In quest of reversibility of Friedel-Crafts acyl rearrangements in the pyrene series

Friedel-Crafts acyl rearrangements in PPA of diacetylpyrenes (80-120 degrees C), dibenzoylpyrenes (80-200 degrees C), and bis(4-flurobenzoyl)pyrenes (80-120 degrees C) and Scholl reactions in AlCl3/NaCl of dibenzoylpyrenes (140-200 degrees C) have been studied. The substrates were 1-AcPY, 2-AcPY, 1,3-Ac2PY, 1,6-Ac2PY, 1,8-Ac2PY, 2,7-Ac2PY, 1-BzPY, 1,6-Bz(2)PY, 1,8-Bz(2)PY, 1-4FBzPY, 1,6-4FBz(2)PY, 1,8-4FBz(2)PY. The mixtures of pyrene, 1-AcPY, 2-AcPY, 1,3-Ac2PY, 1,6-Ac2PY, 1,8-Ac2PY, and 2,7-Ac2PY were separated by HPLC. The following reversible intermolecular isomerizations were established: 1,6-Ac2PY 1,8-Ac2PY, 1,6-Bz(2)PY 1,8-Bz(2)PY, and 1,6-4'FBz(2)PY 1,8-4'FBz(2)PY, albeit not in high yields. The results substantiate Gore's 1955 proposition that The Friedel-Crafts acylation reaction of reactive aromatic hydrocarbons is a reversible process. The isomerizations reported here differ from the few previously reported completely reversible intramolecular Friedel-Crafts acyl rearrangements. At >= 140 degrees C, in PPA and in AlCl3/NaCl, 1,6-Bz(2)PY and 1,8-Bz(2)PY underwent a highly regioselective double Scholl reaction to give pyranthrone (3) and deacylations to 1-BzPy (and pyrene), followed by mono-Scholl reactions to give 8H-dibenzo[def,qr]chrysen-8-one (1), and 11H-indeno[2,1-a]pyren-11-one (2). The formation of 3 and not the expected tribenzo[a,ghi,o]perylene-7,16-dione (4) from 1,8-Bz(2)PY indicates that 1,8-Bz(2)PY has first undergone isomerization to 1,6-Bz(2)PY. The present study confirms the linkage between Friedel-Crafts acyl rearrangements and the Scholl reaction.