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Journal of Electroanalytical Chemistry 507 (2001) 157 – 169 www.elsevier.com/locate/jelechem Electrochemical synthesis of organophosphorus compounds with PO, PN and PC bonds from white phosphorus Yu.M. Kargin b,c , Yu.H. Budnikova a, *, B.I. Martynov b,c , V.V. Turygin b,c , A.P. Tomilov b,c a A.E. Arbuzo Institute of Organic and Physical Chemistry, Russian Academy of Sciences, 420088, Kazan, Russia b Kazan State Uniersity, Kremlieskaya Street 18, 420008, Kazan, Russia c State Research Institute of Organic Chemistry and Technology, 23 Shosse Entusiasto, Moscow, 111024 Russia Received 31 August 2000; received in revised form 26 October 2000; accepted 20 November 2000 Abstract The conditions of electrochemical oxidation of white phosphorus are studied, and direct electrochemical methods for the synthesis of esters of phosphoric, phosphorous and phosphonic acids, amidoesters and trialkyl and triaryl phosphines from white phosphorus were elaborated. Equipment allowing the processes to be carried out continuously and under fire-safe conditions using synthesis of triethyl phosphate was tested as an example. The proposed processes are suitable for industrial technology. © 2001 Elsevier Science B.V. All rights reserved. Keywords: White phosphorus; Electrosynthesis; Esters of phosphoric acids; Esters of phosphorous acid; Esters of phosphonic acid; Amidoesters; Trialkylphosphines; Triarylphosphines 1. Introduction At the present time, organophosphorus compounds (OPC) are of great practical importance in various branches of medicine, engineering and agriculture. Ac- cording to technologies accepted in industry, phospho- rus trichloride or phosphorus oxotrichloride is the starting material for the synthesis of phosphorus-con- taining organic compounds. As a result of the use of traditional methods for OPC synthesis, significant amounts of by-products difficult to use, such as hydro- gen chloride or other chlorides, are formed [1]. From the viewpoint of environmental hazards, direct methods of synthesis from elemental phosphorus, bypassing an intermediate chloration stage, are of great interest. This problem has received the bulk of recent attention [2,3]; the interest in OPC electrosynthesis based on white phosphorus is due to a number of advantages com- pared to usual chemical methods. In order to involve white phosphorus in the reaction, we used a method which consists in the formation of reactive intermediates, including radical-anions of te- traphosphorus and metal cations (from anodic mate- rial), in an undivided cell at the cathode and at the anode. These intermediates produce a joint action on P 4 resulting in the formation of various derivatives of phosphorus acids or phosphine, depending on the con- ditions. Our preliminary studies in this field had shown that two directions of tetraphosphorus transformation are of particular interest, namely synthesis of esters of phosphoric, phosphorous, phosphonic acids, and preparation of phosphines and phosphine oxides. 2. Experimental The method of cyclic voltammetry was used to study the electrochemical behaviour of white phosphorus. Measurements with rotating electrodes of platinum and glassy carbon were carried out on a computerized in- stallation QP-3080 from ECI-Technology Company (USA). Platinized titanium served as a counter elec- trode. Measurements with a stationary electrode were * Corresponding author. Tel.: +7-8432-359512; fax: +7-8432- 752253. E-mail address: [email protected] (Y.H. Budnikova). 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(01)00435-1

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Page 1: Electrochemical synthesis of organophosphorus compounds with PO, PN and PC bonds from white phosphorus

Journal of Electroanalytical Chemistry 507 (2001) 157–169www.elsevier.com/locate/jelechem

Electrochemical synthesis of organophosphorus compounds withP�O, P�N and P�C bonds from white phosphorus

Yu.M. Kargin b,c, Yu.H. Budnikova a,*, B.I. Martynov b,c, V.V. Turygin b,c,A.P. Tomilov b,c

a A.E. Arbuzo� Institute of Organic and Physical Chemistry, Russian Academy of Sciences, 420088, Kazan, Russiab Kazan State Uni�ersity, Kremlie�skaya Street 18, 420008, Kazan, Russia

c State Research Institute of Organic Chemistry and Technology, 23 Shosse Entusiasto�, Moscow, 111024 Russia

Received 31 August 2000; received in revised form 26 October 2000; accepted 20 November 2000

Abstract

The conditions of electrochemical oxidation of white phosphorus are studied, and direct electrochemical methods for thesynthesis of esters of phosphoric, phosphorous and phosphonic acids, amidoesters and trialkyl and triaryl phosphines from whitephosphorus were elaborated. Equipment allowing the processes to be carried out continuously and under fire-safe conditions usingsynthesis of triethyl phosphate was tested as an example. The proposed processes are suitable for industrial technology. © 2001Elsevier Science B.V. All rights reserved.

Keywords: White phosphorus; Electrosynthesis; Esters of phosphoric acids; Esters of phosphorous acid; Esters of phosphonic acid; Amidoesters;Trialkylphosphines; Triarylphosphines

1. Introduction

At the present time, organophosphorus compounds(OPC) are of great practical importance in variousbranches of medicine, engineering and agriculture. Ac-cording to technologies accepted in industry, phospho-rus trichloride or phosphorus oxotrichloride is thestarting material for the synthesis of phosphorus-con-taining organic compounds. As a result of the use oftraditional methods for OPC synthesis, significantamounts of by-products difficult to use, such as hydro-gen chloride or other chlorides, are formed [1]. Fromthe viewpoint of environmental hazards, direct methodsof synthesis from elemental phosphorus, bypassing anintermediate chloration stage, are of great interest. Thisproblem has received the bulk of recent attention [2,3];the interest in OPC electrosynthesis based on whitephosphorus is due to a number of advantages com-pared to usual chemical methods.

In order to involve white phosphorus in the reaction,we used a method which consists in the formation ofreactive intermediates, including radical-anions of te-traphosphorus and metal cations (from anodic mate-rial), in an undivided cell at the cathode and at theanode. These intermediates produce a joint action on P4

resulting in the formation of various derivatives ofphosphorus acids or phosphine, depending on the con-ditions. Our preliminary studies in this field had shownthat two directions of tetraphosphorus transformationare of particular interest, namely synthesis of esters ofphosphoric, phosphorous, phosphonic acids, andpreparation of phosphines and phosphine oxides.

2. Experimental

The method of cyclic voltammetry was used to studythe electrochemical behaviour of white phosphorus.Measurements with rotating electrodes of platinum andglassy carbon were carried out on a computerized in-stallation QP-3080 from ECI-Technology Company(USA). Platinized titanium served as a counter elec-trode. Measurements with a stationary electrode were

* Corresponding author. Tel.: +7-8432-359512; fax: +7-8432-752253.

E-mail address: [email protected] (Y.H. Budnikova).

