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 used to investigate the title reactions, and transition states were located for the E2(anti), E2(syn), and S(N)2 pathways. In cyclopentyl chloride, 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-membered ring forces the syn elimination to be periplanar, and the enhanced pi-overlap leads to a unusually stable syn transition state. In cyclohexyl chloride, the ring has little effect on the eliminations, and the barriers to syn (5.4 kcal/ mol) and anti (-9.9 kcal/mol) elimination are similar to those found in acyclic analogs. The results are compared with DePuy's original experimental studies of syn eliminations (J. Am. Chem. Soc. 1962, 84, 1314). The calculations also predict that elimination will dominate over substitution in these systems.