Geometric isomerism cycloalkenes

As is the case with acyclic alkenes, geometrical isomerization becomes the major photoreaction observed for medium-sized cycloalkenes upon direct excitation, although the carbene-derived products are still obtained as minor products. Thus, direct irradiation of cyclohexene 31Z in aprotic... 

For compounds that show geometric isomerism, add the appropriate prefix cis- or trans-, or E- or Z-. Cycloalkenes are assumed to be cis unless named otherwise. 

If high selectivities also applied to trisubstituted olefins, a predictable consequence [Eq. (12)] would be that cycloalkenes unsymmetrically substituted on the double bond would yield polymers that, except possibly for geometrical isomerism, would be translationally invariant. 

As is the case with acyclic alkenes, cycloalkenes undergo geometrical isomerization upon direct excitation. Thus, irradiation at -200 nm of cyclohexene (13Z) in aprotic media affords a stereoisomeric mixture of [2 -I- 2]-cyclodimers (16), in 1.6 2.3 1 ratio. This photocyclodimerization is interpreted in... 

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. 

A number of reviews have been devoted to the photoisomerism of alkenes, with the photochemistry of cycloalkenes treated as a subcategory.The geometrical photoisomerization of alkenes was reviewed also in the previous edition." However, the photobehavior of cycloalkenes is not a simple extension of that of acyclic alkenes, particularly in the cases of small- and medium-sized cycloalkenes. Medium-sized (E)-cycloalkenes produced photochemicaUy are highly constrained and rapidly react with alcohols in the presence, or even in the absence, of dilute acid. An account of the photoprotonation-driven cationic reactions, such as addition, isomerization, rearrangement, and fragmentation of cyclohexenes and cyclo-heptenes, in protic solvents has been published. The protonation of strained (E)-cycloalkenes was also reviewed in the previous edition. ... 

---