0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 -0728 (01 )00435 -1

Page 2: Electrochemical synthesis of organophosphorus compounds with PO, PN and PC bonds from white phosphorus

Y.M. Kargin et al. / Journal of Electroanalytical Chemistry 507 (2001) 157–169158

performed with a potentiostat PI-50-1. A glassy carbonplate was used as the working electrode, and platinumwire served as an auxiliary electrode. The Ag � AgClhalf cell was the reference electrode in this case; allpotentials are given with reference to it. The solutionswere bubbled during experiments by dry hydrogen.

Preparative electrolyses were performed by means ofthe direct current source B5-49 in a thermostaticallycontrolled cylindrical undivided electrolyser (a three-electrode cell) with 150 cm3 volume Ag � AgNO3 (0.01mol l−1 solution in CH3CN) as a reference electrode.Platinum and steel cylinders with surface areas of 100and 80 cm2, respectively, were used as the anode andthe cathode. An argon flow was bubbled through theelectrolyte in the course of electrolysis. During electro-lysis, the electrolyte was stirred with a magnetic stirrer.

Organophosphorus compounds with P�O and P�Nbonds were synthesized from P4 using the followingprocedure. A solution with a total volume of 150 mlwas prepared by dissolving the required amount ofalcohol (phenol, amine) in acetonitrile (DMF). In theresulting solution, 1 g (32 mmol) of white phosphoruswas dispersed in an argon atmosphere and heated to53°C. The electrolysis was carried out at a 1–3 mAcm−2 anodic current density in the galvanostatic mode.The amount of electricity passed through the electrolytewas five electrons per molecule of phosphorus.

The electrosynthesis of organophosphorus com-pounds with P�C bonds was carried out according tothe following general procedure. The anode was acylindrical magnesium, zinc or aluminium rod (diame-ter 1.3 cm) surrounded by a platinum cylinder. Asolution of Bu4NBF4 (0.005 mmol), NiBr2bipy orNi(BF4)2bipy (0.5 mmol), organic halide (35 mmol) and7 mmol of white phosphorus dispersed in DMF (100ml) was electrolysed at 0.5 mA cm−2 cathodic currentdensity in an argon atmosphere until the RX wasconsumed completely. The solution was hydrolysed us-ing 0.1 M NH4Cl and extracted with ether; the organiclayer was washed with water and dried, and the solventwas evaporated. The products were purified by columnchromatography on silica gel with a pentane+ethermixture as the eluent.

After completing the electrolysis, the solvent wasdistilled from the catholyte, the residue was extractedby a mixture of ether and chloroform, concentrated byevaporation and purified either by vacuum distillation,in the case of liquid substances, or by recrystallizationfrom hexane, in the case of crystalline substances. Indi-vidual products were isolated from the mixture bycolumn chromatography on silica gel and were iden-tified by measuring physical constants and spectra ac-cording to appropriate literature data.

For simulation of a continuous process, an installa-tion with a flow-through filter-press electrolyser and adetached reactor for dissolving phosphorus was used.

Electrolysis products were analysed by NMR andchromato-mass spectrometric methods. The water con-tent in the electrolyte was found by Fischer’s method.Halide ion concentrations before and after electrolysiswere measured by the Volhard method. The amount ofPH3 evolved in the electrolysis was determined by thedecrease of iodine content in a solution of potassiumiodide when the exhaust gases were passed through it.The excess of iodine was titrated by a 0.05 M solutionof sodium thiosulphate.

Chromatographic analysis of reaction mixtures andsynthesized compounds was carried out on a Chrom-4gas-liquid chromatograph using helium as the carriergas and a flame ionization sensor-detector. Glasscolumns filled with a 5% Silicone SE-30 packing onchromaton N-AW (0.0125–0.160) were used. TheNMR spectra were recorded in CD3CN using a VarianT-60 spectrometer with a working frequency 60 MHz.The NMR 31P spectra were recorded using a CXP-100Brucker spectrometer (85% H3PO4 as an external stan-dard) with and without decoupling of protons.

3. Synthesis of compounds with P�O and P�N bonds

3.1. Voltammetric beha�iour of white phosphorus

White phosphorus exists in solutions as a P4 moleculewith the structure of a symmetric tetrahedron. Becauseof the strain in the small rings in the tetraphosphorusmolecule, these particles are able to participate in vari-ous chemical reactions. White phosphorus is reduced inaprotic solvents (acetonitrile, dimethyl formamide) at amercury dropping cathode at potentials of approxi-mately −1.55 to −1.73 V (with reference to a mercurypool), and 1.5–2 electrons are consumed by one phos-phorus atom [4]. In alcohol solutions, phosphorus re-duction is observed in the region of approximately−1.88 to −1.90 V (with reference to a mercury pool).The number of electrons participating in the electrodeprocess is approximately three to one phosphorus atom[5].

It is shown that the reduction proceeds through thestage of an intermediate chemisorbed compound ofphosphorus with mercury, P4Hgx. Phosphorus oxida-tion in aqueous solutions of sulphuric acid and sodiumhydroxide at the electrode from graphite or amalgam-silver matrices impregnated by solid or liquid whitephosphorus proceeds at potentials of 0.84�1.08 V [6].

Taking into account that phosphorus forms interme-diate compounds with mercury, we studied the voltam-metric behaviour of P4 at solid electrodes. Comparisonof polarization curves recorded at rotating electrodesfrom platinum and glassy carbon (GC) in non-aqueousethanol in the presence of sulphuric acid (Fig. 1a,b)shows that a significant inhibition of the electrode

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Y.M. Kargin et al. / Journal of Electroanalytical Chemistry 507 (2001) 157–169 159

process is observed during both cathodic and anodicpolarization at platinum. In this connection, the follow-ing investigations of phosphorus in ethanol with vari-ous supporting electrolytes were performed atstationary GC electrodes. Phosphorus is electrochemi-cally active in a base electrolyte of sulphuric acid (Fig.2). The observed increase of current on the polarizationcurves can be caused by both direct oxidation of phos-phorus, which begins at 0.5 V, and partial or completetermination of the inhibition of the electrochemicalprocess. Addition of halide ions to the supportingelectrolyte generates an increase of anodic current, dueto discharge of these ions; however, in the presence ofchloride ions (Fig. 3a), the rate of phosphorus oxida-tion is not practically changed. A detectable accelera-tion of oxidation is observed when bromide (Fig. 3b) oriodide (Fig. 3c) is involved. Their discharges begin at0.5 and 0.2 V, respectively.

