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| - Cycloheptyne Intermediate in the Reaction of ChiralCyclohexylidenemethyliodonium Salt with Sulfonates
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| - Reactions of (R)-4-methylcyclohexylidenemethyl(phenyl)iodonium salt and its 3-trifluoromethylphenyl and 4-methoxyphenyl derivatives (1) with tetrabutylammonium mesylate and triflate werecarried out in chloroform at 60 °C. The products include (S)-4-methylcyclohexylidenemethyl sulfonate(2) and (R)-5-methylcyclohept-1-enyl sulfonate (3) as well as iodoarene. Reactions of (S)-1 wereconfirmed to provide the counterpart results. The rearranged triflate (R)-3Tf formed in the reactionwith triflate maintains mostly the ee (enantiomeric excess) of (R)-1, while the ee of the mesylateproduct 3Ms is largely lost. The 13C-labeling at the exocyclic position of 1 results in the isotopicscrambling of C-1 and C-2 of 3Ms in the mesylate reaction. The degree of the scrambling agreeswell with that of the loss of ee of (R)-3Ms obtained from (R)-1, implying that the racemization isnot 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 the4-methylcycloheptyne intermediate was obtained in 24% yield, but the normal products 2Ms and3Ms were still formed. The 3Ms obtained here in a low yield maintains the high ee of 1. Theseresults indicate that the cycloheptyne is an intermediate responsible for the formation of racemicproduct 3Ms in the mesylate reaction. It is also concluded that the unrearranged products 2 areformed via the competitive pathways of in-plane and out-of-plane SN2 reactions.
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