Microwave photochemistry III: Photochemistry of 4-tert-butylphenol

Influence of UV and combined MW-UV irradiation on 4-tert-butylphenol (4TBP) transformation was investigated in the presence and in the absence of sensitizers with different value of singlet and triplet energy and in the presence of solvents with different polarity. Irradiation by UV and also by MW-UV gave CO and CC products: 4′,5-di-tert-butyl-2-hydroxydiphenyl ether (1) (ortho-O), 5,5′-di-tert-butylbiphenyl-2,2′-diol (2) (ortho-ortho) and 2TBP as an isomerization product. Their ratio depends upon the nature of solvents and sensitizers used. In non-polar solvents (hexane, heptane, toluene), direct 4TBP radiation led to the formation of products 1, 2 and 2TBP where ortho-ortho dimer 2 is predominant over ortho-O ether 1. In acetonitrile ortho-ortho dimer 2 was selectively formed under UV and MW-UV irradiation. In 1,4-dioxane and in methanol, no product formation was observed. Influence of solvent polarity (acetone, hexane) on the relative products distribution of 4TBP photoreaction was also observed in the presence of sensitizers: acetone, naphthalene, acetophenone and benzophenone. The acetone-sensitized reaction was found to give all three products: ortho-O ether 1, ortho-ortho dimer 2 and 2TBP where ortho-O ether 1 is predominant. In the presence of singlet sensitizer naphthalene, the reaction of 4TBP in hexane gave mainly ortho-O ether 1 and 2TBP, whereas ortho-ortho dimer 2 was not found. Triplet-sensitized reaction with acetophenone in hexane led selectively to ortho-O ether 1, while with benzophenone gave moreover 2TBP and its ortho-ortho3 and ortho-para4 dimers.