Adsorption chirally modified

There are two views on the origin of enantiodifferentiation (ED) using Pt-cinchona catalyst system. In the classical approach it has been proposed that the ED takes place on the metal crystallite of sufficient size required for the adsorption of the chiral modifier, the reactant and hydrogen [8], Contrary to that the shielding effect model suggest the formation of substrate-modifier complex in the liquid phase and its hydrogenation over Pt sites [9],... 

Surface faceting may be particularly significant in chiral heterogeneous catalysis, particularly in the N i/P-ketoester system. The adsorption of tartaric add and glutamic acid onto Ni is known to be corrosive and it is also established that modifiers are leached into solution during both the modification and the catalytic reaction [28]. The preferential formation of chiral step-kink arrangements by corrosive adsorption could lead to catalytically active and enantioselective sites at step-kinks with no requirement for the chiral modifier to be present on the surface. 

The chapter Chiral Modification of Catalytic Surfaces [84] in Design of Heterogeneous Catalysts New Approaches based on Synthesis, Characterization and Modelling summarizes the fundamental research related to the chiral hydrogenation of a-ketoesters on cinchona-modified platinum catalysts and that of [3-ketoesters on tartaric acid-modified nickel catalysts. Emphasis is placed on the adsorption of chiral modifiers as well as on the interaction of the modifier and the organic reactant on catalytic surfaces. 

The modification of platinum-group metals by adsorbed chiral organic modifiers has emerged as an efficient method to make catalytic metal surfaces chiral. The method is used to prepare highly efficient catalysts for enantioselective hydrogenation of reactants with activated C = O and C = C groups. The adsorption mode of the chiral modifier is crucial for proper chiral modification of the active metal surfaces. The most efficient chiral modifiers known today are cinchona alkaloids, particularly CD, which yields more than 90% enantiomeric excess in the hydrogenation of various reactants. 

In the hydrogenation of ethyl pyruvate in the presence of Pt-Al203 modified by 10,11-dihydrocinchonidine, alkaloid adsorption leads to a marked increase in reaction rate (44). The actual hydrogenation involves two kinds of reactive sites, chirally modified Pt (Ptm) and unmodified metal (Pt ). Accordingly, the reaction is analyzed in terms of a general two-cycle mechanism (Scheme 18). The first cycle is ligand-... 

Ramamurthy and coworkers have utilized zeolites modified with chiral organic compounds [17,18]. Zeolites are crystalline aluminosilicates with open framework structures. In this approach, the zeolite is first loaded with a chiral inductor and the compound to be photolyzed is then added in a second, separate adsorption step. Asymmetric induction ensues as a result of the close proximity enforced between reactant and chiral inductor in the confined space of the zeolite supercage. The zeolite method has the disadvantage that the size of the substrate is limited by the pore size of the zeolite being used. Most of the work using the chirally modified zeolite approach was compared with the ionic chiral auxiliary method by Scheffer and coworkers. The enantiodifferentiations by the zeolites are usually low to moderate. 

The history of heterogeneous enantioselective catalysis with chiral modification of the metal surface extends back even further than that of homogeneous molecular metal catalysis. However, successful precedents which result in a practicably useful stereoselectivity (e.g. of over 80%) involve only three types, all of which involve the hydrogenation of unsaturated bonds. Initially, these reactions were realized by achieving the correct solution to address all requirements for the chiral modifier. That is, the adsorption of the modifier must occur on all of the active... 

Pd modified by cinchona, vinca, or ephedra alkaloids is a moderately efficient catalyst but Pd is still the catalyst of choice for the enantioselective hydrogenation of olefins with a functional group in the a position [8,20]. Modification of Pd with cinchonidine is as simple as for Pt, but Pd requires a considerably lower substrate/ modifier ratio than Pt, probably because of weaker adsorption and/or partial degradation (hydrogenation) of the modifier during reaction. Another drawback is that the reactions are not accelerated but decelerated by the chiral modifier (by a factor of up to 140 [21]). This phenomenon can rationalize the moderate performance of chirally modified Pd. 

It has recently been proposed that kink sites of high Miller index metal surfaces should be considered as chiral when the step lengths on either side of the kink are unequal [22], Two such surfaces, which are not superimposable, can be defined - by analogy with the Cahn-Ingold-Prelog rules - as, e. g., Ag(643) and Ag(643). Theoretical calculations predicted that adsorption of chiral molecules should be stereospecific on such surfaces, but the only experimental evidence yet available is the electro-oxidation of d- and L-glucose on Pt(643) and Pt(531) surfaces [23]. It was speculated that with the polycrystalline metal catalyst, which contains equal numbers of (/ )- and (S)-type kink sites, preferential adsorption of a chiral modifier on one type of kink site would leave the other type of site free for catalysis. 

T Biirgi, Z Zhou, N Kunzle, T Mallat, A Baiker. Enantioselective hydrogenation on chirally modified platinum new insight into the adsorption mode of the modifier. J Catal 183 505-408, 1999. 

Supported chiral Ni and Ni-Cu catalysts i 9-i60-24s,248,249 special interest because they allow the elucidation of the nature of the metal-support interaction and the asymmetric adsorption of modifier and substrate molecules by use of the IR spectra of adsorbed molecules. 

In addition to simultaneous in situ spectroscopic studies, accompanying ex situ investigations also provide valuable information about the specific interaction of the substrates (imines and respective hydrogenation products) with both the chiral modifier (P-acid) and the solid catalyst. Thus, FTIR spectroscopic analysis of the catalyst after adsorption of the imine points to a strong interaction of the latter with the catalyst surface, in particular with the support, which is reflected by marked band shifts. It could be shown that the surface of the catalyst is mainly covered by the imine after use in the hydrogenation reaction, besides some small quantities of the product [11]. 

One of the simplest approaches to the creation of an enantioselective catalyst is the adsorption of a chiral molecule (often referred to as a modifier) onto the surface of a metal catalyst. The metals most commonly employed for this type of catalysis have been the Pt group metals and Ni [29]. The most successful chiral modifiers have been naturally occurring alkaloids (Pt) and tartaric acid (Ni) (Scheme 5.2). Each system has primarily been used for hydrogenation reactions with Pt/cinchona producing ee values of greater than 90% for the hydrogenation of a-ketoesters [42, 43] ... 

Adsorption of fluorescent molecules on chirally modified surfaces and sol-gel films can also generate enantioselective effects [39]. 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

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