Radical cations of alkylazulenes: an EPR and ENDOR study

Abstract

Azulene (1) and its alkyl derivatives 2–15 were oxidized in a UV-irradiated stationary system by mercury(II) trifluoroacetate in dichloromethane. These derivatives are: 4,6,8-trimethyl-(2), 1,4,6,8-tetramethyl-(3), 6-tert-butyl-1,4,8-trimethyl-(4), 2,4,6,8-tetramethyl-(5), 1,3-dimethyl-(6), 1,3-ditert-butyl-(7), 1,3,5-tri-tert-butyl-(8), 1,3,6-trimethyl-(9), 6-tert-butyl-1,3-dimethyl-(10), 1,3,5,7-tetramethyl-(11), 1,3,4,6,8-pentamethyl-(12), 1,3,4,8-tetramethyl-6-propyl-(13), 6-tert-butyl - 1,3,4,8-tetramethyl-(14) and 1,2,3,4,6,8-hexamethyl-azulene (15). Only radical cations substituted in both reactive positions C-1 and -3 (6˙⁺-15˙⁺) were sufficiently persistent to be characterized by their hyperfine data with the use of EPR spectroscopy. Those bearing tert-butyl substituents at C-1 and -3 (7˙⁺ and 8˙⁺) or having three alkyl groups at the seven-membered ring in addition to the 1,3-dimethyl substituents (12˙⁺-15˙⁺) were also amenable to ENDOR and TRIPLE resonance studies. In contrast, the radical cations with none (1˙⁺, 2˙⁺ and 5˙⁺) or only one methyl group in the positions C-1 and -3 (3˙⁺ and 4˙⁺) rapidly reacted to yield follow-up products. For ¹˙⁺-4˙⁺these products were identified by EPR and EN DOR spectroscopy as the radical cations of correspondingly substituted 1, 1′-biazulenyls (1a–4a). A mechanism is proposed for the formation of the primary (6˙⁺-15˙⁺) and the secondary radical cations (1a˙⁺-4a˙⁺). The failure to detect the radical cation of biazulenyl 5a˙⁺, starting from 5, must be due to instability of 5a˙⁺ in which two pairs of methyl substituents sterically interact. The high reactivity of the azulene radical cations in the positions C-1 and -3 is consistent with the unusually high π-spin populations ρ_1.3 at these centres, as manifested by the largest by far coupling constants |a_H1,3| of the α and methyl β-protons.