Chiral templating of surfaces: adsorption of (S)-2-methylbutanoic acid on Pt(111) single-crystal surfaces

The adsorption and thermal chemistry of (S)-(+)-2-methylbutanoic acid ((S)-2MBA) on Pt(111) single-crystal surfaces was characterized by using temperature programmed desorption (TPD) and reflection-adsorption infrared (RAIRS) spectroscopies. Particular emphasis was placed on the characterization of the chiral superstructures formed upon the deposition of the submonolayer coverages of enantiopure (S)-2-methylbutanoate species that are produced by thermal dehydrogenation of the (S)-2MBA. The enantioselectivity of the empty platinum sites left open on those structures were identified by their difference in behavior toward the adsorption of the two enantiomers of propylene oxide. It was found that a significant enhancement in adsorption is possible on surfaces with the same chirality of the probe molecule, specifically that the uptake of (S)-propylene oxide is larger than that of (R)-propylene oxide on (S)-2-methylbutanoate adsorbed layers. This contrasts with the lack of enantioselectivity previously reported for the same adsorbate on Pd(111). Detectable differences in adsorption energetics of (R)- vs (S)-propylene oxide on the (S)-2-methylbutanoate/Pt(111) overlayers were measured but deemed not to be the controlling factor in the enantioselectivity reported in this system.     

Adsorption states and modifier-substrate interactions on Pt(111) relevant to the enantioselective hydrogenation of alkyl pyruvates in the Orito reaction

Reflectance FTIR spectroscopy (RAIRS) was used to study the chemisorption and intermolecular interactions of methyl pyruvate and (+/-)-1-(1-naphthyl)ethylamine (NEA) on Pt(111). NEA serves, in this study, as a tractable model of a chiral modifier in the asymmetric hydrogenation of alpha-dicarbonyls on alkaloid-modified platinum surfaces-the Orito reaction. The results show the presence of a majority enediolate state on the clean surface. A perpendicularly adsorbed trans conformation state is populated at close to full-monolayer coverage on the clean surface. The latter state desorbs at 185 K. The enediolate undergoes dissociation at 230 K. NEA displays hydrogen-bond association at high coverages. Coadsorption studies show that NEA inhibits the formation of the enediolate state. Multilayer methyl pyruvate shows a clear hydrogen-bond interaction with chemisorbed NEA, leading to a reorientation of the ethylamine group. The high-coverage trans-chemisorbed methyl pyruvate state also hydrogen-bonds to chemisorbed NEA. The latter interaction renders the trans state stable to above 300 K. A new schematic mechanism for the Orito reaction is proposed on the basis of these data.     

Enantioselective chemisorption on a chirally modified surface in ultrahigh vacuum: adsorption of propylene oxide on 2-butoxide-covered palladium(111)

The enantioselective chemisorption of (S)- and (R)-propylene oxide is measured on a Pd(111) surface chirally modified using (S)- and (R)-2-butanol. Reflection-absorption infrared spectroscopic (RAIRS) data suggest that adsorbed 2-butanol forms 2-butoxide species when heated to approximately 150 K and converts to a ketone with a concomitant loss in chirality at 200 K. Methyl ethyl ketone, ethylene, methane, CO, and hydrogen are found as products in temperature-programmed desorption (TPD). Propylene oxide adsorbs reversibly on Pd(111) at 80 K without undergoing any thermal decomposition, thus providing an ideal probe of surface chirality. The coverage of (R)-propylene oxide adsorbing on an (R)-2-butoxide-covered surface, ratioed to that on one covered by (S)-2-butoxide, reaches a maximum value of approximately 2 at a relative 2-butoxide coverage of approximately 25% of saturation and decreases to unity at a coverage of approximately 50% of saturation. This implies that the enantioselectivity depends critically on coverage and arises due to chiral "pockets" formed on the surface.     

