Chiral Modifier Research Articles

Highly oxidized Pt species stabilized inside carbon nanotubes for asymmetric hydrogenation

The chemical state and its influence on Pt species in or outside of the channels of CNTs and the effect on the asymmetric hydrogenation of α-ketoester were investigated. XPS analysis showed that 13% Pt species in a highly oxidized state (Pt4+) were stabilized inside the channels in the presence of Na+. There were more highly oxidized Pt species inside the CNTs than outside. The highly oxidized Pt species promoted the interaction between the nanoparticle and chiral modifier, which is crucial for high enantioselectivity. Hydrogen temperature programmed desorption showed that Pt nanoparticles confined in the channels can better activate hydrogen, which contributed to their high activity even at low hydrogen pressure.

Visual chiral recognition of mandelic acid enantiomers with l-tartaric acid-capped gold nanoparticles as colorimetric probes

We report a method for visual differentiation between the l- and d-forms of mandelic acid (MA). It is based on the chirality of l-tartaric acid-capped gold nanoparticles (l-TA-capped AuNPs, ca. 13nm in diameter) that can be used as chiral selector for l- and d-forms of MA. On addition of l-MA to a solution of the l-TA-capped AuNPs, a color change from red to blue can be seen, while no color change is found on addition of d-MA. The chiral assay can be used to determine the enantiomeric excess of l-MA in the range from −100% to 100%. The effect can be detected with bare eyes and quantified by spectrophotometry. Notably, this method does not require the complicated chiral modification. The method excels by its sensitivity, low cost, good availability of materials, and its simplicity.

ChemInform Abstract: Surface Chemistry for Enantioselective Catalysis

AbstractReview: 128 refs.

Separation of catechin epimers by complexation using ion mobility mass spectrometry.

Ion mobility coupled with mass spectrometry provides a fast and repeatable method to separate catechin epimers by previous complexation with selected chiral modifiers and transition metals. Several combinations with chiral ligands such as D- and L-amino acids and/or additional metal cations, chiral crown ethers, tartaric acid and heptakis(2,6-di-O-methyl)-β-cyclodextrin were screened for their ability to affect the separation efficiency. The clusters having the form of [2M + D-amino acid + Cu(2+) - 3H](-) (M stands for (-)-epicatechin or (+)-catechin) showed improvement in stereodifferentiation between two epimeric catechins in comparison to the analysis of pure epimers, where no separation was observed or the separation was hampered by the formation of mixed dimer complexes. Among various examined D-amino acids only those possessing hydrophobic side chains induced the improvement of separation efficiency. The best peak-to-peak resolution (Rp-p) was determined to be 0.71 for [2M + D-Leucine + Cu(2+) - 3H](-) clusters.

Impact of branching on the supramolecular assembly of thioethers on Au(111).

Alkanethiolate monolayers are one of the most comprehensively studied self-assembled systems due to their ease of preparation, their ability to be functionalized, and the opportunity to control their thickness perpendicular to the surface. However, these systems suffer from degradation due to oxidation and defects caused by surface etching and adsorbate rotational boundaries. Thioethers offer a potential alternative to thiols that overcome some of these issues and allow dimensional control of self-assembly parallel to the surface. Thioethers have found uses in surface modification of nanoparticles, and chiral thioethers tethered to catalytically active surfaces have been shown to enable enantioselective hydrogenation. However, the effect of structural, chemical, and chiral modifications of the alkyl chains of thioethers on their self-assembly has remained largely unstudied. To elucidate how molecular structure, particularly alkyl branching and chirality, affects molecular self-assembly, we compare four related thioethers, including two pairs of structural isomers. The self-assembly of structural isomers N-butyl methyl sulfide and tert-butyl methyl sulfide was studied with high resolution scanning tunneling microscopy (STM); our results indicate that both molecules form highly ordered arrays despite the bulky tert-butyl group. We also investigated the effect of intrinsic chirality in the alkyl tails on the adsorption and self-assembly of butyl sec-butyl sulfide (BSBS) with STM and density functional theory and contrast our results to its structural isomer, dibutyl sulfide. Calculations provide the relative stability of the four stereoisomers of BSBS and STM imaging reveals two prominent monomer forms. Interestingly, the racemic mixture of BSBS is the only thioether we have examined to date that does not form highly ordered arrays; we postulate that this is due to weak enantiospecific intermolecular interactions that lead to the formation of energetically similar but structurally different assemblies. Furthermore, we studied all of the molecules in their monomeric molecular rotor form, and the surface-adsorbed chirality of the three asymmetric thioethers is distinguishable in STM images.

