Surface Chemistry for Enantioselective Catalysis
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A Perspective is offered on the lessons learned from surface-science studies on enantioselective chemistry on solid surfaces performed by the author’s groups. Our emphasis is on studies on model systems, mainly metal single-crystal surfaces under controlled environments, but extension of such research to more realistic samples relevant to heterogeneous catalysis is also briefly discussed. Enantioselective chemistry on surfaces is here divided into three guiding modalities, depending on the underlying mechanism. First, enantioselective chemistry resulting from the use of intrinsically chiral surfaces, which can be made from achiral solids such as metals by exposing the appropriate planes, is discussed. Next, the imparting of enantioselectivity to achiral surfaces by modifying them with adsorbates is classified in terms of two operating mechanisms: first, via the formation of supramolecular surface ensembles with chiral adsorption sites, and second, by relying on the effect of the local chiral environment intrinsically provided by the chiral modifiers through a one-to-one interaction between the modifier and the reactant. A discussion is then provided on studies with more complex samples involving metal nanoparticles and high-surface-area porous oxides. Finally, the present state of our understanding of enantioselective surface chemistry and the prognosis for the future are provided.
KeywordsChiral Enantioselectivity Templating Surface science Catalysis
While much of the early work in catalysis focused on improving activity, defined as the rate at which reactants are converted into products, more recently the emphasis has shifted to improving selectivity, to make more effective use of the reactants and to minimize waste products [1, 2, 3]. Enantioselectivity, in which one enantiomer of a chiral product is formed in favor of the other, is perhaps the subtlest and most difficult-to-control example of selectivity: in a chiral compound, the two enantiomers (typically denoted R and S, or sometimes d- and l-) are identical except for the stereographic arrangement of their atoms, which generates enantiomer pairs that are not superimposable on one another. Enantiomers exhibit identical chemical behavior except when reacting with other enantiomerically pure chiral compounds. Since much of the biochemistry of living organisms relies on single enantiomers of chiral compounds, enantioselectivity has acquired increasing relevance in those industries producing or using bioactive chemicals.
Indeed, from a practical point of view, controlling enantioselectivity in catalysis is critical to the synthesis of chiral pharmaceutical and agrochemical compounds [4, 5]. Many pharmaceuticals are currently synthesized by homogeneous-phase catalysts, and those, in many instances, are non-enantioselective. Moreover, homogeneous processes require subsequent steps for the separation of products from catalysts. The development of enantioselective heterogeneous catalysts for the production of chiral pharmaceuticals would eliminate the large amounts of waste generated with current methods, making the process greener, would reduce the cost incurred in the production of chiral compounds as racemic mixtures and their subsequent separation into enantiomerically pure components, and would also eliminate the difficulties associated with the separation of the catalyst from the final product.
There have been significant efforts to develop such heterogeneous-phase catalytic systems by trial-and-error methods, but those have so far shown only limited success, yielding large enantiomeric excesses (ee’s) almost exclusively in hydrogenation reactions with selected α- and β-ketoesters [6, 7, 8]. There is a need to develop a fundamental understanding of the principles that underpin enantioselective catalysis in order to be able to design and develop new chiral catalyst systems. Since the central chemistry controlling heterogeneous catalysis occurs at the interface between gases or liquids and the solid surfaces used as catalysts, advances in enantioselective heterogeneous catalysis at a molecular level requires a focus on understanding chiral surface chemistry. During the past decade, our three research groups have been working in a coordinated way to contribute to the advancement of this area. In this Perspective, we discuss the main lessons learned so far from that effort.
Naturally chiral surfaces, where surfaces have intrinsically chiral atomic structures (Fig. 1a). Chiral bulk materials such as quartz or calcite inherently expose chiral surfaces, but achiral bulk structures can also expose chiral surfaces in some instances. In fact, the majority of the high-Miller index surfaces of single crystals are chiral . Interactions of chiral adsorbates with naturally chiral surfaces are enantiospecific.
Chirally templated surfaces, where molecules adsorbed on achiral surfaces self-assemble into ordered chiral structures with long-range periodicity (Fig. 1b). Imparting chirality in this way is expected for chiral molecules, but can in fact also occur for molecules that are not chiral in the gas phase as long as adsorption of such prochiral molecules removes their mirror plane of symmetry (although, in the absence of a chiral symmetry, they would form equal amounts of left- and right-handed domains).
One-to-one chiral modifiers, where an isolated chiral modifier on an achiral surface interacts enantiospecifically with a prochiral adsorbate to form a one-to-one complex, orienting the adsorbate enantiospecifically upon adsorption on the surface (Fig. 1c). Such a one-to-one interaction does not require the modifier to form an ordered array on the surface, although this may still happen. In this classification, the dominant interaction in one-to-one modification is between a prochiral molecule and the chiral modifier.
