FREE-RADICAL ALPHA BROMINATION OF CYCLOPROPYL COMPOUNDS BY N-BROMOSUCCINIMIDE

Attempts to carry out free-radical bromination of norcarane using N-bromosuccinimide (NBS) resulted solely in rearrangement of the NBS to β-bromopropionyl isocyanate. However, with the model systems cycloprop[2,3]indene, benzylcyclopropane, trans-1-benzyl-2-methyleyclopropane, and bicyclo[4.1.0]hept-3-ene, in which the carbons a to the cyclopropane rings are activated toward radical formation by adjacent phenyl or vinyl substituents, azobisisobutyronitrile-initiated NBS bromination in carbon tetrachloride solution proceeds smoothly and rapidly. Product formation in each case is derived predominantly via initial hydrogen atom abstraction from a carbon a to a cyclopropane ring. Bromide products derived both from cyclopropylcarbinyl and rearranged allylearbinyl radical intermediates were observed. In certain cases, products resulting from ion-pair rearrangements and eliminations from initially formed, highly reactive cyclopropylcarbinyl bromides were also found. © 1969, American Chemical Society. All rights reserved.