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 seve ral aldehydes with crotyl and propargyl halides using zinc powder as t he condensing agent in cosolvent/water(salt) media have been extensive ly studied. Radical ions of well-defined geometry such as [CH2=CHCH(CH 3)Br](.-) are thought to be formed through an electron-transfer proces s, which seems to be accelerated by the high interfacial energy develo ped by the water. These radicals are trapped by the aldehyde molecules to form isomeric mixtures of threo/erythro-beta-methylhomoallylic alc ohols or alpha-allenic/beta-acetylenic alcohols. The stereochemistry s eems to be determined by the structure of the radical anions adsorbed on the zinc particles and is independent of the geometry of the organi c halides, the cosolvents, or the salts. Reactions performed in the pr esence of Bu(3)SnCl show no discrimination between the coupling reacti ons of the allyl bromide with aldehydes and with Bu(3)SnCl, both homoa llylic alcohols and allyltributylstannanes being formed. In contrast, in the presence of Bu(2)SnCl, only beta-methylhomoallylic alcohols are formed, this being a function of the differential allylstannation of aldehydes and crotyldibutyltin halides, which are intermediate product s of the zinc-mediated process. The zinc-mediated method gives satisfa ctory results with acetals if workup is made under the catalytic media tion of Bu(2)SnCl(2).