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Radical Brominations of Alkanic Positions by Bromine and by N-Bromosuccinimide.

During the past decade, the details of mechanism of rudical brominations of alkanes, and in particular the influence of a bromo substituent on the selectivity in the hydrogen-abstraction step, have been studied and debated by several groups of investigators. Central to the controversy is the mechanism of bromination by N-bromosuccinimide (NBS), for some of the proposals are supported almost solely by the pre­ sumed sameness of bromination mechanism for both molecular bromine and NBS reagents. That sameness was established for brominations of benzylic positions by kinetic studies and has been assumed for other systems. Some earlier data in the literature on bromocyclohexane halogenations indicate that the isomer distribution obtained with NBS reagent is closer to that obtained with chlorine than to that with bromine, even though the data were presented from the viewpoint that bromine and NBS bromina­ tions involve the same hydrogen-abstracting and bromine-transfer species. Our study suggests that a different mechanism is involved when bromination occurs on alkanic positions. That is, the hydrogen-abstracting agent is different when molecular bromine is used than when NBS is used. Both by intermolecular competition experiments (butyl bromide vs. cyclo­ hexane and cyclohexane vs. cyclopentane) and by reactions with single sub­ strates (cyclopropane and norbornane), the two brominating reagents have been shown to be different. With cyclopropane, the identities of the radicals which attack the substrate are shown to be different from the two reagents, and with norbornane, the identities of the bromine-transfer reagents which react with the alkyl radical are shown to be different.