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 C_α-O bond is broken without any progress in the breakage of C_β-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.