Phosphonium Salts and P-Ylides

Irina L. Odinetsa

DOI: 10.1039/9781849730839-00094

1. Introduction

This chapter covers the most significant developments during 2008 in theabove area the importance of which is increasing in various fields ofchemistry ranging from medicinal chemistry, organic synthesis to materialsciences. As the abstraction of a proton from the corresponding conjugateacid is a classical method for preparing ylides, some publications on syn-thesis and chemistry of P-ylides in some respects are intimately connectedwith that of phosphonium salts.

2. Phosphonium salts

2.1 Preparation

As is well known, quaternization of the corresponding phosphines is the mosttypical and easy to perform procedure for the preparation of phosphoniumsalts. This approach was mostly used for the synthesis of salts used as ionicliquids, precursors in organic synthesis, and in the one-pot version of theWittig reaction, which will be discussed below. A careful study of this re-action (involving Diels-Alder cycloaddition, 1,2,4-triazole and amide bondformation) with a multiple fiber-optic probe for temperature measurements,has revealed that in the case of microwave assistance all effects are purelythermal in nature and are not related to the microwave field.1 Their intrinsicability for quaternization was used for immobilization of di- and tetra-phosphines with a rigid scaffold on silica (affording phosphonium salts, e.g.,(1)), in order to prevent interactions of metal complexes with oxide supportsin metal complex-catalysed reactions.2 In this context, the bis-phosphoniumhexafluorophosphate salt (2) obtained by quaternization of 1,8-di(bromo-methyl)naphthalene, followed by anion exchange, was shown to be a selectivenaked eye chemosensor for fluoride anion.3 Similarly, the synthesis of aseries of novel triphenylphosphine-derived phosphonium salts, mostly havinga carboxylic acid ester or amide residue in one of the substituents at thephosphorus atom, was performed. These salts were tested as antitumoragents, hexadecyl(triphenyl)phosphonium bromide [Ph3P(CH2)15CH3]Brshowing a significant inhibitory rate on human cervix cancer cell lines. Theorigin of this activity, thought to be related to an interaction of the com-pound with DNA, was confirmed by surface-enhanced FT-Raman spec-troscopy in conjunction with electronic absorption spectroscopy.4 However,in some cases quaternization requires the use of a catalyst. Thus, an efficientmethod to synthesize tetraarylphosphonium salts (3) involves the palladium-catalyzed (Pd2(dba)3, 1mol%)5 and the nickel-catalyzed (anhydrous NiBr2,5mol%)6 coupling reaction between triphenylphosphine and functionalizedaryl iodides, bromides or triflates. Both these couplings are compatible with a

aA.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,Moscow, Russia

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�c The Royal Society of Chemistry 2010

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wide range of functionalities such as hydroxyl, hydroxymethyl, carbonyl,amido and phosphino groups. The synthesis and applications of fluorousammonium and phosphonium salts gradually emerging as an alternative totypical phase transfer catalysts especially in the area of fluorous biphasiccatalysis (FBC), have been a subject of a detailed review.7 The new fluoro-philic phosphonium salt (4), synthesized via alkylation of the correspondingtriarylphosphine with dimethylsulfate, has found use as an anion-exchangesite in the first potentiometric fluorous-membrane anion-selective electrode.This salt demonstrated a solubility of at least 14 mM in nonpolar fluoroussolvents and therefore has high potential for use as a catalyst in FBC.8 Theinterest in easy to form phosphine-dihalogen adducts R3PX2 (X¼ I, Br, Cl),which are considered to be close to the structural boundary line between ionicand molecular species, has been rekindled. The adducts [R3PI]I for a series ofdialkyl(carboranyl)phosphines9 and tris- and di(dialkylamino)phosphines10

with molecular iodine were obtained and investigated by NMR spectroscopyand X-ray analysis. The phosphonium species are essentially ionic butdisplay long soft-soft I . . . I interactions in a solid state. The reaction of R3P[R3¼ (Et2N)3, (n-Pr2N)3, (pyrrolidin)3] with (Ph2Se2I2)2 lead to the formationof phenylseleno-phosphonium salts [R3PSePh]I which did not show any soft-soft interactions between the selenium and iodine atoms. Interestingly, thereaction of trialkylphosphine selenides bearing tert-butyl and isopropylgroups at the phosphorus atom with 2 equivalents of arenetellurenyl iodidesyielded novel trialkyl(aryltelluroseleno)phosphonium diiodotellurate(II)salts. Loss of a mesityl group in the reaction with MesTeI gave a uniqueiodotelluroselenophosphonium salt (5) with chelating cation-anion TeII . . .I–TeIV contacts.11

P(Ph)3(Ph)3P

PF6 PF6

E

PR2R2P

PR2

PR2

Et Et

O O O OSiR′R′Si

O O

(1)

E=C, Si, Sn; R = Ph, Et, Cy, tBu

X = Br, I , Tf

FG = Me, Ph, 4-Br-C6H4, 4-HOCH2-C6H4,4-C(O)NHMe-C6H4,C(O)H, OMe, etc

Ph3PFG

X

(2) (3)

P

(CF2)6CF3(CF2)6CF3

(CF2)6CF3

(CF2)6CF3

F3C(F2C)6

F3C(F2C)6 S

O

O

OO

PPri

Se

PriiPr

Te

II

I

Te

II I

I

3

P iPrSe

iPriPr

TeI

(4) (5) (6)

PO

Cl

Ph

Cl

Cl

PCl6

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Naturally, synthetic approaches to phosphonium salts are not limited byquaternization. The reaction of 4-phenyl-2,6-dichlorobenzo[e]-1,2-oxa-phosphorinine 2-oxide with PCl5 resulted in 2,2,6-trichlorobenzo[e]-1,2-oxaphosphorinin-2-onium hexachlorophosphate (6) formed via theintermediate 4-phenyl-2,2,2,6-tetrachlorobenzo[e]-1,2-oxaphosphorinine.12

Unexpectedly, the multistep strategy developed for the synthesis ofo-phosphino-phenols from methoxymethyl-protected phenols led to thenew hydroxymethyl-substituted salt (7). The proposed mechanism of thereaction involves intramolecular nucleophilic attack of the phosphorusatom on the C-atom of the protective group in the deprotection step.13 Anunusual synthesis of phosphonium salt (8) having a binaphthyl skeleton isbased on the activation of two C–H bonds in BINAP [2,20-bis(dip-henylphosphino)-1,10-binaphthyl] and a consequent double C–P bondformation under the action of an equimolar amount of copper(II) tri-fluoromethylsulfonate.14 Mazurkiewich et al. have developed methods forhydrolysis of N-acylglycine derived phosphoranylidene-5(4H)-oxazolonesand the products of their C-alkylation to yield hitherto unknown N-acyl-a-triphenylphosphonio-a-amino acids (9).15 Further decarboxylation ofcompounds (9) either under vacuum at elevated temperature or in thepresence of diisopropylethylamine at 20 1C resulted in a-(N-acylami-no)alkyltriphenylphosphonium salts useful as bifunctional reagents forconstruction of a variety of heterocyclic systems. Under solvent-freeconditions nucleophilic substitution of 2-halopyridines with Ph3P (per-formed without any difference either under classical heating or using themicrowave irradiation) readily led to the corresponding phosphonium saltsonly in the case of 2-bromo- and 2-iodopyridines while for 2-chloro- and2-fluoro-substituted substrates the presence of equimolar amounts of alkalimetal salts (especially lithium ones) was required for successful reaction.No reaction was observed for 3-halopyridines and only oxidation wasobserved on the case of trialkylphosphines such as Bu3P and Cy3P.

16 Incontrast to reactions of inosine nucleosides with BOP or Ph3PdI2 wherein situ formed unstable phosphonium salts readily undergo subsequentreactions to yield O6-(benzotriazol-1-yl)inosine derivatives,17 O6-benzyl-30,50-bis-O-(tert-butyldimethylsilyl)-20-deoxyxanthosine reacts with BOPyielding the stable nucleoside C-2 tris(dimethylamino)phosphonium hex-afluorophosphate salt (10).18 The latter can be effectively used for thesynthesis of N2-modified 2-deoxyguanosine analogues via SNAr displace-ment reactions with a broad range of amines and in a new synthesis of anacrolein adduct with 20-deoxyguanosine.