Chiral supramolecular nanoparticles: The study of chiral surface modification of silver nanoparticles by cysteine and its derivatives

Many studies dealing with modified nanoparticles are based on the assumption that organic molecules form single-layer structure on their surface. In this work, we suggest a novel approach to study the important phenomenon of supramolecular chirality at metal nanoparticle surfaces. Self-assembly layers of several ligands, namely l-cysteine, l-penicillamine and α-methyl-l-cysteine, on different silver nanoparticles were studied by means of absorption and electronic circular dichroism spectroscopy in UV–vis range and surface enhanced Raman scattering spectroscopy. The amount of ligands bonded to the surface of nanoparticles was quantified by absorption spectroscopy. Quantification was performed indirectly by measuring concentration of free ligands in solution after incubation with absorption labels 4-toluenesulfonyl chloride or 1-fluoro-2,4-dinitrobenzene. Data were obtained for ligands at concentration levels from 10−6 to 10−3 mol L−1. The results showed the significantly lower adsorbed molecules of methyl derivatives of cysteine than for cysteine themselves. This fact could explain the unexpected behavior of silver nanoparticles that provide ECD bands in the presence of cysteine and not in the presence of its derivatives.

Highly enantioselective adsorption of small prochiral molecules on a chiral intermetallic compound.

Intrinsically chiral surfaces of intermetallic compounds are shown to be novel materials for enantioselective processes. Their advantage is the significantly higher thermal and chemical stability, and therefore their extended application range for catalyzed chiral reactions compared to surfaces templated with chiral molecular modifiers or auxiliaries. On the Pd1 -terminated PdGa(111) surface, room-temperature adsorption of a small prochiral molecule (9-ethynylphenanthrene) leads to exceptionally high enantiomeric excess ratios of up to 98 %. Our findings highlight the great potential of intrinsically chiral intermetallic compounds for the development of novel, enantioselective catalysts that can be operated at high temperatures and potentially also in harsh chemical environments.

Highly Enantioselective Adsorption of Small Prochiral Molecules on a Chiral Intermetallic Compound

AbstractIntrinsically chiral surfaces of intermetallic compounds are shown to be novel materials for enantioselective processes. Their advantage is the significantly higher thermal and chemical stability, and therefore their extended application range for catalyzed chiral reactions compared to surfaces templated with chiral molecular modifiers or auxiliaries. On the Pd1‐terminated PdGa(111) surface, room‐temperature adsorption of a small prochiral molecule (9‐ethynylphenanthrene) leads to exceptionally high enantiomeric excess ratios of up to 98 %. Our findings highlight the great potential of intrinsically chiral intermetallic compounds for the development of novel, enantioselective catalysts that can be operated at high temperatures and potentially also in harsh chemical environments.

Ferrites catalyzed aerobic oxidation of benzoin and its extension to enantioselective version

Ferrites have been used as efficient catalysts for the oxidation of α-hydroxy ketones in the presence of air. The enantio selective version of the benzoin oxidation gave 30% yield with 43% ee using (L)-tartaric acid as chiral modifier on magnetite (inverse spinel).

Asymmetric Hydrogenation of Benzalacetone Catalyzed by TPPTS-Stabilized Ru in Ionic Liquids

Achiral monophosphine TPPTS [TPPTS: P(m-C6H4SO3Na)3]-stabilized Ru was successfully applied to catalyze the asymmetric hydrogenation of benzalacetone in ionic liquids using (S,S)-DPENDS [disodium salt of sulfonated (S,S)-1,2-diphenyl-1,2-ethylene-diamine] as chiral modifier. Under the optimized reaction conditions, the conversion of benzalacetone, chemoselectivity, and enantioselectivity of 4-phenyl-3-buten-2-ol could be reached up to 99.5%, 98.8%, and 74.6%, respectively. The TPPTS-stabilized Ru catalyst immobilized in ionic liquids could be easily separated from the resulting products by extraction with n-hexane, which could be recycled and reused five times without significant loss in activity, chemoselectivity, and enantioselectivity.

Single-chiral-catalytic-surface-sites: STM and DFT study of stereodirecting complexes formed between (R)-1-(1-naphthyl)ethylamine and ketopantolactone on Pt(111)

Revealing the set of most stable bimolecular complexes formed by a prochiral molecule and a chiral modifier on Pt(111).

Chirally-modified metal surfaces: energetics of interaction with chiral molecules.

