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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 (2...

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Published in:European journal of organic chemistry 2017-06, Vol.2017 (22), p.3249-3255
Main Authors: Harrington, Laura E., Britten, James F., Casey, Michael, Grealis, John, Ortin, Yannick, Müller‐Bunz, Helge, McGlinchey, Michael J.
Format: Article
Language:English
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Summary: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. Thermolysis and protonation of tetraphenylcyclopentadienone yield two isomeric rearrangement products. These observations are rationalized in terms of specific rotations about different single bonds prior to double ring closure.
ISSN:1434-193X
1099-0690
DOI:10.1002/ejoc.201700019