Zweistufige Singulett-Photocycloadditionen

Abstract

The mechanism of cyclovinylogous 2 + 2-, 2 + 4-, and 4 + 4- photoadditions and reversions is discussed with respect to their kinetic and stereochemical behavior. In the 2 + 2-additions the occurance of short lived singlet photointermediates may be used to explain the apparent inability of 1 (n > 2) and various 1,4- cyclohexadienes for intramolecular cyclobutane formation to compete with alternative deactivation processes on the basis of orbital overlap and steric hindrance considerations. Irradiation of all mono- and polycyclic 1,4-cyclohexadienes studied yields as major monomeric products the corresponding aromatics, which are more readily prepared by oxidation with 2-chloro-2-nitropropane. The 2 + 4-photoaddition of maleic anhydride, dimethylmaleate and dimethylfumarate with anthracene is non stereospecific, as both diesters yield the same adduct 25b preferentially. On irradiation 26a, 26b, and 25b (as well as triptycene) strongly fluoresce and partly revert to anthracene and olefin. Thus cleavage of 26a occurs with a quantum yield of 0.022. Less eflicient is the fluorescence of the 4 + 4-dimer 28. Its photocleavage into two mols of anthracene at 269 nm follows a first order rate law with a quantum yield of 0.55. Some spectroscopic (NMR, UV) data for the new compounds are reported as well as corrected fluorescence data of the aromatic esters 13, 19, and 21.