[organophosphorus chemistry] organophosphorus chemistry volume 37 || phosphine chalcogenides,...

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Phosphine chalcogenides, phosphonium salts and P-ylides G. Keglevich DOI: 10.1039/b704708h 1 Phosphine chalcogenides Trofimov et al. found that the reaction of three equivalents of 4-chloromethylstyrene with red phosphorus in the presence of aqueous potassium hydroxide afforded tris(4-vinylbenzyl)phosphine oxide (Scheme 1). 1 The Michael-Arbuzov rearrangement is a basic reaction for the preparation of 4-coordinate species including phosphine oxides from 3-coordinate phosphorus esters, such as phosphinous esters. In most cases, the reaction requires a prolonged heating above 100 1C. Odinets et al. have now succeeded in carrying out the Arbuzov-reaction of ethyl diphenylphosphinite with a variety of alkyl halides in ionic liquids at or below 110 1C in short reaction times, mostly within half an hour. The best ionic liquid was 1-hexyl-3-methylimidazolium bromide (Scheme 2). 2 The recovered [hmim]Br could be recycled at least five times without a decrease in activity. The next examples for the preparation of phosphine chalcogenides are based on the reactions of phosphines with oxygen, sulfur and selenium. Ortho- and meta- pyrazolylphenyl–diphenylphosphine oxides were obtained by making available the corresponding triarylphosphines and treating them by hydrogen peroxide (Scheme 3). 3 Bakos et al. developed bis(4-trifluoromethylphenyl-)arylphosphines with strong p- acceptor character that were converted to the corresponding selenides (Scheme 4). 4 Scheme 1 Scheme 2 Scheme 3 Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, Mu ¨egyetem rkp. 3, Budapest, Hungary, H-1111 Organophosphorus Chem. , 2008, 37, 73–115 | 73 This journal is c The Royal Society of Chemistry 2008 Downloaded by Duke University on 02 March 2013 Published on 04 February 2008 on http://pubs.rsc.org | doi:10.1039/B704708H

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Page 1: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

Phosphine chalcogenides, phosphonium salts

and P-ylides

G. Keglevich

DOI: 10.1039/b704708h

1 Phosphine chalcogenides

Trofimov et al. found that the reaction of three equivalents of 4-chloromethylstyrenewith red phosphorus in the presence of aqueous potassium hydroxide affordedtris(4-vinylbenzyl)phosphine oxide (Scheme 1).1

The Michael-Arbuzov rearrangement is a basic reaction for the preparation of4-coordinate species including phosphine oxides from 3-coordinate phosphorusesters, such as phosphinous esters. In most cases, the reaction requires a prolongedheating above 100 1C. Odinets et al. have now succeeded in carrying out theArbuzov-reaction of ethyl diphenylphosphinite with a variety of alkyl halides inionic liquids at or below 110 1C in short reaction times, mostly within half an hour.The best ionic liquid was 1-hexyl-3-methylimidazolium bromide (Scheme 2).2

The recovered [hmim]Br could be recycled at least five times without a decrease inactivity.

The next examples for the preparation of phosphine chalcogenides are based onthe reactions of phosphines with oxygen, sulfur and selenium. Ortho- and meta-pyrazolylphenyl–diphenylphosphine oxides were obtained by making availablethe corresponding triarylphosphines and treating them by hydrogen peroxide(Scheme 3).3

Bakos et al. developed bis(4-trifluoromethylphenyl-)arylphosphines with strong p-acceptor character that were converted to the corresponding selenides (Scheme 4).4

Scheme 1

Scheme 2

Scheme 3

Department of Organic Chemistry and Technology, Budapest University of Technology andEconomics, Muegyetem rkp. 3, Budapest, Hungary, H-1111

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Page 2: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

In a study of factors affecting the optical properties of the dibenzophosphaborinsystem, the P-sulfide and P-selenide were prepared by Kawashima et al. (Scheme 5).5

Morpholine- andN-methylpiperazine P(III) compounds have been stabilized as thecorresponding sulfides and selenides (Scheme 6).6

N-Carbazolyl phosphines of the type PPh3�n(NC12H8)n (n = 1–3) that are P(III)species with bulky and electron-withdrawing substituents were also converted to thecorresponding selenides.7 The 9,10-dicyanoanthracene- and biphenyl-photosensitizedoxidation of triarylphosphines with air was studied in acetonitrile (Scheme 7) and theinvolvement of a peroxy radical cation (Ar3P

+–O–Od) intermediate was substantiated.8

Scheme 5

Scheme 6

Scheme 4

Scheme 7

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Page 3: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

Zhang et al. studied the oxidation of triarylphosphines with singlet oxygen(Scheme 8), and they found that the rate of removal of singlet oxygen showed agood correlation with the Hammet s parameter.9

This research group also investigated the intramolecular arene epoxidation ofbinaphthyl phosphine derivatives. Singlet oxygen reacts with 1,10-binaphthyl-2-di-tert-butylphosphine to form the corresponding phosphine oxide–epoxide intermedi-ate with complete retention of stereochemistry. This then undergoes a slow ‘‘NIH-rearrangement’’ to form the 1-phosphinoxido-10-hydroxy binaphthyl derivative(Scheme 9).10

An unusual oxidation of phosphines involving water as the oxygen source andtris(benzene-1,2-dithiolate)molybdenum(VI) as the oxidant was also described.11

In the next section, methods based on the modification of phosphine oxides toprovide novel phosphine oxides are discussed. New star-shaped and rod-shapedfluorescent phosphine oxides were obtained by the Pd-catalyzed Sonogashiracouplings of the corresponding PQO-functionalized arylacetylenes with appropriatearylhalogenides. The elegant syntheses are shown in Schemes 10 and 11.12

Scheme 8

Scheme 9

Scheme 10

Scheme 11

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Page 4: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

Linear biscatechols and triangular triscatechols incorporating a central phosphineoxide function have also been prepared. 1-(4-Bromophenyl-)2,5-dimethylpyrrole(regarded as a protected bromoaniline) was treated with dichlorophenylphosphineafter conversion to an organometallic reagent. The bis(pyrrolylphenyl)phenylphos-phine oxide obtained by oxidation was subjected to cleavage of the pyrrole groups byhydroxylamine hydrochloride to afford the bis(aminophenyl)phenylphosphine oxidethat on reaction with two equivalents of 2,3-dimethoxybenzoyl chloride led tothe corresponding bis(amide). Dealkylation of the four methoxy groups by tri-bromoborane furnished the corresponding bischatecol (Scheme 12).13 A trischatecolwas prepared in a similar way using O-benzotriazol-1-yl-N,N,N0,N0-tetramethyl-uronium hexafluorophosphate (HBTU) in the acylation step (Scheme 13).13

Tris(2-alkyl-1-methyl-3-indolyl)phosphine oxides were synthesized by the reactionof the corresponding indole with phosphoryltribromide in the presence of pyridine(Scheme 14).14 The products are configurationally stable molecular propellersexhibiting eight possible stereoisomers (as four enantiomeric pairs). Theoreticalcalculations suggested that in the case of R = Me, the antipodes are not stable

Scheme 12

Scheme 13

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Page 5: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

configurationally at room temperature, whereas with R = Et and R = iPr theenantiomers become more stable, as represented by racemization half lives of 1month and 104 years, respectively. Attempts to resolve the phosphine oxides weresuccessful.

