Microwave photochemistry: II. Photochemistry of 2-tert-butylphenol

Influence of UV and combined MW–UV irradiation on 2-tert-butylphenol (2TBP) 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 combination MW–UV gave three CC dimers: 3,3′-di-tert-butylbiphenyl-2,2′-diol (1) (ortho–ortho), 3,3′-di-tert-butylbiphenyl-2,4′-diol (2) (ortho–para) a 3,3′-di-tert-butylbiphenyl-4,4′-diol (3) (para–para). Their ratio depends upon the nature of solvents and sensitizers used. In non-polar solvents (hexane, heptane, benzene, toluene), direct 2TBP radiation led to the formation of two products 1 and 2 where ortho–para 2 is predominant over ortho–ortho dimer 1. Irradiation in dipolar aprotic solvent 1,4-dioxane led only to ortho–ortho 1 and in case of acetonitrile only to ortho–para dimer 2. In polar solvents (acetic acid, water, methanol/water = 1:1 v/v), it led to the formation of all three products. In methanol, the reaction did not proceed. Influence of solvent polarity (hexane, acetonitrile) to relative product distribution of 2TBP photoreaction was also observed in the presence of sensitizers: acetophenone, benzophenone, naphthalene, acridine and anthracene. Reaction in hexane gave ortho–ortho 1 and ortho–para 2 dimers with all tested sensitizers. The acridine-sensitized reaction in acetonitrile was found to give ortho–para 2 and a greater amount of para–para 3 dimer. Singlet-sensitized reaction (naphthalene, anthracene) in acetonitrile led to ortho–para dimer 2. Nonsensitized as well as singlet-sensitized reaction of 2TBP proceeded from singlet state situated under singlet state energy of anthracene (320 kJ mol−1). Triplet state of 2TBP was found to be under triplet state of benzophenone (290 kJ mol−1).