In Situ and Reactor Study of the Enantioselective Hydrogenation of Acetylacetone by Ruthenium Catalysis with the New Chiral Diphosphine Ligand (R)-(R)-3-Benzyl-2,4-bis(diphenylphosphino)pentane

The new optically pure C1-symmetric diphosphine ligand (R)-(R)-3-benzyl-2,4-bis(diphenylphosphino) pentane (BDPBzP) has been synthesized by enantioselective reduction of 3-benzyl-2,4-pentanedione with [((S)-BINAP)Ru(p-cymene)Cl]Cl, followed by the reaction of potassium diphenylphosphide with the bis(mesylate) intermediate (S)-(S)-PhCH2CH(CH- (OMs)CH3)2. The Ru(II) complexes [(BDPBzP)Ru(p-cymene)I]Iâ2CH2Cl2 (5), [(BDPBzP)- (DMSO)Ru(í-Cl)3RuCl(BDPBzP)] (6a), and [(BDPBzP)RuCl(CH3CN)3]OTf (7) have been prepared and characterized by multinuclear NMR spectroscopy. The p-cymene complex 5 has been authenticated by a single-crystal X-ray analysis. All Ru(II) complexes are effective catalyst precursors for the enantioselective hydrogenation of acetylacetone to (R)-(R)-2,4- pentanediol. The best catalytic performance in terms of enantioselective discrimination (ee's up to 99%) has been observed for the dimer 6a. An in situ high-pressure NMR study in catalytic conditions has shown that the nonclassical polyhydride dimer [(BDPBzP)(è2- H2)ClRu(í-H)2RuCl(è2-H2)(BDPBzP)] is the only species observed all over the catalytic cycle. The monohydrogenated intermediate (R)-4-hydroxypentan-2-one is obtained in appreciable yields only at low temperature or low conversion. The relative yields and ee's of the monoand dihydrogenated products are consistent with the effect of a double stereodifferentiation process.