Aromatics arenes, heterocycles

The coordination of aromatic molecules to certain strong metal itt-bases leads to (jj -aromatic)ML complexes (aromatic = arenes and aromatic heterocycles). This kind of r] coordination can activate the aromatic molecules toward an electrophilic attack since d - tt back-donation into a... 

A variety of biochemical pathways are known which may lead to reactive quinoid derivatives. They include dihydroxylation of aromatic or heterocyclic compounds and epoxide formation and hydrolysis to -diphenolic compounds (Booth and Boyland 1957) o- and p-hydroxylations of phenols or arylamines (In-SCOE et al. 1965 Miller et al. 1960 Booth and Boyland 1957) and rearrangement of -hydroxyarylamines to o-aminophenols (Miller and Miller 1960). It now appears that aromatic hydroxylations proceed via highly reactive arene oxides, i.e., compounds in which a formal aromatic double bond has undergone epoxidation. Depending on the compound, arene oxides may give rise to other electrophilic reactive species, including quinoid structures, but react as such readily with nucleophiles and thus provide a basis for understanding covalent attachment of aromatic hydrocarbon derivatives to protein and nucleic acids (Jerina and Daly 1974). 

Numerous reactions have been described in which the oxygen of the oxepin system is removed to give benzene derivatives. The formation of the aromatic products can be rationalized by an arene oxide as intermediate. A suitable reagent for the elimination of an oxygen atom from this heterocycle is triphenylphosphane, e.g. formation of l,24 2a,12 and 2b.1,9... 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

Nitrogen heterocycles continue to be valuable reagents and provide new synthetic approaches such as NITRONES FOR INTRAMOLECULAR -1,3 - DIPOLAR CYCLOADDITIONS HEXAHYDRO-1,3,3,6-TETRAMETHYL-2,l-BENZISOX AZOLINE. Substituting on a pyrrolidine can be accomplished by using NUCLEOPHILIC a - sec - AM IN O ALKYL ATION 2-(DI-PHENYLHYDROXYMETHYL)PYRROLIDINE. Arene oxides have considerable importance for cancer studies, and the example ARENE OXIDE SYNTHESIS PHENANTHRENE 9,10-OXIDE has been included. An aromatic reaction illustrates RADICAL ANION ARYLATION DIETHYL PHENYLPHOSPHONATE. 

In this chapter, we will be concerned with the ortho photocycloaddition of arenes to the benzene ring. This implies that photocycloadditions to larger aromatic systems and to heterocyclic aromatic molecules will not be discussed. The photoadditions of alkynes to benzene and derivatives of benzene, however, are included in this review. The material is organized in sections, according to the pathway that is followed from the ground state of the addends to the ortho photocycloadducts. 

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