Carbenes are neutral divalent carbon compounds with short lifetimes that are especially important in organic synthesis.1-2 They can exist in singlet and triplet states. In the singlet state, there is an empty pπ orbital on the carbene carbon, and the nonbonding electrons occupy an sp2 orbital in the plane of the molecule. In the triplet state, the nonbonding electrons have parallel spins and occupy an sp2 orbital as well as an orbital with substantial p character. Singlet carbenes are expected to show electrophilic and nucleophilic behavior because of the lone pair and the vacant orbital, whereas triplet carbenes are expected to exhibit diradical reactivity 3 (Scheme 1).
