Study 6.2 Asymmetric synthesis enantiodifferentiating photoisomerization

Case Study 6.2 Asymmetric synthesis - enantiodifferentiating photoisomerization 

Selected theoretical and computational photochemistry references.16 480 534 535 538 603-610 Electrocyclic Photorearrangements 

Only the singlet state interconversions of conjugated dienes and cyclobutenes are taken into consideration here, because the triplet state is generally not involved in this process.596 There are two important factors that affect electrocyclization (1) reaction 

Apart from photocyclization reactions, simple trienes undergo many types of other phototransformations, including E Z photoisomerization (Section 6.1.1), sigmatropic shifts (see later) or photoadditions (Section 6.1.4). For example, conformational control and possibly equilibration between the excited conformers result in a 

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Recent studies on enantiodifferentiating photoisomerization reveal that the photosensitization of C Cg (Z)-cycloalkenes with optically active aromatic esters affords chiral ( )-cycloalkenes in good to excellent ees. The mechanism involves enantiodifferentiating rotational relaxation within the exciplex intermediate formed from the excited singlet state of chiral sensitizer and prochiral substrate. The exciplex structure and hence the stereochemical outcome are very sensitive not only to the internal factors, such as sensitizer energy and structure, but also to the external entropy-related factors such as temperature, pressure, and solvent. This leads to a novel idea of multidimensional control of product chirality by several environmental factors. The asymmetric photosensitization can be applied to the photochemical asymmetric synthesis and also be used as a powerful tool for exploiting the reaction mechanisms in the excited-state chemistry. 

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