J. Phys. Chem. C 2011, 115, 1163–1170
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In Situ Surface-Enhanced Raman Spectroscopic Studies and Electrochemical Reduction of r-Ketoesters and Self Condensation Products at Platinum Surfaces † Neil V. Rees,‡ Robert J. Taylor, Yu Xiong Jiang, Ian R. Morgan, David W. Knight, and Gary A. Attard* Cardiff Catalysis Institute, School of Chemistry, Cardiff UniVersity, Main Building, Park Place, Cardiff, CF10 3AT United Kingdom ReceiVed: July 13, 2010; ReVised Manuscript ReceiVed: NoVember 8, 2010
The adsorption and hydrogenation of ketopantolactone (KPL) and ethyl pyruvate (EP) on Pt electrodes, measured by surface-enhanced Raman spectroscopy, are compared. In addition, pure samples of selfcondensation products of ethyl pyruvate including a dimer (in both linear and cyclized forms) and a lactone have been synthesized. These dimeric and aldol condensation intermediates have previously been proposed by ourselves as playing a crucial role in explaining the significant rate enhancement observed during enantioselective hydrogenation of R-ketoesters at supported platinum catalysts. The adsorption of the linear dimer at platinum leads to cyclization. At hydrogen evolving potentials in dimer-free aqueous sulfuric acid, the dimer may be hydrogenatively desorbed readily from the electrode surface. KPL is found to accumulate at the electrode surface under hydrogen evolving conditions. From comparison with the behavior of EP under similar hydrogenating conditions, it is deduced that self-condensed dimers do not form when EP is being either hydrogenated using gas phase hydrogen or electrocatalytically hydrogenated in aqueous sulfuric acid. In addition, unlike EP, KPL does not form a long-lived half-hydrogenated state surface intermediate, a significant contributory factor in rate enhancement observed when EP is hydrogenated at supported platinum catalysts. Hence, the role of cinchona alkaloids in establishing rate acceleration cannot be ascribed simply to a destabilization of self-condensation products of ethyl pyruvate under reaction conditions. Models to explain the relative degrees of rate acceleration in KPL and EP are discussed. Introduction The field of heterogeneous chiral catalysis has experienced significant growth over the past 30 years, driven in major part by the pioneering work of Alfons Baiker and others.1–3 The asymmetric hydrogenation of prochiral R-ketoesters by means of chirally modified supported platinum catalysts has been of particular interest, since the discovery by Orito et al. of the hydrogenation of methyl pyruvate (MP) to (R)-methyl lactate on cinchonidine-modified platinum.4 Although the obvious advantages of utilizing heterogeneous catalysts in asymmetric catalysis (for example ease of separation of the catalyst from enantiopure product) is a major attraction, the enantiomeric excesses (ee) reported have generally not reached a sufficient level so as to compete with existing homogeneous catalysts, although under optimum conditions, ee in excess of 95-98% have been obtained.1–5 The Orito reaction is usually carried out at room temperature and elevated pressures with catalytic amounts of the alkaloid modifier using either toluene or acetic acid as solvent. Despite many studies, however, no consensus has been reached on a single mechanism that can fully explain the entire range of reported phenomena associated with this reaction.7–22 Several mechanisms have been proposed for the Orito reaction. The 1:1 modifier-substrate model due to Wells11 and Baiker7 assumes that hydrogen bonding occurs between the †
Part of the “Alfons Baiker Festschrift”. * Corresponding author. E-mail:
[email protected]. Tel: +44 (0)29 2087 4069. Fax: +44 (0)29 2087 4030. ‡ Present addresses: Deptartment of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ United Kingdom.
