Photochemistry of Alkenes. 7. E/Z Isomerization of Alkenes Sensitized with Benzene and Derivatives

Article abstract of DOI:10.1021/jo00331a006

Contrary to numerous previous reports placing the (E/Z)_pss ratios from benzene-sensitized isomerization of various alkenes at unity, the (E/Z)_pss ratios for alkenes 1-4 (Table I) have been found to (a) differ from unity, with the less highly strained isomer predominating, (b) vary depending on the structure of the alkene, and (c) bear an apparently linear relationship with the triplet excitation energy of the sensitizer.The results are tentatively interpreted in terms of "nonvertical" energy transfer, with transfer occuring more efficiently to the thermodynamically less stable isomer.Possible difficulties with the quantitative aspects of this interpretation are dicussed.Of several sensitizers evaluated for efficiency in effecting E/Z isomerization of 3,4-dimethyl-2-pentene (2) (Table III), p-xylene and phenol were found to be superior.The latter has the added advantage of being easily separated from the alkene by extraction with base.