Oxidation-reduction reactions chlorosulfonic acid

Elaboration of the thiophene ring can be achieved by standard electrophilic substitution reactions, which are normally favored at the a-carbons for thiophene and at the 3-position for benzo[fc]thiophene. Reductive alkylation of aminocarbamate 72 with butyraldehyde in the presence of selenophenol gave thiophene 73 . Treatment of 3-methylthiophene with chlorosulfonic acid followed by tcrt-butylamine led to selective sulfonamidation producing 74 . Similarly, selective 2-iodination of 3-alkylthiophenes was ob.served upon treatment with iodine in the presence of mercuric oxide . Exhaustive bromination of thieno[3,2-7i]thiophene 27 gave 2,3,5,6-tetrabromothieno[3,2-ii]thiophene which was... 

The thioindigo derivative Pigment Red 88 can be obtained by initial reaction of p-dichlorobenzene with chlorosulfonic acid followed by reduction with zinc, S-alkylation with chloroacetic add, and ring closure with AICI3 to yield the intermediate 2,5-dichlorothioindoxyl. Oxidation with atmospheric oxygen then yields Pigment Red 88. 

Protonation and Related Reactions. TfOH is one of the strongest monoprotic organic acids known. The acid, and its conjugate base (CF3S03 ), have extreme thermal stability, are resistant to oxidation and reduction, and are not a source of fluoride ions, even in the presence of strong nucleophiles. They do not lead to sulfonation as do sulfuric acid, fluorosulfuric acid, and chlorosulfonic acid in some reactions. TfOH is therefore effectively employed in protonation reactions. 

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