Radical cation cycloaddition reactions of 2-vinylbenzofurans and 2-vinylfurans by photoinduced electron transfer

Abstract

2-Vinylbenzofuran (2a) and 2-isopropenylbenzofuran (2b) can be successfully employed in cycloaddition reactions involving photoinduced electron transfer with 1,3-cyclohexadienes 4, styrenes 5 and acyclic 1,3-dienes 6, yielding [4+2]-and/or [2+2]-cycloadducts. 2,4,6-Tri(4-methoxyphenyl)pyrylium tetrafluoroborate (1a) acts as an efficient sensitizer. Products are benzofuryl substituted cyclobutanes. 1,2,3,4-tetrahydrodibenzofurans, 2-benzofuryl bicyclo[2.2.2]-octenes, and benzofuryl substituted cyclohexenes. Of 2-vinylfurans, only the 5-bromo substituted derivative 3 reacted with 1,3-cyclohexadiene. The 2-vinylbenzofurans behave like styrenes in these photoinduced cycloaddition reactions mostly generating intermediate cyclobutyl radical cations. 1,3-sigmatropic C- and H-shifts taking place at the radical cation stage are likely in these reactions. The mechanisms are discussed in detail, supported by quantum chemical calculations. © Acta Chemica Scandinavica 1998.