Dehydrogenative Boron Homocoupling of an Amine-Borane

摘要

The dehydrogenative coupling of amine-boranes as catalyzed by transition-metal fragments offers the potential for controlled hydrogen release and the formation of oligomeric and polymeric materials in which head-to-tail coupling yields products with B—N bonds that are isoelectronic with technologically pervasive polyolefins. Because of this, the area has received considerable attention recently and there are now a wide range of catalysts available, which operate using inner-sphere- or outer-sphere-type mechanisms, that dehy-drogenatively couple amine-boranes of the general formula H3BNRR'H (R, R' = H, alkyl) to give monomeric, cyclic, or polymeric amino-borane materials based on H2B=NRR'. By contrast, the homocoupling of amine-boranes to form well-defined products with B—B single bonds has not been reported, although dehydrogenation of H3B-NH3 by Pd-(NCMe)4BF4 has been reported to form insoluble polymeric materials with B—B bonds. This preference for heterocoupling likely stems from the fact that B-H/N-H activation of amine-boranes gives amino-boranes that are well set up for further oligomerization through the formation of dative B—N bonds, a process that is also driven thermo-dynamically by the differences in relative o-bond strengths between B-B (70 kcalmol~(-1)) and B-N (107 kcalmol~(-1)) single bonds. Well-defined homocoupling of boranes, as mediated by transition metals, is essentially limited to B—B bond formation in polyhedral boranes, for example pentaborane(9) (A), guanidine bases (B), and most recently the homo-coupling of HBCat and related derivatives to give the corresponding diboranes (C) (Scheme 1). By contrast, the homocoupling of phosphines or silanes is well estab-lished.