Enantioselective hydrogenation of α-ketoesters with cinchona-modified platinum catalysts: Effect of acidic and basic solvents and additives

Enantioselective hydrogenation of activated ketones in the presence of Pt–cinchona catalysts. Is the proton transfer concept valid?

“…Pyridine and quinoline increased both rate constants and ee (compare entries 13 and 15-16 in Table 2), too. In an earlier study a similar phenomenon attributed to some sort of base effect of quinoline [25]. Over CD-Pt/Al 2 O 3 catalyst the highest ee value was obtained when aromatic basis (quinoline) and achiral tertiary amine (quinuclidine) was introduced together (entry 17 in Table 2).…”

Bioreductive preparation of ACE inhibitors precursor (R)-2-hydroxy-4-phenylbutanoate esters: Recent advances and future perspectives

“…The enantioselective resolution of rac-HPBE and rac-hydroxynitrile might represent effective approaches, however, limited to maximal yield of 50% [12,17]. Asymmetric transfer hydrogenation (ATH) of ketones plays an important role in asymmetric synthesis of chiral secondary alcohols [18][19][20][21][22]. Enantioselective hydrogenation of ethyl 2-oxo-4-phenylbutanoate is likely one of the most economic and efficient method.…”

“…The highest enantioselectivity reported was 92% ee (R) using 10,11-dihydrocinchonidine (DHCd) as the chiral modified, and 94% ee (R) with the hydroxyl group in the DHCd modifier methylated. However, both cases were achieved using acetic acid as solvent, and under rigorous and commercially impractical conditions of high pressure (100 bar) and high temperature (400°C) hydrogen pretreatment of the Pt/Al 2 O 3 catalyst [22]. After optimization, this process could be carried out at mild reaction condition (17°C and 5.8 bar) resulting enantioselectivity of merely 91% [23].…”

Enantioselectivity on Surfaces

“…[22] Despite the large number of factors influencing the enantioselectivity of templated Pt catalysts, excellent results have been achieved by empirical tuning of pH, solvents, particle sizes, and comodifiers as was done with the Ni/tartaric acid system. Blaser et al [23] obtained an ee = 94% for the hydrogenation of methyl pyruvate. As in the case of the hydrogenation of b-ketoesters on Ni, the exact impact of these parameters must ultimately be elucidated to attain enantioselectivities approaching 100%.…”