Aromatic and heteroaromatic compounds

The archetypal aromatic compound benzene [29] is comprehensively discussed in 5.3 and the spectrum shown in Fig. 5.5, that of CeDg is shown in Fig. 8.1. Benzene has also been widely used as an adsorbate in catalyst studies ( 7.3.2.10). 

The INS spectrum of toluene is shown in Fig. 3.31. 1,4-dimethylbenzene (p-xylene) [30,31] has been studied in order to investigate the spectral effects of interacting methyl groups. INS spectra of the other isomeric xylenes have also been investigated [32]. 

5- trimethylbenzene (mesitylene) [33] and the 2,4,6-trihalo-mesitylenes (bromo, chloro and iodo) [34,35] have been studied. The trihalomesitylenes provide a textbook example of how the combination of crystallography, spectroscopy and modelling can provide a coherent picture of the structure and dynamics and allow insights into the intra-and intermolecular forces present [36]. 

3- and 4-fluorost3n ene [39,40] have been studied. In solution, the former exists as a mixture of two conformers and to accormt for the solid state spectrum it was necessary to include bodi conformers. 

Monocyclic aromatic compounds exhibit a strong characteristic band at a group frequency near 400 cm, this is the shown in Fig. 8.7. 

---

The thiazolyl radicals are, in comparison to the phenyl radical, electrophilic as shown by isomer ratios obtained in reaction with different aromatic and heteroaromatic compounds. Sources of thiazolyl radicals are few the corresponding peroxide and 2-thiazolylhydrazine (202, 209, 210) (see Table III-34) are convenient reagents, and it is the reaction of an alky] nitrite (jsoamyl) on the corresponding (2-, 4-, or 5-) amine that is most commonly used to produce thiazolyl radicals (203-206). The yields of substituted thiazole are around 40%. These results are summarized in Tables III-35 and IIT36. 

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). 

Table 2.5. Typical HH coupling constants (Hz) of aromatic and heteroaromatic compounds...

In the case of alkenes and aromatic and heteroaromatic compounds, analysis of a single multiplet will often clarify the complete substitution pattern. A few examples will illustrate the procedure. 

In the chemical shift range for alkenes and aromatic and heteroaromatic compounds enol ether fragments (furan, pyrone, isoflavone, 195-200 Hz) ... 

Scheme 10.3. Activation Hardness for Aromatic and Heteroaromatic Compounds ...

Silver(I) triflate is widely applied to the preparation of various derivatives of triflic acid, both covalent esters [66] and ionic salts For example, it can be used for the in situ generation of iodine([) triflate, a very effective lodinatmg reagent for aromatic and heteroaromatic compounds [130] (equations 65 and 66)... 

Diazo Chemistry I Aromatic and Heteroaromatic Compounds. By Heinrich Zollinger Copyright 1994 VCH Verlagsgesellschaft mbH ISBN 3-527-29213-6... 

For the introduction of fluorine into aromatic and heteroaromatic compounds the photolytic fluoro-de-diazoniation sometimes has advantages compared with the corresponding thermal dediazoniation (Balz-Schiemann reaction, see Sec. 10.4). For aromatic substrates the reaction was studied by Rutherford et al. (1961), Christie and Paulath (1965), Petterson et al. (1971), and Becker and Israel (1979). Hexafluorophos-phates sometimes give better yields than tetrafluoroborates (Rutherford et al., 1961). In analogy to Balz-Schiemann reactions in solution (Fukuhara et al., 1987), photolytic fluoro-de-diazoniations of benzene derivatives with electron-withdrawing substituents give lower yields. 

Alkyl radicals generated efficiently from allylsulfones in 80% aqueous formic acid induced a cyclization reaction on aromatic and heteroaromatic compounds to provide polycyclic aromatic and heteroaromatic derivatives (Eq. 7.17).37... 

Jutz, J. C. Aromatic and Heteroaromatic Compounds by Electrocyclic Ringclosure with Elimination. 73,125-230(1978). 

Reaction with Aromatic and Heteroaromatic Compounds 4.1 Benzene and its Derivatives... 

Pearlman, R. S., Yalkowsky, S. H., Banerjee, S. (1984) Water solubilities of polynuclear aromatic and heteroaromatic compounds. J. Phys. Chem. Ref. Data 13, 555-562. 

Light irradiation of styrenes in the presence of aromatic and heteroaromatic compounds leads to the formation of u./V-diarylnitrones in good yields (331, 332). 

Quite recently, the same research group compared the electrophilicity of 6-nitro-tetrazolo[l,5- ]pyridine and 6,8-dini-trotetrazolo[l,5- ]pyridine 11 with a series of electron-deficient aromatic and heteroaromatic compounds <2005JOC6242>. As reference nucleophiles, fV-methylpyrrole, indole, fV-methylindole, and some morpholino enamines were used. The reactivity of the electrophiles studied followed the linear-free energy relationship defined by Mayr et al. <2003ACR66>. 

---