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Five methods (A to E) were examined for the preparation of 5-aryl- or 5-alkyl-dibenzothiophenium salts and 10-aryl- or 10-alkyl-9, 9-dimethylthioxanthenium salts, and the scope and limitation of these methods were determined. Behavior of dibenzothipohene 5-oxide (I) and of 9, 9-dimethylthioxanthene 10-oxide (XXI) was compared and it was found that I was quantitatively reduced with conc. hydrochloric acid at room temperature to give dibenzothiophene (XIII). Reaction of I and 70% perchloric acid-phosphoryl chloride afforded 5-(2-dibenzothienyl)dibenzothiophenium perchlorate (XXVIII). The mechanism for the formation of XXVIII was found to be the Friedel-Grafts type condensation of I', formed by protonation of I, and XIII formed by the reduction of I.
Hori et al. (Mon,) studied this question.