Stereoselectivity in Organoborane Rearrangement: Relationship to the Mechanism of Hydroboration

Article abstract of DOI:10.1021/jo00152a028

Hydroboration of 1,2-dimethylcyclohexene and subsequent rearrangement of the tertiary to the primary alkylborane occur with substantial (<*>99:1) suprafacial selectivity.Similar though less pronounced behavior is found for the rearrangement of the tertiary to the secondary alkylborane.These results rule out, as the lowest energy pathway, dissociation (dehydroboration) to the free olefins followed by readdition with reversed regiochemistry, since hydroborations of these olefins exhibit little selectivity.The observed stereoselectivity provides strong support for an intra molecular process, most likely involving an intermediate ? complex, which must give rearranged alkylborane faster than dissociated entities.Similar stereochemical results are obtained for the rearrangement in the presence or absence of THF, showing that solvent plays no critical role in the intramolecular migration.As a further mechanistic probe, B2D6 was employed, and the deuterium content was examined in various products.Evidence for an exchange process at the tertiary center β to boron was found.In general, the results are compatible with the proposed ?-complex mechanism.An unusual feature is the incorporation of deuterium at the borane migration terminus, for which a free-radical mechanism is suggested.The rearrangement results are considered in the context of the mechanism of hydroboration.Although a ?-complex intermediate has been suggested for hydroboration, it is concluded that such an intermediate (if it exists) must be fundamentally different from that involved in the rearrangement.This conclusion is surprising considering the close similarities of the two processes.