Since iodine elimination takes place at less positivepotentials than phosphorus electrooxidation, the mainprocess in the solution of iodide ions consists in theirdischarge followed by the interaction of halide formedwith phosphorus. In the presence of bromide, jointoxidation of phosphorus and bromide ions proceeds atthe anode. The rate of phosphorus oxidation is not Fig. 3. Voltammetry at a stationary GC electrode in ethanol: (1) 0.5

mol l−1 H2SO4; (2) 0.5 mol l−1 H2SO4, 0.05 mol l−1 HCl (a); 0.5mol l−1 H2SO4, 0.05 mol l−1 NaBr (b); 0.5 mol l−1 H2SO4, 0.05 moll−1 NaI (c); (3) the same+0.05 mol l−1 P4.

Fig. 1. Voltammetry at a rotating electrode from platinum (a) andglassy carbon (b) in ethanol: (1) 0.5 mol l−1 H2SO4, (2) 1+0.05 moll−1 HCl, (3) 2+0.025 mol l−1 P4.

Fig. 4. Voltammetry at a stationary GC electrode in ethanol: (1) 0.5mol l−1 (C2H5)4NBr (a), (C2H5)4NI (b); (2) the same+0.025 mol l−1

P4.

Fig. 2. Voltammetry at a stationary GC electrode in ethanol: (1) 0.5mol l−1 H2SO4, (2) the same+0.025 mol l−1 P4.

increased due to chloride ions. Due to its high activityand relatively low concentration of phosphorus, atomicchlorine evolved at the anode is mostly used for ethanoloxidation.

When tetraalkylammonium halides are used as sup-porting electrolytes, the phosphorus activity is signifi-

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Fig. 5. Cyclic voltammetry at a stationary GC electrode in ethanol:(1) 0.1 mol l−1 (C2H5)4NBF4 as supporting electrolyte, (2) the firstanodic cycle (with addition of 0.025 mol l−1 P4), (3) followinganodic–cathodic cycles in a solution of 0.1 mol l−1 (C4H9)4NBF4+0.025 mol l−1 P4.

3.2.1. Alcohol and alcohol-containing solutionsThe main electrochemical processes in alcohol and

alcohol-containing solutions are:at the cathode with moderate hydrogen overvoltage,discharge of hydrogen ions or (in their absence)alcohol molecules with formation of hydrogen;at the anode, discharge of halide ions and, in theirabsence, oxidation of alcohol molecules.Initiation of the P4 cage opening in such solutions

depends appreciably on the pH of the medium. Inacidic media, the formation of particles I and II is mostprobable, i.e. addition of the electron is the primary actfollowed by protonization of the radical anion andformation of radical II, which can react by a radical-chain mechanism with halogen generated at the anode(Eq. (2)):

(2)

and so on. A following rupture of the P�P bonds is ableto occur, also, without participation of radical particles.At the same time, halogen is replaced by an alkoxygroup (Eq. (3)):

(3)

In the absence of free protons in the medium, aninteraction between a phosphorus molecule and alcoho-late ion generated in the reduction of the alcoholmolecules at the cathode and generation of particle IIIare possible. The anion (III), as a strong base (pKa ofconjugated acid is 29 [6]), is protonated in a mediumcontaining ROH. A course of further transformationsdepends on the relationship between the rates of activa-tion of phosphorus molecules and their subsequentoxidation. In the absence of free halogen or with exten-sive reduction of P4 (at the cathode with high over-voltage of hydrogen), large quantities of radicalparticles II collect, followed by their reduction at thecathode (Eq. (4))

(4)

to phosphine and other hydrides and also by polymer-ization to linear and cross-linked structures, for exam-ple (Eq. (5)):

The polymers arise first at the surface of the cathode.It is known [7] that the molecule of white phosphorusitself reacts readily with various phosphide ions, andbecause of this, in the absence of free protons, espe-cially at the initial stages of electrolysis, a process of

cantly lower than in the acidic solutions discussedabove, and some inhibition of the anodic process isobserved in the base electrolyte of Et4NI. In solutionsof Et4NBr and Bu4NI (Fig. 4a,b), as with the back-ground of H2SO4 in the presence of Br− and I− (Fig.3b,c), peaks are observed on the reverse scan of thecurve which are caused by reduction of the Br2 and I2

formed. These peaks disappear on addition of P4 to thesolution, suggesting the existence of a stage of interac-tion between phosphorus and halide generated at theanode. An effect of anodic activation of phosphorus inthe medium free from halide ions with the backgroundof 0.1 mol l−1 Bu4NBF4 was recorded rather distinctly(Fig. 5). The first cycle was recorded with potentialscanning from 0 to 2.5 V, the second cycle was recordedfrom 0 to −2.5 V and further up to 2.5 V. Uponaddition of phosphorus, the oxidation current increasedin the second cycle compared to the first one.

3.2. Mechanism of electrochemical transformations ofwhite phosphorus

Participation of the molecule P4 in electrode pro-cesses can be represented by the formal scheme (Eq.(1)):

(1)

Which of the paths followed is determined by theelectrolysis conditions, but formation of the particle Vseems to be unlikely, taking into account the chemicalproperties of P4.

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Y.M. Kargin et al. / Journal of Electroanalytical Chemistry 507 (2001) 157–169 161

(5)

and (8) compete in neutral and weakly alkaline media.A temperature increase and the presence of alcoholateions are favourable for (RO)3P rearrangement, allowingdialkyl alkyl phosphonate to be formed in detectablequantities in the absence of free halogen.

Dialkyl phosphite reacts readily with halogen elimi-nated at the anode, resulting in trialkyl phosphate in analcohol medium. Because of this, the concentration ofdialkyl phosphite in solution is low and depends on theratio between the rates of its formation and transforma-tion into trialkyl phosphate. The yield of dialkyl phos-phite is high when electrosynthesis is carried out insolution containing iodine as softly acting halogen. Theoverall reaction of electrosynthesis with the use ofhydrogen chloride as the electrolyte can be presented asfollows:

P4+16ROH+4HCl�4(RO)3PO+4RCl+10H2 (9)

The molar ratio RCl/(RO)3PO is usually less than 1.0and depends primarily on the HCl concentration.

The use of bromides and iodides allows the isolationof halogen-containing products to be excludedpractically:

P4+16ROH�4(RO)3PO+4RH+10H2 (10)

since the resulting alkyl halide is subjected to reductionto the alkane:

RHal+ROH ����+2e−

RH+Hal−+RO− (11)

In addition, some part of the alkyl halide interactswith ethylate-anion formed at the cathode according tothe Williamson reaction:

RO−+RHal�ROR+Hal− (12)

Water, which is present in alcoholic solutions, is notonly competitive with alcohol at the stage of halogensubstitution by the P�Hal bond, but can also partici-pate in the stage of nucleophile generation followed byan attack of the monomeric or oligomeric molecule ofphosphorus. It is expected that the possibility of form-ing the acids (RO)2P(O)OH, ROP(O)(OH)2, H3PO4,H3PO3 and others increases as the water concentrationis increased.

