Heteroaromatic azo compounds

Azobenzenes, (29), and analogous heteroaromatic azo compounds, (30), are in aprotic solvents reduced in two sequential one-electron steps to the radical anion and the dianion [61-66]. Disproportionation of the radical anion to the dianion is favored by the presence of Li+ [67]. The dianion is considerably more basic than the radical anion, and the dianion is only stable in very dry nonacidic solvents [64, 65, 67, 68]. Both the dianion and the radical anion derived from (29) have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pK units) than the dianion but more basic than... 

The synthesis of both symmetric and unsymmetric aromatic azo compounds including heteroaromatic azo compounds in high yields is achieved through an oxidative dimerization of aromatic amines using Bu OI, generated in situ from t-butyl hypochlorite and sodium iodide, in acetonitrile under mild conditions. Spectroscopic study indicates that formation of the N=N bond is the key step in the oxidative reaction. ... 

With azo compounds With aromatic and heteroaromatic azo compounds Co(4-C5H4N-C HN =N)2Cl2 room 8849 4.59 Gouy tetrahedral coordination suggested 73B22... 

Aromatic diazonium compounds became industrially very important after Griess (1866a) discovered in 1861/62 the azo coupling reaction, by which the first azo dye was made by C. A. Martius in 1865 (see review by Smith, 1907). This is still the most important industrial reaction of diazo compounds. Hantzsch and Traumann (1888) discovered that a heteroaromatic amine, namely 2-aminothiazole, can also be diazotized. Heteroaromatic diazonium compounds were, however, only used for azo dyes much later, to a small extent in the 1930 s, but intensively since the 1950 s (see Zollinger, 1991, Ch. 7). 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

There are also some couplings in which hydrazones are formed but for which the azo tautomer is not detectable and probably does not exist. This is the case in some coupling reactions involving methyl groups of aromatic heterocycles (see, for example, 12.48 and 12.49 in Sec. 12.5). Replacement of a methyl proton by an arylazo group (Scheme 12-3) would result in an azo compound containing an sp3-hybridized — CH2 — group (12.1). The latter is less stable than the tautomeric hydrazone (12.2), in which there is a n-n orbital overlap from the heteroaromatic to the aromatic system. 

Fig. 12-2. Correlation between azo coupling rates of heteroaromatic diazonium ions with the 2-naphthox-ide-3,6-disulfonate trianion and the chemical shifts of protons at the position of the diazonio group in H NMR spectra of the corresponding heteroaromatic parent compounds (Diener and Zollinger, 1986).

Aminobenzimidazoles and 2-aminobenzothiazoles can be oxidized with sodium hypochlorite to afford the corresponding 2,2-azo compounds. A rather unusual autooxidation of a heteroaromatic (7-amino group occurs on treatment of l-benzyl-2-aminobenzimidazole 671 with excess sodium or potassium in liquid ammonia. The products are 2,2-azobenzimidazole 674 and 2-nitrobenzimidazole 675, formed in 60 and 40% yields, respectively. It is supposed that... 

Azo coupling of aromatic or heteroaromatic diazonium salts with 0x0 TPs at the 6-position gives azo compounds (92JSC165) or tautomers (e.g., 95) (97JCR(M)2378). 

Azo Compounds, Azirines, Diazirines, Diazo Compounds, Diazonium Salts, Azides,. V-Oxides, Nitrite Esters and Heteroaromatic Compounds Photofragmentation and Photorearrangement... 

Azo compounds can be regarded as substitution products of diimine HN=NH, but, unlike this parent, azo compounds of general formula RN=NR are stable. R and R may be the same or different aromatic or heteroaromatic groups or R may be aromatic and R aliphatic however, wholly aliphatic azo compounds are also known. Further, the azo group may be linked to an aromatic or aliphatic acyl group (giving an acylazo compound). Finally, R and R may be secondary amino groups (1,1,4,4-tetrasubstituted 2-tetra-zenes). 

