Photosensitized Cis/Trans Izomerization of 1-(1-Propenyl)cycloalkenes

Article abstract of DOI:10.1021/jo00281a032

The photosensitized cis/trans isomerization of a series of 1-(1-propenyl)cycloalkenes is reported.A plot of the photostationary state trans/cis ratio vs the sensitizer triplet energy for 1-(1-propenyl)cyclopentene shows a constant trans/cis ratio of ca. 1.0 with high-energy sensitizers (E_T>61 kcal/mol).The plot shows one maxima at E_T ca. 55 kcal/mol with low-energy sensitizers (E_T<61 kcal/mol).This type of plot is very similar to those obtained with acyclic dienes such as piperylene.The 1-(1-propenyl)cyclohexene system shows a similar plot with high-energy sensit izers, but with low-energy sensitizers this system shows two maxima occurring at 56 and 47 kcal/mol, respectively.This double-maxima plot is rationalized by an unsually low trans/cis decay ratio for the s-cis relaxed triplet state of the 1-(1-propenyl)cyclohexene system.This double maxima is not observed in other diene systems due to a high trans/cis decay ratio for their s-cis relaxed triplet states.The photosensitized cis/trans isomerization of 2-ethylidene-10-methyl-1(9)-octalin was also studied as a model for a conformationally locked s-trans system.The 1-(1-propenyl)cycloheptene system undergoes photosensitized cis/trans isomerization, but photostationary cis/trans isomerization data could not be obtained due to a very efficient photosensitized dimerization of this diene system.