Enantioface-Differentiating (Asymmetric) Hydrogenation of β-Ketoester with Modified Raney Nickel Catalyst (MRNi). XXXI. A Comparative Study of Reaction Rates and Optical Yields

Abstract

Enantioface-differentiating hydrogenations of methyl acetoacetate over (S)-alanine-, (S)-2-aminobutyric acid, (S)-valine-, (S)-leucine-, (S) -malic acid-, and (R,R)-tartaric acid-MRNi were carried out under atmospheric pressure of hydrogen and the effects of the modifying reagent on the rates of the formation of enantiomer, vS and vR, were investigated. The rates of hydrogenation, v=vS+vR, were the same for all the amino acid-MRNi and also for all the hydroxy dicarboxylic acid-MRNi. However, the vR⁄vs ratios were different. These results are explained in terms of the independence of the rate-determining and enantioface-differentiating steps in the reaction pathway of hydrogenation. The rate-determining step of hydrogenation was shown to be the hydrogen addition to the adsorbed substrate, while the enantioface-differentiating step was expected to occur prior to the rate-determining step.

The role of catalyst activation in the enantioselective hydrogenation of methyl acetoacetate over silica-supported nickel catalysts

“…The apparent rates, over the range of particle sizes, were considerably higher for the modified samples. This observation is in direct conflict with earlier reports (7,12,17) where the adsorption of TA was observed to inhibit hydrogenation activity by blocking the active surface metal. However, Wells and co-workers (48) and Blaiser and co-workers (49) studying the enantioselective hydrogenation of a-ketoesters over platinum catalysts have also observed an increase in the hydrogenation rate due to the presence of the adsorbed cinchona alkaloid modifier.…”

“…With regard to the published reports of MAA hydrogenation on nickel catalysts, reaction rates have either been omitted or included as addenda to the enantioselectivity data. It has, how-'present address: Department of and ever, been concluded in a number of instances (7,12,17 ing part of the active metal surface. In addition, several workers have noted a difficulty in obtaining reproducible product compositions (10,12,18,23,24,31).…”

“…The reaction has been carried out over a variety of supported (3,4,(18)(19)(20)28,30,31,37,38) and unsupported (1-3, l l , 2 8 , 2 9 , 32, 36) nickel systems, principally in the liquid phase (3,4, 11, 19,20,27-32, 34, 35, 37, 38), but gas phase transformations have also been reported (23,26,30,36). In attempting to define the nature of the surface sites that promote enantiodifferentiation, the effects of varying the modification conditions have been considered (1,3,4,18,24,25,27,28,32,33) and the reaction variables have also been altered to optimize the enantioselectivity process (1,4,10,12,14,17,34). Regardless, there is still no consensus of opinion regarding the actual source of the enantiocontrol and, to date, the efforts made to relate optical yield to the coverage of the catalyst surface by the modifier have been largely unsuccessful (20,27,33,35,36).…”

Role of Acetic Acid Added to the Reaction Media for the Enantio-differentiating Hydrogenation of Methyl Acetoacetate Over a Tartaric Acid-modified Nickel Catalyst

“…In order to determine the mechanism of the enantiodifferentiation over a tartaric acid-modified nickel catalyst, kinetic studies have been studied using premodified catalysts [10][11][12][13][14][15]. However, it is difficult to evaluate the effect of the adsorption of the modifier on the hydrogenation activity over a pre-modified catalyst, because pre-modification in an aqueous solution at pH 3.2 and 373 K is a corrosive procedure that changes the nickel surface conditions and the surface area during the modification.…”

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

“…In order to understand the features of the modified catalyst, which is necessary for the development of a more efficient enantio-differentiating catalyst (having a higher e.d.a. and a higher hydrogenation activity) for various substrates, the kinetic studies of unmodified and modified catalysts have been carried out [11][12][13][14][15][16]. However, even for the effect of the tartaric acid modification on the hydrogenation rate, the experimental results were quite diverse.…”