RADICAL CATIONS OF ALKYLAZULENES - AN EPR AND ENDOR STUDY

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-di-tert-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 ENDOR 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 pi-spin populations rho(1,3) at these centres, as manifested by the largest by far coupling constants \a(H1,3)\ of the alpha- and methyl beta-protons.