The Pschorr Synthesis and Related Diazonium Ring Closure Reactions

DeTar, DeLos F.


In the middle 1890s, three groups of chemists independently discovered a new cyclization reaction of certain appropriately constituted diazonium salts. Fischer and Schmidt reported that an aqueous solution of 2-benzylbenzenediazonium chloride furnished fluorene on heating. Graebe and Ullman reported that 2-benzoylbenzenediazonium chloride yielded fluorenone and Staedel reported a somewhat similar result from the action of nitrous acid on 2,2′-diaminbenozpehnone, a reaction that produced a little 1-hydroxyfluorenone. Two years later, Robert Pschorr applied the ring closure reaction to the diazonium salt derived from trans-2-amino-alpha-phenylcinnamic acid to obtain phenanthrene-9-carboxylic acid. The principal utility of these cyclizations reactions has been the synthesis of substituted ring structures in which the positions of the substituents are known.