Two-Dimensional Self-Assembly and Catalytic Function: Conversion

J. Phys. Chem. C 2010, 114, 7291–7298

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Two-Dimensional Self-Assembly and Catalytic Function: Conversion of Chiral Alcohols into Self-Assembled Enols on Pt(111)† Vincent Demers-Carpentier, Marc-Andre´ Laliberte´, Ste´phane Lavoie, Gautier Mahieu, and Peter H. McBreen* De´partement de Chimie, UniVersite´ LaVal, Que´bec, Canada G1K 7P4 ReceiVed: September 14, 2009; ReVised Manuscript ReceiVed: October 18, 2009

The chemical transformation and subsequent self-assembly of chiral alcohols on platinum was studied using three different pairs of prochiral ketones and their alcohol products. The ketones were chosen because they represent three different types of substrates in the asymmetric hydrogenation on chirally modified platinum catalysts. Scanning tunneling microscopy and high-resolution electron energy loss vibrational spectroscopy data were combined to show that methyl lactate transforms into the enol tautomer of methyl pyruvate on Pt(111) at room temperature. Specifically, the chiral alcohol undergoes dehydrogenation leading to the same adsorbed enol assemblies that are formed directly through the adsorption of the prochiral R-ketoester. Similarly, 1-phenylethanol transforms into assemblies of the enol tautomer of acetophenone. The interrelationship between surface reactivity and self-assembly was further explored by studying the oxidation of 1-phenyl-2,2,2trifluoroethanol to form CH · · · O bonded 2,2,2-trifluoroacetophenone assemblies. In terms of catalytic function, these self-assembly processes provide insight on the optimization of the asymmetric hydrogenation of activated ketones on chirally modified platinum catalysts. Introduction Investigations of molecular self-assembly on metal surfaces are generally carried out using coinage metal substrates. These surfaces range from being weakly reactive (gold) to mildly reactive (copper) and hence are ideal for the design of assemblies and 2D networks without the complication of extensive molecular decomposition. They also offer the advantage of fine control of thermal activation processes such as deprotonation of carboxyl groups,1 deprotection steps,2 and chemical synthesis,3 as well as of nonthermal activation processes such as electron beam induced reactions.4 In contrast, there are relatively few studies of molecular self-assembly on highly reactive metals, where bond breaking or strong chemisorption is the rule rather than the exception.5 Yet, self-assembly is a key component in catalytic asymmetric induction on chirally modified surfaces.6 Here, we report on chemical transformations and selfassembly processes involving R-ketoesters, R-hydroxy esters, R-phenyl alcohols, and R-phenyl ketones on platinum, a metal which displays high activity for catalytic reactions involving these compounds (Scheme 1). For example, R-ketoesters may be hydrogenated at room temperature on chirally modified Pt to R-hydroxy esters, as in the efficient (98% ee) enantioselective conversion of methyl pyruvate to methyl lactate.6 Similarly, 2,2,2-trifluoroacetophenone (TFAP) may undergo efficient (92% ee) asymmetric hydrogenation on Pt to 1-phenyl-2,2,2-trifluoroethanol (TFPE).7 The asymmetric hydrogenation of these intramolecularly activated ketones on cinchona-modified Pt catalysts is referred to as the Orito reaction.8 Acetophenone, which is not an intramolecularly activated ketone, undergoes inefficient (20% ee) asymmetric hydrogenation to 1-phenylethanol on cinchona-modified Pt.9 The oxidation of alcohols to ketones, as in the conversion of 1-phenylethanol to acetophenone

SCHEME 1: Platinum-Catalyzed Transformations of the Six Molecules Investigated in This Study: (A) Hydrogenation of Activated Ketones to Chiral Alcohols: Conversion of Methyl Pyruvate to Methyl Lactate and Conversion of TFAP to 1-Phenyl-2,2,2-trifluoroethanol (TFPE); (B) Oxidation of Alcohols to Ketones: Conversion of 1-Phenylethanol to Acetophenone; (C) Keto-Enol Tautomerization of Methyl Pyruvatea

a All of these processes can occur at room temperature on platinum catalysts.

