Ru(II)-PBTNNXN complex bearing functional 2-(pyridin-2-yl)benzo[d]thiazole ligand catalyzed α-alkylation of nitriles with alcohols
Six tridentate NNN ligand precursors derived from 2-(pyridin-2-yl)benzo[d]thiazole(PBT) with different linkers, PBT NN X N (X = NH, NMe, O, S) ( 1a – 1f ), have been successfully prepared. The electronic properties of PBT NN X N ligands are well tunable by differing linkers between PBT skeleton and the pyridine ring, and/or by introducing electron-donating/withdrawing groups on the pyridine ring (R = OMe or F). The ligand precursors and representative complexes Ru ( PBT NN NH N )Cl 2 (PPh 3 ) ( 2a ), Ru ( PBT NN NMe N )Cl 2 (PPh 3 ) ( 2b ), and Ru ( PBT NN S N )Cl 2 (PPh 3 ) ( 2f ) have been characterized by NMR spectroscopy, high-resolution mass spectroscopy, and Fourier transform infrared (FT-IR). The molecular structures of 1f , 2a , and 2f have been determined by X-ray diffraction study. The results indicate that PBT NN NH N ligand in the complex presented coplanar with two five-membered chelating rings. It should be noted that 2a featuring a NH group exhibits superior performance compared to those with other linkers (such as NMe, O, or S). A variety of heterocyclic and aromatic nitriles with aromatic and aliphatic alcohols have been explored in α -alkylation for good to excellent yields. Based on kinetic experiments and mechanistic studies, a proposed mechanism was put forward. Ru-H species and benzaldehyde, which was oxidized from benzyl alcohol, were detected in the catalytic cycle.
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