Facile Preparation of Allylzinc Species from Allyl Bromides and Unactivated Zinc Induced by a Catalytic Amount of Aluminum Chloride and Their Reactions with Carbonyl Compounds and Acetals.

Abstract

The effects of Lewis acids (ZnCl 2 , AlCl 3 , BF 3 , and TiCl 4 ) on the generation of allylzinc species from allyl bromide and unactivated zinc powder in dry tetrahydrofuran (THF) were examined by trapping the organozinc compound with benzaldehyde, that is, Grignard-type allylation of the aldehyde. Among the Lewis acids employed, AlCl 3 was found to be the promoter of choice. The allylzinc species preformed in the presence of a catalytic amount of AlCl 3 effectively allylated carbonyl compounds. Various aromatic and aliphatic aldehydes as well as ketones were converted into homoallylic alcohols in good to excellent yields. Under the reaction conditions employed, ester, hydroxy, acetal, and aromatic nitro and halide groups were tolerated. In the case of α,β-unsaturated carbonyl compounds, selective 1,2-addition was observed. Substituted allyl bromides such as prenyl, crotyl, cinnamyl, and 2-cyclohexenyl bromides were smoothly converted to the corresponding allylzinc compounds, which reacted with carbonyl compounds to give substituted homoallylic alcohols in excellent yields. The diastereoselectivity in crotylation, cinnamylation, and 2-cyclohexenylation depended upon the structures of both the organic metals and the electrophiles. The origin of the observed selectivity is discussed. The allylation of dimethyl and cyclic acetals accompanied with carbon-oxygen bond cleavage also proceeded in excellent yields provided that two equivalents of AlCl 3 was present