prochiral carbonyl (on the R-ketoester) and the quinuclidine moiety (of the alkaloid). This, along with steric repulsion experienced between the ester group (R-ketoester) and the quinoline ring (alkaloid), is suggested to be the source of the stereoselection. The rate enhancement relative to racemic reaction, also a notable feature of the Orito reaction, has been attributed to a combination of chemisorption (to the Pt surface), and activation of the prochiral carbonyl by the hydrogen bond to the protonated quinuclidine substituent (the ligand acceleration effect).22 Recently, Lavoie et al.14,23,24 have proposed an alternative model of enantioselection based on the existence of a 2-point hydrogen bonding interaction: one between the ester carbonyl (R-ketoester) and quinuclidine moiety, and a second between the prochiral carbonyl (R-ketoester) and the hydrogen atoms of the quinoline ring (alkaloid). It has previously been shown that aromatic molecules adsorbed onto Pt can polarize the C-H bonds of the π-system sufficiently to form hydrogen bonds with coadsorbed carbonyl groups,25,26 and Lavoie and co-workers suggest this to be the real source of the metal specificity of the reaction. This model has been applied to a wide range of literature data and has been shown to be qualitatively consistent.14 We have proposed, based on selective blocking of terrace and step/kink sites by inert adatoms such as Bi that chiral edge kink sites at the junction of the platinum catalyst particle with the support may provide particularly enantioselective active sites during reaction13,27 although recent work by Baiker favors the {111} terrace site as being the most enantioselective.28 In the enantioselection, the role of the C5′-H and C6′-H was also verified by NMR measurements too.29 In ref 13 it was also noted
10.1021/jp106495b 2011 American Chemical Society Published on Web 12/09/2010
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that the adsorption of Bi at steps/kinks produced an increase in the rate of reaction over and above the already rate-accelerated cinchona-modified Orito reaction. This effect was postulated as being due to the suppression, by Bi adatoms, of a sitepoisoning side reaction such as homoaldol condensation of ethyl pyruvate at edge and defect sites. In this model, rate acceleration is associated with an increase in available platinum sites rather than the more commonly held view of ligand acceleration by adsorbed cinchonidine. Since that time, other workers have challenged our model. For example, Bartok and co-workers30,31 examined the hydrogenation of EP and KPL with different modifiers in a flow reactor and found the hydrogenation rate slowed when switching from quinine to cinchonine but then accelerated when the flow of quinine was restored. In the case of KPL, the hydrogenation proceeded faster with cinchonine, indicating that overall the difference in rates was more likely due to kinetic effects (e.g., ligand-substrate interactions) than a suppression of surface poisoning. Indeed, rate acceleration has been observed for many prochiral ketones although it is emphasized that, under optimal reaction conditions, the observation of dimeric or polymeric side products has not been reported.9,21 In the present study, we use a combined electrochemicalsurface enhanced Raman spectroscopy (SERS) approach to investigate the catalyst surface under electrochemical hydrogenating conditions. The inherent difficulties of obtaining SERSactive Pt surfaces have recently been surmounted by either (a) pinhole-free electrodeposition of Pt onto preroughened, SERSactive Au electrodes,32,33 (b) direct activation of Pt itself,34 or (c) depositing a thin shell of Pt on SERS-active gold nanoparticles.35 Also, it has been established that the hydrogenation of R-ketoesters at supported platinum catalysts may be performed electrocatalytically.36 Hence, in situ SERS at a hydrogen evolving electrode may offer a unique aspect to view heterogeneous catalytic hydrogenations of some complexity with all reacting elements present simultaneously. Previous spectroscopic investigations have identified several intermediate and decomposition species of EP on platinum such as ethyl pyruvate homoaldol products.13,37,38 Indeed, extensive polymerization of ethyl pyruvate has been reported on Pt surfaces under ultrahigh vacuum39 and also on alumina supports,40 although in the case of the former, it has been demonstrated that the surface polymer under hydrogen limited conditions is actually a self-assembly of enolic-MP adsorbates.41 In this paper, we present results from SERS investigations into the adsorption and hydrogenation of EP, its dimer by selfcondensation, and a nonenolizable substrate (KPL) at a Au@Pt nanoparticle surface. The effects on the SERS spectrum of each of these adsorbates both singly adsorbed and coadsorbed with cinchonidine are reported. For completeness, the SERS data is compared to Raman spectra of the bulk linear and cyclic dimers together with the lactone produced by cyclization of the dimer, following loss of ethanol.37 We establish that self-condensation fragments of EP are not observed under enantioselective hydrogenation conditions and hence cannot be invoked to any significant extent in models of rate acceleration. In addition, the nature of the adsorbed intermediate is found to be different in the cases of EP and KPL and hence, the nature of rate enhancement in Orito-type reactions is revaluated. Experimental Section Ethyl pyruvate (EP, 1; Alfa Aesar, 98%) was purified by vacuum distillation (0.1mbar) at room temperature. Very conveniently, diethyl 2-hydroxy-2-methyl-4-oxo-pentanedioate
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Figure 1. Experimental set up and flow cell cross section.