†

Part of the “Martin Moskovits Festschrift”. * To whom correspondence should be addressed. E-mail: peter.mcbreen@ chm.ulaval.ca.

(Scheme 1B), also takes place on platinum catalysts at room temperature.10 Furthermore, we have previously reported that

10.1021/jp908877b  2010 American Chemical Society Published on Web 10/30/2009

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SCHEME 2: Schematic Drawing of a Proposed Pro-(R) 1:1 Cinchonidine/Methyl Pyruvate Diastereomeric Complex12a

Figure 1. STM of chain structures formed by methyl pyruvate (A) and methyl lactate (B) on Pt(111) at room temperature.

a Cinchonidine-modified Pt catalysts convert methyl pyruvate into (R)-methyl lactate with up to 98% ee. The groups indicated by (A) and (B) in the chiral modifier are hydrogen bond donors. The quinoline group (B) is an aryl-CH hydrogen bond donor.12 The quaternary ammonium group (A), located in the quinuclidine substituent, is an NH+ hydrogen bond donor.6 The OH group in the modifier is indicated in red. The substrate, methyl pyruvate, present as the keto tautomer, contains two carbonyl group hydrogen bond acceptors, the keto carbonyl and the ester carbonyl. The illustrated diastereomeric complex is pro(R). The assumption is made that the keto carbonyl is activated by both its chemisorption interaction with the platinum surface and by its hydrogen bonding interaction with the quinoline group.15

in the absence of gas-phase hydrogen methyl pyruvate undergoes keto-enol tautomerization (Scheme 1C) on Pt(111) at room temperature.11 Asymmetric hydrogenation on cinchona-modified Pt catalysts is an area in which the fields of chemical reactivity and selfassembly directly overlap, both in terms of activity and selectivity.6 Specifically, it is widely accepted that a 1:1 modifier to prochiral substrate interaction is at the origin of asymmetric induction in the hydrogenation of methyl pyruvate to methyl lactate on chirally modified Pt. A stereodirecting methyl pyruvate/cinchonidine complex proposed by Lavoie et al.12 is illustrated in Scheme 2, and a number of other suggested modifier-substrate interactions are to be found in the literature.6 A key aspect of the complex illustrated in Scheme 2 is that the prochiral carbonyl is simultaneously subject to a chemisorption interaction with the metal and a C-H · · · O bonding interaction to the aromatic, quinoline, group of the modifier. The latter interaction may also contribute to the activation of the carbonyl bond as in hydrogen bond organocatalysis where small organic molecules are used to direct and activate asymmetric processes.13 Hydrogen bond activation and stereocontrol are also believed to play key roles in the Orito reaction,6,14,15 although the specific interactions involved are the subject of debate. While the structure of 1:1 complexes formed under reactions conditions is not known, the Orito reaction is clearly a case of where local molecular assembly plays a determining role in catalytic function. Since there is a very extensive catalysis literature on how to optimize the catalytic function for a wide range of substrate-modifier pairs, the role of parameters such as modifier structure, substrate structure, modifier coverage, solvent, H2 pressure, and metal used as a catalyst are comprehensively documented.6,7 As a result, surface science data when placed in the context of the catalysis literature provide a way to further understand the molecular details of the reaction.

