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 using density functional theory (DFT). The influence of the substituent X and Y groups on the activation energy (Ea) of 1,2‐H shifts were examined. The ‘by stander’ 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 by stander 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 (ΔE_S‐T) of the carbene, i.e., E_a of a carbene is larger as its ΔE_S‐T in creases due to an X substitution. The X effect was also found to be related to the magnitude of the exothermicity: Ea of reaction is smaller when the reaction is more exothermic. Origin of the Y effect is attributed to the inter play between two factors: ‘lateness’ of transition state on the potential energy surface, and the exothermicity of the reaction.