(DHP, 4) was the resulting residue (ca. 2-3% of the sample). The lactone species 10 was prepared by refluxing 4 for 4 h in toluene and was found to exist as both tautomers in deuterochloroform: the isotetronic acid (ITA, 11) and ethyl tetrahydro2-methyl-4,5-dioxofuran-2-carboxylate (ETD, 10). In all cases, 1 H and 13C NMR (CDCl3; 400 and 100 MHz, respectively) was used to confirm the identity and purity of the samples.42 Hydrogen (BOC Gases, 100%), cinchonidine (CD; Fluka, purum grade >98%), ketopantolactone (KPL; Aldrich 97%), and sulfuric acid (BDH, Aristar grade) were used as received and SERS active Au@Pt nanoparticles were prepared using the methodology of Tian et al.35 Electrolyte solutions were prepared using ultrapure water (Millipore) with a resistivity of not less than 18.2 MΩ cm-1. Raman spectroscopic measurements were performed using a LabRam Raman Microscope (Horiba JobinYvon Ltd., Middlesex, U.K.) fitted with a HeNe laser (λ ) 633 nm, output power 16 mW), with data recording and analysis performed using the proprietory LabSpec software (Horiba JobinYvon) run on a Dell Optiplex PC. The flowcell used for experiments has been described elsewhere43 and is shown schematically in Figure 1. Electrode materials used were Pd and Au (both from Advent Research Materials Ltd., Eynsham, Oxon, U.K., 99.95%) together with a Ag/AgCl reference electrode. Voltammetry was performed using a three electrode arrangement within the flowcell (reference electrode was Ag/AgCl), using a CH Instruments potentiostat (model CHI750). In order to obtain in situ SERS data under hydrogenation conditions, two methods of hydrogen dosing were used. In the first, hydrogen gas was flowed through a bubbler containing the R-ketoester. The hydrogen gas exiting the bubbler was thus saturated with EP and subsequently entered the spectroelectrochemical flow cell as indicated in Figure 1. In the second method, hydrogen gas was evolved directly at the electrode surface itself, derived from electrolytic breakdown of the aqueous electrolyte. A potential of -0.33 V versus a Ag/AgCl electrode reference electrode was found sufficient for this purpose. It should be noted that by flowing electrolyte through the cell, any hydrogen bubbles forming at the surface of the electrode (which would preclude detection of any SERS signal) could readily be dislodged from the electrode surface when the electrolyte was flowing. Electrochemical potential cycling was used to clean the SERS-active electrode. The potential range utilized for cleaning purposes was between -0.3 V and +1.05 V at 0.1 V s-1 in 0.1 M sulfuric acid followed by -0.95 V and +0.25 V at 0.1 V s-1 in 0.1 M sodium hydroxide versus Ag/ AgCl until stable voltammograms were achieved. The electrode
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Figure 2. Various forms of dimeric ethyl pyruvate (EP).
would then be rinsed with flowing ultrapure water for 15 min. In the case of gas phase dosing, dry argon gas was also passed through the cell for 20-30 min prior to dosing. Results and Discussion 1. Raman Spectra of Pure Species. Figure 2 shows the possible dimeric species that may be derived from 1 including the cyclic lactones. The above equilibria illustrate the possible aldolic reaction of EP and the ensuing cyclizations, including the keto 1, enol 2 tautomers and H-bonded conformer 3 of ethyl pyruvate, the keto 4, enol 7 tautomers and respective intramolecular H-bonded conformers 5 and 8 of the dimer, the cyclic keto 6 and enol 9 forms of the dimer, and the keto 10, enol 11, and H-bonded conformer 12 of the lactonic species, formed by loss of ethanol. The Raman spectrum of pure EP 1 was recorded and band assignments made based on standard references,44 and comparison with known assignments for related species.45,46 Figure 3 shows the recorded spectrum of 1 along with those for the pure, linear dimer 4, the cyclic form of the dimer (6, 9), and the cyclic lactone (10-12) with assignments listed in Table 1. It is striking from the spectra displayed in Figure 3 how similar the bulk molecular spectra are between monomeric EP and its linear homoaldol dimer (4). The main differences are related to the CO wag, clearly resolved into 609 and 623 cm-1 in EP but weak and broadened in the linear dimer. Also, the linear dimer has more structure around the carbonyl region 1644-1785 cm-1 whereas pure EP exhibits only a single peak at 1734 cm-1 reflecting the presence of a small amount of enol. By comparing panels b and d in Figure 3, it is evident that the small peaks in the range 1640-1790 cm-1 are consistent with the presence of trace quantities of 6/9. Hence, enolization is only favored for cyclic forms of the dimer. The weak C-H stretches at 3070 cm-1 confirm that the enol is present in the cyclic dimer and the lactone 10/11 although enolization is found
to be much more extensive in the latter as signified by the magnitude of this peak. Very weak features at 2750 cm-1 may be ascribed to chelation to form a 5-membered hydrogen bonded ring within the linear dimer, cyclic dimer, and the lactone. The presence of carbonyl stretches around 1770 cm-1 in the spectra is also consistent with the presence of a 5-membered ring generated by either hemiacetal (cyclic dimer) or lactone formation (via loss of ethanol). The sharp bands at 1660 and 1680 cm-1 in 6/9 and 10/11 are ascribed to enol forms and intramolecular hydrogen-bonding, respectively, which are consistent with both literature values24,47 and their absence in the spectrum of the nonenolizable KPL (see later). Note that the relative intensities of the enol peaks between 1600 and 1700 cm-1 are reversed in the lactone compared to the cyclic dimer. There is some broadening in the 1770 cm-1 band since, according to the literature,44 5-ring R-diketones should exhibit two closely spaced bands at approximately 1760 and 1770 cm-1. Another noteworthy feature from the bulk spectra depicted in Figure 3 is the diminution in the intensity of the 1734 cm-1 carbonyl peak in the cyclic forms and the corresponding growth in a feature at 803 cm-1 which we ascribe to C-C vibrations of the 5-membered ring since it is absent in the linear dimer and pure EP but again is present in the Raman spectrum of KPL. This means that the intensity of this peak is a signature of both the cyclic dimer and the cyclic lactone. 2. SERS Characterization of KPL and Dimeric EP. In this section, the hydrogenation of the dimeric species and KPL is studied together with the effect of cinchonidine coadsorption. Dimer 4 was first adsorbed onto the working electrode from a 0.1 M aqueous solution of sulfuric acid at a potential of -0.33 V versus a Ag/AgCl reference electrode. At this potential, the platinum electrode was evolving hydrogen freely. The concentration of the dimer was approximately 10-4 M. Figure 4 shows both the bulk Raman spectra for 4 and 6/9 together with SERS for the adsorbed dimer in steady state with the dimer containing
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Figure 3. Bulk Raman spectra of (a) 1, (b) 6/9, (c) 11/12, and (d) 4.
