Alkaloids metal surface adsorption

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. 

Enantiomeric excess diminished with time on stream when the modification concentration was 0.34mM, but remained constant when the concentration was 3.4mM (Figure 3). It is known for Pt/silica that alkaloid adsorption can occur on both the metal and the support and in our catalysts there may have been diffusion of alkaloid molecules between the support and the active phase during hydrogenations. In such a case, modification at a higher concentration would have provided more extensive adsorption on the support, and hence a larger reservoir of cinchonidine would have been available to sustain a steady state concentration on the Pt surface or to replace any modifier rendered inactive by partial or complete hydrogenation. 

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] ... 

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