Facile Synthesis of Polycyclic Aromatic Hydrocarbons: BrOnsted Acid Catalyzed Dehydrative Cycloaromatization of Carbonyl Compounds in 1,1,1,3,3,3-Hexafluoropropan-2-ol

The cycloaromatization of aromatic aldehydes and ketones was readily achieved by using a BrOnsted acid catalyst in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP). In the presence of a catalytic amount of trifluoromethanesulfonic acid, biaryl-2-ylacetaldehydes and 2-benzylbenzaldehydes underwent sequential intramolecular cationic cyclization and dehydration to afford phenacenes and acenes, respectively. Furthermore, biaryl-2-ylacetaldehydes bearing a cyclopentene moiety at the -position underwent unprecedented cycloaromatization including ring expansion to afford triphenylenes. HFIP effectively promoted the cyclizations by suppressing side reactions presumably as a result of stabilization of the cationic intermediates.