Chiral Modification of Rh and Pt Surfaces: Effect of Rotational

Feb 21, 2008 - The adsorption behavior of cinchona-type chiral modifiers on model rhodium and platinum catalysts has been studied using attenuated tot...
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J. Phys. Chem. C 2008, 112, 3866-3874

Chiral Modification of Rh and Pt Surfaces: Effect of Rotational Flexibility of Cinchona-Type Modifiers on Their Adsorption Behavior Erik Schmidt, Davide Ferri, Angelo Vargas, and Alfons Baiker* Institute for Chemical and Bioengineering, ETH Zurich, Ho¨nggerberg, HCI, CH - 8093 Zurich, Switzerland ReceiVed: NoVember 15, 2007; In Final Form: January 7, 2008

The adsorption behavior of cinchona-type chiral modifiers (β-isocinchonine, cinchonidine, and cinchonine) on model rhodium and platinum catalysts has been studied using attenuated total reflection infrared (ATRIR) spectroscopy and density functional theory (DFT). The ATR-IR studies performed under conditions closely resembling those encountered during enantioselective hydrogenation as well as in the absence of hydrogen revealed significant differences in the adsorption mode of the modifiers, depending on the rotational flexibility of the modifier, the platinum group metal, and the presence of hydrogen. The more rigid β-isocinchonine showed preferential tilted adsorption on both metals, independent of the presence or absence of hydrogen. Cinchonidine and cinchonine were adsorbed on Pt predominantly with the quinoline ring nearly parallel to the surface, irrespective of the presence or absence of hydrogen, whereas on Rh in the presence of hydrogen only tilted species were observed. The latter behavior is attributed to the fast hydrogenation of the aromatic anchoring group (quinoline) on Rh that interferes with the adsorption process. DFT calculations confirmed that the energy difference between tilted and adsorbed species decreases in the case of the β-isocinchonine. This combined spectroscopic and theoretical investigation indicates a greater bias toward the tilting of the anchoring group in the case of β-iCN as compared to the natural occurring alkaloids cinchonidine and cinchonine. Such behavior explains the greater stability toward quinoline ring saturation observed for β-isocinchonine on rhodium-supported catalysts during enantioselective hydrogenation. This study thus helps define the delicate equilibrium between formation of chiral surface sites and their stability in strongly reducing conditions in terms of the structure and consequent adsorption behavior of the modifier molecule, as a function of the metal catalyst of choice.

Introduction Cinchona alkaloids such as cinchonidine (CD) and cinchonine (CN) are chiral alkaloids bearing a complex but well-studied stereochemistry.1-3 They are readily accessible in nature in relatively large amounts (chiral pool) and have found since the nineteenth century wide application in medicine as antimalaric drugs and more recently as efficient chiral auxiliaries for enantioselective catalysis.4,5 In particular, in the field of heterogeneous enantioselective catalysis they are used as chiral modifiers in the Orito reaction,6 that is, the hydrogenation of activated CdO bonds over chirally modified supported Pt6-10 and Rh11-20 catalysts. Also, CdC bonds can be enantioselectively hydrogenated over supported Pd21-29 catalysts in the presence of such alkaloids, which makes the cinchona-modified platinum group metals the most efficient and promising heterogeneous catalysts for asymmetric hydrogenation.30 Nonetheless, important differences exist within these catalytic systems. Modified Pt/Al2O3 is the most studied catalyst for the Orito reaction and affords excellent enantioselectivity with remarkable rate enhancement, although the understanding of the reaction mechanism is not yet complete.31-34 On the contrary, Pd- and Rh-based asymmetric heterogeneous catalysts are not yet as efficient as chirally modified Pt and their homogeneous counterparts35 and are also characterized by a lesser degree of mechanistic understanding. It has been shown31 that the critical structural features of cinchona alkaloids consist of three conceptually different * Corresponding author. E-mail: [email protected].

functional units: (i) the quinoline ring, acting as anchoring unit necessary for binding the molecule to a metal surface, (ii) the quinuclidine group, bearing a tertiary amine likely responsible for binding the substrate, and (iii) a stereogenic region localized at the C(8) and C(9) atoms. Changing the absolute configuration at these two stereogenic carbon atoms results in the inversion of the sense of enantiodifferentiation (Figure 1) in the asymmetric hydrogenation of activated ketones.6 Another fundamental aspect of cinchona alkaloids is their conformational complexity that has been extensively addressed using theoretical and experimental methods.36-38 The topology of the surface chiral sites largely depends on the conformation assumed by the modifier upon adsorption;39,40 furthermore, hydrogen exchange with the metal has been shown to be allowed by the conformational degrees of freedom of the adsorbed alkaloid.41 In the past years, some structural isomers of CN, namely R-isocinchonine (R-iCN) and β-isocinchonine (β-iCN), obtained by thermal ethereal cyclization of CN in strong acids, have been tested as surface modifiers in asymmetric reactions.42-48 At first, their use was restricted to the assessment of the hypothesis that open conformers (conformers where the quinuclidine N-atom points away from the quinoline anchoring moiety) of the alkaloids were implicated in the mechanism of enantiodifferentiation, because such conformers lose one rotational degree of freedom and are constrained in an open conformation.42,43 At this stage, it seemed that their performance as modifiers (on platinum) was analogous, only with lesser efficiency, compared to the parent natural alkaloid. Later, it was recognized that β-iCN

