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

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.

The Critical Role of Tilted Cinchona Surface Species for Enantioselective Hydrogenation

“…6,9,[17][18][19][20]29,[34][35][36] The flat-lying surface species, also referred to as -bound species, is the most strongly bound cinchona species on Pt or Pd metal catalysts, according to theoretical calculations 20 and experimental observations [28][29][30] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 27 including this experimental work. Weaker bound tilted cinchona species have also been found in previous studies on Pt 28,29 and Pd, 30 but strong surface anchoring via parallel -bonding of the aromatic moiety has been considered favorable for enantioselection 6,9,29,[34][35][36] and theoretical studies have focused on the most strongly bound species.…”

“…Weaker bound tilted cinchona species have also been found in previous studies on Pt 28,29 and Pd, 30 but strong surface anchoring via parallel -bonding of the aromatic moiety has been considered favorable for enantioselection 6,9,29,[34][35][36] and theoretical studies have focused on the most strongly bound species. [17][18][19][20] In the presented operando spectroscopic study we accomplished to distinguish between four different regimes depending on the chiral modifier surface coverage under reaction conditions and such a detection is a necessary prerequisite for assessing the enantiodifferentiating ability of differently bound chiral modifier surface species. These unprecedented experimental observations reveal that a cinchona species more weakly bound via a tilted aromatic QN moiety is in fact responsible for the very high enantioselectivity in the studied Pd-catalyzed reaction, and we have to discuss whether these mechanistic implications also hold true for other chirallymodified catalysts, that is, for catalytic systems involving other substrates, solvents, chiral modifiers and also Pt catalysts.…”

“…In the field of chirally-modified metals calculations on diastereomeric surface complexes involving a prochiral reactant and a chiral modifier currently focus on the most stable adsorbed species of the chiral modifier, which is bound with its aromatic ring system parallel to the catalytic metal surface due to strong π-electron interaction. [17][18][19][20] Nevertheless, to guide and refine such theoretical calculations comparison to experimental observations is inevitable.…”

Disordered Two-Dimensional Self-Organization of Ethyl Pyruvate Molecules on the Pt(111) Surface

“…In the study from the same group, the enol isomer of methyl pyruvate was also discussed to be the active species in 1:1 chiral complexes between the chiral modifier (1-naphthyl)ethylamine (NEA) and methyl pyruvate. 41 In contrast to these adsorbate structures derived for Pd(111), McBreen et al 46 proposed the formation of enediolate species formed from methyl pyruvate on the pristine Pt(111) surface. In this adsorption configuration, both carbonyl groups directly interact with two underlying Pt atoms.…”

“…This is one family of the broader class of α-ketoesters on transition-metal surfaces, which was extensively investigated in previous studies, employing rigorous surface-science approaches. − α-Ketoesters belong to the most important prochiral precursors for enantiopure lactates, which are produced in a heterogeneously catalyzed way over chirally modified transition-metal surfaces . Several groups reported atomistic-level studies on the surface chemistry of the simplest α-ketoesters methyl pyruvate or ethyl pyruvate, either over pristine or chirally modified Pd(111) − and Pt(111) ,, metal surfaces. On pristine Pd(111), methyl pyruvate was proposed to undergo keto-enol tautomerization on the carbonyl group to the enol form of the adsorbate.…”