieties which are im-mediately reduced to the compound (2), namely, theacylphosphonium ions (4), have less negative poten-tial. The more cathodic peak is assigned to the reduc-tion of acylphosphonium ion (3). The values of thepeak potentials indicate the sequence of reactivity ofthe (3) ion in respect of Ph3P: aromatic carboxylicacid derivatives > primary aliphatic acids > secondaryaliphatic acids > tertiary aliphatic acids.

AnodePPh3 + RCO2H

32e3, 3H+ 1

R

O

OP+Ph37776

Ph3P

3Ph3PO R

O

P+Ph3e3

Cathode

R

O3

P+Ph3

e3, H+

RCHO47773Ph3P, 3H+

H2O

RR P+Ph3 P+Ph3H

OH OH347

H+

4

.

5

3

2 7 6Note that stable formation of (7) ion in the solution

at the electrolysis als explains conversion ofL-a-aminoacids into L-a-aminoaldehydes without ra-cemization. According to the studied mechanism ofthe process, one can assume that phosphonium ion (6)formed at the two-electron reduction of (4) ion can betaken as an equivalent of acyl-anion and the neutralradical (5) generated at the one-electron reduction ofthe ion (4) as an equivalent of acyl radical in variousreactions [69].

Electrochemically generated acyloxyphosphoniumion Ph3P

+3OCOR in reactions with various nucleo-philes form esters, amides andb-lactams under mildconditions [67]. Thus, oxidation of Ph3P in CH2Cl2 in



Table 2. Bicyclic a-ketones obtained from cetoacidsÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÒÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ

Substrate ³ Yield º Substrate ³ YieldÄÄÂÄÄÄÂÄÄ´ ÇÄÄÂÄÄÄÂÄÄ´³ (2+3),% º ³ (2+3),%³ n ³ m³ (2 : 3) º ³ n ³ m³ (2 : 3)

ÄÄÅÄÄÄÅÄÄÅÄÄÄÄÄÄÄÄÄÄ×ÄÄÅÄÄÄÅÄÄÅÄÄÄÄÄÄÄÄÄ1a³ 2 ³ 1 ³68 (86 : 14) º1e³ 8 ³ 1 ³ 54 (83 : 17)1b³ 1 ³ 1 ³29 (only 3) º1f ³ 2 ³ 2 ³ 37 (59 : 41)1c³ 3 ³ 1 ³73 (72 : 28) ºIg ³ 2 ³ 3 ³ 3

1d³ 4 ³ 1 ³63 (69 : 31) º ³ ³ ³ÄÄÁÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÐÄÄÁÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄ

the presence of a carboxylic acid and an alcohol leadsto the corresponding carboxylic acid ester in a yieldnot higher than 60%.

Ph3P76 Ph3P+.776 Ph3P+

3OCOR3e3

RCOOH

3e3, H+

7776 RCOOR + Ph3PO + Et3NH+R`OH, Et3N

A method was proposed for theb-lactam electro-synthesis [67]:

Ph3P76 Ph3P+.7777776

3e3(2) Et3N, PhCH=NR(1) PhOCH2COOH,e3

9<9<ei

NeqOPhO Ph

R

R` = i-Pr (63%), Bu (66%).

Reaction of such phosphonium ions with primaryaliphatic amines has been studied [67]. Yield of cor-responding amides achieves 75383%.

Ph3P76 Ph3P+.776 Ph3P+

3OCORRCOOH

3e3 3e3 , 3H+

777776 RCONHR.CH2Cl2, LutClO4

R`NH2

Several routes can be considered for the amideformation:

Ph3P+3OCOR776 RCONHR + Ph3PO + H+,

R`NH2

Ph3P+3OCOR7776 (RCO)2 + Ph3PO + H+,

RCOOH

or RCOO3

(RCO)2O + RNH2 76 RCONHR + RCOOH,

Cathode: 2H+ + 23e 76 H2.

The amide formation via alkylaminophosphoniumion Ph3P

+3NHR` wich could appear in the absence ofcarboxylic acid is little probable. The method isrestricted to the use of carboxylic acids possessing nofunctional groups which could be oxidized at lesspositive potentials than Ph3P.

Cathode reduction of acyltributylphosphonium ion

generates above-mentioned acyl-radical equivalentswhich enter cyclization to form cyclopentanones [70].

7777776CH2Cl2, Me3SO3H

3e3R1R2C=CH(CH2)nCH2CH2C.(O3)P+Bu3

92

(CH2)nCH2CH2C(O)CH3CHR1R2

77777777g g+ R1R2C=CH(CH2)nCH2CH2CHO

Bu3P + R1R2C=CH(CH2)nCH2CH2COOH

This reaction is restricted to the use of unsubsti-tuted and 6-phenyl-substituted hexenoic acids.

The active particles ofRekO3 type generatedat the electrochemical oxidation of Bu3P in the pre-sence of a carboxylic acid can be used as a new syn-thone in organic synthesis, as an acyl-anion equivalent.They were used in various reactions, in particular forthe carbon-carbon bond formation [71, 72]. Forexample, electrochemical synthesis of bicyclica-hydroxyketones fromd-ketoacids in the presence ofBu3P in a cell with no separator has been described[71, 72].

O

n m CO2H

Bu3P7763e3

OH O OOH

n mH Hn

+

The process total yield was 54373% (Table 2). Inthe cyclization with a bicyclic system formation thetrans-selectivity (n = 4, 5, 6, 10) prevails. However,with n = m = 1 only the ring (3) is formed and cor-responding aldehyde is the main reaction product.

While the sixmembered ring nevertheless is formedfrom thee-ketoacid (1f), cyclopentanon is not formedfrom the compound (1g) under the same conditiond.The formation stereoselectivity of bicyclic compoundsfrom the acids (1h) and (1i) is improved when thealkyl substituents are introduced to C4 and C6 posi-tions of cyclohexane ring (1a) (Table 3).

Change in the cyclization stereochemistry at theintroduction a methyl group to C2 position of com-pound (1a), namely, predominating formation ofcisisomer (3j) from compound (1j) is of special interest.Besides, formation of a bicyclic system, the derivativeof the acid (1b) with a methyl group in C2 position atthe electrolysis of compound (1k) is found to proceed



R3

R2

R1

n

m

O

CO2H

776Bu3P

3e3

R2 R2

R1 R1O O

n n

m m

OH

H H

OH

more effectively. The cyclization is also typical ofacyclic d- and e-ketoacids (1l31n).

