Gas phase reaction of F- with chlorocyclopropane. Ab initio study of elimination and substitution

The stereochemistry of elimination in chlorocyclopropane was investigated using ab initio (6-31(+)G*//HF/6-31(+)Gi) calculations. The competitive S(N)2 route was also considered. Transition states were located for the E2(anti), E2(syn) and S(N)2 reactions. The three-membered ring has a pronounced effect on the syn/anti preference of E2 elimination; the barrier of syn elimination (22.2 kcal mol(-1)) is only 3.6 kcal mol(-1) larger than that of anti elimination. The small syn/anti difference is related to the inherent periplanarity of the syn transition state and the torsional ring strain of the anti transition state. Comparing the reaction of F- with cyclopropyl, cyclopentyl and cyclohexyl chlorides, it was found that the tendency for anti elimination is especially reduced in the three-membered ring because of the non-periplanar conformation of the anti transition state. According to theory, the initial reactant complex can react further via the E2(anti) pathway or S(N)2 substitution. The barrier to substitution is 2.5 kcal mol(-1) and the S(N)2 pathway dominates over elimination in this system. At the MP2/6-31(+)G*//HF/6-31(+)G* level, the S(N)2 pathway has a lower activation barrier by 7.3 kcal mol(-1) compared with the E2(anti) pathway. (C) 1997 Elsevier Science B.V.