Photocyclisations of 1,4-diarylbut-1-en-3-ynes. Part II. Mechanism of the reaction

The photocyclisation of 1,4-diarylbutenynes into aromatic compounds (Scheme 1) proceeds in aprotic solvents as a radical reaction in which the singlet excited cis-isomer of the parent compound abstracts a hydrogen atom from the solvent or some other hydrogen donor, and the resulting radical undergoes intramolecular ring closure (Scheme 3). With methanol as the solvent the same end product arises via ionic intermediates, formed by photoprotonation of the starting compound and subsequent, intramolecular, electrophilic substitution (Scheme 4).