SQSQjoR

OCO2H( )n 776

Bu3P, e3

NoOgcOH

R( )n

1l (R = Me, n =1) 4l (27%)1m (R = Ph, n =1) 4m (43%)1n (R = Ph, n =2) 4n (22%)

For extension of synthetic applications of a uniquereaction generating equivalent amounts of acylradicals at the electrolysis [70] was proposed to usedicarbonyl compounds (5) as initial substances. Theytransform into the products of radical cyclization withhigher yield owing to the well-known effect ofgeminal bisubstitution [72].

Unexpectedly, compound (6a) was only obtainedin the reaction. Up to now no success was achievedin rearrangement of two carbon fragments initiated byaddition of radical or anion particles to carbonylgroups, therefore the studied reaction is of greatinterest in the view of synthesis.

A possibility was established for the addition oftrialkyl phosphates to cyclohexanone in an electro-chemical process [73]. Thus, electrochemical oxida-

Table 3. Bicyclic ketones obtained from alkyl-substitutedketoacidsÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄ

Substrate ³Yield (2+3), %

ÄÄÄÄÂÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄ³(2:3)

³ n ³ R1 ³ R2 ³ R3 ³ÄÄÄÄÅÄÄÄÄÅÄÄÄÄÄÅÄÄÄÄÄÅÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄ

1h ³ 1 ³ H ³ t-Bu ³ H ³ 54 (90:9)1i ³ 1 ³ H ³ H ³ CH3 ³ 58 (90:10)1j ³ 1 ³ CH3 ³ H ³ H ³ 50 (only 3)1k ³ 0 ³ CH3 ³ H ³ H ³ 44 (only 3)ÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄ

tion of triethyl phosphite in the presence of cyclo-hexanone gave a mixture of three products: diethyl-1-ethoxycyclohexylphosphonate (I ), diethyl phos-phonate (EtO)2P(O)Et (II ) (yields 38 and 58% on theparent phosphite) and 2-(1-cyclohexene-1-yl)cyclo-hexanone (III ) (26% on the nonreacted cyclo-hexanone). The process was performed in a cell withseparated anode and catode spaces at the continuous(EtO)3P adding.

7ei(EtO)2P(O)

EtO 7<7FpO

I III

As cyclohexanone is difficultly oxidized comparedto triethylphosphite, the process obviously proceedsvia the intermediates of (EtO)3P oxidation. Theabsence of vinylphosphonate anda-ketophosphonatein the reaction product expected at the direct interac-tion of (EtO)3P

+. cation radical with the ketoneindicates that the process proceeds via formation ofquasiphosphonium salt (EtO)3

+P

+P(OEt)3 which

initiates addition of triethyl phosphite to cyclohexa-none and cyclohexanone condensation. Catalysis ofaddition of phosphite to ketone by the quasiphos-phonium salts was proven in a separate experiment.

Electrochemical Reduction of Phosphorus(III)Compounds in Low Coordination

Electron paramagnetic resonance in conjuction withelectrochemical methods is very effective for obtain-ing data on the electron density distribution in organicradicals containing a heteroatom, and whenab initiocalculation applied this spectroscopy providesexact description of structure of these particles. Investi-gation of short-living organophosphorus anion ra-dicals which can be used as the key intermediatesdefining pathways of various chemical transforma-tions is of special interest. The organic moleculescontaining an atom of trivalent bicoordinated phos-



phorus are extremely reactive. Nevertheless, they canbe isolated at room temperature in the case of sterichindrances of the organic group attached to the phos-phorus atom by ordinary bond [74]. In the recentyears, chemical and electrochemical properties of low-coordinated phosphorus compounds were widelystudied [14, 74380]. The attention to these compoundsis connected with their ability of formationn1- andn2-complexes with transition metals and with unusualproperties of stable anion radiclas and, less often,cation radicals obtained electrochemically.

Electrochemical investigations showed that reduc-tion of compounds containing phosphaalkene frag-ments (RP=C<) strongly depends on the nature oforganic groups R bounded to the phosphaalkenylcarbon atom. The reduction is irreversible when R isan alkyl group [81] and close to reversible when R isphenyl [75, 77]. In the last case, EPR spectra of theanion radical products can be registered. Diphos-phenes RP=PRrepresent another class of the com-pounds where a trivalent phosphorus atom is involvedin formation of a double bond, and in the case ofR = aryl such anion radicals are stable enough forregistration of EPR spectrum in liquid phase [82, 83].

A comparative study of methylenophosphinesR1P=CR2R3 [R1 = t-Bu, NEt2, N(SiMe3)Bu-t, R2 = R3

= SiMe3], iminophosphines R1P=NR2 (R1 = R2 = t-Bu) and diphosphenes R1P=PR2 [R1 = R2 =N(SiMe2Bu-t)2, C5Me5] has been performed [81]. Theproperties of complex formation by such systemconstruct one of the most interesting areas in themodern inorganic chemistry. The P(III) compoundswith double bond show ambident behavior due totheir two frontier orbitals close by energy. A varietyin chemical behavior of the compounds of theseclasses puts a question about the energy of their fron-tier orbitals, that is, about their electron affinity. In afirst approximation, oxidation and reduction potentialsare the quantitative measures of the energy of thefrontier orbitals, HOMO and LUMO, even for ir-reversible processes. The choosen methylenophos-phines enter irreversible oxidation and reduction,while dialkyliminophosphines and diphosphenes arereversibly reduced to form anion radicals. The firstvertical ionization potential of methylenophosphinescorrelates with the values of anode oxidation peaks.The peak potentials depend strongly on the substi-tuents bounded to thep-system. Their influence onthe HOMO and LUMO energy is defined on the basisof the Hammet substituent constants [81].

Electrochemical behavior of ArP=C(H)Ph (1)(Ar = tri-tert-butylphenyl) and hyperfine splittingconstants (isotropic and anisotropic) in the anion

radical compared to those in the ArP=PAr anionradical obtained under the same conditions has beenstudied [75]. The phosphaalkene (1) is reduced withone one-electron reduction wave on Pt, Hg or glass-carbon electrode. Cathode and anode potentials of thepeaks are:Ep

j 32.15, Epp 31.82 V. It was found that

31P splitting constants are almost identical for thephosphaalkene and diphosphene anion radicals. Asshown, the unpaired electrone in (13

.) is delocalized

over p* orbital consisting of the phosphorus 3p andcarbon 2p orbitals of the phosphaalkene and benzenering. Due to its orthogonal orientation to the CPCplane [77, 84], the aromatic ring bounded to the phos-phorus atom is not involved in the formation of thisorbital. As compared to the styrene anion radical, thepresence of phosphorus atom induces shift of a partof electron density from the phenyl ring to the doublebond.