Tetra-ortho-substituted P-functionalized biaryls were synthesized starting from2-nitro-6-bromophenylacetylene. Its phosphorylated derivative was subjected tocyclization with a suitable diene to lead eventually to a biphenyl, whose hydroxygroup was benzylated at the end of the sequence (Scheme 15).15

The methoxy groups of bis(anisyl)phosphine oxides were substituted by twoprolinol-based chiral lithium amide units to provide pyrrolidinyl substituted aryl-phosphine oxides (Scheme 16).16

Aiming at bifunctionalized allenes, alkynols were phosphorylated with diphenyl-chlorophosphine. The resulting intermediate underwent a spontaneous [2,3]-sigma-tropic rearrangement in the presence of hydrochloric acid as catalyst to afford thecorresponding allenyl phosphine oxide that was converted in two steps to thesulfonyl derivative (Scheme 17).17

Scheme 14

Scheme 15

Scheme 16

Scheme 17

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Page 6: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

A meso-porphyrinylphosphine oxide was prepared from the corresponding bro-moporphyrin by the Stille method, using Ph2PSiMe3 in the presence of a Pd(II)catalyst. The yield of the reaction was as low as 2% (Scheme 18/1).18 Starting fromthe iodoporphyrin, the efficiency was 20%. In the synthesis of another porphy-rinylphosphine oxide, an Z1-palladioporphyrin obtained from a suitable bromo-porphyrin was reacted with a Ph2P source. (Scheme 18/2).18

In the presence of the porphyrin moiety, the phosphine-functions were easilyoxidized by air to the corresponding P-oxides.Beletskaya et al. prepared phosphine oxides by hydrophosphination of substituted

olefins followed by oxidation (Scheme 19).19

New steroidal phosphine oxides were obtained by the Pd(II)- and base-catalyzedaddition of diphenylphosphine to a,b-unsaturated steroidal esters (Scheme 20),followed by oxidation at phosphorus.20

Van der Eycken et al. elaborated the four-step synthesis of 2,5-diphenylphos-phinoxidonorbornane. According to this, 2,5-norbornadione is transformed to thebis(phosphinoxido)norbornadiene via the bis(enolate) and the unsaturations areremoved by catalytic hydrogenation (Scheme 21).21 The racemic bis(phosphinoxido)-norbornane was resolved to give the precursor of bidental P-ligand (2S, 5S)-DIPHONANE.

Scheme 18

Scheme 19

Scheme 20

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Page 7: [Organophosphorus Chemistry] Organophosphorus Chemistry Volume 37 || Phosphine chalcogenides, phosphonium salts and P-ylides

Hydrazobenzenes with a chalcogenophosphoryl group were prepared by thereduction of 2-iodoazobenzene with hydrazine hydrate followed by a Pd(II)-cata-lyzed cross-coupling with diphenylphosphine. The P(III) function was subsequentlyoxidised by hydrogen peroxide, sulfur and selenium (Scheme 22).22 It was possible torestore the azobenzene structure by oxidation. The radical phosphination of aryliodides by chlorodiphenylphosphine in the presence of tris(trimethylsilyl)silane, 1,10-azobis(cyclohexane-1-carbonitrile) (V-40) and pyridine led to aryldiphenylphos-phines that were converted to the P-sulfides (Scheme 23).23

Zhao et al. accomplished the arylation of 4P(O)H species in the presence ofcopper(I) iodide and a base in toluene or DMF. The use of proline or pipecolinic acidas ligands greatly improved the efficiency of the coupling reactions (Scheme 24).24

Fiaud et al. described the Pd(II)-catalyzed coupling of optically active 2,5-diphenylphospholane oxide and aryl-/heteroaryl iodides in the presence of 1,3-bis(diphenylphosphanyl)propane (dppp) as the ligand and diisopropylethylamineas the base in DMSO (Scheme 25/1).25 1-Aryl-2,5-diphenylphospholane oxides werealso prepared in another way, by the reaction of the corresponding phosphinoylchloride with aryl lithium reagents (Scheme 25/2).

Scheme 21

Scheme 22

Scheme 23

Scheme 24

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Snieckus et al. added dicyclohexylphosphine oxide to the electron-poor doublebond of a quinone derivative in the presence of DBU. The hydroquinone formedafter aromatization was dimethylated and the resulting species was treated withtrichlorosilane. Surprisingly, instead of the expected deoxygenation, the 1-methoxygroup was cleaved to take advantage of the presumed coordination effect. However,after removing the 1-methoxy group with Ti(iPrO)4/(EtO)3SiH, the deoxygenationtook place (Scheme 26).26

Trofimov et al. accomplished the addition of bis(phenylethyl)phosphine-oxide and-sulfide to aldehydes and ketones to afford the corresponding a-hydroxyphosphineoxides/sulfides (Scheme 27).27

It was also possible to achieve reduction of the resulting hydroxy function afteraddition of a 4P(X)H species on a carbonyl group in a one-pot procedure. Hence,the reaction of diphenylphosphine sulfide with N,N-disubstituted formamides gavethe corresponding aminomethyl-diphenylphosphine sulfides in the presence of excessof sodium hydride. A representative example is shown in Scheme 28.28

Scheme 26

Scheme 25

Scheme 27

Scheme 28

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The phospha-Mannich condensation of 2- or 4-pyridinecarboxyaldehydes, pri-mary amines and secondary phosphine oxides was carried out in different variations,applying achiral reactants, optically active phosphine oxides and/or (+)-a-phenyl-ethylamine (Scheme 29).29

4P(X)H species undergo radical addition to the double-bond of 1-octene onmicrowave irradiation, in the absence of conventional initiators (Scheme 30).30 Theorder of reactivity was:

Ph2P(S)H 4 (EtO)PhP(S)H 4 PH2P(O)H B (EtO)2P(S)H 4 (EtO)2P(O)H.

