Photocycloaddition of cyanonaphthalenes with acetylacetone: enhancement of quantum yields by sulphuric acid

Abstract

In acetonitrile or methanol, singlet excited 1-cyano- and 2-cyano-naphthalenes (1-NpCN and 2-NpCN) react with acetylacetone to give vicinally substituted acetyl–acetonyl addition products, 1 and 3, that are assumed to be formed by a [2 + 2] photocycloaddition followed by ring opening. The diketone 1 was photolysed to give an oxetane by an intramolecular addition reaction. The photocycloaddition was quenched by typical singlet quenchers such as tributylamine, di-methylaniline, biacetyl, oxygen and methyl iodide; these substrates also quenched cyanonaphthalene fluorescence. The rate constant for the interaction of singlet excited 2-NpCN, determined by monitoring the quantum yield of 3, was much smaller than that obtained by monitoring fluorescence intensities. Therefore, the reactive species is related to the spectroscopic singlet state but by an unknown mechanism. The quantum yields of the photocycloadditions of NpCN were remarkably enhanced in the presence of H₂SO₄ at ≤ 10^–3 mol dm^–3. Over this sulphur acid concentration in acetonitrile, the fluorescence intensitity of *2-NpCN was not significantly affected; at higher sulphur acid concentration (0.0006–0.03 mol dm^–3), the quenching by H₂SO₄ was analysed by the Stern–Volmer relation to give a rate constant of 0.55 × 10⁹ dm³ mol^–1 s^–1, a relatively low value.