A series of azulene analogues of triphenylmethyl cation (tri(1-azulenyl)methyl, di(1-azulenyl)phenylmethyl, and (1-azulenyl)diphenylmethyl hexafluorophosphates) were synthesized by hydride abstraction from the corresponding methane derivatives with DDQ. In order to examine the effect of substituents on the cations, and to enhance their stabilities, a series of cations bearing 3-methyl, 3-methoxycarbonyl, 3,6-di-t-butyl, 6-t-butyl, or 3-t-butyl groups on each of the azulene rings were also synthesized. Their pKR+ values showed that the stabilities of these cations dramatically increase with the number of azulene rings. Tris(3,6-di-t-butyl-1-azulenyl)methyl cation showed the highest pKR+ value (14.3) ever observed. The high stabilities of these cations were attributed to a large conjugative effect between the central cation and the azulene ring(s). The dynamic stereochemistry of these cations was also studied based on the temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism. Low-temperature NMR studies indicated that tri(1-azulenyl)methyl cations exist in two types of propeller conformations (symmetrical and unsymmetrical propellers, which have C3 (A) and C1(B) symmetries, respectively), and at higher temperature the NMR reflect the rapid isomerization. The lower activation energy of the process B → (or → A), compared with that of B → , indicates that the threshold rotation mechanism for the cation is a one-ring flip. This is the first example of a molecular propeller with a threshold rotation mechanism comprising a one-ring flip.

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Extremely stable carbocations, tris[6-(dimethylamino)- and 6-morpholino-1-azulenyl]methyl (2e and 2f), bis[6-(dimethylamino)-1-azulenyl][4-(dimethylamino)phenyl]methyl (3e), and [6-(dimethylamino)-1-azulenyl]bis[4-(dimethylamino)phenyl]methyl (4e) cations, were prepared, and their properties were fully characterized. These cations showed extremely high stabilities with high pK R + values. The values of 2e and 3e were determined spectrophotometrically in DMSO/water as 24.3 ± 0.3 and 21.5 ± 0.2, respectively, which are higher than those of tri(1-azulenyl)methyl and di(1-azulenyl)phenylmethyl cations (2a and 3a) by 13.0 and 11.0 pK units. The value of 4e was determined to be 14.0 ± 0.1 in 50% aqueous acetonitrile and 14.3 ± 0.2 in DMSO/water, which is higher than that of (1-azulenyl)diphenylmethyl cation (4a) by 11.0−11.3 pK units. The extreme stability of these methyl cations is attributable to the dipolar structure of the azulene rings, in addition to the contribution of the mesomeric effect of three dimethylamino groups. The electrochemical reduction of 2e, 3e, and 4e showed a wave at −1.26, −1.22, and −1.14 V (V vs Ag/Ag+), respectively, upon cyclic voltammetry (CV). The relatively high reduction potentials, compared with those of unsubstituted parent 1-azulenylmethyl cations (2a, −0.78; 3a, −0.66; and 4a, −0.48 V), also exhibited the electrochemical stabilization of these methyl cations by the dimethylamino substituents. The oxidation of 2e, 3e, and 4e exhibited an irreversible, barely separated two-step, one-electron oxidation wave to generate a trication species at a potential range of 0.50−0.75 V upon CV.

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The reaction of azulenes with several sulfoxides in the presence of acid anhydrides to afford the corresponding 1-azulenylsulfonium and 1,3-azulenediyldisulfonium ions is reported. The subsequent conversion of these ions in high yields into 1-azulenyl methyl and phenyl sulfides and 1,3-bis(methyl-and phenylthio)azulenes through treatment with diethylamine is also described. Reaction of the 1-azulenyl sulfides with MCPBA afforded 1-azulenyl sulfoxides, which were then efficiently transformed into 1,1Ј-biazulene deriva-