P

PPhPh

Ph

Ph

(OTf)2

(7)

tBu

OH

PPh2

HO

ClR N

H

O R′ PPh3

OH

O X

(8) (9)

X = BF4 or I; R= Me, tBu, Ph;R′ = H, Me, MeOCH2

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O

RO

RO

N

N

N

N

OPh

O

P

PF6

Me2NNMe2

NMe2

(10)

Ph3P (CH2)n S

O

n = 1, 4

(11)

O

O

O

BN

NPh3P

O

O

Br

(12)

The other approach to new phosphonium salts comprises transformationvia different reactions of functional groups in precursor salts. Thus, 2-(N-disubstituted amino)ethyltriphenylphosphonium bromides, used as im-portant intermediates in the synthesis of different active pharmaceuticalssuch as acrivastine, pyrrobutamine, triprolidine and zimelidine, were pre-pared in quantitative yields and high purity by reacting secondary amineswith 2-methoxyethyltriphenylphosphonium bromide under aqueous con-ditions.19 However, reactions with water-insoluble amines such as prolinemethyl ester, sarcosine methyl ester, diphenylamine, and N-methylanilinewere unsuccessful even in aqueous methanol. Bromination of hydroxyalk-ylphosphonium bromides, obtained in turn through the quaternization oftriphenylphosphine with bromoalcohols, followed by the reaction withpotassium thioacetate resulted in two new o-thioacetylalkylphosphoniumbromides (11).20 These salts function as masked thiolates and under mildreducing conditions readily lose the acetyl groups and form stable phos-phonium-functionalized water-soluble gold nanoparticles of ca. 5–10 nm insize. Note that the role of phosphonium group is crucial, as the relatedo-thioacetylalkylphosphine oxide does not act as stabilizing ligand in goldnanoparticles formation. Phosphonium-modified bifunctional dye (12),mitochondria peroxy yellow MitoPY1, useful as a new targeted fluorescentprobe selectively detecting hydrogen peroxide in mitochondria of livingcells, was synthesized via alkylation of a preformed fused system, having aboronate (peroxide responsive) element and a piperidine heterocycle, withtriphenyl(4-iodobutyl)phosphonium iodide.21 Reinvestigation of the one-pot synthesis of 4-(N,N-dialkylamino)benzyltriphenylphosphonium iod-ides, known as useful intermediates in the synthesis of nonlinear opticalcompounds via the Wittig reaction, has revealed that the initial reaction stepcomprises the condensation of triphenylphosphine and formaldehyde (inthe presence of sodium iodide) to give the hydroxymethylphosphonium saltfollowed by nucleophilic substitution at the para-position of the benzenering of dialkylanilines. Interestingly, while hydroxymethylphosphoniumiodide reacts with dialkylanilines to form the above products, its reactionwith aniline in EtOH results in (N-phenylaminomethyl)triphenylpho-sphonium iodide.22 The deacylation of easily accessible b-(N-acylamino)-vinylphosphonium salts under the action of nucleophiles such as methanol,phenol, benzylmercaptan, aniline or benzylamine, was suggested as a con-venient alternative to Schweizer’s method for the synthesis of b-aminovi-nylphosphonium salts useful for the synthesis of quinoline derivatives,E-allylamines and optically active g-aminoacid derivatives.23 N-heterocyclic

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phosphenium trifluoromethanesulfonates were obtained via the reaction ofthe corresponding cyclic chlorophosphines with Me3SiOTf or ArOTf.Reacting with 1,3-butadienes, these phosphenium salts gave the expectedcycloaddition products, spirocyclic phospholenium salts (13).24 The re-action of polymer-supported diphenylphosphine oxide with triflic anhydridegave a polymer-supported Hendrikson ‘POP’ reagent, this being an equi-librium mixture of polymer-supported triphenylphosphine ditriflate andphosphonium anhydride.25

PN

N E

E

Ar

Ar

E= CH, NMes; Ar=Ph, 4-MeOC6H4, Dipp, Mes

OTf

(13)

B PMesMes

PhMe

Ph

F

(14)

P

B

R R

Mes Mes

R = Cy, tBu

(15)

Phosphonium-borane adducts continue to attract attention. Gabbaiet al.26 have demonstrated that 1-dimesitylboryl-2-phosphoniobenzene ef-fectively binds the fluoride ion, forming the cyclic chelate (14) with abinding constant exceeding that measured for the related 1,4-substitutedanalogue reported previously by the same authors. Yamaguchi et al.suggested the route to so called ladder phosphonium-borate bridgedcompounds, e.g., (15), based on the nucleophilic cascade reaction in bo-rylphosphino-substituted diaryl(hetaryl)ethynylenes and more extendedhomologues.27 Poly(methylene phosphine) n-Bu[MesP-CPh2]nH wastransformed into a regular phosphonium-borane polymer in reaction withBH3.

28

The concept of ‘‘frustrated Lewis pairs’’ (FLP) in which Lewis acid-basecouples formed by sterically crowded phosphines and pentafluoro-phenylboranes are sterically precluded from adduct formation, which wasestablished and significantly developed by Stephan’s group, has been re-viewed by the latter.29 FLP open alternative reaction pathways and thediscovery of their reactivity, even in its infancy, brings a new perspective tothe area of small molecule activation and applications in catalysis.

Of special interest is the rapid and reversible reaction of FLP with H2,splitting it into proton and hydridic components and giving the corres-ponding zwitterionic salts of the type [R3PH

þ ][HBR3� ]. These can serve as

reducing agents for hydrogenations. The FLP derived from a 1,8-dipho-sphino-naphthalene, in combination with hydrogen, catalytically reducessilyl-enol ethers.30 The pair P(C6H2Me3)3/B(C6F5)3 is an efficient catalystfor the direct hydrogenation of imines and protected nitriles, and the re-ductive ring-opening of aziridines with H2 under mild conditions.31 TheFLP (16) with an ethylene linker is employed in the catalytic reduction ofbulky ketimines and enamines at room temperature (2.5 bar H2) andcatalyzes the unique P/B hydrogenation of the frustrated Lewis pair with aP-vinyl moiety (17), which itself is inactive toward H2, to yield the zwit-terionic hydrogenation product (18).32 In the case of FLP derived from

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a-(dimesitylphosphino)ferrocenes (19) and (20) and B(C6F5)3, the reactionwith H2 yields an unfunctionalized ferrocenophane or non-phosphorylatedferrocene and the phosphinoborane adduct [Mes2PH

.B(C6F5)3].33 The un-

quenched Lewis acidity and basicity of the FLP (formed by tBu2RP(R¼ tBu, 2-C6H4(C6H5)) and B(C6F5)3) activate the B–H bond in the re-action with cathechol borane to give a species that can be formally describedas borenium cation (21) or boryl-phosphonium salts (22).34 DFT data gavereasons to support the latter.

