Studies of the effects of the modification conditions on the hydrogenation rate for the enantio-differentiating hydrogenation of methyl acetoacetate over a tartaric acid–NaBr-modified nickel catalyst

Abstract

The effects of the modification conditions on the hydrogenation rate and the enantio-differentiating ability (e.d.a.) for the enantio-differentiating hydrogenation of methyl acetoacetate were studied over an (R, R)-tartaric acid-NaBr-in-situ-modified nickel catalyst. It was revealed that a tartaric acid modification increased the hydrogenation rate irrespective of the presence of the auxiliary modifier, NaBr. In the presence of the tartaric acid, NaBr would have two roles, i.e., Na + activates the enantiodifferentiating sites through the interaction with tartaric acid, and Br ) deactivates both the enantio-differentiating sites and nonenantio-differentiating sites. The e.d.a. values and the hydrogenation rate would be determined by the combination of these effects of Na + and Br ) .

Asymmetric Catalysis at Chiral Metal Surfaces

“…Effective and widely used procedures are (i) the corrosive pretreatment of Raney Ni, 55,76,300,306 (ii) prereduction of supported Pt 143,203,293 and Pd 69,391,395,464 in hydrogen at elevated temperature in the gas phase or in solution, and (iii) ultrasonication of Pt, 56,[465][466][467] Pd, 347,369 and Ni [468][469][470] in solution in the presence of the modifier. These empirically developed techniques are highly catalyst specific.…”

“…Crucial requirements that the modifier has to fulfill are the presence of two carboxyl groups and at least one hydroxyl group . The original, time-consuming, and corrosive catalyst preparation technique has been improved remarkably in the past years. ,,− A general obstacle is the relatively low activity of Ni as compared to that of Pt group metals; completion of the reactions requires high pressure and elevated temperature. In an extreme case, in the hydrogenation of racemic 3-cyclopropyl-2-methyl-3-oxopropanoate, 93% ee and high diastereomer ratio were obtained by dynamic kinetic resolution but the conversion was only 23% after 1 day at 80 °C …”

“…An illustration to the complex and poorly understood role of metal surface is the sometimes dramatic influence of catalyst pretreatments. Effective and widely used procedures are (i) the corrosive pretreatment of Raney Ni, ,,, (ii) prereduction of supported Pt 143,203,293 and Pd ,,, in hydrogen at elevated temperature in the gas phase or in solution, and (iii) ultrasonication of Pt, ,− Pd, , and Ni − in solution in the presence of the modifier. These empirically developed techniques are highly catalyst specific.…”

Selective hydrogenation of ethyl-benzoylacetate to 3-hydroxy-3-phenyl-propionate catalyzed by Pd/C in EtOH as a solvent in the presence of KOH: The role of the enolate ion on the reaction mechanism

“…The literature relating this item is large and several review and articles are available on the argument, in particular Ni, Pt and Pd supported and unsupported metal catalysts promoted by chiral modifiers and salts are studied [19][20][21][22][23][24][25][26]. In particular, these authors studied the influences of modifiers and operative variables on reaction rates and enantiomeric excess.…”

“…They observed that, together with a high enantiomeric excess obtained in the hydrogenation of prochiral ketones, in some cases the chiral modifier enhances also the catalytic activity [19,20]. In these papers, surface complex between the ketones and the modifiers are claimed and the chiral nature of the modifier is responsible for the enantioselective hydrogenation [18][19][20][21][22][23][24][25][26]. However, the role of the enolate ion on the reaction mechanism is not investigated and only in the old paper is mentioned [4,5].…”