Ground state conformational control photoreactivity

Molecules can be classified both structurally and dynamically in terms of their intramolecular reactivity. The three structural classes are (1) those with a reactive geometry as the most stable or only conformation, (2) those with a reactive geometry that interconverts with other more stable but unreactive conformations, and (3) those with no reactive conformations populated. Dynamically, the three classes of excited states are (1) those in which interconversion among conformers is much faster than decay reactions of the individual conformers, such that conformational equilibrium is established before reaction (2) those in which conformational interconversions are all slower than decay, such that ground-state conformational preferences control photoreactivity and (3) those in which unreactive conformers undergo irreversible rotation into a reactive conformer, which reacts faster than it can rotate back into the unreactive geometry. In this third case, reaction of the excited state is subject to rotational control, the intramolecular equivalent of intermolecular diffusion control. 

The purpose of this review is to examine the recent progress in the field of asymmetric photoreactions involving a chiral reactant, and to discuss the factors that control the diastereodifferentiation. In the excited state as in the ground state, the level of asymmetric induction is strongly related to the conformational flexibility of the reactants. When carried out in the solid state, in a confined cavity of zeolites or in supramolecular scaffoldings, restrictions of the mobility occur, and photoreactions can become highly stereoselective. These aspects of asymmetric induction will be covered in separate chapters, and this review will consider only diastereoselective photoreactions carried out in solution. 

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