[Organophosphorus Chemistry] Organophosphorus Chemistry Volume 27 || Ylides and related compounds

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  • 6 Ylides and Related Compounds

    BY 6. J. WALKER

    1 Introduction

    An edition of Chem. Rev. (1994, 94) entirely dedicated to phosphorus chemistry has been published and contains two articles relating to ylide chemistry (see refs. 1 and 45).

    Fewer new studies of the Wittig mechanism have appeared this year than previously. However, stereochemical control in phosphorus- based olefina tions continues to be a topic of substantial interest. In addition to further studies of the control of olefin stereochemistry there have been a number of reports of enantioselective olefinations and the use of ylides and phosphonate carbanions to achieve high levels of asymmetric induction.

    The use of complex phosphonates, phosphine oxides and phosphonium salts, in one case with a molecular mass of approximately 800, in the synthesis of a wide range of large molecules continues and increases.

    2 Methylenephosphoranes

    2.1 Preparation and Structure. - The nature of bonding in phosphonium ylides, phosphines and phosphine oxides has been critically reviewed in an article that perhaps finally ends speculation that d-orbitals are involved in such bonding by showing that involvement is both highly unlikely and unnecessary.

    a-Oxy ylides, e.g. (l), are suggested to be formed in the electrolysis of keto acids with triphenylphosphine (Scheme 1). Compounds (1) appear to act as novel acyl anion equivalents since cyclic a-hydroxy ketones are the products isolated.2 Dichloro- (2)3 and difluor0-(3)~ methylene ylides have been generated electro- chemically from the corresponding phosphonium salts and undergo Wittig reactions with aldehydes to give gem-dihalogenoalkenes in good yields. Ylide intermediates, e.g. (9, have been proposed for the phosphine-catalysed, unusual y-addition of alcohols to ester-activated a l k y n e ~ . ~ The reaction is illustrated here by an intramolecular example (4) but also occurs intermolecularly.

    Tellurophosphoranes (6) have been prepared and shown to react with alde- hydes in siru to give vinylic tellurides with (Z)-stereoselectively.6 The first example of an a-selenoarsonium ylide (7) has been reported and shown to undergo (2)- stereoselective Wittig reactions. a-Mercurio-substituted phosphonium ylides (8) have been synthesised by the reaction of alkylidenephosphonium ylides with bis[di(trismethylsilyl)amino] mercury.* Compounds (8) undergo normal Wittig reactions to give vinyl mercury compounds.

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  • 6: YIides and Related Cotnpounh 265

    Reagents: i , BusP, MeS03H, CH2C12, PhCH2NEt3CI-; ii, 28; iii, H 2 0

    Scheme 1

    (M@J)3P = CX2 (2) X=CI (3) X = F

    R

    TePh PhSP

    Ph3P=C: Ph&= CHSeR Ph

    (6) (7) (5)

    A number of mixed phosphine-phosphonium ylides have been prepared. The ylide (10) is the ultimate product from the iodine-induced coupling reaction of the bisphosphine carbanion (9)9 and a range of ylidic-dihalogenophosphines (1 1) have been prepared by the reaction of phosphonium ylides with phosphorus trihalides.'* The structures of (1 1) have been studied by NMR spectroscopy and X-ray crystallography. A number of, in some cases novel, heterocyclic phos- phorus ylides, e.g. (12) and (13), have been synthesised and their structures determined. * Phosphine carbanions, including tetralithiated dibenzyl(methy1)-

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  • 266 Organophosphorus Chemistry

    phosphine (14), have been generated and their structures determined.'* The 1 ,215-azaphosphete (1 6) has been synthesised in excellent yield from the azirine (15) and fully characterised by 13C, 31P, and 29Si NMR spectroscopy.13

    4 Li+.2 tmeda (14)

    The thermally stable acylbismuthonium ylides (1 7) have been prepared by base-treatment of the corresponding salt. l4 The formation of a,P-epoxyketones in moderate to good yields on reaction with aldehydes is typical of the reactivity of (1 7). The presumed intermediate (1 8) in boron-Wittig reactions has been synthe- sised and its structure and decomposition investigated.Is

    Ph3Bi =CHCOR' + R2CH0 - Ph3Bi+ R2hH H C02R'

    (17)

    MeS.. - Me X K+OI 8-cs Ar -Pg Ar =

    M 9 4 & C F 3 NAr

    CF3 (19) X = NH (20) x=o (18)

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  • 6: Ylides and Related Compoundy 267