Imparting chirality to non-chiral metal surfaces by adsorption of chiral modifiers is a highly promising route to create effective heterogeneously catalyzed processes for the production of enantiopure pharmaceuticals. One of the major current challenges in heterogeneous chiral catalysis is the fundamental-level understanding of how such chirally-modified surfaces interact with chiral and prochiral molecules to induce their enantioselective transformations. Herein we report the first direct calorimetric measurement of the adsorption energy of chiral molecules onto well-defined chirally-modified surfaces. Two model modifiers 1-(1-naphthyl)ethylamine and 2-methylbutanoic acid were used to impart chirality to Pt(111) and their interaction with propylene oxide was investigated by means of single-crystal adsorption calorimetry. Differential adsorption energies and absolute surface uptakes were obtained for the R- and S-enantiomers of propylene oxide under clean ultrahigh vacuum conditions. Two types of adsorption behavior were observed for different chiral modifiers, pointing to different mechanisms of imparting chirality to metal surfaces. The results are analyzed and discussed in view of previously reported stereoselectivity of adsorption processes.

Adsorption and stability of chiral modifiers based on 1-(1-naphthyl)-ethylamine for Pt catalysed heterogeneous asymmetric hydrogenations

Catalytic and in situ ATR-IR spectroscopic investigations provide mechanistic insight relevant to heterogeneous asymmetric hydrogenation on Pt catalyst using naphthylethylamine-based modifiers.

Chiral modification of copper exchanged zeolite-Y with cinchonidine and its application in the asymmetric Henry reaction.

Chirally modified Cu(2+) exchanged zeolite-Y was synthesized by direct adsorption of cinchonidine under ambient conditions. The chirally modified materials were characterized using various spectrochemical and physicochemical techniques viz. BET, FTIR, MAS ((1)H and (13)C NMR), XPS, SEM, cyclic voltammetry and PXRD. Characteristic peaks of cinchonidine observed in the supported materials confirmed the adsorption of cinchonidine and its coordination with the Cu(2+) active site on copper exchanged zeolite-Y. (13)C SSNMR and XPS analysis however confirmed for the half encapsulation process, only the quinoline ring of cinchonidine gets coordinated to the internal metal sites via the N atom while the quinuclidine moiety extends out of the host surface. Cinchonidine supported Cu(2+)-Y zeolites were found to exhibit good catalytic performance in the asymmetric Henry reaction. (1)H SSNMR studies also confirmed the protonation of the N atom of the quinuclidine ring during the course of the Henry reaction. Heterogeneous chiral catalysts were effective for up to two consecutive cycles. Leaching of cinchonidine after the second cycle was found to have a negative result in the catalytic performance.

Chiral modification of platinum: ab initio study of the effect of hydrogen coadsorption on stability and geometry of adsorbed cinchona alkaloids.

The cinchona alkaloids cinchonidine and cinchonine belong to the most efficient chiral modifiers for the noble metal-catalyzed enantioselective hydrogenation of C=O and C=C bonds. Under reaction conditions these modifiers are coadsorbed on the noble metal surface with hydrogen. Using density functional theory, we studied the effect of coadsorbed hydrogen on the adsorption mode of cinchonidine and cinchonine on a Pt(111) surface at different hydrogen coverages. The theoretical study indicates that the presence of coadsorbed hydrogen affects both the adsorption geometry as well as the stability of the adsorbed cinchona alkaloids. At all hydrogen coverages the cinchona alkaloids are found to be adsorbed via anchoring of the quinoline moiety. In the absence of hydrogen as well as at low hydrogen coverage the quinoline moiety adsorbs nearly parallel to the surface, whereas at higher hydrogen coverage it becomes tilted. Higher hydrogen coverage as well as partial hydrogenation of the quinoline part of the cinchona alkaloid and hydrogen transfer to the C[double bond, length as m-dash]C double bond at 10, 11 position of the quinuclidine moiety destabilize the adsorbed cinchona alkaloid, whereas hydrogen transfer to the nitrogen atom of the quinoline and the quinuclidine moiety stabilizes the adsorbed molecule. The stability as well as the adsorption geometry of the cinchona alkaloids are affected by the coadsorbed hydrogen and are proposed to influence the efficiency of the enantiodifferentiating ability of the chirally modified platinum surface.

Ligand-Acceleration by a Chiral Modifier in the Enantioselective Hydrogenation of Methyl Acetoacetate on a Raney Nickel Catalyst: Effect of a Modifier Configuration

Abstract Reaction rate and enantioselectivity of asymmetric hydrogenation of methyl acetoacetate were studied over Raney nickel catalysts modified with (R,R)-tartaric acid, malic acid, or succinic acid to reveal the impacts of the modifier configuration. Catalysts comodified with two different acids were also examined to confirm the conclusions. From analysis of the enantiomer ratio of the hydrogenation product and initial reaction rate, tartaric acid (TA) was found to have dual functions as the modifier during the hydrogenation; effective suppression of the racemic catalysis on bare Ni surface and extensive enantiodifferentiating ligand acceleration by adsorbed TA. It was demonstrated that each adsorbed chiral modifier molecule independently takes part in the enantiospecific hydrogenation.

