Alkylidenephosphoranes, Wittig reaction

A particularly interesting reaction is the C-formylation[2] of alkylidenephosphoranes by A-formylimidazole to give a-formyl derivatives (C6H5)3P=C(R)CHO which can be further transformed into a,/ -unsaturated aldehydes by Wittig reaction, or hydrolyzed to produce aldehydes RCH2-CHO. 

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 Wittig reaction proceeds more rapidly in dimethyl sulfoxide than in other customary solvents due to the enhanced deprotonation of the phosphonium salts to alkylidenephosphoranes [327]. Even camphor, a rather unreactive ketone, undergoes the Wittig reaction easily in base/CH3SOCH3 systems [327]. 

The generation of an ylide from a phosphine oxide and an alkyne is of interest because it represents the reversal of the known intramolecular Wittig reaction of acylated alkylidenephosphoranes. ... 

In a paper edited in 1953, concerned with the preparation of the stereoisomeric forms of pentaphenylphosphorus, Wittig and GeiBler described the reaction of methylene-triphenylphosphorane 1 and benzophenone 2, forming 1,1-diphenylethylene 3 and triphenylphosphine oxide 4 (Scheme 1). Soon afterwards, it could be demonstrated that alkylidenephosphoranes (phosphine alkylenes, phosphorus ylides) generally react with carbonyl compounds such as aldehydes and ketones to give alkenes with the formation of phosphine oxide 1,2). 

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,... 

Alkylidenephosphoranes (a.La. phosphorus yUdes) of the general formula Ph3P=CR R (1) have been frequently used in key steps of heterocycle synthesis. Numerous papers and review articles [1-4] testify their versatility in the construction of rings with sizes ranging from three to well beyond 20 and with virtually any number, kind and distribution of heteroatoms. The Wittig alkenation of carbonyl groups is doubtless the most common, though not the only, reaction of P-ylides that has been employed in the cyclization of bifunctional precursors. The cycloaddition between acyl ylides (1 = H,... 

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