Deoxygenative Hydroboration of Amide Group Using Homogeneous Pincer Ni (II) Catalysts
Catalyst-assisted hydroboration of amides represents a valuable process of transforming relatively unreactive amide moieties into boryl precursors for further synthesis. In an attempt to obtain less expensive catalysts, ways of introducing first-row transition metals, particularly nickel, instead of heavier transition metals have been investigated. Various iminophosphinite POCN Ni pincer complexes were studied as catalysts in amide reduction with pinacol borane. Differences in reported complexes included various compositions of nitrogen arm of the pincer ligands (alkyl or aryl substituted nitrogen atom) and alteration of the second ligand of the central metal (alkyl, aryl, alkoxide or halogen). These aspects influenced the stability, yields of synthesis and efficiency of the reported molecules as catalysts in the reaction of interest. Ni pincer complexes, that were stable enough to be isolated, were fully characterized with NMR and X-ray diffraction techniques. With certain ligand combinations, the reaction set up worked best for secondary amides with NMR yields being as high as >99%. Interestingly, the reported systems showed better results for primary amides than for tertiary amides, which contradicts general reactivity trend of amides where reactivity decreases from tertiary amides to secondary and finally to primary. This observation allows for potential uncommon selectivity in amide hydroboration.