n-Tributylphosphine was found to catalyze the a-P addition of H-phosphine oxidesand H-phosphonates to the triple bond of alkynes bearing a –P(O)Ph2 moiety(Scheme 31).31

Hydrotris(3,5-dimethylpyrazolyl)borate-Rh(PPh3)2 and the corresponding cyclo-octadiene derivative were tested as catalysts in the hydrophosphination of alkynes.They showed activity, but were less active as the Wilkinson’s catalyst.32

Scheme 29

Scheme 30

Scheme 31

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The C–P bond forming reaction between 1-alkynes and tetraphenylbiphosphinecould be catalyzed by a Rh-complex in the presence of 2,4-dimethylnitrobenzene toafford alkynylphosphine oxides after a subsequent oxidation by hydrogen peroxide(Scheme 32).33

a-Iminophosphine oxides were synthesized by the palladium-catalyzed insertionof isocyanides into the P–H bond of sec-phosphine oxides. Rhodium-catalysis led tothe selective formation of bis(phosphinoyl-)aminomethanes (Scheme 33).34

The reaction of bis(phenylamino)phosphine oxide with AlCl3 and SiCl4 resulted inthe formation of new phosphazane derivatives (Scheme 34).35

Nitrobenzene has been involved in a regioselective nucleophilic aromatic substitu-tion with a phosphorus-stabilized carbanion. The anion, generated fromCH3CH2P(O)Ph2 by butyllithium, attacked nitrobenzene in the para position(Scheme 35).36

Scheme 32

Scheme 33

Scheme 34

Scheme 35

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b-Silylvinylphosphine oxides were prepared regioselectively by the silylcuprationof 1-alkynylphosphine oxides followed by hydrolysis (Scheme 36). The organosilyl-copper(I) reagents were prepared in situ from CuCN and 2 equivalents of organo-silyllithium.37

The rhodium-catalyzed asymmetric hydroarylation of diphenylphosphinylalleneswith arylboronic acids furnished b,g-unsaturated diphenylphosphine oxides(Scheme 37).38

The chemo- and regioselective semihydrogenation of an 1,2-allenylphosphineoxide and related compounds was accomplished in the presence of a Pd[bis-(arylimino)acenaphthene](alkene) catalyst (Scheme 38).39

Palacios et al. described the stereoselective synthesis of fluoroalkyl-substitutedaziridine-phosphine oxides by the diastereoselective addition of Grignard reagents tofunctionalized ketoxime-phosphine oxides. Aziridines were used as intermediates forthe regioselective synthesis of fluorine containing b-aminophosphine oxides. Pro-ducts of the latter type could also be obtained by reduction of the ketoxime-phosphine oxides with sodium borohydride (Scheme 39).40

A variety of dihydrofuran-based phosphine oxides was made available. A b-keto-g0,d0-dihydroxyphosphine oxide was involved in intramolecular cyclization in twodifferent ways to afford the dihydrofuran as distinct diastereomers. The resultingisomers were converted to the corresponding phenylthio derivatives. One of theintermediates was useful in the synthesis of an azide, while the other one found use in

Scheme 37

Scheme 36

Scheme 38

Scheme 39

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the preparation of a tetrahydrofuran derivative (Scheme 40).41 Treatment oftris[2-(phenylthio)ethyl]phosphine oxide with sodium amide in tetrahydrofuran ledto a trivinylphosphine oxide via thiol elimination (Scheme 41).42

The synthesis, reactivity and conformation of 1,2-dihydrophosphinine oxides(e.g., A in Fig. 1), 1,2,3,6-tetrahydrophosphinine oxides (e.g., B and C), as well as1,2,3,4,5,6-hexahydrophosphinine oxides (e.g., D and E) have been reviewed byKeglevich (Fig. 1).43

The twist-boat conformation was sensitive to substitution effects and the3-P(O)Ph2-substituted cyclic phosphine oxides proved to be excellent precursors ofbidentate P-ligands.A valuable method was explored for the optical resolution of 3-methyl-1-phenyl-

2,5-dihydro-1H-phosphole 1-oxide via the formation of a supramolecular adductwith any antipode of TADDOL or its analogue (Fig. 2).44

Scheme 40

Scheme 41

Fig. 1

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The novel inverse Wittig type reaction of cyclic phosphine oxides and dialkylacetylenedicarboxylates, yielding b-oxophosphoranes, was accomplished undermicrowave conditions (Scheme 42). Hence, the reaction time was only ca. 3 h(instead of 2 weeks) at 150 1C and the yields were higher.45

The UV light-mediated fragmentation-related phosphinylation of methanol wasstudied utilizing 2,5-dihydro- and 2,3,4,5-tetrahydro-1H-phosphole oxides, as well as7-phosphanorbornene 7-oxide derivatives as the precursors (Scheme 43). It wasproved that the reactivity of the cyclic P-oxides is governed by ring-strain and theUV light absorption properties.46 In the case of UV-inactive substituents, such ascyclohexyl, ethyl and benzyl, only the strained phosphanorbornenes were suitableprecursors (Scheme 44).

Scheme 42

Scheme 43

Fig. 2

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A suitable 7-phosphanorbornene 7-oxide was transformed to the corresponding 2,3-oxaphosphabicyclo[2.2.2]octene in a Baeyer-Villiger oxidation that was a suitable pre-cursor for metaphosphonate used in the phosphorylation of simple alcohols (Scheme 45).A dual mechanism comprising elimination–addition and addition–elimination pathwayswas substantiated for both thermally- and photochemically-initiated reactions.47

Remaining with P-heterocycles, gem-diphenyltetrafluorophosphazene was preparedand treated with FcCH2P(S)(CH2OLi)2 to result in the formation of the endo- andexo,ansa-substituted fluorophosphazenes, together with the spiro-isomer (Scheme 46).48

A monofunctional phosphine oxide containing the endcapping reagent, 4-car-boxyl–phenyl–diphenylphosphine oxide, was prepared for applications in the synth-esis of telechelic polyester oligomers. The diphenyl-4-methylphenyl phosphine oxideobtained in Grignard reaction was subjected to oxidation to afford the carboxyl–phenyl derivative (Scheme 47).

Scheme 46

Scheme 47

Scheme 45

Scheme 44

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The utilization in polyester synthesis is shown in (Scheme 48).49 Eight new flame-retardant poly(amide-imide)s incorporating phosphine oxide moieties were synthe-sized starting from bis(3-aminophenyl-)phenylphosphine oxide that was convertedto a bis(3-carboxyl-phthalimido) derivative.

