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STM Study of Ketopantolactone/(R)-1-(1-naphthyl)ethylamine Complexes on Pt(111): Comparison of Prochiral and Enantiomeric Ratios and Examination of the Contribution of CH…OC Bonding Yi Dong, Katrine L. Svane, Jean-Christian Lemay, Michael Nelson Groves, and Peter Hugh McBreen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02590 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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STM Study of Ketopantolactone/(R)-1-(1-naphthyl)ethylamine Complexes on Pt(111): Comparison of Prochiral and Enantiomeric Ratios and Examination of the Contribution of CH…OC Bonding

Yi Dong a, Katrine Svaneb, Jean-Christian Lemay a, Michael N. Grovesc and Peter H. McBreena* aDepartment

of Chemistry, Laval University, Quebec City, Quebec, G1K 7P4, Canada of Chemistry, University of Bath, Bath, BA27AY, UK cDepartment of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, CA, 92834, USA bDepartment

[email protected] Abstract

We report a comprehensive model surface science study, using scanning tunnelling microscopy (STM), of the regioselective and stereospecific complexation of a prochiral substrate molecule to a chiral modifier molecule on a metal surface. The system is chosen so as to compare the prochiral ratio (pr) measured directly by STM for the model system with the reported enantiomeric ratio (er) for the hydrogenation of the prochiral molecule using the same chiral modifier and metal under true reaction conditions. Specifically, diastereomeric complexes formed between ketopantolactone (KPL) and (R)-1-(1naphthyl)ethylamine, ((R)-NEA), on Pt(111) were studied as a function of the ratio of KPL to (R)-NEA. Only KPL molecules in complexes are detected in the STM experiments, performed between 237 and 250 K, due to rapid diffusion of free KPL on the surface. The prochirality of KPL in almost all bimolecular and termolecular, (KPL)2/(R)-NEA, complexation configurations is assigned as pro-R or pro-S using the contrast within the submolecularly resolved STM motifs. While the overall pr is relatively constant over wide ranges of ratios of KPL to (R)-NEA, pr values specific to individual complexation configurations vary strongly. The overall pr measured at low to medium ratios closely matches er values reported in the literature for the hydrogenation of KPL on a (R)-NEAmodified Pt catalyst at atmospheric pressure. The large set of data collected also permits an

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investigation of the contribution of arene-CH…O interactions to the formation of abundant complexes.

Keywords: Chirally Modified Platinum Surfaces, Heterogeneous Asymmetric Catalysis, Scanning Tunneling Microscopy, Ketopantolactone, (R)-1-(1-naphthyl)ethylamine, AreneCH…O Interactions, Prochiral Ratios, Surface Diastereomeric Complexes

Introduction

There is increasing progress in the design of chirally modified metal particles for heterogeneous catalytic enantioselective reactions,1-5 where modification of the catalyst surface is achieved by adsorbing a chiral molecule to create asymmetric reaction sites.6 The great potential of the approach is exemplified in recent work by Kobayashi and co-workers describing asymmetric carbon-carbon bond forming reactions using RhAg,7 Rh8 and AuPd9 nanoparticle catalysts modified with a secondary amide-substituted chiral diene. More extensive work has been carried out in the area of heterogeneous asymmetric hydrogenation.3,4 For example, several groups have investigated the hydrogenation of the C=C double bond of isophorone using proline-treated Pd catalysts, finding evidence, depending on the reaction conditions, for both kinetic resolution and heterogeneous enantioselective hydrogenation.5,10,11 The most investigated systems are tartaric acid modified Ni catalysts used for the hydrogenation of β-ketoesters,12 and cinchona alkaloid modified Pt and Pd catalysts. The latter two systems are used, respectively, for the hydrogenation of activated keto-carbonyl bonds3,6,13-17 and for C=C bond hydrogenation in α,β-unsaturated carboxylic acids.18 Recently, Moores et al. reported asymmetric hydrogenation on cellulose supported Pd in the absence of any additional chiral modification.19 The interesting level of new activity in the field is further evidenced by the report by van Leeuwen et al. on asymmetric hydrogenation of ketones on chiral secondary phosphine oxide capped Ir nanoparticles20 and the report by Morris et al. of transfer hydrogenation of ketones using chirally-modified Fe nanoparticles.21


