Substituted Benzenes, Naphthalenes, and Anthracenes

Class B. Substituted benzenes, naphthalenes, and anthracenes. The compounds discussed under this heading are ... 

B. Substituted Benzenes, Naphthalenes, and Anthracenes 1. Substituted benzenes... 

Recently, Wallis and Kochi reported on the photochemical and thermal osmylation of benzene, substituted benzenes, naphthalene, and anthracene (331, 332). Complexes of the form Ar-0s04 are formed and were studied by laser flash photolysis. In the presence of donor... 

Molecular complexes of OSO4 with various substituted benzenes, naphthalenes, and anthracenes have been identified by their charge-transfer absorption, which follows the Mulliken correlation in Eq. 8 [114, 161], The arene-0s04 complexes are quite stable when kept in the dark and only very slowly form osmium(VI) cycloadducts by thermal osmylation (Eq. 30). 

Fluorescence spectra of analytical and pharmaceutical interest arise from functionally substituted aromatic molecules, particularly those derived from benzene, naphthalene, and anthracene or their heteroaromatic analogs pyridine, quinoline, isoquinoline, and acridine. 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

There are many other aromatic hydrocarbons, i.e. compounds like benzene, which contain rings of six carbon atoms stabilised by electron delocalisation. For example, if one of the hydrogen atoms in benzene is replaced by a methyl group, then a hydrocarbon called methylbenzene (or toluene) is formed. It has the structural formulae shown. Methylbenzene can be regarded as a substituted alkane. One of the hydrogen atoms in methane has been substituted by a or —group, which is known as a phenyl group. So an alternative name for methylbenzene is phenylmethane. Other examples of aromatic hydrocarbons include naphthalene and anthracene. 

The aromatic ring to which the side chain becomes attached may be that of benzene itself, certain substituted benzenes (chiefly alkylbenzenes and haloben-zenes), or more complicated aromatic ring systems like naphthalene and anthracene (Chap. 30). 

Because the substituted benzene chromophores absorb in the 205- to 280-nm range and have low e values, the solvents used must be transparent down to at least 250 nm. This requirement is unnecessary for the more conjugated naphthalene and anthracene chromophores. The usual polar, nucleophilic solvents that have been used to observe ions and ion-derived products are various alcohols, water, or water mixed with a cosolvent such as dioxane for solubility reasons. Recently, and particularly for the observation of the intermediate carbocations by laser flash photolysis (LFP) methods, the strongly ionizing (high Tore values) but weakly nucleophilic Oow N values) alcohols, 2,2,2-trifluoroethanol (TFE) and l,l,l,3,3,3,-hexafluoro-2-propanol (HFIP), have been more commonly used. A limited list of polar solvents and their properties is given in Table 2 [23,24]. 

Most luminescence spectra of pharmaceutical interest arise from functionally substituted aromatic molecules. Consequently, the compounds of interest in this chapter are those derived from drugs possessing aromatic rings, such as benzene, naphthalene, or anthracene, or their heteroaromatic analogs pyridine, quinoline, acridine, etc. As is the case for absorption spectra, the luminescence spectra of these substances may often be understood in terms of the electronic interactions between the simple aromatic structures and their substituents. 

A large number of polycyclic heterocycles containing two or more six-membered rings that share two or more carbon atoms are known. For the majority of these systems, only one of the six-membered rings contains the heteroatoms and the other rings are fused carbocyclic rings such as cyclohexane, benzene, naphthalene, and in rare examples, anthracene. Systems in which more than one of the six-membered rings contain heteroatoms are also briefly discussed these include spiro-cyclic compounds and substituted naphthalenes. 

Cycloaddition reaction between two aromatic substrates yields dimerised products except benzene. For example, 2-substituted naphthalene and anthracene give dimerised product on photolysis. 

Takai et al. reported rhenium-catalyzed formal [2- -1- -2- -1] cycloaddition of a-substituted- 3-ketoesters 121, electron-rich alkynes 122, and electron-deficient alkynes 124, leading to densely substituted benzenes 125 (Scheme 6.34) [47b,48], They proposed that this reaction proceed via retro-aldol reaction (C—C singlebond cleavage) to form 2-pyranones followed by aromatization with the electron-deficient alkyne (Scheme 6.34). The use of in situ-generated arynes 128 in place of electron-deficient alkynes 124 afforded naphthalene and anthracene derivatives 129 (Scheme 6.35) [47b]. 

