Reduction of Carbonyl Compounds via Hydrosilylation. 4. Highly Regioselective Reductions of α,α-Unsaturated Carbonyl Compounds

Highly regioselective reduction of α,β-unsaturated carbonyl compounds giving the corresponding unsaturated carbonyls or allylic alcohols as predominant product was effected by hydrosilylation catalyzed by tris(triphenylphosphine)chlororhodium followed by the methanolysis of the resulting adducts. Regiospecific deuteration was also achieved by using deuteriosilanes. The regioselectivity in the hydrosilylation was found to depend markedly on the nature of hydrosilanes used. In general, monohydrosilanes afforded silyl enol ethers (1,4 adduct) while dihydrosilanes gave silyl ethers (1,2 adduct). Other factors controlling the regioselectivity, e.g., the structure of α,β-unsaturated carbonyl compounds, the hydrosilane/substrate ratio, solvent, and the reaction temperature, also were investigated. A spin trapping experiment on the hydrosilylation of β-ionone with diphenylsilane was carried out, and the observed EPR signals were interpreted. Possible mechanisms that can accommodate the observed high regioselectivity are discussed.