5djgjPo6jdd

Poj

5djgd

t-BuH H

t-Bu t-BuBu-t Bu-t

Bu-t

3

The reversible reduction of the phosphaalkene (3)takes place at the potential31.91 V [76]. Experi-mental hyperfine splitting constants on31P and1H areconsistent with the free rotation around the P=C bondin the Cphosphaalkene3Cbenzeneat room temperature [77].This phosphaalkene forms a complex with Pd(II),which also is reduced reversibly in THF at31.23 Vto form the paramagnetic particles which shows aspectrum characteristic of ion radical which differsfrom the spectrum of free ligand and shows hyperfineinteraction with two equivalent31P nuclei. TheLUMO p* orbital of the phosphaalkene is not of a highenergy, and it was shown that the additional electronoccupiesp-nonbonding orbital of the ligand ratherthan the metal [76].

The paramagnetic particles with the structures(ArP=C(H)Ar)3

., and (ArP=PAr)3

., obtained by reduc-

tion of organophosphorus compounds in comparisonwith the structure (Ar2PPAr2)

+ has been studied byEPR in frozen state [74]. Thep* structure was con-firmed for the phosphaalkene and diphosphene anionradicals (with negligible contribution of phosphorusand carbons orbitals to HOMO) as well as nonbond-ing character of the HOMO in the diphosphine cation.

Electrochemistry and EPR methods were used forthe study of polarity of the phosphaethylene3P=C<bond and for explaining interaction mechanism for thephosphaalkene with metals [78]. Series of compounds



including phosphafulvenes (5) and dibenzofulvenes(6) were studied. It has been shown [75] that reduc-tion of the systems containing a phenyl ring attachedto the phosphaalkene carbon (4) leads to the anionradical with spin density on the phosphorus less than0.5: the unpaired electron is delocalized over bothphosphaethylene bond and phenyl ring. Due to re-sonance stabilization of cyclopentadienide anion,including the phosphaalkene carbon to the cyclo-pentadiene ring in (5) and (6) significantly modifiesspin distribution by increasing contribution of phos-phinyl mesomeric structure (7).

2jP=CdHj

Ar4

1cP.h

Ar7

M[P=CiAr

5

4"4

P=CiAr

6

By the method of cyclic voltammetry was shownthat phosphafulvene (5) and dibenzophosphafulvene(6) in DMF are reduced at31.200 and31.349 V,respectively. The EPR spectra of the anion radicalsformed were compared with those of phosphaalkeneanion radicals and a great difference in their electronicstructure was discovered. The unpaired electron isdelocalized over all the P=C(H)R fragment in thephosphaalkene anion, while in the phosphafulveneanion it is localized on the phosphorus atom.

Phosphaalkene (8) and (9) molecules where thecarbon atom of the C3P=C group is bound to a hetero-cycle furan or thiophene are shown entering to quasi-reversible reduction [79].

LY\Od

CoPd

Ar

iH LY\Sd

CoPd

Ar

iH

8 9

Ar = 2,4,6-(t-Bu)3C6H2.

Replacement furan or thiophene ring for thebenzene ring decreases slightly the phosphaalkenereduction potential: from31.98 to31.90 and31.82 V,respectively. Stability of anions of the heterocycle-containing phosphaalkenes is, however, not high. Thevalues of spin density in the anion radicals (8) and (9)confirm localization of the unpaired electron on thep*

orbital constructed from the five-membered ring andthe phosphaalkene double bond. Note that these anionradicals, including that of phosphaalkene with phenyl

substituent show similar electron delocalization overthe double bond.

Diphosphaallenes RP=C=PR can be electrochemi-cally oxidized [14] and reduced as well into the anionradicals, in THF at low temperature [80]. The anionradical structure was shown not corresponding to thatof the neutral molecule. The unpaired electron islocalized on the two equivalent phosphorus atoms,the radical ArP=C3PAr. trans configuration is morestable.

Ar. 3C3 =PAr 46 Ar3C3 =P.Ar

We hope that the recently obtained information onthe red-ox properties of bicoordinated phosphoruscompounds and electronic structure of their anionradical will be useful at the performing of theirvarious transformations and for the development ofnew methods for the synthesis of organoelementcompounds.

Electrochemical Reduction of the PhosphorusAcids Esters

Electrochemical reduction of the phosphoprus(III)acids derivatives has poorly been studied. Trialkyl-and triphenylphosphite are not reduced in the availab-le potential range. The ability to electrochemicalreduction show only the compounds with phos-phorus(III)3p-system bonding. Earlier the process ofreduction of diethyla- and b-styrylphosphonites inDMF on a mercury electrode [85] and reduction ofchlorophosphines to polyphosphines [86] have beendescribed. The reduction of styrylphiosphonitesproceeds reversibly to form [(EtO)2PCH=CHPh].

anion radical.

However, in recent years electrochemical reductionof organophosphorus compounds was significantlydeveloped, first of all by studying mediators such asanion radical metal complexes, etc. This allowed touse new possibilituies for extension organic syn-thesis and more detailed studying the mechsnisms ofelectron transfer, bond splitting and other electro-chemically initiated processes.

For the studying the mechanism of reactions andelectron transfer, nature of reaction center, syntheticapplication of intermediate dual reacting in electro-chemical processes certain attention should be paid torelatively poorly studied phosphoric esters. Reductionof triphenylphosphate on a mercury or platinum elec-trode proceeds irreversibly with transfer of two elec-trons [87]. Preparative electrolysis of (PhO)3PO inDMF with Bu4NBF4 background leads to formationof diphenyl butyl phosphate, benzene and tributyl-



amine as the final reaction products, that is, the reduc-tion is accompanied with Ph3O bond splitting.

(PhO)3PO + 2e3 76 Ph3 + (PhO)2P(O)O3

29

29HD 3Bu3N Bu4N

+

PhH (PhO)2P(O)OBu

The irreversibility of the total reduction processallows to apply the homogenous reduction methodusing stable anion radicals for the study the regulari-ties of intermolecular electron transfer and followingchemical reactions [88]. The homogenous reductionof triorganyl phosphates BX with the anion radicalsA3. of selected mediators at 25oC features in a slowelectron transfer on the substrate molecule and fastfollowing step of O3Ar bond splitting [89, 90].