TABLE 1: Band Assignments for SERS Spectra of Bulk Ethyl Pyruvate and Related Species ethyl pyruvate, 1/cm-1 EP dimer, 4/cm-1 DHP cyclic dimer, 6, 9/cm-1 lactone, 10-12/cm-1ETD/ITA 600-630 (s)
550 (m) 600-630 (m)
857 (s) 1022 (m) 1092 (m) 1106 (m) 1265 (m-w) 1295 (m-w) 1391 (m-w) 1430 (m-w) 1446 (m)
857 (s) 1022 (m-w) 1094 (s) 1108 (s) 1266 (m) 1295 (m) 1387 (w)
1732 (s)
1736 (s)
1449 (s)
2750 (w) 2850-3000 (s)
2850-3000 (s)
band assignments
550 (m) 600-630 (m) 800 (s) 857 (s) 1022 (w) 1096 (m-s) 1113 (m-s) 1265 (w) 1302 (m) 1396 (vw)
550 (m) 600-630 (m) 800 (s) 857 (s) 1022 (w) 1096 (m-s) 1113 (m-s) 1265 (w) 1302 (m) 1396 (vw)
hydrogen bonded CdO def CdO def cyclic skeletal C-C str/CH2 rocking CH3-CH2 str CO-CO str C-CH3 rock
1452 (s) 1654 (m) 1682 (m-s) (s) 1736 (w) 1777 (s) 2750 (w)
1452 (s) 1658 (vs) 1687 (s) 1736 (w) 1776 (s) 2750 (w)
2850-3000 (s) 3070 (w)
2850-3000 (s) 3070 (w)
CH2C-H3 def enol CdC str H-bonded enol CdO stretch keto CdO str keto-lactone CdO str O-H str of chelating 5-membered H-bonded ring. C-H str enolic C-H stretch
bulk electrolyte and after swapping the electrolyte for pure aqueous sulfuric acid. The peaks at 2040 and 2090 cm-1 under electrochemical conditions may be ascribed to linearly bonded CO(ad) and Pt-H(ad), respectively. The CO(ad) is assumed to be a decomposition product of the organic adsorbate. The peak at 470 cm-1 may then be assigned to the Pt-C stretch of adsorbed carbon monoxide. In comparison with the bulk Raman spectra of linear and cyclic dimer, although the linear dimer was adsorbed, the presence of a prominent peak at 803 cm-1 signifies that the adsorbed species is cyclized under hydrogenating conditions. Peaks situated at 1457, 1311, 1020, 857, 763, 640-660, and 548 cm-1 confirm that the dimeric species is adsorbed intact at the electrode surface although the relative
C-Oester str COC-H3 def
intensities of all of these peaks are modified somewhat relative to the spectra of the bulk species. The absence of surface peaks in the range 1600-1780 cm-1 suggests that any carbonyl groups are aligned parallel to the surface based on surface selection rules. The presence of peaks between 600 and 640 cm-1 also suggests however that CdO deformation modes are SERS active in this surface configuration. In addition, the extremely weak (in the bulk phase) C-Oester stretch or CH3-C rocking is now observed clearly at 1167 cm-1 when the dimer is adsorbed. There is a small peak at 1621 cm-1 that may be ascribed to some molecular fragment of the dimer since it is also observed after hydrogenative desorption. In fact, inspection of the upper spectrum in Figure 4 illustrates the fact that the dimeric species may readily be removed from the Pt surface when coadsorbed
Enantioselective Hydrogenation of R-Ketoesters
Figure 4. Raman and SER spectra of (from bottom up): bulk linear homoaldol dimer (4), bulk cyclic homoaldol dimer (6), (4) adsorbed on SERS active Pt electrode under HER (hydrogen evolution reaction) conditions in steady state with solution phase dimer, and finally (top) after hydrogenation of surface adsorbed dimer with dimer-free electrolyte.
