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 C-1-symmetric diphosphine ligand (R)-(R)-3-benzyl-2, 4-bis(diphenylphosphino)pentane (BDPBzP) has been synthesized by enantiosel ective 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) complex es [(BDPBzP)Ru(p-cymene)I]I . 2CH(2)Cl(2) (5), [(BDPBzP)-(DMSO)Ru(mu-Cl)(3) RuCl(BDPBzP)] (6a), and [(BDPBzP)Ru(Cl(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) compl exes are effective catalyst precursors for the enantioselective hydrogenati on of acetylacetone to (R)-(R)-2,4-pentanediol. The best catalytic performa nce in terms of enantioselective discrimination tee's up to 99%) has been o bserved for the dimer 6a, An in situ high-pressure NMR study in catalytic c onditions has shown that the nonclassical polyhydride dimer [(BDPBzP)(eta(2 )-H-2)ClRu(mu-H)(2)RuCl(eta(2)-H-2)(BDPBzP)] is the only species observed a ll over the catalytic cycle, The monohydrogenated intermediate (R)-4-hydrox ypentan-2-one is obtained in appreciable yields only at low temperature or low conversion. The relative yields and ee's of the mono-and dihydrogenated products are consistent with the effect of a double stereodifferentiation process.