Deoxygenation of Epoxides with Carbon Monoxide

The use of carbon monoxide as a direct reducing agent for the deoxygenation of terminal and internal epoxides to the respective olefins is presented. This reaction is homogeneously catalyzed by a carbonyl pincer-iridium(I) complex in combination with a Lewis acid co-catalyst to achieve a pre-activation of the epoxide substrate, as well as the elimination of CO₂ from a γ-2-iridabutyrolactone intermediate. Especially terminal alkyl epoxides react smoothly and without significant isomerization to the internal olefins under CO atmosphere in benzene or toluene at 80-120 °C. Detailed investigations reveal a substrate-dependent change in the mechanism for the epoxide C-O bond activation between an oxidative addition under retention of the configuration and an S_N 2 reaction that leads to an inversion of the configuration.