Evidence for a Stepwise Addition of Carbenes to Strained Double Bonds: Reactions of Dihalocarbenes with Cyclopropenes

摘要

The most thoroughly investigated pathway of carbene stabilization is addition to carbon-carbon double bonds. Early studies by Skell and Woodworth consider the singlet carbene addition a one-step process in which two new bonds are formed simultaneously. Even though the addition of singlet carbenes is concerted, it cannot be synchronous according to orbital symmetry considerations. Jones et al. suggested that the concept of nucleophilicity and electrophilicity, as applied to inter-molecular additions of carbenes to alkenes, can be interpreted as the different contributions of the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs). For instance, during the electrophilic attack of a dihalocarbene. charge is transferred from theolefin's HOMO to the empty p orbital of the carbene (LUMO). Calculations of activated complexes for addition of dihalocarbenes to simple olefins support this direction of charge transfer. These calculations also show a shorter distance from the carbene carbon to one of the carbon atoms of the double bond.