Stereoselective Z,E-photoisomerization of formyl-substituted (E,E,E)-1,6-diphenylhexa-1,3,5-triene in solution

Abstract

Direct irradiation of (E,E,E)-1,6-bis(4-formylphenyl)hexa-1,3,5-triene in chloroform under argon atmosphere led to a fast equilibrium between E,E,E(62%) and Z,E,E(38%) isomers. Neither isomers nor photoproducts were detected by HPLC in the photostationary mixture. This is entirely different from the reaction of (E,E,E)-1,6-diphenylhexa-1,3,5-triene on irradiation under similar conditions, in which dimers and solvent adducts were formed via various kinds of Z isomers. The high stereoselectivity observed in the Z,E-photoisomerization of the formyl-substituted diphenylhexatriene can be explained by the assumption that a terminal twisted transient state has a zwitterionic character which is stabilized by the polarization of carbonyl groups.