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Updated: May 12, 2026

Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene
Published on: March 20, 2017
Nonamethylcyclopentyl cation rearrangement mysteries solved
Dean J Tantillo1, Paul von Ragué Schleyer
1Department of Chemistry, University of California-Davis, 1 Shields Avenue, Davis, California 95616, United States. djtantillo@ucdavis.edu
Abstract:
The C1 nonamethylcyclopentyl cation minimum undergoes complete methyl scrambling in SbF5 with a 7 kcal/mol barrier. This corresponds to the rate-limiting conformational interconversion of enantiomeric hyperconjomers via a C(s) transition structure (above right). A remarkable, more rapid, second process only exchanges methyls within sets of four and five (blue and red, see above), as has been observed experimentally at low temperatures. The computed ∼2 kcal/mol barrier involves a C(s) [1s,2s] sigmatropic methyl shift transition structure (above left).
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