Photocycloaddition Reactions of Olefins and Polyenes

Many examples of photocycloaddition reactions of olefins and polyenes yielding cyclobutane derivatives have been reported. Irradiation of mixtures of butadiene and 1,1-dichloroethylene in the presence of a 

With l,l-difiuoro-2,2-dichloroethylene, butadiene yielded cyclobutane (31) in the presence of benzophenone  

Both cyclobutane products (30) and (31) are thought to arise from the most stable diradical intermediate  

Irradiation of equimolar mixtures of butadiene and 2-acetoxyacryIo-nitrile in the presence of sensitizers is reported to yield cross-adducts (32)-(34) in addition to butadiene dimers  

The product distribution in this reaction was found to be dependent upon the triplet energy of the sensitizer. Variation of the relative amounts of cyclobutanes (32) and (33) to cyclohexene (34) with sensitizer triplet energy 

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The subject of the first section of this chapter is the photodimerization and photocycloaddition reactions of olefins. In this category we include only those compounds in which the photoreactive olefin or polyene is not part of an aromatic system although it may bear an aromatic substituent, such as in styrene. 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

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