Isotope effects and the mechanism of deoxygenation of epoxides with dichlorocarbene

The deoxygenation of styrene oxide with dichlorocarbene is studied by a combination of isotope effects and theoretical calculations. A normal 13C isotope effect of 1.016 is observed for the α-carbon of the styrene oxide but a surprising inverse isotope effect of 0.995 is observed at the β-carbon. This is indicative of a highly asynchronous process in which the Cα–O bond is broken without any progress in the breakage of Cβ–O bond. Theoretical calculations support this interpretation. This coarctate reaction is formally concerted as it avoids involving a high-energy intermediate, but it appears uninfluenced by transition state aromaticity.