Halogenations dibenzothiophene

Halogenation. Dibenzothiophene displays a unique reactivity depending on the oxidation state of the sulfur atom. For example, in the sulfide form, halogenation takes place at the 2- and 8- positions due to its electron-donating ability. In contrast, in the sulfone form, which is electron-withdrawing group, halogenation occurs at the 3- and 7- positions. 

Oxidation of thiophene with Fenton-like reagents produces 2-hydroxythiophene of which the 2(570 One isomer is the most stable (Eq. 1) <96JCR(S)242>. In contrast, methyltrioxorhenium (Vn) catalyzed hydrogen peroxide oxidation of thiophene and its derivatives forms first the sulfoxide and ultimately the sulfone derivatives <96107211>. Anodic oxidation of aminated dibenzothiophene produces stable radical cation salts <96BSF597>. Reduction of dihalothiophene at carbon cathodes produces the first example of an electrochemical halogen dance reaction (Eq. 2) <96JOC8074>. 

An unsuccessful attempt to repeat this reaction was made by Bird however, he was able to show that dibenzothiophene 5-oxide did, in fact, react with either thionyl chloride or phosphorus oxychloride to yield 2-chlorodibenzothiophene in good yield. Since all methods of nitration of dibenzothiophene yield a mixture of 2-nitrodibenzothio-phene and dibenzothiophene 5-oxide, which have identical melting points, it was concluded that the earlier workers had in fact been working with the sulfoxide and not the nitro compound. The reaction was rationalized as being a deoxygenative halogenation of a heterocyclic 5-oxide akin to the Meisenheimer reaction of A-oxides, which already had precedents in the sulfoxide field. Unfortunately the 2-chlorodibenzothiophene prepared by this route is contaminated with 2,8-dichlorodibenzothiophene which cannot be removed by crystallization. The best method of preparation of this compound is therefore via a Sandmeyer reaction on 2-aminodibenzothiophene. ... 

Reaction of 5-bromobenzo[6]thiophene with one mole of n -butyllithium leads to 5-bromo-2-benzo[6]thienyllithium with two moles of n-butyllithium, the 2,5-dilithio derivative is formed (70AHC( 11)177). Monolithio derivatives of dibenzothiophene have been prepared by halogen-metal exchange from the 1-, 2- and 3-bromo compounds. Dilithio-dibenzothiophenes have also been prepared by the same procedure (74AHC(i6)l8i). 

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