10.1021/jp7108994 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/21/2008

Rotational Flexibility of Cinchona-Type Modifiers

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Figure 1. Molecular structure of cinchonidine (CD), cinchonine (CN), and β-isocinchonine (β-iCN) in their minimum energy structure. The arrow indicates the direction of rotation used to draw the graph, displaying the dependence of the energy (PM3) upon rotation of the torsional angle τ (C(4a′)-C(4′)-C(9)-C(8)) of β-iCN.

exhibits significant peculiarities with respect to CN when used as chiral surface modifier. Inversion of the absolute configuration of the hydrogenation product occurred when changing the solvent from toluene to acetic acid, using β-iCN as modifier in the Pt-catalyzed hydrogenation of ethyl pyruvate,44,46 ketopantolactone,48 and several esters of phenylglyoxylic acid.47 This effect was not observed for CD. More recently, it was found that on platinum catalysts β-iCN is less efficient concerning achievable enantiomeric excess (ee) than CD and CN,45,47 whereas on rhodium catalysts β-iCN performs remarkably better than the natural alkaloids.49 β-iCN appears more stable under the strongly reducing conditions compared to CD and enables low modifier/substrate ratios compared to platinum catalysts. The anchoring moiety of the latter is hydrogenated thus causing a loss of the surface chiral sites, whereas β-iCN resists hydrogenation even on rhodium and allows more efficient surface modification on this metal. This interesting behavior is likely to originate from the structural differences between CD, CN, and β-iCN, and in particular the higher rigidity of the oxazatwistane structure of β-iCN (Figure 1). The numerous conformational changes possible for CD and CN are reduced to a single degree of freedom, namely rotation around C(4′)-C(9). This rotation is further hindered upon adsorption; therefore the number of relevant surface processes is strongly reduced compared to the conformationally more flexible CD and CN. To the best of our knowledge, only a recent mechanistic investigation involving theoretical calculations and NMR spectroscopy exists on this topic, mainly dealing with solution and gas-phase species.50

Insight into the adsorption behavior of β-iCN on platinum-group metals is not yet available. Several characterization methods have been used to elucidate the adsorption and reactivity of cinchona alkaloids at metal surfaces.51-55 Attenuated total reflection infrared (ATR-IR) spectroscopy56 has been shown to deliver valuable information about the structure of molecules at the metal-solution interface. Most ATR-IR investigations of the mechanism of the Orito reaction concerned the study of the adsorption of CD57-60 and its derivatives61,62 on supported Pt model catalysts. The present study aims at determining the adsorption geometry of β-iCN on rhodium and platinum using ATR-IR spectroscopy. The adsorption behavior and reactivity toward hydrogenation of the quinoline anchoring moiety of β-iCN will be compared to corresponding properties of CD and CN. We have shown that CD is much more prone toward hydrogenation on Rh/Al2O3 films63 than on Pt and that additional equilibria to the ones observed for intact CD on Pt are generated on the Rh surface by the presence of hydrogenated species. Density functional theory (DFT) calculations on model rhodium and platinum surfaces39,64-66 are performed in the present work to complement the information obtained by experiment and to determine submolecular details of the adsorbed species. Main differences between the behavior of the more rigid modifier β-iCN and that of the classical natural cinchona modifiers, CD and CN, will be elucidated and discussed in the context of the known performance of these modifiers in enantioselective hydrogenation.

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TABLE 1: Assignment of the Major Normal Modes of β-iCN in Solution, Adsorbed (Rh and Pt) and Calculated (see Experimental for Details)a β-iCN

CD

solutionb

calcc

on Rhd

n.o.e

n.o. 1606 1581 1558

n.o. 15953 1572(1)3 n.o. 15113 1495 1475

1594 1572 1509 1486 1474

1498 1481 1471 1467

on Ptd

solutionb

assignment

n.o.

1635

15943 15701,2,3 15322 15113 1495 1473

1593 1570

ν(CdC), vinyl ring stretch, QN ring stretch, QN ring stretch, QN ring stretch, QN ring stretch, QN δ(C-H), QD δ(C-H), QD δ(C-H), Me δ(C-H), QD δ(C-H), QD + Me δ(C-H), QD + Me δ(C-H), QD ring stretch, QN δ(C-H), QD δ(C-H), CH2 ring stretch, QN δ(C(9)-H) + ring stretch, QN δ(C-H), Me δ(C(9)-H) + ring stretch, QN

1509

1463 1460

1457 1454 1449 1446

1460

1460 1454

1418 1370 1352

1432 1412 1377 1368 1358

n.o.

n.o.

