Heteroaromatic ring

Doerksen R J and Thakkar A J 1999 Quadrupole and octopole moments of heteroaromatic rings J. Phys. Chem. A 103 10 009... 

Reactions of aromatic and heteroaromatic rings are usually only found with highly reactive compounds containing strongly electron donating substituents or hetero atoms (e.g. phenols, anilines, pyrroles, indoles). Such molecules can be substituted by weak electrophiles, and the reagent of choice in nature as well as in the laboratory is usually a Mannich reagent or... 

Heteroaromatics such as furan, thiophene, and even the 2-pyridone 280 react with acrylate to form 281(244-246]. Benzene and heteroaromatic rings are introduced into naphthoquinone (282) as an alkene component[247]. The pyrrole ring is more reactive than the benzene ring in indole. 

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

The cross-coupling of aromatic and heteroaromatic rings has been carried out extensively[555]. Tin compounds of heterocycles such as oxazo-lines[556,557], thiophene[558,559], furans[558], pyridines[558], and seleno-phenes [560] can be coupled with aryl halides. The syntheses of the phenylo.xazoline 691[552], dithiophenopyridine 692[56l] and 3-(2-pyridyl)qui-noline 693[562] are typical examples. 

The main chain of these polymers contains, as the principal component, five- or six-membered heteroaromatic rings, ie, imides, which are usually present as condensed aromatic systems, such as with benzene (phthalimides, 3) and naphthalene (naphthalimides, 4) rings. 

By changing the substituents on the ethylenic linkage and exchanging phenyl rings for heteroaromatic rings, photochromic systems that are thermally reversible are transformed into systems that are thermally irreversible but photochemically reversible. The transition between the ben2othiophene-derivative... 

Heteroaromatic ring stmctures can also be incorporated into poly(arylene vinylene) stmctures using the same precursor polymer method shown for PPV. Poly(thienylene vinylene) (13) (113—118) and poly(furylene vinylene) (14) (119,120) have been prepared in this manner. In addition, alkoxy-substituted poly(thienylene vinylenes) (15) (119,121) have been synthesized. Various copolymers containing phenjiene, thienylene, and furylene moieties have also been studied (120,122,123). 

In general, five-membered heteroaromatic ring systems with one heteroatom all undergo preferential a rather than /3 electrophilic substitution. This is rationalized in terms of the more effective delocalization of charge in the intermediate (36) leading to a substitution than in the intermediate (37) leading to /3 substitution. 

This process is not as common as the other oxidative procedures and usually involves ring closure onto an aromatic or heteroaromatic ring. The following examples illustrate the structural types required for this cyclocondensation. 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

The position selectivity for electrophilic substitution in the simple five-membered heteroaromatic rings is usually 2 > 3. This reflects the more favorable conjugation in intermediate A than in intermediate B. In structure A the remaining C=C bond can delocalize the positive charge more effectively than in B. Substituents on the ring can easily override this directive influence, however. 

Bonds within aromatic and heteroaromatic rings are not strategic. 

The photochemical introduction of a trifluoromethyl group into aromatic and heteroaromatic rings, such as uracil, can be performed also with trifluoromethyl bromide [/5/] (equation 131)... 

The most characteristic feature of the infrared spectra of all 5-mono- and -di-substituted-amino-l,2,3,4-thiatriazoles is a strong band in the 1540-1590 cm range, which without doubt is due to the C=N and N=N stretching vibrations of the heteroaromatic ring system. Various infrared bands between 889 and 1122 cm have been assigned to skeletal vibrations of the thiatriazole ring and a band... 

Some investigations have been inspired by another special circumstance concerning the structure of the fundamental heteroaromatic rings like the parent aromatic homocyclic hydrocarbons, these structures are readily amenable to theoretical treatment by the approximation methods of quantum mechanics. Quantitative studies are clearly desirable in this connection for a reliable test of the theory and, indeed, they have been utilized to this end. ... 

Reactions involving monocyclic six-membered heteroaromatic rings have not been studied sufficiently extensively to allow a quantitative treatment of substituent effects. However, comparison with aza-naphthalene reactivities indicates that aza- and polyaza-benzene systems must also be highly selective. 

I.3.2.2.3. 1,3-Diazepines Fnsed to Heteroaromatic Rings Synthesis by Ring-Closure Reactions... 

Various 1.4-benzodiazepines with a five-membered heteroaromatic ring fused to the 1,2-bond, such as 17, where A represents a heteroaromatic ring, have been prepared by such methods and tested for activity on the central nervous system, as cholecystokinin antagonists and as antagonists of platelet activating factors.245... 

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