While these categorizations are, to some extent, artificial, and while a particular modifier may operate by a mixture of two or more of these types of interaction, the division described above represents a useful conceptual framework for organizing this Perspective. In the following sections we discuss some of the recent advances coming from our laboratories on the understanding of these three modalities of surface enantioselectivity. The focus is on work using model systems, typically single-crystal surfaces under ultrahigh vacuum (UHV) conditions, but some data are also reported from our recent studies with more realistic catalytic samples and conditions. The emphasis of this review is on the contribution to the field by the authors, but reference is also made to some of the key reports from other groups.
2 Naturally Chiral Surfaces
It was realized some time ago that high-Miller-index surfaces of single crystalline materials consisting of kinked steps separated by flat low Miller index terraces are intrinsically chiral [12, 14, 15, 16]. These surfaces are therefore expected to exhibit enantioselective surface chemistry. In their original work articulating these ideas, Gellman and coworkers used a chiral molecule, 2-butanol, to probe such enantioselective behavior on Ag(643)R versus Ag(643)S single-crystal surfaces . Although no detectable differences in adsorption energetics or in the kinetics of dehydrogenation reactions could be seen by temperature programmed desorption (TPD) between homochiral (R-2-butanol on Ag(643)R and S-2-butanol on Ag(643)S) and heterochiral (R-2-butanol on Ag(643)S and S-2-butanol on Ag(643)R) pairs of reactants and surfaces, later work with similar systems did show clear enantiospecific differences in reaction energetics. Indeed, Sholl et al., based on quantum mechanics calculations, predicted that chiral molecules such as trans-1,2-dimethyl-cycloalkanes and limonene should display enantiospecific differences in adsorption energies on Pt(643)R&S of up to 2 kcal/mole [17, 18]. Subsequent experimental studies on the electro-oxidation of d- and l-glucose on chiral Pt(643)R&S and Pt(531)R&S surfaces revealed differences in the rate of oxidation of as much as a factor of 3 depending upon the handedness of the reactant with respect to that of the surface [19, 20].
The kink sites on chiral single-crystal surfaces have also been shown to be dynamic in nature [34, 35]. Clean chiral surfaces such as those shown in Figs. 1a and 4 are prone to thermal roughening in which atoms diffuse along step edges and result in coalescence of the kinks. In spite of this thermal roughening these surfaces retain their net chirality. An interesting consequence of this behavior is that even achiral surfaces may show chiral kinks at a local level, although on average a racemic mixture of R- and S-kinks is formed on the surface at a macroscopic level. The adsorption of chiral molecules on surfaces can further complicate the dynamic changes in surface structure. They can cause straight step edges to form chiral kinks, flat low Miller index surfaces to facet into chiral high Miller index planes, and the extraction of atoms from an achiral surface to form chiral arrays of adatoms. This adsorbate-induced restructuring of solid surfaces can be used to “imprint” new chiral structures into surfaces that are initially achiral [36, 37, 38, 39]. For instance, recent scanning tunneling microscopy (STM) images have revealed that adsorbed tartaric acid extracts Cu atoms from an achiral Cu(110) surface to form highly ordered, chiral adatom arrays capped by a continuous molecular layer . In some instances, these homochiral domains, made on achiral surfaces, may extend over reasonably large distances.
3 Chirally Templated Surfaces
On surfaces that are not intrinsically chiral, enantioselectivity may be imparted via the adsorption of chiral molecules. This new chirality can be provided entirely by the molecular structure of the individual molecules bonded to the surface, as discussed in the next section, but can also be created as a result of the formation of supramolecular structures on the surface. This latter modality is particularly important in cases where the added chiral modifiers are small or simple molecules not capable of directing enantioselective surface pathways on their own.
Chiral molecules adsorbed on single-crystal metal surfaces have been shown to form complex two-dimensional structures, many with specific enantiomeric characteristics [41, 42]. In fact, because surfaces lower the symmetry of adsorbates, chiral molecules can display more complex behavior on surfaces than in gas or liquid phase, and so-called prochiral molecules, which are not chiral in their isolated state but can become chiral upon adsorption, are also capable of forming ordered structures with enantiospecific characteristics [43, 44]. The idea that these supramolecular chiral structures on surfaces may bestow enantioselectivity to achiral surfaces was first introduced by Wells and coworkers , but it is thanks to chiral titration experiments devised by the group of Tysoe to measure enantioselectivity of chiral overlayers in UHV that this possibility can be tested experimentally .