Polycondensation of this biscarboxylic acid with a variety of diamines gave thetarget poly(amide-imide)s (Scheme 49).50

The above-shown bis(3-aminophenyl-)phenylphosphine oxide was used also as areactive additive in halogen-free flame-retarded epoxy resins.51 Polyimides werederived from a bis(3-aminophenyl)-4-(4-adamantylphenoxy-)phenylphosphine oxideand bis(anhydrides) by polycondensation, utilizing two units of phthalic acidanhydride as end-groups (Scheme 50).52

Scheme 48

Scheme 49

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A new type of bismaleimide resin containing a P-heterocyclic unit was prepared bythe reaction of a bis(epoxide) and two units of N-(4-carboxylphenyl)maleimide tomake available a new flame retardant (Scheme 51).53

Interestingly, a phosphine oxide was used as a reagent in a Wittig-Hornerolefination aiming at the synthesis of the key intermediate of vitamin D3

(Scheme 52).54

Silanechalcogenides were synthesized by the reaction of a cyclic silylene withsuitable chalcogen precursors. The sulfur atom was transferred by trimethylphos-phine sulfide (Scheme 53).55

A catalytic asymmetric cyano-phosphorylation of aldehydes by diethyl cyano-phosphonate in the presence of a YLi3tris(binaphthoxide) complex and

Scheme 51

Scheme 52

Scheme 53

Scheme 50

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tris(2,6-dimethoxyphenyl)phosphine oxide led to the corresponding cyanohydrin O-phosphates.56 In this instance, the phosphine oxide was an additive. Chiral phos-phine oxides have been used as catalysts. Thus, e.g., the enantioselective aldolreactions of benzaldehyde and trichlorosilyl enol ethers were catalyzed by BINA-PO.57 The application of secondary phosphine oxides as preligands in catalysis wasreviewed by Ackermann.58 In solution, secondary phosphine oxides exist in equili-brium between pentavalent and trivalent tautomeric structures. In the presence oflate-transition metals, the equilibrium is shifted via coordination through phos-phorus to yield a cyclic complex with the participation of a deprotonated secondaryphosphine oxide (Scheme 54).58

The complexes of (N,N-dibutylcarbamoylmethyl)diphenylphosphine oxide(CMPO) with the proton and its hydrates in wet dichloroethane solution werestudied by IR and 31P, as well as 13C NMR spectroscopy. The formation of twogroups of complexes has been determined as shown in Fig. 3.59

Leung et al. described titanium(IV) terminal hydroxo complexes containingchelating bis(phosphine oxide) ligands, such as BINAPO.60 The metal ion extractingability of mainly a-keto-phosphine oxides and related compounds (Fig. 4) based oncomplex formation has been investigated by quantum chemical calculations.61

Special substrates comprising a phosphine oxide function and a phenolic moietyserved as asymmetric bifunctional catalysts after complexation with rare earth metal

Fig. 3

Fig. 4

Scheme 54

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ions (Scheme 55). The asymmetric polymetallic catalysts made possible a reversedenantioselectivity and a higher activity (e.g., in the ring opening reaction ofmeso-aziridines with TMSCN).62

A novel O,C,S-coordinating pincer-type ligand was made available that was usedin the synthesis of a triorganotin cation stabilized by sulfur and oxygen donor atoms(Scheme 56).63

A modified DIOP monophosphine oxide with 3,5-dimethyl-4-methoxyphenylsubstituents instead of the phenyl groups was successfully applied as a ligand inthe rhodium-catalyzed enantioselective phenylation of aromatic aldehydes with phe-nylboronic acid. The application of the hemilabile P–PQO ligand was based on theconcept of conformational control.64 A new series of iodocarbonyl ruthenium(II)complexes with unsymmetrical phosphine–phosphine sulfide ligands of typePh2P(CH2)nP(S)Ph2, where n = 1–4 was developed and studied.65 An oxime–phosphine oxide ligand (o-C6H4(P(O)Ph2)(CQN–OH)) has been utilized in thecopper-catalyzed coupling of aryl-iodides and -thiols.66 Complexes were preparedby Levason et al. from phosphine oxides and Group II metal nitrates. Hence, amongothers, [Be(OPPh3)2(NO3)2], [Mg(OPPh3)2(NO3)2], [Ca[Ph2P(O)CH2P(O)Ph2]2-(NO3)2] and [Ca[o-C6H4(P(O)Ph2)2]2(NO3)2] were characterized.

67 In another paper,tin(IV) fluoride pseudooctahedral complexes of phosphine oxide ligands, such as[SnF4L2] (L = Ph3PO and Me3PO) and [SnF4(L–L)] (L–L = Ph2P(O)CH2P(O)Ph2,

Scheme 56

Scheme 55

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o-C6H4(P(O)Me2)2, o-C6H4(P(O)Ph2)2 were described that were prepared fromSnF4(MeCN)2 and the appropriate ligand.68

There has been interest in the preparation of platinum(II) complexes withphosphine sulfides. Hence, [Pt(Ph3PS)(Me2SO)Cl2] and [Pt(Ph3PS)(Me-pTolSO)Cl2]were made available and found to be efficient catalysts in hydrosilylation.69 A seriesof thorium(IV) perrhenato- and pertechnetato-complexes with phosphine oxideligands (L), [Th(MO4)4(L)4] (where M = Re or Tc), were prepared and character-ized.70 Triphenylphosphine oxide (L) and bis(diphenylphosphineoxido)methane (L–L) were used in the preparation of uranyl complexes of the type UO2L4 andUO2L5.

71 The extraction properties of trialkylphosphine oxides in kerosene havebeen investigated by mathematical modelling. The basicity of the extractants to HClwas measured. Dependence of the extraction equilibrium on the basicity of theextractants was also studied.72 The solubilities of tri(p-methoxyphenyl)phosphineoxide and tri(p-hydroxyphenyl)phosphine oxide were determined in connection withtheir catalyst-binding role.73

2 Phosphonium salts

Quaternization is the most basic method for the preparation of phosphoniumsalts. Alkyl-tris(2-pyridyl)phosphonium salts were synthesized fromtris(2-pyridyl)phosphine and alkyl chlorides. The phosphonium salts obtainedfrom butyl bromide and benzyl chloride were found to decompose rapidly in hotacetone to 2,20-bipyridinium bromide and (2-Py)Bn(C(OH)Me2)P(O), respectively(Scheme 57).74

In another procedure, 1-alkylphosphonium salts were synthesized by the Pd-catalyzed reaction of terminal alkenes, triarylphosphines and bis(trifluoromethane-sulfonyl)imide. The product was formed according to the anti-Markovnikov or-ientation (Scheme 58).75

Fluorous quaternary phosphonium salts bearing four ‘‘ponytails’’ weresynthesized by the quaternization of a variety of fluorous phosphines withmostly perfluorinated alkyl halogenides (Scheme 59).76 This work of Gladyszet al. was aimed at exploring new candidates for phase-transfer catalysts and ionicliquids.

