Intermolecular hydroboration using activated amine boranes.

Abstract

The use of iodine activated pyridine·borane (py·BH₃) for intermolecular hydroborations reactions at room temperature has been demonstrated. This methodology allows for facile access to monoalkyl borane adducts, such as potassium monoalkyl trifluoroborate salts as well as monoalkyl pinacol boronate esters. Furthermore, good functional group compatibility with the py·BH₂I hydroborating reagent was observed. The generation of py·BH₂I as an active hydroborating reagent allowed access to what is believed to be unique S_N2-like mechanism involving displacement of the iodide leaving group by an alkene and subsequent hydroboration. Generation of a more reactive amine borane hydroborating reagent that could react via an S_N1-like mechanism was of interest. To that end catalytic amounts of TrB(C₆F₅)₄ were used to activated py·BH₃ in hopes that upon hydride abstraction by trityl cation, the highly reactive py·BH₂ cation would form and act as a competent hydroborating reagent. While TrB(C₆F₅)₄ activation of py·BH₃ did allow for rapid hydroboration of a variety of olefins, it was subsequently determined that this activation method facilitates B-N bond cleavage to generate borane in situ. The mechanism of this trityl cation catalyzed B-N bond cleavage was envisioned to be analogous to other acid catalyzed B-N bond cleavage mechanisms. Attempts to extend this methodology to the enantioselective hydroboration of olefins was largely unsuccessful. Several chiral amine boranes were synthesized, but were found to suffer from low reactivity when activated with I₂. The use of HNTf₂ instead of I₂ as the activating agent increased the reactivity of amine boranes for hydroboration, but does not give useful enantioselectivity. However, the reasonable levels of enantioselectivity in hydroborations using a nopol derived chiral amine borane and HNTf₂ activation is encouraging for the future development of other chiral amine borane hydroborating reagents.