Understanding the Surface Chemistry of Enantioselective

May 14, 2009 - (S)-Glutamic Acid on Ag(100): Self-Assembly in the Nonzwitterionic Form. M. Smerieri , L. Vattuone , T. Kravchuk , D. Costa , and L. Sa...
0 downloads 0 Views 318KB Size
Download PDF
10706

J. Phys. Chem. C 2009, 113, 10706–10711

Understanding the Surface Chemistry of Enantioselective Heterogeneous Reactions: Influence of Modification Variables on the Interaction of Methylacetoacetate with (S)-Aspartic Acid Modified Ni{111} Karen E. Wilson and Christopher J. Baddeley* EaStCHEM School of Chemistry, UniVersity of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9ST United Kingdom ReceiVed: March 18, 2009; ReVised Manuscript ReceiVed: April 29, 2009

It has long been established that the adsorption of R-amino acids onto Ni surfaces results in the formation of catalysts capable of the enantioselective hydrogenation of β-ketoesters. Reflection absorption infrared spectroscopy (RAIRS) has been employed to investigate the chiral modification of Ni{111} surfaces by (S)aspartic acid as functions of modification temperature and pH. Correlations are drawn between the coverage and molecular conformation of aspartic acid on the modified model catalysts and the effectiveness of chirally modified Ni catalysts prepared under analogous conditions. In addition, we investigated the interaction of the simplest β-ketoester, methylacetoacetate, with chirally modified surfaces and identified how the tautomeric form of methylacetoacetate is strongly influenced by both modification pH and temperature. We discuss the implications of our findings for understanding the enantioselective behavior of aspartic acid-modified Ni surfaces. Introduction The increasing demand for enantiomerically pure chiral molecules for use as, for example, pharmaceutical products is a strong motivation for the development of enantioselective catalysts. The use of chirally modified surfaces to catalyze enantioselective reactions has been extensively studied for several decades - in particular the hydrogenation of R-ketoesters over modified Pt catalysts, and the Ni catalyzed hydrogenation of β-ketoesters.1-6 Despite this extensive research there are still unanswered questions regarding the mechanism of the surface catalyzed processes. Hydrogenation of the simplest β-ketoester, methylacetoacetate (MAA), over unmodified Ni yields a racemic mixture of (R)- and (S)-methyl-3-hydroxybutyrate (MHB). However, the modification from aqueous solution of Ni catalysts with R-hydroxy or R-amino acids results in the reaction becoming enantioselective3 (Figure 1a). The Ni or Pt surfaces are thought to provide sites for the dissociative adsorption of H2 to facilitate the hydrogenation reaction.1 In each case, the ketoester species are prochiral and close to planar. If the prochiral reagent is able to lie flat on the metal surface with the carbonyl group parallel to the surface, attack by H(ads) from underneath the >CdO functionality will produce a C-OH bond pointing away from the surface and establish a chiral center. Controlling which face of the molecule lies down on the surface controls the ultimate enantioselectivity of the reaction. In order to exclusively produce adsorption via one molecular enantiotopic face, one may envisage creating ordered arrangements of chiral molecules containing chiral pores of dimensions similar to the pro-chiral reagent. This was the basis of the Template model proposed by Wells in the Pt system which was subsequently ruled out as a likely mechanism.7 Alternatively, a 1:1 interaction (possibly via H-bonding) between the pro-chiral reagent and an adsorbed chiral molecule may be sufficient to energetically * Corresponding author. E-mail: [email protected].

