Aromatic rings polycyclic arenes

In the aromatic-ring-annelated oxepin series the resonance effect is clearly the major influence dominating other factors (e.g. temperature, solvent, etc.) which affect the oxepin-arene oxide equilibrium. It is however very difficult to exclude the presence of a minor (spectroscopically undetectable) contribution from either tautomer at equilibrium. This problem has been investigated by the synthesis of chiral arene oxides from polycyclic aromatic hydrocarbons (PAHs). The presence of oxepin (26) in equilibrium with naphthalene 1,2-oxide has been excluded by the synthesis of the optically active arene oxide which showed no evidence of racemization in solution at ambient temperature via the achiral oxepin (26) <79JCS(Pl)2437>. 

In addition to the temperature, solvent, and substituent effects, a preference for either the arene oxide or oxepin form may be achieved by localization of one double bond as part of an aromatic ring system. Thus the reluctance to form a cyclobutadiene ring causes 10 to exist preponderantly as its oxide form. Naphthalene 1,2-oxide 11 is the simplest arene-oxide member in the polycyclic aromatic hydrocarbon (PAH) series and exists exclusively in that tautomeric form. In contrast, naphthalene 2,3-oxide exists exclusively as the oxepin form 12 since the C4-CS bond in the oxepin ring forms part of an aromatic ring. ... 

The simplest member of the polycyclic aromatic hydrocarbon (PAH) series, naphthalene, may in principle form four possible arene oxide-oxepin tautomeric pairs (A-D). In practice, the valence tautomers that have an intact aromatic-ring structure 11, 12, 100, 101 predominate. This discussion of arene oxide synthesis... 

Both the intermolecular [3 -I- 2] photocycloaddition of arenes to alkenes (see Section 3.2) as well as its intramolecular counterpart (one example is given in Houben-Weyl, Vol. 4/5 a, p 495) afford polycyclic compounds containing a cyclopropane subunit. In recent years it has been shown that by the appropriate choice of substituents on either the alkenyl side chain or on the aromatic ring, the regio- and stereochemical outcome of such 5-phenylpent-l-ene to tet-racycloundecene photoisomerizations can be controlled. Thus the parent hydrocarbon, and... 

Polycyclic arene(tricarbonyl)chromium complexes.h These complexes arc best prepared by treatment of polycyclic arcnes with (NHj),CT(CO)37 and BFj ethcratc. As in complcxation with Cr(CO), the terminal or most aromatic ring is complexed selectively. However, the lower temperatures used in the newer method are advantageous with thermally labile polycyclic arcnes. These complexes are useful for substitution reactions at positions that arc not available by electrophilic substitution of the arenc directly. One such reaction is hydroxylation effected by simultaneous reaction with a base (BuLi or I. DA) and tributoxyborane (excess) followed by H2O2/HOAC workup. Rcgiosclectivc silylation is effected by reaction of the complex with LiTMP and (CHj SiCI with... 

The NICS values for the linear polycyclic arenes show a pattern of increasing negativity (aromaticity) toward the center ring. In contrast, for phenanthrene, the center ring has the lowest NICS, consistent with the more localized nature of this ring. ... 

Other T Ugarkis Polycyclic arenes such as naphthalene also bind to low-valent metals. In this case t) binding is still common but the tendency to bind T) is enhanced because, as we saw for indenyl, this allows the uncomplexed ring to be fully aromatic. If one ring is different in some way from the other, different isomers, called haptomers, can exist in which the metal is bound to one or the other ring. The metal can migrate from one ring to the other in a haptomeric equilibrium. 

Although the attack on (or by) the aromatic ring that leads to substitution can be brought about by free radical, electron-rich (nucleophilic) or electron-poor (electrophilic) reagents, the surfeit of electrons associated with aromaticity provides a richness that dictates that the latter should be most common. Nonetheless, aU three substitution pathways will be considered. Indeed, the three that will be examined are benzene (CeHe) and the polycyclic arenes (naphthalene [CioH ], anthracene [C14H10], and phenanthrene [CmHjo]). 

Members of a class of arenes called polycyclic aromatic hydrocarbons possess subslanlial resonance energies because each is a colleclion of benzene rings fused logelher... 

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