1355

n.o.

a

The assignment of the major modes of CD in solution is also reported for comparison. For adsorbed β-iCN, two major species are observed. 1 ) flat adsorbed, 2 ) R-quinolyl, and 3 ) N-lone pair bonded species. See text for details. bIR spectrum of a 10 mM solution in CH2Cl2. c B3LYP-6-31G(d,p), scaling factor of 0.962; dAdsorbed from a H2-saturated solution in CH2Cl2, 0.1 mM. eNote that β-iCN does not bear any vinyl group. Vibrational mode description using the following notation: QN, quinoline group; QD, quinuclidine group; Me, methyl group; n.o., not observed; νs, symmetric stretching; νas, asymmetric stretching; δ, deformation.

Experimental Section Materials. β-isocinchonine (β-iCN) was synthesized according to a method described in literature45 with some crucial changes in reaction and separation conditions49 (99.5% by GC/ MS, no cinchonine present). Cinchonidine (CD, Alfa Aesar, 99%) and cinchonine (CN, Fluka, >98%) were used as received. Dichloromethane solvent (Baker) was stored over activated 5 Å molecular sieves. All gases were supplied by PANGAS (N2 99.995 vol %; H2 99.999 vol %). Rh (99.9%) and Pt (99.99%) wires as well as Al2O3 pellets (99.98%) used as targets for electron beam vapor deposition were supplied by Umicore. Thin Film Preparation. The metal/Al2O3 model films were coated on a trapezoidal Ge internal reflection element (IRE, 50 × 20 × 2 mm3, Komlas) by physical vapor deposition.67 The IRE was cleaned before each film deposition by polishing with 0.25 µm diamond paste and rinsing with ethanol. A BAE-370 (Balzers) vacuum system was used for evaporation of the target materials from a graphite crucible (8 kV; up to 0.13 mA for Rh and 0.15 mA for Pt) at a maximum base pressure of 1 × 10-5 mbar. First, a 100 nm thick film of Al2O3 was deposited at 1.0 Å/s followed by 1 nm of Rh (Pt) at 0.5 Å/s. The film thickness was measured using a quartz crystal microbalance. A detailed characterization of the Rh and Pt films and their suitability as model catalysts is available in the literature.63,67 Spectroscopy. ATR-IR spectra of the solid-liquid interface were recorded on a Bruker IFS-66 spectrometer equipped with a dedicated ATR accessory (Optispec) and a liquid nitrogencooled MCT detector. The coated IRE was mounted in a homemade stainless steel flow-through cell and maintained at 15 °C during the measurements. After cell mounting and optics alignment, the probe chamber was purged with nitrogen overnight to remove water and carbon dioxide. Adsorption studies on the solid-liquid interface were carried out after flowing N2-saturated CH2Cl2 (0.8 mL/min by means of a peristaltic pump) over the surface for about 90 min until stabilization of the signals was reached. Prior to adsorption of

the molecule under investigation, the metal surface was cleaned by flowing H2-saturated CH2Cl2 for 15 min, a procedure that was shown to be efficient for Pt,57,67 Pd,68 and Rh63 thin films. Adsorption of the chiral modifiers was studied by pumping a H2-saturated solution in CH2Cl2 (0.1 mM) through the cell. The solution concentration is chosen to provide only signals that are predominantly from adsorbed species. Stabilization of the IR signals related to the modifier was usually observed within 10 min, whereas about 30 min were necessary for β-iCN. Additional experiments were performed under the same conditions to explore the influence of the modifier concentration in solution (10-6 to 5 × 10-4 M) and of the atmosphere (N2 or H2). The interpretation of the infrared spectra is based on the previous study of CD adsorption on Rh/Al2O3.63 Computational Methods. Calculations of the chiral modifier adsorbed on rhodium (111) and platinum (111) were performed using the density functional theory ADF code.69 The Pt and Rh(111) surfaces were modeled by means of clusters of 38 metal atoms, as already shown elsewhere in more detail.39,40,65 During all calculations, the clusters were kept rigid to bulk parameters while all degrees of freedom of the alkaloid were set free to optimize. A zero-order regular approximation (ZORA) Hamiltonian was used to account for relativistic effects,70 together with relativistically corrected core potentials. The BeckePerdew exchange and correlation functional was used.71,72 Atoms were modeled using a frozen core approximation reaching 4f for Pt, 3d for Rh, and 1s for second row elements. The basis set used was double-ζ plus polarization functions for second row elements and double-ζ for platinum and rhodium, within an especially core-steep set of functions optimized for use with the ZORA formalism. Normal-mode analyses were performed with the Gaussian 03 set of programs,73 using density functional theory with a B3LYP hybrid functional71,74-76 and the 6-31G (d,p) basis set of Pople and co-workers.77,78

