A noVel ruthenium complex prepared from a chiral tridentate amine functionalized phosphine has been characterized by X-ray crystallography and been found to be actiVe in the hydrogenation of an unprecedented range of CdO and CdN double bonds, including the enantioselectiVe hydrogenation and transfer hydrogenation of normally unreactiVe bulky ketones with up to 90% ee.Reduction of CdO and CdN double bonds using molecular hydrogen is, due to its low cost and complete atom efficiency, a very important process in industrial organic syntheses. 1-3 There has consequently been a massive research effort aimed at developing homogeneous catalysts that can carry out this goal with high efficiency, chemoselectivity, and, in the case of prochiral ketones, enantioselectivity. Asymmetric hydrogenation of -keto esters and related substrates has been an industrial process for some time. 3 Homogeneous hydrogenation of unfunctionalized ketones could not be carried out with sufficient efficiency or chemoselectivity until Noyori's pioneering research on ruthenium complexes containing both diphosphine and diamine ligands. 4 These catalysts are highly chemoselective for CdO bonds, 5 show industrially relevant turnover numbers, and if the catalyst shown in Figure 1 is used, extremely high enantioselectivity for a range of acetophenone derivatives. Since Noyori's publications, several other catalysts have appeared that are based on the same design blueprint and have also given excellent results for reduction of acetophenone derivatives. [6][7][8][9][10] The key to the massively enhanced reactivity relative to that of simple Ru-phosphine catalysts is proposed to be the unique mechanism in which the substrate hydrogen bonds to the NH functionality in the diamine ligand. 11,12 However, [Ru(BINAP)(DAIPEN)Cl 2 ] and related catalysts do have some important limitations and are far less effective for the hydrogenation of tetralones, dialkyl ketones, bulky ketones, and some heterocyclic ketones and imines. We therefore initiated a project aimed at successfully hydrogenating these difficult substrates, with the general impression that a departure from the [Ru(diphosphine)(diamine)Cl 2 ] blueprint would be required. In this communication, we report preliminary results showing the promise of ruthenium complexes of chiral tridentate ligands as hydrogenation catalysts. Phosphine-amine ligands have provided some interesting niche applications in catalysis, 13-15 and given the absence of data on hydrogenation catalysis using tridentate ligands, 16,17 ruthenium complexes of tridentate P ∧ N ∧ NH 2 type ligands seemed worthy of investigation. This type of ligand could form octahedral ruthenium complexes with a more open coordination environment, thus increasing substrate scope or reactivity in hydrogenation. Another feature of interest was applying a single ligand to play the roles carried out by both diphosphine and diamine ligands in Noyori catalysts. [18][19][20] Ligand 1 is readily available in both racemic and enantiomerically pure forms from chea...

A study on the enantioselective hydrogenation of tertiary alkyl ketones catalysed by a novel class of tridentate-Ru complex is reported. In contrast to the extensively studied [RuCl(2)(diphos)(di-primary amine)] complexes, this new class of hydrogenation catalyst smoothly reduces these less reactive bulky ketones with up to 94 % ee. The same catalyst system can also selectively reduce other potentially problematic substrates such as bulky heterocyclic ketones. Unusually for a pressure hydrogenation catalyst, similar enantioselectivity can be obtained under transfer hydrogenation conditions. The transfer hydrogenations are somewhat slower than the pressure hydrogenations, but this drawback is readily overcome, since we have discovered that a microwave accelerated transfer hydrogenation of the above ketones occurs within 20 min at about 90 degrees C with similar selectivity to that obtained in the pressure hydrogenation system.

Alcohols Q 0230Enantioselective Hydrogenation and Transfer Hydrogenation of Bulky Ketones Catalyzed by a Ruthenium Complex of a Chiral Tridentate Ligand. -The novel ruthenium complex (RTL) shows activity in both enantioselective pressure and transfer hydrogenation of ketones which do not react when established ketone hydrogenation catalysts are used. -(DIAZ-VALENZUELA, M. B.; PHILLIPS, S. D.; FRANCE, M. B.; GUNN, M. E.; CLARKE*, M. L.; Chem. Eur.