Tris[6-(dimethylamino)-1-azulenyl]methyl Hexafluorophosphate. Extremely Stable Methyl Cation with the Highest pKR+ Value

Abstract

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 pKR+ 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 dimethy...