Experimental surface science studies which focus on the chemisorption of the substrate have already contributed much new information.16 This study focuses on the chemisorption of the chiral product. It is essential to study the product because the reaction is carried out at approximately room temperature. At such mild temperatures, the surface residence time will be relatively large and the possibility that the product undergoes dissociative chemisorption and/or supramolecular assembly on the surface needs to be considered. Experimental Section The experiments were performed in two ultrahigh vacuum chambers on separate polished Pt(111) single crystals. One chamber houses an Omicron variable temperature STM and the second an LK-3000 high-resolution electron energy loss spectrometer (HREELS). All of the STM measurements were performed using -1.0 V sample bias and 1 nA tunneling current. All of the HREELS measurements were performed at 6.3 eV impact energy with the sample held at 100 K. Off-specular measurements were performed at 15°. The platinum surfaces were cleaned by cycles of Ar ion bombardment at 600 K and oxygen (1 × 10-7 Torr) treatment at 900 K. The sample used in the HREELS studies was flashed to 1000 K following the cleaning cycles. The purity of the products used were as followssmethyl pyruvate, >95% Fluka; methyl lactate, 98% Aldrich; acetophenone: >99% Fluka; 1-phenylethanol: 98% Aldrich; TFAP, 99% Aldrich; TFPE, 98% Aldrich. The products were further purified by repeated freeze-thaw cycles in the gashandling line. Results Figure 1 shows STM images for methyl pyruvate (panel A) and methyl lactate (panel B) on Pt(111) at room temperature. It may be immediately seen that both molecules form very similar self-assembled structures. The majority structure is in the form of dimer chains some of which are branched. Sequences of images taken at intervals of 10 min reveal that the chains can break apart or fuse together. The chains are of varying degrees of compactness, and the full width of the most compact structures is ∼1.3 nm. A minority triangular structure, where the sides are formed around a stable trimer core, is also present for both adsorbates (Figure 2). In most cases the sides of the triangles are not fully occupied. Given that no selfassembled structures are observed for either adsorbate on introduction at 150 K, the strong similarity between the structures formed at room temperature suggests that both molecules transform into the same chemical state. The structures formed by methyl pyruvate were attributed, in a previous study, to keto-enol tautomerisation leading to enol-enol assembly.11 Spectroscopic support for the formation

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Figure 2. STM images of minority triangular structures formed by methyl pyruvate (A) and methyl lactate (B) on Pt(111) at room temperature.

Figure 4. Off-specular HREELS spectra for MP and ML on Pt(111): (A) ML at 155 K; (A′) ML following an anneal to 300 K; (B) MP at 155 K; (B′) MP following an anneal to 300 K.

TABLE 1: Comparison of Vibrational Frequencies (cm-1) for Gas-Phase19 Vinyl Alcohol (CH2dCH-OH), and a Pt-Vinyl Alcohol Complex,20 with HREELS Data for the Common Chemical Species Formed by MP and ML on Pt(111) at Room Temperature

Figure 3. In-specular HREELS spectra of methyl pyruvate and methyl lactate on Pt(111): (A) methyl lactate (ML) at 155 K; (A′) methyl lactate following an anneal to 300 K; (B) methyl pyruvate (MP) at 155 K; (B′) methyl pyruvate following an anneal to 300 K.

of the enol form was provided by the observation of bands at 1280 and 1590 cm-1 in the reflection absorption infrared spectroscopy (RAIRS) spectra.11 This pair of frequencies is highly characteristic of enol or enolate species.17,18 In this study, HREELS measurements were used to verify if the methyl pyruvate and methyl lactate structures imaged at room temperature (Figures 1 and 2) are due to a common enol state. Inspecular (Figure 3) and off-specular (Figure 4) spectra show that both adsorbates undergo changes on heating from 155 K to room temperature to yield spectra displaying only minor differences in the relative intensities of the loss peaks. In both cases, new bands or significantly increased intensity is observed in the off-specular spectra at 390, 1300, 1595, 2960, and 3030 cm-1. A comparison is made in Table 1 between the room temperature spectrum and literature data19 for the enol tautomer of acetaldehyde, vinyl alcohol, which we use as a model for the enol moiety of the R-enolester. The good agreement between the vibrational frequencies strongly suggests that methyl lactate undergoes dehydrogenation leading to the same adsorbed enol state that is formed directly through adsorption of methyl

assignment

vinyl alcohol

vinyl alcohol complex

methyl lactate methyl pyruvate

CdC stretchinga CH2 bending CH2 rocking CCO bending CdC torsion o.p.b CCH bending CH2 wagging CCH bending o.p. COH bending OH torsion o.p.

