Syntheses and properties of extremely stable di(1-azulenyl)phenylmethyl and (1-azulenyl)diphenylmethyl cations having dimethylamino substituents on their phenyl groups

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 pK(R+) values of these cations dramatically increased with the dimethylamino substituents on their phenyl groups. The values of 5a-e (pK(R+) 13.2-13.8) and 6a-c (pK(R+) 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 H-1 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.