Phenanthrene and anthracene both react preferentially in the center ring. This behavior is expected from simple resonance considerations. The c-complexes that result from substitution in the center ring have two intact benzene rings. The total resonance stabilization of these intermediates is larger than that of the naphthalene system that results if substitution occurs at one of the terminal rings. ... 

The versatility of poly(phenylcne) chemistry can also be seen in that it constitutes a platform for the design of other conjugated polymers with aromatic building blocks. Thus, one can proceed from 1,4- to 1,3-, and 1,2-phenylene compounds, and the benzene block can also be replaced by other aromatic cores such as naphthalene or anthracene, helerocyclcs such as thiophene or pyridine as well as by their substituted or bridged derivatives. Conceptually, poly(pheny ene)s can also be regarded as the parent structure of a series of related polymers which arc obtained not by linking the phenylene units directly, but by incorporation of other conjugated, e.g. olefinic or acetylenic, moieties. 

The complexes Cr(CO)3L, with L = phenanthrene, naphthalene, or anthracene, are more active for diene hydrogenation than with L = substituted benzenes (see also Section VIII), and this is attributed to an easier displacement of the arene by the diene substrate, the phenanthrene type being asymmetrically bonded, having two longer and more readily cleaved chromium-carbon bonds (198, 199). 

Aromatic molecules with no polar substituent include benzene derivatives or other, more polyaromatic molecules, such as naphthalene, phenanthrene, and anthracene. These are polarizable. Paraffins are not polarizable by comparison. In gas-liquid systems, aromatic molecules will show stronger interactions with polar stationary phases that paraffins of comparable boiling point and, thus, polar stationary phases can aid in improving separation of substituted aromatics. 

Solid benzylic halogens are easily substituted with gaseous dialkylamines. Monoalkylamines are less suitable for uniform reactions due to secondary substitution of the initial product by the benzylic halide present. Some characteristic 100% yield conversions are listed in Scheme 31. The benzene (230) and naphthalene derivatives (231) started from the solid bromides, the anthracene derivatives (232) from the solid chlorides [22]. 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition reactions. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that the net loss in stabilization energy for the first step in electrophilic substitution or addition decreases progressively from benzene to anthracene therefore the reactivity in substitution and addition reactions should increase from benzene to anthracene. 

Many different combinations of carboxylic acid and hydroxyl groups have been tested to form LCPs. An aromatic structure (benzene, naphthalene, anthracene, etc) is required that has its functional groups symetrically arranged on opposite sides of the molecule. Examples are a 1,4-substituted benzene compound or 2,6-substituted naphthalene compound. These monomers are often complex and expensive molecules and account for a significant portion... 

Benzene (1) is the simplest aromatic hydrocarbon upon which our knowledge of aromatic chemistry is based. This hydrocarbon, the alkylbenzenes (2), the arylmethanes [e.g. diphenylmethane (3)], the biphenyls [e.g. biphenyl (4)] and the condensed polycyclic systems [e.g. naphthalene (5) and anthracene (6)] all exhibit chemical reactivity and spectroscopic features which are markedly different from their aliphatic and alicyclic hydrocarbon counterparts. Indeed the term aromatic character was introduced to specify the chemistry of this group of hydrocarbons and their substituted functional derivatives, and it was soon used to summarise the properties of certain groups of heterocyclic compounds having five- and six-membered ring systems and the associated condensed polycyclic analogues (Chapter 8). 

There are principally two different approaches of correlating experimental rate data of electrophilic substitution with reactivity indices (1) Correlating the index with the rate data of a given reaction, e.g. bromination. For example, a satisfying correlation of Dewar reactivity numbers with the log of rate constants of the bromination of benzene, naphthalene (1- and 2-position), biphenyl (4-position), phenanthrene (9-position), and anthracene (9-position) has been observed [55]. In correlations of this type the reactivity index corresponds to the reactivity constant in the Hammett equation while the slope of the linear correlation corresponds to the reaction constant (see also Sect. 3) (2) correlating the index with experimental a values. 

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