Chirally modified metal surfaces

Chirally Modified Metal Surface for Heterogeneous Asymmetric Hydrogenation 17... 

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... 

Among the various strategies [34] used for designing enantioselective heterogeneous catalysts, the modification of metal surfaces by chiral auxiliaries (modifiers) is an attractive concept. However, only two efficient and technically relevant enantioselective processes based on this principle have been reported so far the hydrogenation of functionalized p-ketoesters and 2-alkanons with nickel catalysts modified by tartaric acid [35], and the hydrogenation of a-ketoesters on platinum using cinchona alk oids [36] as chiral modifiers (scheme 1). 

Instead of the absorption of chiral modifiers on metal surfaces, a new method using a slightly different approach attaches chiral moieties directly to metal surfaces through chemical bonds. Chiral silyl ethers have been attached to Pd surface atoms these new catalysts have the form (Pd)s=Si-0-R(,< orS) 42 Their synthesis arose from studies of the effects of siliconation on the catalytic activities and selectivities of dispersed, supported Pd and Pt.43-47 The results from... 

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). 

Nickel and other transition metal catalysts, when modified with a chiral compound such as (R,R)-tartaric acid 5S), become enantioselective. All attempts to modify solid surfaces with optically active substances have so far resulted in catalysts of only low stereoselectivity. This is due to the fact that too many active centers of different structures are present on the surface of the catalysts. Consequently, in asymmetric hydrogenations the technique of homogeneous catalysis is superior to heterogeneous catalysis56). However, some carbonyl compounds have been hydrogenated in the presence of tartaric-acid-supported nickel catalysts in up to 92% optical purity55 . 

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. 

The generally low-percent asymmetric synthesis in asymmetric heterogeneous hydrogenations may be due, in part, to a nonuniform distribution of chiral modifying agents over the catalytic surfaces. In the case of silk fibroin, metal clumping on the chiral support or dissociation of the metal from the fibroin may allow some reduction to occur in an achiral local environment. 

At the basis of the application of zeolites in fine chemicals reactions is the rich variety of catalytic functions with which zeolites can be endowed. Bronsted acidity, Lewis acidity and metallic functions are well known from classical bifunctional chemistry but for specific reactions, unusual sites, e.g. Lewis acid Ti4+ centres, have been introduced into zeolites. Moreover, zeolites can acquire more or less weakly basic properties metal complexes can be entrapped in zeolite pores or cavities, and enantioselective reactions have been performed by decorating the zeolite surface with chiral modifiers. 

Finally, metal colloids can adsorb chiral molecules such surface-modified particles can catalyze hydrogenations in high optical yields. An example is platinum colloids treated with cinchona alkaloids.18... 

Surface science results have thrown new light on modified (chiral) metal surfaces, highlighting new phenomena such as complex adsorption phases, two-dimensional organization and the creation of extended chiral surfaces [307]. New strategies to immobilize an asymmetric catalyst onto a support (adsorption, encapsulation, tethering using a covalent bond and electrostatic interaction) also result in an... 

Chirally modified Attaching chiral metal surface auxiliary to reactant... 

Heterogeneous Enantioselective Hydrogenation on Metal Surface Modified by Chiral Molecules... 

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... 

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