Generation of Highly Electrophilic Cationic Boron Complexes and their Application toward Formation of C-B Bonds.

Abstract

The focus of our research was to achieve better understanding of the processes underlining the chemistry of highly electrophilic boron cations, and to develop the applications of such cations to C-B bond formation. The reactions of Lewis base borane complexes with potent electrophiles such as the trityl cation, bistriflimide and tris(pentafluorophenyl)borane were explored, and the product structures were assigned based on the X ray crystallographic and spectroscopic data. Hydride abstraction from Lewis base borane complexes by trityl cation was shown to be a viable method for generation of unstabilized primary borenium species, and several such L-BH2+ cations were shown to exist in the condensed phase in the form of diborane(6)-type dicationic dimers. We also explored the aliphatic and aromatic borylation chemistry involving B-H borocations, and developed a procedure for the intramolecular C-H borylation of amine boranes that uses only a catalytic amount of a strong electrophile, and produces H2 gas as the sole byproduct. Additionally, the methodology for generation of more substituted unstabilized borocations derived from 9-BBN was developed, and subsequently applied to the borylation of electron rich heteroaromatic compounds. An unusually hindered boronium cation possessing a 9-BBN cage chelated by 1,8-bis(dimethylamino)naphthalene was structurally characterized and shown to be a potent borylating reagent.