THEORETICAL-STUDIES OF THE RING-OPENING REACTIONS OF BICYCLOALKYL RADICALS - ELECTROCYCLIC AND DEGENERATE REARRANGEMENTS IN THE BICYCLO[3.1.0]HEX-3-EN-2-YL RADICAL

Ab initio MCSCF calculations have been performed on the ring-opening and circumambulatory degenerate rearrangements in the bicyclo[3.1.0]hex-3-en-2-yl radical (3). The thermal ring opening of 3 to give cyclohexadienyl radical 5 is calculated to be a symmetry-forbidden electrocyclic process that takes place via a highly unsymmetric transition state lying about 16 kcal/mol above 3. The degenerate 1,4-migration of the methylene group in 3 is predicted to be a two-step process, involving the formation of the cyclopentadienylmethyl radical (6) as the stable intermediate, rather than a pericyclic suprafacial 1,4-sigmatropic rearrangement. The calculated potential energy barrier for the first step (i.e., the ring opening 3 --> 6) of this degenerate rearrangement is 12.1 kcal/mol, while the second step of (i.e., the cyclization 6 - 3) is predicted to be about 6 kcal/mol lower. The lower activation energy calculated for the 3 --> 6 ring opening mode, as compared with that calculated for the alternative 3 --> 5 ring-opening mode, is attributed to the fact that the SOMO in 3 can overlap to some extent with the exocyclic cyclopropane bonds, while this orbital is orthogonal to the inter-ring bond.