Observation of Elementary Steps in the Catalytic Borane Dehydrocoupling Reaction

Abstract

Step‐by‐step: Elementary steps of the catalytic borane dehydrocoupling reaction have been observed by using platinum complexes, and key intermediates have been isolated. By combining these observations with DFT calculations, a global mechanism for the catalysis was proposed, and the experimentally observed effect of rigid versus flexible phosphine ligands on the reaction was explained (see scheme).

Formation and Reactivity of Electron‐Precise B−B Single and Multiple Bonds

“…Motivated by the desire to find am ore atom-efficient route to the widely used diborane(4) reagents,B 2 Pin 2 and B 2 Cat 2 ,i n2 010 our group successfully accomplished the first synthetically viable dehydrogenative coupling of pinacolborane (HBPin) and catecholborane (HBCat) to the corresponding diboranes(4) using homogeneous Group 10 transitionmetal precatalysts (Scheme 5a). [38] Furthermore,anearlier study had evidenced scrambling of B 2 Cat 2 and B 2 Cat' 2 (Cat' = 4-tertbutylcatecholato) in the presence of [(Cy 3 P) 2 Pt],asevidenced by the formation of B 2 CatCat' and the mixed cis-bis(boryl) Pt II complex, 14.T hese findings suggest both reversible oxidative addition of diborane and the existence of at etraboryl Pt IV intermediate (Scheme 5c). [36] While homogeneous catalysis only provided low turnover numbers (TON max = 106), the use of ah eterogeneous catalyst (Pt on alumina, 0.006 mol %) led to significant improvements (Scheme 5a).…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…In 2013, Weller and MacGregor reported the first well-characterised example of the B-B homocoupling of an amine-borane to yield the diborane (4) Me 3 N · BH 2 BH 2 · NMe 3 ligand sigma bound to rhodium [62]. B-B homocoupling of boranes has been otherwise limited to B-B bond formation in polyhedral boranes [73,74], guanidine bases [75] and catechol-and pinacolboranes [76][77][78] Sigma complexes of aminoboranes have also been isolated, where donation from the B-H bonds into a vacant metal orbital is reinforced by π back-donation from the metal into the π* B-N orbital of the aminoborane [80]. Various examples have been characterised with rhodium [13,66,80,81], iridium [17,19,80,82] and ruthenium [24,80,83,84], and a selection is presented in Fig.…”

Dehydrogenative Boron Homocoupling of an Amine‐Borane

“…By contrast, the homocoupling [5] of amine-boranes to form welldefined products with BÀB single bonds has not been reported, although dehydrogenation of H 3 4 ] has been reported to form insoluble polymeric materials with B À B bonds. Well-defined homocoupling of boranes, as mediated by transition metals, is essentially limited to B À B bond formation in polyhedral boranes, for example pentaborane(9) (A), [7,8] guanidine bases (B), [9] and most recently the homocoupling of HBCat and related derivatives to give the corresponding diboranes (C) [10][11][12] (Scheme 1). Well-defined homocoupling of boranes, as mediated by transition metals, is essentially limited to B À B bond formation in polyhedral boranes, for example pentaborane(9) (A), [7,8] guanidine bases (B), [9] and most recently the homocoupling of HBCat and related derivatives to give the corresponding diboranes (C) [10][11][12] (Scheme 1).…”