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

Ab initio computations of cyclobutylcarbene
(c-C4H7CH) 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 bent 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 N-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 c-C4H7 ring-puckering, :CH-group rotation, and :CH-group
bending. The singlet’s bent :CH-group favors bicyclo[2.1.0]­pentane
and cyclopentene formation.