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The rapid development of the field of heterogeneous asymmetric catalysis is greatly aided by the direct investigation of interactions between chiral modifiers and prochiral substrate molecules on catalyst surfaces.22-24 Notably, in-situ and operando infrared spectroscopy investigations by Baiker and co-workers have revealed NH…O hydrogen bonding interactions between ketopantolactone and chemisorbed cinchonidine on Pt catalysts.25 In parallel, model surface science studies are helping to reveal the combination of chemisorption and intermolecular forces operating in chiral modifier-prochiral substrate complexes on single crystal metal surfaces.26-35 In previous publications, we described the results of scanning tunneling microscopy (STM) and density functional theory (DFT) studies of diastereomeric complexes formed between α-ketoesters and (R)-NEA, (R)-1-(1naphthyl)ethylamine, on Pt(111). In particular, we reported investigations of noncovalently bonded diastereomeric complexes formed between (R)-NEA and methyl-3,3,3 trifluoropyruvate (MTFP)30,34 and between (R)-NEA and ketopantolactone (KPL).31,35 In both cases, N-H…O bonding to the modifier was found to play a key role in complexation, in agreement with the results of operando spectroscopy studies of KPL/cinchonidine25 and KPL/(R,S)-pantoylnaphthylethylamine36 complexes on supported Pt catalysts. In the present study, we seek to compare measurements made under reaction conditions with measurements made on the model surface science KPL/(R)-NEA /Pt(111) system.

Herein, we present a comprehensive STM investigation of the formation of KPL/(R)NEA diastereomeric complexes on Pt(111). The principal objective of the experiment is to compare STM determined prochiral ratio (pr) values to enantiomeric ratios (er) measured under reaction conditions and reported in the literature.36,37 We define the pr as the ratio of pro-R to pro-S complexation states of KPL in KPL/(R)-NEA/Pt(111) complexes, measured by visually inspecting large sets of STM data. The STM experiments were carried out at various temperatures from 237 to 250 K, where in each case the surface was first exposed to (R)-NEA and KPL, in sequence, at room temperature. While complete images of KPL monomers are not observed due to rapid diffusion, KPL is temporarily immobilized in complexes such that it can be resolved with sub-molecular resolution below 260 K. In contrast, all adsorbed (R)-NEA molecules can be observed. Hence, the ratio of KPL to (R)NEA used in this report refers to the number of KPL molecules observed (in complexes)


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relative to the number of (R)-NEA molecules observed. The high-resolution of the STM images allows us to inspect individual bimolecular and termolecular (KPL)n/(R)-NEA complexation configurations and, with the help of previously published DFT simulations of STM motifs,31 to quantify the contribution of individual pro-R and pro-S states to the overall prochiral ratio (pr) induced by (R)-NEA. The secondary objective of the study is to examine the role that CH…O bonding plays in determining the relative abundances of KPL/(R)-NEA complexation configurations on Pt(111). As in supramolecular organometallic asymmetric catalysis38 and organocatalytic enantioselective transformations,39,40 strong hydrogen bonding, such as NH…O interactions, plays a key role in stereocontrol on chirally modified catalysts.3 However, the role of typically weak interactions, such as CH…O hydrogen bonding, is less well appreciated, although surface science measurements strongly suggest that they should be considered.30,31,41 Indeed, structures of abundant diastereomeric complexes, taken from our work on KPL/(R)-NEA and MTFP/(R)-NEA, provide examples where multiple non-covalent interactions combining N-H…O and arene-CH…O bonding are in operation. The large amount of data acquired in this study permits the determination of the percentage of complexes formed through the interaction of KPL with (R)-NEA at the naphthyl group in the absence of any N-H…O bonding. The latter complexes are manifestations of arene-CH…OC bonding. There are several examples in the organic synthesis literature of stereocontrol through CH…O interactions.42-48 Furthermore, chemical physics experiments on gas phase clusters clearly show examples where CH…O interactions determine chiral recognition.49,50 Similarly, there are many examples in the literature on organic self-assembly on metal surfaces where CH…O interactions are determinant in the structures formed.41,