1644 1300 1260 1098 960 948 817 699 486 413

1550 1330 1270 1145 968

1595 {1300 1100 {970

The assignments are for gas phase vinyl alcohol.19 b o.p. ) out of plane. a

pyruvate. This conclusion is further supported by reference to literature data for Pt-coordinated vinyl alcohol.20 The π-complex displays a ν(CdC) band at 1550 cm-1 and strong bands at 1330, 1270, 968, and 497 cm-1.20a The scope of this novel surface chemistry, in which an alcohol undergoes transformation into an enol state, was explored by studying acetophenone and its hydrogenation product 1-phenylethanol. In order to facilitate the analysis of the data for these two R-phenyl molecules it is first necessary to consider results for TFAP and its hydrogenation product TFPE. The latter systems are simpler, since the CF3 group prevents enolization and only dehydrogenation of the alcohol to the ketone can occur. In addition, previously published STM images for TFAP on Pt(111) are understood on the basis of DFT calculations15 that show aryl-CH · · · O intermolecular bonding. This interaction

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Figure 5. STM images of self-assembled structures formed (A) by TFPE and (B) by TFAP on Pt(111) at 300 K. The insert to (B) shows a trimer formed by TFAP.

results in the formation of homochiral dimer and trimer structures (Figure 5B). The most stable calculated dimer structure is illustrated in Scheme 3A.15 The calculated H · · · O distance in the dimers is 2.38 Å.15 This distance may not be inferred from a visual inspection of the images. The bright spot in each molecular image of TFAP only indicates the side of the ring on which the COCF3 group is located. A combination of HREELS (Figure 6) and STM (Figure 5A) data show that TFPE undergoes oxidation to TFAP on Pt(111). The HREELS spectrum for TFPE at 180 K (Figure 6A) displays losses at 1045 and 2875 cm-1 which we attribute to the ν(C-O) and ν(C*H) vibrations, respectively, where C* is the chiral center.21 Both of these losses are removed at room temperature and the resulting spectrum (Figure 6B) is essentially identical to that for adsorbed TFAP (Figure 6C) confirming that the alcohol transforms into the prochiral ketone. In agreement, at room temperature, TFPE is imaged as homochiral trimers and dimers (Figure 5A), which are essentially identical to those formed by TFAP (Figure 5B). By analogy to the analysis of the TFAP data, acetophenone homochiral dimer and trimer assembly at 150 K (Figure 7A) is attributed to aromatic to carbonyl C-H · · · O bonding. However, while each acetophenone molecule is imaged as a one-headed feature at 150 K, heating to room temperature leads to the formation of two-headed features which cluster together into a variety of dimer and trimer configurations (Figure 7B-E). Furthermore, in contrast to the structures formed by TFAP, the structures produced by acetophenone are not limited to homochiral configurations; both homochiral and heterochiral structures are formed in roughly equal amounts. The high-contrast image of acetophenone in Figure 7F shows that a distinction can be made between the two heads of the structure formed at room temperature. The protrusion separated from the body of the molecule is tentatively attributed to the COH group of enolacetophenone while the protrusion merged to the body of the

Figure 6. HREELS data for TFAP and TFPE on Pt(111): (A) TFPE at 180 K; (B) TFPE following an anneal to 300 K; (C) TFAP at 300 K.

molecule is attributed to the enol CH2 group in π-conjugation with the phenyl group. As illustrated in Scheme 4, an interpretation of the room temperature images in terms of enol formation is consistent with a range of plausible OH · · · OH bonded dimer and trimer structures. The arrows in Scheme 4 indicate that some variation exists in the intermolecular angles observed for structures 1, 2, and 4. This is consistent with the flexibility allowed by OH · · · OH bonding. The image shown in Figure 7G is rarely observed. It is tentatively attributed to the keto tautomer of acetophenone. The interpretation of the room temperature acetophenone STM images in terms of keto-enol tautomerisation is supported by HREELS measurements (Figure 8). The in-specular losses at 575, 1200, and 1665 cm-1 observed for acetophenone at 180 K are assigned to CdO bending, C-CH3 stretching, and CdO stretching modes, respectively.22 The latter mode is downshifted by ∼40 cm-1 with respect the gas-phase value indicative of lone-pair carbonyl adsorption. The off-specular spectrum at 180 K shows strong intensity for the CH3 modes22 at 920, 1375, and 1435 cm-1. Heating to room temperature greatly attenuates those bands, as well as the in-specular losses at 575, 1200, and 1665 cm-1, suggesting a change in the acetyl moiety. The intense band which appears at 1640 cm-1 is assigned to the enol ν(CdC) mode by reference to the data in Table 1. The room temperature conversion of 1-phenylethanol into the enol tautomer of acetophenone is confirmed by the combination

