Cyclopropanes from Olefins by the Oxygen-accelerated Zinc-carbenoid Reaction

Oxygen greatly accelerates the cyclopropanation of olefins by the diethylzinc-chloroiodomethane or -diiodomethane system. Olefins, including cyclohexene, cyclooctene, 1-heptene, 1-octene, 2-heptene, indene, n-butyl vinyl ether, styrene, and methyl oleate, gave the corresponding cyclopropanes in yields of ca. 70–100%. The methylene addition to cis- and trans-2-butene proceeded in a stereospecific manner in the presence of oxygen. Methyl methacrylate gave no appreciable amount of cyclopropanated product, but polymerization proceeded. Styrene inhibited the formation of the zinc-carbenoid reagent from diethylzinc and dihalomethanes under a nitrogen atmosphere. AIBN and UV light initiated the cyclopropanation of styrene with diethylzinc and chloroiodomethane. A free-radical-chain mechanism is proposed for the formation of the zinc-carbenoid reagent from diethylzinc and dihalomethanes.