favor one enantioface adsorption geometry. This type of interaction is the favored mechanism in the Pt/cinchona system.5 Though ultrahigh vacuum (UHV) surface science has provided much insight into chiral surface chemistry in the past decade, UHV is far removed from the liquid-solid interface environment employed in both the modification step and in the catalytic reactions. In order to address this issue, we have carried out a number of vibrational spectroscopic studies of chirally modified Ni surfaces prepared at the liquid-solid interface and of the interaction of MAA with modified surfaces. In both the Ni/tartaric acid8 and Ni/glutamic acid9 systems, we established that, under modification conditions where the catalyst operates most effectively for the production of (R)-MHB, the dominant tautomeric form of MAA on the modified surface is the diketo form. In the case of (S)-glutamic acid we also showed that, under conditions where the catalyst becomes enantioselective toward (S)-MHB, the enol tautomeric form of MAA dominates on the Ni surface.9,10 In this article, we use reflection absorption infrared spectroscopy (RAIRS) to study the adsorption of (S)-aspartic acid on Ni{111} from aqueous solution as functions of modification pH and temperature. (S)-aspartic acid differs by just one methylene unit from (S)-glutamic acid, yet it is considerably less effective as a chiral modifier11 exhibiting rather different correlations between catalytic behavior and modification conditions. The effect on catalytic behavior of varying the modification temperature and pH has been investigated by Izumi et al.11 It was shown that, although there was a decrease in enantioselectivity with increasing temperatures up to 348 K (Figure 1b), there was no evidence of a switch in enantioselectivity as exhibited by (S)-glutamic acid modified Ni catalysts.3 Izumi and coworkers11 also reported that (S)-aspartic acid modified Ni produces (R)-MHB in excess, with enantioselectivity showing a sharp maximum following modification at pH 5.0. In Figure 1c, the data pertinent to the pH behavior of the (S)-aspartic acid

10.1021/jp902430s CCC: $40.75  2009 American Chemical Society Published on Web 05/14/2009

Enantioselective Heterogeneous Reactions

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10707

Figure 1. (a) Reaction scheme of the enantioselective hydrogenation of MAA showing both tautomeric forms. (b) Effect of variation of modifying temperature on enantioselectivity of Raney Ni (RNi) using (S)-Aspartic acid (b) as a modifier. Hydrogenation of methyl acetoacetate was carried out at 60 °C. Adapted from ref 11. (c) Effect of modifying pH of (S)-aspartic acid (b) on enantioselectivity of Raney Ni (RNi). [Modifying conditions: 0 °C. Reaction conditions: MAA (neat), 60 °C, 80-100 kg/cm2] Adapted from ref 11.

modified Ni system are presented. These data have been extracted from the work of Izumi and co-workers.11 Experimental Section A mechanically polished Ni{111} single crystal was annealed at 1273 K for 8 h in a 5% H2/argon stream and allowed to cool, in the same atmosphere, to room temperature. The IR data were collected using a Digilab FTS7000 FTIR spectrometer with a liquid-N2-cooled mercury-cadmium-telluride detector. PEMRAIRS was used to analyze the cleanliness of the annealed Ni{111} single crystal surface by checking for any surface molecular contaminants. The sample was immersed in a 10 mM (S)-aspartic acid (Fluka Biochemika 97%) aqueous solution for 900 s at 300 K under constant agitation as a function of pH. The pH was altered by addition of 1 M NaOH or concentrated HCl. After modification a RAIR spectrum (256 scans) was recorded and ratioed against a background spectrum of the unmodified surface. Additionally, the rovibrational bands associated with small changes in gas-phase water levels in the beam path were subtracted from the spectra. The modified surface was then rinsed for several seconds in a flow of Millipore water (18.2 MΩ) and another RAIR spectrum was taken. The sample was subsequently immersed at 300 K for 900 s under constant agitation in a 50:50 mixture of MAA (Fluka g99%) in tetrahydrofuran (THF) and a further RAIR spectrum obtained. For RAIRS experiments carried out in the absence of the chiral modifier, the pH of the aqueous solution was adjusted by the addition of conc. HCl or 1 M NaOH. Results Adsorption of (S)-Aspartic Acid on Ni{111} as a Function of Modification pH at 300 K. Figure 2 shows the RAIR spectra of (S)-aspartic acid modified Ni{111} at 300 K as a function of solution pH. At pH 1.8, four main absorption bands are observed at 1741, 1656, 1518, and 1419 cm-1, with the most intense band appearing at 1656 cm-1. At pH 3.0, a broad band appears between 1750 and 1500 cm-1, centered at 1631 cm-1 with a

Figure 2. RAIR spectra following adsorption of 10 mM (S)-aspartic acid on Ni{111} at 300 K as a function of pH.