SCHEME 3: (A) Calculated Structure of a TFAP Dimer on a Three-Layer 72-Atom Model of a Pt(111) Surface.15 (B) Schematic Drawing of the Calculated Structure



Figure 7. STM images of acetophenone on Pt(111): (A) homochiral trimers at 150 K. The individual molecules are imaged as protrusions displaying a light and a bright area. These are referred to as one-headed protrusions in the text. They are similar to the protrusions observed for TFAP (Figure 5B); heating to room temperature leads to the formation of two-headed protrusions and assembly into both homochiral and heterochiral dimers and trimers; (B) homochiral trimer formed at room temperature; (C) homochiral and heterochiral trimers at room temperature; (D) heterochiral dimer at room temperature; (E) homochiral dimer at room temperature; (F) high-contrast image of a two-headed acetophenone feature. (G) A very small number of one-headed dimers are also observed at room temperature.

SCHEME 4: Schematic Drawings of Plausible OH · · · OH Bonded Self-Assembled Structures Formed by the Enol Tautomer of Acetophenone on Pt(111)a

a The proposed structures are made on the basis of STM and HREELS data showing that acetophenone undergoes keto-enol tautomerization on Pt(111) at room temperature leading to the formation of homochiral and heterochiral dimers and trimers. The arrows in structures 1, 2, and 4 illustrate the fact that there is some variation in the intermolecular alignment in the imaged structures.

of STM and HREELS data shown in Figures 9-11. In addition to a minority close-packed domain structure formed by intact 1-phenylethanol, the alcohol produces the same types of twoheaded homochiral and heterochiral dimers and trimers as acetophenone at room temperature, consistent with enolization and enol-enol assembly. The HREELS data in Figure 10 show that heating adsorbed 1-phenylethanol from 180 K to room temperature causes the band at 1140 cm-1, which we attribute to an alcohol C-O stretching mode,23 to disappear and the enol ν(CdC) band at 1640 cm-1 to grow in. Figure 11 concisely displays the HREELS evidence for molecular rearrangement of the R-phenyl ketone and its chiral hydrogenation product into the same enolic species at 300 K. While the spectra of the two molecules at 180 K are very different they give essentially superimposable new spectra at room temperature. Discussion The data for methyl lactate and 1-phenylethanol show that chiral alcohols can undergo room temperature transformation into self-assembled enol-enol structures on Pt(111). There are two possible reaction channels for the alcohol to enol transformation. In one, oxidation to the ketone occurs first, followed by tautomerization. A clear example of the oxidation reaction is given by the data for TFPE, showing conversion to TFAP at room temperature. In the second, the alcohol converts directly to the enol without forming a chemisorbed ketone intermediate. The direct transformation requires two CH bond breaking

Figure 8. HREELS data for acetophenone on Pt(111). A′, B′ are inspecular spectra and A, B are off-specular spectra. (A and A′) Introduction with the surface held at 180 K; (B and B′) Spectra taken on annealing to 300 K.

reactions, one at the chiral center and one in the CH3 group R to the chiral carbon. Formation of the enol via a ketone

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Figure 11. Superimposed off-specular HREELS data for acetophenone (red) and 1-phenylethanol (blue) at (A) 180 and (B) 300 K.

Figure 9. STM images of 1-phenylethanol at room temperature. (A) Large area scan showing a close packed domain and two-headed dimer and trimer structures; (B) high-contrast image of a two-headed protrusion; (C) homochiral dimer; (D) homochiral trimer; (E) heterochiral trimer and dimer. The numbers 1-4 refer to the schematic drawings in Scheme 4. The structures 1-4 are formed by both 1-phenylethanol and acetophenone on Pt(111) at room temperature.