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Results and Discussion β-Isocinchonine in Vacuum. β-isocinchonine (β-iCN) is characterized by a reduced conformational flexibility with respect to the natural cinchona alkaloids. Figure 1 shows a computed model of the minimum energy conformer of β-iCN and a graphic of the dependence of the energy (as calculated within the PM3 semiempirical scheme)79,80 from the torsional angle τ. With respect to natural cinchona alkaloids, the torsional angle normally referred to as τ2 is lost for this molecule,39,40 because the quinuclidine moiety is not able to freely rotate around the C(8)-C(9) bond. Because of the energy profile shown in Figure 1, we expect to find only one conformer for this molecule, with a value for the torsional angle C(4a′)-C(4′)-C(9)-C(8) of about 100°. When the structure is refined in this minimum using the B3LYP method and the 6-31G(d,p) basis set, an optimal value of 74° is found for this dihedral. These results are in complete agreement with the structures proposed by other authors on the basis of both calculations and NMR experiments.46,81,82 The normal-mode analysis included in the present investigation was performed on this structure and the most relevant vibrational modes for the discussion are reported in Table 1 with the corresponding assignment. Adsorption on Rh. Before entering into the detailed presentation of the results, it is useful to recall the main spectroscopic features of the adsorption mode of cinchonidine (CD) on noble metals.55,57,83 These are (i) adsorption via the quinoline ring parallel to the surface plane (flat adsorption) strongly perturbs the spectrum of the alkaloid and is revealed by a single signal at ca. 1570 cm-1;57 (ii) adsorption via the quinoline ring tilted with respect to the surface plane (N-lone pair adsorption) results in a spectrum resembling closely that of the modifier in solution and thus exhibits signals at ca. 1610, 1590, and 1511 cm-1; and finally, (iii) adsorption via the quinoline ring tilted with respect to the surface plane and with R-hydrogen extraction (R-quinolyl species) generates a new signal at 1530 cm-1 compared to the solution spectrum. This species was observed on Pt57 but not on Pd68 and Rh.63 The spectroscopic investigation of the adsorption of CD on Rh revealed a complex surface chemistry that is matching reported catalytic experiments.63 The quinoline ring of CD quickly hydrogenates to 1′,2′,3′,4′,10,11-hexahydrocinchonidine on Rh so that only tilted species can be observed during adsorption. This is shown in Figure 2a by the presence of signals at 1593 and 1511 cm-1 that are stronger than that at 1572 cm-1. However, the flat species is the precursor of the hydrogenated product. The tilted species are most stable because the flat species are immediately hydrogenated and the loss of adsorption bond strength due to saturation of the anchoring moiety enhances desorption. During adsorption, the tilted species replaces the hydrogenated species on the metal surface.63 This process will be discussed later on for β-iCN as well. The ATR-IR spectra of β-iCN, CN, and CD adsorbed on Rh/ Al2O3 in the presence of hydrogen exhibit similar features (Figure 2a-c) and indicate a similar interaction mode with the metal surface. Spectral features common to all three adsorbed modifiers in the 1650-1500 cm-1 spectral region result from vibrational modes of the aromatic quinoline ring.57,83 The weakness of the signal at 1572 cm-1 compared to those at ca. 1590 and 1510 cm-1 suggests that for all modifiers flat adsorption is not dominant under these conditions. At a closer look the spectrum of β-iCN adsorbed on Rh (Figure 2c) exhibits some peculiarities. For instance, the ratio between the signals at 1595 and 1572 cm-1 (A1595/A1572) is

Figure 2. ATR-IR spectra of (a) CD, (b) CN, and (c) β-iCN adsorbed on Rh/Al2O3 from a 0.1 mM solution in CH2Cl2/H2. Spectrum (d) is the scaled transmission spectrum of a solution of β-iCN in CH2Cl2 (10 mM, 1:20).