shoulder at 1712 cm-1. Treatment of the Ni{111} sample with (S)-aspartic acid at pH 4.6 and 7.4 results in the observation of similar bands to those exhibited in the spectrum following modification at pH 3.0 albeit with a weaker intensity. At pH 10.3, an absorption band at 1656 cm-1 dominates the spectrum, and bands emerge at 1712 and 1697 cm-1. The broad feature present at lower pH values disappears. Influence of Washing (S)-Aspartic Acid Modified Ni{111} Surfaces. Figure 3 illustrates the RAIR spectra obtained after washing the (S)-aspartic acid modified Ni{111} sample prepared at 300 K. The intensity of bands associated with (S)-aspartic acid adsorption decrease by a factor of at least 3. Indeed samples

10708

J. Phys. Chem. C, Vol. 113, No. 24, 2009

Figure 3. RAIR spectra following post modification water wash of (S)-aspartic acid modified Ni{111} at 300 K as a function of pH.

Figure 4. RAIR spectra following immersion of (S)-aspartic acid modified Ni{111} at 300 K in an MAA:THF solution at 300 K as a function of pH (MAA spectra ratioed against the spectrum taken following the washing of the modified model catalyst).

modified in the pH range 3-7 display few recognizable bands after washing. Interaction of Methylacetoacetate with (S)-Aspartic Acid Modified Ni{111}. Figure 4 displays the RAIR spectra following MAA adsorption onto the (S)-aspartic acid modified Ni{111} surface at 300 K. To aid in the identification of IR bands associated with adsorbed MAA compared to those of the chiral modifier, each single beam spectrum of the MAA exposed surface is ratioed against the single beam spectrum of the modified and washed surface. Positive bands are associated with a chemical change, desorption or reorientation of the modifier

Wilson and Baddeley

Figure 5. RAIR spectra following immersion of (S)-aspartic acid modified Ni{111} at 300 K in an MAA:THF solution at 300 K with no post modification water wash as a function of pH (MAA spectra ratioed against the spectrum of the (S)-aspartic acid modified Ni surface).

while negative bands signify adsorption of MAA. Following modification at pH 1.8, MAA adsorption is identified by a series of negative bands. There is a main band at 1760 cm-1, with a shoulder at 1728 cm-1, as well as a number of bands in the 1450-1150 cm-1 region. With increasing pH, the bands in the 1700-1800 cm-1 range decrease in intensity and new peaks at 1643 and 1251 cm-1 are observed and become the most intense features following exposure of MAA to the Ni{111} surface modified at pH 10.3. Figure 5 shows MAA adsorption at 300 K on (S)-aspartic acid modified Ni{111} at modification temperature of 300 K, with no post modification water wash. It can be seen that there are peaks present at 1757 cm-1 (with a shoulder at 1720 cm-1) at all values of modification pH, while peaks at 1660, 1637, and 1257 cm-1 emerge at higher pH. Figure 6 shows RAIR spectra following MAA adsorption at 300 K on unmodified Ni{111}. The aqueous solution pH was adapted accordingly using HCl or NaOH. At all values of modification pH employed, the major bands are observed at 1753 and 1720 cm-1 and there is only a weak band in the 1600 cm-1 range of the spectra. Figure 7 compares the immersion of Ni{111} modified with (S)-aspartic acid at pH 5 in a 1:1 MAA:THF solution at 300 K, as a function of modification temperature. At the lower temperatures of 300 and 350 K, there is no change in the major peaks present (1759 and 1726 cm-1). However, following modification at 373 K, the bands at 1643 and 1253 cm-1 become more prominent. Discussion Aspartic acid has an isoelectric point of 2.77 and pKa values of 2.1, 3.9, and 9.8, corresponding to the removal of protons from the carboxylic acid functionality of the amino acid, the aliphatic carboxylic acid, and the NH3+ group, respectively. Therefore, the experimental pH values used in this work were chosen to be lower or higher than each of the pKa values in order to control the nature of the dominant species in solution.

