Enhanced hydrogenation activity and diastereomeric interactions of methyl pyruvate co-adsorbed with R-1-(1-naphthyl)ethylamine on Pd(111).

Mausumi Mahapatra,Wilfred T Tysoe,Michael Garvey,Dilano K Saldin,Yun Bai,Luke Burkholder

doi:10.1038/ncomms12380

Mausumi Mahapatra, Wilfred T Tysoe + Show 4 more

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https://doi.org/10.1038/ncomms12380

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Abstract

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity. Here, this effect is explored on a R-1-(1-naphthyl)ethylamine (NEA)-modified Pd(111) model catalyst where temperature-programmed desorption experiments reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. NEA+MP docking complexes are imaged using scanning tunnelling microscopy supplemented by density functional theory calculations to allow the most stable docking complexes to be identified. The results show that diastereomeric interactions between NEA and MP occur predominantly by binding of the C=C of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H2N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds thus providing a rationale for the catalytic observations.

Highlights

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity
To explore the diastereomeric interactions between MP and chiral modifiers, and to investigate the origins of the enhanced hydrogenation activity, experiments are performed on a model Pd(111) single-crystal catalyst modified by R-1-(1-naphthyl)ethylamine (NEA) since the cinchona alkaloids are difficult to introduce into vacuum
To understand the enhancement in hydrogenation activity, we first explore the hydrogenation of MP on NEA-modified Pd(111) using temperature-programmed desorption (TPD) to establish that a Pd(111) single crystal adequately reproduces this enhancement effect

Summary

Introduction

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity. The CH3–C 1⁄4 O bond in methyl pyruvate (MP) can be enantioselectively hydrogenated to methyl lactate over cinchona-modified catalysts in the so-called Orito reaction where cinchona operates as a one-to-one modifier[12,13,14,15,16,17,18,19,20,21,22,23,24] In this case, it has been found that hydrogenation rates are substantially increased at chirally modified sites compared with the unmodified catalyst[17,22,24,25,26,27], thereby enhancing the effect of the diastereomeric interaction to produce enantiomeric excesses (ee) approaching 100%. Since C 1⁄4 C double bonds hydrogenate significantly more than C 1⁄4 O bonds[40], this provides a plausible explanation for the enhanced hydrogenation activity found on chirally modified catalysts

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Conclusion