[Organophosphorus Chemistry] Organophosphorus Chemistry Volume 16 || Ylides and related compounds

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  • YIides and Related Compounds

    BY B. J. WALKER

    1 Introduction During the past few years it has become increasingly obvious that the different phosphorus-based olefin syntheses are largely complementary. These reactions now offer excellent methods for highly stereospecific synthesis of most olefins, with the exception of some conjugated E alkenes and very highly substituted alkenes.

    2 Methylenephosphoranes Recent advances in the chemistry of ylides in general' and of P-heterosubstituted phosphorus ylides2 in particular have been reviewed.

    2.1 Preparation and Structure. - The electronic structure of the simplest ylide ( H 3 6 - E H 2 ) has been calculated using ab initio molecular orbital theory at the GVB+POL-CI level .3 Both n.m.r. spectroscopic data and X-ray analysis show that the carbanionic centre in cyc lobu ty l idene t r ipheny lphosphorane is not quite planar and that the ylide P-C bond has a considerable degree of double- bond character. 4

    Continuing their investigation of carbodiphosphoranes, Schmidbaur's group has reported attempts to prepare those based on the 1 , 3-diphosphanindane skeleton. The semi-ylides ( 2 1 are formed by base-treatment of the cyclic bisphosphonium salt (11, but are readily hydrolysed to (3). Diylides of the type ( 4 ) , but not carbodiphosphoranes, can be prepared from (2) by transylidation. Carbodiphosphoranes of types ( 5 ) and ( 6 ) can be prepared from their semi-ylide salts by treatment with butyl-lithium. The cyclic bis(ylide)-bis(salt) ( 8 ) is formed by spontaneous dimerization of solutions of the vinylphosphonium salt ( 7 ) through Michaeladdition of phosphine to the activated alkene.6 been confirmed by X-ray analysis. A series of P-amino-substituted carbodiphosphoranes ( 9 ) have been prepared and shown to have fairly predictable properties. However, an &-ray study of hexakis- (dimethy1amino)carbodiphosphorane ( 9 , R =R =R =NMe 1 indicates a previously unobserved linear arrangement at ylidic carbon. '

    The structure of ( 8 ) has

    1 2 3 2

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  • 8: Ylides and Related Compounds 283

    # \R2 ( 5 ) R = Ph, R2 =Me

    (6 ) R 1 = R 2 = Ph

    2 ;;H2 1- - +,c\

    Ph2P PPh, Me

    P h ,PMe

    1 2 3 R ,P=C = P(NMe, )R R

    ( 9 )

    Ph Me

    (4)

    2 121 1- Surprisingly, the lithiated phosphine (10) undergoes reaction with electrophiles at phosphorus, rather than carbon, and thus provides a route to a variety of P-substituted phosphonium ylides including

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  • 284 Organophosphorus Chemistry

    1 R' R H a l Ph2P- CEiMe,), - Ph2P=C(SiMe3I2 (10) ( 1 1 )

    Li

    '12 XHal, I X

    Ph,P' 'PPh,

    (Me3Si ),C C(SiMe,), II It

    Ph,P-PPh , I I I

    (Me,Si ),C CH(SiMe,),

    (13)

    (15)

    PPh, I I

    2+

    2 B r -

    (16) n = 3 , 4

    the unknown !-,acylphosphoranes (11, R1=R2CO) . A variety of di- ylides ( 1 2 ) have been synthesized by an analogous method and in one case shown to be in the alternative prototropic form ( 1 3 ) in the solid state. In an investigation of tris(phosphoniurn1 methanide salts and ylides, Schmidbaur has shown by 31P n.m.r. spectroscopy that the salt (151, formed by phosphinylation of hexaphenylcarbodiphosphorane ( 1 4 ) , has non-equivalent triphenyl- phosphorus groups due to restricted rotation of the diphenyl- phosphinyl group .9 salts (16).

    Other examples synthesized include the cyclic

    Vinylphosphonium salts have been used extensively in the generation of ylides. The potentially useful synthetic inter- mediates (17) have been synthesized by the reaction of phosphonium ylides with tetrarnethylformamidinium chloride . lo Acid hydrolysis,

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  • 8: Ylides and Related Compounds 285

    CI -

    / ( l l ) Hydrolysis R 0-

    NMe2 /

    Ph3P +7==lH (1 9)

    0

    Ph3P +X H

    Me (20)

    +

    KSR Reagents: i, NaH, THF; i i , Ph3P

    Scheme 1

    followed by treatment with base, converts ( 1 7 ) into the formyl ylides (181, and (17, R = H ) is deprotonated with sodium amide to give the allenylidene ylide ( 1 9 ) . The reaction of l-mercaptovinyl- phosphonium salts with the anions of ketones has been used to prepare a variety, e.g. (201 , of highly functionalized cyclo- pentenes via intramolecular Wittig reactions (Scheme 1) .I1 analogous phosphonium salt ( 2 1 ) has been generated in situ and shown to react with various enolates to give ylides, e.g. ( 2 2 ) . 1 2 Depending on the conditions used, these ylides react to give a variety of synthetically useful products; however, attempts to use the reaction to generate the fused BC ring fragment ( 2 3 ) of bruceantin gave only low yields . I 3 1-Phosphorylenamines ( 2 5 ) , prepared from the phosphonate ( 2 4 1 , give on base-treatment the delocalized anions ( 2 6 ) . I 4 In almost all cases ( 2 6 ) react regio-

    -- The

    -

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  • 286 Organophosphorus Chemistry

    -bcmMI *PPh3 COOMe SPh

    ( 2 1 ) ( 2 2 )

    COOMe

    Me0

    R ' ! R4

    i , v i - lv 1.