Enantioselective Heterogeneous Reactions

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10709 TABLE 1: Vibrational Assignments of Adsorption Bands (in cm-1) of (S)-Aspartic Acid Adsorbed from Aqueous Solution onto Ni{111} Compared to Solution Spectra of (S)-Aspartic Acid in H2O (ref 12) 300 K

Figure 6. RAIR spectra following immersion of unmodified Ni{111} at 300 K in an MAA:THF solution at 300 K as a function of pH.

Figure 7. RAIR spectra of Ni{111} modified with (S)-aspartic acid at pH 5 in a 1:1 MAA:THF solution at 300 K, as a function of modification temperature.

Figure 8. Speciation of aspartic acid. Adapted from ref 12.

Figure 8 shows the variation in the predominant aspartic acid species at increasing pH: from cationic through to dianionic. Roddick-Lanzilotta and McQuillan reported in situ IR studies of 100 mM aqueous (S)-glutamic acid solutions on a bare ZnSe prism as a function of pH.12 They also studied (S)-aspartic acid using similar experimental methods and found that the spectra closely resembled the solution spectra for glutamic acid (Table 1). The structural distinction of the R and distal carboxylic acid groups in aspartic acid being closer in space than in the case of

assignment

low pH

ν(CdO) of CdO group ν(CdO) of distal CdO group νasym(OCO-) of R OCO- group δasym(NH3+) δsym(NH3+) δsym(OCO-)

1741 1712

high pH

1631 1656 1516 1419

aspartic acid in H2O 13 1725 1720 1620 1600 1520 1400

glutamic acid causes a slight change in the spectrum as the two -COOH groups are spectroscopically more similar and so only a single peak is observed for the antisymmetric carboxylate stretch. We are thereby able to assign key bands associated with the different aspartate species, in conjunction with an infrared study by Pearson and Slifkin on amino acids and dipeptides.13 In addition, important IR bands for aspartic acid are approximately assigned from the Ni/glutamic acid studies.9 At dianionic pH values, the asymmetric and symmetric ν (OCO-) stretches of the amino acid carboxylate occur at ∼1600 and ∼1400 cm-1 respectively. At anionic pH values, the amine is protonated which gives asymmetric and symmetric δ(NH3+) bands at ∼1620 and ∼1520 cm-1. In its zwitterionic form, the distal carboxylic acid is protonated. This is identified by the presence of a band at ∼1712 cm-1, as well as a decrease in the ∼1400 cm-1 band. Finally, at cationic pH values, both carboxylate groups are in their protonated form. A band at 1722 cm-1 is due to a contribution from both COOH groups. Adsorption of (S)-Aspartic Acid at 300 K as a Function of Solution pH. The spectra illustrated in Figure 2 shows that there are clear differences between the (S)-aspartic acid species at pH 1.8 and 10.3. In the spectrum acquired following modification at pH 1.8, the most intense band at 1656 cm-1 corresponds to the asymmetric δ(NH3+) band. The peak at 1741 cm-1 corresponds to the carboxylic acid groups. The smaller peak at 1516 cm-1 can be assigned as the symmetric δ(NH3+) band. This confirms that, under highly acidic conditions, the (S)-aspartic acid adsorbate is in a cationic state. At pH 3.0, there is a change in the RAIR spectrum with a peak at 1712 cm-1 corresponding to the protonation of the distal carboxylic acid. At pH 4.6 and 7.4, aspartic acid is in its anionic form with the continuing presence of a peak at 1656 cm-1 (asymmetric δ (NH3+) band) as well as the emergence of broad peaks at 1631 cm-1 and 1419 cm-1 which have been assigned as the asymmetric and symmetric ν(OCO-) stretches of the amino acid carboxylate. The broadness of the peaks are due to contributions from the asymmetric ν(NH3+) band.9 At pH 10.3, bands corresponding to the ν(OCO-) stretches are still present, as well as the asymmetric δ(NH3+) band which suggests that, although it is expected that the dianionic species will be present, there are still contributions from the anionic form of aspartic acid. The RAIRS results are summarized in Table 1, alongside absorption bands recorded by Pearson and Slifkin of the IR solution spectra for aspartic acid in H2O.13 Effect of Washing the Modified Surface on the Nature of the (S)-Aspartic Acid Modified Ni(111) Surface. Figure 3 shows the RAIR spectra following post modification wash of (S)-aspartic acid modified Ni{111} at 300 K, with the band intensity decreasing by at least a factor of 3 across the pH range. At moderate pH values between 4 and 7 we are unable to detect