(ArO)2P(O)O3 + Bu4N+776 (ArO)2(BuO)PO + Bu3N.

slow

k6

A + (ArO)3PO 7647

k2

k32

A + [(ArO)3PO]3. ,

A + e3 7647 A3. ,

k1

k31

[(ArO)3PO]3. 76 (ArO)2P(O)O3 + Ar.k3

A3. + Ar. 7647

k4

k34

Ar3 + A,

Ar3 + [H+] 7647

k5

k35

ArH,

A common feature of the process is varying inactivation entropyDS# from a big negative value forthe reaction with maximumDE1/2

A3BX to near zero(within experimental error) for the carrier with lowDE1/2

A3BX (in this latter case the electron transfer isnear outspherical one). The mediators possessingheteroatoms give the anion radicals with significantcharge delocalization (benzonitrile, cardiamine) withmuch lowerDH #. The transition state stabilization isprobably defined by the ability of partial bonding(may be, with partial splitting of other bond) whichreducesDH #. However, this results in ordering thetransition state leading to negative activation entropy.

The plots lnkef25oC3DE1/2

A3BX (activation is the reac-tion [moving force]) are linear for all phosphates, withslope 1/120 mV31 corresponding to the electronhomogenous transfer factor 0.5. This confirms activa-tion character of the process (Fig. 1).

A possibility of stabilization of the electron transfertransition state defining the process enthalpy andentropy values was shown using a mediator [90].

Preparative reduction of triorganyl phosphates inaprotic solvents on Alk4N

+ salts background can beregarded as a usefull method for the synthesis of

ln kef

10

8

6

4

00.17 0.33 0.49

DE1/2A 3BX , V

6 5 48 7 32 1

Dependence lnkef reductiontriorganylphospates onDE1/2

A3BX (kef, l mol31 s31). (1) (n-BrC6H4O)3PO,(2) (n-ClC6H4O)3PO, (3) (PhO)3PO, (4) (o-MeC6H4O)3 .PO, (5) (n-t-BuC6H4O)(Ph)

2PO, (6) (n-t-BuC6H4O)3PO,(7) (Ph)2(BuO)PO, and (8) (Ph)(BuO)2PO.

mixed phosphates by peresterification under mildconditions (Table 4).

The heterogenous and homogenous electron trans-fer reactions of phosphorus acids derivatives meritattention not only in connection with the role of thephosphorus atom environment in the kinetics andthermodunamics of reactions of phosphorus bondssplitting, but also because they can give certain resultsin the synthesis. In development of investigationswere described mechanism and kinetical regularities

Table 4. The products of (RO)3PO electroreduction onAlk4NI backgroundÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄ

Substrate³

Alk4NI³

Reaction product³Yield,

³ ³ ³ %ÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄ(PhO)3PO ³Et4NI ³(PhO)2(EtO)PO ³ 90(PhO)3PO ³Bu4NI ³(PhO)2(BuO)PO ³ 91(o-MeC6H4O)3PO³Bu4NI ³(o-MeC6H4O)2 . ³ 91

³ ³(BuO)PO ³ 91(p-t-BuC6H4O)3 . ³Bu4NI ³(p-t-BuC6H4O)2 . ³ 92PO ³ ³(BuO)PO ³ 92(PhO)2(EtO)PO ³Et4NI ³(PhO)(EtO)2PO ³ 89(PhO)2(BuO)PO ³Bu4NI ³(PhO)(BuO)2PO ³ 88ÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄ



Table 5. Characteristics of the polarography waves ofsubstrate reduction (BX) and electron carriers (A) in DMF(acetonitrile) on the background of 0.1 M Bu4NI solution(Bu4NBF4) (cA,BX 01033 mol l31)ÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ

BX

³3E1/2, ³ DE/ ³

n, 3e

³

Aa

³DE1/2

A3BX,³ V ³ Dlog [i/ ³ ³ ³V³(bottom³(id 3 i)],³ ³ ³

³mercury)³ V ³ ³ ³ÄÄÄÄÄÄÄÅÄÄÄÄÄÄÅÄÄÄÄÄÄÅÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄPh(Et)P3 ³ 1.890 ³ 0.073 ³1.62³PhC(O)Ph³ 0.305SEt ³ ³ ³ ³9,10-DMA ³ 0.360Ph2PSBu³ 1.750 ³ 0.063 ³1.83³4,4-bipy ³ 0.373

³ ³ ³ ³Anthracene³ 0.288³ ³ ³ ³p-Methoxy-³ 0.298³ ³ ³ ³benzo- ³³ ³ ³ ³phenone ³³ ³ ³ ³Isonicotin- ³ 0.223³ ³ ³ ³amide ³

Ph2PCl ³ 1.075 ³ 0.190 ³1.30³PhNO2 ³ 0.275Ph2P(S)3 ³ 1.920 ³ 0.073 ³1.57³2,2-bipy ³ 0.285CH2Ph ³ ³ ³ ³9,10-DMA ³ 0.390ÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄa DMA is diethoxyanthracene, bipy is bipyridyl.

of electron transfer reactions on the some diorganyl-phosphine molecules and for comparison the relatedmolecules with tetracoordinated phosphorus Ph2P(S).CH2Ph [91,92]}. These reactions are useful modelsfor the study of electron transfer processes with bondsplitting.

Electrochemical reduction of several tricoordinatedphosphorus acids derivatives R3P (R = EtS, EtO,Et2N) was studied [91] by the polarography method.Trialkyl- and triamidophosphites are electrochemicallyinactive, thiophosphites gives one irreversible two-electron wave. Commutation curves contain anodicwaves which were assigned to the processes ofmercury oxidation in the presence of anionic productsof thiophosphite oxidation.

R2PSEt + 23e 76 R2P3 + EtS3

By the method of hidden limit current was shownthat the formed anionic products differ by their ba-sicity: in the presence of ammonium perchlorate asproton donor one of them is protonated (probably,R2P

3). The final electrolysis product in the absence ofa proton donor is diphosphide.

R2P3 + Ph2P3PPh2 + EtS3.

Polarograms of diorganylphosphines and the tetra-coordinated phosphorus derivatives [92] contain anirreversible wave with thelimit current not correspond-

ing to an integer number of transferred electrons. Thisfact proves proceeding of chemical reactions decreas-ing the depolarizator flow to the electrode. The reac-tions are fast enough:[simple] criterions, e.g.,dependence ofi lim3t 1

31/6 on t1 (t1 is the mercury elec-trode dropping period) do not indicate kineticalcharacter of the wavelimit current. In the case ofPh2PCl at the more negative potentials the secondwave is also observed, it is partially reversible, withthe limit current ratio (anodic commutated to ca-thodic) ia/id equal to 0.3.

The corresponding reduction peaks on the cycliccurves were irreversibletill the highest rates ofpotential change (Table 5). The second reduction peakof Ph2PCl is quasi-reversible.

