Photochemical transformation of β,β′-dithienyl substituted o-divinylbenzenes leading to 1,2-dihydronaphthalenes or fused pentacyclic compounds: first evidence of electrocyclization process via 2,3-dihydronaphthalene intermediates

The photochemical behaviour of 2,2′-(o-phenylenedivinylene)dithiophenes (7a–c), 3,3′-(o-phenylenedivinylene)dithiophene (8a) and 3,3′-(o-phenylenedivinylene)dibenzothiophene (8b) was studied under the low concentrations. An intramolecular reaction via the 2,3-dihydronaphthalene intermediate has been observed in all studied examples accompanied by dimerization and polymerization. The 1,2-dihydro-2,3-dithienylnaphthalenes (9a–c) were isolated (7–9%) from the 2-thiophene derivatives while the 3-thiophene derivatives gave polycyclic structures (13–44% yield).