An Experimental and Computational Study about the Effect of a Spirocyclopropane Group on the Solvolysis Rates of Bridgehead Triflates.

7',7'-Dimethylspiro[cyclopropane-1,2'-norbornan]-1'-yl triflate (9) was obtained and its solvolysis rates in buffered 60% aqueous ethanol were determined at different temperatures. The solvolytic behavior of 9 and other bridgehead derivatives (13-17, see Table 1) was studied by force-field, semiempirical and ab initio [B3LYP/6-31g(d)] methods. Cation 9(+) is a slightly pyramidal cyclopropylcarbinyl cation in a nearly perpendicular conformation, showing an sp(2)-like hybridization. Its high electron demand provokes an enhancement of the sigma-participation of the C(5)-C(6) and C(4)-C(7) bonds. The introduction of a cyclopropyl group adjacent to the bridgehead cation leads to an increase of the strain energy. This fact has two opposite effects on the solvolysis rate: (a) the strain energy hinders the flattening of the cation and, hence, originates a rate depression, and (b) the strained C-C bonds are prone to stabilize the cation at C(1) by sigma-delocalization, which provokes a rise of the solvolysis rate. Both effects are accounted for only by the B3LYP/6-31g method. The claimed electron-withdrawal effect of the cyclopropyl group as a basis for the small k(17)/k(16) rate ratio is not confirmed by our calculations.