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) photocycloadditions to the benzene ring. The reaction mode selectivity and regiochemistry 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 which influences the furan reaction. While (4 + 3) and (4 + 4) additions occur with toluene, the (2 + 2) and (2 + 3) processes predominate with anisole, and fluorobenzene and alpha,alpha,alpha-trifluorotoluene yield mainly (2 + 2) and (4 + 2) adducts. The directing influence of the nitrile substituent towards a regio- and stereo-specific (4 + 3) addition of furan is considered to arise from polar and/or secondary orbital interactions. The photoexcited cyanoanisoles are, however, essentially unreactive towards furan, and the benzene-furan non-conjugated bichromophores 41 and 42 do not yield photoisomers.