Mes2P B(C6F5)2

Mes2P R

B(C6F5)2H

Mes2P R

B(C6F5)2H

H

H

H2, rt

(16) (17) (18)

cat.=(16)

OB

OP(R)tBu2

[HB(C6F5)3]

OB

OP(R)tBu2

[HB(C6F5)3]

(19) (20) (21) (22)

Fe

PMes2

Fe PMes2

2.2 Application in synthesis

Phosphonium salts are known as useful reagents, catalysts and intermedi-ates in general organic synthesis. Using a-naphthyltriphenylphosphoniumperoxodisulfate (23), which can be regenerated and reused, alcohols,hydroquinones, tetrahydropyranyl and trimethylsilyl ethers as well as oxi-mes, semicarbazones and phenylhydrazones were efficiently converted tothe corresponding carbonyl compounds in acetonitrile at reflux.35 Aneffective one-pot transformation of complicated alcohols, including optic-ally active ones, to alkyl iodides and alkyl bromides (70–80% isolatedyields) is based on disproportionation of the phosphonium salt[(Et2N)2P(OR)R 0]X (R0 ¼Me, Et) obtained in the reaction of the alcoholwith bis(diethylamino)chlorophosphine followed by treatment with loweralkyl halides.36 The reaction of the bromophosphonium adduct [Ph3PBr]Brand acetonylphosphonium chloride with 3-hydrazino-1,2,4-triazine yieldeda range of linear and cyclic phosphorus-substituted triazine derivativespossessing considerable molluscicidal activity.37 The combination of thebromine adduct [Ph3PBr]Br (formed in situ) and n-Bu4NNO2 presentsan effective reagent for the preparation of N-nitrosamines and azides fromthe corresponding amines and hydrazines, respectively.38 The reaction of(2-aminobenzyl)triphenylphosphonium bromide with aromatic aldehydesor a,b-unsaturated aldehydes under microwave-assisted conditions consti-tutes a new one-pot synthesis of 2-substituted indoles in high yields (81–97%).39 The reaction proceeds via the formation of a phosphonium-sub-stituted Schiff base that undergoes cyclization under the action of a base.Phosphonium salts such as PyBroP, PyBOP, BroP and BOP, well-known asreagents for the coupling of carboxylic acids with amines, were useful forPd-catalyzed direct arylation of tautomerizable heterocycles with aryl

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boronic acids via C-OH bond activation. The Br-derived reagents(PyBroP and BroP) were more active in this series and the reaction wassluggish in the absence of water. This methodology was successively used inthe cross-coupling of the purine ribonucleosides.40 Phosphonium salts canalso be useful as precursors of organometallic compounds. For example, amulticomponent template reaction using an air-stable bisphosphoniumdimer (24) led initially to the first enantiopure bis-tridentate iron complexesmer-[Fe(PN-N)2]

2þ and then to new tetradentate trans-[Fe(MeCN)2(P-N-N-P)]2þ .41 1,2,3-Triphenylcyclopropenyl-phosphonium bromides (25)(obtained by quaternization of tertiary phosphines with triphenylcyclo-propenyl bromide) react with sodium polyphosphides generated in situaffording a new convenient method for the preparation of sodium 3,4,5-triphenyl-1,2-diphosphacyclopentadienide in good yield.42 Furthermore,air- and moisture-stable and easy to handle phosphonium salts are usefulintermediates in the synthesis of the respective phosphine ligands. Thus,simultaneous intramolecular P-alkylation in 4-(2-bromoethyl)-4-phenyl-1-phospha-4-silacyclohexane afforded bicyclic benzylphosphonium salt(26) which upon the reduction with LiAlH4 afforded a caged nonvolatiletrialkylphosphine ligand with Me3P-like steric and electronic character.43

Bis(phosphine-aminophosphonium) dihydrohalide (27) readily obtainedfrom bis(diphenylphosphino)methane (dppm) (reaction with molecularbromine followed by treatment with ethylenediamine) under deprotona-tion with MeLi led to a tetradentate mixed phosphine-iminophosphoraneligand in high yield.44 Chiral enantiopure 1-alkyl-2,5-diphenylphospho-lanium salts (28, R¼H) obtained through alkylation of phospholane

PPh3

2

S2O82-

(23)

PPh2

Ph2P

OH

HOBr2

(24)

Ph Ph

Ph PR3

Br

PR3=PPh3, PMe2Ph

(25)

Si

P

CH2Ph

Br

(26)

P P PhHN

HN

PP

PhPh

Ph

Ph

Ph

Ph

Ph

Br

Br

(27)

P

Ph

Ph

R

R′OTf

R = H, Me, Et, 2-MeOC6H4R′ = Me, Et, Bu, Oct

(28)

P

Ar

Ar

Bu

Bu

Ar = Ph, 3,5-(CF3)2C6H3

(29)

PN

NH

NH

HN

Me

PhPh

Ph

Ph

(30)

RCOO

with alkyl triflates, are stable precursors of air-sensitive electron-rich cyclictrialkylphosphines.45 Additional alkylation of these phosphines with alkyltriflates yielded chiral 1,1-dialkyl-2,5-diphenylphospholanium salts (28, Rother than H), having potential applications as chiral phase-transfercatalysts.

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Despite the growing interest in asymmetric organocatalysis, only a fewpublications dealing with applications of phosphonium salts as organoca-talysts have been mentioned. Thus C2-symmetric chiral tetraalkylpho-sphonium salts (29) obtained by double alkylation of dibutylphosphine withaxially chiral (S)-2,20-bis(bromomethyl)-3,30-diaryl-1,1 0-binaphthyl, wereused as phase-transfer catalysts in the asymmetric amination of b-ketoesters, providing high yields and ca. 90% enantiomeric excess in the case ofthe salt bearing a 3,5-bis(trifluoromethyl)phenyl group.46 Ooi et al. reportedexcellent enantioselectivity in direct Mannich-type reation of azalactones,applying the salt (30) having a P-spirocyclic tetraaminophosphoniumframework and carboxylate counteranion.47 In the same context, anorganic-inorganic phosphonium catalyst (31) prepared by coupling of3-(triethoxysilyl)propyl(triphenyl)phosphonium bromide and mesoporoussilica displayed high activity for the production of cyclic carbonates fromCO2 and epoxides. This hybrid catalyst (1 mol%) provided more than 97%yield after ten-times reuse.48

R PCy2H

BF4

R = PhCH2, Ph(CH2)3, Bu

O

O

O

Si(CH2)3PRRR′

(31)

Br

HPPH

2 BF4

CyCyCy Cy

(CH2)n

(32) (33)

PPh

Rh

Ph2P CO

I

I

PPh2(12CH3)

CH3

13

I

(34)

The idea of using easy to store air-stable phosphonium salts instead of therespective phosphine ligands and their transformation to these ligands insitu under the action of a base directly over the course of the metal-catalyzedreaction has continued to develop. Plenio et al. described an efficient large-scale synthesis of 9-alkyl-9-fluorenyl phosphonium tetrafluoroborate salts(32) as precursors of the corresponding electron rich and bulky 9-fluo-renylphosphines.49 Synthesis of a dicyclohexyl(2-sulfo-9-(4-sulfophenyl)-propyl-9H-fluoren-9-yl)phosphonium tetrafluoroborate salt provided ahighly water-soluble fluorenylphosphine ligand, whose Pd-complex enabledthe Suzuki coupling of chlorosubstituted N- and S-heterocyclic substrates aswell as aryl chlorides.50 Furthermore, the synthesis of the related bispho-sphonium salts (33) opened an easy access to bidentate phosphines, highlyactive for Buchwald-Hartwig amination and Suzuki and Sonogashiracouplings.51 In discussing the theme of homogeneous metalocomplexcatalysis, it is interesting to note that formation and isolation of phospho-nium salt (34) from rhodium-TRIPHOS complexes under methanol car-bonylation conditions, confirmed that the observed loss of phosphineoccurs via a simple dissociation mechanism of one arm of the ligand fromthe metal centre.52

Completing this section, it should be mentioned that nucleophilic phos-phine organocatalysis generally starts via nucleophilic addition of phos-phines to generate reactive zwitterionic intermediates, i.e. phosphoniumsalts. In rare cases such intermediate salts were either isolated or their

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structures were established on the basis of NMR data but mostly theirformation was proposed from a general chemistry point of view. The reviewof Tang et al. covers the data up to 2007 on phosphine-catalysed synthesisof functionalized cyclic compounds and gives insight on the mechanisticaspects of these reactions.53 Among a great many of more recent publi-cations in the field of phosphine catalysis, in this chapter we will mentionsome of them which discuss the formation of intermediate phosphoniumsalts and their structure. They include the triphenylphosphine-inducedBiginelli synthesis of 3,4-dihydropyrimidin-2-(1H)-ones(thiones)54 and an-nulation of allylic compounds with N-tosylimines affording either 3-pyr-rolines or (E)-dienylimines, depending on the reaction conditions andnature of the starting substrate,55 annulations of alkyl allenoate with avariety of aromatic aldehydes leading to 2-pyrones56 and alkylidenemalo-nonitriles to give highly functionalized cyclopentenones,57 creation of thetetrahydrofuran ring from propargyl alcohol and Michael acceptors,58 andcatalytic cyanomethylation using highly basic tris(2,4,6-trimethoxyphenyl)phosphine.59