Chiral imidazolidinone and proline-derived surface modifiers for the Pt-catalysed asymmetric hydrogenation of activated ketones

A series of imidazolidinone and proline derivatives have been synthesised and tested with regard to their suitability for the chiral modification of platinum used for the asymmetric hydrogenation of activated ketones. Hydrogenations of ketopantolactone (KPL), methyl benzoylformate (MBF) and trifluoroacetophenone (TFAP) performed at low hydrogen pressures (1 and 10bar) were selected as test reactions. With some of these synthethic modifiers, encouraging levels of enantioselectivity were achieved (up to 73.5:26.5 e.r.) without optimisation of the reaction conditions. Moreover, performance enhancement of l-proline derived-modifiers, as a consequence of molecular editing with fluorine, was found to be significant (OH→F, Δee up to 23%) contributing to the growing interest in modulating catalyst performance by fluorine introduction.

Surface Chemistry for Enantioselective Catalysis

A Perspective is offered on the lessons learned from surface-science studies on enantioselective chemistry on solid surfaces performed by the author’s groups. Our emphasis is on studies on model systems, mainly metal single-crystal surfaces under controlled environments, but extension of such research to more realistic samples relevant to heterogeneous catalysis is also briefly discussed. Enantioselective chemistry on surfaces is here divided into three guiding modalities, depending on the underlying mechanism. First, enantioselective chemistry resulting from the use of intrinsically chiral surfaces, which can be made from achiral solids such as metals by exposing the appropriate planes, is discussed. Next, the imparting of enantioselectivity to achiral surfaces by modifying them with adsorbates is classified in terms of two operating mechanisms: first, via the formation of supramolecular surface ensembles with chiral adsorption sites, and second, by relying on the effect of the local chiral environment intrinsically provided by the chiral modifiers through a one-to-one interaction between the modifier and the reactant. A discussion is then provided on studies with more complex samples involving metal nanoparticles and high-surface-area porous oxides. Finally, the present state of our understanding of enantioselective surface chemistry and the prognosis for the future are provided.

Theoretical Modeling of the Interaction Chiral Modifier/Substrate as a Key Step in the Enantioselective Hydrogenation of α-ketoesters and Vicinal Diketones

This paper deals with the computational modeling of the chiral modifier/substrate interaction for chiral modifiers studied in our laboratory, different from those conventionally used in enantioselective hydrogenation reactions. (S)- (+)-1-aminoindane and (R)-(-)-1-aminoindane were chosen as chiral modifiers and the selected substrates were methyl pyruvate, ethyl pyruvate and 1-ethyl-4,4-dimethyl-pyrrolidinae-2,3,5-trione. The geometry of each of the chiral modifier/substrate complexes was optimized using DFT calculations and a BLYP functional. The theoretical enantiomeric excess was calculated from the energy of each of the proposed complexes. The calculations were carried out considering different reaction solvents through the use of COSMO program. It was found that this simple model allows predicting the experimental values of both the sense of enantiodifferentiation and the enantiomeric excess with a good approximation. It was also able to predict the inversion of configuration when using the (S)-(+)-1-aminoindane as chiral modifier in polar solvents such as acetic acid and 2-propanol. Keywords: Theoretical modeling, Enantioselectivity, DFT, Hydrogenation, Chiral modifiers, Aminoindane.

Reduced graphene oxide supported chiral Ni particles as magnetically reusable and enantioselective catalyst for asymmetric hydrogenation

A reduced graphene oxide (rGO) supported chiral-modified Ni catalyst was synthesized, characterized and employed for asymmetric hydrogenation. The prepared hybrid catalyst could produce each enantiomer with d- or l-tartaric acid as chiral modifier and exhibited a high TOF (20160h−1) and enantioselectivity (enantiomeric excess, 98.5%) for asymmetric hydrogenation of methyl acetoacetate. The high catalytic activity and enantioselectivity were mainly attributed to the unique properties of the support rGO, as it had a large specific surface area to sustain and stabilize Ni particles and its high charge carrier mobility could enable the readily transfer of electrons in the reaction process. Besides, the catalyst could also gain an enhanced reactant sorption with the support of rGO, thus achieved a greatly catalysis enhancement. The ferromagnetism of Ni made the catalyst easier for separation and reuse. The catalytic and recycling performance of the prepared chiral Ni catalyst demonstrated that rGO was indeed a promising support to improve activity, enantioselectivity and durability of catalysts, and the prepared catalysts were promising reusable heterogeneous catalysts for asymmetric hydrogenation.