10710

J. Phys. Chem. C, Vol. 113, No. 24, 2009

the presence of modifier on the Ni{111} surface. In the catalytic preparation, a water wash is routinely employed to remove any excess modifier from the surface of nickel particles in order to leave submonolayer quantities of adsorbate on the surface. The issue of metal leaching in this catalytic system is important and it has been shown previously that leached Ni from a surface can itself be extracted and used as an effective hydrogenation catalyst.14 Recent work by Lambert and co-workers has shown that, in the proline-modified Pd-catalyzed chiral hydrogenation of isophorone, the key enantioselective step occurs homogeneously in the liquid phase and not on the Pd surface.15 In our case, there is a strong indication from the post modification washing step that there is a significant loss of modifier from the Ni{111} surface in all cases, and that at pH values between 4 and 7, the levels of aspartic acid remaining on the surface are essentially undetectable, i.e. a very small fraction of monolayer coverage. Under the very conditions where the catalytic enantioselectivity is optimized, the modifier coverage is minimized. If the reaction is catalyzed heterogeneously, there seem to be two plausible explanations for this behavior. Consider the case where two types of active site exist on the surface, one which is a “bare Ni” site and catalyzes the racemic reaction and one which is a chirally modified and enantioselective site whereby an MAA species interacts directly with an isolated aspartate species. One would anticipate that the enantioselectivity would increase with modifier coverage up to a point where the modifiers become relatively tightly packed and activity drops off. One explanation for the relatively high enantioselectivity at very low modifier coverages is that the influence of the modifier on the enantioface adsorption of MAA spreads much further than just a nearest neighbor interaction. Such chiral amplification effects have been reported following the coadsorption of (chiral) tartaric acid and (achiral) succinic acid on Cu{110}.16,17 A second explanation is that the corrosive adsorption of (S)-aspartic acid on Ni results in the formation of a high density of chiral step-kink defects which themselves act as enantioselective catalytic sites. There are many examples where chiral carboxylic acids have been shown to cause chiral faceting of metal surfaces.18-22 The faceting reported in these studies occurs at high molecular coverage and involves a chiral redistribution of the metal atoms. After washing, though the molecular coverage is low, it is likely that the chiral arrangement of metal atoms will be preserved. Step-kink defect sites have been shown to contribute to the enantioselectivity of Pt catalysts for ethyl pyruvate hydrogenation.23 Interaction of Methylacetoacetate with (S)-Aspartic AcidModified Ni(111). Figure 1a shows the reactant MAA molecule existing in its two tautomeric forms, enol and diketo. The -OCH3 group favors the diketo tautomeric form and the CH3 group favors the enol form. IR matrix spectroscopy and DFT calculations have been carried out recently to assign bands in the IR spectrum to the two different forms.24 In our RAIR spectra, the three bands for the enol form of MAA are observed at 1251 cm-1 (ν(C-OH)) and 1643 and 1668 cm-1 (coupled ν(CdC) and ν(CdO) bands). The diketo form has an intense band at 1760 cm-1 and a lower frequency shoulder at 1728 cm-1 (ν(CdO) bands). These band assignments are summarized in Table 2. Figure 4 illustrates the RAIR spectra following adsorption of MAA onto the modified Ni{111} surface at 300 K. At low pH, it is clear that the diketo form of MAA is the majority tautomeric form, with no evidence of IR bands corresponding to the enol form. With increasing pH, the enol contribution grows, culminating with a majority enol species present at pH

Wilson and Baddeley TABLE 2: Vibrational Assignments of Adsorption Bands (cm-1) of Adsorbed MAA from THF Solution onto (S)-Aspartic Acid Modified Ni{111} assignment

