Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 996−1001
pubs.acs.org/JPCL
Revealing Catalytically Relevant Surface Species by Kinetic Isotope Effect Spectroscopy: H‑Bonding to Ester Carbonyl of trans-Ethyl Pyruvate Controls Enantioselectivity on a Cinchona-Modified Pt Catalyst Fabian Meemken* and Laura Rodríguez-García Department of Chemistry and Applied Biosciences, Institute of Chemical and Biomolecular Engineering, ETH Zürich, Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland S Supporting Information *
ABSTRACT: Monitoring active surface species on an operating technical catalyst is a challenging task due to the presence of multiple different adsorption sites and the abundance of bulk species. In this work, kinetic isotope effect (KIE) spectroscopy is introduced to capture the signals of catalytically relevant hydrogenation species from the IR spectroscopic detection in attenuated total reflection mode. The catalytic interface formed between a cinchona-modified Pt/Al2O3 catalyst and the solvent toluene is sensitively probed directly at the rate limiting step(s) during the asymmetric hydrogenation of ethyl pyruvate by measuring the effects of substituting H2 by D2 kinetically and spectroscopically in the same operando experiment. The application of KIE spectroscopy provides unprecedented molecular level insight into the structure of the diastereomeric intermediate surface complex and the phenomenon rate enhancement, which revolutionizes our understanding of chirally modified metal catalysts. atalytic selective hydrogenation plays a pivotal role in fine chemical production, and is of great interest in both industry and academia.1 For the development of heterogeneous hydrogenation catalysis, information about the surface processes governing the formation of reaction intermediates and products would be particularly valuable.2,3 Operando spectroscopy is a suitable approach to obtain such structure− activity−selectivity relationships, because it aims at investigating the catalyst under operating conditions. Despite the significant advances in operando spectroscopic characterization,4 it remains challenging to capture the signals, which resemble efficient catalytic turnover, due to the presence of multiple different adsorption sites on technical catalysts and the abundance of bulk species, especially in heterogeneous liquid phase catalysis.5−8 In the general hydrogenation mechanism, dissociated hydrogen is considered to participate in the rate-determining step(s).9 While the sequence of elementary steps and the key molecular surface structures are the subject of debate,3,8,10−15 most of the proposed mechanisms focus on a stepwise addition of the two H atoms to the unsaturated bond.8,9,11−13,15 Accordingly, the reduction of a carbonyl bond involves the formation of a half-hydrogenated state9 as exemplarily shown in Scheme 1 for the hydrogenation of ethyl pyruvate (EP) to the alcohol ethyl lactate (EL). Originally discovered by Orito et al., addition of cinchonidine (CD) to the Pt-catalyzed hydro-
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Scheme 1. Proposed Stepwise H Addition Mechanism in the Orito Reaction
genation of pyruvate esters leads to strong enantioselective control,16 and the reaction was observed to be up to 100 times more active.17 The scope of chirally modified noble metal catalysts has been extended to asymmetric hydrogenation of other functionalized substrates,18 but generally those reactions are highly structure-sensitive, and acceleration of the rate has Received: December 20, 2017 Accepted: February 8, 2018 Published: February 8, 2018 996
DOI: 10.1021/acs.jpclett.7b03360 J. Phys. Chem. Lett. 2018, 9, 996−1001
Letter
The Journal of Physical Chemistry Letters only been observed in hydrogenations of a few activated ketones and diketones using a Pt catalyst.18−21 The rate enhancement (RE) has been proposed to originate from a shift of the rate-determining step,22 mitigation of catalyst deactivation,19 as well as from additional activation of the carbonyl bond induced by intermolecular H-bonding.21,23,24 The structure of the diastereomeric intermediate surface complex formed in the Orito reaction has been studied intensely,24−29 and a 1:1 interaction involving H-bonding to the aliphatic amine moiety of the modifier is widely accepted to be at the origin of enantioselectivity.18,23,26,30 Surface science as well as spectroscopic studies indicate that the substrate can adopt multiple configurations leading to the formation of various different diastereomeric surface complexes.24−28 Due to the difficulty connected to resolving the surface structure of the catalytically relevant species during operating catalysis,30,31 it could only be speculated whether the intermolecular Hbonding interaction involves the keto or ester group in transEP or even both carbonyl groups in cis-EP, respectively. The two prevailing mechanistic models differ in this particular respect, but both focus on cis-EP.18,23,26 Herein, the application of kinetic isotope effect (KIE) spectroscopy is introduced to discriminate the signals of catalytically relevant surface species from the IR spectroscopic detection in attenuated total reflection (ATR) mode. Operating a spectroscopic flow-through reactor cell at low conversion (
