Stereoselectivity of hydrogen 1,2-shifts in singlet alkylcarbenes: a quantitative theoretical assessment of ground-state orbital alignment, and torsional, and steric effects

Abstract

The influence of ground-state {sigma}{sub CH} alignment on the rates of singlet carbene hydrogen 1,2-shifts has been studied with ab initio molecular orbital theory. Geometry optimizations were performed on five model systems with restricted Hartree-Fock calculations and the 3-21G or 6-31G* basis sets. These systems are dimethylcarbene, cyclohexylidene (carbenacyclohexane), carbena-2-norbornane, carbena-2-norbornene, and brexan-5-ylidene (carbena-5-brexane). The effect of electron correlation was included with use of second-order Moller-Plesset theory with the 6-31G* basis on the optimized 6-31G* geometries. Electron correlation was found to have no effect upon the relative activation energies, which are in excellent accord with experiment. The exo selectivities observed and predicted for 1,2-shifts in rigid systems are explained by torsional and steric interactions that develop in the transition structures. Ground-state orbital alignment induces significant geometric distortions yet has little influence on the rates of rearrangements of singlet alkylcarbenes. Because the planar double-bond geometry of the product is already achieved in the 1,2-shift transition structure, there is no difference in the activation energy for axial and equatorial hydrogen migration in cyclohexylidene.