Dirhodium(ii,ii) catalyst optimisation for chemoselective hydroaminomethylation: towards efficient amine synthesis

Abstract

The optimisation of a suitable mixed ligand dirhodium(II,II) catalyst for chemoselective hydrogenation of imines and enamines was carried out using four previously synthesised heteroleptic dirhodium(II,II) acetato-bipyridyl complexes (1–4). As such, optimised reaction conditions incorporating changes in temperature, H₂ pressure, catalyst loading, and reaction time were applied to model substrates to determine the selectivity towards target amine product(s). The title complexes were evaluated under the optimised conditions for the hydrogenation reaction with the trifluoromethyl substituted complex (3) showing the highest hydrogenation activity for the series. Varying the partial pressure of CO relative to H₂ in the syngas mixture to optimise the conversion of olefin substrate to form the target amine products was then applied with catalyst precursor (3). The amine reactants were varied, and the use of aromatic amines resulted in low conversion to target products, while aliphatic amine substrates showed good to excellent production of both secondary and tertiary amines in combination with a range of olefins. With the optimised catalyst and reaction conditions in hand, the hydroaminomethylation of a suitable substrate using catalyst precursor 3 to afford two analogues of a known opioid analgesic, Tramadol® was carried out.