Preparative and Mechanistic Studies on Copper-Mediated 1,6-Addition Reactions to a 2,4-Dienone

The 2,4-dienone 4a-methyl-4,4a,5,6-tetrahydro-3H-naphthalen-2-one (1) was found to react with cyano-Gilman reagents R2CuLi·LiCN (R=Me, n-Bu, Ph) or Grignard reagents in the presence of catalytic amounts of copper 2-(dimethylaminomethyl)thiophenolate with complete 1,6-regioselectivity and high trans-diastereoselectivity. NMR-spectroscopic investigations revealed that these Michael-additions proceed via π-complexes 5 which may be transformed into the 1,6-adducts 6 via short-lived copper(III) species.