Alkenes E Z isomerization

Entry Starting material3 Product(s) Mechanism Section 

Rearrangement and decomposition are the major reactions observed when small-ring cycloalkenes are photolysed. E Z photoisomerization becomes an important 

The multiplicity of the excited state that is responsible for photoisomerization may vary with the nature of the stilbene phenyl ring substituents (Table 6.2). When the coupling 

Case Study 6.1 Supramolecular chemistry - photoresponsive stilbene dendrimers 

Sensitization is often indispensable for inducing reaction from the triplet state and it is practical in the case of aliphatic non-conjugated alkenes, which absorb weakly over 200 nm. The triplet energy (ET) of the sensitizer should be higher than that of the acceptor to afford efficient energy transfer (Section 2.2.2). Nevertheless, triplet 

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It is known that simple p,y-unsaturated ketones 69 do not undergo ODPM rearrangement by the acetophenone-sensitization. Instead, under these conditions 69 undergoes alkene E/Z isomerization. However, when thioxanthone is used as a sensitizer, the [ly-unsaturated ketones 69 react to form ODPM rearrangement photoproducts 70 (Scheme 4.28) [41 ]. This result shows clearly that the energy of the triplet sensitizer plays an important role in determining the outcome of the triplet ODPM rearrangement of p,y-unsaturated ketones. 

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