The chiral supramolecular templating effect has since been observed with other templating molecules (2-methylbutanoate and 2-aminobutanoate species ), with other templating/probe pairs (proline/2-butanol  and 1-(1-naphthyl)ethylamine (NEA)/2-butanol ), and on other surfaces, including Au/Pd(111) , Pt(111) (with 2-butanol , 2-methyl butanoic acid , and NEA  as templating molecules), and Cu(100) (with a PO/lysine probe/template pair ). On the other hand, no enantioselectivity could be detected with PO on alanine-modified Cu(110), PO on alaninol-modified Cu(111), PO on 2-butanol-modified Cu(111), PO on 2-butoxide-modified Cu(100), PO on 2-butoxide-modified Cu(111), R-3-MCHO on 2-butoxide-modified Cu(100), or R-3-MCHO on 2-butoxide-modified Cu(111) . These differences have been tentatively explained on the basis of possible hydrogen bonding among adsorbates, but more studies are required to develop a definitive answer to the question of what makes chiral templating work in some cases but not in others.
The role of hydrogen bonding in this chiral templating chemistry has been explored in more detail by the group of Tysoe and coworkers by looking into the details of possible deprotonation steps and zwitterion formation on surfaces. It was found that, in the case of tartaric acid (TA) on Pd(111), deprotonation of the carboxylic groups happens readily at room temperature. Accordingly, the lack of enantioselectivity in PO uptake on the TA-modified surface may be justified by the inability of the hydrogen-bonding proton acceptor site in PO to bind to the –OH groups on TA . Other chiral probes are being tested at the present time to corroborate this hypothesis.
In a separate study, RAIRS spectra of saturated monolayers of racemic NEA adsorbed from CCl4 solutions onto Pt surfaces were shown to differ from those of enantiopure monolayers, a behavior that was proposed to result from the formation of racemate pairs via hydrogen bonding at the amine moiety also responsible for bonding to the surface [54, 66]. The effect of these differences on the ability of such chiral modifiers to generate chiral adsorption sites on metal surfaces is to be explored next.
4 One-to-One Chiral Modifiers
When the chiral modifier is relatively large, it may be possible for individual molecules to provide the chiral environment on the surface required to bestow enantioselectivity to the catalysts without the need to form chiral supramolecular ensembles. This is believed to be the case in the so-called Orito reaction, the enantioselective hydrogenation of α-ketoesters catalyzed by platinum-based catalysts modified by chiral cinchona alkaloids. This reaction has been studied extensively and many aspects of the process have been explained, but many questions remain still [6, 8, 67, 68, 69, 70]. Moreover, it has not been possible to extend the process to a broad range of reactions or reactants. Nevertheless, a general picture has emerged in which the cinchona modifier forms a weakly bonded complex with the α-ketoester around its chiral center, forcing adsorption of the carbonyl group of the reactant preferentially into one of its two possible orientations [71, 72, 73].
Given that ultrahigh-vacuum (UHV) surface analysis studies with cinchona alkaloids are difficult, much research has been carried out with a simpler representative of such chiral modifiers, 1-(1-naphthyl)ethylamine (NEA). Even though NEA has a much smaller chiral group than the cinchona alkaloids, it has been shown to bestow moderate enantioselectivity to platinum catalysts [70, 85, 86]. Indeed, NEA contains the main functionalities believed to be responsible for the modifying behavior of the cinchona alkaloids, namely, an aromatic ring and an amine group near a chiral center [8, 87]. Based on surface-science studies under UHV [88, 89, 90], adsorption of the modifier on the metal has been proposed to occur through the aromatic ring. Extensive work by McBreen and coworkers, mainly relying on surface imaging using STM but also using vibrational spectroscopies, have provided nice experimental evidence for the formation of hydrogen-bonded complexes between NEA and a number of model reactants on platinum single-crystal surfaces [91, 92, 93, 94, 95, 96, 97, 98, 99, 100].
5 Studies with More Realistic Catalyst Models
The Zaera group has also explored the possibility of using self-assembled monolayers as a way to improve the performance of Pt nanoparticles for the Orito reaction . They found that the addition of alkyl thiol self-assembled monolayers (SAMs) to colloidal platinum nanoparticles suspended in the liquid phase leads to significant improvements in both activity and enantioselectivity in the hydrogenation of ethyl pyruvate modified via the addition of cinchonidine to the solution (compared to those seen with naked Pt nanoparticles). This may be explained by an increase in cinchonidine residence time on the surface. Those cases involved both Pt nanoparticles and the cinchona alkaloid in solution, but even though more nuanced compromises between activity and selectivity were identified when using supported catalysts, reasonable performances were still found to be possible with all-heterogeneous Pt/Al2O3 catalysts covered with thiol-tethered cinchonidine SAMs.