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Indeed, CH…O

interactions were used to design specific porous structures on Cu(111),56 to explain the formation of a two-dimensional quasicrystalline structure on Au(111),57 and to develop a method for the low temperature synthesis of graphene on Cu(111).58

The interpretation of the STM results draws from our previously published DFT calculations on the KPL/(R)-NEA system31 and new DFT results presented herein. Figs. 1 and 2, adapted from a prior publications,31,59 are used to facilitate the reader in the visual inspection the STM motifs of complexes, in relating the motifs to molecular structures, and


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in determining the prochirality of KPL in complexes. Fig. 1 shows the structures of chemisorbed (R)-NEA59 and KPL31 on Pt(111). The (R)-NEA structures shown in Figs. 1A and B, respectively, are the two conformers formed on adsorption: the exo-conformer, (R)NEA-1, and the endo-conformer, (R)-NEA-2. The DFT-predicted most stable adsorption geometry of KPL on Pt(111) is shown in pro-R and pro-S configurations in Figs. 1C and D, respectively.

Figure 1. DFT-calculated most stable chemisorption structures of (R)-NEA and KPL on Pt(111). (A, B): (R)-NEA is present in two different adsorption geometries, an exoconformation ((R)-NEA-1) and an endo-conformation ((R)-NEA-2). Adapted from 59. Copyright 2014, Elsevier. (C, D): Pro-S and pro-R adsorption configurations of KPl. Adapted from 31. Copyright 2015, Royal Society of Chemistry.

An STM image of a KPL/(R)-NEA-2 complex is shown in Fig. 2A. The same image is presented in Fig. 2B, using numbered circles to indicate three distinguishable protrusions in the STM motif of complexed KPL. The molecular complexation structure to which the image is assigned is shown in Fig. 2C. As discussed in detail in the previous publication,31 the three protrusions are roughly attributed, on the basis of a comparison to DFT simulated STM images, to the methyl groups (protrusion 1), the CH2 group (protrusion 2) and the ester


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moiety (protrusion 3), respectively. Using these assumptions, the prochirality of KPL in any given well-resolved complex can be inferred from the sub-molecular contrast of the STM motif.31 The prochirality indicates the enantiomeric product that would be formed by adding 2H from the surface to the enantioface turned towards the surface. KPL in the complex is turned towards (R)-NEA so as to interact with the amine group, through NH…O hydrogen bonding (to the ester-carbonyl for the complex shown in Fig. 2C). The ketocarbonyl, the prochiral functional group, is α to the dimethyl group (located by protrusion 1 in Fig. 2 B). Looking down on the STM motif shown in Fig. 2B, and applying the Cahn-IngoldPrelog priority rules, shows decreasing priority in the clockwise sense: -O, -C-O and -C-C. This analysis identifies the KPL motif in Fig. 2B as arising from pro-R KPL. In the present study, roughly 95% of the observed KPL/(R)-NEA motifs show the level of sub-molecular contrast required to assign the prochirality.

Figure 2. Illustration of the proposed relationship between the STM motif of a KPL/(R)-NEA complex and the molecular structure of the complex. (a) STM motif of a complex. Tunneling conditions: V = 1 V; it= 0.22 nA; T = 245 K. (b) The same motif is labeled to indicate three protrusions arising from complexed KPL: 1 is the brightest protrusion and 3 the dimmest protrusion. The brightest protrusion, 1, is attributed to the dimethyl group, based on DFTsimulated images.31 (c) The DFT calculated molecular and chemisorption structure of a complex that is consistent with the STM motif. KPL is in a pro-R configuration in the complex. Adapted from 31. Copyright 2015, Royal Society of Chemistry.

