Photochemistry of Alkenes and Dienes

The reaction is ordinarily stereoselective, favoring the more stable adduct, and a long-lived triplet diradical intermediate is implicated  

This diradical is believed to be preceded on the reaction path by a complex of the alkene with excited-triplet benzophenone. This reaction, particularly its stereochemistry and regioselectivity, will be considered in more detail in Part B, Chapter 6. 

The photochemistry of alkenes and dienes has already been examined in part, since these compounds are particularly illustrative of the principles of orbital symmetry control in electrocyclic processes. The orbital symmetry rules for cycloadditions and electrocyclic processes were covered in Section 11.2. Cycloadditions are also considered, from a synthetic viewpoint, in Part B, Chapter 6, Section 6.2. This section will emphasize unimolecular rearrangements of alkenes and dienes. 

A classic reaction in olefin photochemistry is the photochemical interconversion of CIS- and trans-isomers. Usually, the trans-isomer of a disubstituted olefin is the more thermodynamically stable form. A photochemical steady state is established on irradiation that is usually richer in the cis -isomer than is the ground-state equilibrium mixture. As a result, irradiation provides a means of converting trans-alkenes to the cis-isomers. 

The isomerization of olefins is believed to take place via an excited state in which the two sp -carbons have been twisted 90° relative to the ground state. This state is referred to as the p (perpendicular) geometry  

It is easy to see that if this twisted geometry were attained, there would be a possibility of returning to either cis or trans ground state compound. 

Especially detailed study of the mechanism of configurational isomerism has been made with cis- and -stilbene. Spectroscopic data have established the energies of the singlet and triplet states of both cis- and trans-stilhene and of the twisted excited state that is formed from both isomers. This information is summarized in Fig. 11.8. As might be deduced from their similarity in energy, it is believed that the geometry of the species and are very similar. The state is believed to vibrationally convert rapidly to p. Excited states from either the cis-or trans-stilhtnt can readily attain the common p states. 

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