The rate of hydrogenation of γ ketoesters MeCOCH 2 CH 2 COOR (R = Et, Pr i , Bu t ) in the presence of the chiral Ru II -BINAP catalyst (BINAP is 2,2´ bis(diphenylphosphino) 1,1´ binaphthyl) greatly increases upon the addition of 5-10 equivalents of HCl with respect to ruthenium. In the hydrogenation of ethyl levulinate, the optically active γ hydroxy ester ini tially formed would cyclize by ~95% to give γ valerolactone with optical purity of 98-99% ee. When the Ru(COD)(MA) 2 -BINAP-HCl catalytic system is used (COD is 1,5 cyclooctadiene, MA is 2 methylallyl), complete conversion of the ketoester (R = Et) in EtOH is attained in 5 h at 60 °C under an H 2 pressure of 60-70 atm.Key words: enantioselective catalytic hydrogenation, ruthenium(II), 2,2´ bis(diphenyl phosphino) 1,1´ binaphthyl (BINAP), esters of levulinic acid, esters of γ hydroxyvaleric acid, optically active γ valerolactone.The asymmetric catalytic hydrogenation of ketoesters is a highly efficient method for the preparation of opti cally active hydroxycarboxylic acids and their derivatives, valuable multi purpose building blocks. The asymmetric hydrogenation of α and β ketoesters catalyzed by chiral ruthenium(II) complexes has been studied rather compre hensively in recent years. 1-5 However, little is known about the asymmetric hydrogenation of γ ketoesters, which are much less reactive than α and β ketoesters. In the presence of the RuCl 2 (BINAP) catalyst (BINAP is 2,2´ bis(diphenylphosphino) 1,1´ binaphthyl), pre pared in situ from Ru(OAc) 2 (BINAP), 6 or the [RuCl 2 (BINAP)(DMF)] n , 7 [RuCl 2 (BINAP)] 2 NEt 3 , 8,9 [RuCl(p MeC 6 H 4 Pr i )(BINAP)]Cl, 10 and [Me 2 NH 2 ]{[RuCl(SEGPHOS)] 2 (µ Cl) 3 } 11 catalysts (SEGPHOS is 5,5´ bis(diphenylphosphino) 4,4´ bi(1,3 benzodioxolyl)), a high conversion of γ ketoesters com bined with a high hydrogenation enantioselectivity can be attained in 2 to 10 days at 30-65 °C and 50-100 atm H 2 .The purpose of this study is to search for a more effi cient catalytic system for the enantioselective hydrogena tion of γ ketoesters. Esters of levulinic acid (1a), com pounds 1b-d, were chosen as model substrates. They were hydrogenated (Scheme 1) in the presence of the Ru 1-Ru 4 catalytic systems (Tables 1 and 2).The plot shown in Fig. 1 indicates that in the presence of the Ru 1 catalytic system, containing only 2 equiv. of HCl (this amount of the acid is required for the formation of RuCl 2 (BINAP)), the conversion of ketoester 1b is less than 5% under the chosen hydrogenation conditions. When HCl is added up to 5 equiv. with respect to ru thenium, hydrogenation occurs much faster and gives γ hydroxy ester 2b, which would cyclize into lactone 3 bỹ 95% under the reaction conditions. Further increase in the amount of HCl induces a gradual increase in the conversion of ketoester, which reaches 100% for the ratio [HCl]/[Ru] = 10. This ratio was chosen as the standard one in the subsequent experiments. It is significant that the growth of the medium acidity does not influence the Scheme 1

No abstract

Enantioselective hydrogenation of ethyl 4 chloro 3 oxobutyrate in ionic liquid systems containing tetraethylammonium bromide, 1 butyl 3 methylimidazolium hexafluorophosphate, or 1 butyl 3 methylimidazolium trifluoromethanesulfonate in the presence of the chiral cata lyst Ru BINAP ensures a high level of asymmetric induction after at least threefold recycling of the catalyst.Key words: enantioselective hydrogenation, ethyl 4 chloro 3 oxobutyrate, ethyl 4 chloro 3 hydroxybutyrate, chiral ruthenium complexes, (R) and (S) BINAP, ionic liquid, tetraethyl ammonium bromide, recycling of the catalyst.

A comparative study of asymmetric hydrogenation and deuteration of methyl levulinate catalyzed by the Ru II -(S) BINAP-HCl system (BINAP is 2,2´ bis(diphenylphosphino) 1,1´ binaphthyl) in MeOH and MeOD was carried out. The results obtained suggest an impor tant role of the protic solvent in the formation of catalytically active ruthenium complexes.Key words: asymmetric catalytic hydrogenation, deuteration, hydrogen heterolysis, H/D ex change, ruthenium complexes, 2,2´ bis(diphenylphosphino) 1,1´ binaphthyl (BINAP), levulinic acid ester, chiral γ valerolactone.Earlier, 1 we have investigated asymmetric hydroge nation of levulinic acid esters catalyzed by the Ru II -(S) BINAP-HCl system (BINAP is 2,2´ bis(di phenylphosphino) 1,1´ binaphthyl). The resulting (S) γ hydroxy esters underwent in situ transformation into (S) γ valerolactone with up to 99% ee.Here we studied the role of solvent in the aforemen tioned reactions with methyl levulinate (1) as a model substrate (Scheme 1). Scheme 1 R = H (a), D (b) ExperimentalCommercial (S) BINAP, (COD)Ru(H 2 C(Me)C=CH 2 ) 2 (COD is cycloocta 1,5 diene) (Acros), and D 2 and MeOD (pi lot plant Prikladnaya Khimiya, St. Petersburg) were used. Methyl levulinate was prepared from levulinic acid (Acros) ac cording to a standard procedure. 2 Gaseous H 2 and D 2 were purified by passing through columns with a nickel-chromium catalyst and molecular sieves. All solvents were dehydrated be fore use. Catalytic hydrogenation of oxo ester 1 and analysis of the reaction products were carried out as described in the pre ceding paper. 1 Here the catalyst for the asymmetric hydrogena tion and deuteration of oxo ester 1 was prepared in situ without isolating the intermediate complex [((S) BINAP)RuCl 2 ].

No abstract