Fundamental Aspects of Enantioselective Heterogeneous Catalysis:  the Surface Chemistry of Methyl Pyruvate on Pt{111}

Abstract

STM, NEXAFS, XPS, and TPR have been used to characterize the adsorption and reactivity of methyl pyruvate on Pt{111}. In the absence of coadsorbed hydrogen, methyl pyruvate polymerizes at room temperature yielding polymer chains, partly dendritic. The STM and XPS data provide independent estimates of the average length, found to be ∼9 monomer units. NEXAFS shows that this polymer contains CdO bonds and no CdC bonds; the CdO bonds are inclined at 64°( 5°with respect to the metal surface. It is proposed that polymerization occurs by hydrogen elimination from the monomer, followed by an aldol condensation that involves elimination of methanol. This mechanism is in excellent accord with the intramolecular bonding, shape, and reactivity of the polymer deduced from the NEXAFS, STM, and TPR results. Coadsorbed hydrogen completely suppresses polymerization. These findings suggest that irreversible deactivation during start-up or steady-state operation of Pt catalysts during enantioselective hydrogenation of alkyl pyruvates can be due to hydrogen starvation which results in polymerization of the prochiral reactant.

Adsorption of activated ketones on platinum and their reactivity to hydrogenation: a DFT study

“…The adsorption of EP has been experimentally studied with different techniques on Ni and Pt [ [38][39][40][41]. While the preferred adsorption of molecular methyl pyruvate (MP) on Ni was found to be a bifurcated version of the η 1 discussed for AC, the studies on Pt point to a mixture of adsorption modes of the ethyl pyruvate.…”

Electrochemical studies of irreversibly adsorbed ethyl pyruvate on Pt{hkl} and epitaxial Pd/Pt{hkl} adlayers

“…Bonello et al have reported STM, NEXAFS and XPS [82][83][84] results in UHV which showed that MePy polymerises on Pt{1 1 1} in the absence of co-adsorbed hydrogen. The mechanism for polymerisation was thought to involve deprotonation of the MePy monomer followed by aldol condensation, with the irreversible loss of methanol.…”

Aspects of Heterogeneous Enantioselective Catalysis by Metals

“…Very strikingly, coadsorbed hydrogen completely suppressed polymerization, thus 211 confirming the importance of avoiding hydrogen starvation at all stages of catalyst operation. 41 The hydrogenation of β-ketoesters and β-diketones using chirally modified Raney Ni has also 220 been extensively studied, with tartaric acid as the most often used chiral modifier ( Figure 3). Other 221 chiral glutamate and glutamic acid modifiers have also been used with Raney Ni, the earliest example of 222 this approach being that of Stewart and Lipkin who in 1939 used d-glucose but achieved only a less than 223 1%.…”