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 nBuLi to various arsonium halide derivatives: [Ph2As(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. Dissymmetric semi‐stabilised arsonium ylide anions react chemoselectively with aldehydes in olefination reactions. An allyl side chain is transferred selectively over a prenyl chain leading to terminal 1,3‐dienes with very high E/Z ratios (see scheme).