Low-temperature irradiation of 1-substituted naphthalenes

Abstract

An unprecedented photodimerization of 1-substituted naphthalenes is reported. Upon irradiation of naphthalene-1-carbonitrile or methyl 1-naphthoate through a Pyrex filter at –78 °C, the corresponding syn-[2+2] cyclodimer was isolated as a single product. However, no cyclodimer was found in the irradiation of 1-methylnaphthalene, 1-methoxynaphthalene, and naphthalene under the same irradiation conditions. By a triplet-sensitized photoreaction and low-temperature NMR study of the reaction mixture irradiated at low temperature, the syn-[2+2] cyclodimer was found to be produced through a facile Cope rearrangement of the exo-[4+4] cyclodimer which was the primary product formed by a singlet-state 1,4–4′,1′ dimerization of the substituted rings.