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The dependence of cycloaddition rate constants on solvent polarity is a significant mechanistic criterion. Various model reactions are discussed. The slow step of the 2 + 2 cycloaddition of tetracyanoethylene (TCNE) to enol ethers is the formation of a zwitterionic intermediate. The log k2 values show an unusually high influence of solvent polarity and are linearly related to the empirical solvent parameter ET. The inclusion of alcohols as solvents calls for the application of the Koppel-PaIm four-parameter equation; the non-specific Coulombic term is found to be of greatest importance.
Rolf Huisgen (Tue,) studied this question.