Experimental

The experiments were performed under ultrahigh vacuum conditions in a surface science apparatus equipped with a SPECS Aarhus STM-150 variable temperature scanning tunneling microscope. KPL (purity 97%) and (R)-NEA (optical purity 98%) were purchased


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from Sigma-Aldrich and further purified by pumping and freeze-thaw cycles in the gas handling vacuum line prior to dosing onto the clean surface held at room temperature. The Pt(111) crystal, purchased from MaTecK Gmbh, was cleaned in the chamber prior to each experiment by Ar+ (1.0×10-5 Torr) bombardment at 600 K and oxygen (2×10-7 Torr) treatment at 900 K followed by a flash annealing to 1000 K. After exposure to the gases at room temperature, the sample was cooled down to between 237 and 250 K prior to taking STM measurements. All images were acquired at a bias voltage of 1 V and at a constant tunneling current of 0.22 nA. WSxM image treatment software was used in the adjustment of brightness and contrast.60 Complexes formed between KPL and (R)-NEA were counted and catalogued using the accelerated procedure described previously.34 The accelerated procedure is solely based on a visual analysis of each complex and does not involve any direct use of image recognition algorithms on the raw data. The data were recorded in sequences of measurements on the same scanning area (apart from displacements due to thermal drift) at constant temperature, whereas in our previous study31 images were acquired by scanning different areas. The experiments were performed at a rate of one frame per 52.4 s, with an interval of 0.2 s for the tip to move back to the start point. Complexes are counted such that if a complex apparently appears in n consecutive frames it is taken as n observations. The choice of counting procedure was made in order to take into account the apparent lifetimes of specific complexes.

Density functional theory (DFT) calculations were performed for selected complexes where KPL interacts with the naphthyl group of (R)-NEA, distant from the ethylamine group. We use the grid-based projector augmented wave (GPAW)61,62 software with a gridspacing of 0.175 Å. Dispersive interactions are taken into account by use of the optB88vdW63 exchange-correlation functional. Periodic boundary conditions are used in the x- and y-directions, while the calculations are aperiodic in the z-direction (perpendicular to the slab) with a minimum of 6 Å of vacuum between the atoms and the unit cell boundary. All structures are relaxed with 2x2 k-points to a maximum force of 0.025 eV/Å. The complexation energies are calculated with the separated molecules on the surface as a reference. The parameter setup and reference is identical to those used for the calculation of complexes between KPL and the ethylamine part of NEA in Ref. 31 and the results in this


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publication are thus, in principle, directly comparable to the complexation energies calculated there.

Results

Fig. 3 is used to provide a qualitative introduction to STM images of ensembles of KPL/(R)-NEA/Pt(111) complexes. Fig. 3A and B presents large scan (15 nm x 10 nm) STM images acquired at 246 K, showing both isolated (R)-NEA and KPL/(R)-NEA complexes. Blue and red squares are used in Fig. 3A to demonstrate that the STM experiment can visually distinguish between the two chemisorption conformers, (R)-NEA-1 and (R)-NEA2,29,59 as also reported by Tysoe et al.28, 64-66 It may be seen that (R)-NEA-1 produces an STM motif that is quite distinct from the boot-shaped (R)-NEA-2 motif. The average value, over fifteen experiments, for the (R)-NEA-1:(R)-NEA-2 ratio was found to be 3:2 (Fig. 3C). This value, determined on the basis of the 46709 observations of 1118 (R)-NEA molecules is lower than the 7:3 ratio reported in a previous publication.29 These endo- and exostructures are formed on adsorption and only very rare events among the STM observations can plausibly be interpreted in terms of interconversion between the two conformers. A dotted contour is used in Fig. 3B to indicate the location of the ethylamine group, distinguishable as the brightest protrusion in the STM motif of (R)-NEA. The consecutive images in Fig. 3A, B show an example where KPL apparently moves from one (R)-NEA-1 to a close neighbour (R)-NEA-1. Otherwise, negligible exchange between bound and monomer KPL can be inferred from time-lapsed STM images taken at this temperature. In contrast, in experiments performed at room temperature, decomplexation occurs frequently on the time-scale of the experiment.35