2.3 Application as ionic liquids

The unique feature of phosphonium salts as room temperature ionic liquids(ILs) with higher thermal stability than the nitrogen-based ones and beingreasonably cheaper at an industrial scale remains an active research areafocused both on application as efficient promoting reaction media in asynthetic procedures and other purposes. According to the widely cited viewthat ‘‘Ionic liquids are starting to leave academic labs and find their wayinto a wide variety of industrial applications’’, the critical review of Seddonet al. demonstrated the parallel and collaborative exchanges between re-search and industrial developments dealing both with nitrogen and phos-phonium ionic liquids.60 As for academic research, the highly reactive andselective reductive carbonylation of mono- and dinitroarenes to the cor-responding mono- and di-urethanes was carried out in the trihexyl(te-tradecyl)phosphonium IL, ([C14H29(C6H13)3P][PF6]), under mild reactionconditions in the presence of a PdCl2/Phen catalytic system without anycocatalyst.61 The IL having the same trihexyl(tetradecyl)phosphoniumcation and bromide as an anion was found to be the best reaction mediumfor the synthesis of 3-substituted isoindolin-1-one derivatives either via thepalladium catalyzed carbonylation-hydroamination reaction of 1-halo-2-alkynylbenzenes with amines or by the Sonogashira coupling-carbonyla-tion-hydroamination one-pot reaction of dihalides, alkynes, and amines.62

Application of ionic liquids as recyclable reaction media for the directamidation of diphenylphosphoryl acetic acid in the presence of triphenylphosphite as an activator provides an effective access to carba-moylmethylphosphine oxides (CMPO), useful for processing of radioactivewastes. In terms of reaction rate and the product yields trihexyl(te-tradecyl)phosphonium chloride ([P(C6H13)3(C14H29)]Cl) along with 1-butyl-3-methylimidazolium bromide ([bmim]Br) proved to be the best media.63

Phosphonium ILs, most notably trihexyl(tetradecyl)phosphonium decano-ate, were demonstrated as suitable solvents for bases such as Grignard

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reagents, isocyanides, Wittig reagents and N-heterocyclic carbenes (NHCs)and the stability of organometallic species in such ILs was found to beanion dependent.64 NHCs and phosphoranes were generated in phospho-nium ILs and used for benzoin condensation and Kumada-Corriucross-coupling reactions, and PQO olefination, respectively. Very rapidlipase PS-catalysed transesterification of secondary alcohols was observedwhen 2-methoxyethyl(tri-n-butyl)phosphonium bis(trifluoromethylsulfonyl)amide (MEBu3P][NTf2]) was used as a solvent.65 Furthermore, a new ionicliquid, tridecylmethylphosphonium tribromide, prepared by the reaction oftridecylmethylphosphonium bromide with molecular bromine, allows easybromination procedures of unsaturated substrates with high stereo-selectivities.66 The strong influence of a counter anion was mentioned in thepalladium-catalyzed thiocarbonylation of iodoarenes with thiols in trihex-yl(tetradecyl)phosphonium ILs as reactive media, where the hydrophobichexafluorophosphate anion provided the highest yields and allowed facilerecovery and recycling of the catalyst.67 As one of the key features con-cerning the current importance of ionic liquids is their potential to dissolve awide range of organic and inorganic materials, the mutual solubility of anumber of mixtures of commonly used ionic liquids (including phospho-nium ones) with partially fluorinated n-alcohols (C7–C10) and per-fluoroheptane68 and gas solubility (CO2, ethylene, propylene, 1-butene and1,3-butadiene) at low pressure and its dependence on the viscosities of Ils69

were thoroughly investigated. Some unique properties of ILs expand theirareas of application. Trihexyl(tetradecyl)phosphonium bis(trifluoromethyl-sulfonyl)amide ([C14H29(C6H13)3P][NTf2], together with ILs having methyl-imidazolium and methylpyrrolidinium cations and NTf�2 anions, werefound to be suitable solvents for successful electrodeposition of titaniumfrom its halides.70 The same phosphonium IL was also used as a thermo-metric fluid in liquid-in-glass thermometers with ranges of operation tunedto general and speciality applications71 and as a mutually immiscible ILin combination with 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)amide for the separation of aromatic and aliphatic hydro-carbons, exemplified in this study by benzene and hexane, by solventextraction.72 Phosphonium phenylthioacetates bearing butyl and phenylgroups at the phosphorus atom are shown to be efficient co-initiators for thephotoinduced free-radical polymerization of vinyl monomers, providingmore than twice the acceleration compared with the known initiators.73 Therelated phosphonium ionic liquids having a persulfate anion were alsosuggested as a new class of radical initiators giving rise to radical specieswithout gas evolution and therefore to products without bubbles.74

Understanding free radical polymerization and oxidation in ILs requiresknowledge and characterization of reactive free radicals and for thefirst time the formation of a muoniated cyclohexadienyl radical, C6H6Mu(muonium (Mu) has a positive muon with a spin 1/2, a mass of approxi-mately one-ninth of the proton and magnetic moment ca. 3.2 times thatof the proton as its nucleus), was reported in trihexyl(tetradecyl)phos-phonium chloride.75 Therefore, muonium is an effective probe for thestudy of transient radicals in ILs. The new synthetic approach towardsionic liquids (imidazolium, pyrrolidinium and phosphonium) bearing the

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hydroxytris(pentafluorophenyl)borate anion [B(C6F5)3OH]� was elabor-ated. This borate anion protonates the Zr-Me bond in the Cp2ZrMe2complex forming CH4 and provides the first reported example of anionic[Cp2Zr(Me)OB(C6F5)3]

� species.76 Finally, trihexyl(tetradecyl)phospho-nium chloride can be used for gold recovery from HCl solutions both inliquid/liquid extraction systems (using toluene and hexane as solvent) andafter being immobilized in a biopolymer composite matrix.77. This IL wasvery efficient at removing Zn(II) from HCl solutions (optimum found be-tween 2 and 4M HCl), being immobilized on an Amberlite XAD-7 mat-rix)78 and for the recovery of mercury from concentrated HCl solutions(0.1–5M HCl), being immobilized on a composite polymer made of gelatineand alginate.79 Interestingly, the presence of competitor metals did not af-fect sorption capacity except when stable chloro-anionic species wereformed. Zn(II) can be easily desorbed using a number of eluents (includingwater and 0.1M solutions of HNO3, H2SO4, and Na2SO4) while Hg(II) canbe desorbed using 6 M nitric acid solutions. In both cases the sorbent can berecycled for at least six sorption/desorption cycles without significant de-crease in the sorption performance. In all the above cases metal ionswere believed to be removed as anionic chlorocomplexes (AuCl�4 , ZnCl2�4 ,HgCl2�4 ) by an ion exchange mechanism. Further work has been reportedon the directed synthesis of trihexyl(tetradecyl)phosphonium ionic liquidscontaining different paramagnetic anions such as [FeCl4]

� , [CoCl4]2� ,

[MnCl4]2� , [Co(NCS)4]

