“…Indeed, the ultimate product composition for the two representative modified systems was unchanged after a further 100 h of bubbling/agitation in hydrogen. Such a time independent optical yield is in stark contrast to the range of dependencies that are described in the literature. ,,,, 5 Effect of t reaction on mol % conversion to MHB over the unmodified catalyst (◇) and over a catalyst that had been modified in aqueous (▵) and ethanolic (□) media: t mod = 2 h; T mod = 298 K; [TA] = 0.03 mol dm -3 ; T reaction = 343 K. Inset: The variation of ee with time, symbol as above.…”
An enantioselective Ni/SiO2 catalyst has been prepared by modification with aqueous, methanolic, ethanolic, and butanolic solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral β-keto ester (methyl acetoacetate) to a β-hydroxy ester ((R)-(-)-methyl 3-hydroxybutyrate). The effects of systematically varying the modification time and temperature, modifier concentration, and the incorporation of NaBr as a comodifier on the uptake of TA and on the ultimate activity and enantioselectivity are discussed. The nature of the interaction of TA with the surface nickel metal is considered and data on the corrosive leaching of nickel from the catalyst surface are presented. The degree of TA adsorption was found to increase and the extent of nickel dissolution to decrease with the following sequence of modifier solvents: water, methanol, ethanol, 1-butanol. Changes in hydrogenation activity and enantioselectivity are reported as a function of the reaction time, temperature, and the reactant to catalyst molar ratio. Modification of the catalyst with TA not only induced enantioselectivity but also promoted the rate of hydrogenation where the alcoholic treatments yielded the highest activities and selectivities. The reaction exhibits typical Langmuir-Hinshelwood behavior and reaction orders with respect to methylacetoacetate and hydrogen concentrations and apparent activation energies are provided. The effect of the reaction solvent on product composition is discussed, and a linear relationship between the apparent rate and dielectric constant of the reaction medium, in the case of alcoholic solvents, is presented. The action of thiophene, acetic acid, and water as additives is identified, and catalyst durability is addressed.
“…Indeed, the ultimate product composition for the two representative modified systems was unchanged after a further 100 h of bubbling/agitation in hydrogen. Such a time independent optical yield is in stark contrast to the range of dependencies that are described in the literature. ,,,, 5 Effect of t reaction on mol % conversion to MHB over the unmodified catalyst (◇) and over a catalyst that had been modified in aqueous (▵) and ethanolic (□) media: t mod = 2 h; T mod = 298 K; [TA] = 0.03 mol dm -3 ; T reaction = 343 K. Inset: The variation of ee with time, symbol as above.…”
An enantioselective Ni/SiO2 catalyst has been prepared by modification with aqueous, methanolic, ethanolic, and butanolic solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral β-keto ester (methyl acetoacetate) to a β-hydroxy ester ((R)-(-)-methyl 3-hydroxybutyrate). The effects of systematically varying the modification time and temperature, modifier concentration, and the incorporation of NaBr as a comodifier on the uptake of TA and on the ultimate activity and enantioselectivity are discussed. The nature of the interaction of TA with the surface nickel metal is considered and data on the corrosive leaching of nickel from the catalyst surface are presented. The degree of TA adsorption was found to increase and the extent of nickel dissolution to decrease with the following sequence of modifier solvents: water, methanol, ethanol, 1-butanol. Changes in hydrogenation activity and enantioselectivity are reported as a function of the reaction time, temperature, and the reactant to catalyst molar ratio. Modification of the catalyst with TA not only induced enantioselectivity but also promoted the rate of hydrogenation where the alcoholic treatments yielded the highest activities and selectivities. The reaction exhibits typical Langmuir-Hinshelwood behavior and reaction orders with respect to methylacetoacetate and hydrogen concentrations and apparent activation energies are provided. The effect of the reaction solvent on product composition is discussed, and a linear relationship between the apparent rate and dielectric constant of the reaction medium, in the case of alcoholic solvents, is presented. The action of thiophene, acetic acid, and water as additives is identified, and catalyst durability is addressed.
“…major product K, resulted in Raney Ni with lower hydrogenation activity) produced a modified Ni with a higher enantioselectivity than the W-6 method (digested at 323 K, resulted in Raney Ni with higher hydrogenation activity) [58]. The simple preparation of a high temperature digestion, such as at 373 K for 1h in a 20% NaOH solution is sufficient for attaining a high enantioselectivity and a satisfactory hydrogenation activity [26].…”
The state of the art for the enantio-differentiating hydrogenation over an asymmetrically modified nickel catalyst is reviewed along with the background development of the catalyst. An enantioselectivity up to 98% was obtained for the hydrogenation of β-functionalized ketones. The durability of the enantioselectivity was significantly improved using an in-situ-modified nickel powder catalyst (80-90% enantioselectivity for over 20 runs). Recent mechanistic investigations based on surface science are also summarized for the discussion of the enantio-differentiating model.
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