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

New Journal of Chemistry Pub Date: 2021-05-31 DOI: 10.1039/D1NJ01698A

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-OxNNNEt2)–H; R’ = H (3a), R′ = Me (3b)], and chiral ligands, (R)-R′-oxazolinyl-2-C6H4-NH-C(O)CH2NEt2 [(R)-(R′-OxNNNEt2)–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-OxNNNEt2)NiCl [R′ = H (4a), R′ = Me (4b)] and (R′-OxNNNEt2)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.

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