Journal of Photochemistry and Photobiology A: Chemistry 134 (2000) 147–154 Photochemical dimerisation of 2-vinylfuran and 2-vinylthiophene derivatives bearing electron-withdrawing groups Maurizio D’Auria ∗ , Lucia Emanuele, Giacomo Mauriello, Rocco Racioppi Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy Received 16 December 1999; received in revised form 3 March 2000; accepted 6 March 2000 Abstract The photochemical dimerisation of 1,3-diheteroaryl-2-propen-1-one derivatives gave a mixture of dimers in agreement with the hypoth- esis that the reaction is under frontier orbitals control and preferential formation of only the more stable isomers. On the contrary, the reaction of 2-heteroaryl-nitroethylene derivatives were not observed to give cyclobutane dimers but only coupling products with the loss of one nitro group. This behaviour is in agreement with the nature of the HOMO–LUMO orbitals which do not allow the superimposition of the reagents. This result allows to formulate the hypothesis that the photochemical dimerisation of 1-heteroaryl-2-EWG-ethylenes must occur as a concerted reaction. Finally, 2-heteroaryl-1,1-dicyanoethylene derivatives gave the corresponding dimers. Also in this case, we obtained only the head-to-head dimer and the more stable one. © 2000 Elsevier Science S.A. All rights reserved. Keywords: LUMO-S 0 ; HOMO-S 0 ; LSOMO-S 1 ; HSOMO-T 1 ; Dimer 1. Introduction Cycloaddition reaction of alkenes to give cyclobutane dimers is one of the most studied reaction in organic photo- chemistry [1]. Some years ago we found that the irradiation of methyl 3-(2-furyl)acrylate (1) in the presence of ben- zophenone gave a mixture of two isomeric dimers 2 and 3 (Scheme 1) [2]. Subsequently, we found that this reaction occurs in the triplet state of the molecule and that this triplet state is obtained via energy transfer from benzophenone [3]. The regiochemical behaviour of methyl 3-(2-furyl)acrylate was explained assuming a reaction between the LSOMO of the triplet state of 1 with the LUMO of 1 in its ground state. Furthermore, the stereochemistry observed in the re- action 1 → 2 could be explained assuming the formation of the more stable isomers [4]. On the basis of these re- sults we tested our hypothesis on the reaction mechanism of the dimerisation reaction in solution in the presence of arylacrylonitrile derivatives 4 and of esters of urocanic acid 8 (Scheme 1) [5,6]. In this paper, we want to report our further results on the photochemical dimerisation reaction of furyl and thienyl substituted alkenes bearing electron withdrawing sub- ∗ Corresponding author. E-mail address: dauria@unibas.it (M. D’Auria) stituents. In particular we describe the dimerisation reactions of 1,3-diheteroaryl-propenones (10, Scheme 2), nitroethy- lene derivatives (15, Scheme 3), and 1,1-dicyanoethylene derivatives (20, Scheme 4). 2. Materials and methods Mass spectra were obtained with a Hewlett-Packard 5971 mass-selective detector on a Hewlett-Packard 5890 gas chro- matograph (OV-1 capillary column between 70 and 250 ◦ C (20 ◦ C min −1 )). NMR spectra were recorded on a Bruker 300 AM instrument. Elemental analyses were obtained by using a Carlo Erba Elemental Analyser 1106. Compounds 10a–d were obtained through aldol condensation between 2-substituted heterocyclic carboxyaldehydes and hetero- cyclic methyl ketone derivatives. In particular compound 10a was prepared following the method reported in [7,8], compounds 10b–d were prepared on the basis of the proce- dure of [9]. Compounds 15a–b were obtained through aldol condensation between 2-substituted heterocyclic carboxy- laldehyde and nitromethane [10]. Compounds 20a–b were prepared via Knoevenagel condensation between the corre- sponding aldehydes and malononitrile: in particular 20a was prepared following the procedure described in [11], while 20b was obtained using the methodology described in [12]. 1010-6030/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S1010-6030(00)00260-4