Photochemical dimerisation of 2-vinylfuran and 2-vinylthiophene derivatives bearing electron-withdrawing groups

The photochemical dimerisation of 1,3-diheteroaryl-2-propen-1-one derivatives gave a mixture of dimers in agreement with the hypothesis 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. (C) 2000 Elsevier Science S.A. All rights reserved.