T. Varea, B. Abarca, R. Ballesteros, G. Asensio 1203 z Electron-Transfer in the Light-Promoted 1,6-Cyclodimerization of 1,l-Di-2-thienylethylene. A Thermal and Photochemical Study Teresa Varea, B e l h Abarca, Rafael Ballesteros, and Gregorio Asensio" Departamento de Quimica Organica, Facultad de Farmacia, Avda. Biasco TbAfiez 13, E-46010 Valencia, Spain Received October 11, 1990 Key Words: Dithienylethylene zyxwvuts / Cyclodimerization / Photoinduced electron transfer l,l-Di-2-thienylethanol zyxwvutsr (1) undergoes light-induced zyxwvutsr (h > 400 nm) dehydration to afford l,l-di-2-thienylethylene zyxwvuts (2). Corn- pound 2 is selectively converted into 1,6-cyclodimers under thermal or photochemical conditions. In these latter case the non-aromatic Diels-Alder adduct 3 can be characterized. An ET mechanism involving a methyldithienylcarbenium ion (9) as electron acceptor and a chain mechanism is proposed to account for the products. Arylethylenes are known to undergo several types of dimerization processes under thermal or photochemical conditions to afford 1,4- and 1,6-cycloadducts as well as open-chain dimers. The relative amount obtained of each class of products depends both on the nature of the subtituents attached to the aryl groups and the reac- tion conditions. Photochemical dimerizations are usually promoted by sensitizers and proceed via intermediate radical cations I). The light-promoted dimerization of olefins substituted by strong elec- tron donors involves a chain mechanism 2! An alternative thermal methodology to promote cycloadditions via cation radicals by chain reactions is based on the use of ammoniumyl salts as electron acceptors3). However, in spite of the extensive studies performed on the reactivity of diarylethylenes, the behavior of their hetcroaro- matic counterparts towards the dimerization remains unknown. We report now on our study on the dimerization of 1,l- di-Zthienylethylene (2) under thermal and photochemical conditions. Ethylene 2 is an unstable compound which polymerizes4) very easily and is therefore difficult to charac- terize and to handle. However, the precursor alcohol 1 is stable and can be purified in the dark. Compound 2 is ob- tained by thermal dehydration of 1, but dimers and poly- merization products are formed simultaneously. Surpris- ingly, irradiation zyxwvuts (A > 400 nm) of 1 in non-nucleophilic sol- vents for a short time gives rise to quantitative formation of 2. Heat-Promoted Cyclodimerization of 2 l,l-Di-2-thienylethanol(l) is heated in the dark at 150°C for 60 minutes in a sealed tube to afford the aromatic cy- cloadduct 4 in more than 70% yield besides about 15% of its dehydro derivative 7 and lJ-di-2-thienylethane (8). The formation of these products can be easily rationalized by assuming the initial formation of the Diels-Alder adduct 3 which is converted into the products by two alternative pathways (see Scheme 1). In the main way (path A), the bridgehead hydrogen migrates to C-7 leading to the aro- matic dimer 4. In the secondary reaction (path B) the be- havior of 3 resembles that of other aryl-substituted olefins with respect to their polymerization processes5). Thus, the hydrogen is abstracted by a molecule of unreacted olefin 2 to afford a pair of radicals zyxw 5 and zyxw 6, which are the immediate precursors of compounds 7 and 8 due to simple hydrogen Scheme 3 1 2 Th Th Th Th Th 3 2 path B I I"Y Th 4h + CH3 Th 6 5 Th 8 7 Th = 2-Thienyl Th 4 Chem. Ber. 124 (1991) 1203- 1206 (c VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1991 ooO9-2940/91/0505-1203 $ 3.50+.25/0