Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene.

Ab initio computations of cyclobutylcarbene (-CHCH) were performed using the UMP4(fc)/6-311++G(2df,2p)//UMP2(full)/6-311++G(d,p) theoretical model. The carbene's most striking feature is its :CH-group. It is markedly toward the elongated C1'-C2' bond in the singlet ground-state but not in the triplet state, which is at least 1.1 kcal/mol higher in energy. Nonclassical 3C2E bonding among the C1, C1', and C2' atoms is prominent in the HOMO{-1}. The electron-donating ability of the nonbonding HOMO is thereby enhanced. The intensified nucleophilicity of the singlet carbene is manifested in quantifiable ways. For example, its hard and soft acid and base (HSAB) hardness, HSAB absolute electronegativity, and gas-phase proton affinity rival those of ylide-stabilized -heterocyclic carbenes. It is computed to act as a nucleophile toward alkenes with higher HSAB hardness values. Transition states from singlet cyclobutylcarbene to bicyclo[2.1.0]pentane, cyclopentene, and methylenecyclobutane were computed and confirmed by intrinsic reaction coordinate calculations. Activation energies depend on the singlet's conformation with regard to -CH ring-puckering, :CH-group rotation, and :CH-group bending. The singlet's bent :CH-group favors bicyclo[2.1.0]pentane and cyclopentene formation.