Enantioselectivity and catalyst morphology: step and terrace site contributions to rate and enantiomeric excess in Pt-catalysed ethyl pyruvate hydrogenation

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“…We could demonstrate that by using low EP concentrations (0.01-0.1 mol L −1 ) in a batch reactor, two mechanistic cornerstones for the Orito reaction were no longer valid-that is, the reaction rate was not proportional to the modifier concentration, and a high ee could be obtained with no rate acceleration or even in the presence of rate deceleration. These experimental observations lend support to the hypothesis proposed by Jenkins et al [8] that rate acceleration is due to increased number sites rather than to increased turnover frequencies in a small amount of sites. Rate acceleration could be observed in our batch reactor experiments at higher reactant concentrations (0.3-2.0 mol L −1 ), in line with numerous previous reports on rate acceleration.…”

“…The increased turnover rate over modified sites has been considered a key mechanistic feature of reactant-modifier interactions, supported by the experimentally observed correlation of modifier concentration, enantiomeric excess (ee), and reaction rate, as well as the up to 100-fold-higher hydrogenation rate in the presence of modifier. The increased turnover rate mechanism has been challenged by recent experimental observations [3][4][5][6][7][8], however. Ligand acceleration is readily observed in homogeneous catalysis, and in enantioselective hydrogenation over heterogeneous catalysts is associated mainly with ethyl pyruvate (EP) hydrogenation over cinchonidine (CD)-modifier Pt catalysts (Orito reaction) [2], whereas for other reactants the reaction rate does not differ much in racemic and enantioselective reactions and sometimes even rate deceleration is observed [9].…”

Enantioselective hydrogenation of ethyl pyruvate over cinchonidine-Pt/Al2O3 catalyst. A reaction kinetic approach

“…These values allow to differentiate between enantioselective and the racemic hydrogenations. These two terms reflect the rate acceleration phenomenon observed by most of the research groups [2,8,11,23,24].…”

“…Recently it has been suggested by different authors that in the enantioselective hydrogenation of a-keto esters the rate acceleration effect does not exist [24,25], i.e., the rate enhancement cannot only be explained by the base catalysis induced by tertiary amines, such as CD [20,26]. It has also been suggested that CD suppresses both the decomposition of ethyl pyruvate to form chemisorbed CO at the Pt surface and formation of oligomers with general formula of C x H y O z [27].…”

“…It has also been suggested that CD suppresses both the decomposition of ethyl pyruvate to form chemisorbed CO at the Pt surface and formation of oligomers with general formula of C x H y O z [27]. Other authors proposed that the rate increase is due to the suppression of the formation of oligomers at the Pt surface by CD or other tertiary amines [24]. It is well known that oligomers are formed when CD is mixed with ethyl pyruvate [28] or when the Pt surface interacts with the substrate [29].…”