    R RY-:: 3 R RYooH 4 3 4 Reagents: i , 6u"Li; ii, ( M e S I 2 ; i i i , A ; iv , A X ; v , H 2 0 ; v i , R X

    Scheme 2

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  • 8: Ylides and Related Compounds 287

    specifically and stereospecifically in the u-position with electro- philes. Hydrolysis of the alkylated products provides a route to a large variety of multiply-substituted carboxylic acids (Scheme 2 ) .

    +- Ph36CFzBr 3; + R,P - [Ph3PCF2] + R3PBr

    Ph,P + [CFz]

    + R3PBr2 R,P + R,PCF,Br Br- - ~ R3P=CF,

    Difluoromethyl ylides (27) are thought to be formed and to dissociate to give difluorocarbene in the exchange of the bromo- difluoromethyl group which takes place when (bromodifluoromethyll- triphenylphosphonium bromide is treated with tertiary phosphines or phosphites . I5 of sodium hydroxide, mixtures of chloroform, triphenylphosphine, and benzaldehyde give l,l-dichloro-2-phenyl ethene. l6 suggestion that the reaction involves the formation of dichloro- carbene, and hence dichloromethylene ylide in situ, is supported by similar reactions of other halides which are known to give carbenes under these conditions. The method was used to prepare a variety of vinylcyclopropane derivates, e.g. -- ( 2 8 ) , related to pyrethrins, although in some cases direct olefination with the appropriate phosphonate anion was shown to be superior. The reaction of dibromomethylenetr iphenylphosphorane with 2-halopent- 2-enals leads to the formation of the expected diene ( 2 9 ) and in some cases, depending on the conditions, 1,1,4-tribromohex-l-en-3- one (30) . I7 (30).

    ketones from straight-chain dialdehydes .I8

    Under phase-transfer conditions in the presence

    The

    A mechanism has been proposed for the formation of

    Bestmann has reported a new route to unsaturated macrocyclic Stereospecif ic 7-

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  • 288 Organophosphorus Chemistry

    1 R SCIC-CH

    A'SCHC12 + Ph,P +

    a C H O Ph3P, CBg *

    X = F, CI, Br

    NaOH T E B A

    Ph,P=CH(CH,), COO- Na'

    ( 3 4 )

    olefination of ( 3 1 ) with the appropriate carboxylate ylide ( 3 2 ) followed by reaction with the cumiilene ylide ( 3 3 ) offers a new method of generating B-keto ylides ( 3 4 ) . Deprotection and intra- molecular Wittig reaction gives (E,Z)-cyclodienones ( 3 5 ) . The insoluble polv(styrene)-bound phosphonium salts ( 3 6 ) and ( 3 7 ) have been prepared and, throiigh conversion to the corresponding ylides and reaction with aldehydes, used to prepare isomeric mixtures of

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  • 8: Ylides and Related Compounds 289

    w l : H 2 C ~ M e l - - CH COOEt Ph

    Br- (36)

    @ = polymer support

    @53 Ph I Me Br-

    (37)

    Reagents: i , NaOEt, EtOH; i i ,

    Me Me CH=CH C = CHCH= CHC = CHCOOEt

    Me

    i , iii /

    MwcHo Me ... I l l , EtOOC a COOEt

    Scheme 3

    ethyl retinoate (Scheme 3 ) .19 idene)phosphorane (38) offers a method of introducing the 3 , 4 - unsaturated aldehyde function v i a the Wittig reaction.

    Triphenyl(4-trimethylsiloxybutyl-

    20

    Ph,P=CH (CH2),0Si Me,

    (38)

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  • 290 Organophosphorus Chemistry

    The reaction of arylhyrlrazonyl chlorides ( 3 9 ) with triethyl- amine and triethylphosphine gives arylazomethylenetriphenyl- phosphoranes (41) via reaction of the intermediate nitrile imine (40) with phosphine.2

    22 disulphide to give 4-aryl-1,3,4-thiadiazoline-5-thiones (42). The first examples of d i a l k y l b o r o n - s u b s t i t u t e d phosphonium

    The phosp oranes (41) react with carbon

    ylides (43) have been prepared by reaction of dialkylchloroboranes with alkylidene-ylides. 2 3 The compounds are crystalline, but rather

    CI PPh, ArNHN==C - A r f i N = k O O E t - ArN=NC/

    (39) (40) (41 1 \COOE t COOEt

    CCOOtt s 4 c \ s /

    1 2 1 2 + 1 2 Ph3P=CHR + RZBCI Ph,P=CR B R Z + Ph,PCH,R CI -

    (43)

    Ph

    (44 1

    unstable thermally. Transition-metal complexes, e.g. (44), of a number of reactive phosphorus 24y25 and arsenicz4 Z d e s have been prepared by reaction of the appropriate ditertiary phosphine or arsine with sulphur ylide complexes.

