The Chemistry of Heteroarylphosphorus Compounds, Part 14 [1] Effects of Heteroaryl Substituents at Phosphorus on the Steric Course of Wittig Reactions of Semistabilised and Stabilised Ylides

Alkene cis-trans Ratio, Betaine Mechanism, Cycloaddition Mechanism The cis-trans ratio of the stilbenes formed in Wittig reactions of semistabilised ylides (derived from benzyltri(hetero)arylphosphonium salts in ethanolic ethoxide) with benzaldehyde decreases markedly in the series 2-furyl >2-thienyl >phenyl > 1-methylpyrrol2-yl. The 2-furyl group favours a greater proportion of the ci«-isomer than m-trifiuoromethylphenyl, whereas the l-methylpyrrol-2-yl group favours a greater proportion of the trans isomer than jo-methoxyphenyl. Similarly, in Wittig reactions of carbonyl-stabilised ylides with benzaldehyde and acetaldehyde, the presence at phosphorus of 2-furyl groups results in a significant increase in the proportion of the cis-alkene compared to that formed from the related triphenylphosphonium ylide. These results are discussed in terms of both betaine and cycloaddition mechanisms for the Wittig reaction. Also discussed is their relevance to recent proposals concerning the elimination of alkene from the intermediate oxaphosphetan.