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Figure 3. (A, B) Time-lapsed images of KPL/(R)-NEA on Pt(111) at 246 K. Blue and red squares are used to indicate (R)-NEA-1 and (R)-NEA-2, respectively. The arrow indicates the location of the ethylamine group of an (R)-NEA-2 molecule. The circle guides the eye to an apparent event where KPL moves from one (R)-NEA-1 to a near-neighbour (R)-NEA-1. The two (R)-NEA-1 molecules are rotated with respect to each other by 120° due the rotational symmetry of the surface (Fig. 1A). Tunneling conditions: V = 1 V; it= 0.22 nA; frame rate of one per 52.6 s. (C) Relative surface coverages of the (R)-NEA conformers as measured by counting from images acquired at eight different (increasing from 1 to 8) temperatures in the 237-250 K range.

Figs. 4(A-C) present STM images, taken at low, medium and high KPL to (R)-NEA ratios, respectively. The observed ratios, measured directly from many STM scans, refer strictly to KPL in complexes. The most striking change on going from low to high ratios is an increase in the abundance of termolecular, (KPL)2/(R)-NEA, structures. Tetramolecular, (KPL)3/(R)-NEA, complexes, formed at even higher ratios, either involve two KPL molecules bound at the ethylamine group and a third that is exclusively bound to the naphthyl group (Fig. 4I shows such a (R)-NEA-2 complex) or three KPL in proximity to the ethylamine group (Fig. 4F shows such a (R)-NEA-1 complex). In addition, some tetramolecular (KPL)2/((R)-NEA)2 complexes (Fig. 4J-L) are observed. In these, each KPL appears to be in proximity to both ethylamine groups, thereby forming a closed structure. The image in Fig. 4J involves two (R)-NEA-1 while Fig. 4K shows two (R)-NEA-2 . The closed structures likely involve simultaneous NH…O bonding of each KPL to both (R)-NEA-2 molecules.


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Fig. 4 : Large scan STM images (15 nm x 15 nm) for the coadsorption of (R)-NEA and KPL on Pt(111) at KPL to (R)-NEA ratios of approximately 0.2 (A), 0.8 (B) and 1.7 (C). (D-I) Selected images of complexes involving single (R)-NEA molecules. (J-L) Selected images of (KPL)2/((R)-NEA)2 complexes. Tunneling conditions: V = 1 V; it= 0.22 nA. The images were acquired at T = 240 K, apart from C which was acquired at 244 K.

It can be seen from Fig. 4 that in the vast majority of complexes, KPL is located at the bright protrusion of the (R)-NEA image. That is, KPL interacts with the ethylamine group, and complexation is driven by the formation of NH…O bonds. However, the observation of complexes where KPL interacts exclusively with the naphthyl group, distant from the ethylamine group, such as in the tetramolecular complex shown in Fig. 4I, implies the existence of structures that are stabilized only by arene-CH…O interactions. 196 out of 15686 observations of KPL in bimolecular complexes are due to binding to (R)-NEA-1 exclusively on the naphthyl group distant from the ethylamine group. Similarly, 139 out of 15467 KPL data points for bimolecular complexs are due to exclusive bonding to the


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naphthyl group of (R)-NEA-2. DFT calculations were performed (Fig. 5) to estimate the binding energy of KPL to the naphthyl moiety of (R)-NEA-1. The predicted values of 0.21 eV are attributed to an ester-carbonyl interaction with a naphthyl CH donor.

