Other Aromatic Systems

The aromatic compounds C5 H5 and C7 H7 have also been studied and shown to be basically similar to benzene [77]. The partial aromatic character of such compounds as thiopene [78] and similar materials is also reflected in their raised CH-stretching frequencies and general spectral character. 

Langseth and LotA, Danske Vidensk Selskab. Math. Fys., 1938,16, 6. 

Bailey, Hale, Ingold, Leckie, Raisin, Thompson and Wilson,/. Chem. 

Herzberg, Infra-red and Raman Spectra of Polyatomic Molecules (Van 

Barnes, Gore, Liddel and Van Zandt Williams, Infra-red Spectroscopy 

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Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

It is interesting to note that all the new aromatic systems, as described, undergo displacement polymerizations in DMAC solvent by the K2CO3 method, except perfluoroalkylene [10] and amide activated polymerization [9], which were performed in NMP solvent. The displacement polymerization in DMAC solvent was carried out at 155-164°C. poly(aryl ether ketones) require less reaction time (3-6 h) than other aromatic systems for synthesis of polyethers [15]. Synthesis of the fluorinated polyether as reported by Irvin et al. [16] was carried out at room temperature for 16 h (Mw = 75,000), whereas the same polymer by Mercer et al. [17] was synthesized at 120°C for 17 h (Mw = 78,970). 

We have discussed these contours of benzene in some detail since they will help us understand localized orbitals which appear in other aromatic systems. 

A A,A-dimethylcarboxamido group attached to other aromatic systems can be hydrolyzed enzymatically as demonstrated by the metabolism of the ring-opened 1,4-benzodiazepine derivative 4.84 in dogs and rats [53], Here also, hydrolysis was shown to proceed via the secondary and primary amide formed by A-demethylation. 

Reduction of aryl esters.1 Alkyl benzoates are reduced to 1,4-dihydro compounds by sodium in THF NH3 if 1 2 equivalents of water is present before addition of the metal (equation I). The presence of water can be useful for reduction of other aromatic systems (equation II). 

In our discussion of the Hammett correlation, we have confined ourselves mostly to benzene derivatives. Of course, a similar approach can be taken for other aromatic systems, such as for the derivatives of polycyclic aromatic hydrocarbons and heterocyclic aromatic compounds. For a discussion of such applications, we refer to... 

On benzene and some other aromatic systems, the donor or acceptor nature of a substituent is critical for the MCD sign, whereas on others such as pyridine it is not, at least for weak substituents. Further, the MCD is helpful for the analysis of a wide field of chemical events such as conformational changes coupled with substitutions, proton tautomerism, proton dissociation on aromatics, or through-space interaction in bicyclic aromatic systems 299-318>. 

This early work was followed by the study of many other aromatic systems and the molecules were generally found to be essentially planar. During the 1930 s the X-ray work was mainly two-dimensional and the accuracy was not high by modem standards. Limits of error in the region of 0-05 to 0-10 A can generally be assumed. Many of the important structures have now been refined by more accurate methods and the naphthalene and anthracene molecules are known to be planar to within 0-01 A. 

Oxadiazole (1) is a thermally stable neutral aromatic molecule (65JA5800). Other aromatic systems are 1,3,4-oxadiazolium cations (2) and the exocyclic-conjugated mesoionic 1,3,4-oxadiazoles (3) and 1,3,4-oxadiazolines (4). Also known are derivatives of the non-aromatic reduced systems, 2,3-dihydro-l,3,4-oxadiazole (A2-l,3,4-oxadiazoline 5),... 

Benzene is aromatic because it has six electrons in a cyclic conjugated system. We know it is aromatic because it is exceptionally stable and it has a ring current and hence large chemical shifts in the proton NMR spectrum as well as a special chemistry involving substitution rather than addition with electrophiles. This chapter and the next are about the very large number of other aromatic systems in which one or more atoms in the benzene ring are replaced by heteroatoms such as N, O, and S. There are thousands of these systems with five- and six-membered rings, and we will examine just a few. 

Although it is easy to demonstrate that benzene and other "aromatic" systems are stabilized, it is not as easy to determine the exact origin of the stabilization. Both valence bond and molecular orbital theories can provide a formalism for "explaining" the stabilization, and the latter can quantitatively account for the energy of benzene and its low reactivity. However, they do not provide a physical model for the stabilization. The latter must come from a consideration of the electron density distribution, for that alone determines the energy of a molecule. 

The first example of this type of complex was the molecule Fe(C5H5)2, now known as ferrocene, in which the 6jr-electron system of the ion C5H5 is bound to the metal. Other aromatic systems with the magic numbers of 2, 6, and 10 for the aromatic electronic configuration are the carbocycles ... 

Photochromic systems have been described in which a 2,5-dihydrothiophene acts as the bridging ethene unit between two thiophene rings. The novelty here is that the core system has two 2-iodothiophene moieties which can be used to couple with a variety of other aromatic systems by standard reactions <2006SL737>. An example is given in Equation (6). 

Thiazolidines/thiazoles in fusion with other aromatic systems are also potential bioactive scaffolds. Riluzole (Fig. 14)—a benzothiazole analogue—is known to intervene in epilepsy, a cerebral disorder of the central nervous system, via a glutamatergic mechanism [121]. The anticonvulsant activity of riluzole has been attributed to its direct action on the voltage-dependent... 

The unusual stability of the aromatic sextet suggests that benzene will be resistant to oxidation and reduction of the ring, since both processes will destroy the aromaticity. Although this is generally the case, both types of reaction are possible under certain conditions. This chapter is restricted to benzene and its derivatives, but other aromatic systems are more easily oxidized and reduced (see Chapter 12). 

The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R", adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO)3 cationic intermediate (XVI) loss of a proton from this intermediate generates the substituted cyclobutadiene -Fe(CO)3 complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation (XVII), as well... 

In common with other aromatic systems, halogen can be displaced from azapentalenes by nucleophiles. Examples include displacement of Br by or in derivatives of 402, by CN in... 

For a long time aromatic compounds were believed to be stable when exposed to ultraviolet irradiation. The interest in the photochemistry of arenes only started in the late 1950s when several groups observed both isomerization and addition reactions of benzene and its derivatives. Among the pioneers who were active at that time are the groups of Bryce-Smith and Schenck. Since then the photochemistry of aromatic compounds has become the subject of innumerable papers dealing with their conversion to other aromatic systems (by substitution or isomerization) or even to nonaromatics. 

Benzene is typically thought of as a combination of two equivalent resonance structures. These could be written as the SMILES C1=C-C=C-C=C1 and C1-C=C-C=C-C=1. In order to have just one representation for benzene and other aromatic systems, SMILES handles these aromatic systems specially, treating the atoms in an aromatic ring as a special aromatic type and the bonds as a special aromatic type. The lowercase symbol is used to denote an aromatic atom in SMILES and SMARTS. The SMILES for benzene then becomes clcccccl. A bond between aromatic atoms is an aromatic bond, unless otherwise spelled out. For example, biphenyl can be written as clcccccl-clcccccl. 

Shaw et al. [27] indicate that anthracene is more readily hydrogenated than other aromatic systems (e.g. naphthalene). The hydrogenation of anthracene can be described by the following equation (9.14) ... 

Extension of this study of the efficiency of directing groups to other aromatic systems should provide further insight into the reliability of the above ranking. 

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