Investigations of Novel Uses for Boron Compounds in Organic and Inorganic Chemistry
Cross-Coupling , Regeneration of Ammonia Borane
Herein I describe a model study to determine the feasibility of organic hydride donors as a source of hydride in the regeneration of ammonia borane. Hydride transfer was observed in the model system comprised of Hantzsch ester and several analogues, as the organic hydride donor, and tris(pentafluorophenyl)boron, as the boron-based hydride acceptor akin to BBr3. Side reactions could be minimized by varying the reaction conditions. We determined that a Lewis acid-base adduct was forming between the carbonyls of the donor and the hydride acceptor, that this adduct was dynamic in the case of Hantzsch ester and that it could be inhibited by bulkier ester groups or promoted by reducing the steric bulk at the carbonyl in the case of a methyl ketone. The thermodynamics of the hydride transfer reaction with an N-substituted analogue were probed via variable temperature NMR and compared to two differently substituted analogues. In addition, the scope of the sp2-sp3 Suzuki-Miyaura cross-coupling previously developed in our lab was extended to include 2-(1,2-diaryl)ethane pinacolborane scaffolds. In order to access this asymmetric scaffold, reaction conditions for the cross-coupling of a primary boronic ester in the presence of a secondary one were developed. Yields achieved for the linear cross-coupling were in the 70 % range and varied from 42 % to 69 % for the secondary position. These latter yields are in the same range as the hydroborated styrene scaffolds described in our first account demonstrating the broad scope of these reaction conditions.