clearly in favor of the high-energy signal (and of that at 1509 cm-1) more than in CD and CN and decreases in the order β-iCN > CN > CD. Therefore, the surface concentration of flat adsorbed species of β-iCN on Rh in the presence of H2 appears even lower than that of CD (and CN). Note that this also holds for CN with respect to CD. The spectrum of adsorbed β-iCN closely resembles that measured in transmission (Figure 2d) in the region of the quinoline signals, so that it can be concluded that adsorption on Rh in the presence of H2 does not perturb the quinoline ring. Hence, flat adsorption of β-iCN is not favored on Rh. Major differences are observed for the signals in the spectral region corresponding to the C-H deformation modes of the quinuclidine (oxazatwistane) ring (1500-1400 cm-1).57,83 This is the submolecular unit differing in the three cinchona alkaloids (Figure 1). For β-iCN (Figure 2c) two strong signals appear in the surface spectrum at 1475 and 1460 cm-1 together with an additional weaker feature at 1495 cm-1. The spectrum calculated at the B3LYP 6-31G(d,p) level of theory indicates that the signals at 1495 and 1475 cm-1 belong effectively to modes of the quinuclidine ring (δ(CH2)) showing a larger energy gap than in CD (Table 1, cf. with refs 57 and 83). The signal at 1495 cm-1 is additionally blue-shifted (∆ν ) 9 cm-1) with respect to the spectrum of β-iCN in solution and displays the same value as in the spectrum of the solid alkaloid, meaning that the quinuclidine ring can be considerably perturbed upon contact with the Rh surface. As in CD, the signal at 1460 cm-1 results from the overlap of modes very close in energy that belong to quinuclidine but also to the methyl group. The intensity of these signals is significantly increased upon adsorption compared to the signals characteristic of the quinoline ring in the transmission spectrum. Moreover, the signal at 1475 cm-1, which is only a shoulder of the signal at 1460 cm-1 in the transmission spectrum is enhanced in the adsorbed layer (note the increased A1475/A1495 ratio). This

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Figure 3. ATR-IR spectra taken after adsorption from a 0.1 mM solution in CH2Cl2 saturated with different gases. Bold spectra, inert atmosphere (N2); normal, hydrogen atmosphere; (a) CD, (b) CN, (c) β-iCN.

enhancement might be due to the influence of the absorption loss at ca. 1400 cm-1 (removal of CHx species formed from the decomposition of CH2Cl2 solvent on the clean metal),57,63 which can considerably decrease the intensity of the neighboring signal at 1460 cm-1. However, we prefer a second possibility, a preferential orientation as a result of the interaction with the metal surface. At least two modes (calculated at 1457 and 1454 cm-1) display a dynamic dipole moment oriented by more than 60° with respect to the metal surface and are therefore easily detectable on a surface.84 Interestingly, in a hypothetical flat adsorbed species, these two signals would be silent so that they can be readily assigned to β-iCN adsorbed with the quinoline ring tilted to the surface plane. The spectrum of β-iCN is also characterized by a strong broad feature at ca. 1355 cm-1 which might originate from the overlap of the umbrella mode of the methyl group and the deformation mode of the C(9)-H bond (Table 1). Although the direction of the dynamic dipole moment of the former mode indicates that it could also belong to a possibly flat species, the signal is more reasonably assigned to the tilted adsorbed species, as it will become clear later. If the signal belongs to the umbrella mode, the methyl group would need to be positioned above the skeleton of the molecule in the adsorbed species. During adsorption in the absence of dissolved hydrogen flat adsorbed CD starts to accumulate on the surface.63 Figure 3 shows the spectra obtained after 120 min adsorption time in the presence and in the absence (bold spectra) of H2 for the three modifiers. It should be noted that the unusual long time needed for stabilization of the spectral features in the case of the experiment where the modifier solution is saturated with N2 is a result of the slow removal (consumption) of residual H2 adsorbed during the cleaning step. The difference between CD and β-iCN (CN) is rather obvious. Although similar to CD, the signal at ca. 1572 cm-1 exhibits a slightly increased intensity for β-iCN (Figure 3c) and CN (Figure 3b) after adsorption from a N2-saturated solution. Adsorption of β-iCN (and CN) results

