Reactivity of Radical Cations: Generation, Characterization, and Reactivity of 1,3-Diene Radical Cations1

Abstract

Conjugated diene radical cations have been generated by either photoionization of the parent diene or photosensitized electron transfer using triplet chloranil or 1,4-dicyanonaphthalene/biphenyl in acetonitrile and have been characterized by laser flash photolysis. The radical cations have a moderately strong absorption in the 340−400 nm region with a broader and weaker band at longer wavelength, in agreement with literature data for the same species in matrices. The radical cations react with anionic nucleophiles with rates that approach the diffusion-controlled limit and are insensitive to oxygen. Reaction of the diene radical cation with its precursor diene is the predominant decay pathway in the absence of added nucleophiles or alkenes and occurs with rate constants on the order of 108−109 M-1 s-1. Product studies for 2,5-dimethyl-2,4-hexadiene indicate that this reaction does lead to dimer formation although the major dimeric product results from substantial rearrangement of the initial adduct. The complex chemistry is consistent with the observation of negative curvature at high diene concentrations in the quenching plots for reaction of the diene radical cations with their precursors. Rate constants for reaction of the diene radical cations with methanol range from 1.2 × 105 M-1 s-1 for 2,5-dimethyl-2,4-hexadiene to 1.8 × 108 M-1 s-1 for 2,4-hexadiene and demonstrate the effects of alkyl substitution on the radical cation reactivity. The reactivity of the diene radical cations toward a number of alkyl- and alkoxy-substituted ethylenes was also measured and provides some of the first kinetic data for cross-cycloaddition reactions for simple nonaryl-substituted radical cations.