    New chemistry of a number of low co-ordinate P' ylide like structures has been reported. Imido-( 19) and oxo(imido)-(20) phosphoranes have been prepared and their structures confirmed by X-ray crystallography.16 Monomeric tri(imino)- metaphosphates, e.g. (21), have been generated and their X-ray structures reported. l 7 The 2,4-di-rert-butyl-6-( 1 -piperidino)phenyl group has been used to stabilise the diselenoxophosphorane structure and X-ray analysis shows that this is achieved with nitrogen co-ordination to phosphorus to form a four-membered ring (22) with a highly distorted P-C-C bond angle.'* Yet another theoretical study of the simplest phosphonium ylide (23) and its Wittig reaction with formaldehyde has appeared.I9 However, this latest report also includes a study of ylide formation from the corresponding phosphonium salt and a comparative study of the formation, structure and reactivity of ylides derived from the other elements in group 15. Explanations of the differing stabilities,. structures and modes of reaction of the different ylides are provided by the results. The energy and structure of bis(methy1ene)phosphorane (24) have been calculated for both singlet and triplet states using an ab initio MP4/6-3 lG**//MP2/6-3 1G** met hod. 2o

    Reports concerning the structure of stabilised ylides include a detailed 'H, I3C, and 31P NMR study of the conformation, and the factors influencing the conformation, of phosphonium ylides (25), (26), (27), and (28) derived from succinic acid2* and a chemical, spectroscopic and X-ray structural study of the

    NAr

    NAr A ~ N H P ~

    'I'

    nBuLi

    THF

    LT.4 THF

    + (22)

    CH~CO~R' ,CHMeC02Et P h,P = C: Ph,P=C,

    C02R2 CO2Et

    (26) R' = H, R2 = Et (27) R' = R2 = Et

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  • 268 Organophosphorus Chemistry

    mesomeric phosphonium ylides (29).22 The relative contributions from ylide and betaine structures in (29) depend on the nature of X.

    Silylmethylene aminophosphonium ylides (30) have been prepared by silylation of the corresponding methylene aminophosphonium ylide and their structures investigated by 'H, 13C, I5N, "0, 29Si, and "P NMR.23 The neutral ylide complex (31) has been isolated and its structure determined by X-ray crystal- 10graphy.~~ Methylenetriphenylphosphorane reacts with 2,4,6-trimethylphenol to give the expected phosphonium aryloxide which X-ray crystallography shows to exist as a dimer, aggregated solely through CH-O hydrogen bonding by alkyl and aryl hydrogens of the phosphonium cation, in the solid state.2s

    + 2(R2N)3P=CH2 + C12Si(OR')2 - 2(R2N)$=CHS(OR1)2 + (R2N13PCH3 CI-

    I CI

    (30)

    2.2 Reactions of Methylenephosphoranes 2.2.2 Aldehydes. - The stereochemistry and mechanism of the Wittig reaction has been the subject of a major review26 although few significant new mechanistic studies have been reported this year. It has been reported that Wittig reactions of unstabilised ylides give equilibrium mixtures of ( E ) and (2) disubstituted alkenes when carried out under daylight lamp irradiati~n.~' According to a recent report "remarkable rate enhancements and dramatic reductions of reaction times" are achieved by using microwave irradiation in Wittig reactions of stable ylides with aldehydes2* Kinetic resolutions of racemic aldehydes, e.g. (33), by reaction with chiral phosphonates (32) have been in~es t iga ted .~~ Good levels of diastereoselec- tivity and (2)-selectivity were achieved but generally not in the same reactions. An enantioselective synthesis of 4(R)-4-hydroxycyclopntanones (35) in one-pot by the Wittig reaction of chiral glyoxals (34) followed by Pd-induced de- carboxylation (Scheme 2) has been reported.30

    OCHO (32) R = Me, Et, P4, CFaCHz (33)

    f.3-Enehydrazino phosphonium salts (36), and B-hydrazono phosphine oxides (37) and phosphonates (38), have been prepared and the derived ylides and carbanions used in olefination reactions for the synthesis of a,B-unsaturated

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  • 6: YIides and Related Compoundr

    0

    269

    Reagents: i, Ph3P=CHCOCH2C02Allyl; ii, Pd(Ph3P)*, morpholine, THF Scheme 2

    (37) R = Ph (38) R =OEt

    hydrazones, pyrazolines and pyrazole derivative^.^' Enolates of acyl phosphor- anylidene carboxylate esters (39) react with aldehydes to form adducts (40) which, on oxidation with ozone, followed by acid-catalysed dehydration, give 3- hydroxyfuran-2-carboxylates (41) (Scheme 3).32 (E,Z)-Dienes have been synthe- sised with stereoselectivities of between 20:l and 2:l over the (E,E)-isomers by treatment of butadienyltriphenylphosphonium bromide with an ionic nucleophile to generate the appropriate ylide, followed by addition of an aldehyde (Scheme 4).33 The effect of the nature of the nucleophile and the aldehyde on the stereochemistry of the diene formed was also investigated. Further evidence34 that epi-phosphonium salts (43) are intermediates in the stereoselective synthesis of alkenes by treatment of 2-hydroxyalkylphosphines with PC13 and triethy- lamine35 is provided by the independent generation of (43) from the a- bromobenzylphosphonium salt (42).