It has been possible to direct the process towardformation of one final product, trialkyl phosphate, inhigh yield, using certain limits of the molar ratio of thereagents in the electrolyte. Only hydrogen is a by-

interaction between III and a further molecule of P4 ispossible, which is in competition with the protonationreaction. Then, similar polyphosphide chains with ter-minal alkoxy groups arise which are terminated duringprotonation at various stages of their growth. When thefree access of phosphorus to the cathode is confined bya membrane, no cathode activation occurs and interac-tion of P4 molecules with the halogen formed at theanode is hampered. In the case of chloride, the oxida-tion of alcohol proceeds first. In the presence of bro-mide or iodide, elemental halogen accumulates in thesolution. The resulting products of electrolysis are ob-tained with low current efficiency. The formation ofradical particles and phosphide anions is precluded, andbecause of this there are no conditions for polymeriza-tion even with a high content of phosphorus insolution.

With free halogen in solution, radical chain reactionsin acidic and neutral media are terminated fast just inthe vicinity of the cathode surface and the formation ofpolymer structures is not detected. No elimination ofphosphine was detected either. Intensive polymerizationproceeds in alkaline medium (in the presence of RO−

ions), since protonation of particles III is hindered.Thus, the course of the phosphorus transformations

depends not only on the pH of the medium but also onthe membrane, the catalysts and on the electrode mate-rials. Since all P�P bonds in such polyphosphidesshould be equivalent to a first approximation, theirsuccessive cleavage proceeds, eventually, with high se-lectivity, as confirmed by isolation of only one productfrom tetraphosphorus [8]. The terminal reactionproduct (2), (3) as well as the result of the completedissolution of all phosphorus oligomers is trialkyl phos-phite which is unstable under these conditions andsubsequently undergoes the following chemicaltransformations:

(RO)3P+HHal� (RO)2P(O)H+RHal (6)

(RO)3P����[RHal]

(RO)2P(O)R (7)

(RO)3P+Hal2+ROH� (RO)3PO+RHal+HHal(8)

The reaction (6) proceeds so fast in acidic solutionthat phosphonate formation on the catalytic action ofalkyl halide according to reaction (7) is not establishedunder these conditions. On the contrary, reactions (7)

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product in this case. The overall equation of the processis as follows:

P4+12ROH+4H2O������20e−

4(RO)3PO+10H2� (13)

Intermediates arising during successive rupture ofchemical P�P bonds in the molecule of tetraphosphorusact as polyfunctional chemical reagents, which are ableto enter into further transformations with various reac-tive centres leading to various products. Thus, theproducts which arise can be considered as a result ofambiguous reaction of some intermediates, and our aimis to suppress some reaction paths and to stimulateothers by the control of the conditions.

It is assumed by analogy with dialkyl phosphites thata more stable hydrophosphoryl group is formed evenbefore complete cleavage of all P�P bonds. This groupis able to be functionalized further (reaction (14)):

(14)

However, it is complicated to prove the existence ofsuch a process. Rauhut [9] believed that the �P�(O)Hform in cyclic intermediate compounds is stable, be-cause a rearrangement into the four-coordinated hy-drophosphoryl form, �P�(O)H should lead toincreasing strain of the ring. However, Maier [10] al-lowed the possibility of such tautomerism, especially atthe final stages of cleavage of P�P bonds in phosphorusoligomers. Assuming that the equilibrium in strainedrings is not completely shifted to the right (reaction(15))

(15)

it may be proposed that the anion forming as a resultofdissociation (reaction (16))

(16)

has to display rather strong nucleophilic properties,when the tetraethylammonium cation is a counterion[9] and it is capable of attacking the electrophilic cen-tres of the P�P group (reaction (17)):

(17)

In this case, a group with an anhydride bridge ariseswhich can lead, for example, to pyrophosphites (un-stable in this system) or to pyrophosphates [11].

3.2.2. Solutions of phenols and aminesA study of phenol electrolysis in acetonitrile solution

in the presence of white phosphorus emulsion sup-

ported the general regularities of the process. It isknown that phenols, like alcohols, are capable of beingreduced on transition metals with a low hydrogen over-voltage with tetraalkylammonium salts as supportingelectrolytes and form phenolates which will react withwhite phosphorus under favourable conditions similarlyto other nucleophilic reagents, although they areweaker nucleophiles. Triphenyl phosphite formationduring electrolysis of phenol solutions of Et4NI in anundivided cell proceeds by the above path but somedissimilarities are found at subsequent stages of thetransformation of this intermediate. The sole product,an appropriate phosphate, is formed from triphenylphosphite. The emergence of phosphate among theelectrosynthesis products may be the result of hydroly-sis of unstable iodine derivatives of phosphorus,(PhO)3PI2, by residual water in the electrolyte [12]:

(PhO)3P+ (I2)� (PhO)3PI2���H2O

(PhO)3PO (18)

We succeeded in establishing pentaphenoxyphospho-rane (�= −85 ppm) and, possibly, (PhO)3PI2 (�=104ppm), as the intermediates in the process of triphenylphosphite transformation to phosphate, as might besupposed, as a consequence of the following reactions:

(PhO)3P�I2

(PhO)3PI2 ����2PhOH

−2HI(PhO)5P�

?(PhO)3PO (19)

The last stage here is of special interest, because dataabout possible routes and conditions for the transfor-mation of phosphorane to phosphate under similarconditions are virtually non-existent in the literature.Phosphite synthesized by a chemical method is con-verted completely to triphenyl phosphate when elec-trolysed in an undivided cell in an acetonitrile solutionof phenol and Et4NI after passing of 2.5 electrons permolecule of (PhO)3P. The presence of benzene in thesolution was shown by GLC analysis of the solventremoved from the reaction mixture. Pentaphenoxyphos-phorane is reduced irreversibly at the platinum elec-trode in acetonitrile to triphenyl phosphate at lessnegative potentials than phenol and gives one two-elec-tron wave. Results of preparative electrolysis of pen-taphenoxyphosphorane at a nickel cathode inacetonitrile with Et4NI as supporting electrolyte in adivided cell supported this scheme of the process;triphenyl phosphate and benzene were isolated from thereaction mixture [13]:

(PhO)5P+2e−���[H+]

(PhO)3PO+PhH+PhO− (20)

For greater insight into the mechanism of the processand to widen the scope of synthesis, investigation of theinteraction of P4 with amines under conditions of itscathodic activation and in the presence of anodicallygenerated halogen is of particular interest. Amide an-ions are formed in the reduction of amines at thecathode with a low overvoltage of hydrogen:

2R2NH+2e−�2R2N−+H2 (21)

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Table 1Products of P4 electrolysis in a mixture of alcohol and acetonitrile inthe presence of Et4NI (M(P):M(ROH)=1:20, 6.2 electrons/atom P,T=18–25°C) [14–16]; yield of products isolated on fractionation

ROH Yield on P/%Product

MeOH 11(MeO)2PHO70(MeO)3PO

3(MeO)2P(O)Me(MeO)3PO 51

28(MeO)2P(O)Me9EtOH (EtO)2PHO

69(EtO)3PO65(EtO)3PO14(EtO)2P(O)Et7BuOH (BuO)2PHO

66(BuO)3PO(BuO)3PO 60

15(BuO)2P(O)Bu

electrolyte by a reaction similar to reaction (18). Theoverall scheme of the process is given below:

P4+12R2NH+4H2O ��20e−

4(R2N)3PO+10H2 (22)

Hence, the utilization of various reagents, which arepotential participants in the functionalization of whitephosphorus, is of special interest to elaborate the reac-tion mechanism of phosphorus and its oligomers.

