Alkenes, Arylalkenes and Cycloalkenes

Regardless of the type of activation either by direct irradiation into the (7t,Tt ) absorption band or by sensitization, the double bond of an alkene is changed in its bond order which has a strong impact on its reactivity. As a consequence, generally six types of reactions are 

In addition, there are other proeesses caused by excitation into a Rydberg state which readily undergo nucleophilic addition. The interested reader is recommended to look for details in the references given below. 

-Isomerizations of 1,2-disubstituted alkenes are well documented and can occur either by direct irradiation via the singlet state or via the triplet by sensitization. Examples of electron transfer activation have also been reported and mainly involve radical cations of alkenes. For the reasons discussed above, in general, the sensitization is the method of choice. There are numerous examples collected in the textbooks and the reviews. The ZE-isomerization of cyclooctenes presented by Y. Inoue belong to rare examples of a singlet sensitization. The advantage of such a process is that the reaction probably involve exciplex intermediates. By choosing a chiral sensitizer an asymmetric Z,E-isomerization is possible with an enantiomeric excess up to 53% (at -85 °C). 

This unique rearrangement was discovered by Zimmerman in the late sixties and generally belongs to sigmatropic rearrangements of the type [1,2]. Since it is not limited to 

4-dienes but may also applied to oxa, aza and other analogues and since it has got a strong impact on synthetic chemistry often a single textbook chapter is devoted to this process. The numerous examples presented here still reflect the topicality of the di-rt-methane rearrangement which allows the synthesis of enantiomeric pure compounds due to its stereospecificity. Whereas acyclic 1,4-diene systems often require direct irradiation in order to avoid Z, -isomerization via triplet formation, constrained 1,4-dienes can also be triplet sensitized and thus, this procedure often leads to higher product yields. 

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