Reactivity of Aldehydes with Semi‐Stabilised Arsonium Ylide Anions: Synthesis of Terminal ( E )‐1,3‐Dienes

A study of the reactivity of semi‐stabilised arsonium ylide anions in olefination reactions is presented. The different ylide anions were generated by the addition of n BuLi to various arsonium halide derivatives: [Ph 2 As(R)R′] + X − , where R and R′ are methyl, allyl, prenyl or benzyl groups. By using diallyldiphenylarsonium bromide (R=R′=allyl) an olefination protocol was optimised allowing the efficient transformation of aliphatic aldehydes into terminal 1,3‐dienes with a high selectivity for the E isomer ( E / Z ratios ranging from 90:10 to 97:3). The olefination reactions of aldehydes with dissymmetric arsonium halides (R≠R′) are very chemoselective; with arsonium ylide anions the benzyl moiety is more reactive than the allyl moiety which is much more reactive than prenyl and methyl groups. Based on the experimental results, a mechanism is proposed for the reaction.