l,4-benzodithianes, l,4-benzothiazines and l,4-benzoxathianes, when treated with bromine in anhydrous chloroform, undergo regiospecific monobromination at the aromatic ring. Based on semiempirical quantum mechanical calculations, the regiochemistry of the reaction in the case of l,4-benzothiazines and l,4-benzoxathianes was shown to be consistent with a charge-controlled electrophilic aromatic substitution reaction where the Br+ species comes from monobromosulfonium ions (as 2). On the contrary, in the case of l,4-benzodithianes, the bromination is accomplished by Br- via an orbital-controlled vicarious nucleophilic substitution of hydrogen occurring on dibromodisulfonium ions (as 4).
AROMATIC RING BROMINATION IN 1,4-BENZOTHIAZINES, 1,4-BENZOXATHIANES AND 1,4-BENZODITHIANES: THE OCCURRENCE OF VICARIOUS NUCLEOPHILIC SUBSTITUTION OF HYDROGEN AND ELECTROPHILIC AROMATIC SUBSTITUTION MECHANISMS
DE NISCO, Mauro;
1996-01-01
Abstract
l,4-benzodithianes, l,4-benzothiazines and l,4-benzoxathianes, when treated with bromine in anhydrous chloroform, undergo regiospecific monobromination at the aromatic ring. Based on semiempirical quantum mechanical calculations, the regiochemistry of the reaction in the case of l,4-benzothiazines and l,4-benzoxathianes was shown to be consistent with a charge-controlled electrophilic aromatic substitution reaction where the Br+ species comes from monobromosulfonium ions (as 2). On the contrary, in the case of l,4-benzodithianes, the bromination is accomplished by Br- via an orbital-controlled vicarious nucleophilic substitution of hydrogen occurring on dibromodisulfonium ions (as 4).Pubblicazioni consigliate
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