Stabilization of Cyclopropenium Ion and Cyclopropenone by Guaiazulene

C₃Cl₃ ⁺AlCl₄^- (from tetrachlorocyclopropene and aluminum chloride) reacts with 2 molar equiv of guaiazulene (8) in dichloromethane solution to give, after aqueous workup, di-3-guaiazulenylcyclopropenone (7). An analogous reaction of C₃Cl₃⁺AlCl₄^- with 3 molar equiv of 8 followed by treatment of perchloric acid (70%) afforded tri-3-guaiazulenylcyclopropenium perchlorate (6). The dipole moment of 7,5.13 D, is analyzed in terms of the dipole moment orientations of the azulenyl groups relative to the cyclopropenone moiety. The ¹H NMR spectra of 6 and 7 are analyzed in comparison with the corresponding spectra of 8 and various of its 3-acyl derivatives. The interactions of the guaiazulenyl groups with the cyclopropenone (in 7) and the cyclopropenium ion (in 6) are discussed. The high pK_R⁺ of 6, >10, indicates the remarkable effect of the three guaiazulenyl groups in delocalizing the positive charge of the three-membered ring.