Alkylidenephosphoranes, olefination

The reaction of the phosphonium salts 28, generated from the -halogencarboxylic esters 26, with sodium ethoxide in DMF or sodium hydride in DMSO yields the phosphoranes 29 which, on treatment with aldehydes 30, are converted into (Z)-alkenoic esters 31 49,51 53). To synthesize As-unsaturated acids and higher homologous acids of type 31 (m 11) Bergelson et al. 49,54) olefinated ethyl co-formyl-alkanoates 34, obtained from co-iodoalkanoic esters 32, with alkylidenephosphoranes 35 to the ethyl esters 31 (Scheme 7). 

By this method (Z)-monounsaturated fatty acids and esters could be obtained with an ( )-isomer content of less than 10% this stereoselectivity being however inferior to that of the commonly used acetylenic approach 55,56). However, the salt-free techniques used today in Wittig reactions allow (Z)-alkenoic acids to be synthesized with less than 2% of the ( )-isomers. Thus, Bestmann et al. prepared methyl and ethyl esters of (Z)-4,5,6,7,8,9,ll- and 13-alkenoic acids of different chain lengths 35,57 62), which served as intermediates in the synthesis of insect pheromones, both by reaction of co-alkoxycarbonyl-substituted alkyl-triphenyl-phosphonium salts with simple alkanals and of co-formylalkanoic esters with alkylidenephosphoranes. As the starting material for the synthesis of -substituted alkyl-phosphonium salts co-chloro- and -bromocarboxylic esters were used. The corresponding -substituted aldehydes can usually be obtained by ozone cleavage of suitable olefin derivatives or by oxidation of alkohols 57,58). 

The olefination of cyclopropanone hemiacetales with alkylidenephosphoranes has also been published 121) but the yields are rather low. 

Olefins with electrophilic substituents, but not those with electron-donating substituents, add tertiary phosphines 61 the phosphonium betaines first formed stabilize themselves under some circumstanes by forming their tautomers, the alkylidenephosphoranes. Thus, for example, the ylide (5) is obtained in more... 

Alkylidenephosphoranes have recently achieved great importance by their use in Wittig olefination. The manifold applications of that reaction are supplemented by the reactions involving introduction of acyl or alkyl groups the latter have been reviewed by Bestmann.455-457 It should be noted that,... 

Scheme 16 Olefination of alkylidenephosphoranes under Schlosser s conditions...

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