In the electrolysis of diorganylphosphinous acidsolutions at the limit current potential of firstwaveonly about one electron per a molecule is consumedin any case, and the main isolated product is tetra-organyldiphosphine {in the case of Ph2P(S)CH2Ph amixture of the corresponding diphosphine disulfide[Ph2P(S)]2 and diphenylphosphine sulfide Ph2P(S)H in~2 :1 ratio was formed}. Such a difference in distri-bution of electrolysis products reflects the differencein nucleophilic reactivity of the intermediates andelectrophilicity of the substrates. The higher nucleo-philicity of Ph2P

3 anion compared to that of Ph2P(S)3

anion promotes P3P bond formation. Besides, as isknown in an element-elenent ambident system(>P3S3) with the elements bearing a lone electron pairoccurs anomalous weakening in phosphorus donatingproperties and increase of its electrophilicity [93].The total effect of these factors defines proceeding ofreaction by the route of diphosphide formation. Thediphosphide reduction potential is the same as that ofPh2PCl second reduction wave [94]. This all can beexplained by proceeding of the following reactions:

Ph2PX + 3e 7647 [Ph2PX]. 76 Ph2P. + X3,

Ph2P. + 3e 76 Ph2P3,

Ph2P3 + Ph2PX 76 [Ph2P]2 + X3

9976 Ph2PH

H+

The process irrevercibility can be explained by fastsplitting of P3X leading to formation of phosphynylradical, which fast accepts an electron and formsthe corresponding anion. Its oxidation anode peak at30.68 V was successfully registered. Reaction of thephosphinyl anion with the initial substrate moleculeexplains diminishing the wave height below two-elec-tron level due to decrease in the substrate concentra-tion at the electrode surface. Probably the competitive



dimerization of the Ph2P. radicals should be excluded:

addition of cyclohexene to the electrolyte did not giveeven trace amount of the radical addition product.Obviously the Ph2P

. radical reduction proceeds withthe rate close to diffusion and with much lowercathode potential, that is, the second electron transferaffects only total cathode current and the processstereochemistry.

The irreversibility of the total process of diorganyl-phospinites RRPX and Ph2P(S)CH2Ph reduction (X =CH2Ph, SEt, SBu, Cl) allows applying homogenousreduction method using stable anion radical carriersfor the study of regularities in the process of homo-genous intermolecular electron transfer and sub-sequent chemical reactions (Table 5). The basicscheme of the catalysis is as follows [92]:

k2

A3. + Ph2PX 7647 A + [Ph2PX]3.,k32

A is an organic mediator (anthracene etc.).

kef # f(cA),

k3[Ph2PX]3. 76 Ph2P. + X3,

k4A3. + Ph2P. 7647 Ph2P3 + A,

k5Ph2P3 + Ph2PX 76 Ph2P3PPh2 + X3,

bipy3.Ph2PSBu776 Ph2P(S)Bu (kef grows with growingcA).

The reduction regularities are found depending onthe mediator nature. For the studied substrates, de-pending on the mediator used thekef value candepend oncA or not and the activation energy isnoticeably higher than was published for the homo-genous reduction of certain substrates. Some of thecarriers although formally obey requirements do notshow catalytic activity under the conditions of polaro-graphy. At the reduction of Ph2PSBu by anthraceneanion radical increase in the carrier concentration doesnot affect the catalytic property which indicates slowelectron exchange between the carrier anion radicaland the substrate molecule. The constancy of the effec-tive rate constantkef at the change incA indicates afast P3S bond splitting (k3 >> k32cA). The reductionproduct in this case is (Ph2P)2. It is found that 4,4-bi-pyridyl anion radical promotes isomerization ofPh2P(S)Bu andkef grows with growingcA. Such asignificant difference is defined by the necessity ofthe latter mediator to coordinate with the substrate forthe electron transfer or to be involved in the transitionstate formation.

Electrosynthesis with White Phosphorus

A problem of direct functionalization of whitephosphorus became more actual in view of rapid de-velopment of chemistry and extension of practicalapplications of organophosphorus compounds decele-rated by difficulty in their synthesis and ecologicalunsafety of their manufacturing.

The experimental material on the synthesis ofvarious organophosphorus compounds and of keycompounds in particular from white phosphoruspoints to necessity of applying rigid thermal require-ments or using complex multicomponent systems. Themost prospective route for overcoming the problem ofthe synthesis of organophosphorus compounds fromelemental phosphorus is the process based on the jointaction on the white phosphorus of nucleophilic andelectrophilic ragents. It is known, in part from quan-tumchemical calculations, that the phosphorus atomsin tetrahedral P4 molecule show very weak nucleo-philicity. However, due to the rigidity of the three-membered rings and weakening of P3P chemicalbonds the phosphorus atoms show clear electrophilicproperties and easily react with various nucleophiles.

POPP

PPO

P9977 77Nu3 + 76POPP

P

OP39977 77

gNu

76E+

POPP

P

OP3E9977 77

gNu

76 ...

This general consideration was used for develop-ment of conditions for involvement white phosphorusto various transformations. Thus, it was attractive tocombine anvantage of ordinary homogenous che-mistry in solution with the possibility of electro-chemical generation of reagent on an electrode surfacefollowed by its transfer to the reaction system[953100].

For the white phosphorus functionalization a re-action of alcohol O3H bond splitting with formationof akoxylate ion on a cathode with low hydrogenovertension (2ROH + 2

3e 76 2RO3 + H28) and

ability of P4 to react easily with nucleophiles havebeen applied. The necessary electrophilic mediatorwas the halogen generated at the anode by oxidationof halide ion [95, 96, 1013103].

In the presence of water it is hydroxide ion thatobviously attacks the phosphorus molecules and splitsthe P3P bond (RO3 + H2O 76 HO3 + ROH).



RO3 + PQSPPg>>::P76 QSP3

Pg>>::PRO3P 776

ROH3RO3 QSP3H

Pg>>::PRO3P 776

ROI3HI QSP3OR

Pg>>::PRO3P 76 ... 76 (RO)3P

3ROH,+3e3

33/2H29

9

97777776

7777776

3ROH, H2O, +2e3

33/2H2

(RO)3P

(RO)2PHO

7777776I3, 2ROH, +2e3

(RO)3PO

777777777982

9

3RI, 3H2, 3RO3

3HI, 3H2, 3RO3

I3, 2ROH, +2e33RI HI1/4P4 7


Reaction of white phosphorus under the action ofcathode-generated nucleophilic reagents proceeds bytwo routes, monomeric (solved part of phosphorus)and polymeric (via polymer, the product of poly-merization of liquid phase of the white phosphorus)[96]. This is supported by the data on relation betweenthe amount of electricity consumed for the formationof monomeric phosphorus-containing products of theprocess, to the whole amount of electricity at a giventime. An example with the white phosphorus emul-sion in hexyl alcohol shows that approximately a halfof the electricity is consumed for polymerization ofthe phosphorus and functionalization of the polymerwithout formation of soluble compounds. However, inthe following steps the yield on the current of thesoluble phosphorus-containing products increasessignificantly [96].

