Achiral and chiral NNN-pincer nickel complexes with oxazolinyl backbones: application in transfer hydrogenation of ketones

Abstract

We describe the synthesis of new NNN-oxazolinyl-pincer nickel complexes and their application in the transfer hydrogenation of ketones. Achiral NNN-ligands, R′₂-oxazolinyl-2-C₆H₄-NH-C(O)CH₂NEt₂ [(^R′2-OxNNN^Et2)–H; R’ = H (3a), R′ = Me (3b)], and chiral ligands, (R)-R′-oxazolinyl-2-C₆H₄-NH-C(O)CH₂NEt₂ [(R)-(^R′-OxNNN^Et2)–H; R′ = Ph (3c), R′ = CH₂Ph (3d), R′ = ⁱPr (3e), R′ = CH₂ⁱPr (3f)], were efficiently synthesized. Treatment of these ligands with (DME)NiCl₂ afforded the desired amido-pincer nickel complexes, (^R′2-OxNNN^Et2)NiCl [R′ = H (4a), R′ = Me (4b)] and (^R′-OxNNN^Et2)NiCl [R′ = Ph (4c), R′ = CH₂Ph (4d), R′ = ⁱPr (4e), R′ = CH₂ⁱPr (4f)], in good yields. All the ligand precursors and nickel complexes were thoroughly characterized by various analytical techniques. The molecular structures of 4a, 4d and 4f were established by X-ray crystallography. The developed nickel complexes were found to be efficient catalysts for the transfer hydrogenation of ketones using ⁱPrOH as a viable hydrogen source. Enantioselectivity in hydrogenation was not observed with the developed chiral catalysts.