Schmidt et al. in the A1595/A1572 ratio in favor of the tilted geometry. The signals at ca. 1590 and 1511 cm-1 are slightly attenuated in the absence of H2, whereas their intensity is considerably reduced for CD compared to the experiment with H2 (Figure 3a). In essence, if there is striking evidence that flat adsorbed CD can populate the Rh surface, this species may as well exist for β-iCN under the same conditions but its surface concentration is much lower. Figures 2 and 3 show that β-iCN and CN strongly prefer the tilted adsorption geometry on Rh irrespective of the presence of H2. The large difference in the spectra of CD on Rh in the presence and in the absence of hydrogen reveals that hydrogenation of the quinoline ring occurs and competes with CD adsorption/desorption, as mentioned above. The slight difference in the spectra of β-iCN recorded for the same set of experiments (Figure 3) suggests that this process occurs for β-iCN as well but to a lesser extent. Intriguingly the different adsorption behaviors of the modifiers are also reflected in their resistance toward hydrogenation. It has been reported that CD is readily hydrogenated on Rh/Al2O3 catalysts,49 whereas β-iCN appears to resist more to hydrogenation. Additionally, chemoselectivity is also different; hydrogenation of the heteroaromatic ring is favored for CD, whereas the nonheteroaromatic ring appears favored with β-iCN. Interestingly, CN shows an intermediate behavior. Hydrogenation of Modifiers on Rh. As predicted by the spectra shown in Figure 3, during rinsing with H2-saturated solvent after adsorption, the hydrogenation of the quinoline ring of β-iCN was observed on Rh similarly to the process reported for CD.63 Hydrogenation of the heteroaromatic ring results in the disappearance of the signals at 1590, 1572, and 1511 cm-1 and in the evolution of a new signal at 1601 cm-1. Figure 4 shows that this occurs with both CD (Figure 4a) and CN (Figure 4b). The spectrum of β-iCN (Figure 4c), taken after 2 min under a flow of H2-saturated solvent, exhibits no significant intensity decrease for the signals at 1595, 1572, and 1511 cm-1 and for the modes at 1500-1450 cm-1. The signal at 1601 cm-1, which is the major evidence for the presence of the hydrogenation product of the quinoline ring, appears with a clear delay (after 3-6 min on stream). Note that after comparable time on stream, almost all CD or CN had been hydrogenated. The longer persistence of the signals of the tilted species of β-iCN indicates that on the Rh surface and under these conditions hydrogenation of the quinoline ring is significantly retarded, corroborating that β-iCN is more stable against hydrogenation than CN and CD. This observation is in perfect agreement with the preferential adsorption of tilted species rather than with flat species and strongly indicates that the former are considerably more stable on Rh and do not easily convert into flat species, opposite to the behavior of CD.63 Additionally, although the signals of β-iCN at 1595 and 1511 cm-1 were already stronger than in the case of CD, the persistence of the signals of the quinuclidine ring and the relatively high intensity of the signal at 1600 cm-1 compared to CD indicate that the hydrogenation product 1′,2′,3′,4′-tetrahydro-β-isocinchonine (TH-β-iCN) is relatively stable on the surface. This result agrees well with the absence of decahydro-β-isocinchonine in the catalytic experiments, but the presence of TH-β-iCN seems to stand in contrast with the observed chemoselectivity for quinoline hydrogenation with a preference toward 5′,6′,7′,8′TH-β-iCN.49 This apparent contradiction is explained by the fact that the possible 5′,6′,7′,8′-tetrahydro-isomer found for β-iCN in ref 49 is not clearly distinguishable in the ATR-IR spectra from the already present adsorbed species.57,63

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Figure 5. ATR-IR spectra of (a) CD, (b) CN and (c) β-iCN adsorbed on Pt/Al2O3 from a 0.1 mM solution in CH2Cl2/H2. Spectrum (d) is the scaled transmission spectrum of a solution of β-iCN in CH2Cl2 (10 mM, 1:40).

Figure 4. ATR-IR spectra taken after 0-10 min of rinsing previously adsorbed modifier layers with hydrogen saturated solvent. (a) CD, (b) CN, (c) β-iCN. The top panel shows the 1600-1440 cm-1 spectral region, whereas the bottom panel is a magnification of the features in the 1600-1570 cm-1 spectral region.

Although the spectra measured during desorption of CD and CN are at first glance very similar, a more detailed comparison reveals certain differences. After about 2 min of rinsing, only about half of the tilted CN (signals at 1511 and 1593 cm-1) was still present on the surface and the same holds for CD. At the same time, the flat species (1572 cm-1) had already vanished in both cases. The signal at 1460 cm-1 was noticeably attenuated in the case of CN, and the signal at 1601 cm-1 was of much weaker intensity for CN than for CD. In the spectra acquired after 2 min, it exhibits almost the same intensity as the signal at 1593 cm-1 in the case of CD, whereas for CN it is only a weak shoulder. This shows that the surface concentration of CN decreased faster and formation of the hydrogenation product was retarded compared to CD. This is likely the result of the lower adsorption strength of CN on Rh compared to CD, in agreement with the nonlinear effect observed on Rh/Al2O3 catalysts.49 Therefore, CN is primarily desorbed during rinsing, and hydrogenation occurs to a lesser extent. In summary, the hydrogenation experiments shown in Figure 4 indicate that the stability of the three modifiers on Rh follows the order tilted β-iCN > CN > CD. Although flat adsorbed β-iCN may appear on Rh as well (as indicated in Figure 3c)

