Alkene bromination with N-bromosuccinimide (NBS) has long been valued for its selectivity for allylic substitution, however, the selectivity of further bromination is not well known. For example, cyclohexene with 2 equiv NBS has been reported to yield 3,6-dibromocyclohexene as the major product (49 % isolated yield). We brominated 3-bromocyclohexene (CCl4, 275-W sunlamp, reflux) and here report that five dibromocyclohexene isomers result in GC-MS yields of trans-3,6- (41 %), cis-3,6- (39%), trans-3,4- (15 %), cis-3,4- (3 %), and 1,3- (2%). The relative product yields are invariant with conversion of the reactant. The major products (trans/cis-3,6- and trans-3,4-) have long been thought to arise solely from allylic hydrogen abstraction at C-6 (path A). However, preliminary DFT calculations suggest a bromine bridge can stabilize the C-4 radical and thus favor hydrogen abstraction at C-4 (path B). This bridged intermediate would be the same as that formed via allylic resonance in path A and both the 3,4- and 3,6- products could arise from abstraction at either C-4 or C-6. Our results hint at a bromine bridged intermediate with the trans-3,4- isomer forming in greater yield than the cis-3,4- isomer (15 % compared to 3 %). We are attempting to explore this idea more closely by brominating trans-4-bromo-2-hexene to look for the products of hydrogen abstraction at C-5; 4,5-dibromo-2-hexene and 2,5-dibromo-3-hexene.

Selectivity of Wohl-Ziegler brominations of cyclohexene and trans-2-hexene

255th National Meeting (2018)

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