Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products ( approximately 98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.

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The electro-oxidation of D-and L-glucose has been investigated using the chiral electrode Pt{643} R and its enantiomorph Pt{643} S . Both electrodes are demonstrated to be enantioselective. We ascribe this behavior to the inherent (left or right) "handedness" of kink sites present at the surface. In contrast, no difference in D-and L-glucose oxidation could be detected using stepped Pt{211} and Pt{332} electrodes. Stepped surfaces are achiral, since they lack the prerequisite necessary for the observation of chirality, namely kink sites. For Pt{531}, a surface which contains only kink sites, a diastereomeric product excess of ∼80% is estimated for D-and L-glucose oxidation. This compares with a value of ∼60% for Pt{643}. Hence, the chiral discrimination appears to scale with the surface density of kink sites. These findings constitute the first experimental proof that chiral discrimination is an intrinsic property of kinked single-crystal surfaces.

The combination of electrochemical methods with the use of well-defined kinked metal surfaces allows the experimentalist to examine many fundamental aspects of asymmetric reactions at solid surfaces in a new and detailed manner. For example, by systematically changing both the stereogenic center on a particular chemisorbing molecule and also the molecular architecture of the metal surface, relationships between adsorption geometry and reactive sites on the metal surface may be deduced. In addition, by measuring the differential rate of chemisorption of chiral auxiliaries on R and S metal surfaces insights in to the mechanism of the so-called "Orito" reaction (whereby enantioselective hydrogenation of R-ketoesters is observed) may be deduced. In the present article, a brief resume ´of the relevant electrochemical and heterogeneous catalysis literature is expounded together with an analysis of the problem of chirality in two dimensions in order to show that, by utilizing an electrochemical surface science approach, the nature of asymmetric surface reactivity my be tackled in a systematic and accessible manner. Important questions regarding the surface stability or otherwise of chiral modifiers under reaction conditions may be addressed together with the relevance of surface structure in the chemisorption of such species. Some conclusions regarding the mechanism of the Orito reaction on the basis of new experimental data are discussed.

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