Calculations on tunneling in the reactions of noradamantyl carbenes

Noradamantylchlorocarbene has been found experimentally to undergo ring expansion to 2-chloroadamantene at cryogenic temperatures. The rate constant, calculated with inclusion of small-curvature tunneling, is within a factor of 2 of the rate constant measured at 9 K in a nitrogen matrix. Our calculations predict that noradamantylfluorocarbene will not be found to rearrange under these conditions. The rate constant for carbon tunneling in the ring expansion of noradamantylmethylcarbene (1d) to 2-methyladamantene at T ≠10 K is calculated to be lower by more than 8 orders of magnitude than the rate constant for formation of 3-vinylnoradamantane from 1d by hydrogen migration.