Azulene, basicity

The figures for seven azulenes are summarized in Table 24. Again, the basicity of the unsaturated system increases with the substitution of methyl groups. The differences are such that Mq (the Hq- value for P = 1) represents a characteristic quantity which is useful for the identification of azulene derivatives, and for testing their purity. 

Wassermann (1965) carried out spectroscopic and conductimetric measurements of the interaction of azulene with trichloracetic and dichloracetic acids in benzene as the solvent. The basicity constant determined in these systems cannot, for the reasons explained in Section IVA, be compared with the o (-P = 1) values and with the basicities of... 

Long and Schulze (1961) determined spectroscopically, in perchloric acid, the Hp-value for which the concentration ratio Cah+/ca = 1- This value can be regarded as an approximate uncorrected pAg-value. These investigations were extended to other azulene derivatives, permitting a study of the effect of polar substituents on the basicity (Long and Schulze, 1964). 

In calculations on proton addition complexes of azulene and methyl-azulenes by means of the HMO method, Heilbronner and Simonetta (1952) took the effect of methyl groups into aecount by adjustment of the a and j8 values, as had already been done by PuUmann and collaborators (1950) in calculating the spectra of the methylazulenes. These calculations correctly reproduce the effeet of a methyl group on the basicity of azulene. In contrast to the ease of naphthalene, the position of addition of the proton remains independent of the position of the methyl group ... 

Triphenylpyrylium tetrafluoroborate is a versatile and useful stable starting material. Its reaction with nitromethane under basic conditions has made 2,4,6-triphenylnitrobenzene easily available. In addition, pyrylium salts are readily converted to a variety of pyridine derivatives i i . 20 including alkyl- and arylpyridinium salts, to thiopyrylium salts," and to substituted azulenes. ... 

In the phenylazoazulene [52], the more basic of the two nitrogens is the nitrogen in the j3-position to the azulene ring (Gerson and Heilbronner, 1959), as predicted from molecular orbital calculations. In jO-dimethylaminophenylazoazulene [53], the same nitrogen re... 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

Azulene is measurably basic (pXBH + —1.7) and is roughly half-protonated in 2.2 M aqueous perchloric acid [181]. The rate coefficients shown in (102)... 

Structures comprising Two Five-Membered Rings and One Seven-Membered Ring (5,5,7).—Azuleno[2,l-d]thiazoles [C3NS-CS-C7]. The reaction between the substituted azulenes (171 R = H or C02Et) and neutral or basic alumina gives the title compounds (172). ... 

Azulenes are basic compounds (see Section 4.1.1). The formation of carbocyclic azulenium cations in the presence of strong adds is a transformation to tropylium salts that is accompanied by a large hypsochromic shift and the color changes to yellow (55FCF(3)334, p. 380 66CZ691). Azulenes fused to heterocyclic nitrogen bases, which are primarily N-protonated, do not behave uniformly. 

Similarly azulene reacts with sodamide in liquid ammonia to give a somewhat unstable red basic product which is probably... 

A maximum yield of 70% was obtained if ring-closure was carried out in a high-boiling basic organic solvent such as benzidine, and by a continuous process in which only small amounts of the intermediate fulvene were present at any time. Alkylcyclopentadienes react similarly to give 1-substituted azulenes, and substituted pyridines miy also be used to give azulenes substituted in the seven-membered ring. 

The simplest example of electrophilic attack is the reversible formation of azulcnium salts, as discussed above. The basicity of azulene approaches that of nitroaniline [80]. Azulenium salts have been isolated as crystalline products by the action of perchloric acid [81] and of tctrafluoroboric acid in ether [82] on azulene derivatives. These salts are colourless or pale brown addition of water instantaneously restores the blue colour of the parent azulene. Other 1-substituted azulenium salts have also been isolated as crystalline compounds, for example [83] ... 

Perhaps the most obvious reaction index to use for the earliest stages of electrophilic or nucleophilic reactions is the n-electron density. If Cl is attracted to tt charge, it should be attracted most to those sites where tt density is greatest. (Such an ion should be attracted to sites having excessive a charge density also, but our basic HMO assumptions ignore any variations in a density.) For an alternant hydrocarbon like naphthalene, all tt densities are unity, so this index is of no use. For nonaltemant molecules, however, it can be quite helpful. Azulene has varying HMO n densities (XXI). (More sophisticated calculations described in future chapters are in qualitative... 

It should be mentioned that similar pyrroles, indoles and azulenes bearing COCF3 group all reacted with alkali with haloformic removal of CHCF3 the strange behavior of 3-COCF3 indolizines was explained by their higher basicity and possibility of substitution of trifluoroacetate ion by ipso-protonation [14],... 

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