MECHANISTIC STUDIES ON ZINC-INDUCED ADDITION OF CF2BR2 TO OLEFINS - ANOVEL 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 reacti ons hitherto reported. Both cyclopropane derivatives and 1:1 addition products were obtained from this reaction. A free-radical chain additi on reaction induced ; by single electron transfer (SET) is proposed. T his reaction is inhibited by hydroquinone and completely quenched by p -dinitrobenzene. The nature of the 1:1 adducts formed revealed the cha racter of a typical radical reaction. The nature of the cyclopropane d erivatives formed was also much different from the usual cyclopropane adduct derived from carbene addition. When diallyl ether (15) and norb ornadiene (16) were used as substrates, only rearranged 1:1 addition p roducts could be obtained, and no cyclopropane product could be detect ed. The results of addition reactions with 4-octenes (27) showed also that the cyclopropanation reaction was not stereospecific. On the basi s of the above-mentioned results, a novel reductive debromocyclopropan ation reaction of the gamma-bromopropyl radical intermediate proceedin g on the zinc metal surface is suggested.