Reflection absorption infrared spectroscopy and temperature programmed desorption investigations of the interaction of methanol with a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the adsorption of methanol (CH(3)OH) on the highly oriented pyrolytic graphite (HOPG) surface. RAIRS shows that CH(3)OH is physisorbed at all exposures and that crystalline CH(3)OH can be formed, provided that the surface temperature and coverage are high enough. It is not possible to distinguish CH(3)OH that is closely associated with the HOPG surface from CH(3)OH adsorbed in multilayers using RAIRS. In contrast, TPD data show three peaks for the desorption of CH(3)OH. Initial adsorption leads to the observation of a peak assigned to the desorption of a monolayer. Subsequent adsorption leads to the formation of multilayers on the surface and two TPD peaks are observed which can be assigned to the desorption of multilayer CH(3)OH. The first of these shows a fractional order desorption, assigned to the presence of hydrogen bonding in the overlayer. The higher temperature multilayer desorption peak is only observed following very high exposures of CH(3)OH to the surface and can be assigned to the desorption of crystalline CH(3)OH.     

Reflection absorption infrared spectroscopy and temperature-programmed desorption studies of the adsorption and desorption of amorphous and crystalline water on a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two- and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.     

Aspects of enantioselective heterogeneous catalysis: structure and reactivity of (S)-(-)-1-(1-naphthyl)ethylamine on Pt[111

The molecular orientation, spatial distribution, and thermal behavior of the powerful chiral catalyst modifier precursor (S)-naphthylethylamine adsorbed on Pt[111] have been studied by NEXAFS, XPS, STM, and temperature programmed reaction. At 300 K, both in the presence and in the absence of coadsorbed hydrogen, the strongly tilted molecules do not form ordered arrays. These results constitute the first direct evidence against the template model and are at least consistent with the 1:1 interaction model of chiral induction in the enantioselective hydrogenation of alkyl pyruvates. Raising the temperature beyond 320 K (the temperature of enantioselectivity collapse) leads either to irreversible dimerization with hydrogen elimination or to dissociation of the ethylamine moiety, depending on whether coadsorbed H(a) is present. Either way, the stereogenic center is destroyed. These findings provide the first direct clue as to the possible origin of enantioselectivity collapse, by a mechanism not previously considered. When NEA and methyl pyruvate are coadsorbed in the presence of H(a), STM reveals entities that could correspond to a 1:1 docking complex between the prochiral reactant and the chiral modifier.     

Changes in the Enantiomeric Composition of Chiral Mixtures Upon Adsorption on a Non‐Chiral Surface

The adsorption of propylene oxide, a chiral molecule, on a Pt(111) single‐crystal surface was studied as a function of enantiomeric composition by temperature programmed desorption (TPD) and molecular beams. Two opposing trends were observed leading to variations in the enantiomeric excess (ee) of the chemisorbed layers with respect to the composition of the gas‐phase mixtures: a kinetic effect dominant during the initial uptake, with a preference toward the formation of enantiopure layers, and a steady‐state effect seen after approximately monolayer half‐saturation, at which point the preference is toward racemization. These effects may account for important phenomena such as the bias toward one chirality in biological systems and the selective crystallization of some chiral compounds, and may also be used in practical applications for chemical separations and catalysis.     

Mechanistic insights into the proline-directed enantioselective heterogeneous hydrogenation of isophorone

The adsorption rates onto a range of platinum single-crystal surfaces of key species involved in the proline-directed heterogeneous enantioselective hydrogenation of isophorone were investigated by electrochemical means. Specifically, the uptakes of the prochiral reactant (isophorone), the chiral hydrogenation product (3,3,5-trimethylcyclohexanone), and the chiral directing agent ((R)- and (S)-proline) were examined. The effects of R,S chiral kink sites on the adsorption of (R,S)-proline were also studied. The reactant adsorbs approximately 105 times faster than the chiral modifier so that under conditions of competitive adsorption the latter is entirely excluded from the metal surface. Supplementary displacement and reaction rate measurements carried out with practical Pd/carbon catalysts show that under certain reaction conditions isophorone quickly displaces preadsorbed proline from the metal surface. Thus both kinetics and thermodynamics ensure that the chiral modifier can play no role in any surface-mediated process that leads to enantiodifferentiation. These results are fully consistent with the recent proposal1 that the crucial step leading to enantiodifferentiation occurs in the solution phase and not at the metal surface. In addition, it is found that there is no preferred diastereomeric interaction between (R,S)-proline and R,S step kink sites on Pt{643} and Pt{976}, implying that such sites do not play a role in determining the catalytic behavior of supported metal nanoparticles.     

