Osmium(II)/R-pybox vs ruthenium(II)/R-pybox complexes in the catalytic asymmetric transfer hydrogenation of arylketones

The reaction of the complexes trans-MuCl(2)(eta(2)-C2H4){(S,S)-Pr-i-pybox}] (1a) and trans-[RuCl2(eta(2)-C2H4){(R,R)-Phpybox}] (1b) with nitrogen heterocyclic ligands, provide the complexes trans-[RuCl2 (L)(R-pybox)] (L = py (3a,b), 3-Br-py (4a,b), isoquinoline (5a,b), pyrazine (6a,b), 1-methylimidazole (7a,b), 1-benzylimidazole (8a,b), pyrazole (9a,b), 3-methylpyrazole (10a,b), and 1H-1,2,4-triazole (11a,b)). The complexes trans-[OsCl2(L){(S,S)Pr-i-pybox}] (L = py (12), 3-Br-py (13), 3-CN-py (14), 3-MeO-py (15), 3-NO2-py (16), 4-CN-py (17), 4-MeO-py (18), isoquinoline (19), 1-methylimidazole (20), 1-benzylimidazole (21), pyrazole (22)) have been similarly synthesized by the substitution of ethylene from the precursor complex trans-[OsCl2(eta(2)-C2H4) {(S,S)Pr-i-pybox}] (2) by the corresponding N-donor ligand in refluxing toluene. Moreover, the dinuclear complexes [(RuCl2{(S,S)-Pr-i-PYbox})(2)(mu-N,N-C4H4N2)] (23a), ((RuCl2{(R,R)-Ph-pybox})(2)(mu-N,N C4H4N2)] (23b) and [(OsCl2{(S,S)-Pr-i-pybox})(2)(mu-N,N-C4H4N2)] (24) have been prepared by the reaction of the complexes 1 and 2 with pyrazine (1:0.5 M ratio for 23 and 1:1.5 for 24). The structure of the complexes 9a, 12, 23a and 24 has been determined by single-crystal X-ray diffraction analysis. The ruthenium 3a,b, 6a and 10a,b and osmium complexes 12-22 and 24 have been assayed as catalysts for the asymmetric transfer hydrogenation reaction. Among them, the osmium complexes 12, 15, 16, 18 and 24 have proven more efficient in the reduction of a variety of aromatic ketones affording the (R)-benzylalcohols with very high conversion and moderate enantioselectivity up to 73% e.e.