Formation and EPR spectral detection of methyl-substituted naphthalene radical cations and products of their further transformations: binaphthalene formation

Abstract

The EPR spectral behaviour of a number of methyl-substituted naphthalenes (ArH), as well as the corresponding 1,1′-binaphthalenes and perylenes, upon oxidation by any of the four methods mentioned below has been investigated. A study of these compounds by cyclic voltammetry is also reported. The substrates ArH included 1-methyl-, 1,2-, 1,3-, 1,4-, 1,5-, 1,7- and 1,8-dimethylnaphthalene as well as acenaphthene, and the oxidation methods were: (i)–(iii) photolysis of ArH and a good electron acceptor [tetranitromethane, DDQ or Hg^II trifluoroacetate] in dichloromethane–TFA (0.4–0.8 mol dm^–3) at a temperature between –60 and –70 °C, or (iv) thermal oxidation by TI^III trifluoroacetate in dichloromethane–TFA (0.4–0.8 mol dm^–3) at room temperature, followed by recording of the EPR spectrum at –60 °C. Except in cases where a peri methyl group can interfere (1,4- and 1,5-dimethylnaphthalene) with σ coupling at the 1-position, these methods directly generated the EPR spectra of the radical cations of the corresponding 1,1′-binaphthalenes Ar–Ar˙⁺, previously interpreted as originating from π dimer radical cations, (ArH)₂˙⁺. No evidence was obtained for further oxidation of the 1,1′-binaphthalenes to perylene derivatives under the prevailing reaction conditions.