Cyclooctenes cycloalkenes, photochemical

Constrained (E)-isomers of medium-sized cycloalkenes and cycloalkadienes have long been known to be chiral and indeed the enantiomerically pure or enriched (I -cyclooctene/cyclooctadienes were resolved optically and/or synthesized via thermal reactions in the 1960s [26] photochemical preparation of ( )-cyclooctene from the (Z)-isomer was also reported in the late 1960s [27]. Nevertheless, the... 

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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