Photoinduced cross-coupling reaction of 5-bromo-1,3-dimethyluracil to electron-rich aromatics

On irradiation with Pyrex-filtered light, 5-bromo-1,3-dimethyluracil 1 coupled with methyl- and methoxynaphthalenes 2–7 to give 5-naphthyl-1,3-dimethyluracils 8–13. No coupling product was formed by triplet sensitization except in the cases of 2,3-dimethioxynaphthalene 3 and 2-methoxynaphthalene 5. indicating that the singlet excited states of the naphthalenes are involved in the unsensitized coupling reaction. The k_qτ values of the fluorescence quenching of 1,4-dimethoxynaphthalene 2 and 1-methoxynaphthalene 4 by 1 in acetonitrile were comparable with those obtained from the kinetics of the coupling reactions. On the basis of this fact and the fluorescence-quenching rate constants k_q in acetonitrile ranging from 10⁸ to 10⁹ M^?1 sec^?1, involvement of an electron-transfer process possibly via a singlet exciplex is proposed for this cross-coupling reaction.