Syntheses of Azulene Analogues of Triphenylmethyl Cation: Extremely Stable Hydrocarbon Carbocations and the First Example of a One-Ring Flip as the Threshold Rotation Mechanism for Molecular Propellers

Abstract

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.

Cyanine–cyanine hybrid structure as a stabilized polyelectrochromic system: synthesis, stabilities, and redox behavior of di(1-azulenyl)methylium units connected with electron-accepting π-electron systems

“…10 The first reduction potential of bis(3-methoxycarbonyl) derivative 5b + is slightly less negative compared with that of 5a + , as similar to the results on those of cations 15a + and 15b + (E 1 red = −0.49 V). 10 The oxidation of 5b + also exhibited a wave at 1.38 V on CV ascribed to the oxidation of an azulene ring to generate a dicationic species, but without reversibility. The reduction of 8a + and 8b + showed same tendencies for 5a + and 5b Electrochromic analysis.…”

“…Cation 5a + (pK R + = 9.5) is not strongly destabilized by the 4-nitro substituent compared with that of 15a + (pK R + = 12.4), 10 in spite of the existence of the strong electron-withdrawing nitro substituent as an end group.…”

“…We selected di(1-azulenyl)methylium units as an end group for the construction of the C-C hybrid due to their high stability and also their strong absorption in visible region owing to the cyanine-like structure as demonstrated by our previous work. [10][11][12][13][14][15][16][17] Two di(1-azulenyl)methylium units with bis(3,6-di-tert-butyl and 3-methoxycarbonyl) substituents were applied to examine the substituent effect on the azulene rings (Schemes 6-9). Introduction of the tert-butyl group into the azulene rings should increase their thermodynamic stabilities and also reversibility of both electrochemical reduction and oxidation states.…”

Synthesis and Reactivity of 23‐tert‐Butyl‐ and 23‐Phenyltetraarylazuliporphyrins: an Analysis of the Effect of Bulky Substituents on Oxidative Ring Contractions to Benzocarbaporphyrins

“…[1,[7][8][9] In order to further investigate this chemistry, azuliporphyrins with bulky substituents at the 2 3 -position ( Figure 1) were synthesized from 6-tert-butyl-and 6-phenylazulene. [14,15] The symmetry of the precursors allows the azuliporphyrins to retain a plane of symmetry, while the placement of large substituents at the 6-position of azulene would be expected to have little effect on macrocycle formation as these groups are orientated away from the 1,3-positions where the carboncarbon bond-forming reactions take place. [16] Results and Discussion 6-tert-Butylazulene (8a) and 6-phenylazulene (8b) are easily synthesized from the related 4-substituted pyridines using a literature method (Scheme 2).…”

Electrophilic ipso‐Substitution and Some Unique Reaction Behavior of 1,3,6‐Tri‐tert‐butylazulene

“…Previously, we had reported the synthesis of 1,6-di-tert-butylazulene (1; Scheme 2, vide infra) by Friedel-Crafts alkylation of 6-tert-butylazulene with tert-butyl chloride in the presence of AlCl 3 . [4] As would be expected, the reaction produced significant amounts of 1,3,6-tri-tert-butylazulene (2; Scheme 1) as a byproduct. We were now able to put our hands on an azulene derivative suitable for the exploration of electrophilic ipsosubstitution reactions.…”