Photochemical Michael reaction of silyl enol ethers with 2′-nitro-and 2′,2′-dicyanostyrenes

Photolysis of silyl enol ethers with 2′-nitro- and 2′,2′-dicyanostyrene acceptors yields the corresponding Michael adducts in moderate yield and diastereoselectivity. The desilylation of the silyl enol ethers to the corresponding ketones and cis-trans isomerization of the olefinic acceptors competes with the photochemical Michael addition. Under similar conditions, α,β-unsaturated carbonyl compounds give only cis-trans isomerization. A mechanism involving the regiospecific addition of silyl enol ether to the π-π* excited Michael acceptor to form a zwitterionic intermediate is proposed.