Dehydrocoupling reactions of borane-secondary and -primary amine adducts catalyzed by group-6 carbonyl complexes: formation of aminoboranes and borazines.

Photoirradiation of a solution of BH3·NHR2 (1a: R = Me, 1b: R = 1/2C4H8, 1c: R = 1/2C5H10, 1f: R = Et) containing a catalytic amount of a group-6 metal carbonyl complex, [M(CO)6] (M = Cr, Mo, W), led to dehydrogenative B−N covalent bond formation to produce aminoborane dimers, [BH2NR2]2 (2a−c, f), in high yield. During these reactions a borane σ complex, [M(CO)5(η1-BH3·NHR2)] (3), was detected by NMR spectroscopy. Similar catalytic dehydrogenation of bulkier amineboranes, BH3·NHiPr2 (1d) and BH3·NHCy2 (1e, Cy = cyclo-C6H11), afforded monomeric products BH2═NR2 (4d, e). The reaction mechanism of the dehydrocoupling was investigated by DFT calculations. On the basis of the computational study, we propose that the catalytic dehydrogenation reactions proceed via an intramolecular pathway and that the active catalyst is [Cr(CO)4]. The reaction follows a stepwise mechanism involving NH and BH activation. Dehydrocoupling of borane−primary amine adducts BH3·NH2R (1g: R = Me, 1h: R = Et, 1i: R = tBu) gave borazine...