Density functional study on the reactivity of carbenes toward 1,2-H shifts

The 1,2-H shift reactions of simple carbenes (CH2Y-C-X) have been studied u sing density functional theory (DFT). The influence of the substituent X an d Y groups on the activation energy (E-a) of 1,2-H shifts were examined. Th e 'bystander' Y substituents lower E-a in the order of Me, F > Cl, Br > H. Our analysis shows that the X effect is more significant than the bystander Y effect. X substitutions increase E-a of carbenes in the order of F > Cl > Br > Me > H. The influence of X on E-a is governed by the singlet-triplet energy separation (DeltaE(S-T)) of the carbene, i.e., E-a of a carbene is larger as its DeltaE(S-T) increases due to an X substitution. The X effect was also found to be related to the magnitude of the exothermicity: E-a of reaction is smaller when the reaction is more exothermic. Origin of the Y e ffect is attributed to the interplay between two factors: 'lateness' of tra nsition state on the potential energy surface, and the exothermicity of the reaction.