ESR STUDIES OF THE RING OPENING OF CYCLOPROPANE RADICAL CATIONS IN FREON MATRICES

Article abstract of DOI:10.1016/S0040-4020(01)88093-5

The radical cations of cyclopropane and several of its methyl derivatives have been characterized by ESR spectroscopy following their generation by γ irradiation of dilute solutions of the parent compounds in Freon matrices at 77 K.In the CFCl3, CF3CCl3, and CF2ClCCl3 matrices, only the ring-closed species is usually observed in the accessible temperature range up to ca 160 K.In the CFCl2CF2Cl matrix, however, the ring-closed radical cations initially formed at 77 K undergo ring opening between 83 and 110 K, the more highly substituted radical cations requiring a higher temperature for this transformation.The ring-closed radical cations are ²A₁ species for C_2V symmetry, the most substituted cyclopropane C-C bond being elongated with the spin density largely confined to the basal carbons in a face-to-face (90^o, 90^o) structure.In the ring-opened radical cations, the radical center is localized on the most substituted carbon atom following the breaking of the weakened C-C bond of the ring-closed species.The radical conformations of the ring-opened species have been determined, the RCH2CH2^. center produced from cyclopropane having a bisected conformation while the RCH2CMe2^. center obtained from 1,1,2,2-tetramethylcyclopropane is eclipsed, as expected for the presence of α-methyl substituents at the radical site.The nature of the putative carbocation center in the ring-opened radical cations is discussed with reference to recent proposals that this center is strongly coordinated to an electrophile (Cl^- or RCl) thereby negating the requirement for an orthogonal structure.Consideration of the strong matrix dependence of the ring-opening reaction suggests a possible solvation effect, however, in which the CFCl2CF2Cl matrix assists the twisting of one of the CR1R2 groups at the most substituted bond, leading to the rupture of this one-electron ? bond.A strong solvation effect also explains why ring-opening can occur in a suitable polar solvent despite theoretical calculations of unfavorable energetics for a similar gas-phase reaction.Experiments are also described on spiro<2.5>octane, the cyclopropane ring undergoing scission at the CH2-CH2 bond of this radical cation to give an RCH2^. radical center.This radical then undergoes a H-atom abstraction with a neutral spiro<2.5>octane molecule in the CFCl2CF2Cl matrix at higher temperature to give the spiro<2.5>oct-6-yl radical.