Theoretical Studies of Eliminations. 6. The Regiochemistry and Stereochemistry of the Gas-Phase Reactions of 3-Halocyclohexenes with Fluoride. An ab Initio Study

The gas phase reactions of fluoride with 3-chlorocyclohexene and 3-fluorocyclohexene have been investigated with ab initio calculations up to the MP2/6-31+G level. Transition states were located for 1,4-eliminations and 1,2-eliminations as well as S(N)2 and S(N)2' substitutions. For each reaction, a variety of stereochemical outcomes was explored. For both systems, the preferred pathway is an anti 1,4-elimination, but syn 1,4-eliminations and anti 1,2 eliminations have transition states that are nearly as stable. The two modes of substitution (S(N)2 and S(N)2') have similar barriers, but are much less favorable than elimination. The results are compared to recent experimental studies of related systems.