Hydrogenation chirally modified metal surface

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

Keywords. 3-Functionalized ketones, a-Keto acid derivatives. Cinchona modified Pt catalysts. Chiral imprints. Chiral metal surfaces. Chiral polymers. Cyanohydrin formation. Cyclic Dipeptides, Epoxidation catalysts. Heterogeneous catalysts. Hydrogenation catalysts. Modified metal oxides. Polypeptides, Tartrate-modified Nickel catalysts... 

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). 

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. 

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... 

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... 

Several approaches can be used to design solid enantioselective catalysts [1-5]. In general, the solid material must combine catalytic activity with stereochemical control. The active site should be regarded as an ensemble of surface metal atoms which adsorb and activate the reactant and hydrogen and can also accommodate a soluble chiral modifier. For example, in the hydrogenation of ethyl pynivate over cinchona-modified Pt an ensemble of about 15-20 metal atoms is required to accommodate the bulky modifier, substrate, and hydrogen [6]. 

Chiral molecules on the surface of the metal colloid can induce enantioselective control. Following this concept a new type of enantioselective platinum sol catalyst stabilized by the alkaloid dihydrocinchonidine was designed [120, 121]. Chirally modified Pt catalyst precursors have been prepared in different particle sizes by the reduction of platinum tetrachloride with formic acid in the presence of different amounts of the chiral alkaloid. Optical yields up to 80% ee were obtained in the hydrogenation of ethyl pyruvate. This type of catalyst was demonstrated to be structure insensitive since turnover frequencies (ca. 1 s ) and enantiomeric excess are independent of the particle size. 

---