Upon excitation by light, molecules reach states of higher energy, so called excited states. The energy input allows the molecule to access reaction pathways which are not possible in a conventional (thermal) chemical reaction. As a consequence, products are generated which display unique structural features and which can only be formed by light-induced (photochemical) processes. The quintessential reaction is the [2+2] photocycloaddition of alkenes leading to strained cyclobutanes. In the course of many photochemical reactions, stereogenic centers are formed making the products of the reaction chiral. Chirality in chemistry refers to the fact that molecules exist in two configurations which cannot be superimposed despite the fact that their spatial coordinates are identical. Our hands, which is where the name stems from, have the same property; they behave as image and mirror image. The interaction of small molecules within biological systems is determined by the orientation of its atoms in three-dimensional space. As a consequence, enantiomeric molecules may have completely different pharmacological properties and this fact has stimulated extensive research efforts towards the synthesis of enantiomerically pure compounds. With regard to organic compounds, the key issue is to install the first stereogenic carbon atom with high enantioselectivity as the creation of consecutive stereogenic centers will lead to diastereoisomers. Although chiral compounds available from nature still play an important role as starting materials in organic synthesis, the most important and effective way to create enantiomerically pure compounds de novo is based on enantioselective (asymmetric) catalysis. The Nobel Prizes for Chemistry in 2001 and 2021 recognized pioneering efforts towards the development of thermal enantioselective reactions. Modern pharmaceutical drug production, a market of almost one trillion €, relies heavily on enantioselective catalytic methods.
Given the continuing success that enantioselective catalysis has encountered over the last few decades, it is striking that it has – until very recently – not played a role in the photochemical synthesis of chiral molecules. The fact, that photochemistry lags behind the development in enantioselective catalysis, is critical since there is a large number of biologically relevant molecular scaffolds that are only accessible by photochemical but not by thermal reactions. Major reasons for this lack in progress towards enantioselective light-induced catalysis are intrinsic difficulties encountered when designing potential catalysts for such a process. Efficient photochemical reactions have relatively low activation barriers and product formation is rapid while many thermal reactions have significant activation barriers and proceed with very low reaction rates even at elevated temperature. The goal of the ELICOS (Enantioselective Light-Induced Catalysis for Organic Synthesis) project was to develop catalytic methods that would allow for the enantioselective synthesis of a variety of versatile compound classes by light-induced reactions. It was meant to address the intrinsic challenge of modulating excited state reactivity and to advance the field by new concepts and mechanistic insights.
