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.

The titled stable monocations, di(1-azulenyl)(2- and 3-thienyl)methyl cations 7a,b and 8a,b and dications composed of two di(1-azulenyl)methylium units connected with 2,5-thiophenediyl and 2,5-thieno[3,2-b]thiophenediyl spacers 9a,b and 10a,b were prepared by hydride abstraction of the corresponding methane derivatives. These mono- and dications 7a,b, 8a,b, 9a,b, and 10a,b showed high stability with large pK(R)+ values. The values of monocations 7a,b and 8a,b were 11.2-11.8 +/- 0.1 and 11.4-12.4 +/- 0.1, respectively. Two cation units in dications 9a,b and 10a,b were neutralized via one step at the pH of 11.1-11.7 +/- 0.1, which corresponds to the average of the pK(R)+ values of the dications and half-neutralized monocations. Electrochemical behavior of 7a,b, 8a,b, 9a,b, and 10a,b was examined by cyclic voltammetry (CV). Formation of the thienoquinoid products 18a,b and 19a,b from 9a,b and 10a,b was characterized by UV-vis spectroscopy under electrochemical reduction conditions. Chemical reduction of 9a,b and 10a,b with Zn powder in acetonitrile afforded 18a,b and 19a,b as deep-colored crystals, which exhibited rather high electron-donating ability.

A series of extremely stable di(1-azulenyl)phenylmethyl and (1-azulenyl)diphenylmethyl cations having dimethylamino substituents on their phenyl groups, i.e., di(1-azulenyl)[4-(dimethylamino)phenyl]methyl (5a) and (1-azulenyl)bis[4-(dimethylamino)phenyl]methyl (6a) cations and their 3-methyl-1-azulenyl (5b and 6b) and 3,6-di-t-butyl-1-azulenyl (5c and 6c) homologs, were synthesized by hydride abstraction from the corresponding hydrides. Their properties were fully characterized. As expected, the pKR+ values of these cations dramatically increased with the dimethylamino substituents on their phenyl groups. The values of 5a—c (pKR+ 13.2—13.8) and 6a—c (pKR+ 12.6—13.3) are higher by 1.4—2.7 and 8.7—9.6 pK units than those of the corresponding analogous phenyl- and diphenylmethyl cations. The redox behavior of each cation was also affected by the substituents. The oxidation of 5a—c in acetonitrile exhibited a barely separated two-step oxidation wave at potential ranges of +0.75 — +0.87 and +0.89 — +1.01 V vs. Ag/Ag+ upon cyclic voltammetry (CV), as occurs in the oxidation of tri(1-azulenyl)methyl cations. The wave is ascribed to the oxidation of the two azulene rings to generate a trication species. The oxidation of 6a—c also showed two waves at a narrow potential range at +0.74 — +0.92 V. Although the reduction of 6a—c exhibited an irreversible wave at −0.94 V, that of 5a—c showed a reversible wave at −0.87 — −0.95 V upon CV. The temperature-dependent 1H NMR spectra of 5b and 6b were also examined to clarify their stereochemistries. Stereoisomerizations due to the pyramidal dimethylamino substituents in addition to the propeller conformations of three aromatic rings were observed by low-temperature NMR studies. At higher temperature the NMR reflects the rapid isomerization of these stereoisomerisms.

Extremely stable carbocations, tris(6-methoxy-1-azulenyl)methyl (8), bis(6-methoxy-1-azulenyl)(4-methoxyphenyl)methyl (9a), and (6-methoxy-1-azulenyl)bis(4-methoxyphenyl)methyl (10a) cations and a series of di(1-azulenyl)phenylmethyl and (1-azulenyl)diphenylmethyl cations having methoxy substituents on each phenyl group, i.e., di(1-azulenyl)(4-methoxyphenyl)methyl (9b) and (1-azulenyl)bis(4-methoxyphenyl)methyl (10b) cations and 3-methyl-1-azulenyl (9cand 10c) and 3,6-di-t-butyl-1-azulenyl (9d and 10d) analogues, were synthesized by hydride abstraction from the corresponding methane derivatives and their properties were fully characterized. The pKR+ values of 8 and 9a were well beyond 14.0. The value of 10a was determined as 13.2, which is higher by 10.2 pK units than that of (1-azulenyl)diphenylmethyl cation. The value also considerably increased by the methoxy substitution on each phenyl group. The values of 9b—d (pKR+ 11.7—13.4) and 10b—d (pKR+ 5.2—7.0) are higher by 1.0—1.4 and 2.2—2.4 pK units than those of the corresponding analogous benzyl and diphenylmethyl cations. The electrochemical reduction of 8, 9a—d, and 10a—d showed a wave at −0.88, −0.71 — −0.83, and −0.56 — −0.71 V (V vs. Ag/Ag+), respectively, upon cyclic voltammetry (CV). The relatively high reduction potentials also exhibited the stabilization of the methyl cations by the methoxy substituents. The oxidation of 8 in acetonitrile exhibited barely separated two-step, one-electron oxidation waves at a potential range of +0.90 — +0.98 V upon CV, although 9a—d and 10a—d did not show two similar waves at a narrow potential range. The wave is ascribed to the oxidation of two azulene rings to generate a trication species.

Acid-catalyzed condensation of azulenes 8a-c with 4-hydroxy- and 3,5-di-tert-butyl-4-hydroxybenzaldehyde leads to substituted di(1-azulenyl)(4-hydroxy- and 3,5-di-tert-butyl-4-hydroxyphenyl)methanes 7a- f, which are easily converted into substituted di(1-azulenyl)(4-hydroxy- and 3,5-di-tert-butyl-4-hydroxyphenyl)methyl cations 5a-f by oxidation with DDQ. The spectroscopic data are consistent with the protonated cationic structures of 5a-f. The electrochemical reduction of 5a-f showed a reversible wave at -0.74 to -0.86 V (V vs Ag/Ag(+)) upon cyclic voltammetry (CV), although 5d and 5e showed an irreversible wave at -0.79 V. The relatively high reduction potentials of 5a-f, compared with those of di(1-azulenyl)phenylmethyl cations 2a-c, exhibit the stabilization by 4-hydroxy substituents on the phenyl groups. These salts (5a-f.PF(6)(-)) bearing 4-hydroxyl groups on the phenyl rings have been converted by treatment with bases to alpha,alpha-di(1-azulenyl)-1,4-benzoquinone methides 6a-f, which revert to 5a-f.PF(6)(-) upon reprotonation with HPF(6). These quinone methides (6a-f) are highly polarized by the extreme-electrodonating properties of 1-azulenyl groups. The highly polarized properties of 6a-f reflected to the high pK(a) values of their conjugate acids (5a-c, 6.5-7.1, and 5d-f, 3.4-3.8). The strong solvatochromic effects also provide strong evidence of a large contribution of dipolar forms (6') in the ground state. The relatively low oxidation potentials of 6a-f (+0.35 to +0.47 V vs Ag/Ag(+)) reflected facile formation of phenoxy radical cations, stabilized by two 1-azulenyl groups.