3.3. Electrosynthesis of trialkyl phosphates

A mixture of products is obtained after full dissolu-tion of all phosphorus oligomers at 25–50°C in anon-aqueous solution of alcohol and acetonitrile. Thismixture contains trialkyl phosphate, dialkyl phosphiteand dialkyl alkyl phosphonate (Table 1) [14–16]. Alkyliodide is reduced at the cathode and was detected in theelectrolyte only in minor quantities. Hence, some by-products are formed under these conditions, other thantrialkyl phosphate. It was possible to obtain a singleultimate product for certain limits of the molar ratio ofwater and alcohol (Table 2) [11].

This variant of the synthesis method is practicallywithout waste and allows difficulties existing in themodern production of trialkyl phosphates, which areassociated with the formation of alkyl chlorides due todestruction of trialkyl phosphate and intermediates byhydrogen chloride, to be avoided.

The use of additional solvents, from the viewpoint ofindustrial production of trialkyl phosphates, adds un-warranted complexity to the process pertaining to theirrecycling. Furthemore, salts of quaternary ammoniumbases are easily decomposed and are too costly, com-pared with halides of alkaline metals and hydrogen. Inthis connection, the method of trialkyl phosphate syn-thesis by electrolysis of white phosphorus in appropri-ate alcohols with addition of suitable supportingelectrolytes was tested (Table 3).

Hydrogen chloride has considerable solubility in allmixtures. However, the trialkyl phosphate obtained iseasily decomposed under the action of HCl in sec-ondary and, especially, tertiary alcohols, decreasing

Table 2Yields of trialkyl phosphates at M(P):M(H2O):M(ROH)=1:(1–1.1):(20–50), CH3CN, 50°C, Et4NI, ja=1–3 mA cm−2 [11]

Yield on P4 % Current efficiency/%(RO)3PO

93 89(MeO)3PO92 88(EtO)3PO89(PrO)3PO 8680(i-PrO)3PO 7795(BuO)3PO 91

8083(s-BuO)3PO(AmO)3PO 8689

Reactions of white phosphorus with amines (amideions) are practically non-existent in the literature. Pro-tons bonded to a nitrogen atom are less acidic thanprotons bonded to oxygen. However, it is possible toassume that the successive cleavage of P�P bonds withthe subsequent substitution of the halogen by an amidogroup proceeds in the same manner as in the presenceof alcohol or phenol. Thus, triamidophosphite is pro-duced as a result of the main reaction. Probably, itstransformation to phosphate occurs at the expense ofhydrolysis of unstable iodine derivatives by water in the

Table 3Electrosynthesis of (RO)3PO in ROH medium

Electrolyte cel/mol−1 l−1 Cathode Anode j/kA m−2Number T/°CR Yield a on P4/% Current efficiency/%

1 CH3 HCl 1.4 Pt C b 0.6 8CH3 NaBr 0.4 C2 Pt 0.6 16 83 60i-C3H7 HCl 1.5 Pt3 C 0.4 22 20 10i-C3H7 NaI 0.5 Ti4 C 0.7 14–17 60 45

2.0HCl 658520–240.4Cn-C4H9 Pt56 NaI 0.6 Tin-C4H9 C 1.0 16–17 30 257 n-C5H11 457022–280.6CPt2.0HCl8 6010–120.4CPt1.5HClCH2ClCH2 30

a Yield of product isolated on distillation.b Graphite.

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Fig. 6. Dependence of the molar ratio of (EtO)2PHO/(EtO)3PO,obtained from white phosphorus, on the concentration of HI insolution. Preparative yield of (EtO)2PHO at the peak area is 65% onP4.

after additional optimization of electrolysis conditionsand isolation of the products.

3.4. Electrosynthesis of dialkyl phosphites, dialkylalkyl phosphonates and tetraalkyl pyrophosphates

Electrolysis of aqueous alcohol solutions of HI andHBr in the presence of an emulsion of white phospho-rus shows that a complete conversion of phosphorus totarget products is achieved under these conditions. Di-alkyl phosphite is the main product in a wide range ofconcentration of HI (Fig. 6). Inorganic phosphorusacids, tri-, di and monoalkyl phosphates are otherproducts.

In this way, dialkyl phosphorous acid is preparedfrom white phosphorus in mixtures of aprotic solvent,alcohol and hydrogen iodide with yields of up to 65%.Temperature elevation in the course of electrolysis ofwhite phosphorus in non-aqueous solutions of alcoholsin acetonitrile and in the presence of alcoholate ions isfavourable for (RO)3P rearrangement to phosphonateunder the action of alkyl iodide formed at intermediatestages of the process, by reaction (7) (Table 4) [14–16].

Synthesis of dialkyl alkyl phosphonates is also possi-ble in neutral alcohol solutions. It was shown fromNMR spectra that diethylethyl phosphonate is one ofthe products of white phosphorus electrolysis in ethanolsolutions of sodium bromide or iodide, at an initialtemperature of 16–17°C [�(31P)=33 ppm]. The ratio ofphosphonate to TEP reaches 1:3.

The behaviour of white phosphorus in secondary andprimary alcohols is different. Phosphorus electrooxida-tion in a butanol solution of NaI at elevated tempera-ture results in a quantity of dibutyl butyl phosphonate[�(31P)=30 ppm], along with tributyl phosphate, butthe appropriate phosphonate was not formed inmethanol solution of sodium bromide, at least up to18°C.

In experiments with a reduced relative content ofalcohol and water in the medium of aprotic solvent, itwas found that, during the electrolysis of solutions withan initial molar ratio P:H2O:ROH ca. 1:0.03:2, thegeneral direction of the electrochemical process changesand tetraalkyl pyrophosphate becomes the mainproduct [11]. Decreasing alcohol content suppresses thereaction of trialkyl phosphate formation from the inter-mediates, (RO)2PHO and (RO)3P in the general processscheme, and increases the probability of (RO)3P acidiccleavage. Thus, a mixture of trialkyl phosphate andtetraalkyl pyrophosphate in molar ratios of approxi-mately 1:1.5 for linear aliphatic alcohols and 1:(8–10)for iso alcohols is obtained as a result of electrolysis ofwhite phosphorus emulsion under the conditions de-scribed with low alcohol content in the electrolyte.Yields of [(i-RO)2P(O)]2O are ca. 80%, based on P4

(Table 5).