2� , and [GdCl6]3� .80 The synthesis was accom-

plished via the simple reaction of the neat IL having a chloride counter-anion, with metal halides and the formation of discrete MClz�x anions wasconfirmed by the Raman spectra. These ILs being liquids under ambientconditions display simple paramagnetic behaviour over the temperaturerange 50–350K and remain intact in water over several months with theapplication of an external strong magnetic field. The potential thus exists fortheir use for magnet transport in aqueous systems. Furthermore, for tri-ethyl(methoxymethyl)phosphonium bis(trifluoromethyl-sulfonyl)amide aclear increase of ionic conductivity was demonstrated in the phase changeupon cooling which differed from conventional room temperature ILs. It isnoteworthy that no such phase changes were observed for the related ILhaving the triethyl(ethoxymethyl)phosphonium cation.81 Concerning theproperties of phosphonium salts, those containing perfluorophenyl as asubstituent displayed extensive anion-p-interactions that lead to structurallyflexible relative orientation of the aromatic moiety and the anion.82

2.4 Coordination properties

In continuation of the discussion dealing with anionic metal complexesformed by phosphonium ionic liquids, other phosphonium salts are also ofundoubted interest for target design of metal complexes for different ap-plications. The recent review of Brownie and Baird83 surveyed methods forthe synthesis and coordination chemistry of phosphonium cyclopentadie-nylide (35) (cyclopentadienylidene ylide) ligand systems. In a relatedtopic, the phosphonium bridged ansa-metallocene calcium complex (36),displaying a high degree of flexibility in bending at the Cipso of the

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cyclopentadienyl ring similar to the silyl-bridged counterparts and struc-turally related to organolanthanides Cp02LnX, was obtained by the reactionof the phosphonium salt [Me(tBu)P(C5Me4H)2]I with 1 equiv of KH fol-lowed by Ca[N(SiMe3)2]2. Interestingly, the treatment of the same salt-precursor with 2 equiv of KH produces the coordination polymer {K[Me(t-Bu)P(C5Me4)2]thfN}.84 An unexpected reaction of zirconium tetra-chloride with a new ferrocenyl tetraphosphine resulted in a mixed dipho-sphonium-diphosphine cation associated with two [ZrCl5dTHF]� anions(37). The mechanistic proposal for such selective protonation of peripheralphosphine groups comprises the reaction of the intermediate LZrCl4 com-plex (with coordination of two phosphine groups) with the solvent followedby the ring opening of the latter which also functions as a proton source.85

FePPh2R

(36)

MeP

But Ca-N(SiMe3)2

PPh2R

(35)

P(H)Ph2

But PPh2

PPh2

P(H)Ph2

But 2 ZrCl5.THF

(37)

OHN

PPh3

RePh3P

NOBr

PPh3

ReO4

(38)

4-Methylbenzyl(triphenyl)phosphonium chloride was used as a template forconstruction of metal-organic frameworks in the reaction with zinc acetateand nicotinic acid (NA) affording one-dimensional anionic chains of thezigzag polymer [ZnCl2(NA)]n with phosphonium countercations.86 Simi-larly, tetraphenylphosphonium 1,3-dimethylcyanurate was shown to formanionic complexes of Cr, Mo, W with coordination via the nitrogen atomand tetraphenylphosphonium as the counterion.87 In the case of Pþ -sub-stituted salicylaldimines, an additional triphenyl(aryl)phosphonium groupdoes not influence the complexing properties and the ligands form Zn andCd complexes via an O,N-chelating mode.88 The reaction with bismuthiodide in acetone, pyridine or DMSO with alkyl(triphenyl)phosphoniumiodides form the complexes with a wide range of bismuth-containinganions, e.g., [BiI5]

2� , [BiI5(Py)]2� , [Bi2I9]

3� , [Bi3I12]3� etc., depending on

the molar ratio of reactants.89 The complex phosphonium salts with thetungsten-containing anion [WI3(CO)4]

� , the structures of which were elu-cidated by X-ray analysis, were obtained in the reaction of tris(ferroce-nylmethyl)substituted salts [(FcCH2)3P

þ (CH2OH)I� ] with WI2(CO)3(NCMe)2. The starting compounds could be readily obtained through thestep-by-step substitution of hydroxy group of P(CH2OH)3 by the ferrocenylmoiety under the action of [FcCH2NMe3]I.

90 The first example of a complex(38) containing a triphenylphosphonium-aminophenolate ligand, formed bythe nucleophilic attack of PPh3 on the rhenium coordinated aminopheno-late ligand, has also been reported.91 Since plasma and mitochondrialtransmembrane potentials are negative, cationic molecules with appropriatestructural features can be driven electrophoretically through these mem-branes and accumulate in mitochondria of tumor cells and triarylpho-sphonium cations are especially useful for mitochondrial-targetingPET-radiotracers. For the further development of this area, a comparative

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biological evaluation of new 64Cu-labeled triaryl(alkyl)phosphonium cat-ions bearing 4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane or1,4,7-tris(carboxymethyl)-1,4,7-triazacyclononane complexing cavities wasperformed to elucidate the influence of structural features (the nature of arylgroups, type of the linker, and the chelating moiety) on the radiotracertumor uptake.92 Sulfonatocalix[4]arene and a bis(triphenyl)phosphoniumsalt derived from 1,4-di(chloromethyl)benzene form self-assembled arraysin the presence of lanthanide cations, building up new materials based oncations of both types and featuring a 2D channel system with a scaffoldingrole by phosphonium cations.93 Finally, R4P

þ cations (R¼Me, Et, nPr,nBu) are able to form ‘guest-host’complexes, e.g., those with cucurbit[7]uril,which, with considerable size selectivity, comprise the smaller cationicguests inside its cavity rather than at the carbonyl-linked portals.94 The self-assembling of phosphonium iodides was also mentioned in anionic co-ordination networks with 1,3,5-trifluoro-2,4,6-triiodo-benzene.95

3. P-ylides (phosphoranes)

3.1 Preparation

The three-component reactions of triphenylphosphine (TPP), dialkyl ace-tylenedicarboxylates (DAAD) and various OH-, NH-, SH- and CH-nucleophiles stabilizing by protonation a reactive intermediate generatedfrom the reaction of TPP and DAAD followed by addition of a conjugatedbase to the vinyltriphenylphosphonium salt formed, is a well-establishedroute to highly functionalized stabilized phosphorus ylides (39a) (existingmostly as zwitterionic vinylphosphonium salts (39b)). The above approachto P-ylides has been discussed in detail in a review of Iranian researchers,who made a noticeable contribution to this area.96 Further examples in-volving new nucleophiles (in a practically boundless series) into this reactiongave rise to novel representatives of the above general structure. Amongthem are compounds produced using aromatic aldehyde semicarbazones,97

benzanilides,98 urethanes,99 arylsulfonylhydrazides and aryl hydrazines,100

NH- and SH- heterocyclic compounds such as 5-mercapto- and5-aminotetrazoles,101 as well as CH-acids such as malonodinitrile102

and ethylcyanoacetate,103 and also 4,4,4-trifluoro-1,3-butadiones104 and2-naphthalenethiol.105 These reactions can be performed in typical organicsolvents, under solvent-free conditions and also in water, which is especiallyadvantageous from a ‘green chemistry’ point of view. Under aqueousconditions the corresponding ylides were obtained using both differentNH-heterocyclic systems106 and such CH-acids as diethyl malonateand cyclohexane-1,3-dione.107 Generally, functionalized P-ylides obtainedvia this procedure are stable, but some undergo further transformationsaffording both phosphorus-containing products and some which have lostthe triphenylphosphine group. Thus, the above mentioned reactionswith 4,4,4-trifluoro-1,3-butadiones are accompanied by decarboxylationand loss of a trifluoromethyl group while in the case of 2-naphthalenethiolfurther intramolecular cyclization affords a P-ylide having a fused

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2-oxo-1,2-dihydronaphtho[2,1-b]thiophen-1-yl system. Often such trans-formations comprise the intramolecular Wittig reaction as, e.g., in thesynthesis of dialkyl 5-oxo-1,2-dihydro-5Y-chromeno[4,3-b]pyridine-2,3-dicarboxylates using enaminocarbaldehydes as the third component108 or inthe reaction of PPh3, diaroylacetates and arylidenemalononitriles affording3-aroyl-2,5-diaryl-2,4-cyclopentadiene-1,1-dicarbonitriles.109 In this con-text, it should be noted that the normal intermolecular Wittig reactions arenot so typical for P-ylides of the above structure excluding those with highlyelectron-poor carbonyl groups. The other way of transformation com-prising 1,2-proton shift and elimination of PPh3 resulted in symmetric andasymmetric anthracenyl-2-butenedioates (the reaction with 1,8-dihydrox-yanthraquinone serving as C-nucleophile),110 vinylpyrazoles (the reactionwith 3,5-diphenylpyrazole as N-nucleophile),111 g-spirolactones (the re-action with benzofuran-2,3-diones),112 E-isomers of vinylamides (the re-action with NH-amides),113 substituted alkyl acrylates (the reaction witha-ketoamides),114 while subsequent intramolecular amidation in the re-action with ethyl acetamidocyanoacetate afforded N-acetyl a,b-unsaturatedg-lactams.115 Note that in such cases triphenylphosphine can be used incatalytic amount (3–5mol%).