Figure 10. HREELS data for 1-phenylethanol on Pt(111). A′, B′ are in-specular spectra and A, B are off-specular spectra. (A and A′) Introduction with the surface held at 100 K followed by annealing to 180 K; (B and B′) Introduction with the surface held at 100 K followed by annealing to 300 K.

intermediate requires two additional steps, OH bond breaking, and OH bond formation. The data for the six molecules studied show that all of the above reaction steps can occur on Pt(111) at room temperature. Hence, there is a possibility that both channels contribute to the alcohol to enol transformation. Enolization occurs because, at room temperature, the platinum surface can break a CH bond in the normally inert CH3 group.24 By analogy to the analysis given by Hu et al. of LangmuirHinshelwood processes,25 hydrogen from CH bond scission in the alcohol will be captured by the surface. The metal then

provides a source of atomic hydrogen which can diffuse and react to form enol OH groups. The diffusion step is expected to be very rapid since hydrogen also undergoes recombinative desorption just above room temperature.26 Enolization on platinum is consistent with DFT calculations27 which show that acetone may form stable enol or enolate states on Pt(111) and surface vibrational spectroscopy studies which show an enolate state of acetone on Ni(111).18 Baddeley and co-workers17 have shown that the β-ketoester, methylacetoacetate, undergoes keto-enol tautomerization on Ni(111). Enol tautomers of β-ketoesters may be stabilized by forming an intramolecular six-membered ring through OH to carbonyl hydrogen bonding. In contrast, since R-ketoesters cannot form such a six-membered ring, H-bonding must occur intermolecularly. The stabilization of the enol state is attributed to the combined effect of a strong interaction of the CdC bond with the surface and OH · · · OH intermolecular bonding in the self-assembled structures. A plausible structure for compact chain formation is suggested in Scheme 5. Several other arrangements involving OH · · · OH bonding may also be envisaged, particularly if both cis and trans enol isomers are considered, and this may the reason why most of the chains are not in the most compact form. The alcohol-to-ketone and alcohol-to-enol transformations are driven by the catalytic and chemisorption properties of the metal, and we propose that they may, in turn, impact on the rate and enantioselectivity of the Orito reaction. First, a chiral alcohol product reacting back to a prochiral ketone re-enters the catalytic process and has a probability to undergo hydrogenation at nonmodified racemic sites, thereby lowering both the enantiomeric excess and the conversion. The conversion of methyl lactate to chemisorbed methyl pyruvate on Pd(111) has been recently reported.28 Second, self-assembled enol structures could compete with the formation of 1:1 R-ketoester-modifier complexes. Furthermore, as pointed out by Lavoie et al.,12 even if a 1:1 modifier to enol complex is formed, it may form in such a way (Scheme 6) as to lead to the undesired enantiomer. Indeed, the opposite enantiomer is formed in excess on cinchonidine modified Pd under optimized conditions, where the enol form is known to be present.29 A number of groups have shown that the enol tautomer of MP does not play a role in the optimized Orito reaction on Pt.29,30a Nevertheless, evidence for enolization under reaction conditions (293 K and 30 bar H2) is provided by a study by Jenkins et al.30b The present results suggest that enolization may be relevant to the platinum-catalyzed reaction performed under



SCHEME 5: Schematic Drawing of a Proposed Structure for Self-Assembled Enol Chains Formed on Pt(111) at Room Temperature by Both MP and Its Hydrogenation Product ML

tion, secondary self-assembly processes may be relevant to the optimization of the Orito reaction. Under optimized reaction conditions, TFAP undergoes enantioselective hydrogenation on cinchonidine modified Pt to yield of (R)-TFPE up to 92% ee.7 The same type of catalyst may be used to hydrogenate MP to (R)-ML up to 98% ee.6 However, optimal conditions are quite different in both cases; a much higher surface coverage of the modifier is required when TFAP is the substrate.7 Furthermore, evidence for significant rate enhancement has been reported by several authors for MP and related substrates. In contrast, monoketone substrates bearing an R-phenyl group, such as TFAP, show small or negligible apparent rate enhancement on the modified surface as compared to the rate of the racemic reaction on nonmodified platinum.6 We have proposed15 that TFAP displays small or negligible rate enhancement because carbonyl bond activation in C-H · · · O bonded TFAP dimers and trimers is similar to that for TFAP undergoing C-H · · · O bonding in a 1:1 complex with cinchonidine. In both cases, arylCH · · · O bonding is activated by the chemisorption interaction between the aromatic and the platinum surface. This interpretation implies that the formation of a racemic mixture of homochiral TFAP dimers and trimers is in direct competition with substrate-modifier formation and hence in direct competition with the desired asymmetric induction. This may explain why ∼5-10 times higher concentrations of cinchonidine are used to optimize the ee for TFAP as compared to ethyl pyruvate.7b Presumably, the higher coverage of modifier is required to disrupt substrate-substrate assembly.

