Quinoxaline aromaticity

Ortho quinones (and also aromatic a-diketones, o-phenylenediamine to yield quinoxalines as follows. 

Tautomerism questions arise with hydroxyphenazines and quinoxalines which contain hydroxyl groups in the aromatic ring. Does 5-hydroxyquinoxaline (62) show any tendency to exist as the tautomer (63) or 1-hydroxyphenazine (64) to exist as (65) Analogous tautomeric forms can also be written for 6-hydroxyquinoxaline and 2-hydroxyphenazine. 

A considerable number of non-cross-linked aromatic and heterocyclic polymers has been produced. These include polyaromatic ketones, aromatic and heterocyclic polyanhydrides, polythiazoles, polypyrazoles, polytriazoles, poly-quinoxalines, polyketoquinolines, polybenzimidazoles, polyhydantoins, and polyimides. Of these the last two have achieved some technical significance, and have already been considered in Chapters 21 and 18 respectively. The most important polyimides are obtained by reacting pyromellitic dianhydride with an aromatic diamine to give a product of general structure (Figure 29.17). 

The classical synthesis of quinoxalines involves the condensation of an aromatic o-diamine and an -dicarbonyl compound. 

The condensation reactions of aromatic o-diamines and sugars and sugar derivatives have been studied in detail and quinoxaline derivatives have been prepared recently from osones, osonehydrazones, and dehydro-L-ascorbic acidd ... 

Diphenol/thiophenol is one of the most important polymer precursors for synthesis of poly(aryl ethers) or poly-(aryl sulfides) in displacement polymerizations. Commonly used bisphenols are 4,4 -isopropylidene diphenol or bisphenol-A (BPA) due to their low price and easy availability. Other commercial bisphenols have also been reported [7,24,25]. Recently, synthesis of poly(aryl ethers) by the reaction of new bisphenol monomers with activated aromatic dihalides has been reported. The structures of the polymer precursors are described in Table 2. Poly(aryl ether phenylquinoxalines) have been synthesized by Connell et al. [26], by the reaction of bisphenols containing a preformed quinoxaline ring with... 

This is by far the most used type of primary synthesis for quinoxalines. It usually involves the cyclocondensation of an o-phenylenediamine (or closely related substrate) with a synthon containing an oxalyl [—C(=0)—C(=0)—] or equivalent [e.g., HC(=0)—C=N] grouping. For convenience, discussion of this synthesis is subdivided according to the type of synthon used to produce formally aromatic quinoxalines the formation of similar ring-reduced quinoxalines (mostly from related synthons at a lower oxidation state) is included in each such category. 

Aromaticity. The aromaticity indices (based on deviations in peripheral bond orders) for quinoxaline and related azaheterocycles have been calculated and they show good correlation with independently calculated resonance... 

Note The C-alkylation of quinoxaline has been done by addition (with or without subsequent aromatization see also preceding subsection), by reductive alkylation, or by homolytic alkylation. 

Note A few typical examples of the aromatization of alkylated or arylated hydroquinoxalines and of the oxidative ring fission of quinoxalines are given here. More mundane examples are given passing mention in other chapters. 

Ortho quinones (and also aromatic a-diketones, e.g., benzil) react with o-phenylenediamine to yield quinoxalines as follows. Dissolve the substance... 

The preparation of helically well-ordered polymers with stable screw-sense, which is able to be transmitted to newly formed polymer main-chains effectively, is highly desired for the development of new methodology for the synthesis of optically active helical polymers. An aromatizing polymerization of 1,2-diisocyanobenzenes is promoted by methylpalladium(II) complexes, producing poly(quinoxaline-2,3-diyl)s.146-148 The polymerization proceeds with successive insertion of the two isocyano groups of the diisocyanobenzene to the carbon palladium bond of... 

Experiments with quinoxaline complex 23 and phenazine complex 24 established that additional binding interactions were available in the form of aryl-aryl stacking between aromatic subunits in the components. In the case of quinoxaline this accounts for about 1.6 kcal or a factor of 15 in Ka. In 24, these attractive forces are partially offset by steric effects introduced by the remote ring as shown. 

Pyridine A-oxides were converted to tetrazolo[l,5-a]pyridines 172 by heating in the presence sulfonyl or phosphoryl azides and pyridine in the absence of solvent <06JOC9540>. 3-R-5-Trinitromethyltetrazolo[l,5-a]-l,3,5-triazin-7-ones 173 have been prepared from the alkylation of 5-trinitromethyltetrazolo[l,5-a]-l,3,5-triazin-7-one silver salt with different alkylation agents <06CHE417>. The use of 2-fluorophenylisocyanide in the combinatorial Ugi-tetrazole reaction followed by a nucleophilic aromatic substitution afforded tricylic tetrazolo[l,5-a]quinoxaline 174 in good yields and with high diversity <06TL2041>. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

An interesting variation of this quinoxaline synthesis is outlined by the synthesis of sydnoquinoxalines shown in Scheme 103. The starting material is phenylsydnone 288 with an iminophosphorane group in an o-position. With isocyanate or isothiocyanate carbodiimide intermediates 289 are formed by an electrophilic aromatic substitution at the sydnone ring (4 position), the 4-(arylamino)sydno[3,4-a]quinoxalines (290) are obtained (91S745). 

Directed ort o-metalations are also applied to quinoxalines possessing 2-chloro, 2-methoxy, and pivaloylamino substituents <1993JHC1491>, but successful syntheses via the lithio intermediates are far fewer compared to pyrazines. In fact, no example of metalation on aromatic carbons can be found in the literature since 1994. However, lithiation on benzene carbons of 6-chloro-2,3-dimethoxyquinoxaline was reported <1999T5389>. 

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