COOR

Nu

Ph3P

ROOC

Ph3P COOR

Nu

(39b)(39a)

P

Ph

MeOOC

COOMeCOOMe

MeOOC

P

Ph

MeOOC

COOMe

O

(40) (41)

R=Me, Et

Me

Me

Me

Me

OR

O

R1

O

P

R2HN R3

OR

R

X

X= Ph, NMe2, NiPr2R=OMe, OBn, NMe2, NiPr2R1, R2 = Me, EtR3 = NH2, NHPh, OtBu

(42)

PN

R1 R2

AlkOPh

NH2

O

O

(43)

NN

RO

P

O

NAlk2Alk2N

Alk2N

(44)

R1, R2 = Me, Et R = NH2, NHPh, OtBu

Furthermore, the possibilities of this approach to form heterocyclic sys-tems via functionalized P-ylides, generated in the reaction of PPh3 andacetylene derivatives in the presence of an acid, may be further expandedusing a one-pot four-component reaction. The examples include synthesis oftetrasubstituted pyrroles using butane-2,3-dione in combination with am-monium acetate,116 2,5-bis(amino)furans and electron-poor imides frombenzoic acid and alkylisocyanide,117 pyrazol-4-yl-2-butenedioates usingacetylacetone and phenylhydrazine,118 bisfuramides using a,o-alkylenedia-mines and diketene (a pseudo-five component synthesis)119 as well as adiastereoselective route to 2H-indeno[2,1-b]furans using different alcoholsas an acid and subsequent intermolecular Wittig reaction with ninhydrin.120

As mentioned above, the nucleophilic addition of phosphines to DMADresults in unstable reactive species, which undergo further transformations.

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However, reinvestigation of the dimethyl-phosphole reaction with DMADhas revealed the formation of the surprisingly stable ylide (40) in the pres-ence of DMAD excess and an oxo-ylide (41) due to oxidation when thereaction was performed in the presence of AgNO3.

121

The other approach to P-ylides based on the nucleophilic addition ofphosphorus(III) reagents to the terminal carbon atom of 1,2-diaza-1,3-butadienes has continued to find application. The reaction with dialkyl-phosphonites or phosphorus(III) amides under solvent-free conditions (andin the presence of atmospheric moisture) was found to be a convenientapproach to a-phosphanylidene hydrazones (42).122 The linear ylides (42) inTHF solution undergo further intramolecular transformations to give1,2,3l5-diazaphospholes (43) in the case of phenylphosphinite as a startingsubstrate or 5-oxo-4-phosphoranyidene-4,5-dihydro-1H-pyrazoles (44)using tris(dialkylamino)phosphine.

3.2 Reactions

3.2.1 Wittig reactions. Without doubt, the term ‘phosphorus ylides’ isindissolubly linked with the Wittig reaction, this being probably the bestknown example of the use of organophosphorus reactants in the prepar-ation of organic compounds and one of the most popular and powerfulmethods for C–C bond generation, and its organic applications. Con-sequently, the Wittig reaction of different P-ylides was applied in a numberof publications for the target design of olefins without affecting otherfunctional groups. Applications of Wittig (along with the aza-Wittig) re-actions in various cyclizations including both intramolecular ring closuresand multistep procedures which were reported during the last decade, arethe subject of an extensive review that also includes discussion of the re-action mechanism of the transformations.123 Wittig olefinations are keysteps in the development of a practical synthesis of the marine anticanceragents discodermolide and dictyostatin, reviewed by Florence et al.124

The double Wittig reaction of 2-(2-naphthyl)isophthalaldehyde with13CH2QPPh3 was the key step in the synthesis of 13C-labelled analogues ofthe carcinogenic benzo[a]pyrene (the prototype of polycyclic aromatichydrocarbons) and its active metabolites that react with DNA leading tomutations.125 Traditionally, the Wittig reaction was used for the targetsynthesis of biologically active compounds such as cucurbitaxantin A,cycloviolaxantine and capsantin 3,6-epoxide,126 allenoic acid (obtained as aracemate, followed by resolution of diastereomeric salts in W95% ee)127 orcompounds having biological potential such as substituted 1,2,4-tria-zines.128 Two alternative approaches to pyrrolo[2,3-c]quinoline-2,4-dionesof interest from a pharmacological point of view were developed based onboth the intermolecular Wittig olefination of 3-aminoquinoline-2,4(1H,3H)-diones with ethyl (triphenylphosphoranylidene)acetate and the intra-molecular Wittig reaction using the phosphonium salt (45) as ylideprecursor.129 Further successful examples of a Wittig approach performedunder aqueous conditions comprise synthesis of unsaturated potassiumorganotrifluoroborates (with dominating E-olefins) from a variety of

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trifluoroborato-substituted aromatic and heterocyclic aldehydes and a-keto,a-carbalkoxy, a-carbamoyl, and a-cyano stabilized phosphorus ylides(90 1C, 12 h, yields in the range of 60–90%).130 In the same context, Tiwariand Kumar131 highlighted an unusual temperature-dependent effect ofprohydrophobic additives (LiCl and NaCl, ‘salting-out’ agents) and anti-hydrophobic additives (guanidinium chloride GnCl, ‘in-salting’ agent) onthe rates of the ‘in water’ or more precisely ‘on water’ Wittig reaction, whichwas exactly opposite at two temperatures: 298K (25 1C) and 338K (55 1C).Investigation of nucleophilic substitution in a series of mixed phosphonio-iodonic ylides (46) revealed the easy substitution of the iodonium group forhalogenides under the action of Me3SiX (X¼Cl, Br, I), and S-containingnucleophiles such as the thiocyanate anion and thiourea. Consistent com-bination of nucleophilic substitution for halogen and Wittig reaction inone-pot process resulted in a number of substituted a-haloacrylates withpreferential formation of Z-isomer.132 However, another one-pot synthesisof a-haloacrylates, also with a high Z/E ratio, using the combination ofPh3PQCHCOOAlk, halodimethylsulfonium halide and aldehyde or alco-hol (DCM, � 78 1C to r.t.) seems more convenient.133

Typically, in the case of non-stabilized ylides the Wittig reaction is per-formed as the one-pot version with in situ generation of the ylide fromthe corresponding phosphonium salts in the presence of a base. For suchprocedures trialkylgallium bases were suggested and organogalliumintermediates gave (Z)-enynes predominantly in the reaction of propargyl-phosphonium salts with aliphatic and aromatic aldehydes.134 Applyingmore conventional bases, one-pot multiple Wittig reactions of phospho-nium salt (47) and phthalaldehyde provided a simple synthetic route (underdilution conditions) to isomeric mixtures of tetrabenzo [16]- and hex-abenzo[24]annulene systems, separated by analytical HPLC (a separationon preparative scale failed).135 The one-pot version of PQO olefination wasalso used as one of the key steps of multistep approaches to syntheticanalogues of natural biologically active compounds such as antibacterial(–)-kendomycin,136 rac-Glyoceollin and both its enantiomers, known fortheir cardiovascular effects,137 chemopreventive thiomethylsubstituted stil-benes (inhibitors of cytochrome CYP1A1, CYP1A2 and CYP1B1 activ-ities),138 phytosphingosine and dihydrosphingosine,139 synthetic analoguesof all-trans-retinoic acid,140 and the lipid-lowering agent rosuvastatin.141