    Arsonium ylides continue to be studied and in some cases provide useful

    1- 0

    i i i , iv I

    Reagents: i , LDA; ii, R3CHO; iii, 03; iv, H30+ Scheme 3

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  • 270 Organophosphorus Chemistry

    + I - -/

    Ph,P Br-

    0 - 0-

    I ii m R R' 0

    Reagents: i, 2Li+, - 78 "C - 0 "C, 1 h; ii, RCHO, THF, 10 rnin / / R

    Scheme 4

    Ph x Ph Br- PhCH= CHPh - 50-80% (2 )

    alternatives to the phosphonium analogues. The previously unknown 1 -alkyl analogues (45) of l-(alkoxycarbony1)-methylenetriphenylarsonium salts have been prepared by quarternisation of triphenylarsine with the appropriate triflate ester (44). Wittig reactions of (45) with aldehydes using alumina-supported potassium fluoride as base provide good yields of the corresponding Wittig product, exclusively as the ( E ) - i ~ o m e r . ~ ~ Attempts to carry out similar reactions

    F? + 8 Ph3As + CF3S-O-CHR2C02R1 - Ph3AsCHR2C02R1 X -

    (44) R3COR4 (45' R3CH0

    R'0& R&T C02R' H C02R' (46)

    with ketones gave, as major products, alkenes (46) derived from coupling of the arsorane ligand. The yield of (46) is optimised in reactions carried out in the absence of the carbonyl component. Many ar-selenophosphonium ylides and a- selenophosphonate carbanions are not sufficiently reactive to undergo the Wittig reaction. However, acyl- (47)37 and alko~ycarbonyl-(48)~~ (phenylseleny1)methy- lenetriphenylarsonium ylides have now been prepared by phenyiselenation of the corresponding stabilised arsonium ylide and, as expected in view of the greater reactivity of arsonium ylides, shown to react with aldehydes to give moderate

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  • 6: Yiides and Related Compounds 27 1

    yields of predominantly (2)-a-phenylselenyl a,p-unsaturated ketones and esters (49) (Scheme 5).

    + 2 Ph3As=CHCOR + PhSel - Ph3AsCH2CORI - + Ph,As=C-COR

    (47) R=Alkyl SePh (48) R = 0 Alkyl 1 ii

    H COR

    RHSePh

    (49) R = alkyl. 0 alkyl

    Reagents: i, Et20, MeOH, R.T.; i i , R'CHO, CHCl3, 55-65 "C

    Scheme 5

    2.2.2 Ketones. - The stereochemistry of the Wittig reaction of substituted-phenyl 3-pyridyl ketones (50) with unstabilised a-carboxyalkylphosphonium ylides derived from salts ( 5 1) has been inve~t iga ted .~~ Both 3- and 4-sulfonamidophenyl 3-pyridyl ketones provide high levels of (@-selectivity, whereas other substituents generally lead to poor levels of (2)-selectivity. An explanation, involving H- bonding or salt bridging in the oxaphosphetane intermediates, is presented and supported by both experimental and theoretical results.

    x-$iJ Br- NaHMDS.

    THF

    (51)

    An intensive study of the use of the Wittig reaction of methylenetriphenyl- phosphorane to introduce a methylene group in the synthesis of a number of conjugated caradienes, e.g. (52), has been reported. All the known problems with this type of reaction, low yields, enolate formation, isomerisation, side reactions of phosphonium salt to give benzene etc., are encountered. Phosphoranes (53) derived from a,P-unsaturated esters react with 1,2-diacy- lethenes at room temperature to give poor to excellent yields of double bond isomers, e.g. (54), of cyclopentadienes and none of the alternative cyclohexadiene product^.^' The reaction of 2-allyl-1,2-dihydroindol-3-ones (55) with stabilised phosphonium ylides under vigorous conditions provides a synthesis of 3-substi- tuted indole acetates (56) in moderate to good yields via a tandem Wittig

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  • 272 Organophosphorus Chemistry

    (52)

    +RR R Ph,P=CHACHC02Et + $COCH=CHCOR2 -

    R2COCH2 C02Et

    (53) (54) X

    Ac Ac

    (55) (56)

    reaction/Cope rea~~angernent.~~ Enantiopure a-aminoketones (57) are converted enantiospecifically, presumably ultimately by an intramolecular Wittig reaction, into 3-pyrrolines (58) by treatment with sodium hydride followed by vinylphos- phonium salts (Scheme 6).43 The aziridyl ketones (59) have been converted, in one-pot and diastereospecifically, into the unsaturated cis-piperidines (60) by reaction with methylenetriphenylphosphorane via a combination of Wittig olefination and aza-Wittig rearrangement.44

    (57) X = SOZPh, COPh (58)

    SPh Reagents: i, NaH; ii, ++

    PPh3

    BuO~C 0 +

    R 2 Ph3PCH3 Er-

    BULL OM. R.T.

    H

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  • 6: Ylides and Related Compoun& 273

    2.2.3 Miscellaneous Reactions. - The synthetic applications of metallated phosphonium ylides and metallated phosphine imines have been reviewed.45 Cyanomethylenetributylphosphorane (6 1) has been investigated as an alterna-

    tive to DEAD/triphenylphosphine in the Mitsunobu reaction.46 A similar inter- medi...

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