STEREOMUTATION OF CYCLOPROPANE REVISITED - AN ABINITIO INVESTIGATION OF THE POTENTIAL SURFACE AND CALCULATION OF SECONDARY ISOTOPE EFFECTS

The transition states for single and double methylene group rotation on the singlet trimethylene potential surface have been located at the GVB/6-31G* level of theory, with one pair of electrons correlated. The preference for conrotatory double rotation over single rotation is found to be 0.97 kcal/mol at the GVB level, and is expected to increase slightly with inclusion of additional electron correlation, on the basis of SD-CI calculations on (0,0)- and (0,90)-trimethylene. At the SD-CI level, the transition state for disrotation is expected to be quite close in energy to that for single rotation, and passage over either of these transition states can lead to net one-center epimerization. The H/D kinetic isotope effects for passage over all three of these transition states have been calculated and found to be moderate and normal, with the beta-isotope effects much smaller than the a effects. On the basis of the isotope effects computed for conrotatory double methylene group rotation, the kinetic preference for cleavage of an isotopically less substituted bond in cyclopropane-1,2-H-2(2) (1) is calculated to be z12 = 1.13. This value is in good agreement with that assumed by Berson et al. in their analysis of the stereomutations of 1. However, this value is very different from the value of z12 = 0.4 that is required in order to apply a common mechanistic interpretation to the experimental results obtained recently for cyclopropane-1-C-13-1,2,3-H-2(3) (2) by Baldwin and co-workers, and to those obtained for 1 by both Berson et al. and Baldwin et al.