Protonation of Tetraphenyl- and 2,3,4-Triphenyl-cyclopentadienone: An NMR and X-Ray Crystallographic Study

The protonation of 2,3,4,5-tetraphenylcyclopentadienone (tetracyclone) yields 6,11-diphenyl-5H-benzo[a]fluoren-5-one (17) and 2,3,4,5-tetraphenylcyclopent-2-en-1-one (18) as the major products. This contrasts with the pyrolysis of tetracyclone that yields 5,6-diphenyl-11H-benzo[a]fluoren-11-one (22) a structural isomer of 17. Mechanisms are presented that rationalize these observations. The protonation of 4-hydroxy-2,3,4-triphenylcyclopentadienone, the precursor to 2,3,4-triphenylcyclo¬pentadienone, generates 2-(2′-oxo-3′,4′,5′-triphenylcyclopent-3′-enyl)-3,4,5-triphenyl-cyclopenta-2,4-dienone, (30) which has been unequivocally characterized by X-ray crystallography. The establishment of the conformation of molecule 30 in the solid state, which correlates with its structure in solution, provides a rationale for the subsequent formation of the tetrahydro-pentaphenyl-as-indacene-3,4-dione, 32, upon further protonation.

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