Figure 5. DFT calculated complexes, and complexation energies, for structures where KPL interacts uniquely with the naphthyl group of (R)-NEA.

Fig. 6 shows the fractions of (R)-NEA-1 and (R)-NEA-2 participating in 1:1, 2:1 and 3:1 complexes, relative to the total population of the conformer, as a function of the KPL to modifier ratio. A greater fraction of (R)-NEA-2 is in complexes over the entire range. Indeed, (R)-NEA-2 forms the majority of complexes at low ratios (≤0.5) even though it is the minority conformer.31 Abundant termolecular complexes (abundance greater than 10%) are only consistently seen for ratios above ~0.8. Tetramolecular complexes play a significant part only at very high KPL to (R)-NEA ratios.

Figure 6. The fraction of (R)-NEA-1 (blue) and (R)-NEA-2 (red) in complexes, relative to the total population of (R)-NEA-1 and (R)-NEA-2, respectively, as a function of the KPL to (R)-


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NEA ratio observed by STM. The fractions in 1:1, 2:1 and 3:1 (KPL)n/(R)-NEA complexes are indicated by progressively lighter shades of blue and red. The black line indicates the measured ratio of KPL to (R)-NEA (right axis). The data were measured in the 237-250 K temperature range.

STM images of the most abundant bimolecular KPL/(R)-NEA complexation configurations are shown in Fig. 7. In order to facilitate the discussion of the complex multiconfiguration system, a labeling system and a schematic depiction of the STM motifs is also included in Fig. 7. In the schematic depiction, the two (R)-NEA chemisorption conformers are drawn using blue and red ellipses to represent the naphthyl groups of (R)-NEA-1 and (R)-NEA-2, respectively. Yellow ellipses are used to represent the ethylamine group. The three-protrusion images of complexed KPL are schematically represented by the combination of a yellow circle (the brightest protrusion), a grey ellipse (the second brightest protrusion) and a black ellipse (the least bright protrusion). These protrusions are indicated by 1, 2 and 3, respectively, in Fig. 2B. The three protrusions are visible in roughly 95% of the observed complexes, permitting the determination of the prochirality of KPL in each case.

The labeling system for the complexation configurations shown in Fig. 7 specifies the prochiralty as (S)- or (R)-, the modifier conformation as 1 or 2, and the position (left (L), right (R) or top (T)) of the substrate with respect to the ethylamine group. A subscript is used in cases where there is significant spatial overlap between two complexation configurations. For example, (S)-2R1 specifies the pro-S configuration of the KPL in a complex with (R)-NEA-2 on the right-hand side of ethylamine group. The schematic depictions roughly indicate the connectivity between the substrate and the modifier. For example, for the complexes labeled (S)-2R2 and (R)-2R2, only the ester-carbonyl (roughly located by the black ellipse in the schematic depiction) is in proximity to the modifier. In contrast, both the keto- and the ester-carbonyls are in proximity to the modifier in (R)-2R1. We assume that the KPL/(R)-NEA-2 structure shown in Fig. 2C is (R)-2R2.


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Figure 7. (Left Panel) The ten most abundant STM motifs observed for KPL/(R)-NEA bimolecular complexes on Pt(111). A schematic illustration of the STM motif is included in each case. The naphthyl group of (R)-NEA is illustrated by blue and red ellipses for (R)-NEA1 and (R)-NEA-2, respectively. A yellow ellipse is used to represent the ethylamine group. The three-protrusion STM motifs of KPL in complexes (described in Fig. 2) are represented in yellow (the methyl groups), grey (the CH2 group) and black (the ester moiety). The complexes are labeled to specify, in sequence, the prochirality of KPL, the conformation of (R)-NEA, and whether KPL is located to the right, top or left of the ethylamine group. The label for the pro-R KPL/(R)-NEA-2 configuration (R)-2R2 is distinguished from that for the spatially overlapping configuration (R)-2R1 by the subscript. (Right Panel) Schematic illustration of the ten most abundant STM motifs for KPL/(R)-NEA complexes. Tunneling conditions: V = 1 V; it= 0.22 nA. The images were acquired at 248 K.

