A THEORETICAL-MODEL FOR THE ORIENTATION OF CARBENE INSERTION INTO SATURATED-HYDROCARBONS AND THE ORIGIN OF THE ACTIVATION BARRIER

Transition states for carbene insertion reactions into C-H bonds can be classified as sigma or pi approaches. In the pi approach, the empty orbital of the carbene is aligned with the carbon p orbital of the pi(CH2) fragment orbital; in the sigma approach, the empty carbene p orbital is aligned with the sigma(CH2) fragment orbital. Concerted hydrogen migration to the larger lobe of the carbene lone pair is energetically favorable. The transition state for CH2 insertion into methane has been calculated at the HF/6-31G*, MP2/6-31G*, and QCISD/6-31G* levels. At all levels, the sigma approach is slightly favored over the pi approach. The barrier at HF is too high, and the C-C bond in the transition state is too short. A small barrier (0.4 kcal/mol) relative to a long-range complex was found at the QCISD/6-31G* level, but none is found at QCISD(T)/6-31G* and QCISD/6-311G**. Transition states have also been optimized at the HF/6-31G* and MP2/6-31G* levels for HCCH3, C(CH3)2, CHF, and CF2 inserting into methane. The sigma approach is again slightly favored over pi. For vinylidene (C=CH2) there is a slight preference for the pi approach for insertion into both methane and ethane. Transition states for insertion of CH2, HCCH3, C(CH3)2, CF2, vinylidene, and silylene into ethane have been optimized at HF/6-31G* and MP2/6-31G*. The sigma(CH2) pathways are favored over pi(CH2) and pi(CHCH3) With sigma(CHCH3) lying significantly higher. For the carbenes considered, there is a wide variation in the barrier heights. The transition state with the carbene in an inverted orientation and the hydrogen migrating to the wrong side of the carbene lone pair is typically 5-10 kcal/mol higher. Contrary to expectations, the barrier heights do not correlate with the HOMO or LUMO energies or the HOMO-LUMO gap. Instead, the trend correlates with the singlet-triplet energy differences in the carbenes. The valence bond state correlation method has been used to develop an explanation for the barrier heights.