Electrophilic substitution azulenes

Azulyl sulphoxides 127 have also been prepared by a reaction involving a direct electrophilic substitution on the azulene ring by alkane- or arenesulphinyl chlorides186 (equation 69). Preparation of the methyl and phenyl sulphoxides of 4,6,8-tri-methylazulene and 4,6,8-tri-isopropylazulene by this method resulted in fair yields (57— 72%). However, the substitution on azulene itself gave only low yields of the corresponding sulphoxides. 

In keeping with its aromatic character and unsymmetrical charge distribution, azulene undergoes certain typical electrophilic substitution reactions at the 1 and 3 positions. Thus Friedel-Crafts acylation leads to a mixture of 1-ethanoylazulene and 1,3-diethanoylazulene ... 

Only a few investigations of electrophilic substitution reactions of pseudo-azulenes containing a pyrrole-type nitrogen have been reported. There are many examples of alkylations (see Table VI). An alkylation always takes place at the nitrogen of the five-membered ring. For 7H-pyrrolo[2,3-b]-pyridine 68 azocoupling and reaction with dithiolium salts have been reported.166... 

Morriii, T. C., Opitz, R., Repiogie, L. L., Katsumoto, K., Schroeder, W., Hess, B. A., Jr. Correspondence between theoretically predicted and experimentally observed sites of electrophilic substitution on a fused tricyclic heteroaromatic (azulene) system. Tetrahedron Lett. 1975, 2077-2080. 

In an analogous manner, we succeeded finally in synthesizing the 5-carbonitrile of the kata-condensed tetracyclic azuleno(l, 2-f)azulene (776) in a 50% yield by condensation of (44) with azulene-2-acetonitrile (149) to the red-violet aminonitrile (174) and smooth thermolysis of it78. The hydrocarbon (176) is the first known member of the theoretical 12 isomeric azulenoazulenes of which considerable stability and aromatic properties are predicted79. Indeed, (7 76) can readily be formylated by dimethylformamide-phosphorylchloride in a typical aromatic-electrophilic substitution to the 1-carbaldehyde (7 77)78. ... 

Besides protonation, electrophilic substitution at the 1- and 3-positions is another typical feature of azulene reactivity (55FCF(3)334, p. 367 59MI2, p. 310 61FOR(19)32 66MI1 84MI1 85HOU(5/2c)127, p. 249). Assisted by their polarity (see structures 232), azulenes have been even described as "pacemakers" (typical model compounds) of new electrophilic reactions (66CZ691) since they are colored, substitution can easily be observed and optimal conditions can be worked out. Examples mentioned earlier in the synthetic Section 2 are those of thiocyanation (Scheme 32) and alkylation (Scheme 38). In the case of occupied 1- and 3-positions reactions at C-5 and C-7 can be observed. 

The oxidative formation of azulenoquinones from linearly fused azulenoheterocycles, on the other hand, exactly follows the rules of electrophilic substitution on azulenes (cf. Section 4.2.1). Thiophene 45a or furan 47a (Scheme 68) can be oxidized by bromine/aqueous acetic acid, phenyltrimethylammonium perbromide (PTAB) or pyridinium perbro-mide to yield mixtures of quinones 274a and 275a or 274b and 275b, respectively (96BCJ1149, 03H(61)271). 

Reactions with Electrophiles. Electrophilic substitution of azulenes takes place at the 1- and 3-positions if these are vacant, reaction proceeding according to the path ... 

A Study of the nitrosation of azulene and of its 1-nitro-derivative showed that the rates were characteristic of electrophilic substitution, with encounter of the reactants in the case of azulene, and proton loss from the protonated intermediate in the case of nitro-azulene, being rate limiting [104],... 

Among other electrophilic substitution reactions undergone by azulenes arc mercuriation [106,132] and aminomethylation [133,134]. 

There is an example of electrophilic substitution occurring in a side-chain as well as in the azulene ring. Sulphenylation of guaiazulene with trifluoromethylsulphonyl chloride leads to substitution in the 4-methyl and 7-isopropyl groups as well as at the 3-position [138]. 

It is interesting to compare the various indices we have discussed for a single molecule to see how well they agree. Data for azulene are collected in Table 8-4. Experimentally, azulene is known to preferentially undergo electrophilic substitution at positions 1 (and 3), nucleophilic substitution at positions 4 (and 8), and radical addi-... 

This mechanistic consideration is in accord with the result from arylation of indene (7) [5]. The arylation occurs exclusively at the 1,3-positions, those with the highest electron density, of intermediary indenyl anions (9). The related arylation of azulene also proceeds exclusively at 1,3-positions, interpreted as electrophilic aromatic substitution, again by an electrophilic aryl-Pd species [10]. 

Electrophilic aromatic substitution reactions of compounds 10 occur in a fashion characteristic for heterocyclic analogues of azulene, and are specific at positions 5 and 7 <1994CB1479>. Thus, 10a (R = H, R = Ph) was successfully brominated, formylated, and acylated, as shown in Scheme 7. 

Electrophilic reactions at ring atoms 4.2.1 Substitution at the azulene ring system... 

The fact that electrophilic or nucleophilic attack results in substitution reactions indicates, however, that reformation of the azulene system is energetically favourable. 

Attack by an electrophile on azulene results in the formation of a tropyliura ion loss of a proton from this intermediate provides a substituted azulene ... 

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] ... 

Apart from the usual spectroscopic structural evidence, other support for a-polymer-linkages for most CPs is fairly strong 2,5-disubstituted pyrrole monomers fail to polymerize, and the polymerization of 2-monosubstituted monomers stops at the dimer stage 1- and 1,3-substituted azulenes (Scheme 5-Y) fail to polymerize [58] theoretical electron density studies on azulene indicate that electrophilic attack, as with a radical cation on a neutral monomer, would occur at the 4-position, yielding a 1,4-linkage rather than the 1,3-linkage actually observed [59]. 

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