Reactions that produce only one of two mirror-image forms of a molecule are a hot topic in organic synthesis. A light-driven catalyst provides good results, and the technique could be generally applicable. Chirality — the non-identity of a molecule with its mirror image — is ubiquitous. It occurs not only in biomolecules (amino acids, sugars, DNA and RNA are examples of chiral molecules), but also in man-made chemicals, materials and drugs. Catalytic asymmetric synthesis — the use of chiral catalysts to transfer and amplify chirality in chemical reactions — has therefore become a central topic in molecular science. Bach and colleagues (this issue, page 1139) now combine two approaches to asymmetric synthesis — the thermal and the photochemical— to control the spatial arrangement of the atoms in a chiial reaction product. The results could be seminal in the field of chiral photochemistry. In conventional thermal asymmetric synthesis, vibrational energy is supplied to a reaction in the presence of a chiral catalyst or enzyme. This activates ground-state reagent molecules to achieve an asymmetric transformation in which one of two enantiomers — mirror-image forms of a molecule — of a reaction product will be preferentially synthesized. The aim of photochemical asymmetric synthesis is the same, but its tools are different: it uses short-lived, weakly inter acting molecular states that have been excited not by heat but by absorbed light. This technique is more difficult to control than its thermal counterpart, and has therefore been less extensively studied, despite its inherent advantages — the low activation energy required for such reactions and the ability to create unstable molecules unique to photochemical reactions, for example.
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