Ring isomerization, aromatic compounds

These are subdivided into (a) compounds isomeric with aromatic compounds in which the ring contains two double bonds but also an hybridized carbon (7 systems Scheme 6) or a quaternary nitrogen atom (9 systems Scheme 7). 

The 1,2,3-thiadiazole 1 possesses three contiguous heteroatoms in a five-membered ring and exists as a remarkably stable neutral aromatic compound. It is isomeric with the ring-opened a-diazothioketone 2 (Equation 1) although there is evidence that it reacts through this intermediate, all structural methods, including X-ray diffraction, point to 1 as the structure for a 1,2,3-thiadiazole. 

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

Quinoline and isoquinoline, known as benzopyridines, are two isomeric heterocyclic compounds that have two rings, a benzene and a pyridine ring, fused together. In quinoline this fusion is at C2/C3, whereas in isoquinoline this is at C3/C4 of the pyridine ring. Like benzene and pyridine, these benzopyridines are also aromatic in nature. 

There are several compounds that possess some measure of aromatic character typical of benzene, but do not possess a benzenoid ring. Appropriately, they have (4n + 2) it electrons and are classified as nonbenzenoid aromatic compounds (see Section 21-9). An example is azulene, which is isomeric with naphthalene and has a five- and a seven-membered ring fused through adjacent carbons ... 

Unlike phenols (Section 26-l), structural analysis of many of the hydroxy-substituted aza-aromatic compounds is complicated by isomerism of the keto-enol type, sometimes called lactim-lactam isomerism. For 2-hydro xypyrimidine, 19, these isomers are 19a and 19b, and the lactam form is more stable, as also is true for cytosine, 15, thymine, 16, and the pyrimidine ring of guanine, 18. 

Cytochromes P-450 also catalyze the hydroxylation of aromatic rings. In most cases, these reactions involve the intermediate formation of arene oxides derived from the epoxidation of a double bond of the aromatic compound and an isomerization of these very reactive epoxides into the corresponding phenols. 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

A number of nitrations, particularly of phenols, may in practice be nitrosations followed by oxidation of the nitroso compound to form the nitro compound by nitric acid. Although the underlying pattern of the nitration ortholpara or meta) is determined by the substituent(s) on the aromatic ring, the ratio of the different isomeric nitro compounds... 

In aromatic isomeric bicyclic compounds, those with bridge rings have lower b.ps. than those with two isolated or condensed rings. 

The sulfonic acid resins such as Dowex-50 and Amberlyst-15 have been used to promote the alkylation of the more active aromatic rings but attempts to increase their acidity generally resulted in the degradation of the solid. 2 The more strongly acidic perfluorinated resin sulfonic acid, Nafion-H,2>3 has, however, been used to promote the alkylation of benzene and other aromatic compounds. Nafion-H catalyzed the vapor phase reaction between toluene and methanol. When nm at 185°C a 12% yield of the isomeric xylenes was obtained with the ortho isomer the major product. 0 Methylation of phenol at 205°C over this catalyst gave, at 63% conversion. 37% anisole and 10% of a mixture of the ortho and para cresols in a 2 1 ratio. Reaction of anisole with methanol under these conditions resulted in a 14% selectivity to the methyl anisoles at 40% conversion, with the ortho and para isomers formed in nearly equal amounts. ... 

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