Monobenzofused 1,4-Azaborines: Synthesis, Characterization, and Discovery of a Unique Coordination Mode

We report the first general synthesis of boron-substituted monobenzofused 1,4-azaborines using ring-closing metathesis of an enamine-containing diene as a key synthetic strategy. As part of our investigations, we discovered that the B-C3 moiety in a 1,4-azaborine can serve uniquely as a eta(2)-L-type ligand. This functionality is exemplified by two kappa(2)-N-eta(2)-BC Pt complexes of a boron-pyridyl-substituted monobenzofused-1,4-azaborine. Single-crystal X-ray diffraction analysis of the Pt complexes shows a strong structural contribution from the iminium resonance form of the monobenzofused-1,4-azaborine ligand. We also demonstrate that a palladium(0) complex supported by a 1,4-azaborine-based phosphine ligand can catalyze hydroboration of 1-buten-3-yne with unique selectivity. In view of the importance of arene-metal pi-interactions in catalytic applications, this work should open new opportunities for ligand design involving the 1,4-azaborine motif as an arene substitute.