Iridium(III) homo- and heterogeneous catalysed hydrogen borrowing C—N bond formation

Pentamethylcyclopentadienyl iridium(III) complexes ofbidentate carbene–triazole ligands were found to be excellent homogeneouscatalysts for the hydrogen borrowing mediated coupling of alcohols with amines.The reactions are highly efficient, able to reach completion in under 6 h at100 °C at low catalyst loadings of 0.5 mol%, and are environmentally benign,the only by-product is water. The Ir(III) catalysts promoted C—N bond formationacross a range of alcohol and amine substrates, including biologically relevantmotifs. Covalent attachment to a carbon black surface generated a well-defined "hybrid" heterogeneous catalyst which gave good conversion to products in thecoupling reaction of aniline with benzyl alcohol, and could be recycled with nocatalyst leeching. This represents the first report of a covalently linkedheterogeneous iridium catalyst on carbon used for hydrogen borrowing. Turnovernumbers (TON) for the heterogeneous were found to be significantly higher thanthe corresponding homogeneous reaction. To elucidate the homogeneous reactionmechanism, ¹H NMR studies inconjuction with deuteration experiments allowed amechanism to be postulated.