Direct Synthesis of Amides from Coupling of Alcohols and Amines Catalyzed by Ruthenium(II) Thiocarboxamide Complexes under Aerobic Conditions

Four octahedral ruthenium­(II) thiocarboxamide
complexes of the
general formula [RuClCO­(AsPh3)2­(L)]
(L = N-substituted pyridine-2-thiocarboxamide) incorporating
carbonyl and triphenylarsine have been synthesized from the reaction
of 1 equiv of ruthenium precursor [RuHClCO­(AsPh3)3] with 1 equiv of thiocarboxamide ligands in refluxing ethanol
in the presence of base. All the new complexes have been fully characterized
by means of elemental analysis, IR, UV–vis, and NMR spectral
methods. Molecular structures of all the complexes were determined
by X-ray crystallography, which confirm the coordination mode of thiocarboxamide
and reveal the presence of a distorted octahedral geometry around
the Ru ion. All the ruthenium­(II) thiocarboxamide complexes were generated
as highly efficient catalysts for synthesis of secondary or tertiary
amides by coupling of amines and alcohols with low catalyst loading,
and the maximum yield was obtained up to 97%. The coupling reaction
can be readily carried out under mild aerobic conditions, and release
of water is the only byproduct. Further, the effect of substituents
of the ligand, solvents, reaction temperature, time, and catalyst
loading on the catalytic activity of the complexes has been investigated.
A plausible mechanism is proposed for the synthesis of amides via
hemiaminal as intermediate through an oxidation of an alcohol to aldehyde.