Microwave-Enhanced Asymmetric Oxidative Lactonization of meso-Diols

Microwave irradiation often accelerates asymmetric catalytic reactions at the expense of stereoselectivity because the rate enhancement is primarily derived from thermal effects. Herein, we report an iridium-catalyzed asymmetric oxidative lactonization of meso-diols that exhibits significant rate enhancement without erosion of enantioselectivity. These observations are consistent with microwave-associated acceleration beyond conventional thermal effects. One possible explanation is facilitation of formation of a reactive substrate conformation during the initial oxidation step leading to the hydroxyaldehyde/lactol manifold. Mechanistic studies also revealed an unexpected catalytic asymmetric dehydrogenative pathway under degassed conditions.