INFLUENCE OF ARENE SUBSTITUENTS ON THE MODE AND REGIOCHEMISTRY OF PHOTOCYCLOADDITION OF FURAN TO THE BENZENE-RING

Furan undergoes (2 + 2), (2 + 3), (4 + 3) and (4 + 4) photocycloadditi ons to the benzene ring. The reaction mode selectivity and regiochemis try of the addition are markedly dependent on the arene substituents, but unlike the photoaddition of ethenes to the benzene ring, the ease of electron transfer between the addends is not the dominant factor wh ich influences the furan reaction. While (4 + 3) and (4 + 4) additions occur with toluene, the (2 + 2) and (2 + 3) processes predominate wit h anisole, and fluorobenzene and alpha,alpha,alpha-trifluorotoluene yi eld mainly (2 + 2) and (4 + 2) adducts. The directing influence of the nitrile substituent towards a regio- and stereo-specific (4 + 3) addi tion of furan is considered to arise from polar and/or secondary orbit al interactions. The photoexcited cyanoanisoles are, however, essentia lly unreactive towards furan, and the benzene-furan non-conjugated bic hromophores 41 and 42 do not yield photoisomers.