Polysubstituted anthracenes

Both anthracene and phenanthrene undergo electrophilic substitution, with a few exceptions, liowevcr, these reactions are of little value in synthesis because of the formation of mixtures and polysubstitution products. Derivatives of these two hydrocarbons are usually obtained in other ways by electrophilic substitution in 9.10-aiithraquinone or 9,10-dihydrophenanthrene, for example, or by ring closure methods (Secs. 30.18 and 30.19). 

Polysulfonation is especially pronounced with polycyclic compounds. Hydrocarbons such as anthracene or phenanthrene polysulfonate so easily that the preparation of a monosulfonate always involves the formation of substantial amounts of the undesired polysubstituted compounds, even in the presence of unreacted hydrocarbon. Benzene and naphthalene are, however, easily monosulfonated without substantial disulfonation. 

Table4.2. CARCINOGENIC INDICES (COLLATED FROM SEVERAL SOURCES) OF ACTIAfE POLYSUBSTITUTED BENZ[fl]ANTHRACENES...

Practically any aromatic hydrocarbon or aryl halide can be sulfonated if the proper conditions are chosen. As the compound becomes more complex, however, the tendency toward the production of by-products and mixtures of isomers is increased. It is usually difficult to prevent polysubstitution of a reactive hydrocarbon. For example, even when phenanthrene is sulfonated incompletely at room temperature, some disulfonic acids are formed. The sulfonation of anthracene follows such a complex course that the 1- and 2-sulfonic acid derivatives are made from the readily available derivatives of anthraquinone. The foUowii sections include comments.on the accessibility of the reaction products of the commonly available hydrocarbons and aryl halides. The examples cited and still others are listed in Tables I-XIII. 

Anthracene (Table XI, p. 188). Anthracene is sulfonated so readily that even at low temperatures and with a mild sulfonating reagent some polysubstitution occurs. It is striking, therefore, that dilution of the reaction mixture with acetic acid (but not with water) decreases the extent of disubstitution, even with the more reactive sulfonating agents, chlorosulfonic acid or oleum. Under these conditions only 20% of the product consists of disulfonic acids the 1-sulfonic acid is formed in 50% yield, the 2-acid in 30% yield. Substitution in the 2-position is favored by high temperatures, but this is not a result of conversion of the 1-acid into the 2-acid (as is probably true in the case of the naphthalenesulfonic... 

Takahashi s group reported an effective copper-promoted method for the coupling of 1,2-dihalobenzenes and 1,2,4,5-tetrahalobenzenes with zirconacyclopentadienes to produce polysubstituted naphthalenes and anthracenes, respectively (X = Y = halogen in Scheme 25.1 see also Chapter 11) [lb,2]. 

Interestingly, the reactions of 2-naphthyl- and 2-anthrylboron reagents also proceed to form the corresponding anthracene and tetracene derivatives selectively (Schemes 25.15 and 25.16). It should be noted that 2-anthrylboronates 26 are preparable via the iridium-catalyzed direct borylation of their parent anthracenes [18]. Combining the homologation with the borylation, polysubstituted tetracenes 27 can be prepared effectively. 

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