The effects of gold and Co-adsorbed carbon on the adsorption and thermal decomposition of acetic acid on Pd{111}

The growth and annealing behavior of ultrathin Au films on Pd{111} were monitored with scanning tunneling microscopy (STM) and medium energy ion scattering (MEIS). The adsorption of acetic acid on both clean and deliberately carbon-contaminated bimetallic surfaces was investigated with reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). We report that the surface chemistry of acetic acid is strongly modified by the presence of Au in the bimetallic surface which acts both to stabilize adsorbed acetate and to decrease the tendency of acetic acid to decompose on adsorption to produce adsorbed carbon. The adsorption of acetic acid at 300 K is found to cause measurable segregation of Pd to the surface for all surface compositions tested.     

Mechanisms of Asymmetric Heterogeneous Catalysis1

Asymmetric catalysis plays an important role in present-day production of pharmaceuticals. It can be predicted that enantioselective catalysis will dominate in the future, replacing conventional stoichiometric methods. Heterogeneous catalysts offer several advantages compared to their homogeneous counterparts. In the present review mechanisms of asymmetric heterogeneous catalysis are discussed and the different ways of chirality transfer are addressed. Enantioselection is possible over chiral supports and chiral metals exhibiting intrinsic chirality, as well as over modified metal supported catalysts. The interactions between a reactant and a modifier are very specific being critical for achieving high enantioselectivities.     

Enantiospecific desorption of chiral compounds from chiral Cu(643) and achiral Cu(111) surfaces

Temperature-programmed desorption (TPD) experiments have been conducted to investigate enantiospecific desorption from chiral single-crystal surfaces. The (643) and (six four three) planes of face-centered cubic metals such as Cu have kinked and stepped structures which are nonsuperimposable mirror images of one another and therefore are chiral. These chiral surfaces are denoted Cu(643)(R) and Cu(643)(S). We have observed that the desorption energies of (R)-3-methylcyclohexanone and (R)- and (S)-propylene oxides from the Cu(643)(R) and Cu(643)(S) surfaces depend on the relative handedness of the adsorbate/substrate combination. Since the (643) surface is comprised of terraces with local (111) orientation which are separated by kinked monatomic steps, it is instructive to perform TPD experiments with these chiral compounds on the achiral Cu(111) surface. These experiments have given some insight into the adsorption sites for the chiral molecules on the Cu(643) surfaces. There are several high-temperature features in the TPD spectra of the chiral compounds that only appear in the spectra from the (643) surfaces and thus are attributed to molecules adsorbed at or near the kinked steps. In addition there are lower temperature desorption features observed on the Cu(643) surfaces which occur in the same temperature range as desorption features observed on the Cu(111) surface. These features observed on the (643) surfaces are attributed to desorption from the flat (111) terraces.     

Factors controlling adsorption equilibria from solution onto solid surfaces: the uptake of cinchona alkaloids on platinum surfaces

The room-temperature adsorption of four closely related cinchona alkaloids and three reference quinoline-based compounds from CCl4 solutions onto a polycrystalline platinum surface was characterized by in situ reflection-absorption infrared spectroscopy (RAIRS). The adsorption equilibrium constants (Kads) were found to follow the sequence cinchonine > quinidine > cinchonidine > quinine > 6-methoxyquinoline > lepidine > quinoline. Some of this ordering can be explained by differences in solubility, but quinidine displays a much larger Kads than expected on the basis of its large relative solubility; bonding to the surface must also play a role in determining its behavior. It was determined that each alkaloid binds differently on Pt at saturation coverages. While the quinoline ring of cinchonidine tilts along its long axis to optimize pi-pi intermolecular interactions, in cinchonine it tilts along the short axis and bonds through the lone electron pair of the nitrogen atom instead, and both quinine and quinidine exhibit additional bonding via the methoxy oxygen atom at intermediate concentrations. Perhaps a more surprising result from this work is the fact that cinchonine displays a higher Kads than cinchonidine, quinine, or quinidine even though, according to previous work, it can be easily displaced from the surface by any of those other cinchona alkaloids. A full explanation of these observations requires consideration of the solvent above the adsorbed species.     

