Theoretical Studies of Eliminations. 6. The Regiochemistry and Stereochemistry of the Gas-Phase Reactions of 3-Halocyclohexenes with Fluoride. An ab Initia 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_N2 and S_N2′ 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_N2 and S_N2′) have similar barriers, but are much less favorable than elimination. The results are compared to recent experimental studies of related systems.