Ruthenium complexes of aminophosphine ligands and their use as pre-catalysts in the transfer hydrogenation of aromatic ketones: X-ray crystal structure of thiophene-2-(N-diphenylthiophosphino)methylamine

Reaction of thiophene-2-methylamine with one or two equivalents of PPh2Cl in the presence of NEt3, proceeds in thf to give thiophene-2-(N-diphenylphosphino)methylamine, la and thiophene-2-(N, N-bis(diphenylphosphino))methylamine, 2a respectively, under anaerobic conditions. Oxidations of la and 2a with aqueous hydrogen peroxide, elemental sulfur or gray selenium in thf gives the corresponding oxides, sulfides and selenides [Ph2P(E)NHCH2-C4H3S] (E: O 1b, S 1c, Se 1d) and [(Ph2P(E))(2)NCH2-C4H3S], (E: O 2b, S 2c, Se 2d) respectively, in high yield. Furthermore, two novel Ru(II) complexes with the P-N ligands la and 2a were synthesized starting with the complex [Ru(eta(6)-p-cymene)(mu-Cl)Cl](2). The complexes were fully characterized by analytical and spectroscopic methods. P-31-{H-1} NMR, DEPT, H-1-13C HETCOR or H-1-H-1 COSY correlation experiments were used to confirm the spectral assignments. The molecular structure of thiophene-2-(N-diphenylthiophosphino)methylamine was also elucidated by single-crystal X-ray crystallography. Following activation by NaOH, compounds 3 and 4 catalyze the transfer hydrogenation of acetophenone derivatives to 1-phenylethanol derivatives in the presence of iso-PrOH as the hydrogen source. [Ru(Ph2PNHCH2-C4H3S)(eta(6)-p-cymene)Cl-2], 3 and [Ru((PPh2)(2)NCH2-C4H3S)-(eta(6)-p-cymene)Cl]Cl, 4 complexes are suitable catalyst precursors for the transfer hydrogenation of acetophenone derivatives in 0.1 M iso-PrOH solution. Notably 4 acts as an excellent catalyst giving the corresponding alcohols in excellent conversions up to 99% (TOF <= 744 h(-1)). This transfer hydrogenation is characterized by low reversibility under the experimental conditions. (C) 2010 Elsevier Ltd. All rights reserved.