Figure 5. SER spectra collected at -0.33 V (Ag/AgCl) of CD (bottom), CD and dimer (middle), and dimer (top).
with hydrogen leaving behind adsorbed carbon monoxide and the unknown molecular fragment. No traces of the 803 cm-1 or other bands associated with adsorbed dimer may be observed in the dimer-free electrolyte after the adlayer of cyclic dimer has been hydrogenated. These observations strongly suggest that both linear and cyclic dimeric species are rapidly and efficiently hydrogenated. Next, the effect of coadsorbed CD on the adsorbed dimer was studied at hydrogen evolving potentials. Figure 5 shows SER spectra of singly adsorbed CD, 4, and coadsorbed CD/4 at hydrogen evolving potentials. The spectrum of the singly adsorbed dimer is almost identical to the spectrum depicted in Figure 4 save for the presence of a weak, broad feature at 1675 cm-1 (which may be ascribed to a hydrogen bonded carbonyl stretch) and a slightly less intense 801 cm-1 band relative to the 860 cm-1 band. The intensity of the 2023 and 470 cm-1 C-O and Pt-C vibrations of linearly adsorbed CO are much reduced in intensity compared to in Figure 4. The spectrum for CD adsorbed on Pt is typical of previous data
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Figure 6. Raman spectra for (bottom to top) bulk ketopantolactone (KPL), bulk pantolactone, and surface-bound KPL on hydrogenation.
collected by ourselves43 and others,47,48 and the various band assignments, based on earlier results from Zaera et al., are listed elsewhere.49 Of note are the two bands at 1602 and 1364 cm-1 associated with the quinoline substituent and the broad peak at 2065 cm-1 due to the Pt-H stretch of electrosorbed hydrogen atoms that are always red-shifted from 2085-2090 cm-1 in the presence of CD.43 The coadsorption of CD with the dimer appears to make very little impact and the resulting SER spectrum may be viewed as a simple superposition of the SER spectra of the singly adsorbed species. Next, a 10-5 M KPL in 0.1 M H2SO4 aqueous electrolyte was introduced into the flowcell. KPL was allowed to adsorb onto the clean Pt surface and then the electrode potential held at a suitably negative potential to evolve hydrogen while the Raman spectrum was recorded. Using bulk KPL and pantolactone as references, the Raman spectra obtained from these species are shown in Figure 6 together with the SERS of KPL adsorbed on platinum. The key changes in the spectrum of KPL on hydrogenation are indentifiable as the emergence of peaks at 418 and 487 cm-1 which may be ascribed to the Pt-C vibration of multicoordinated and linearly bonded adsorbed CO respectively. The corresponding C-O molecular vibrations of these species may be observed at 1809 and 2020/2070 cm-1, respectively. The 487 cm-1 band does also overlap with a small, weak band in bulk KPL. In addition, bands at 1450, 1167, 993, 940, 850, 806, 603, and 545 cm-1 which are found to be present in bulk KPL indicate little molecular perturbation of adsorbed KPL under hydrogenating conditions. In fact the spectrum of adsorbed KPL matches much more closely KPL rather than the hydrogenated product pantalactone (PL), particularly with respect to the absence of PL bands at 603, 880, and 910 cm-1 in the adsorbed intermediate. Moreover, a very weak band at approximately 1600 cm-1 is consistent with KPL bonded to the Pt surface via the keto-carbonyl tilted to the surface. We shall see later that this band becomes more intense in the absence of coadsorbed hydrogen. The very weak feature at 1767 cm-1 corresponding to the CdO stretch of a keto-group in a 5-membered ring indicates that some molecular KPL is also present at the interface in which the bonding to the surface again leaves the molecule relatively unperturbed. Interestingly, the most intense band in the bulk spectrum of KPL at 727 cm-1 is almost completely absent in the adsorbed state. We suggest that this band corresponds to an alkane skeletal vibrational mode
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Figure 7. Hydrogenation of KPL measured by SERS. Hydrogen evolution begins at around -0.26 V (vs Ag/AgCl).
