Secondary amines have been obtained by the selective hydrogenation of imines using ruthenium(II) bis(acetate) complexes Ru(CO)2(OAc)2(PnBu3)[P(C6H5X)3] (OAc = acetate, X = H, Me, OMe) containing two different trans phosphine ligands. Several batch catalytic reactions have been performed to optimize the reaction conditions and the results have been compared with those obtained with analogous complexes as Ru(CO)2(OAc)2(PnBu3)2 or Ru(CO)2(OAc)2(PPh3)2. The presence of two different phosphines and the use of a polar and acidic solvent as trifluoroethanol or methanol produce high conversions and selectivities. The role of the heterodiphosphine monohydride RuH(CO)2(OAc)(PnBu3)(PPh3) has been detected using Operando high-pressure NMR experiments. In fact, the solvent influences the hydrogen activation step and using a protic solvent the monohydride species is prevalently generated by hydrogenation of the bis(OAc) precursor whereas the subsequent transformation in the corresponding dihydride RuH2(CO)2(PnBu3)(PPh3) is reduced. Finally, several Ru(ll) complexes, containing two different phosphines, have been tested as catalyst in the homogeneous hydrogenation of various N-aryl aldimines in order to correlate the ligand effect with the catalytic activity and the selectivity observed for each imine.

Synthesis of amines by homogeneous catalytic hydrogenation of imines

248th National Meeting (2014)

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