MECHANISTIC STUDIES ON ZINC-INDUCED ADDITION OF CF2BR2 TO OLEFINS - A NOVEL RADICAL REDUCTIVE CYCLOPROPANATION ON THE ZINC SURFACE

The addition reaction of dibromodifluoromethane (1) to olefins induced by zinc in ethereal solvent differs surprisingly from addition reactions hitherto reported. Both cyclopropane derivatives and 1:1 addition products were obtained from this reaction. A free-radical chain addition reaction induced ; by single electron transfer (SET) is proposed. This reaction is inhibited by hydroquinone and completely quenched by p-dinitrobenzene. The nature of the 1:1 adducts formed revealed the character of a typical radical reaction. The nature of the cyclopropane derivatives formed was also much different from the usual cyclopropane adduct derived from carbene addition. When diallyl ether (15) and norbornadiene (16) were used as substrates, only rearranged 1:1 addition products could be obtained, and no cyclopropane product could be detected. The results of addition reactions with 4-octenes (27) showed also that the cyclopropanation reaction was not stereospecific. On the basis of the above-mentioned results, a novel reductive debromocyclopropanation reaction of the gamma-bromopropyl radical intermediate proceeding on the zinc metal surface is suggested.