characteristic of a central carbon atom bonded to four other carbon atoms as found for the carbon atom bonded to the two methyl substituents in KPL.44 This mode usually gives rise to strong absorption in Raman.44 Its absence from the SERS is presumably a consequence of the surface selection rules, and it should be expected that a change in molecular orientation might influence this intensity. Later, it will be seen that to some extent this prediction is confirmed. The 806 cm-1 band we ascribe to a C-C stretch associated with the 5-membered ring as found with the cyclic dimer. Furthermore, the peak at 993 cm-1 is also consistent with skeletal vibrations of a 5-membered cyclopentane-like ring structure.44 Again, the 603 cm-1 peak is assigned to a keto-CdO wag/distortion (note its absence after hydrogenation of the ketone to form bulk PL) and the 1167 cm-1 we assume is the same vibration as seen when the cyclic dimer was adsorbed (C-Oester stretch or C-CH3 rocking). In order to investigate the changes in adsorbed KPL as hydrogen is coadsorbed at the surface, the SERS of adsorbed KPL was monitored as a function of increasingly more negative potentials into the hydrogen evolution potential range. The onset of hydrogen evolution may be taken as -0.26 V (Ag/AgCl) (Figure 7). It is evident that in the absence of hydrogen (-0.144 V) the relative intensities of all SER bands is completely different than those reported in Figure 6. For example, the most prominent peaks are clearly those due to adsorbed linear CO (489 and 2038 cm-1) and the µ1 surface-carbonyl stretch at 1600 cm-1. In addition, the appearance of peaks at 1451, 1254, 990, and 1765 cm-1 suggests molecular adsorption of KPL. However, as the potential becomes more negative the relative intensities of the µ1 surface-carbonyl stretch and the C-C skeletal vibrations below 1000 cm-1 begins to change. In fact at -0.266 V, the formation of linear and multiple bonded adsorbed CO dominates the spectrum together with an increase in the intensity of the 851 cm-1 band in particular. In the narrow potential range signifying hydrogen evolution, both the amount of adsorbed CO decreases and the skeletal vibrations (observed below 1000 cm-1) dominate the spectrum. These changes, particularly the variation in the intensity of the 1600 cm-1 band are interpreted as a change in the bonding configuration of KPL toward more tilted (less upright relative to the surface plane) as hydrogen is coadsorbed. It is interesting that Baiker and co-workers report a similar change in the orientation of EP in UHV on Pt{111}
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Figure 8. Changes in the SER spectra of EP adsorbed at Au@Pt following exposure to hydrogen gas (top). The middle spectra corresponds to the EP adlayer formed from adsorption from an EP saturated argon gas flow and the bottom spectrum of liquid EP for comparison purposes.
when hydrogen is present at the surface.50 There is no evidence for Pt-C molecular bonding other than that associated with adsorbed CO. This is to be contrasted with EP (see later). Hence, it appears that prior to hydrogenation, the KPL molecule undergoes a transition from upright to tilted triggered by the presence of electrosorbed hydrogen. It is also noticed that the 725 cm-1 vibration is slightly more prominent when the KPL molecule is in a more upright configuration (absence of hydrogen) compared to when hydrogen is coadsorbed. 3. Gas Phase Hydrogenation of Ethyl Pyruvate. Since both the enolization of EP under hydrogenating conditions and the possible formation of dimeric and other self-condensation side products of EP is a crucial aspect of the present study, data for the gas phase hydrogenation of EP is included in Figure 8. EP offers an interesting contrast with KPL as a substrate since the rate acceleration observed under enantioselective hydrogenation conditions at supported Pt is reported to be significantly reduced for KPL compared to EP.30,31 This might suggest that our earlier model of surface polymer formation may offer an explanation for such behavior since KPL is nonenolizable, and so rate inhibition via polymer formation (and subsequent site-blocking) will be precluded, giving rise to a faster intrinsic hydrogenation rate than EP. Hence, evidence for dimer formation of EP under racemic hydrogenation conditions would be a necessary prerequisite in order to confirm such an interpretation of relative differences in rate acceleration between KPL and EP. Details of the dosing method and DFT calculations to interpret the SERS of EP on Pt in the presence of hydrogen may be found in ref 43. It should be noted that in ref 43 identical spectra to those depicted in Figure 8 under gas phase dosing may also be obtained at hydrogen evolving electrodes. It is seen from Figure 8 that when EP is dosed onto the platinum surface from an EP saturated argon gas flow the resulting SERS corresponds largely to molecular EP together with some adsorbed CO (bands at 485 and 2040 cm-1 corresponding to Pt-C and C-O stretches, respectively). There is some perturbation of the CdO carbonyl vibration at 603 cm-1 in the adsorbed state compared to liquid EP and a rather broad feature at around 1600 cm-1 is exhibited, but in essence, all peaks may be ascribed to a physisorbed layer of EP in
Enantioselective Hydrogenation of R-Ketoesters
Figure 9. Half-hydrogenated state (HHS) of EP on platinum generated following coadsorption of EP with hydrogen.
