Benzenoid Rings, Condense

The reactivity of the hetero ring, with electron deficient 2(3)-positions, has been compared to glyoxal diimine." Quinoxalines are deactivated towards electrophilic substitutions, however, substitution is most likely to occur at the equivalent positions 5 and 8, where electron-localization calculations show the highest electron density to be. Electron-donor substituents in the benzenoid ring facilitate electrophilic substitution and, when activating substituents are present in the hetero ring, the site of reaction depends on the reaction conditions. In deprotonated alkyl-substituted quinoxalines an increased number of resonance possibilities exists and mildly basic conditions arc usually required for condensation reactions. 

The classical synthesis of quinoxalines involves the condensation of benzene-1,2-diamincs with different two-earbon synthons. The reaetion is also useful as a method of characterizing diketones. This reaction was first described by Korner and Hinsberg in 1884, and is still widely used for the synthesis of the parent system as well as for many derivatives bearing substituents in the hetero and/or in the benzenoid ring. 

Balaban, A.T. and Artemi, C. (1989). Chemical Graphs. Part 51. Enumeration of Nonbranched Catafusenes According to the Nmnbers of Benzenoid Rings in the Catafusene and Its Longest Linearly Condensed Portion. Polycyclic Aromatic Compounds, 1,171-189. 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

Peri-condensed benzenoid hydrocarbons or perifusenes can have one or more inner carbon atoms common to three rings, that is, their dualist graph has three-membered cycles. As a well-known rule, Kekulean alternant systems must have even numbers of inner carbon atoms, whereas Kekulean non-altemants may have either even or odd numbers of inner carbon atoms. In Part 3. Perifusenes (Balaban and Randic 2004c) all possible benzenoid perifusenes having four, five and six benzenoid rings were displayed and examined. In Fig. 8.15 we show some of them. 

The effects of the extension of the conjugated s rans (mainly in condensed benzenoid rings, where the reductions of a double bond or a reduction in the side chain are related to the structure of the hydrocarbon), are treated by quantum chemical methods.d)... 

Figure 8. Two hydrocarbons (30 and 31) which may be considered formally to be derived from zethrene (29) by the condensation of two further benzenoid rings. 30 displays bond fixation in the central rings, C, while 31 has aromatic sextets in the corresponding rings, C. He are protons directly attached to the central rings, C, in both cases. (Adapted and reprinted with permission from ref 229. Copyright 1989 The Croatian Chemical Society.)...

The numbering 1,3- is omitted, as there is no other possibility for ring condensation. As I I in the case of benzo[b]furan, the signal for the proton on C-2 appears in the region of - benzenoid protons in the H-NMR spectrum, with a small downfield shift. 

Cata-condensed polycyclic hydrocarbons contain benzenoid rings such that each fused carbon atom is common to no more than two rings. The benzenoid rings may be linearly annelated (acenes), as in the series naphthalene (19), anthracene (20), and naph-thacene (21), or they may be angularly annelated, as in phenanthrene (22), benz[a]anthracene (23), benzo[c]phenanthrene (24), chrysene (25), and tri-phenylene (26). Except for some isolated examples... 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

Another example of bromotropone contraction is derived from Nozoe s benzotropazine chemistry [Scheme 146 89H(29)1459]. Condensation of tropone 552 with further o-aminothiophenol (117b) gave benzenoid compound 554 instead of the expected tris(benzazino)tropylidene. Presumably, this condensation takes place via initial cyclization (to benzothiazine), then cyclocondensation (to spirobenzothiazoline), followed by tautomerization (to norcaradiene 553), ring contraction, and dehydrogenation. 

---