Scheme 58

Scheme 57

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The interaction of 1,7,7,-trimethylbicyclo[2.2.1]heptane-2,3-dione with hexaethyl-phosphoroustriamide led to an intramolecular phosphonium salt that on treatmentwith hydrochloric acid gave a phosphonium chloride (Scheme 60).77

The Mitsunobu reaction involves the alkylation of a nucleophile with an alcoholin the presence of diethyl azodicarboxylate and triphenylphosphine. The Mitsunobualkylation has now been applied to the alkylation of triphenylphosphonium tetra-fluoroborate (Scheme 61).78

(1,3-Dioxolane-2-ylmethyl-)triphenylphosphonium bromide monohydrate(Fig. 5) has been described as an inhibitor against the acid corrosion of iron, steel,zinc and aluminum.79

Scheme 61

Scheme 60

Scheme 59

Fig. 5

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Depending on the phosphine, its interaction with Ph4Se4Br4 resulted in theformation of R3PSe(Ph)Br or [R3PSePh]Br, the latter displaying an ionic structurein solution (Scheme 62).80

[Me3PCRCPh]+Br� in dichloromethane reacted with a slurry of polymericphenylethynylgold to give a clear solution of [Me3PCRCPh]+ [PhCRCAuBr]�

(Scheme 63/1), while the similar reaction of the starting phosphonium salt withpentafluorophenylgold(tetrahydrothiophene) led to [Me3PCRCPh]+ [C6F5AuBr]�

(Scheme 63/2).81

A novel manganese(III) complex of the type PPh4[Mn(malonic acid)2(H2O)2] hasbeen described that incorporates a tetraphenylphosphonium cation and a trans-diaquabis(malonato)manganate(III) unit.82 A diphosphinodiphosphonium dicationwas developed in a two step synthesis. In the first step, dichlorophenylphosphine,triphenylphosphine and trimethylsilyloxy-triflate (TMSOTf) were allowed to reacttogether to give PPhCl–P+Ph3 OTf�. Two units of this intermediate then interactedwith another molecule of Ph3P and TMSOTf to provide the desired dication(Scheme 64).83

Juge et al. have been able to detect the two antipodes of racemic P-chiral orC-chiral phosphonium salts (Fig. 6) by NMR studies in chiral liquid crystallinesolutions.84

Scheme 64

Scheme 63

Scheme 62

Fig. 6

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Tanaka, Scott et al. observed that phosphonium salt host compounds (Fig. 7)formed inclusion crystals with cyclohexane-1,3-diol, cyclohexane-1,4-diol and cyclo-hexane-1,3,5-triol in which the equatorial conformers are selectively included.Hence, the selective crystallization is an elegant method for the separation of thecis and trans isomers of cyclohexane polyols.85

The modification of phosphonium salts is also a possibility to make available newderivatives. Hence, a series of azoles was added to the triple bond of triphenyl-phenylethynyl phosphonium bromide to afford new products (Scheme 65).86

In another example, an unsaturated phosphonium salt was generated fromtriphenyl-(2-phenoxyethyl)phosphonium bromide that on reaction with dimethyl-hydrazine gave the corresponding Michael adduct. After the loss of hydrogen, thehydrogen bromide salt of the hydrazone underwent a double-bond rearrangement(Scheme 66).87

In a related topic, a bis(hydrazone) was treated with two equivalents of aryldi-phenyl-(2-oxoethyl)phosphonium chloride to furnish, after double-bond rearrange-ment, the corresponding bis(phosphonium salt) (Scheme 67).88

Scheme 66

Scheme 67

Fig. 7

Scheme 65

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All components of a complete Wittig reaction sequence, i.e. the starting phos-phine, the phosphonium salts, the phosphorane, the oxaphosphetane and thephosphine oxide (Scheme 68) have been characterized by 31P NMR spectroscopyand the electronic properties of 2-furyl and 3-furyl substituents evaluated byquantum chemical calculations including computational 31P NMR spectroscopy.89

A few phosphonium salts were converted in situ by potassium hydride in paraffin tothe corresponding phosphoranes that were utilized in Wittig reactions (Scheme 69).90

In the presence of a strong base (e.g., NaH or tBuOK) and in acetonitrile,(3-thienylmethyl)triphenylphosphonium salts undergo a homocoupling reaction toform ethenyldithiophenes that is faster than a Wittig reaction with aromatic ketones(Scheme 70). The coupling reaction was also extended to other (hetero)arylderivatives.91

Scheme 70

Scheme 68

Scheme 69

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Optically active, 2,5-diphenyl-tetrahydrophosphole-based phosphonium saltswere utilized in the enantioselective synthesis of allenic esters (Scheme 71).92

A propargylic phosphonium salt was transformed to a cyclic derivative in acobalt-catalyzed Diels-Alder reaction that was utilized in a Wittig olefination. All thethree reaction steps including the formation of the phosphonium salt and its reactionwith aldehydes were accomplished in a one-pot sequence. Moreover, the dihydroderivative was aromatized by oxidation with 2,3-dichloro-5,6-dicyano-1,4-quinone(DDQ). The ratio of the E and Z isomers was in the range ca. 1–2 (Scheme 72).93

The reaction of phosphoranes generated in situ from the respective phosphoniumsalts with the Weinreb amide led to the corresponding aldehyde after acidichydrolysis (Scheme 73).94

Treatment of (2-trimethylsilylphenyl-)methyl phosphonium salts with NaHMDSprovided the corresponding ylide that underwent anionic 1,4-silyl migration to formthe respective phenyl–trimethylsilyl ylides that on reaction with methyl iodide gavephenyl-(1-trimethylsilylethyl)phosphonium iodides (Scheme 74).95

Phosphonium salts may be intermediates in different reactions. The Morita-Baylis-Hillman reaction follows such a protocol. In a typical reaction sequence,a,b-unsaturated carbonyl compounds react with aldehydes in the presence ofnucleophiles, such as a trialkylphosphine, to afford aldol-like products (Scheme75/1), while in another example, unsaturated carbonyl compounds with bromo atomat the end of the chain are cyclized to cycloalkene derivatives (Scheme 75/2). In both

Scheme 74

Scheme 73

Scheme 71

Scheme 72

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cases, phosphonium salts as key-intermediates are formed by the addition of thetrialkylphosphine to the electron-poor double-bond.96