SCHEME 6: Schematic Drawing of a Proposed Pro-(S) Diastereomeric Complex Formed between the Enol Tautomer of MP and Cinchonidine Adsorbed on Platinum12a

a This complex differs from the one illustrated in Scheme 2 in that the keto carbonyl undergoes H-bonding to the ammonium group. The rationale for this proposal is that methyl pyruvate is hydrogenated to (S)-methyl lactate on cinchonidine-modified Pd catalysts. There is a consensus that the asymmetric reaction occurs via the enol tautomer on palladium and hence involves CdC bond hydrogenation.6

nonoptimal conditions where the surface coverage of hydrogen is too low. In general, optimized conditions affording highest ee and reaction rate require high hydrogen pressures and care must be taken to avoid hydrogen starvation due to mass transport problems.31 It would be advantageous to optimize the reaction under the mild conditions offered by low H2 pressures.32 However, catalyst design would have to take into account that enolization might occur under such conditions. We note that the optimization of ee for the hydrogenation of TFAP to TFPE does not appear to require high H2 pressures.7b This may possibly be related to the fact that TFAP cannot undergo enolization. The data for TFAP and TFPE provide another example of how in addition to the primary 1:1 modifier-substrate interac-

Conclusions Surface vibrational and STM data for ML and 1-phenylethanol on Pt(111) show that chiral alcohols can convert into the self-assembled enol-enol structures. Similarly, TFPE transforms into CH · · · O bonded TFAP assemblies. In combination, these systems demonstrate that the Pt(111) surface introduces two mechanisms through which self-assembled structures are formed from alcohols at room temperature. In the first, dissociative activation of R-methyl groups allows enolization and self-assembly to take place. In the second, the chemisorption interaction induces strong interadsorbate arylCH · · · O bonding between the R-phenyl ketone formed through oxidation of the parent R-phenyl alcohol. The observed surface chemistry and self-assembly processes were placed in the context of literature data on the optimization of rate and selectivity in the Orito reaction; the asymmetric hydrogenation of activated ketones such as methyl pyruvate and TFAP on chirally modified Pt catalysts. This is a reaction in which chemisorbed self-assembled structures formed by the chiral modifier and the prochiral substrate determine the catalytic function. Under conditions such as insufficient coverage of hydrogen or of the chiral modifier, the oxidation of the alcohol product to the prochiral ketone, the formation of enol-enol assemblies, enol-modifier complexes, or aryl-CH · · · O bonded substrate dimers or trimers may result in nonoptimal ee. References and Notes (1) (a) Schiffrin, A.; Reichert, J.; Pennec, Y.; Auwarter, W.; WeberBargioni, A.; Marschall, M.; Dell’Angela, M.; Cvetko, D.; Bavdek, G.; Cossaro, A.; Morgante, A.; Barth, J. V. J. Phys. Chem. C 2009, 113, 12101. (b) Stepanow, S.; Strunskus, T.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Lin, N.; Barth, J. V.; Wo¨ll, C.; Kern, K. J. Phys. Chem. B 2004, 108, 19392. (2) (a) Vaughan, O. P. H.; Turner, M.; Williams, F. J.; Hille, A.; Sanders, J. K. M.; Lambert, R. M. J. Am. Chem. Soc. 2006, 128, 9578. (3) (a) Weigelt, S.; Busse, C.; Bombis, C.; Knudsen, M. M.; Gothelf, K. V.; Lægsgaard, E.; Besenbacher, F.; Linderoth, T. R. Angew. Chem.,

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