The set of ten configurations shown in Fig. 7 is chosen by imposing a threshold of 5% or greater for the relative abundance of a specific configuration at any KPL to (R)-NEA ratio. For example, (R)-1T is included as its relative abundance is 8% at KPL to (R)-NEA ratios below 0.4, even though its abundance over the entire range of ratios is only 3%. The relative abundances of the selected complexes, in an ensemble of 31153 STM observations on all 1:1, 2:1 and 3:1 complexes over the entire range of ratios, are (R)-1R (18%), (R)-1L (10%), (S)-1L (6%), (S)-1R (5%), (R)-1T (3%), (R)-2R1 (6%), (R)-2R2 (14%), (S)-2R1 (18%), (S)-2R2 (2%) and (S)-2L (4%). There is a number of other less abundant configurations, and their combined fraction over all observations is 7% for (R)-NEA-1 and 7% for (R)-NEA-2.


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Figure 8. The most abundant STM motifs observed for termolecular (KPL)2/(R)-NEA complexes on Pt(111). The STM motifs are presented in order of decreasing abundance from left to right for the combined data on (R)-NEA-1 and (R)-NEA-2 complexes. Tunneling conditions: V = 1 V; it= 0.22 nA. The images are acquired in the 237-246 K temperature range.

Labeled STM images of termolecular complexes are shown in Fig. 8. In general, the termolecular complexes place KPL to the left- and right-hand sides of the ethylamine group. The exception is the (S)-1R+(R)-1T complex. All of the termolecular structures are combinations of two of the bimolecular configurations shown in Fig. 7. For example, the pro-R states (R)-1R and (R)-1L are the most abundant configurations in (R)-NEA-1 bimolecular complexes and, together, they form the most abundant termolecular (R)-NEA-1 complexes. This close correspondence presumably arises because (R)-1R and (R)-1L localize KPL at opposite sides of the ethylamine group thereby avoiding steric hindrance. The most abundant termolecular (R)-NEA-2 structure combines configurations (S)-2R1 and (S)-2L. The (S)-2L state only becomes abundant when the right-hand states are saturated, that is at high KPL to (R)-NEA ratios. This observation is in agreement with our published DFT predictions of complexation energies,31 which show that that KPL is bound more strongly on the right-hand side of the ethylamine group of (R)-NEA-2.

Fig. 9 shows how the relative abundances of the ten most abundant configurations vary over five selected intervals of KPL to (R)-NEA ratio. Data for 1:1, 2:1 and 3:1 (KPL)n/(R)-NEA complexes are included together. The data, presented separately for the two (R)-NEA conformers and for pro-R and pro-S states, permit an analysis of how multiple


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ACS Catalysis

prochiral states contribute to the overall pr, defined as the pro-R:pro-S ratio for all ethylamine bonded KPL. The right-hand side of the ethylamine group of (R)-NEA-2 is a privileged general position for binding KPL, forming the (R)-2R2 (S)-2R1 and (R)-2R1 configurations. Together, they produce a small pro-S excess at ratios of 0.7 to 1.2. However, since the ratio of (R)-NEA-1:(R)-NEA-2 is 3:2, the contributions of the (R)-1R, (R)-1L and (R)-1T configurations combined with that of (R)-2R2 and (R)-2R1 leads to an overall pro-R excess.

Figure 9. Relative abundances of KPL/(R)-NEA complexation configurations for five ranges of KPL to (R)-NEA ratio. The different complexation configurations are indicated by the color-codes shown in the inserts. The ranges of ratios are, from left to right in each case, approximately [

STM Study of Ketopantolactone - ACS Publications - American

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