In work related to the synthesis of Taxol, an allylic phosphonium salt was reducedby lithium aluminium hydride to give the terminally unsaturated products in a highlydiastereoselective manner. In this instance, the phosphonium moiety served as aleaving group (Scheme 76).97

Phosphonium salts A–C that can be regarded as Mitsunobu-type intermediateswere made available and characterized (Fig. 8). In species A two phenolic moieties

Scheme 76

Scheme 75

Fig. 8

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are involved, while in B only one, but as an anion. The in situ generated intermediateC reacted with ethanol to result in the formation of the corresponding ethyl ester.98

A phosphonium-supported triarylphosphine was prepared in order to control itssolubility in CH2Cl2/Et2O. The amount of Et2O required to obtain quantitativeprecipitation of the arylphosphonium salt was found to be counterion dependent(Br 4 ClO4 4 PF6). The target molecule was synthesized by quaternization followedby change of the anion or by (quaternization followed by) deoxygenation (Scheme 77).99

Charette et al. also described the synthesis of a phosphonium-supported azodi-carboxylate. A bis(quaternization) was followed by ozonolysis and reduction of thealdehyde so obtained. The (4-hydroxymethylphenyl)triphenylphosphonium saltwas then transformed to the target-azodicarboxylate in three steps as shown inScheme 78.99

Bis(phosphonium salts) of the type shown in Fig. 9 were identified as by-productsin catalytic systems involving o-xylylene-a,a0-dihalides, carbon monoxide and Pd–triphenylphosphine complexes, applied in the synthesis of 3-isochromanones.100

Special phosphonium salts, such as (benzotriazol-1-yl-oxy-)tris(dimethylamino)-phosphonium and (benzotriazol-1-yl-oxy-)tris(pyrrolidino)phosphonium salts withhexafluorophosphate counter anion (Fig. 10) have been introduced as couplingreagents in the synthesis of four model peptide sequences.101

Scheme 78

Fig. 9

Scheme 77

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Phosphonium salts can be used as special catalysts in certain cases. It wasobserved that a selective N,N-dimethylation of aniline derivatives with methyl-alkylcarbonates could be accomplished in the presence of phosphonium salts(Scheme 79). Ph3PEtI and

nBu4PBr were especially efficient catalysts. The formationof an amine-salt complex was assumed.102

The catalytic activity of phosphonium salts in the synthesis of cyclic carbonates(e.g., propylene carbonate from propylene oxide and carbon dioxide) was found tobe greatly enhanced by their immobilization onto silica that itself had no catalyticeffect.103 The discipline of ionic liquids keeps developing dynamically. Phosphoniumionic liquids have received much less attention than ammonium salts in the past, butthe situation is changing. Triphenylalkyl-, tetraalkyl- and functionalized-phospho-nium tosylates (Fig. 11) that exhibit melting points mostly above 100 1C wereintroduced and characterized.104

The separation of aliphatic (e.g., alkanes, alkenes, alkynes and alcohols) andaromatic compounds (benzene) was studied using the high molecular weight

Fig. 11

Fig. 10

Scheme 79

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trihexyl-tetradecylphosphonium cation-based ionic liquids with the anions chloride,tetrafluoroborate and bis(trifluoromethanesulfonyl)imide. The selectivity values forphosphonium-based ionic liquids indicated a poorer separation capability than forimidazolium- or pyridinium-based ionic liquids.105 The physical properties ofselected ionic liquids, such as trihexyltetradecylphosphonium chloride/acetate/bis(trifluoromethanesulfonyl)amide were studied under pressure.106 In an effort toexpand the useful temperature range of ionic liquids, large melting point depressions(even 79 1C) were observed for simple phosphonium (and ammonium) salts in thepresence of compressed carbon dioxide. At 150 bar, methyl-tri-n-butylphosphoniumtrifluoromethanesulfonate was found to have a melting point of 40 1C, while atatmospheric pressure it melts at 119 1C.107 Seddon et al. have studied the propertiesof mutually immiscible ionic liquids, some of which are also immiscible withcommon solvents, e.g., water and alkanes. An archetypal biphasic system istrihexyltetradecylphosphonium chloride with 1-alkyl-3-methylimidazolium chloride,where the alkyl group is shorter than hexyl.108 Onium salt phase transfer catalystscan be regarded to be environmentally friendly tools if they can be regenerated afterthe synthesis. Now, Sato and Kawamura have prepared magnetic nanoparticle-supported quaternary phosphonium (Scheme 80) (and ammonium) salts that wereevaluated as phase transfer catalysts. Some of them exhibited activities comparablewith that of tetra-n-butylammonium iodide. The catalysts were easily separatedusing an external magnet and could be reused.109

Phosphonium salts having also a phosphine moiety and bound to solid supportswere studied by 31P CP/MAS NMR spectroscopy. One example is shown onFig. 12.110

Phosphonium cations comprising three or four n-octadecyl chains with iodide,bromide, chloride, fluoride or perchlorate anions were used to gelate and polymerize2-10 wt% solutions of tetraethyl orthosilicate in organic solvents using acid or basecatalysis.111

3 Ylides (phosphoranes)

The discussion of P-ylides is connected in some respects to that of phosphonium salts(see Schemes 68, 69, 71–73).Pinchuk et al. synthesized two pyrazole-based phosphoranes. The chlorination of

a suitable dichlorophosphine eventually afforded the target trichlorophosphorane.Quaternization of the related dimethylphosphino derivative gave the correspondingphosphonium salt that was converted to the trimethylphosphorane by vacuumpyrolysis. A part of the ethyl iodide liberated reacted with the pyrazole moiety(Scheme 81).112

Scheme 80

Fig. 12

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The synthesis and structure of phosphatranes N(CH2CH2NR)3PQCH2 shown inFig. 13 was reported by Verkade et al. The ylide (R = Me) is of extremely strongbasicity within the series N(CH2CH2NMe)3PQE, (E = CH2, NH, lone pair, O andS), in accordance with its low ionization energy (6.3 eV).113

The Wittig reaction of various types of ylide was investigated on realistic systemsusing DFT calculations. The results provide unequivocal support for the generallyaccepted oxaphosphetane mechanism for the salt-free Wittig reaction and rule outthe involvement of a betaine in the mechanism. The selectivities were in very goodagreement with the experimental data. The calculations on the reactions with non-stabilized ylides show that both the initial addition step and the subsequentelimination step can play a role in selectivity. The influence of ylide stabilizationand the effect of P-substituents on the reversibility of oxaphosphetane formation wasalso studied.114 Reference is again made to Scheme 68, a complete Wittig reactionsequence that was studied from the point of view of the effect of the phosphorussubstituents on the 31P NMR shifts.89 The Wittig reaction of 4-substituted cyclo-hexanones with ethoxycarbonylmethylene–triphenylphosphorane was accomplishedunder microwave conditions. The ratio of the exocyclic olefin and the endocyclicisomer was ca. 99:1 at 190 1C in MeCN and ca. 85:15 at 230 1C in DMF (Scheme 82/1). The tendency was the same in the reaction of cyclopentanone and cycloheptanone(Scheme 82/2).

