Templated chiral surface

One of the first attempts to explain e.s. was made by Wells and coworkers [234], who proposed that the L-shaped modifier could generate a chiral surface, by adsorption on Pt in ordered nonclose-packed arrays, allowing preferential adsorption on the metal surface of one of the faces of the prochiral substrate (template model). 

Broadly speaking, there are two types of chiral surfaces those that are templated [2] and those that are naturally chiral. The most commonly used and studied chiral surfaces are those that are templated with chiral organic ligands. These are achiral substrates of any solid that have been modified by the adsorption of a chiral organic compound. The presence of the chiral ligand renders the local structure chiral, and... 

The so-called imprinted polymers are obtained by polymerization of monomers in the presence of chiral template molecules [21]. The templates are washed out of the stationary phase after completion of the polymerization. After this treatment, the polymer is supposed to have a chiral surface. Due to low selectivities. 

Synthetic methods include the use of silanes bearing a chiral group for silylating the surface of the porous sol-gel silica, the use of such silanes as monomers or co-monomers in the polycondensation, the physical entrapment of chiral molecules, the imprinting of SG materials with chiral templates and the creation of chiral pores, and the induction of chirality in the matrix skeleton itself 48... 

ORMOSIL are chemical sponges they adsorb and concentrate reactants at their surface, thereby enhancing reaction rates and sensitivity (in sensing applications). ORMOSIL-imprinted materials with a suitable chiral template such as a surfactant or a protein selectively adsorb (and detect) external reactants. A remarkable example is provided by thin materials that are generally enantioselective, namely where the chirally imprinted cavities can discriminate between enantiomers of molecules not used in the imprinting process, and completely different from the imprinting ones. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

Hence, in this work, we report the heterogeneization of this new chiral macrocycle onto micelle-templated silicate (MTS) surface by substitution of chlorine atom of previously grafted 3-chloropropyl chain. After A-alkylation of the tetraazamacrocycle with propylene oxide and metalation with Mn(lI)Cl2, the catalytic performance of the corresponding hybrid materials was evaluated in the heterogeneous enantioselective olefin epoxidation. 

Of special interest is the eventuality of stabilizing transition states by imprinting their features into cavities or adsorption sites using stable transition state analogs as templates. Studies towards such TSA footprint catalysis have been performed by generating TSA complementary sites as marks on the surface [7.73a] or as cavities in the bulk [7.73b] of silica gel. These imprinted catalytic sites showed pronounced substrate specificity [7.74a,b] (namely in the case of cavities [7.73 b]) and chiral selectivity [7.74c,d]. 

MIP films, applied to a QCM transducer, have been employed for chiral recognition of the R- and 5-propranolol enantiomers [107]. MIP films were prepared for that purpose by surface grafted photo-radical polymerization. First, a monolayer of 11-mercaptoundecanoic acid was self-assembled on a gold electrode of the quartz resonator. Then, a 2,2 -azobis(2-amidinopropane) hydrochloride initiator (AAPH), was attached to this monolayer. Subsequently, this surface-modified resonator was immersed in an ACN solution containing the MAA functional monomer, enantiomer template and trimethylolpropane trimethacrylate (TRIM) cross-linker. Next, the solution was irradiated with UV light for photopolymerization. The resulting MIP-coated resonator was used for enantioselective determination of the propranolol enantiomers under the batch [107] conditions and the FIA [107] conditions with an aqueous-ACN mixed solvent solution as the carrier. The MIP-QCM chemosensor was enantioselective to 5-propranolol at concentrations exceeding 0.38 mM [107]. 

The above mechanism is novel in that it does not require the interaction of a carbonyl moiety with the metal center. Neither a ketone/Ru complex nor a Ru alkoxide is involved in the mechanism, and the alcohol forms directly from the ketone. This non-classical mechanism also explains the high functional selectivity for the C=0 group. When the chiral molecular surface of the Ru hydride recognizes the difference of ketone enantiofaces, asymmetric hydrogenation is achieved. This is different from the earlier BINAP Ru chemistry where the enantio-face differentiation is made within the chiral metal template with the assistance of heteroatom/metal coordination. Similar heterolyses of H2 ligands have been shown by Morris and others (92) to be the critical step in the mechanism of reaction processes related to the Noyori systems. 

In this work, the synthesis of high surface densities of chlororopropyl groups covalently grafted on mesoporous micelle templated aluminosilicates (Al-MTS) of various initial pore diameters is presented. The hybrid chiral materials resulting from halogen substitution are applied in the enantioselective addition of diethylzinc to benzaldehyde. 

We list in Table 2 a few of the systems that have been reported in the recent literature to show metal complexes supported on amorphous, non-porous oxides. We also include a few recent reports on the decorating of mesoporous supports with metal complexes. Complexes have also been introduced into porous, crystalline oxides as well as placed on organic supports." We reported the use of metal complexes as templates for forming familiar crystalline solids and new crystalline materials, some of them adopting the chirality of the metal complex. " Preparations have appeared recently using dinuclear Pd(II) complexes [Pd2Me2Cl2(dppm)2] as the precursor and these were reacted with a silica surface to produce the grafted dinuclear Pd complex with the elimination of methane from the complex. ... 

Later, Hosoya et al. 1931 prepared monodisperse polymer-based CSPs from chiral methacrylamides by co-polymerization onto the surface of polymeric particles. These are synthesized by a staged templated suspension polymerization using a two-step swelling method starting from polystyrene seed particles of 1 pm size used as shape templates, onto which methyl methacrylate and later the chiral methacrylamide is co-polymerized. 

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