Chiral modification can also be attained by tethering chiral molecules to the high-surface-area porous materials often used as supports in heterogeneous catalysts. The tethering, anchoring, or immobilization of homogeneous organometallic catalysts, including compounds capable of promoting enantioselectivity, has been extensively explored in the past [110, 111, 112, 113], and will not be reviewed here. Alternatively, molecular functionality, chiral or otherwise, can be added to porous solids by using so-called “click” chemistry, via the use of an intermediate linking agent [110, 113, 114]. Bestowing enantioselectivity onto a high-surface-area silica catalyst by tethering cinchonidine with propyltriethoxysilane as the linker was successful for the addition of aromatic thiols to unsaturated ketones, a reaction promoted by amines (the tertiary quinuclidine nitrogen atom in the case of cinchonidine) . However, a loss of enantioselectivity was observed upon tethering, which could be accounted for by a combination of at least three effects: (1) the nonselective catalytic activity of the surface of the solid itself; (2) the activity of the OH species generated by hydrolysis of some of the Si-alkoxy groups in the trialkoxy moieties used to bind many linkers to oxide surfaces; and (3) the bonding of the molecule to be tethered directly to the surface . These problems could be minimized by silylation of the active OH groups on the surface of the oxide support, and by a proper selection of linking position within the cinchonidine and the solvent used.
6 Closing Remarks
As already mentioned in the introduction, enantioselective catalysis is of critical importance for the industrial production of many bioreactive compounds, and offers an interesting challenge in terms of the development of a basic molecular-level understanding of the issues that control selectivity in heterogeneous catalysis. Because heterogeneous catalysts are quite complex, modern surface scientists have taken the approach of investigating specific aspects of the chemistry involved, including the chemical adsorption of the relevant compounds on solid surfaces, by using model systems and controlled environments [121, 122, 123]. This Perspective of our work on chiral surface chemistry provides a good example of the advantages of such approach, and illustrates the level of knowledge that can be acquired on the mechanistic details of catalytic selectivity. At the same time, it also highlights its shortcomings, both in terms of the limited amount of information that can be extracted, even when using state-of-the-art surface sensitive analytical techniques, and in the difficulties that may be encountered in trying to extrapolate the information from model systems to realistic catalysts [124, 125, 126].
Clearly, much more work is needed before reaching the point of being able to design enantioselective catalysts based on the mechanistic lessons derived from fundamental research such as ours. Nevertheless, several useful conclusions have already been extracted from the work done so far. First, convincing evidence is now available for the existence of chiral surfaces, even in materials that are not intrinsically chiral as is the case with the transition metals reviewed here. Furthermore, it is also clear that those chiral surfaces can drive enantioselective chemistry, both in terms of adsorption–desorption phenomena and in connection with chemical transformations. The differences in behavior between pairs of enantiomers are typically small, with energetics that differ over a range of only a few kcal/mole at the most, but those are still sufficient to design enantioseparation and enantiodifferentiation processes under appropriate kinetic conditions. Furthermore, these small differences can be amplified into high enantiospecificity by processes, such as surface explosion reactions, with highly non-linear kinetics. These ideas have been proven with model surfaces, but still await extension to more realistic systems.
It has also been shown that achiral surfaces can be modified via the adsorption of chiral molecules to bestow enantioselectivity to catalytic systems. This chiral modification may originate entirely (or mainly) from interactions between the reactant and individual chiral molecules, but it is also feasible with small modifiers because of the possibility of the formation of supramolecular surface ensembles exhibiting chiral adsorption sites. The operation of both of these mechanisms and their impact on adsorption enantioselectivity have been clearly demonstrated by chiral titration experiments on model surfaces, and similar chemistry is likely to also explain the few catalytic examples where enantioselective modification has been achieved (although that remains to be conclusively demonstrated). Current work is aimed at achieving a better understanding of the chemistry between reactants and chiral modifiers, which appears to require hydrogen bonding but is most likely affected by other factors as well. The ultimate goal is to understand the processes imparting chirality to surfaces to the point of being able to identify and design new chiral catalytic systems. Much work remains to before reaching that goal.
Finally, some attempts have been initiated already to characterize more complex chiral systems, in the form of chirally modified metal nanoparticles and high-surface-area oxides. This work is only in its initial stages, but looks promising. We intend to continue this research, and hope to entice many other surface scientists to join us in this effort.
Funding for the projects described in this Perspective has been provided mainly by the U. S. Department of Energy, Basic Energy Sciences, under Grant No. DE-FG02-12ER16330.
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