Fig. 13

Scheme 81

Scheme 82

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The olefinations of bicyclo[3.3.0]octane-3,7-dione and the 1,5-dimethyl analogueat both 190 1C and 230 1C provided the main product in a less regioselective manner(Scheme 83/1). The MW-assisted Wittig reaction of 1,4-cyclohexanedione gave themono-olefinated product and its isomer in a ratio of 94:6 at 190 1C. The bis-olefination product was formed in only 8% yield (Scheme 83/2). The Wittig reactionof 1,2-cyclohexanedione was complicated by the formation of by-products (Scheme83/3). The Wittig reaction of 3-methyl-1,2-cyclohexanedione at 190 1C led to thegeometrical isomers of the exocyclic olefin, but at 230 1C, a butenolide was formedtogether with the endocyclic enone (Scheme 83/4).115

Molander and Figueroa have developed a route to organotrifluoroborates via astereoselective Wittig reaction using aryl-, heteroaryl- and aliphatic aldehydes withpotassium aryltrifluoroborate salts (KF3B–Ar) and cyanoethylmethylene-triphenyl-phosphorane (Scheme 84). The salt 4-KF3BC6H4C(O)H, was also treated with avariety of substituted methylene-triphenylphosphoranes (Scheme 85) to give a seriesof functionalised alkenes.116

Scheme 83

Scheme 84

Scheme 85

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Schlosser et al. prepared substituted 4-trifluoromethyl-2-quinolinones by theWittig reaction of 2-(N-BOC-amino)aryl-trifluoromethyl-ketones and substitutedalkoxycarbonylmethylene-triphenylphosphorane followed by an acid- or base-cata-lyzed cyclization (Scheme 86).117 An a,b,g,d-unsaturated trifluoroethyl ester wassynthesized in complete trans selectivity by the Wittig reaction of cinnamaldehydeand trifluoromethoxycarbonylmethylene–triphenylphosphorane (Scheme 87).118

2,3,4,6-tetra-O-benzylmannoso-1,5-lactone was reacted with tributylphosphoranecontaining an electron-withdrawing substituent in position a to give the correspond-ing (E)-mannosylidene derivatives selectively (Scheme 88).119

The convergent total synthesis of Myxothiazoles involves a Wittig reaction as thelast step. In this procedure, b-methoxyacrylic aldehyde derivatives were treated witha bisthiazole-based phosphonium salt in the presence of a base (Scheme 89).120

A new approach to Combretastatin D2 involves two independent Wittig steps. Thestarting material of the first one was 3-hydroxy-4-methoxybenzaldehyde. Themethoxycarbonylethenyl-product was then converted in several steps to a phospho-nium salt, also containing an aldehyde function, that underwent an intramolecularcyclization (Scheme 90).121

Scheme 86

Scheme 87

Scheme 88

Scheme 89

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The reaction of 4-benzoyl-5-phenylfuran-2,3-dione with a b-oxophosphoranefollowed an unexpected route. The oxaphosphetane did not undergo eliminationof triphenylphosphine oxide, but gave a betaine that was stabilized by a transfer of aproton. Reaction of the b-hydroxyphosphorane so formed with water led to a2-hydroxy-(2-oxoethyl)furan derivative forming an adduct with the triphenyl-phosphine oxide by-product (Scheme 91).122

Chiral stabilized ylides were prepared by the reaction of the Bestmann ylide((triphenylphosphoranylidene)ketene) with camphor-derived lactams that were useddirectly or after modification by alkylation in Wittig olefinations. Representativeexamples are shown in Scheme 92. The products can be regarded as chiral buildingblocks.123

A domino addition-Wittig-rearrangement reaction was described that started withthe addition of an a-hydroxy ester derivative on the 2,3-unsaturation of (triphenyl-phosphoranylidene)ketene, followed by an intramolecular Wittig olefination of theprimarily formed adduct. Under MW irradiation, the furanone derivative so formedunderwent rearrangement of its side chain and finally the newly formed 2-butenylsubstituent was saturated by catalytic hydrogenation (Scheme 93).124

Scheme 90

Scheme 91

Scheme 92

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Other examples were also described by Schobert et al.124 a,b-Unsaturated esterscan be prepared in a one-pot process from a variety of alcohols. In the first step, thealcohols are oxidized selectively by PhI(OAc)2/2,2,6,6-tetramethyl-1-piperidinyloxy(TEMPO) to aldehydes that are then subjected to Wittig olefination with ethox-ycarbonylmethylene- or a b-oxomethylenephosphorane (Scheme 94).125 SubstituentY may include unsaturated, hydroxyalkyl and aryl groups, as well as epoxy-, sugarand pinane-based substituents.

In another one-pot procedure, a Suzuki coupling and a Wittig olefination wererealized back to back. A great variety of aryl- and heteroaryl halogenides andaldehyde-functionalized aryl- and heteroaryl-boronic acids were used along withethoxycarbonylmethylenephosphorane (Scheme 95/1). In another variation, 2-bro-mothiophene, (4-formylphenyl)boronic acid and three different phosphoranes wereused (Scheme 95/2).126 The biaryl/biheteroaryl/arylheteroaryl acrylic or othera,b-unsaturated derivatives are building blocks in organic chemistry.

