Dehydrogenating agents sulfuric acid

An alternative method is to use sulfur as dehydrogenating agent. As early as 1888 Ziegler867 prepared tetraphenylethylene by heating diphenylmethane with sulfur at 240-250° much more recently Toland, Wilkes, and Brutschy868 obtained 4,4 -stilbenedicarboxylic acid from p-toluic acid and sulfur in 57% yield. 

In the classical process a phenylethylamine reacts with a carboxylic acid chloride or anhydride to form an amide, which can be cyclised, with loss of water, to a 3,4-dihydroisoquinoline, then readily dehydrogenated to the isoquinoline using palladium, sulfur, or diphenyl disulfide. Common cyclisation agents are phosphorus pentoxide (P Ojq), phosphorus oxychloride and phosphorus pen-tachloride. The electrophilic intermediate is very probably an imino chloride, or phosphate the former have been isolated and treated with Lewis acids when they are converted into isonitrilium salts, which cyclise efficiently to 3,4-dihy-droisoquinolines. ... 

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