Sh. Wu et al., MECHANISTIC STUDIES ON ZINC-INDUCED ADDITION OF CF2BR2 TO OLEFINS - ANOVEL RADICAL REDUCTIVE CYCLOPROPANATION ON THE ZINC SURFACE, Journal of organic chemistry, 59(4), 1994, pp. 854-857
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.