Ni{111}/(S)-Asp 300 K/MAA 300 K

ν(CdO)keto ν(CdO)keto ν(CdO)enol + ν(CdC)enol δsym(CH3) + δ(CH) + δ(CO-H)enol δsciss(CH2)keto ν(C-OH)enol + ν(CdO)enol ω(CH2)keto t(CH2) ν(C-OH)enol r(CH3) + δ(CH)

1760 1728 1668, 1643 1458 1440 1413 1369 1327 1251 1203, 1155

10.3 (the 1254 cm-1 peak is approximately 2.5 times the intensity of the diketo peaks). The results can be readily explained by the fact that the diketo form of MAA can act as a proton acceptor in forming a hydrogen bond with the protonated cationic aspartic acid modifier, whereas at higher pH the enol form of MAA is stabilized by acting as a hydrogen bond donor to the deprotonated aspartic acid form. The results are similar to those for glutamic acid at the same modification temperature.14 It is interesting that this effect is more dramatic on the washed modified surface than on the unwashed surface. In addition, the switch from diketo- to enol is solely associated with aspartic acid adsorption since modification at pH values controlled by addition of either NaOH or HCl (Figure 6) did not show any dependence on modification pH in the tautomeric form of MAA present on the surface. A comparison of the data presented in Figures 4 and 5 reveal that the washing of the model catalyst after modification influences the diketo:enol tautomeric ratio after exposure to MAA in THF. The higher surface coverage of aspartate seems to correlate with a greater tendency to form the diketo form. This may be one reason why aspartic acid is not as effective as a chiral modifier as tartaric acid. In the latter case, by controlling the modifying pH, it was possible to have control over the modifier coverage such that even after washing, there was sufficient modifier present on the surface to favor the formation of the diketo species whose presence seems to correlate with good enantioselective behavior.8 The optimum modification pH for the Ni/tartaric acid system was found to be pH 5.0, with ∼20% of a complete monolayer present.25 Figure 7 shows that, at modification temperatures around 373 K for aspartic acid at pH 5, there is a clear indication that the enol form of MAA is dominant. These can be compared to the findings of Jones et al. for glutamic acid modified Ni{111},9 in which the modification procedure that optimized catalytic enantioselectivity toward the production of (R)-MHB adsorbed MAA primarily via its diketo form, and via its enol form for the production of (S)-MHB. Therefore, control of the tautomeric form of MAA on the Ni{111} surface appeared to be directly related to the optimization of enantioselectivity of the catalytic reaction. The clear enantiomeric switch in the MHB product was shown to depend on modification temperature, with enol form appearing to be the precursor to the (S)-MHB (dominating at high modification temperature), and the diketo form the precursor to the (R)-MHB (dominating at lower modification temperatures). The previous hydrogenation study on aspartic acid11 does not show an enantiomeric switch, however experiments were only carried out between the range of 273-348 K, so our RAIRS results, which were executed at greater modifica-

Enantioselective Heterogeneous Reactions tion temperatures, show a decrease in the diketo:enol ratio enabling us to predict that aspartic acid modified Ni{111} might, after all, be expected to exhibit a similar switch in enantioselectivity with modification temperature as is observed for Ni/ glutamic acid systems. Conclusions The adsorption of (S)-aspartic acid from aqueous solutions as a function of modification pH on Ni{111} surfaces has been characterized by RAIRS. This allowed the identification of the different chemical forms of aspartic acid present as a function of pH. At room temperature and low pH, aspartic acid is in its cationic/zwitterionic form on the metal surface. As the pH increases, the amino acid ultimately deprotonates to its dianionic form. The subsequent adsorption of MAA onto the modified surface has been shown to be very reliant on modification pH, with the more protonated forms of aspartic acid causing diketone formation. Washing the modified surface has a dramatic effect on the molecular adsorbates depending on the modification pH employed. Of catalytic significance is the fact that under conditions where the catalyst operates most enantioselectively, the amount of adsorbed aspartate is below the detection levels of the RAIRS technique. If the Ni surface is involved in the catalytic steps, this implies that either chiral defects (produced by the corrosive adsorption of aspartic acid) are active sites for enantioselective surface chemistry or that the relatively low coverage of adsorbed aspartate somehow promotes the chiral reaction, perhaps via chiral amplification effects. It is certainly well-established that many different carboxylic acids have the ability to create chiral faceting on surfaces such as Cu(100).26 Finally, our work on the effect of modification temperature found that the diketo:enol ratio of MAA form present on the surface decreased until a switch to the enol form became apparent at temperatures of g373 K. We believe that it is likely that Ni catalysts modified by aspartic acid at ∼373 K would favor the (S)-MHB product as has been observed in the case of Ni/glutamate catalysts.3

