Stereochemical Study of the Allylation of Aldehydes with Allyl Halides in Cosolvent/Water(Salt)/Zn and in Cosolvent/Water(Salt)/Zn/Haloorganotin Media.

The stereochemical course of the allylation and propargylation of several aldehydes with crotyl and propargyl halides using zinc powder as the condensing agent in cosolvent/water(salt) media have been extensively studied. Radical ions of well-defined geometry such as [CH(2)=CHCH(CH(3))Br](*)(-) are thought to be formed through an electron-transfer process, which seems to be accelerated by the high interfacial energy developed by the water. These radicals are trapped by the aldehyde molecules to form isomeric mixtures of threo/erythro-beta-methylhomoallylic alcohols or alpha-allenic/beta-acetylenic alcohols. The stereochemistry seems to be determined by the structure of the radical anions adsorbed on the zinc particles and is independent of the geometry of the organic halides, the cosolvents, or the salts. Reactions performed in the presence of Bu(3)SnCl show no discrimination between the coupling reactions of the allyl bromide with aldehydes and with Bu(3)SnCl, both homoallylic alcohols and allyltributylstannanes being formed. In contrast, in the presence of Bu(2)SnCl(2) only beta-methylhomoallylic alcohols are formed, this being a function of the differential allylstannation of aldehydes and crotyldibutyltin halides, which are intermediate products of the zinc-mediated process. The zinc-mediated method gives satisfactory results with acetals if workup is made under the catalytic mediation of Bu(2)SnCl(2).