the striking difference in the surface concentration with respect to CD suggests that β-iCN prefers a different adsorption geometry (tilted adsorption mode) and creates a chiral site with different environment. This point is interesting in view of the relatively high ee attained on Rh/Al2O3 with this modifier,49 indicating that these tilted species are likely to be involved in the enantiodifferentiation process. Adsorption on Pt. The data obtained on Pt, where flat adsorption is the dominant species in the case of CD55,57,83 and where hydrogenation of the quinoline ring is typically not observed, would imply that β-iCN is expected to prefer a more tilted adsorption. Similar to the adsorption on Rh, the ATR-IR spectrum of β-iCN on Pt (Figure 5c) exhibits signals of adsorbed β-iCN at 1594 and 1511 cm-1 that are stronger than the signal at 1570 cm-1. It is clear from Figure 5 that the ratio between the signals at 1590 and 1570 cm-1 decreases in the order, β-iCN > CN > CD. Moreover, the ATR-IR spectrum taken after adsorption on Pt closely resembles that on Rh during adsorption in the absence of H2 in which the flat adsorbed species is present but is not the dominant species (Figure 3). This observation demonstrates that the chiral sites generated by adsorption of β-iCN on Rh and on Pt are similar and correspond to the molecules adsorbed in the tilted geometry. Very interestingly, the distribution of surface species is not much in favor of the flat species in CN as well.85 Although the tilted species dominates the spectrum in Figure 5c, flat adsorption of β-iCN appears slightly more favored on Pt than on Rh and the R-quinolyl species appears as well (signal at 1532 cm-1). Also, the spectral features related to quinuclidine on Pt in the 1500-1400 cm-1 spectral region strongly resemble those in the corresponding transmission spectrum (Figure 5d) and change little compared to adsorption on Rh. The intensity of these signals is enhanced compared to the signals of the quinoline ring, which is again interpreted with an interaction between the quinuclidine ring and the metal surface. The higher

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Schmidt et al.

Figure 6. β-iCN adsorbed on a Pt-38 cluster. The most stable adsorption mode is shown in panel (a) but the energy difference with the tilted species is reduced compared to CD.

intensity of the signal at 1460 cm-1 on Pt compared to Rh may be associated with the fact that from the calculated spectrum of β-iCN in vacuum (Table 1) this signal can be associated with both tilted and flat species and because relatively more flat species are observed on Pt than on Rh the signal is slightly increased compared to that at 1473 cm-1. Concomitant to the change in distribution of species (relatively more flat and less tilted), the signal at ca. 1355 cm-1 that has been associated with the tilted species is considerably attenuated on Pt. Assuming that flat and tilted species of β-iCN may both contribute to enantioselection as hypothesized above, which appears feasible on Rh, on Pt these species appear less efficient than flat adsorbed CD in terms of ee.45-47 DFT Calculations of the Adsorption Modes of β-iCN on Pt and Rh(111) Model Surfaces. β-iCN was adsorbed on Pt and Rh clusters (Figure 6) to study the adsorption geometry and the relative energies of the resulting sites. Three adsorption geometries were found on platinum: a flat adsorbed mode (Figure 6a) and two tilted adsorption modes (Figure 6b,c) corresponding to the same minimum energy structure in vacuum but exposing a different face toward the surface. Only the flat and tilted(1) adsorption modes were found on rhodium. The Cartesian coordinates of these structures are included in the Supporting Information. Consistent with previous calculations on CD,39,40 the flat adsorption mode results more stable than both tilted adsorption modes. It is important to note that the energy difference between the flat and the tilted adsorption modes of β-iCN is 14 kcal/mol for Pt and 16 kcal/ mol for Rh (for tilted(1)). Previous studies at the same level of theory39,40 showed that for CD adsorbed on platinum the energy difference between the flat and the tilted species is of ca. 20 kcal/mol, meaning that the tilting of β-iCN is energetically more favored than in the case of CD. A similar result is found for calculations on rhodium model surfaces with the only difference being that for this metal the tilted(2) species turns into a flat species. Although the energies of flat and tilted species are closer for β-iCN, a difference by 14 kcal/mol also would be sufficient in theory to populate only the flat species. However, these calculated values do not account for the lateral interactions, typically due to steric or electrostatic repulsive interactions acting through space between two or more species.86 In addition, tilted species are favored by the lesser number of platinum sites with which they are bound, allowing a more efficient coverage. A flat species (Figure 6a) needs in fact seven platinum atoms for chemisorption, while the most stable tilted species (Figure 6c) only needs three. Although such energy terms cannot be quantified yet, it is reasonable to assume that they are constant in first approximation for both β-iCN and CD. Their influence

