Isomerization, alkenes photochemical, cyclooctene

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

Isomerization of ( /Z) isomers is another important transformation. Isomerization of ( ) and (Z-) conjugated amides is effected photochemically " (photo-isomerization " ). There is a rather high energy barrier for the excited state required for (E/Z) isomerization. Isomerization of the C=C units in dienes is also induced photochemically. " Isomerization of cyclic alkenes is more difficult but cyclooctene is isomerized photochemically. " Conjugated aldehydes have been isomerized... 

In 1966, two groups simultaneously discovered photochemical addition reactions of simple alkenes to excited benzenes. Wilzbach and Kaplan [4] reported that upon irradiation of 10% solutions of r/.v-but-2-ene, cyclopentene, and 2,3-dimethylbut-2-ene in benzene with light of 254 nm, meta photocycloadducts are formed. Bryce-Smith, et al. [5] irradiated an equimolar mixture of benzene and cA-cyclooctene with 254-nm light which yielded a mixture of 1 1 adducts. The main component ( 85%) was a 1 1 adduct similar to that found by Wilzbach and Kaplan, a meta photocycloadduct. The minor (10-15%) 1 1 adduct was unstable and underwent thermal isomerization. It was suggested with some reservations that the minor adduct was tetracydo[6.6.0.02,7.03,6]tetradec-4-ene (I, Fig. 1). In 1968, this structure was rejected [37], and in 1973, the product was shown to be the ortho adduct, tricyclo[6.6.0.02,7]tetradeca-3,5-diene [38] (II, Fig. 1). 

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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