equilibrium with the gas phase. In contrast, as soon as hydrogen is switched into the gas feed, new bands appear at 1053, 772, and 657 cm-1 which are not present in the spectra of molecular EP. In particular, it should be noted that the C-O molecular stretch of chemisorbed CO at 2008 cm-1 is of low intensity compared to the argon dosed sample signifying a much lower surface coverage. Moreover, the intensity of the corresponding Pt-C band at 489 cm-1 remains as intense as for the argon dosed sample in spite of the much reduced coverage of CO. This suggests that a second Pt-C stretch at similar frequency to adsorbed CO is present at the surface associated with adsorbed EP. In ref 43 we demonstrate using DFT calculations that the spectrum of EP coadsorbed with hydrogen on Pt is consistent with a long-lived intermediate of EP generated after the first hydrogen atom is transferred to the keto carbonyl. This “halfhydrogenated state” (HHS) is depicted below in Figure 9. A further finding from ref 43 was that, because of the stability of the HHS, it blocked adsorption sites for hydrogen atoms above a certain bulk liquid phase concentration of EP due to the build-up of a steady state HHS surface coverage. Hence, the transfer of the second hydrogen atom to the HHS to form the product ethyl lactate becomes the rate limiting step under racemic hydrogenation conditions at modest to high concentrations of EP. Under enantioselective reaction conditions however, irrespective of bulk EP concentration, it was found that CD afforded sites for the adsorption of chemisorbed hydrogen, and therefore, this was postulated as leading to rate acceleration. Since the HHS state is still observed even under enantioselective hydrogenation conditions,43 the addition of the second H atom should remain the rate limiting step after CD is added to the reaction mixture and thus affording enantiomeric excess. Hence, the availability of chemisorbed H atoms increases under enantioselective reaction conditions facilitating an increase in the rate limiting addition of the second hydrogen atom relative to reaction under racemic conditions. This model is partially consistent with previous kinetic studies51–53 in that, in ref 53 although the addition of the first hydrogen atom was favored as the rate limiting step under racemic conditions (“MV1” in ref 53), it is only marginally so, with the kinetic model based on addition of the second H atom “MV2”53 as rate limiting giving rise to an equally good fit to the experimental data although itself requiring one more kinetic parameter for proper fitting,53 Hence, a possible explanation for the lack of rate acceleration under enantioselective conditions observed both for gas phase hydrogenation54 and at low EP bulk concentrations55 may be ascribed to the greater availability of surface hydrogen atoms at surfaces which are only partially blocked by HHS species. In the present context, it is evident from Figure 8 that there is no evidence for dimers of EP (for example a peak at 803 cm-1) forming during gas phase hydrogenation. Indeed, no such peak was ever observed at a hydrogen evolving electrode43 either. Since dimeric forms of EP are not observed under hydrogenating conditions, they therefore cannot be invoked as central to understanding rate acceleration in the Orito reaction. In fact, it is now known that the polymers originally observed
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1169 in refs 39 and 56 are actually hydrogen bonded networks of enolic monomers41 and that these are completely destabilized in the presence of coadsorbed hydrogen.41,56 Rather, the difference in the relative extents of rate acceleration between KPL and EP may be interpreted in terms of the absence of a longlived and self-blocking HHS in the case of KPL. That some rate acceleration is still observed with KPL may be due to ligand acceleration as described by Baiker and others.57–59 The absence of Pt-C bond formation and hence the formation of a HHS for KPL can only be speculated upon at this stage. It may be that the steric bulk of two methyl groups in KPL simply prevents formation of a long-lived HHS intermediate compared to EP since KPL (unlike EP) cannot lie “flat” on the Pt surface, and so to form a Pt-C bond would bring one of the two methyl groups on KPL into close proximity with the Pt surface. The original aim of the present study was to elucidate the stability or otherwise of structures 1, 4, and 10/11 under Oritotype reaction conditions in order to examine our earlier model of rate-acceleration based on an increase in Pt sites rather than ligand acceleration by CD.13,27 The present results confirm what Baiker et al. had found previously in reactor studies: that hydrogen removed high molecular weight aldol products from a 5% Pt/Al2O3 catalyst surface.40 This study focused on the effect that the aldol products had on the enantioselectivity of the hydrogenation of EP.40 They found that when the reaction mixture was given sufficient time for the aldol products to be formed, the overall enantiomeric excess (ee) of the hydrogenation increased by 2.5%. It was further found that addition of lactone alone to the reaction mixture enhanced the ee by up to 4%. This effect was traced to protonation of CD by the enol form of the lactone; the protonated quinuclidine nitrogen has also previously been shown to enhance ee.58 Margitfalvi et al.60 have also examined the change in reaction kinetics upon addition of the lactone and concluded that results obtained were not consistent with site-blocking by this species leading