Evidence for Cyclohexyne and Cyclopentyne as Intermediates in the Coupling Reactions of Phenyllithium with 1-Chlorocyclohexene and 1-Chlorocyclopentene

Phenyllithium and 1-chlorocyclohexene and 1-chlorocyclopentene react at 150º in ether to form 1-phenylcyclohexene and 1-phenylcyclopentene, respectively. A 1:1 mixture of 1-chlorocyclohexene-2-^(14) and 1-chlorocyclohexene-6 ^(14)- with phenyllithium yielded 1-phenylcyclohexene-x-^(14)C which contained 23 % of the ^(14)C label in the 1-position. When 1-chlorocyclopentene-1-^(14)C was heated with phenyllithium, extensive rearrangement of the ^(14)C label occurred with 48.9, 36.2, and 14.9% of the ^(14)C distributed in the 1-,2-, and 5-positions of 1-phenylcyclopentene, respectively. The observed ^(14)C rearrangements which accompanied these nucleophilic substitution reactions are best rationalized in terms of an elimination-addition mechanism involving cycloalkyne intermediates.