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  • 8: Ylides and Related Compounds 29 1

    2.2 Reactions of Methylenephosphoranes 2.2.1 Aldehydes.- A further study of the Wittig reaction using 31P n.m.r. spectroscopy to observe directly the oxaphosphetane inter- mediates has both confirmed results obtained by other methods and produced some new information. *' triphenylphosphorane, generated as shown, with benzaldehyde gave - cis-(45; in a 3 . 8 : l ratio (Scheme 4). This ratio fell to approximately 1:l as the reaction proceeded and the cis:trans ratio of alkene products (1.5:l) did not reflect the original proportions of the oxaphosphetanes (45) and (46). From these results and a study of a similar reaction with hexanal it is concluded that: (a) cis oxaphosphetanes decompose to alkene faster than trans; ( b ) the

    The reaction of n-butylidene-

    6p -61.4 ppm) and trans-(46; 6p -63.8 ppm) oxaphosphetanes

    R

    Ph3P=CHPr"

    + RCHO

    + Ph3P- 0 Ph3P(CH2I3Me Br-

    (45) ( 4 6 )

    + H Ph, P

    i s v

    + Pr" Ph, P

    HO*-H R

    HO/f-'H R

    Reagents: i, LiHMDS; i i , RCHO; i i i , HBr, iv, Base

    Scheme 4

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  • 292 Organophosphorus Chemistry

    oxaphosphetmes derived frombenzaldehyde, but not those derived from hexanal, undergo equilibration at a rate competitive with the rate of decomposition (possibly y& hetaine reversibility); (c) the smaller alkene z/E_ ratios observed for reactions with aromatic aldehydes, compared to those with aliphatic aldehydes, are caused by oxaphosphetane interconversion in the former case rather than differences in the ylide-aldehyde condensation step. These results are supported by isolation snd ba.se-indiiced decomposition of the erythro (47) and threo (48) 6-hydroxyphosphoniurn salts corresponding to ( 4 5 ) and (46). Some similar experiments were also carried out with e t h y l i d e n e t r i p h e n y l p h o s p h o r a n e since this has been used by both Vede js and Schlosser in their studies; however, the 31P resonances of the isomeric oxaphosphetanes were not well resolved in these cases. Unlike similar reactions of reactive ylides, highly stereoselective olefination of aldehydes using moderately stable ylides (e.g. benzylidene- or allylidene-) has not been possible. Vedejs has now shown that dienes can be synthesized with

    -

    R A CR2 R3 + 1 2 3 4 Ph,(ALkyl)P=CHC=CR R + R CHO - R (49)

    R A CR2 R3 + 1 2 3 4 Ph,(ALkyl)P=CHC=CR R + R CHO - R (49)

    a high degree of E selectivity from salt-free allylic ylides (49) by replacing one of the phenyl groups attached to phosphorus by an alkyl group.27 selectivity in the Wittig reaction by steric interactions with phenyl substituents on phosphorus, these results clearly have implications for the detailed mechanism of that reaction. Vedejs argues briefly but convincingly that a key element of Bestmanns proposals28 are unlikely. olefination of changing the phosphorus group has been further in~estigated.~~ reactions using five- or six-membered cyclic phosphonates than from those using phosphinates or phosphine oxides. The rates of Wittig reactions of stabilized ylides with aldehydes increase markedly and the proportion of trans alkene increases marginally at very

    In view of Schlossers attempts28 to explain cis

    The effect on the stereochemistry of

    Higher yields of z alkenes were obtained from

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  • 8: Ylides and Related Compounds 293

    high pressures.30 effects, even of apparently remote groups, can play in olefination is given by the synthesis of (53) .31 the aldehydic ketone (50) gives (53) in good yield, while a similar

    A demonstration of the important role that steric

    A Wittig reaction of ( 5 2 ) with

    FHO

    Me

    + P h3P= CCOOMe I CH - COOMe

    ( 5 0 ) R, = 0

    Me v

    P h3P= CCOOMe I CH , COOMe

    COOMe

    COOMe

    Me

    + Ph,PMe

    R3Ti , ' P P h 2 . E p (ii) Me1 [.-dwl ( 5 4 )

    reaction of the corresponding ketal ( 5 1 ) gives only traces of the ketal of (53).

    32 New routes to z or E 1,3-dienes have been reported. Reaction of the (dipheny1phosphino)allyltitanium reagent (54) with aldehydes followed by methylation and base-treatment gives moderate to good yields of almost pure z 1,3-dienes. The alternative g dienes are available from reactions of aldehydes with the carbanion of allyldiphenylphosphine oxide. The ylide (561, derived from the iron carbonyl-di...

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