The role of alcohol and water in the competitivefunctionalization of a P3H bond formed at the phos-phide anion protonation can be estimated with use ofassumption that the reactions rates for the functionali-zation of the >P3H fragment of the phosphorusoligomer with alcohol and water and of the >P(O)Hbond in dialkylphosphite are equal, all other condi-tions the same. Thus, the step of functionalizationproceeds under the action of alcohol.

POPP

PPO

P3H + ROH + I29977 77RO3

7632HI

POPP

PPO

P3OR9977 77RO3

Under the optimal conditions at M(P) :M(H2O) :M(ROH) = 1 : (131.1) : (20350) yield trialkyl phos-phates is practically quantitative (80395% on P4).

+203eP4 + 12ROH + 4H2O 776 4(RO)3PO + 10H28,

R = Me, Et, Pr, i-Pr, Bu, s-Bu, Am.

In the experiments with diminished relative alcoholand water content in the electrolyte [M(P) :M(H2O) :M(ROH) = 1 :0.3 :2] the process changes its generaldirection and the main product becomes tetraalkyl-pyrophosphate [(RO)23P(O)]2O [97,101,102]. Forma-tion of an anhydride fragment can be explained inassumption that with Alk4N

+ cation as a counterionthe anion formed at dissociation would show highenough nucleophilicity to be able attack electrophiliccenters of the P3P bonds.

P77O3dj��

��c

+ P3Pdjjd76 P77Odj��

��c

3Pjd+ P3jd

P77OHdj��

��c

7647 P77O3dj�

��c

+ H+,

c is cycle.

Diminishing in the alcohol content leads to depres-sing the reaction of trialkyl phosphate formation fromthe intermediates (RO)2PHO and (RO)3P in thegeneral process scheme and increasing probability ofacidic splitting of (RO)3P. Thus, electolysis of thewhite phosphorus emulsion under the conditions withlow alcohol content a mixture of trialkyl phosphateand tetraalkylpyrophosphate is formed in molar ratio~1 :1.5 in the case of linear aliphatic alcohols and1 : (8310) with iso alcohols. Yield of [(i-RO)2P(O)]2Ois ~80% on P4.

Varying of acidity of the electrolyte containingalcohol and water leads to the change in the state ofcathode-generated reagents and the pathway of trans-formation of white phosphorus.

In a wide range of HBr concentration the main re-action product is dialkyl phosphite (yield up to 65%),and the (EtO)2PHO/(EtO)3PO mole ratio underoptimal condition attains 21.

PI3 776 (EtO)3PEtOH

��2EtOH,H2O��3EtI HI

(EtO)2PHO

P4 + 6I2 76 4PI3



A scheme of electrochemical processes of obtainingphosphorus acid derivatives from P4 looks as follows:

Cathode Anode

0P4

2e3

ROH(H2O)

I3

I+

32e3

(RO)3PO, 90%,(RO)2PHO, 65%,

[(i-RO)2p(O)]2O, 80%, etc.

3 +

With the example of such a simple scheme asphosphorus3alcohol3water is seen that electrochemist-ry is a powerful synthetic method advanced over theclassical methods of organic chemistry: it capable ofsynthesis of organophosphorus compounds from whitephosphorus under mild conditions in a high yield,approaches to creation in many cases of wastelessecological technology (the side product hydrogenprovides inert atmosphere at the synthesis), the elec-trophilic component of the reaction is regenerated onthe anode, the method allows studying mechanism ofthe reaction with P4 (understanding of the mechanismis a base for achieving high selectivity) and creation ascheme with the controlled reaction process leadingto optimal synthesis of the final compounds.

Study of the routes of conversion of white phos-phorus to the products at the electolysis of phenolicsolutions of Et4NI in a diaphragmless cell confirmsthe general regularities of the phosphorus transforma-tions in alcohol media and shows significant featuresof the following steps of transformations of theprimary organophosphorus compound [98].

In DMF only the corresponding triarylphosphatesare formed in practically quantitative yield (75394%),in acetonitrile3 pyridine mixtures the main reactionproduct is triarylphosphite (78%), with completephosphorus conversion.

(PhO)3P is the first relatively stable intermediate atthe reaction of phosphorus nucleophilic and electro-philic reagents generated on electrode after splittingof all P3P bonds in the phosphorus oligomers.

Cathode: 2PhOH + 23e 76 2PhO3 + H28,

Anode: 2I3 3 23e 76 (I2),

+123

eP4 + 12PhOH776 4(PhO)3P + 6H2,

H2O(PhO)3P + (I2) 76 (PhO)3PI2 776 (PhO)3PO,

2PhOH 23

e, H+

(PhO)3PI2 776 (PhO)5P 7776 (PhO)3PO32HI

+ PhH + PhO3.

However, the rest water content (c 0.01 mol l31) istoo low for providing formation of all amount of(PhO)3PO. Another possible route of triarylphosphateformation is transformation of triphenylphosphite withphenol as the oxygen source. Direct transforming oftriarylphosphite to phosphate through (PhO)3PI2 andrearrangement of the quasiphosphonium compound(PhO)4PI is impossible, in distinct to aliphatic phos-phites. However, it was first discovered that penta-chlorophospphorane formed in the intermediate stepsof the process exerts cathodic reduction to triphenyl-phosphate.

At the development of other routes for the functio-nalization of the intermediates at the transformationsof white phosphorus the possibility of use of cationradicals as anode-generated reagents has been con-sidered. In this respect the triarylamine and pheno-thiazine cation radicals formed at the relatively lowpotentials are attractive. Experiments show thatactually the anode-generated reagent affects consi-derably the electrolysis product composition. Electro-lysis of Ar3N and phenothiazine alcohol solutions inthe presence of white phosphorus with phenothiazineleads to formation of a mixture of dialkylphosphiteand trialkyl phosphate, [(RO)2PHO: (RO)3PO = 6.7when R = Me and~1 when R = Et], and with tri(n-bromophenyl)amine only the corresponding phos-phates were obtained [99].

