Heterocycles, quinonoid

The third chapter of this volume is concerned with mono- and diazaquinones. S. R dl (Prague, Czech Republic) reviews compounds that contain one nitrogen or two nitrogen atoms directly within the quinonoid ring, a subject that has not been previously summarized in a comprehensive manner. This is in distinction to the several specialized reviews of quinones themselves. Heterocyclic quinones, in the sense of compounds in which the quinonoid ring is fused to a heterocyclic ring, were covered in Volume 45 of our series in 1989. 

Among the five-membered heterocycles with three nitrogen atoms, triazoles and benzotriazoles are of significant practical importance. In the case of ben-zotriazole, the aromaticity of the benzenoid 1H-benzotriazole (90A, R = H) has been considered to be greater than that of the quinonoid 2/7-benzotria-zole (90B, R = H) (Scheme 43).134 However, the difference in aromaticity between tautomers is greatly dependent on the dielectric constant of the medium. 

Another boron heterocycle with a formula apparently appropriate for a nido-pyramidal structure is the compound H4C4B2F2 (193) (Fig. 28). The fluorine substituents in this molecule, however, can 7r-bond to the boron atoms, thereby generating a quinonoid electronic structure. Elsewhere in boron cluster chemistry, the presence of halogen substituents appears to modify the skeletal electron requirements of the cluster. 

Quinoline is a base since, as for pyridine, the lone pair of electrons on the nitrogen atom is not utilized in its internal resonance. Although it is an aromatic compound, the valence bond description of quinoline shows two of the neutral contributors, A and C (see Scheme 3.1), to the resonance hybrid as quinonoid in character, whereas in B either the carbo-cycle or the heterocycle must exist in the form of a 1,3-diene. The presence of the pyridine nucleus is reflected by the inclusion of doubly charged canonical forms. 

The electrochemical properties of 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo[l,2-A4,5- ]difuran 72 have been investigated by cyclic voltammetry in the search for new materials with enhanced conducting properties <2000JOC2577>. Upon single-electron reduction the central quinonoid ring gains the aromatic stability of benzene, whereas the fused heterocyclic part loses the aromaticity of the furan nucleus. 

The chemistry of the o-quinonoidal heterocycles benzo[c] indole (isoindole, 1) and benzo[< ]thiophene (2) has been previously reviewed in this... 

The electronic structure of 4 (and the other o-quinonoid heterocycles 1,2, and 3) has been discussed in the light of NMR data. For a series of hydrocarbons containing formally cisoid butadiene fragments (10 R and R not explicitly stated), the ratio of the vicinal coupling constants s.e)... 

Linear benz-annelation enhances the reactivity and decreases the stability of benzo[c]furan, as with other o-quinonoid heterocycles.1,3-Diphenyl-naphtho[2,3-e]furan (299) has been obtained from 297 via lactol 298 as deep red glistening plates with mp 148-154°C in 89% yield.Although dried crystals of 299 could be stored for several months in the absence of light and air, solutions in organic solvents decolorized slowly on standing... 

Fusion of pyridine to a furan nucleus gives rise to six isomeric furopyridines (6-11 Scheme 1). Members of all these types are known. As is to be expected and as is shown even by simple HMO calculations (B-78MI31700, B-78MI31701, B-78MI31702), derivatives (6-9) of benzo[6]furan (4) are considerably more stable than the o-quinonoid isomers (10,11) derived from benzo[c]furan (5) (Tables 1 and 2). It is also to be noted that if super-delocalizabilities are considered as indices for reactivities, electrophilic substitution will occur in the furan nucleus both when considering the neutral heterocycles and the N-protonated species. 

The chemistry of thienoquinolines has been explored to a limited degree. Two types (378,379) will be described in this subsection. The pharmacological interest in thieno[2,3-Z>]quinoline (378) and derivatives thereof stems from their isosteric and isoelectronic resemblance to acridine furthermore, (378) constitutes the sulfur analog of furoquinoline (Section 3.17.2.1.5), which is the parent system of a number of alkaloids. Thieno[3,4-6]quinoline (379) is an o-quinonoidal heterocycle which is of interest both for theoretical reasons compared with its isoconjugate analogues and as a synthon in Diels-Alder reactions for the preparation of other condensed heterocycles. 

As reagents causing intramolecular recyclizations of 2-benzopyrylium salts, nucleophiles centered at nitrogen, oxygen, sulfur, or carbon atoms were used. As a fairly general rule, the ring opening of the heterocyclic moiety leads to an orf/io-quinonoid structure, as shown in more detail in Section III,C,4,iv (Structure 201). 

Acyl-chrysene 270 is formed on thermolysis of methoxyisochromene 109 in 50% yield (86UP3). By analogy with behavior of the adducts between 2-benzopyrylium salts and acetoacetic ester 235 (cf. Section III,C,4,d), one cannot exclude the electrocyclic ring-opening of the heterocyclic fragment in 109. The orf/io-quinonoid compound 274 thus formed is a diene, and the vinyl fragment in the unreacted adduct 109 plays the role of an external dienophile. 

Fig. 7.9 Mechanism of formation of tautomeric quinonoid intermediates in the synthesis of L-tryptophan. The conversion of ES III (see also Fig. 7.6) to ES IVA is probably catalyzed by B2 which abstracts the proton on N-l of the heterocyclic ring concomitant with attack of indole on the aminoacrylate. The tautomerization of ES IVA to yield ES IVB is carried out by abstraction of the proton at C3 of indole by a basic group, Bi, concomitant with protonation of N-l by B2H (modified from Ref. 6).

The LCGO calculations of dipole and second moments for a number of five-membered heterocycles agreed with the experimental absolute dipole moment (74JCS(P2)42o). The dipole moments of 4-, 5- and 6-substituted 2,1,3-benzazoles (1), (2) and (5), determined in benzene at 25 °C (73JHC773), show that the 2,1,3-benzoxadiazole (5) exists primarily in the ortho-quinonoid form, while (1) and (2) are more in the benzenoid form. The greatest amount of mesomeric contribution occurs in the selenium compound. 

Enaminones are useful and widely used synthons in organic synthesis of alicyclic, aromatic and heterocyclic compounds. Frequently, it is assumed that enaminones are formed as intermediates in the synthesis of heterocycles. However, when they are not isolated, they are not considered in this article. Moreover, there are many examples of formal analogues of enaminones (e.g. aromatic or quinonoid compounds) as well as heterosubstituted derivatives4 which are also not discussed in the present chapter. 

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