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10711 Acknowledgment. We acknowledge funding from the Engineering and Physical Sciences Research Council (U.K.) to support this project. References and Notes (1) Webb, G.; Wells, P. B. Catal. Today 1992, 12, 319. (2) Baddeley, C. J. Top. Catal. 2003, 25, 17. (3) Izumi, Y. AdV. Catal. 1983, 32, 215. (4) Blaser, H. U. Tetrahedron-Asymmetry 1991, 2, 843. (5) Mallat, T.; Orglmeister, E.; Baiker, A. Chem. ReV. 2007, 107, 4863. (6) Baiker, A. Catal. Today 2005, 100, 159. (7) Simons, K. E.; Meheux, P. A.; Ibbotson, A.; Wells, P. B. Stud. Surf. Sci. Catal. 1993, 75, 2317. (8) Jones, T. E.; Baddeley, C. J. J. Phys. Chem. C 2007, 111, 17558. (9) Jones, T. E.; Rekatas, A. E.; Baddeley, C. J. J. Phys. Chem. C 2007, 111, 5500. (10) Jones, T. E.; Baddeley, C. J. Langmuir 2006, 22, 148. (11) Izumi, Y.; Imaida, M.; Fukuwa, H.; Akabori, S. Bull. Chem. Soc. Jpn. 1963, 42, 2373. (12) Roddick-Lanzilotta, A. D.; McQuillan, A. J. J. Colloid Interface Sci. 2000, 227, 48. (13) Pearson, J. F.; Slifkin, M. A. Spectrochim. Acta Part a-Mol. Spectrosc. 1972, A 28, 2403. (14) Keane, M. A.; Webb, G. J. Chem. Soc.-Chem. Commun. 1991, 1619. (15) McIntosh, A. I.; Watson, D. J.; Lambert, R. M. Langmuir 2007, 23, 6113. (16) Parschau, M.; Romer, S.; Ernst, K. H. J. Am. Chem. Soc. 2004, 126, 15398. (17) Parschau, M.; Kampen, T.; Ernst, K. H. Chem. Phys. Lett. 2005, 407, 433. (18) Bowker, M.; Poulston, S.; Bennett, R. A.; Stone, P. J. Phys.: Condens. Matter 1998, 10, 7713. (19) Chen, Q.; Perry, C. C.; Frederick, B. G.; Murray, P. W.; Haq, S.; Richardson, N. V. Surf. Sci. 2000, 446, 63. (20) Chen, Q.; Frankel, D. J.; Richardson, N. V. Langmuir 2001, 17, 8276. (21) Wang, H.; Zhao, X. Y.; Zhao, R. G.; Yang, W. S. Chin. Phys. Lett. 2001, 18, 445. (22) Zhao, X. Y. J. Am. Chem. Soc. 2000, 122, 12584. (23) Jenkins, D. J.; Alabdulrahman, A. M. S.; Attard, G. A.; Griffin, K. G.; Johnston, P.; Wells, P. B. J. Catal. 2005, 234, 230. (24) Belova, N. V.; Oberhammer, H.; Girichev, G. V. J. Phys. Chem. A 2004, 108, 3593. (25) Tai, A.; Sugimura, T. Chiral Catalyst Immobilisation and Recycling; de Vos, D. E.; Vankelecom, I. F. J.; Jacobs, P. A., Eds. Wiley-VCH: Weinheim, Germany, 2000; Chapter 8, p 173. (26) Zhao, X. Y.; Wang, H.; Zhao, R. G.; Yang, W. S. Mater. Sci. Eng. C 2001, 16, 41.

JP902430S