Cathode: 2ROH + 23e 76 2RO3 + H2,

Anode: Ar3N 33e 7647 Ar3N+.,

PhTh 33e 7647 PhTh +.,

PhTh is phenothiazine.

The initiator of the P4 ring opening is alkoxylateion, however, two pathways can be assumed for thestep of hydrogen replacement in the P3H bond: (1) thecation radicals behave as an oxidizer or (2) the cationradical initially oxidizes the alcohol to form an inter-mediate capable of addition at the P3H bond.

Ar3N+. + 2ROH 7647 [Ar3N(ROH)2]+.,

[Ar3N(ROH)2]+.76 Ar3N + ROH2

+ + RO..

The alcohol eventually gives the correspondingaldehyde.



R`CH2O. 776 R`CHO.Ar3N

3Ar3NH+

Under the conditions of electolysis in the presenceof white phosphorus, aldehyde is formed initially inthe reaction system and then can add to the >P3H.Diaphragmless electolysis of butanol uder the condi-tions given in Table 5 but in the absence of whitephosphorus confirmed formation of butanal as themain final reaction product, in a high yield on theelectricity at the alcohol oxidation. The process canbe represented as follows:

P4 + RO3 76 QSP3

Pg>>::PRO3P 776

ROH QSPHPg>>::P

RO3P

776R`CHO QSP3OCH2R`

Pg>>::

RO3P 76 ...76 (RO)3P

P

Under the conditions of electrolysis the inter-mediate adduct with P3C bond rearranges at the actionof electric current to the final product with P3O bond.

C=Oei

R2

R1+ H3P(O)(OR)2 76 CHOP(O)(OR)2

eiR2

R1e3

Thus, the presented material can be considered asan extension of synthetic application of electro-chemical reactions initiated by nucleophilic attack ofa white phosphorus molecule with involvemet ofanode-generated oxidizers of different nature.

Besides the electrochemically generated alkoxideand phenoxide ions, another reagents for initiation ofabove transformations of white phosphorus werestudied, which could promote involving the P4 mo-lecule to chemical reactions. Electrochemical genera-tion of amide anion on cathode and iodine on anodein the presence of white phosphorus emulsion allowstransformation of the latter into phosphorus acidaminoesters [100].

2R2NH + 23e 76 2R2N3 + H28,

2I3 3 23e 76 I2.

Under the action of cathode-generated amide anionoccurs consecutive splitting of P3P bonds followedby hydrogen substitution in the P3H bonds in thephosphorus oligomers initially by iodine and then byaido group. Thus, the principal reaction results information of triamidophosphite, which then exertsfurther transformations.

123

eP4 + 12R2NH 76 4(R2N)3P + 6H2,

H2O(R2N)3P + I2 76 (R2N)3PI2 776 (R2N)3PO.

The process general scheme can be given asfollows:

203

eP4 + 12R2NH + 4H2O 76 4(R2N)3PO + 10H2.

Yield of (R2N)3PO on P, %: 62 (Me), 60 (Et), 44(Bu).

Another route for (R2N)3P tranformation into finalcompounds was achieved at the adding to electroyteof carbon disulfide as a donor of sulfur. With theamine excess, the carbon disulfide quickly formsthiocarbamate wich in turn reacts with triamidophos-phite leading to triamidothiophosphate.

CS2 + 2R2NH 76 [R2NH2]+[R2NCS2]3,

(R2N)3P + [R2NCS2]3[R2NH2]+ 76 (R2N)3PS

+ R2NC(S)H + R2NH.

The process general scheme is as follows:

+203

eP4 + 16R2NH + 4CS2 76 4(R2N)3PS

+ 4R2NC(S)H + 6H2.

Yield of (R2N)3PS on P is 53368%.

In distinct to the above P4/ROH systems, these re-actions allow avoiding phosphorus polymerization andobtaining after P3P bond splitting triamidophosphiteexerting further transformations.

By similar way, compounds with P3S bonds canbe obtained by electolysis of white phosphorusemulsion in the presence of thiols (or a thiol andalcohol mixture) [104].

+123

eP4 + 12RSH 76 4(RS)3P + 6H2,

+203

eP4 + 6ROH + 6RSH76 (RO)3PO + (RS)3PO

+ (RO)2P(O)SR.

Low selectivity at the electrolysis of mixed estersof phosphoric acid is caused by several effects, amongwhich the presence of two pathways of transformationof the intermediates at P3H bond is probably themost important one.



P3ORei

P3SRei

ROH, 2e3

RSH, 2e3999

7776

7776P3H 7ei

Besides, it can be assumed that the peresterificationreaction rate is high enough.

Detailed studying of mechanism of white phos-phorus reactions at the electrolysis of solution contain-ing protogenous component like alcohol, water andphenol allowed obtaining various compounds whichare useful as such, e.g., trialkyl and triaryl phosphates,dialkyl phosphorus acids, triarylphosphites, tetra-alkylpyrophosphates, triamidophosphates and other.Scientific and practical significance of these processesconsists in promoting creation of basisity of me-chanism of white phosphorus transformations invarious electrochemical systems, elucidating influenceon the process of such factors as reagents ratio,temperature, solvent and other.

The possibility of obtaining from white phosphorusin some events of one specific compound rather thana mixture shows that generally transformation ofphosphorus and reaction intermediates proceedsthrough several highly selective steps. Although somesignificant details of the mechanism, such as sequenceof P3P bond splitting in the P4 tetrahedron and phos-phorus polymers, solvent effects on the polymeriza-tion process (preliminary results indicate great sig-nificance of solvent and in some cases no polymer isformed), relation of rates of certain steps of theprocess are still unclear, nevertheless, the data so faraccumulated allow to emphasize the most significantfactors defining the whole process. Knowing of bothfinal and primary products of electolysis after thesplitting of all P3P bonds under different conditionscan form a basis for elucidation the order and direc-tion of P3P bond splitting in phosphorus structures.For solving this problem seems reasonably to considermodel systems with chemical structure close to thatof separate fragments (P3P, P3H and other bonds invarious environment) which should be attacked bynucleophilic and electrophilic reagents under theconditions same as used in the electrolysis with whitephosphorus. However, selection and detailed study ofsuch systems will be an object of other investigation.

The accumulated material on the regularities intransformations of white phosphorus allowed topropose several process as prospect ways for thesynthesis of selected organophosphorus compoundscompetitive with classical ones; the latter have beenbased on the use of phosphorus chlorides and oxi-chlorides and are characterized first of all by their low

ecological purity and in some cases by low yield offinal compounds, complexity and multistep processing.Thus, in the cases when the process can be directedto initial formation of dialkyl phosphite after splittingof all P3P bonds, the yield of trialkyl phosphates canreach~80393% on phosphorus and 77389% on thecurrent.

