Lewis Base Complexes of Borane as Hydride Sources and C-B Bond Forming Reactions of the Resulting Electrophilic Boron.

Abstract

The last 50 years have seen great advances in the field of boron cations, but most reports have focused on their preparation. Trivalent borenium and even divalent borinium ions have been isolated and characterized, although typically stabilized by bulky, electron-donating ligands. Perhaps these limitations explain the lag in applications of boron cations, the most notable exception being the activated oxazaborolidines developed by Corey as catalysts for ketone reduction and Diels-Alder cycloaddition. The research described in this thesis was directed toward the preparation of relatively unstabilized borenium ions by hydride abstraction from Lewis base-borane complexes (L•BH3). Borenium ions do not accumulate under these conditions due to subsequent rapid reaction with L•BH3 to form B–H–B bonds. However, reversible cleavage of the 3c2e bond releases borenium ion equivalents, as evidenced by the interaction with weak nucleophiles. This reactivity was applied to expand the scope of hydroboration reagents. The reported solvent-assisted decomposition of triphenylmethane indicates that the use of trityl tetrakis(pentafluorophenyl)borate for generation of other reactive electrophiles may warrant closer scrutiny. Trityl activation has also allowed a highly regioselective arene borylation under mild conditions using a number of different heteroatomic directing groups. The observed kinetic isotope effect indicates that the presence of a Brønsted base could accelerate these reactions, but this would require more stabilized borenium cations than L•BH2+. Future development of this methodology could apply the trends that emerge in boron cation literature to find the right balance between stability and reactivity of the cationic boron intermediates. Borane complexes of unsaturated amines and phosphines were used to study hydride transfer to a carbocation formed by protonation of the tethered alkene, achieving directed ionic hydrogenation. Cyclic borane complexes with one face of the intermediate carbocation accessible to the tethered hydride participate in a highly diastereoselective reduction. Amine boranes react by an initial hydride abstraction by the acid, generating an attenuated hydride donor that still reacts with the tethered carbocation. This initial reaction of the strong acid provides an opportunity to introduce a chiral substituent on boron, allowing enantioselective reduction of an unsaturated amine borane.