in tilting flat adsorbed species will then be greater the smaller the energy difference is between flat and tilted species. Therefore, it will be greater for β-iCN than for CD. This strongly supports the spectroscopic results indicating that for β-iCN the weight of the tilted adsorption modes increases (under real conditions of nonzero coverage) on platinum as well as on rhodium. According to the calculated spectrum of β-iCN in the vacuum (Table 1), tilted(1) may actually well represent the tilted species observed in the ATR-IR spectra more than tilted(2). It should be considered that for the enantioselective hydrogenation to occur, the quinuclidine N-atom needs to exchange hydrogen with the metal surface. This implies that the rigid quinuclidine ring of β-iCN can also rotate around C(4′)-C(9) thus orienting the N-atom toward the surface. If such rotation is considered, the enhanced signals of the quinuclidine ring (and of the methyl) on Rh and Pt compared to the solution spectrum (Figure 2 and Figure 5) would really suggest perturbation of the modes of the quinuclidine ring upon interaction with the surface as proposed above. Moreover, careful analysis of the structures of tilted(1) and tilted(2) may reveal that this rotation is actually favored for tilted(1). Rotation around C(4′)-C(9) to orient the N-atom downward for tilted(2) would cause repulsive interaction between hydrogen atoms located on the quinuclidine and on the quinoline rings. Correlation of Spectroscopic Findings and Catalytic Studies. The main results emerging from the previous paragraphs can be summarized as follows: (i) β-iCN is present on a Rh surface mainly as a tilted species. (ii) The relative proportion of tilted species of β-iCN on Rh is larger than that of CD and CN under similar conditions. This is also reflected in the hydrogenation behavior of the quinoline moiety of β-iCN compared to that of CD and CN. (iii) β-iCN persists longer on the Rh surface than CD and CN upon rinsing with H2 saturated solvent, indicating differences in the adsorption modes compared to CD and CN. (iv) Adsorption of β-iCN on Pt shows qualitatively similar, but less prominent tendencies (points i,ii) than observed on Rh. (v) DFT calculations indicate that the energy difference between tilted and flat adsorbed species is decreased for β-iCN as compared to CD. β-iCN-modified Rh shows higher enantioselectivity in the hydrogenation of ketopantolactone than CD-modified Rh, whereas the opposite behavior is observed on Pt.49 The described behavior is consistent with the higher stability of β-iCN on platinum group metal surfaces compared to that of CD and CN. This behavior explains the retention of catalytic activity of this modifier when used in asymmetric catalysis on Rh,49 as compared to CD and CN,45,47 as well as the nonlinear effects observed during catalytic studies.49

Rotational Flexibility of Cinchona-Type Modifiers Conclusion The use of β-iCN instead of the naturally available cinchona alkaloids (CD, CN) as chiral modifiers in platinum group metalcatalyzed enantioselective hydrogenation has triggered considerable interest in the adsorption behavior and stability of this modifier under reaction (hydrogenation) conditions. Our comparative study of the interaction of β-iCN, CD, and CN with Rh and Pt model catalysts by means of ATR-IR spectroscopy and DFT calculations indicates that the restricted rotational flexibility of β-iCN results in significant differences in both the adsorption behavior as well as the stability toward hydrogenation, depending on the metal. On Rh, β-iCN is mainly adsorbed as tilted species, where the quinoline anchoring ring is tilted toward the surface, independent of the presence or absence of hydrogen. This contrasts to the adsorption behavior of CD and CN on this metal, which is strongly affected by the presence or absence of hydrogen. In the presence of hydrogen, mainly tilted species were identified for CD and CN, whereas in the absence of hydrogen flat species were observed as well. The disappearance of flat species on Rh in the presence of hydrogen is attributed to their fast hydrogenation and desorption. The hydrogenation of the quinoline anchoring moiety is strongly retarded in the case of β-iCN, which is attributed to its higher rigidity, that prevents conformational changes, which are possible for CD and CN, and thereby sterically hinders the conversion of tilted to flat species, which is probably a requirement for hydrogenation of the aromatic ring. On Pt, β-iCN also adsorbs preferentially in a tilted mode, irrespective of the presence and absence of hydrogen. This contrasts with the behavior of CD and CN where both flat and tilted species are observed under both conditions. Platinum shows generally much lower reactivity toward quinoline ring saturation than rhodium, which explains the higher stability of flat CD and CN species on platinum. A corresponding relative destabilization of the flat with respect to the tilted adsorption geometry is further proven by DFT calculations. The equilibrium between flat and tilted adsorption is shifted to a higher surface concentration of tilted molecules under otherwise identical conditions. This behavior explains several unusual phenomena observed for β-iCN, therein the retarded hydrogenation of its anchoring quinoline moiety on Rh. The previously observed higher efficiency of β-iCN in asymmetric catalytic hydrogenations on Rh is likely due to its higher resistance toward hydrogenation. The conformational rigidity of this modifier favors tilted adsorption on Rh as well as Pt, whereas the higher rotational flexibility of CD and CN favors flat adsorption. However, flat adsorption of these modifiers on Rh in the presence of hydrogen is not observed due to fast hydrogenation and concomitant loss of adsorption strength of saturated species resulting in their desorption. Acknowledgment. Financial support from the Swiss National Science Foundation and the Foundation Claude and Giuliana is kindly acknowledged. The Swiss Center for Scientific Computing (Manno) and ETHZ are acknowledged for the computational resources. The authors thank A. Dutly for the synthesis of β-iCN. Supporting Information Available: The full references 69 and 73, and Cartesian coordinates of the structures shown in Figure 6. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lyle, G. G.; Keefer, L. K. Tetrahedron 1967, 23, 3253.

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