Surface chemistry of CN bond formation from carbon and nitrogen atoms on Pt(111)

The mechanism of CN bond formation from CH3 and NH3 fragments adsorbed on Pt(111) was investigated with reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The surface chemistry of carbon-nitrogen coupling is of fundamental importance to catalytic processes such as the industrial-scale synthesis of HCN from CH4 and NH3 over Pt. Since neither CH4 nor NH3 thermally dissociate on Pt(111) under ultrahigh vacuum (UHV) conditions, the relevant surface intermediates were generated through the thermal decomposition of CH3I and the electron-induced dissociation of NH3. The presence of surface CN is detected with TPD through HCN desorption as well as with RAIRS through the appearance of the vibrational features characteristic of the aminocarbyne (CNH2) species, which is formed upon hydrogenation of surface CN at 300 K. The RAIRS results show that HCN desorption at approximately 500 K is kinetically limited by the formation of the CN bond at this temperature. High coverages of Cads suppress CN formation, but the results are not influenced by the coadsorbed I atoms. Cyanide formation is also observed from the reaction of adsorbed N atoms and carbon produced from the dissociation of ethylene.     

Aspects of heterogeneous enantioselective catalysis by metals

Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific reference to four different systems where the phenomena that control enantioselection appear to be very different. In the case of glucose electro-oxidation, it is clear that any intrinsic chirality present at the metal surface plays a vital role. With α-keto hydrogenation, achiral surfaces modified by the adsorption of chiral agents become effective enantioselective catalysts and the formation of extended arrays of chiral species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. Hydrogen bonding also plays a central role in β-ketoester hydrogenation, but here fundamental studies indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. Asymmetric C═C hydrogenation, though relatively little studied, has the potential for major impact in synthetic organic chemistry both on the laboratory scale and in the manufacture of fine chemicals and pharmaceuticals. The structural attributes that determine whether a given chiral ligand is effective have been identified; the ability to form strong covalent bonds with the metal surface while also resisting hydrogenation and displacement by the strongly adsorbing reactant under reaction conditions is an essential necessary condition. Beyond this, ligand rigidity in the vicinity of the chirality center coupled with resistance to SAM formation is a critically important factor whose absence results in racemic chemistry.     

X-ray structure and circular dichroism of pure rotamers of bis[guanosine-5'-monophosphate(-1)](N,N,N',N'-tetramethylcyclohexyl-1,2-diamine)platinum(II) complexes that have R,R and S,S configurations at the asymmetric diamine

The use of a sterically hindered diamine ligand (Me(4)DACH) has allowed for the first time, the isolation and characterization, both in the solid state (X-ray crystallography) and in solution (circular dichroism), of pure DeltaHT rotamers of [Pt(Me(4)dach)(5'-GMP)(2)] (compounds 1 and 2 for R,R and S,S configurations of the Me(4)DACH ligand, respectively). Comparison of the CD spectra obtained for each rotamer, which differ only in the chirality of the Me(4)DACH ligand (R,R or S,S) or in the chirality of the HT conformation (Delta or Lambda), allowed us to conclude that, in the 200-350 nm range, the contributions to the overall CD spectrum that stem from diamine chirality and diamine-induced chirality of platinum d--d transitions or from sugar chirality are negligible relative to the exciton chiral coupling that occurs for pi-pi* transitions of the cis guanines. Accurate molecular structures of 1.10 D(2)O and 2.14 D(2)O (conventional crystallographic agreement indexes R(1) convergent to 2.07 % and 2.18 %, respectively) revealed that the crystallized rotamers have a DeltaHT conformation that is in agreement with all previously reported X-ray structures of [Pt(diamine)(nucleos(t)ide)(2)] complexes. This conformation allows the 5'-phosphate to be located in proximity to the Me(4)DACH ligand so that (P)O...HC(N) hydrogen-bond interactions exists in both complexes. For both structures, the canting of the guanine planes on the coordination plane is right-handed (R; canting angle (Phi) of 80.9 degrees and 73.2 degrees, respectively); this indicates that the canting direction is driven by the HT conformation chirality (Delta for both compounds) and not by the chirality of the carrier ligand (different for the two compounds). Density functional theory analysis of the conformational space as a function of Phi indicated a good agreement between the computed and experimental structures. The increase in energy for Phi values below 65 degrees and 55 degrees (for 1 and 2, respectively) is mainly due to the short intramolecular contacts between C(8)H and the cis N-Me groups on the same side of the platinum coordination plane.     