Hence, these strategies required the directed design of novel phosphoniumsalts as ylide precursors. Similarly, the synthesis of ferrocene-containingpyridine ligands via the Wittig reaction led to the elaboration of a simpleone-pot synthesis of 1,10-ferrocenediyl-bis-(methyltriphenylphosphoniumiodide) (48) based on the reaction of 1,10-ferrocenedimethanol with tri-phenylphosphine in the presence of KI and AcOH.142 Furthemore, theone-pot Wittig reaction was used for the synthesis of 1-(2-pyrrolyl)-2-(2-thienyl)ethylene, useful as a catalyst in electropolymerization of thiophene,143

a series of new luminophores with extended p-conjugated chains based oncombinations of biphenyl, carbazole, dibenzothiophene and phenanthrenefragments with alternating phenyl, vinyl or heterocyclic units,144 new ‘‘D-p-D’’ triphenylamine-based chromophores with furan or thiophene rings forlight-emitting diodes,145 a pentiptycene-derived light-driven molecular

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N

O

NR2

R1

R3

COCH2PPh3

O

Br

R1 = H, Me, PhR2 = Me, Bu, CH2Ph, PhR3 = H, Bu

(45)

Ph3PCOOEt

IPh

BF4

(46)

Ph3P

PhI

O

OEt

BF4PPh3

PPh3

2Cl

(47)

PPh3

PPh3

Fe 2I

(48)

brake with a stilbene backcone146 (S,S)-cis-1,4-diphenyl-2-butene-1,4-d2starting from (S)-(þ )-mandelic acid,147 substituted enynes which undergostereoselective syn-intramolecular bromoetherification148 and the liquidcrystal methyl {4-[4-(nonyloxy)-styryl]}benzoate.149 In a similar fashion,allylphosphonium bromide was used for the synthesis of b-butadienyl- andb,b’dibutadienylphorphyrins from the nickel(II) complex of b-fomylphor-phyrin,150 while its substituted analogue was useful for the design of newchelating stilbazonium-like dyes.151 Olefination of dimethyl 4-formyl-2,6-pyridine carboxylate using a [4-(diphenylamino)benzyl]-(triphenyl)pho-sphonium salt resulted in novel donor-p-acceptor type chromophoresshowing strong two-photon absorption.152

Sequential one-pot Wittig and intramolecular Heck reactions were used ina highly regioselective and effective method for the preparation of oxocinederivatives. The former provided the desired precursors for 8-endo trig-cyclization in high yields via the reaction of PPh3MeI and 2-[(2-bromo-benzyl)oxy]benzaldehyde.153 To overcome the stereochemical limitations ofthis approach such as the predominating formation of aliphatic Z-olefinsfrom non-stabilized ylides, and to facilitate the removal of the phosphineoxide side product, new stabilized ortho-substituted phosphonium salts (49)were suggested.154 This work demonstrated an intramolecular alkoxide effectin stereocontrolled PQO olefination that is consistent with ready cis to transisomerization of oxaphosphetane intermediates. Hodgson and Arif155

described the highly E-stereoselective synthesis of alkenyl bromides andiodides via the Wittig-Schlosser procedure via the reaction of alkylidene(triphenyl)phosphoranes with aldehydes followed by in situ lithiation andsubsequent bromination or iodination of the intermediate b-oxidoylide. Thestereochemical outcome was sensitive to the size of the alkylidene moiety (theproportion of E-isomer increased with the increase in size of the latter).Dynamic NMR studies at low temperatures (148–182K) revealed the dy-namic behaviour of the oxophosphetane species (generated from reactionbetween tri(3-furyl)ethylphosphonium iodine and cyclopropyl aldehyde inthe presence of LiHMDS) and their complexation with lithium ions.156

P

H3C X

X= OH, OMe, CH2OH

I

(49)

ArPPh3Ph3P

Br Br

Ar =X,

X = O, S

(50)

N N

OCH2PPh3Ph3PH2C

Br Br

(51)

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CH2PPh3

Ph3PH2C

ORRO

R =

Cl

Cl

(52)

N

R

Ts

Me

O PPh3

R = Et, Ph

N

R

X

Me

Ph3P O

X = Ts, Ms, CH2PhR = Ph, CH=CHArAr = Ph, 2-furyl, 2-thienyl

(53) (54)

The one-pot Wittig reaction starting from the corresponding phospho-nium salts has also been applied widely for the design of new polymericmaterials. Thus, a range of conjugated fluorescent copolymers was obtainedusing arylidene bisphosphonium salts (50) and (51) and carbazole- oroxadiazole-containing dialdehydes.157,158 A similar approach utilizing1,4-diphenyl- or 1,4-dihexyloxy-2,5-xylylene-bis(phosphonium) bromiderespectively, was used in the synthesis of all aromatic poly(2,5-diphenyl-1,4-phenylenevinylene)159 characterized by its very high photoluminescenceefficiency and its 2,5-dihexyloxy analogue.160 In these cases the 2,5-diphe-nyl-benzaldehyde monomer enhances the formation of the cis-product,which in turn enhance the solubility of the polymers in common organicsolvents. However, the luminescence is improved on increasing of portion ofthe trans-configuration. Introduction of a chiral (� )-trans-myrtanoxylgroup into the molecule of the initial bisphosphonium salt (52) resulted in aconjugated polymer framework bearing chiral side groups.161 It should bementioned that in some cases the relative phosphonates were also used forthe synthesis of such conjugated polymers via the Horner-Wadsworth-Emmons version of the reaction.162

3.2.2 Miscellaneous reactions. Among the reactions of phosphorusylides other than Wittig olefinations, the 1,4-addition of non-stabilizedP-ylides to a-phenylselanyl substituted unsaturated ketones yielding cyclo-propanes and/or dihydrofurans, depending on the substitution pattern,should be mentioned.163 New gas-cascade cyclization reactions of stabilizedphosphorus ylides (53) and (54), bearing a suitably substituted 2-amino-phenyl group, were found to be an efficient route to either 3-substitutedquinolines or benzo[c]carbazole and its heterocyclic-fused analogues, de-pending on the above substituents.164 Kinetics, mechanism and products ofthermal or gas-phase pyrolysis of P-ylides of different structure and theirphosphonium precursors were the object of study in a few papers.165

3.3 Coordination properties

Although phosphorus ylides are indissolubly linked with the Wittig reactionand its organic applications, the presence of the ylidic charge, even de-localized, allows using them as ligands for a variety of metals and this areahas attracted the attention of different research groups. The recent findingsin ylide coordination chemistry (up to 2008) are covered in a review byUrriolabeitia, which is focused on the bonding properties of ylides(C- versus X-coordination), their redox properties and their behaviour in

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C–C coupling reactions, as well as their participation in bond activationprocesses (C–H and C–P activation).166 The reactions of a-keto-stabilizedP-ylides with mercury(II) halides were found to result in binuclear trans-likecomplexes (55), involving C-coordination of the above ligands, that are4-12 kcal/mol more stable than the alternative cis-like isomers according toab initio calculations.167 The bridge-splitting reactions of some binuclearcomplexes by DMSO yields mononuclear complexes, also with C-co-ordination of ylides and O-coordination of DMSO. X-ray data has revealedthat the stabilized resonance structures of these ylides are destroyed bycomplex formation in complexes of both types. In the case of PdCl2,complexation of a-keto ylides (1:1 molar ratio) results also in binuclearcomplexes (56) with C-coordination of a ligand but this reaction is ac-companied by orthopalladation at the phenyl ring of the phosphorusunit.168

HgX

XHg

X

X

CH(O)RHAr3P

R(O)C HPAr3

R = 4-NO2-C6H4, 4-Cl-C6H4Ar = Ph, 4-CH3-C6H4X = Cl, Br, I

(55)

