Reactivity of Radical Cations. Effect of Radical Cation and Alkene Structure on the Absolute Rate Constants of Radical Cation Mediated Cycloaddition Reactions1

Abstract

Absolute rate constants for the reactions of styrene, 4-methylstyrene, 4-methoxystyrene, and β-methyl-4-methoxystyrene radical cations with a series of alkenes, dienes, and enol ethers have been measured by laser flash photolysis. The measured rate constants correspond to either addition or electron transfer reactions, with the latter predominating when the oxidation potential of the alkene is lower than that of the styrene. The measured rate constants for the diene additions provide some of the first absolute kinetic data for the initial step in the synthetically-useful radical cation mediated Diels−Alder reaction. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. For example, the reactivity of the radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH3O-β-CH3. The effects of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl ...