Chiral environments at alkaloid-modified platinum surfaces

Abstract

Molecular environments at enantioselective sites are considered with reference to the adsorption of cinchonidine, epiquinidine, brucine, and oxycodone onto 6.3% Pt/silica (EUROPT-1), using the hydrogenation of methyl pyruvate and of butane-2,3-dione as molecular probes. The mode of action of cinchonidine is briefly reviewed, and the ineffectiveness of epiquinidine is interpreted. Pt modified by the strychnos alkaloid brucine is active for the hydrogenation of methyl pyruvate at 10 bar pressure and 293 K giving an enantiomeric excess of up to 20% in favour of S-lactate, but is not enantioselective for butane-2,3-dione hydrogenation. Modelling shows that brucine adsorbed at a step at a Pt surface forms a cavity which provides for the selective enantioface adsorption of methyl pyruvate, but not of butane-2,3-dione. The model locates the site for alkaloid adsorption and the site for enantioselective hydrogenation on the same terrace of metal atoms. Pt modified by the morphine alkaloid oxycodone is enantioselective for the hydrogenation of both methyl pyruvate and butane-2,3-dione at 10 bar pressure and 293 K, the enantiomeric excess being typically 15% with respect to R-product in each case. Oxycodone, like brucine, appears to require a step site for adsorption but in this case modelling indicates that sites for alkaloid adsorption and for hydrogenation are on different terraces. The activity and enantioselectivity exhibited by cinchonidine-modified Pt is inhibited when modification and reaction are conducted under anaerobic conditions. The hypothesis that co-adsorption of alkaloid and oxygen limits the coverage of the former and that access to enantioselective sites becomes possible only after removal of adsorbed-oxygen in the early stages of reaction is tested. Modification has been carried out under atmospheres of propyne or of buta-1,3-diene as co-adsorbent; the resulting catalysts show high activity and enantioselectivity after removal of the co-adsorbent in the early stages of pyruvate hydrogenation. Thus, optimisation of Pt catalysts for enantioselective reaction should take into account both metal particle geometry and alkaloid concentration.