The unstable heteroaromatic benz(cd)indazole ( ) has been characterized in part from its N shift, obtained on enriched material by photolysis of a diazide. Although lying at somewhat higher shielding, the resonance position is consistent with that expected for azo compounds (50). 

C]malonate is sufficient to couple with aryl diazonium salts using aqueous sodium acetate as a base. In the presence of a tautomerizable hydrogen, the azo compound initially formed (e.g., 388) isomerizes to the corresponding hydrazone (e.g., 389). Derivatives of this structural class have been used for the preparation of binuclear heteroaromatic systems. Saponification of the ester groups of hydrazone 389. for example, generated... 

In spite of the large amount of scientific and industrial activity on diazo compounds and azo coupling reactions since the 19th century, heteroaromatic diazo components have been studied intensively only since the 1950s. 

The first group consists of monocyclic heteroaromatic compounds with one heteroatom and without strongly electron-donating substitutents (OH, NH2). Pyrrole, furan, and thiophene are better electron donors than benzene. The order of their reactivities in azo coupling is thiophene > pyrrole > furan > benzene. 

Cardellini et al. (1990, 1991) showed that related heteroaromatic compounds such as 3,3-bisindazolines (12.157) and 2,2-disubstituted 3-oxo- and 3-aryliminoindoline-1-oxyls (12.158, X = 0 and NC6H5, respectively) can be oxidized by arenediazonium ions, but no azo derivatives are formed. 

Shortly after Perkin had produced the first commercially successful dyestuff, a discovery was made which led to what is now the dominant chemical class of dyestuffs, the azo dyes. This development stemmed from the work of Peter Griess, who in 1858 passed nitrous fumes (which correspond to the formula N203) into a cold alcoholic solution of 2-aminO 4,6 dinitrophenol (picramic acid) and isolated a cationic product, the properties of which showed it to be a member of a new class of compounds [1]. Griess extended his investigations to other primary aromatic amines and showed his reaction to be generally applicable. He named the products diazo compounds and the reaction came to be known as the diazotisation reaction. This reaction can be represented most simply by Scheme 4.1, in which HX stands for a strong monobasic acid and Ar is any aromatic or heteroaromatic nucleus. 

The diazonium ions 13 with electron-withdrawing substituents are not h texoaromatic compounds and therefore do not strictly come within the scope of this chapter. They are formally related to the alkenediazonium ions. Nevertheless, they are discussed here because in their properties they bear a close resemblance to heteroaromatic and arenedi-azonium ions rather than to alkenediazonium ions. In particular they can be obtained by direct diazotization of the amines, they are stable in an aqueous medium and they are capable of undergoing an azo coupling reaction. 

The anion BH, formed in Eq. (3), is thermodynamically a stronger base than B and will react with another molecule of substrate, as in Eq. (4). Each PB will therefore consume a total of two protons (and two electrons). An exception to the fast removal of BH by further reduction is sometimes found for radical anion oxygen bases derived from carbonyl compounds, as discussed in Sec. III.B.2. Radical anion EGBs are usually derived from aromatic systems such as aromatic hydrocarbons, A-heteroaromatic systems or azo-arenes. An example was given in Scheme 3 [3]. Radical anion EGBs are normally pro-... 

The N chemical shifts of ring nitrogens in heteroaromatic compounds usually fall in the low field half of the spectrum. For benzo(pyridazines) in DMSO the following shifts, which are typical for azo-type groups, have been observed (values relative to a nitromethane external standard) pyridazine (-1-20), 3-methylpyridazine (N-1, -1-19 N-2, -1-15), phthalazine (-flO), cinnoline (N-1, -f 44 N-2, -1-41), and benzo[c]cinnoline (-1-60). The resonances of the unsymmetrical compounds were assigned on the basis of the two-bond N H coupling of about 10-11 Hz observed for N-1 of... 

Imines, azo derivatives, and heteroaromatic compounds (nitrogen sp -hybridized) form nitrones (2), azoxy derivatives (3), and heteroaromatic N-oxides (4), respectively. Nitriles (nitrogen sp-hybridized) form nitrile N-oxides (5, Equation 99.2). 

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