Table 4Products of P4 electrolysis in non-aqueous solutions of alcohols inacetonitrile in the presence of Et4NI, (M(P):M(ROH)=1:20, 6.2electrons/atom P, T=50–55°C) [14–16]

ROH Product Yield on P/%

(MeO)3POMeOH 51(MeO)2P(O)Me 28(EtO)3POEtOH 65(EtO)2P(O)Et 14(BuO)3POBuOH 60(BuO)2P(O)Bu 15

Table 5Products of electrolysis of P4 emulsion in a solution of alcohol andwater in acetonitrile in a molar ratio of (1:2:(0.3–0.34)) (a deficiencyof alcohol and water), 0.01 mol l−1 Et4NI as the supporting elec-trolyte [11]

ROH Productj/mA cm−2 Yield on P/%T/°C

504.0 (MeO)3PO 65MeOH5[(MeO)2P(O)]2O

[(EtO)2P(O)]2O 50EtOH 4.0 50(EtO)3PO 18

PrOH 52[(PrO)2P(O)]2O523.2(PrO)3PO 17

i-PrOH 4.0 51 [(i-PrO)2P(O)]2O 80(i-PrO)3PO 5

5.5 55BuOH [(BuO)2P(O)]2O 51(BuO)3PO 16

4.7 50i-BuOH [(i-BuO)2P(O)]2O 814(i-BuO)3PO

i-AmOH 524.0 [(i-AmO)2P(O)]2O 79(i-AmO)3PO 4

significantly the yield of product (Table 3, experimentnumber 3). Sodium bromide is freely soluble only inmethanol, therefore, NaI was used as a conductingadditive for other alcohols.

Thus, it is possible to synthesize esters of phosphoricacid with various alkyl radicals in satisfactory yields bydirect electrooxidation of white phosphorus in alcohol

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Y.M. Kargin et al. / Journal of Electroanalytical Chemistry 507 (2001) 157–169 165

3.5. Electrosynthesis of aromatic esters and amidoestersof phosphorus acids

Consideration of the paths for white phosphorustransformation to these products by electrolysis ofEt4NI phenol solutions in an undivided cell allows thegeneral regularities of the white phosphorus reaction inalcohol media to be confirmed and shows some sub-stantial differences in subsequent stages of primaryorganophosphorus compounds. In the electrolysis of aphenol solution in acetonitrile with Et4NI in the pres-ence of white phosphorus emulsion, after passing threeelectrons per atom of phosphorus, a mixture of

triphenyl phosphite and triphenyl phosphate was ob-tained (Table 6) [13].

Incomplete conversion of white phosphorus to targetproducts is observed when P4 is transformed to aro-matic esters of phosphorus acids because a part of thephosphorus is removed from the reaction mass, andpassivation is detected during the electrolysis. If theassumption [17] that passivation of platinum anodes atpotentials of the first wave of iodide ion oxidation iscaused by formation of adsorbed I+ which reacts fur-ther with the solution components is correct, it can beprevented, or weakened, by introducing into the systemadditives (or another solvent) with a donor numberhigher than the acetonitrile number. This allows adecrease of the specific adsorption of iodine cations(+1) at the expense of their increasing energy ofsolvation [18] (iodine in these systems is not solvated).The use of DMF (DN=27) as a solvent allows P4 to becompletely included in the process, and only appropri-ate triaryl phosphates arise in practically quantitativeyields (i.e. the rate of subsequent reactions of intermedi-ate phosphites is higher), and no inhibition of theanode is detected (Table 7). Benzene is detected bychromatography, i.e. the mechanism including forma-tion of pentaphenoxy phosphorane is the same as inacetonitrile. Pyridine (DN=33) addition to acetonitrilealso substantially diminishes the inhibition of the an-odic process, but in this case triaryl phosphite is themain product on full conversion of phosphorus (Table8). Hence, the dependence of product distribution onthe nature of the solvent in the electrolysis of phenolsolutions of Et4NI in the presence of white phosphorusis followed.

Electrosynthesis of amino derivatives under condi-tions described before, also proceeds with complica-tions, decreasing the conversion of initial substance andthe yield of end products [19].

Dimethyl formamide as a reaction medium facilitatesthe reaction markedly: a full conversion of phosphorusto soluble organophosphorus compounds is observed,and polymer forms of phosphorus are not formed.Small amounts of water necessary for quantitative for-mation of phosphate introduced into the electrolyteincrease, as a rule, the yield of phosphate and cause noformation of by-products (Table 9).

This direction of the reaction was proved in indepen-dent experiments when (R2N)3P was oxidized in anundivided cell in the presence of amine and Et4NI.Because compounds with P�O bonds were not detectedamong the electrolysis products, it is assumed thatsubstitution for iodine on the P�I bond, under condi-tions of amine excess, proceeds predominantly underaction of the amine. Otherwise inevitably the acidicphosphate group �P�(O)H formed would no longer beable to be transformed into the amide one.

Table 6Products of electrolysis of phenol solutions in acetonitrile at thebackground of Et4NI in the presence of white phosphorus [13]

Yield on P/% CurrentProductArOHefficiency/%

46 45(C6H5O)3PC6H5OH(C6H5O)3PO 20 19(o-CH3C6H4O)3Po-CH3C6H4OH 38 38(o-CH3C6H4O)3PO 32 32(p-ClC6H4O)3P 10p-ClC6H4OH 10(p-ClC6H4O)3PO 52 52

Table 7Products of electrolysis of solutions of substituted phenols in DMF inthe presence of P4 (50°C, Et4NI, ja=1 mA cm−2, 7.1 electrons/atomP)

ArOH Product Yield on P/%

82(C6H5O)3POC6H5OH78o-CH3C6H4OH (o-CH3C6H4O)3PO

(p-ClC6H4O)3POp-ClC6H4OH 7977p-CH3C6H4OH (p-CH3C6H4O)3PO

o-ClC6H4OH 75(o-ClC6H4O)3PO(p-BrC6H4O)3PO 94p-BrC6H4OH(p-t-C4H9C6H4O)3POp-t-C4H9C6H4OH 84

Table 8Products of electrolysis of solutions of substituted phenols in CH3CNin the presence of P4 (50°C, Et4NI, ja=1 mA cm−2, 3.7 electrons/atom P); (1) without and (2) with an additive of pyridine (4.2×10−2

mol l−1)