Crystallization kinetics and excess free energy of H2O and D2O nanoscale films of amorphous solid water

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.     

Role of sulfur chirality in the chemical processes of biology

Chiral structures profoundly influence chemical and biological processes. While chiral carbon biomolecules have received much attention, chirality is also possible in certain sulfur compounds; just as with carbon, there can be differences in the physiological behavior of chiral sulfur compounds. For instance, one drug enantiomer, Nexium (esomeprazole, a chiral sulfoxide), is used for its superior clinical properties as a proton pump inhibitor over the racemic mixture, Prilosec (Losec, omeprazole). This critical review introduces sulfur stereochemistry and nomenclature, and provides a comprehensive approach to chiral sulfur compounds and their enzymatic reactions in general and secondary metabolism. The major structural types of biological interest are sulfonium salts, sulfoxides, and sulfoximines. (103 references).     

Enantioselectivity of adsorption sites created by chiral 2-butanol adsorbed on Pt(111) single-crystal surfaces

The adsorption and thermal chemistry of 2-butanol and propylene oxide, each individually and when coadsorbed together, were characterized on Pt(111) single-crystal surfaces by using temperature programmed desorption and reflection-adsorption infrared spectroscopies. The formation of chiral superstructures on the surface upon the deposition of submonolayer coverages of enantiopure 2-butoxide species, produced by thermal dehydrogenation of 2-butanol, was highlighted by their difference in behavior toward the adsorption of the two enantiomers of propylene oxide. It was found that a significant enhancement in adsorption is possible on surfaces with the same chirality of the probe molecule, that is, for (R)-propylene oxide adsorption on (R)-2-butoxide layers and for (S)-propylene oxide adsorption on (S)-2-butoxide layers. The propylene oxide probe was found to also adsorb with the ring closer to the surface in those cases. Finally, less butoxide decomposition is seen at higher temperatures from the homochiral pairing, presumably because the coadsorbed propylene oxide forces the alkoxides into a more compact and better packed structure on the surface.     

Intermediates in the destruction of chlorinated C1 hydrocarbons on La-based materials: mechanistic implications

Activity experiments using GC analysis of reactor effluent have been combined with in situ IR spectroscopy to elucidate the reaction steps in the destructive adsorption of CHCl3, CH2Cl2, and CH3Cl over LaOCl. The IR results show that during reaction, LaOCl is covered with carbonate, formate, and methoxy groups. The relative amount of each of these surface intermediates depends on the Cl/H ratio of the reactant. The decomposition of the surface species leads to formation of the reaction products, and is influenced by the temperature and the relative amount of Cl present on the surface. The GC results show that the activity for the destructive adsorption of H-containing chlorinated C1 compounds decreases with increasing hydrogen content of the reactant. The acquired insight into the mechanism of destructive adsorption is crucial to the design of new catalyst materials for the efficient conversion of chlorinated hydrocarbons into nonhazardous products or reusable chemicals.     

Pillararene Host–Guest Complexation Induced Chirality Amplification: A New Way to Detect Cryptochiral Compounds

The contradiction between the rising demands of optical chirality sensing and the failure in chiral detection of cryptochiral compounds encourages researchers to find new methods for chirality amplification. Inspired by planar chirality and the host–guest recognition of pillararenes, we establish a new concept for amplifying CD signals of cryptochiral molecules by pillararene host–guest complexation induced chirality amplification. The planar chirality of pillararenes is induced and stabilized in the presence of the chiral guest, which makes the cryptochiral molecule detectable by CD spectroscopy. Several chiral guests are selected in these experiments and the mechanism of chiral amplification is studied with a non-rotatable pillararene derivative and density functional theory calculations. We believe this work affords deeper understanding of chirality and provides a new perspective for chiral sensing.