Azulenes, electrophilic substitution reactions

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... 

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

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. 

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. 

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

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 ... 

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

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