PPdAr

ArR

O

Cl

ClPd

PAr

ArR

O

(56)

R = Me, Ph, OCH2PhAr = Ph, 4-CH3-C6H4

P

ONi

R1 R2

Ph

L

R1, R2 = Me, Bu, tBu, PhL = PPh3, PyX = OH, OMe, NHMe, NHPh

X

(57) (58)

[M]

P

P

PhPh

PhPh

MeO(O)C

[M] = PtCl2, PdCl2,Pt(CH3)Cl, Pt(CH3)PPh3

+

Similarly, a-keto-phosphorus ylides undergo C–H activation processes inthe reaction with Pd(OAc)2 (1:1 molar ratio) followed by treatment of theintermediate acetate derivatives with LiCl.169 The binuclear C,C-orthome-talated Pd-complexes readily undergo splitting on treatment with tertiaryphosphines, Tl(acac), PPh3, and AgClO4/dppe, giving the correspondingortho-metallated mononuclear complexes as mixtures of stereoisomers. Thereactions of a-keto-stabilized P-ylides with Ni(COD)2 in the presence ofother donor ligands such as PPh3 or pyridine resulted in cyclic complexes(57) with P,O-coordination of ylide. The nickel complexes (57) generatedin situ are active catalysts for ethylene oligomerization with a TON of up to12700 C2H4 (mol Ni).170 Despite the possibility of coordinating to metalsvia several bonding modes, the di-a-keto-phosphorus ylide [Ph3PC(COCH3)(COC6H5)] acts exclusively as an O,O-bidentate chelate ligand in the re-action with uranyl nitrate, forming a 1:1 complex.171 The phosphorus ylidesPh2P(CH2)n(Ph)2PQCHCOOMe (n¼ 1, 2), modified by phosphinoalkylenegroups, afford exclusively cyclic complexes (58) in which the ylide is chelatedto the metal via the phosphine group and the ylidic carbon atom in thereaction with Pd(II)- and Pt(II)- cyclooctadienyl species [M(COD)Cl2] or[Pt(CH3)COD)Cl]. The related ketenylidene Ph2P(CH2)n(Ph)2PQCQCQOreacts with Pt(II) precursors forming (59) as a result of breaking the CQCbond of the –CQCQO group.172 A series of phosphorus bis(ylide) rare-earth metal complexes, stabilized by the Cp* ligand as a strong donor, wereobtained and structurally characterized, revealing a chelating coordina-tion mode and a bridging mode of the [Ph2P(CH2)2]-ligand.

173 A series ofosmacyclic complexes with ylidic backbone (an osmabenzene, a cyclic

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osmium Z2-allene complex, an osmafuran and an a,b-unsaturated ketonecomplex) was obtained by the reaction of osmium phosphine complexOsCl3(PPh3)3 with HCRCCH(OH)CHQCH2 followed by either ligandexchange or thermal decomposition of the initial osmacycle (60).174 Noveliron-dicarbollide complexes (61) were unexpectedly formed in the reactionof a ferracarborane cluster with triethylphosphine in the presence ofMe3NO. In these complexes an ylidic group bonded to a cage carbonatom is also directly s-bonded at the b-position to the adjacent ironvertex, forming a five-membered cycle.175 A combined ab initio and densityfunctional study was performed for binuclear isovalent and mixed-valent gold phosphorus bis-ylide complexes [AuI2(CH2PH2CH2)2],[AuII2Hal2(CH2PH2CH2)2], [AuIII2Hal4(CH2PH2CH2)2], and [AuIAuIIIX2

(CH2PH2CH2)2] with different gold oxidation states in order to understandthe similarities and differences in their chemical properties.176 The analysishas revealed that two- and four-electron oxidation mainly occurs at the goldcenters, resulting in different coordination geometries and spectroscopicproperties of the complexes, and that the triplet-exited states are minimumpoints on the potential energy surface in the case of the first three complexes

COPt

P

PP

P

PhPh

Ph Ph

PhPh

PhPh

(60)(59)

Os

PPh3

PPh3

PPh3ClCl OH

H

H

OR

PEt3

PEt3

= B, = C, = Fe

(61)R = H, Me

N NMes Mes

RuPR3 A

Cl

Cl

(62)

N

PPh3

Cl

Ru ClPPh3

(63)

N

PPh3

Ru ClCl

EtO

OO

OEt

(64)

(HalQCl). An investigation of the rearrangement of the first generationGrubbs complexes has been carried out to get insight into the involvementof catalytically active 14-electron ruthenium intermediates, demonstratingthat carbon monoxide or an aryl isocyanide promote a benzylidene carbenetransfer from ruthenium to tricyclohexylphosphine affording Ru(II) com-plexes RuL3Cl2(PCy3) (LQCO or ArNC) and phosphonium ylideCy3PQCHPh.177 The reaction of Ru(II) phosphonium alkylidenes (62) with1 equiv of ethylene at � 78 1C in the presence of 2–3 equiv of a trappingolefin substrate, also proceed with elimination of vinylphosphosphoniumsalts and yield intermediates relevant to olefin methathesis catalyticcycles.178 In the case of diphenylphosphinomethylpyridine Ru(II) complex(63) the reaction with an excess of diazoacetate at � 60 1C leads to elim-ination of triphenylphosphine ligand from the ruthenium atom as the

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phosphonium ylide Ph3PQCHCOOEt with concomitant formation of anovel diethyl maleate complex (64).179

The phosphorus ylide moiety as a carbene-stabilizing substituent hascontinued to be employed in the modification of a scaffold of carbenespecies of considerable interest as ligands for catalytically active transitionmetal complexes. The cyclic (amino)(ylide)-carbene (AYC) ligands wereshown to possess an excellent electron-donating ability, advantageous forgood catalytic activity. Thus, deprotonation of a suitable phosphonium saltprecursor provided a convenient approach to the (unstable at room tem-perature) AYC ligand (65) which readily forms the corresponding Rh andPd complexes via the carbene center.180 In a generalization of this ‘depro-tonation’ approach, backbones other than indoles were also shown to beable to form phosphorus ylide–stabilized carbenes, e.g., (66), obtained froma vinamidinium tetrafluoroborate salt, and sufficiently stable for directobservation in solutions.181 A further extension of the approach to pyrrole-based phosphonium salts surprisingly resulted in the first stable (both in thesolid state and in solution) lithium adduct of the cyclic (amino)[bis(ylide)]-carbene (67).182 The latter acts as a 1,4-bidentate ligand in transition-metal

N

PPh3

CH3

(65)

N N

PPh3

RR=Me, Ph

N

Dipp

Ph2P CH2

Li(thf)x PR

R

(66) (67) (68)

X Y

A =C

N CH3N B =

CH2

PPh2

X= Y = AX = Y = BX = A, Y = B

Ph2P PPh2

(69) (70)

complexes forming coordinate bonds via the carbene center and the exo-cyclic ylide carbon atom. In a further search for more carbene donor lig-ands, a room temperature stable cyclic vinylidenephosphorane (68), relatedto cyclic carbodiphosphoranes and cyclic push-pull carbodicarbenes (bentallenes), has been synthesized from the corresponding phosphonium salt.183

According to the X-ray crystallographic data and ab initio calculations,phosphorane (68) features a very long P-C bond (1.786 A), indicating itspredominant existence in the depicted resonance form and leaving thecarbon lone pair fully available for coordination with metals. The formationof rhodium complexes via the carbene center supported this supposition.The coordination properties of the diaminocarbene and phosphoniumylide ligand types towards rhodium were systematically investigated usinga set of C,C-chelating ligands (69) containing two moieties of eitherkind, the bis(phosphonium ylide) ligand being found to be the strongerdonor.184 Notably, on the way to bis-ylide ligands (X¼Y¼B), the cyclic

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ylidophosphorane (70) and the bis-ylide Ph3PQCQP(Me)Ph2 were detectedby NMR methods.

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