σ0π2 Singlet Ground State Carbenes Undergo Least-Motion Reactions with H2 and Alkenes

Ground state singlet carbenes commonly feature sigma(2)pi(0) orbital occupations and are known for their concerted sigma-bond insertion and cycloaddition reactions. Despite the facility of these transformations, orbital symmetry conservation forces them into non-least-motion pi-approach reaction pathways. This situation completely changes when the singlet sigma(0)pi(2) electron configuration becomes the ground state, which we show here by means of high-level CCSD(T) geometry optimizations. Carbenes like the experimentally known 2H-imidazol-2-ylidene react with H-2 and ethylene with negligible or no barrier in a sigma-fashion, which effectively corresponds to a least-motion reaction trajectory.