ProductArOH Yield on P/%

21

(C6H5O)3P 46 78C6H5OH(C6H5O)3PO 20 20

70(o-CH3C6H4O)3P 38o-CH3C6H4OH12 25(o-CH3C6H4O)3PO

p-ClC6H4OH (p-ClC6H4O)3P 10 68(p-ClC6H4O)3PO 52 25

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Table 9Yields of (R2N)3PO in electrolysis of amine solutions of Et4NI in thepresence of white phosphorus

RnNH3− Solvent Molar ratio Yield of (R2N)3PS onn P/%P:H2O

Me2NH 10CH3CNDMF 45

1:0.5DMF 62CH3CNEt2NH 12DMF 1:0.6 60

1:0.5DMF 44Bu2NH1:0.5 18BuNH2 DMF

Thus, electrochemical activation of white phoshorusat the cathode in the presence of amines allows thepreparation of amides of phosphorus acids. Triami-dophosphite is the primary product after cleavage ofP�P bonds. It undergoes further transformations totriamidophosphate or to triamidothiophosphate.

4. Synthesis of compounds with a P�C bond

The variations of the electrochemical approach de-scribed above do not permit compounds with P�Cbonds to be obtained. To investigate the feasibility ofthe transformation of P4 into P�C derivatives, an elec-trochemical alternative of the synthetically fruitful reac-tion of oxidative addition was considered. Underfavourable conditions, single bonds of a tetraphospho-rus molecule could also be involved in the above reac-tion. It is known that rupture of the phosphorustetrahedron P4 under the action of organometallic com-pounds, e.g. RMgX and PhLi, takes place by cleavageof P�P bonds leading to formation of a mixture oforganophosphorus products in yields from 5 to 70%:

PhLi+P4+BuX �H2O

PhPBu2+Ph2PBu+Ph2P(0)Bu

+LiX (26)

We proposed that there is a certain chance of involv-ing tetraphosphorus in the reaction under conditionswhen the carbanion intermediate is generated electro-chemically at the electrode in the presence of P4. Withthis in mind, we chose complexes of Ni(0) with 2,2�-bipyridyl generated electrochemically from Ni(II). Theyform nickel organyl �-complexes with organic halidescapable of participating in functionalization of varioussubstrates, reactions of homo- and cross-coupling [20].Electrolyses of organic halide solutions in DMF oracetonitrile in the presence of tetraphosphorus emulsionand Ni(BF4)2bipy3 complex as a catalyst precursor wereperformed to provide experimental verification of thehypothesis developed. In order that electrochemicalgeneration of the nucleophilic participant and its inter-action with the tetraphosphorus molecule should pro-ceed with the help of a Lewis acid, it is reasonable touse a reaction of anode dissolution (aluminium, zinc ormagnesium) in an undivided electrolyser. It is ascer-tained that white phosphorus is converted to com-pounds with P�C bonds, phosphines and phosphineoxides, as shown by treatment of the reaction mixtures(Table 11).

The following reactions proceed in this case. Nick-el(II) complex reduces to nickel(0) complex undergoingan oxidative addition to organic halide forming �-or-ganic complexes, RNiXbipy [20–22], which can enterinto an expected and previously uninvestigated reactionwith participation of tetraphosphorus:

Table 10Yields of (R2N)3PS in electrolysis of amine solutions of Et4NI in thepresence of CS2 and white phosphorus (P:CS2=1:1)

Solvent Yield of (R2N)3PS on P/%R2NH

Me2NH 65CH3CNCH3CNEt2NH 60

68DMFCH3CN 59Pr2NH

Bu2NH CH3CN 53DMF 60

Formation of triamidophosphite as an intermediateproduct can be detected after cleavage of all P�P bonds,if the composition of electrolyte is analysed at theinitial stage of electrolysis (1–1.5 electrons per moleculeof phosphorus) according to the characteristic signal �31P (+118 ppm for (Et2N)3P) in the NMR 31Pspectrum.

Another means of (R2N)3P transformation to thefinal products is, for example, by introduction of car-bon disulphide into the electrolyte. This acts as asulphur source under the electrosynthesis conditionstransforming (R2N)3P to (R2N)3P�S (Table 10), thiofor-mamide being a by-product. The following reactionscheme is proposed. The phosphorous triamide reactswith thiocarbamate, yielding triamidothiophosphate:

CS2+2R2NH� [R2NH2]+[R2NCS2]− (23)

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

� (R2N)3PS+R2NC(S)H+R2NH (24)

The occurence of these processes was substantiatedby independent experiments. Thus, synthesized(Et2N)3P reacted easily with thiocarbamate resulting in(Et2N)3PS, and in the absence of amine under the sameconditions no changes are found.

The overall equation of the process is as follows:

P4+16R2NH+CS2 ���20e−

4(R2N)3PS+4R2NC(S)H

+6R2NH (25)

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Y.M. Kargin et al. / Journal of Electroanalytical Chemistry 507 (2001) 157–169 167

cathode: Ni(II)L3+2e− � Ni(0)L2+L, L=bipy

anode: M−ne−�Mn+, n=2, 3, M=Mg,

Zn, Al (27)

P4+RX�����Ni(0)L2

−X−�P�R (28)

By analogy with the functionalization of white phos-phorus by Grignard reagents, it is believed that a keystage consists in the interaction of organonickel com-pound, RNiIIXLn, with white phosphorus according thefollowing scheme:

Ni(0)bipyn+RX�RNi(II)bipyn−1X���1

4P4

�P�R

+Ni(II)bipy, n=1, 2 (29)

In order to support the reaction path depicted above,the stages of the electrochemical formation of organon-ickel complex, RNiIIXLn, and the stage of functional-ization of white phosphorus by this compound werestudied separately. It was found that the action of asmall excess of PhNiIIXbipyn, where n=1,2, obtainedas a result of two-electron reduction of complexNiBr2bipy in the presence of a two-fold excess of PhBr,on P4 leads also to products of phosphorus arylation,and it is possible to form three P�C bonds, with Ph3Pand Ph3PO (1:1) being produced. Phosphine and phos-phine oxide were also prepared with use of o-TolNiBr-bipy or MesNiBrbipy as model compounds. In theabsence of o-TolBr, the reaction of o-TolNiBrbipy orMesNiBrbipy with P4 in DMF results only in com-pounds of, probably, low-coordinated phosphorus,ArP=Nibipy (� 31P=204 ppm), which proved un-

stable on attempted isolation. Decomposition, proba-bly, leads to TolP(O)(OH)H (� 31P=20 ppm, JPH=531Hz). Evidently, rupture of the P4 tetrahedron and cleav-age of the other P�P bonds occur also under the actionof Ni(0)L or Ni(II)L complexes with formation ofphosphides or polyphosphides of nickel. For example,complex [NiL]2+ (L=MeC(CH2PPh2)3 or tetrakis-[(diphenyl phosphino)methyl]methane) is known to sub-stitute one of the phosphorus atoms in the tetrahedralcluster, leading to a stable complex, [LNi(P3)NiL](BF4)2

containing the cyclotriphosphorus group, �-P3 [23] (re-action (30)):

(30)

We studied the possibility of tetraphosphorus emul-sion interacting directly with complexes Ni(II)bipy andNi(0)bipy, in order to consider other ways of formingP�Ar bonds.

