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

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-C 6 H 4 -NH-C(O)CH 2 NEt 2 [( R′2-Ox NNN Et2 )-H; R' = H ( 3a ), R′ = Me ( 3b )], and chiral ligands, ( R )-R′-oxazolinyl-2-C 6 H 4 -NH-C(O)CH 2 NEt 2 [( R )-( R′-Ox NNN Et2 )-H; R′ = Ph ( 3c ), R′ = CH 2 Ph ( 3d ), R′ = i Pr ( 3e ), R′ = CH 2 i Pr ( 3f )], were efficiently synthesized. Treatment of these ligands with (DME)NiCl 2 afforded the desired amido-pincer nickel complexes, ( R′2-Ox NNN Et2 )NiCl [R′ = H ( 4a ), R′ = Me ( 4b )] and ( R′-Ox NNN Et2 )NiCl [R′ = Ph ( 4c ), R′ = CH 2 Ph ( 4d ), R′ = i Pr ( 4e ), R′ = CH 2 i Pr ( 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 i PrOH as a viable hydrogen source. Enantioselectivity in hydrogenation was not observed with the developed chiral catalysts. NNN-based achiral and chiral (oxazolinyl)amido-pincer nickel complexes are developed and employed for the catalytic transfer hydrogenation of ketones.