Cycloalkenes, photochemical isomerization

One important aspect of the photochemistry of aikenes 301,302) is the E—Z isomerization around the C—C double bond 303). This is also valid for cycloalkenes with the obvious exception of cyclopropenes304a,b) which exhibit a distinct photochemical behaviour, and cyclobutenes and cyclo-pentenes where the ring is to rigid to allow sufficient twisting of the double bond. 

A convenient photochemical procedure for obtaining ( )-cycloalkenes and the products derived therefrom is the photosensitized isomerization of (Z)-isomers, where more conventional instrumentations, such as low/high pressure mercury lamps and quartz/Vycor/Pyrex vessels, can be used. 

In this review, we will concentrate on the E-Z-photoisomerization and subsequent (photo)chemical transformations of cycloalkenes caused by direct and sensitized excitation. Valence isomerization, rearrangement, and fragmentation, observed upon irradiation of cyclopropene, cyclobutene, and cyclopen-tene, are only briefly described, as they have been extensively reviewed elsewhere. - - - Although the photochemistry of acycKc alkenes will not be included in this chapter, we will refer to the photobehavior of conjugated cycloalkenes, such as cycUc dienes, enones, and styrenes, as far as it relates to the cycloalkene functionahty. As the photochemical formation of labile (E)-cycloalkenes is one of the major topics in this chapter, the physical properties of the (E)-isomers will be tabulated and discussed in detail. Recent studies on enantiodifferentiating photoisomerization of cycloalkenes are summarized in one section in view of the rapidly growing interest in asymmetric photochemistry. ... 

Since (E)-cyclooctene (20E) is the smallest member of the (E)-cycloalkenes isolable under ambient conditions, its photochemical behavior has been studied intensively as a prototype or model system of the cycloalkene family selected results are summarized in Table 16.1. When (Z)-cyclooctene (20Z) is irradiated with an equimolar amount of benzene or naphthalene, 1 1 sensitizer-substrate adducts rather than the (E)-isomer are produced.Upon xylene sensitization, however, the adduct formation is retarded and the geometrical isomerization dominates to afford 20E in an acceptable isolated yield. The triplet-sensitized photoisomerization of 20Z has been studied in some detail and has established that the EtZ ratio depends on the alkene concentration. The concentration dependence observed is attributed to the involvement of the excited singlet state, which tends to give arene-cyclooctene adducts such as 49 and 50. Temperature and sohrent-viscosity effects upon triplet-sensitized photoisomerization of cyclooctene have been studied in deta0. In the alkyl-benzene-sensitized isomerization of 20Z in pentane, the photostationary-state E/Z ratio is substantially affected by temperature, being doubled when the temperature is lowered from 20 to —78°C. This result is in contrast with the photoisomerization behavior of acyclic 2-octene. 

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