The non-polar tetrahedric molecule of P4 is charac-terized by weak nucleophilicity, due to the unsharedelectron pairs of the phosphorus atoms, and weakelectrophilicity, caused by the lowest unoccupied anti-bonding d-orbitals [24]. According to data obtained inrecent years [25,26], P4 is liable to show itself as an �1-or as an �2-ligand. The non-polar tetrahedric moleculeis activated due to coordination, as quantum-chemicalcalculations suggest [27]: the energy of P�P bonds de-creases non-uniformly, and the effective charges onP-atoms increase. Coordinated P4, contrary to freephosphorus, becomes a stronger acceptor of electronsand is reduced easily under the action of Ni(0)complexes.

A voltammetric study of NiBr2bipy and Ni(BF4)2-bipy3 behaviour in the presence of white phosphorusdemonstrated that tetraphosphorus is coordinated withnickel complexes, and a reduced form of the metalforms a more stable complex than an oxidative one.The absence of the anodic constituent in reverse scan-ning may be caused by the fast subsequent chemicalreaction of P4 reduction under the action of Ni(0)complex as an electron donor, for example:

Ni2+(P4)bipy+2e− � Ni0(P4)bipyNi0(P4)bipy�Nin+(P4)n−bipy

(31)

The fast subsequent reaction leads to a shift of thereduction curve of NiBr2bipy (Fig. 7, curve 2) relativeto curve 1 to less negative potentials. Preparative reduc-tion of tetraphosphorus under the action of the electro-chemically generated nickel complex Ni(0)bipy confirmsthe possibility of their interaction. Thus, preparative

Table 11Products of electrochemical functionalization of P4 by alkyl and arylhalides catalysed by Ni(0)bipy {from Ni(BF4)2bipy3}

Anode/solventR�X Product Yield on P/%

PhBr Al/DMF Ph3P 28Ph3PO 30

Zn/DMF Ph3P 40Ph3PO 25

9Ph2PH8Ph3PAl/CH3CNPh�Br

Ph3PO 59Mg/DMFPh�Br (PhP)5 60

Ph3P 15Ph�I Mg/DMF Ph3P 30

Ph3PO 30PhI Al/ DMF Ph3P 20

Ph3PO 48Zn/DMF 64Ph�I Ph3P

12Ph2PH38Bu3POBu�Br Al/CH3CN

Mg/DMF 11Bu�Br Bu3P55Bu3PO

7Hex3POHex�I Al/CH3CN

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Y.M. Kargin et al. / Journal of Electroanalytical Chemistry 507 (2001) 157–169168

Fig. 7. Voltammograms for NiBr2bipy (10−2 mol l−1) without (1)and with (2) white phosphorus (10−2 mol l−1). The same solutionafter it was kept in an argon atmosphere for 1 day (3).

for the preparation of compounds of tricoordinatedphosphorus in high yield. However, for example, in thecase of PhBr, a cyclic polyphosphorus compound(PhP)5 with the least strained cycle is formed. Magne-sium ions are known to favour the occurrence of (PhP)5

fragments at the expense of the stabilization of cyclicpolyphosphines [29–31].

Thus, experimental data indicate the decisive role ofmetal ions in the mechanism of rupture of the phospho-rus tetrahedron to end products. Unfortunately, thereasons for this dependence remain to be seen and callfor further investigation. Thus, we showed, for the firsttime, the possibility of electrochemical alkylation andarylation of white phosphorus under mild conditions ofmetallocomplex catalysis. It should be noted that elec-trochemical methods not only allow these processes to

be carried out at high rates but also allow control,achieving a high selectivity in some cases.

5. Conclusions

Electrosynthesis has allowed us to carry out thefunctionalization of white phosphorus and to obtainvarious esters of phosphorus acids depending on theelectrolysis conditions and the reagents present. Using asimple system such as tetraphosphorus alcohol, wedemonstrated that the electrochemical method is a suc-cessful synthetical approach, having a number of ad-vantages compared to classical methods of synthesis;namely:

it allows the synthesis of organophosphorus com-pounds (trialkyl phosphates, dialkyl phosphites, di-alkyl alkyl phosphonates, tetraalkyl pyrophosphates)from white phosphorus under mild conditions withhigh yields in many cases;it opens up approaches to the design of waste-freeand enviromentally safe industrial processes withcyclic regeneration of the oxidant at the anode;it allows a controlled system to be operated and byvaring the synthesis conditions it is possible to obtainspecific products.In addition, we succeeded in the functionalization of

white phosphorus to compounds with P�C and P�Nbonds under mild conditions using electrochemicalmethods.

electrochemical reduction of NiBr2bipy to Ni(0)bipy inthe presence of white phosphorus results in a depositwith elemental composition corresponding to the for-mula C10H8N2Ni2P3Br2. It is likely to be a binuclearcomplex (II):

Compound II was characterized by spectroscopy onnuclei of 31P (� 31P −335 ppm) and 1H (the spectrumconsists of a complex multiplet in the region of thesignals of aromatic protons, 6.8–8.9 ppm). The ESRspectrum shows that the isolated compound is para-magnetic: a signal with a wide maximum is observed.So, e.g. a new mononuclear complex, orange–brown incolour, with a [Ph3P3]-ligand (VII) is produced, i.e.arylation of the cyclo-P3 fragment takes place.

Consequently, such a path to P�Ph bond formationis competitive with the interaction of the �-aryl com-plex, PhNiXbipy, with P4. Thus, white phosphorusfunctionalization can be carried out by electrochemicalmethods with formation of compounds with P�C bondsunder mild conditions.

The results of the preparative synthesis show that thenature of the soluble anode influences drastically thecharacter of the electrolysis products. The use of a zincanode leads to complete conversion of tetraphosphorusto soluble phosphorus compounds, mainly to tertiaryphosphines and, to a lesser degree, to diaryl andmonoaryl phosphines (Table 9). The overall yield ofsoluble organophosphorus compounds at an aluminiumanode is lower, and a major portion of the products isphosphine oxide. This course of the process is likely tobe due to the possibility of forming Lewis acids insolution. Al(III) ions generated at the anode can act asLewis acids, as illustrated by the example of the interac-tion between t-BuCl and P4 in the presence of AlCl3[28]. A magnesium anode, like a zinc one, is favourable

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