Kinetic Studies of a Fast, Reversible Alkene Radical Cation Cyclization Reaction

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 (10). 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 10 and cyclic products, mainly trans-1-(diphenylmethyl)-2-(1-methylethenyl)cyclopentane (11). The rate constants for cyclization at ambient temperature were k = (0.5−2) × 10¹⁰ s^-1, and those for ring opening were k = (1.5−9) × 10¹⁰ s^-1. Laser flash photolysis studies in several solvents show relatively slow processes (k = (2.5−260) × 10⁵ s^-1) that involve rate-limiting trapping reactions for the equilibrating radical cations. In mixtures of fluoroalcohols R_fCH₂OH 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.