Ring isomerization, aromatic

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

Triazoline aromatization to triazoles can be achieved by oxidation, isomerization reactions, and elimination of stable molecular fragments, all of which require the presence of a free hydrogen at position 4 and/or 5 of the triazoline ring. The aromatization reaction affords a selective, synthetic route for the preparation of triazoles of definitive structure, inasmuch as azide addition to acetylenes is not regioselective.33... 

These two isomeric three-ring polycyclic aromatic hydrocarbons have similar values of Kow but different melting points, illustrating the importance of the fugacity ratio term. Both have a molecular formula C14 H10 and a molecular mass of 178.2. 

FIGURE 4-4. Two approaches to the separation of polynuclear aromatics, (a) Reverse-phase separation of isomeric 4-ring polynuclear aromatics using a gradient of 70/30 (v/v) to 100/0 (v/v) acetonitrile/water as shown beneath the chromatogram. Column C,g detection at 254 nm. (b) Normal-phase separation of aromatic hydrocarbons. Column /uPorasil (silica, 10 /urn) 3.9 mm ID x 30 cm (2 columns) mobile phase hexane flow rate 8 mL/min. (Fig. 4-4b reproduced from reference 1 with permission.)... 

Table 6.4 shows the principal photoreactions of aromatic compounds that we discuss in this chapter. Upon irradiation, aromatic compounds, such as benzenes, naphthalenes and some of their heterocyclic analogues, undergo remarkable rearrangements that lead to some non-aromatic highly strained products, such as benzvalene and Dewar benzene (entry 1), which can be isolated under specific conditions. Quantum and chemical reaction yields are usually low however, photochemistry may still represent the most convenient way for their preparation. While bulky ring substituents usually enhance the stability of those products, aromatic hydrocarbons substituted with less sterically demanding substituents exhibit ring isomerization (phototransposition) (entry 2). 

Substitution on benzene raises no regiochemical issues because every hydrogen is equivalent, but as soon as there is one substituent on the ring, isomeric products can result— namely, ortho, meta, and para. The reactivities of the different sites on substituted aromatic rings are quantified by what are known as partial rate factors (where n = o, m, or p for ortho, meta, or para, respectively, and R = substituent). These numbers reflect the rate constants (k ) for reaction of the individual ortho, meta, or para sites with an electrophile compared to the rate constant (k) for addition to benzene itself (Eqs. 10.113 A, B, and C). The rate constants for ortho and meta are divided by two because there are two ortho and meta hydrogens, and the rate constant for benzene is divided by six due to the six hydrogens. 

Aromatic Ring Reactions. In the presence of an iodine catalyst chlorination of benzyl chloride yields a mixture consisting mostly of the ortho and para compounds. With strong Lewis acid catalysts such as ferric chloride, chlorination is accompanied by self-condensation. Nitration of benzyl chloride with nitric acid in acetic anhydride gives an isomeric mixture containing about 33% ortho, 15% meta, and 52% para isomers (27) with benzal chloride, a mixture containing 23% ortho, 34% meta, and 43% para nitrobenzal chlorides is obtained. 

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). 

As would be anticipated, amino groups in the homocyclic ring of 1,2-benzisoxazoles behave as normal aromatic amines, forming mono- and bis-acyl derivatives, etc. (67AHC(8)277,p. 296). In th e isomeric 2,1-benzisoxazoles the 3-amino compound exists as such and not in the tautomeric 3-imino form (65CB1562). Amino groups in 3-phenyl substituents behave as normal aromatic amines (67AHC(8)277,p. 331). 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds ... 

These various photoproducts are all valence isomers of the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the t-butyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the t-butyl groups are coplanar with the ring in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent t-butyl groups. 

The previous sections have dealt with stable C=N-I- functionality in aromatic rings as simple salts. Another class of iminium salt reactions can be found where the iminium salt is only an intermediate. The purpose of this section is to point out these reactions even though they do not show any striking differences in their reactivity from stable iminium salts. Such intermediates arise from a-chloroamines (133-135), isomerization of oxazolidines (136), reduction of a-aminoketones by the Clemmensen method (137-139), reductive alkylation by the Leuckart-Wallach (140-141) or Clarke-Eschweiler reaction (142), mercuric acetate oxidation of amines (46,93), and in reactions such as ketene with enamines (143). 

With a substituted aromatic ring compound 2, mixtures of isomeric coupling products may be formed the ort/zo-product usually predominates. The rules for regiochemical preferences as known from electrophilic aromatic substitution reactions (see for example Friedel-Crafts acylation), do not apply here. 

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