The reaction of dibenzoyldiazine with alkoxycarbonylmethylene-triphenylphos-phoranes led to new 2-[(benzoylhydrazono)phenylmethyl]but-2-enedioic acid diestersin the way shown in Scheme 96/1. Two units of the phosphorane are involved and atriphenylphosphine oxide and a triphenylphosphine molecule form the by-products.The reaction with two equivalents of ethyl 2-(triphenylphosphoranylidene)propionatefollows a double Wittig olefination followed by isomerization (Scheme 96/2).127

Scheme 93

Scheme 94

Scheme 95

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The interaction of dibenzoyldiazine with (triphenylphosphoranylidene)ketenefurnished a novel cyclic adduct with phosphorane function (Scheme 97).127

Interesting results were published by Boulos et al. relating to the reaction of1,2,3,4-tetrahydro-1-naphthylidene-malonitrile and alkoxycarbonylmethylene–triphenylphosphorane that led eventually to cyclization. The b-oxophosphorane soobtained was then olefinated with a second unit of the phosphorane (Scheme 98).128

The tautomeric form of 1-methyl-2-thiohydantoin added easily to the CQCdouble bond of (triphenylphosphoranylidene)ketene. A subsequent intramolecularcyclization afforded a thioxo-(triphenylphosphoranylidene-)hexahydro-furo-[2,3-d]imidazol-5-one (Scheme 99).129

Scheme 96

Scheme 98

Scheme 97

Scheme 99

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A detailed computational study was performed on the H3PCH + C2H4/C2H2

reactions to explore the chemical reactivity of the ylidic radical [H3PCH]d (Fig. 14)towards p-bonded species.130

Bertrand et al. have introduced a stable acyclic a-aminophosphonium salt. Onlybasic phosphines, such as tris(dimethylamino)phosphine, allow for the synthesis ofstable aminophosphonium salts. The species mentioned gave upon deprotonationwith butyllithium the corresponding C-amino phosphorus ylide (Scheme 100/1).131

In contrast, two cyclic a-amino phosphonium salts were found to be stable despitethe presence of weakly basic triarylphosphine moieties. The key intermediates weredicationic aldiminium salts that on treatment with sodium tert-butylate afforded thecyclic a-aminophosphonium salts under discussion (Scheme 100/2 and 3). In Scheme100/2, the carbenoid intermediate involved is also shown. On treatment withLiHMDS or BuLi, the stable phosphonium salts were converted to the correspond-ing P-ylides (Scheme 100/2 and 3). In the second example, the cyclic ylide wastransformed to a phosphinoarylenamine derivative via a carbenoid intermediate(Scheme 100/3).131

Continuing the above line, C-amino P-ylides are either stable (2) or undergofragmentation (1,3) (Scheme 101).132

The above-mentioned stable C-amino P-ylides were applied as bidentate hetero-ditopic (phosphine-aminocarbene) ligands as they reacted with PdCl2(cod) to result

Fig. 14

Scheme 100

Scheme 101

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in the insertion of the PdCl2 moiety into the P–C bond (Scheme 102/1 and 2). In thesecond case, the ylide was generated in situ.133

A new ylide, tris(4-methylphenyl-)benzoylmethylenephosphorane, has beensynthesized and its properties and reactivity studied.134,135 The metallation of thedonor-functionalized ylide ligand (2-methoxyphenyl)methylene–triphenylphosphor-ane with complexes of yttrium and lutetium gave unprecedented phosphoranylidenecomplexes.136 The reaction of triphenyl–methylenephosphorane with Mo(NAr)-(CHCMe3)[OCMe(CF3)2]2, (Ar = 2,6-iPr2C6H3), produced an anionic alkylidynecomplex (Scheme 103).137

An a,b-di(methoxycarbonyl)-b-iminophosphorane was prepared and used incomplexation with Pd(II) and Pt(II) precursors as shown in Scheme 104/1. Theheteroatoms of the b-imino and oxo-functions took part in the complexation.Interaction with Pd(OAc)2 led to a dimeric cyclic complex. It is noteworthy thatthe phenyl ring was covalently bound to the central palladium atom. The dimer wascleaved by triphenylphosphine reactant (Scheme 104/2).138

Scheme 102

Scheme 103

Scheme 104

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In another study, Urriolabeitia et al. investigated the interaction of an ylide-pyridinium salt [Ph3PQCHC(O)CH2N

+C5H5]Cl� with PdCl2(NCMe)2 in the pre-

sence of bases. A four-membered C,C-chelated complex was found to have beenformed (Scheme 105/1). Interestingly, the arsenic-analogue behaved in a similar way(Scheme 105/2).139

The Ramazani group together with other Iranian research groups have continuedto explore the reaction of triphenylphosphine (TPP), dialkyl acetylenedicarboxylates(DAAD) and a variety of nucleophiles. As was shown earlier, a highlyreactive intermediate is formed from the interaction of TPP and DAAD and by asubsequent protonation. The vinyltriphenylphosphonium salt so obtained thenundergoes a Michael addition with the conjugate base of the nucleophile to givethe corresponding stabilized phosphorus ylide/phosphorane (Scheme 106). Theabove reaction in combination with a variety of nucleophiles provides access tomany new compounds.

In the first new example, 1,1,1-trichloroethanol was used together with TPP andDAAD to afford a b-trichloroethoxy phosphorane that on Wittig reaction withthe trioxo derivative formed from ninhydrin by dehydration afforded dialkyl2-(1,3-dihydro-1,3-dioxo-2H-indane-2-yliden-)3-(2,2,2-trichloroethoxy)succinatesas electron-poor alkenes (Scheme 107).140

The next reactions applied phenol- and naphthol-derivatives as the nucleophile toprovide b-aryloxy intermediates that on microwave irradiation and in the presenceof silica gel, or on heating in the presence of K2HPO4, underwent the loss of TPP toresult in the formation of the corresponding aryloxy-olefins (Scheme 108).141,142

Scheme 105

Scheme 106

Scheme 107

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The use of saccharin and 3,5-dimethylpyrazole as N-nucleophiles led to hetero-cyclic-substituted olefins in a similar protocol (Scheme 109).143,144

1,10-Binaphthyl-2,20-diol was also applied as a reagent, but the correspondingphosphorane was transformed by an intramolecular cyclization connected with theloss of TPP to furnish a dinaphthodioxepine derivative (Scheme 110).145

A newer possibility for the intramolecular cyclization of the phosphorane/ylidewas demonstrated by the final outcome of the reaction of TPP, DAAD andcyclohexane-1,3-diones after microwave irradiation of the intermediate. Electron-poor 2H-chromenes were the products of the reactions (Scheme 111).146 Interest-ingly, hydroquinone acted as a C-nucleophile in reaction with the TPP-DAADadduct to result in the formation of a b-aryl phosphorane that provided another2H-chromene derivative on K2HPO4 catalyzed intramolecular ring closure(Scheme 112).147

Scheme 108

Scheme 109

Scheme 110

Scheme 111

Scheme 112

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Returning to the simplest variation of the three component phosphorane synth-esis, about 20 new heterocyclic products were described using indoles,148 2-indoli-none, 3-acetylindole,149 carbazole150 and its derivatives,151 pyrazole152 and imidazolderivatives,153 their benzoanellated analogues152,153 and benzotriazoles154 as theN-nucleophiles (Scheme 113).

The use of C-nucleophiles has also been extended, or earlier reaction systemsmodified by the use of ditBu acetylenedicarboxylate as the reactive alkyne(Scheme 114).155,156

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