Investigations showed that varying of nucleophiliccomponent in a P4 containing system can changesignificantly the general picture of phosphorus trans-formations.

The developed methods for the synthesis of organo-phosphorus compounds from white phosphorus con-sists of only one step and show high yields on bothphosphorus and current. Besides, the electrolytecomposition excludes use compounds aggressive tothe apparatus or destroying the target products, andthis simplifies application of the method; therewith,these methods are practically wasteless, as the solvent,background salt and reagent excess can be easilyregenerated and used in the next cycles.

Realising of the whole process can be taken as aground for modification of certain methods of thesynthesis of organophosphorus compounds which arestill not enough effective but can be developed.

Electrochemically Generated Metal Complexesin the Synthesis of Organophosphorus Compounds

A significant challenge in the preparative chemistryof organophosphorus compounds is development ofnew synthetic pathways to compounds with a P3Cbond on the ground of white phosphorus or phosphoruschlorides under mild conditions. Specific attention ispaid to the synthesis of tertiary phosphines widelyused in organometalic and coordination chemistry.The above described methods based on the simulta-neous action on the white phosphorus of nucleophilicand electrophilic agents in a nonseparated electrolyzerallows obtaining of phosphorus esters, but not thecompounds with a P3C bond. In recent years, con-venient approaches to the synthesis of compoundswith P3C bonds from white phosphorus and organichalides under the action of electrochemically gene-rated Ni(0) complexes or obtained from them orga-nyl s-complexes [105,106]. For this purpose wasperformed electrolysis of organic halides solved inDMF or acetonitrile in the presence of white phos-phorus emilsion and Ni(BF4)2bipy3 complex as acatalyst. The electrolyzer without separation of anodeand cathode spaces was used, with aluminum, zinc ormagnesium rod as anode. The experiment resultsshowed that under the action of electrochemically



generated catalysts, the Ni(0) complexes, whitephosphorus can be transformed to the compoundswith P3C bonds, phosphines and phosphine oxides,in reaction with organic halides: 70% Ph3P in DMF(Zn and Mg anodes), 59% Ph3PO and 47% C6H13POin MeCN (Al anode). The following reactions proceed:

On the cathode: Ni(II)L3 + 23e 7647 Ni(0)L2 + L

On the anode: Al3 33e 76 Al(III),

L is bipyridyl.

P4 + RX 776 P3ReiNi(0)L2

3X3

The key step is reaction of the organonickel com-pound RNiIIXLm with white phosphorus by thefollowing scheme:

Ni(0)Lm + RBr76 RNi(II)L mBr

76 P3R + Ni(II)L meiP4

Functionalization of white phosphorus proceedsunder mild conditions.

Electrochemical generation of SmCl2 in the pre-sence of a mixture of white phosphorus and anorganic halide leads to formation of the productscontaining phosphorus3carbon bonds [107]. WithSmCl3 .5H2O hydrate the main products were thephosphines PhPH2, Ph2PH, Ph3P and phosphoniumsalt Ph4PI. With anhydrous SmCl3 salt as a catalystthe main product was PhPI2, but some amount ofPh4PI was also isolated.

The following scheme can be proposed for thiscatalytic process:

SmCl3 + RX 76 RX3SmCl2 76 1/2R2 + SmCl2X977777777777

2SmCl2X + e3 7647 SmCl2

899977777777777777g

Reduction of RX at the action of Sm(II) ions pro-bably proceeds as an intraspheric electron transfer (asthe products, the dimers R2, differ from the productsof outerspheric transfer RH). Competitive reductionof PhI and P4 under the action of SmCl2 is alsopossible.

SmCl2 + P4 76 [P43_SmCl2

+] 76 P43 + SmCl3

Cl3

29

Products of furtrher transformation

An attainment in the homogenous electrochemical

catalysis is development of approaches to the elec-trosynthesis of tertiary phosphines by coupling ofmono- and dichlorophosphines with aromatic andheteroaromatic halides catalyzed by nickel complexwith 2,2-bipyridyl in a unseparated cell with mag-nesium or zinc anode, and investigation of the phos-phine formation mechanism in the different steps ofthe process [1083113]. Tertiary phosphines are formedin 25387% yield according to the following scheme:

ArX + PhnPCl33n 77776 R33nPhnP,NiBr2Bipy, e3

DMF

Ar =4c;R,4;N ,!?ES ,

3cN

N, NX

NN3Me;cMe

R = Me, MeO, Me2N, CN, CO2Et, Ac; n = 1, 2.

The cross-coupling compound with a P3C bond isformed in the following substitution reaction:ArNiBrbipy + PhnPCl33n 76 Ar33nPhnP + NiBrCl.bipy [in the initial steps with Mg anode, when theprocess occurs only at the potential of the Ni(II)/Ni(0)system]. When approximately a half amount of elec-tricity has passed, the solution contained no initial formof the catalyst NiBr2bipy and electolysis can becontinued at the more cathodic potential in the range31.5 to 31.7 V, depending on the nature of Ar,however, the cross-coupling product is formed evermore effectively than in the initial steps of the elec-trolysis.

ArNiX + 3e 76 ArNibipy + X3 (31.35 V),

ArNibipy + Ph2PCl 76 ArPh2PNibipy,

ArPh2PNiClbipy + 3e 7647 [ArPh2PNiClbipy].

76 ArPh2P + Ni(0) + Cl3 (31.5 to 31.7 V).

For the transformation of aryl halides with donorsubstituents in the ring, an anode made of Mg shouldbe used, with acceptors it should be Zn. The proposedmethod for the synthesis of tertiary phosphines iseffective with aromatic halides containing donor oracceptor substituents and heteroaromatic halides,pyridine, thiophene, pyrimidine and pyrazole halides.Advantage of this process is one step and mild con-ditions (room temperature). The general scheme ofcyclic regeneration of the catalyst is as given below.

Thus, in the electrochemistry of organophosphoruscompounds in the recent years were obtained interest-ing results both in fundamental area, where basicity



Ni(0)or e3 Ph2PCl Ph2P3Ar

Ph2PCl

Ph2P3Ar

Ph2PArNiX ArNi(II)XNi(II)

Ni(0)Ni(0)

ArXArX

2e3

e3

ArNi I

of electrocgemical reactivity under homogenous andheterogenous conditions were established, and in thearea of application, where were developed newmethods for the synthesis of phosphorus derivativesprospective for industrial application.

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