Dehydrocoupling Reactions of Borane−Secondary and −Primary Amine Adducts Catalyzed by Group-6 Carbonyl Complexes: Formation of Aminoboranes and Borazines

Abstract

Photoirradiation of a solution of BH₃·NHR₂ (<b>1a:</b> R = Me, <b>1b:</b> R = 1/2C₄H₈, <b>1c:</b> R = 1/2C₅H₁₀, <b>1f:</b> 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 (<b>2a</b>−<b>c</b>, <b>f</b>), in high yield. During these reactions a borane σ complex, [M(CO)₅(η¹-BH₃·NHR₂)] (<b>3</b>), was detected by NMR spectroscopy. Similar catalytic dehydrogenation of bulkier amineboranes, BH₃·NH^<i>i</i>Pr₂ (<b>1d</b>) and BH₃·NHCy₂ (<b>1e</b>, Cy = <i>cyclo</i>-C₆H₁₁), afforded monomeric products BH₂NR₂ (<b>4d</b>, <b>e</b>). 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 (<b>1g:</b> R = Me, <b>1h:</b> R = Et, <b>1i:</b> R = ^<i>t</i>Bu) gave borazine derivatives [BHNR]₃ (<b>5g</b>−<b>i</b>).