ESR and ENDOR spectroscopy of furan radical cations obtained by cyclisation of 1,4-diketones in electrophilic media

Abstract

Identical ESR spectra are obtained when either (Z)-1,2-dibenzoylethylene (1) or (E)-1,2-dibenzoylethylene (2) react with sulphuric acid or aluminium trichloride in dichloromethane. Both spectra are identified as those of the 2,5-diphenylfuran radical cation (8)^+·. This means that the E-isomer (2) isomerises to the Z-isomer (1). An intramolecular reaction leading to the furan ring follows. Identical behaviour has been found with the (Z)- and (E)-p-methyl substituted dibenzoylethylenes (3) and (4) as well as with 1,2-dibenzoylethane (5), 4,5-dibenzoylcyclohexene (6) and o-dibenzoylbenzene (7). All these 1,4-diketones give, under the above conditions, the corresponding furan radical cations. All ESR spectra have been interpreted and the assignments of the hyperfine splitting constants (hfs) are based on Hückel–McLachlan calculations except for the 4′,4″- and 3,4-positions where the assignments come directly from methyl and chlorine substitution respectively. ENDOR and TRIPLE resonance experiments with the radical cation (9)^+· allowed the determination of the relative signs of the hfs. A mechanism for the cyclisation is proposed.