Toyonobu Asao

Azulene analogues of triphenylmethyl cation; extremely stable hydrocarbon carbocations

A series of azulene analogues of triphenylmethyl cation; tri(1-azulenyl)methyl cation, di(1-azulenyl)phenylmethyl cation, (1-azulenyl)diphenylmethyl cation, and their methyl and methoxycarbonyl derivatives were synthesized. Their pKR+ values showed that the stability of these cations dramatically increases with the number of azulene rings, and the pKR+ values of tri(1-azulenyl)methyl cation (11.3) and its trimethyl derivative (11.4) are the highest ever reported for methyl cations substituted with hydrocarbon groups.

Tris(3,6-di-t-butyl-1-azulenyl)methyl cation; hydrocarbon cation with the highest pKR+ value

Abstract Tris(3,6-di- t -butyl-1-azulenyl)methyl ( 5 ), tri(3- t -butyl-1-azulenyl)methyl ( 10 ), and tri(6- t -butyl-1-azulenyl)methyl ( 11 ) hexafluorophosphates were synthesized. Their p K R + values were determined as 14.3, 13.2, and 13.7, respectively, and the value of 5 showed the highest one as carbocation ever reported. The extreme stability of methyl cation 5 is mainly attributable to steric effects of six bulky t -butyl groups in addition to the contribution of dipolar structure of three azulene rings.

Synthesis and intramolecular pericyclization of 1-azulenyl thioketones.

Several azulene-substituted thioketones, 1-thiobenzoylazulene (1a) and di(1-azulenyl) thioketone (2a) and their derivatives (1b and 2b-d) with alkyl substituents on each azulene ring, were prepared and their intramolecular pericyclization reaction was examined. The thioketones with a 3-alkyl substituent on each azulene ring exhibited the presumed pericyclization reaction under thermal and acid-catalyzed conditions, although the cases of the 1-azulenyl thioketones without the 3-alkyl substituents afforded a complex mixture under similar conditions. The intramolecular reaction following the intramolecular hydrogen transfer afforded the products 13b, 14b, and 14c. The products 13b and 14b were converted into the corresponding cations 18(+) and 19(+), which have structural similarity with that of the phenalenyl cation. These cations exhibited the expected two-step reduction waves upon CV, although the ESR analysis revealed that the neutral radical state did not have the presumed high stability.

Synthesis of [1.1.1.1] (1,3)-2-methoxyazulenophane: azulene analogue of calixarenes

Abstract [1.1.1.1] (1,3)-2-Methoxyazulenophane, azulene analogue of calix [4] arene was synthesized starting from 2-methoxyazulene. The azulenophane had a 1,3-alternate conformer at room temperature and gave crystals including two molecules of benzene.

Vinylic substitution on troponoid catalyzed by palladium complexes, and formation of new C-pd σ-complexes of troponoid

Abstract The reaction of 3-, 4- and 5-halotropolones with olefins catalyzed by palladium (O) complex gave 3-, 4- and 5-(substituted vinyl) tropolones respectively. Similarly, 2-chlorotropone gave 2-styryltropones. Some tropolonyl- and troponylpalladium-σ-complexes were synthesized.

Dynamic stereochemistry of tri(2-methyl-1-azulenyl)methyl cation; steric effect of 2-methyl groups on rotational barriers and mechanism

The stable carbocation, tri(2-methyl-1-azulenyl)methyl hexafluorophosphate (2) was prepared. Steric effect of three 2-methyl groups was investigated by comparison with the 3,3′,3″-trimethyl analogue, tri(3-methyl-1-azulenyl)methyl cation (1). Dynamic stereochemistry of 2 was studied by temperature dependent 1H NMR spectra, which were analyzed by flip mechanism. The threshold rotational mechanism for 2 was a two-ring flip in contrast to a one-ring flip for 1, and the activation energies for 2 (18.6 and 17.5 kcal/mol) were higher than for 1 due to increased crowding in the transition state for the rotation.

Synthesis of dication species stabilized by azulene rings

Abstract Extraordinary stable dications of tetraazulenyl-m-xylylene and tetraazulenyl-p-xylylene were synthesized, and their pK R + values were determined as 11.5 and 11.2, respectively.

Synthesis, stability and bonding situation of tris-, bis- and mono[9-(azuleno[1,2-b]thienyl)]methyl cations

Stable tris-, bis- and mono[9-(azuleno[1,2-b]thienyl)]methyl cations (7a, 8a and 9a) and their derivatives, with a 6-isopropyl substituent on each azuleno[1,2-b]thiophene ring (7b, 8b and 9b) were prepared by the hydride abstraction reaction of the corresponding methane derivatives. The bonding situation of these compounds including the methane derivatives was examined by analysis of the 3J(H,H) values for the seven-membered ring from the 1H NMR spectra. The methane derivatives exhibited a significant alternating pattern in the 3J(H,H) values, which indicated that the pi-system of the azulene core is perturbed by the fused thiophene ring, showing a tendency towards a localized heptafulvene substructure. The 3J(H,H) values of 7b and 8b in the seven-membered ring revealed that the alternating C-C bond lengths in the azulene core still existed. The cations 9a and 9b, which exhibited nearly equal 3J(H,H) values in the seven-membered ring, exhibit the development of a delocalized tropylium substructure in the azulene core. X-ray crystal analysis of 6-isopropylazuleno[1,2-b]thiophene revealed substantial bond-length alternation in the seven-membered ring. Significant bond-length equalization in the seven-membered ring was also confirmed by the X-ray crystal analysis of 9b. The stability of these carbocations was examined by measurement of the pKR+ values and the redox potentials, which revealed that the bond-length alternation in the azulene core does not significantly affect the stability of the carbocations.

Synthesis of 1-, 2-, and 6-azulenethiols

Abstract 1-Azulenethiol was synthesized by reductive cleavage of di-1-azulyl disulfide, and 2- and 6-azulenethiols were synthesized by the conversion of the corresponding hydroxyl group to mercapto group via dimethylthiocarbamates.

Dynamic stereochemistry of tri(3-methyl-1-azulenyl)methyl cation; the first example of one-ring flip mechanism

Abstract Dynamic stereochemistry of tri(3-methyl-1-azulenyl)methyl hexafluorophosphate was studied by 1 H NMR and the spectra were analyzed by flip mechanism. The low temperature NMR indicated the compound exists in two propeller conformations (C 3 and C 1 symmetry), and at higher temperature the spectra reflect rapid isomerization, which were interpreted in terms of one-ring flip mechanism. The process has activation energy of 13.2 kcal/mol.