Intramolecular photocyclization of new dithiophene-substituted o -divinylbenzenes

1. cis,cis-3a Scheme 1. Photoreactivity of 1 cis,trans-3b trans,trans-3c Figure 3.1H NMR spectra (in CDCl3. 300 and 600 MHz) of the aliphatic region of three new dihydronaphtalenes (6, 7, 8) Sheme 2. Photoreactivity of 3-5 Figure 1. Photoisomerization of 2,2'-(1,2-phenylenedivinylene)dithiophene (3) Figure 2. Molecular structure of photoproduct 6 (left) as determined by X-ray crystallography. The molecules in crystals are disordered over two positions (right). Intramolecular photocyclization of new dithiophene-substituted o-divinylbenzenes D.Vidaković1), K. Molčanov 2), B. Kojić-Prodić 2), M. Šindler-Kulyk 1) 1)Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10 000 Zagreb, Croatia 2)Laboratory for Chemical and Biological Crystallography, Department of Physical Chemistry, The Ruđer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb,Croatia dragana.vidakovic@fkit.hr Thiophenes and their polycyclic derivatives represent a class of important and well-studied heterocycles [1]. The interest in this kind of compounds has spread from early dye chemistry to modern drug design and self-assembled superstructures. Unsaturated thiophene derivatives show a versatile photoreactivity and formations of various photoproducts with remarkable electrochemical, optical, physical and biological properties [1]. In aim to investigate the effect of sulphur moiety on photobehaviour of dithiophene-substituted o-divinylbenzenes, the novel 2,2'-(1,2-phenylenedivinylene)dithiophene (3), 2,2'-(1,2-phenylenedivinylene)-5,5'-dimethyldithiophene (4) and 2,2'-(1,2-phenylenedivinylene)-5,5'-dibromodithiophene (5) were prepared, and compared with previously investigated difuro-substituted o-divinylbenzenes[2]. In the case of 2,2'-(1,2-phenylenedivinylene)difuran (1), during irradiation intramolecular [2+2] cycloaddition and intermolecular photocyloaddition took place to give bicyclo[3.2.1.]octadiene (2) and cyclobutane dimers as main products, respectively [2]. Contrary to these results, novelthiophene analogs3-5upon irradiation gave completely different photoproducts, the only observed products were dihydronaphtalenes (6-8), with traces of cyclophane (9).Formation of dihydronaphtalenes 6-8 could be explained by[1,6] electrocyclization and subsequent 1,5-H shift (scheme 2). According to this mechanism, replacement of the furan moiety with thiophene, which is more aromatic than furan, may have influence on the excited state properties of this new hexatriene system and as concequence the formation of diverse photoproducts. [1] E. Campaigne, Thiophenes and their Benzo Derivatives: (iii) Synthesis and Applications in A.R. Katritzky (ed) Comprehensive Heterocyclic Chemistry; vol. 4, Part 3, Pergamon Press, Oxford 1984., 863-934. [2] I. Škorić, N. Basarić, Ž. Marinić, A. Višnjevac, B. Kojić-Prodić and M. Šindler, Chem. Eur. J.2005, 11, 543-551. "Central European Conference on Photochemistry" 10th-14th February, 2008 Bad Hofgastein, Austria