Cycloheptyne intermediate in the reaction of chiral cyclohexylidenemethyliodonium salt with sulfonates

Reactions of (R)-4-methyleyclohexylidenemethyl(phenyl)iodonium salt and its 3-trifluoromethylphenyl and 4-methoxyphenyl derivatives (1) with tetrabutylammonium mesylate and triflate were carried out in chloroform at 60 degreesC. The products include (S)-4-methylcyclohexylidenemethyl sulfonate (2) and (R)-5-methylcyclohept-1-enyl sulfonate (3) as well as iodoarene. Reactions of (S)-1 were confirmed to provide the counterpart results. The rearranged triflate (R)-3Tf formed in the reaction with triflate maintains mostly the ee (enantiomeric excess) of (R)-1, while the ee of the mesylate product 3Ms is largely lost. The C-13-labeling at the exocyclic position of 1 results in the isotopic scrambling of C-1 and C-2 of 3Ms in the mesylate reaction. The degree of the scrambling agrees well with that of the loss of ee of (R)-3Ms obtained from (R)-1, implying that the racemization is not due to the intermediate formation of achiral, primary 4-methylcyclohexylidenemethyl cation. Reaction of 1 with mesylate in the presence of CH3OD provided the 3Ms deuterated at the 2-position. When tetraphenylcyclopentadienone was added to the mesylate reaction system, the adduct of the 4-methylcycloheptyne intermediate was obtained in 24% yield, but the normal products 2Ms and 3Ms were still formed. The 3Ms obtained here in a low yield maintains the high ee of 1. These results indicate that the cycloheptyne is an intermediate responsible for the formation of racemic product 3Ms in the mesylate reaction. It is also concluded that the unrearranged products 2 are formed via the competitive pathways of in-plane and out-of-plane S(N)2 reactions.