THEORETICAL-STUDIES OF ELIMINATION-REACTIONS .4. GAS-PHASE REACTIONS OF F- WITH CYCLOPENTYL AND CYCLOHEXYL CHLORIDE - STEREOCHEMICAL PREFERENCES OF E2 ELIMINATIONS

High-level ab initio calculations (MP2/6-31(+)G*//MP2/6-31+G*) were u sed to investigate the title reactions, and transition states were loc ated for the E2(anti), E2(syn), and S(N)2 pathways. In cyclopentyl chl oride, the barrier to syn elimination (-0.9 kcal/mol) is only 9.3 kcal /mol larger than that of anti elimination (-10.2 kcal/mol). The 5-memb ered ring forces the syn elimination to be periplanar, and the enhance d pi-overlap leads to a unusually stable syn transition state. In cycl ohexyl chloride, the ring has little effect on the eliminations, and t he barriers to syn (5.4 kcal/ mol) and anti (-9.9 kcal/mol) eliminatio n are similar to those found in acyclic analogs. The results are compa red with DePuy's original experimental studies of syn eliminations (J. Am. Chem. Soc. 1962, 84, 1314). The calculations also predict that el imination will dominate over substitution in these systems.