to the rateacceleration effect in Orito reactions. In addition, by careful study of the reaction kinetics of EP, Margitfalvi et al. noted that for enantioselective reaction, the rate of formation of the R product was rate accelerated whereas the S product was rate decelerated61 Again, the association of rate acceleration with oligomer formation could not be established in this work. Finally, more recent work by Bartok and others30,31,62 using continuous flow reactors reported that the formation of homoaldol impurities could not explain the rapid changes in ee and rate observed upon switching the reactant feed containing different cinchona modifiers. Hence, a significant body of work has now accumulated, including the present study, in which the impact of polymeric species of EP upon rate acceleration in Orito-type reactions has not been proven. However, both in the present study and in reference,43 the notion of “self-poisoning” by adsorbed EP under hydrogenating conditions may still be relevant to the observed rate acceleration effect. Hence, the role of the alkaloid and hydrogen together in establishing rateacceleration is ascribed to a destabilization, not of selfcondensation products of ethyl pyruvate under reaction conditions, but rather a more rapid hydrogenation of the long-lived HHS of EP upon CD adsorption, which is speculated to be rate limiting under both racemic and enatioselective conditions. Consequently, an increase in the number of available platinum surface sites as originally postulated in ref 13 and also deduced independently by Murzin55 may still be operative as a rationale for understanding rate acceleration. It should be noted that there is still significant controversy surrounding this assertion63,64 The ”clean-off” of the HHS in a steady state on the Pt surface (the
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coverage of which would be dependent on bulk EP concentration) could provide a general explanation of observations of transient behavior by Blackmond65 and Baiker39,52 of a dependence of ee on the extent of reaction and those of Murzin55 for changes in enantioselective and racemic rate upon increasing the concentration of EP. Conclusion The adsorption and hydrogenation of ketopantolactone (KPL) and ethyl pyruvate (EP) on Pt electrodes has been measured by surface-enhanced Raman spectroscopy (SERS). Unlike EP, KPL does not form a long-lived half-hydrogenated state (HHS) surface intermediate. Rather, KPL changes from an upright µ1keto-Pt configuration in the absence of hydrogen to a more tilted surface bonding arrangement under hydrogenating conditions. In addition, pure samples of self-condensation products of ethyl pyruvate have also been studied. These dimeric and aldol condensation intermediates have previously been proposed by ourselves as playing a crucial role in explaining the significant rate-enhancement observed during enantioselective hydrogenation of R-ketoesters at supported platinum catalysts. At hydrogen evolving potentials in dimer-free aqueous sulfuric acid, the dimer may be hydrogenatively desorbed readily from the electrode surface. When the spectra of EP under hydrogenating conditions are compared with SERS from the adsorbed dimer, it is deduced that self-condensed dimers do not form. Hence, the role of the alkaloid in establishing rate-acceleration cannot be ascribed simply to a destabilization of self-condensation products of ethyl pyruvate under reaction conditions. Rather, it is speculated that the rapid hydrogenative desorption of the HHS upon CD adsorption leads to significant rate acceleration. It is also suggested that the absence of such long-lived surface intermediates in the case of KPL leads to a relatively smaller difference between racemic and enantiomeric rate compared to EP. Acknowledgment. This manuscript is dedicated to Alfons Baiker in recognition of his immense contributions to the field of enantioselective heterogeneous catalysis. N.V.R., I.R.M., and R.J.T. acknowledge the EPSRC for financial support. References and Notes (1) Baiker, A. J. Mol. Catal. A 1997, 115, 473. (2) Wells, P. B. Surf. Chem. Catal. 2002, 295. (3) Blaser, H.-U.; Mu¨ller, M. Stud. Surf. Sci. Catal. 1991, 59, 73. (4) Orito, Y.; Imai, S.; Niwa, S. J. Chem. Soc. Jpn. 1979, 8, 1118. (5) Studer, M.; Burkhardt, S.; Blaser, H.-U. Chem. Commun. 1999, 1727. (6) K.Bala´zsik, K.; Barto´k, M. J. Catal. 2004, 224, 463. (7) Burgi, T.; Baiker, A. Acc. Chem. Res. 2004, 37, 909. (8) Exner, C.; Pfaltz, A.; Studer, M.; Blaser, H.-U. AdV. Synth. Catal. 2003, 345, 1253. (9) Studer, M.; Blaser, H.-U.; Exner, C. AdV. Synth. Catal. 2003, 345, 45. (10) von Arx, M.; Mallat, T.; Baiker, A. Top. Catal. 2002, 19, 75. (11) Wells, P. B.; Wilkinson, A. G. Top. Catal. 1998, 5, 39. (12) Vayner, G.; Houk, K. N.; Sun, Y.-K. J. Am. Chem. Soc. 2004, 126, 199. (13) Jenkins, D. J.; Alabdulrahman, A. M. S.; Attard, G. A.; Griffin, K. G.; Johnston, P.; Wells, P. B. J. Catal. 2005, 234, 230. (14) Lavoie, S.; Laliberte´, M.-A.; Temprano, I.; McBreen, P. H. J. Am. Chem. Soc. 2006, 128, 7588. (15) Diezi, S.; Mallat, T.; Szabo, A.; Baiker, A. J. Catal. 2004, 228, 162. (16) Bonalumi, N.; Vargas, A.; Ferri, D.; Burgi, T.; Mallat, T.; Baiker, A. J. Am. Chem. Soc. 2005, 127, 8467. (17) Cserenyi, S.; Felfeldi, K.; Balazsik, K.; Szollosi, G.; Bucsi, I.; Bartok, M. J. Mol. Catal. A 2005, 247, 108. (18) Murzin, D. Y.; Ma¨ki-Arvela, P.; Toukoniitty, E.; Salmi, T. Catal. ReV. Sci. Eng. 2005, 47, 175–256. (19) Mihaly, B. Curr. Org Chem. 2006, 10, 1533–1567.
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