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

Abstract

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.

First Examples of Povarov Reaction of Cyclopentadienones

“…However, the tetrasubstituted cyclopentadienone used instead of cyclopentadiene in our case, remains an electron poor olefin, even if it is not protonated in the acidic conditions used . So, the nucleophile attack of this quite deactivated double bond on intermediate iminium cation is not obvious and an asynchronous concerted 4 + 2 cycloaddition may also take place.…”