10.1021/jo061868w.s001
John H. Horner
John H.
Horner
Martin Newcomb
Martin
Newcomb
Kinetic Studies of a Fast, Reversible Alkene Radical Cation
Cyclization Reaction
American Chemical Society
2007
rate constants
acyclic product 10
Reversible Alkene Radical Cation Cyclization ReactionThe
fluoroalcohols RfCH 2OH
cation
2007-03-02 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Kinetic_Studies_of_a_Fast_Reversible_Alkene_Radical_Cation_Cyclization_Reaction/3020713
The radical cation formed by mesylate heterolysis from the 1,1-dimethyl-7,7-diphenyl-2-mesyloxy-6-heptenyl radical was studied in several solvents. Computational results suggest that the initially formed
acyclic radical cation is a resonance hybrid with partial positive charge in both double bonds of 1,1-diphenyl-7-methyl-1,6-octadiene (<b>10</b>). Thiophenol trapping was used as the competing reaction for kinetic
determinations. The acyclic radical cation rapidly equilibrates with a cyclic distonic radical cation, and
thiophenol trapping gives acyclic product <b>10</b> and cyclic products, mainly <i>trans</i>-1-(diphenylmethyl)-2-(1-methylethenyl)cyclopentane (<b>11</b>). The rate constants for cyclization at ambient temperature were <i>k</i> =
(0.5−2) × 10<sup>10</sup> s<sup>-1</sup>, and those for ring opening were <i>k</i> = (1.5−9) × 10<sup>10</sup> s<sup>-1</sup>. Laser flash photolysis
studies in several solvents show relatively slow processes (<i>k</i> = (2.5−260) × 10<sup>5</sup> s<sup>-1</sup>) that involve rate-limiting trapping reactions for the equilibrating radical cations. In mixtures of fluoroalcohols R<sub>f</sub>CH<sub>2</sub>OH
in trifluoromethylbenzene, variable-temperature studies display small, and in one case a negative, activation
energies, requiring equilibration reactions prior to the rate-limiting processes. Fast equilibration of acyclic
and cyclic radical cations implies that product ratios can be controlled by the populations of the acyclic
and cyclic species and relative rate constants for trapping each.