Editorial

Catalysis on chiral surfaces

The importance of optically pure chiral products provides ample reason to strive for their efficient production. Among the various methods used, asymmetric catalysis is unique in the sense that with a small quantity of chiral catalyst a large amount of chiral product can be produced. While homogeneous asymmetric catalysis by transition metals with suitable chiral ligands has proven to be a powerful workhorse in practice, the development of efficient heterogeneous chiral catalysts is still somewhat limping behind. However, heterogeneous chiral catalysts, which are the focus of this themed issue, could offer several features that are beneficial for practical application. Among them stability, separation, regeneration as well as easy access to continuous process operation are the most prominent.

In order to function as an efficient chiral catalyst, a solid material has to satisfy two major requirements: it has to be active and enantioselective. Unfortunately, there are only very few natural solid materials known, which are intrinsically chiral and catalytically active. The bulk structures of the metals widely used in catalysis are based on highly symmetric lattices and thus they are achiral. Therefore, suitable strategies have to be applied to bestow chirality to catalytic metal surfaces. The need for an additional source that transfers the stereochemical information to the solid surface together with the intrinsic heterogeneity of surfaces has slowed down the development of solid chiral catalysts. In order to overcome these problems various strategies have been developed for imparting chirality to catalytically active surfaces, among which immobilization, tethering, grafting, trapping or encapsulation of homogeneous chiral transition metal complexes on a suitable support and the chiral modification of active metal surfaces by adsorption of suitable chiral organic compounds have so far been the most successful for practical application. The ultimate goal of a rational design of chiral solid catalysts requires not only a proper understanding of their functioning on the molecular level but also the development of methods to create and control active chiral centers on their surfaces.

Enantioselectivity of a chiral surface is dictated by enantiospecific interactions, which are very sensitive to structural conditions. Generally, enantiospecific energy differences between pairs of enantiomers are in the range of a few kilojoules per mol, which renders their discrimination in a catalytic process very demanding and hitherto still imposes severe limitations regarding accurate theoretical predictions. For achieving high enantioselectivity an ideal chiral catalyst should expose active chiral sites of the same kind and remaining achiral sites should be inactive. This requirement imposes a considerable challenge because on surfaces of solid materials generally various sites with different coordination are exposed. Consequently, sophisticated methods are required for the preparation of chiral solid catalysts. While these design methods are still fairly empirical, there is hope that with a deeper fundamental understanding of the mechanism and of particular enantiospecific interactions of chiral molecules with such surfaces a rational design will come within reach. Nevertheless, this goal needs a multi-faceted approach comprising joint research efforts of scientists with different backgrounds: catalysis, surface science, material science, spectroscopy and theoretical chemistry.

In the past years we have witnessed great progress in various areas, which may accelerate the rational design of solid (chiral) catalysts. Advances in in situ spectroscopy and theoretical calculations have provided new possibilities for studying reaction mechanisms, and new methods have emerged for the structural and chemical tailoring of catalytic materials. However, the complexity of enantiodifferentiating catalysis on chiral surfaces renders a complete understanding of the relevant surface processes still very demanding and substantial further research is needed to eventually reach the goal of a rational catalyst design. It is hoped that the articles in this themed issue will inspire researchers to contribute to the future development of this fascinating research area. I warmly thank all of the authors and referees for their efforts in making this special issue valuable reading for all who are interested in catalysis on chiral surfaces and the fundamental phenomena on which it is based.

---