Revisiting the Stereoselectivity in Organoborane Rearrangement

Hydroboration of 1,2-dimethylcyclohexene (A) and successive rearrangements yield not only tertiary alkylboranes but also primary and secondary ones. In addition, nontypical anti-addition products are detected, whose formation mechanism is not apparent. Herein, we revisit three mechanisms proposed in the literature: an elimination and readdition, an intramolecular process involving an intermediate π-complex, and an intramolecular migration. According to our computations, the formation of all products starts from the tertiary alkylborane obtained by hydroboration of olefin A. This alkylborane then undergoes a sequence of further retrohydroborations and hydroborations in a syn fashion. Interestingly, the conformational changes on the ring affect these transformations and decide the rearrangement mechanism. Free olefin intermediates are generated during these reactions, which are then rehydroborated from their opposite faces, explaining the formation of anti-addition products. Moreover, the temperature effect on the rearrangement reactions is also analyzed.