[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

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