The photoreaction of naphthalene with pyrrole; products and reaction mechanism

Abstract

Irradiation of naphthalene and pyrrole by u.v. light through Pyrex or at 310 nm affords 2-(1,4-dihydro-1-naphthyl)-pyrrole (II) and 2-(1,2-dihydro-2-naphthyl)pyrrole (III). These products together comprise 85% of the mixture, the remainder consisting of two 3-(dihydronaphthyl)pyrroles. Products (II) and (III) were characterized by n.m.r. spectra, and by conversion into the stable, crystalline 5-ethoxycarbonyl derivatives. The mechanism of this addition was studied by use of fluorescence quenching, solvent effects, and a labelling experiment. Pyrrole quenches naphthalene fluorescence at a rate close to the diffusion limit in benzene, acetonitrile, and ethanol. The quenching is similar to that of amines, which is used as model for the pyrrole case. N-Methylpyrrole also quenches naphthalene fluorescence, but does not react. Use of N-deuteriopyrrole as the addendum showed that the deuterium is retained in the methylene groups of compounds (II) and (III). The pyrrole–naphthalene addition does not occur on triplet sensitization, implying that the first excited singlet state of naphthalene is reacting. The ratio of products (II) and (III) is solvent-dependent, and changes in a manner which resembles that observed for certain ground-state, ionic processes. A mechanismis discussed in the light of our results and those in the literature.