Protonation of Tetraphenyl‐ and 2,3,4‐Triphenylcyclopentadienone – 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, which 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‐triphenylcyclopent‐2‐en‐1‐none, the precursor to 2,3,4‐triphenylcyclopentadienone, generates 2‐(2′‐oxo‐3′,4′,5′‐triphenylcyclopent‐3′‐enyl)‐3,4,5‐triphenylcyclopenta‐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 the conformation in solution, provides a rationale for the subsequent formation of the tetrahydro‐pentaphenyl‐as‐indacene‐3,4‐dione (32) upon further protonation.

Protonation of Tetraphenyl‐ and 2,3,4‐Triphenylcyclopentadienone – An NMR and X‐ray Crystallographic Study Journal Articles