Rates and Equilibria of the Methoxyde Attachment to 2,6-Di- t-butyl-4-arylthiopyrylium Cations. Can Ion Pair Formation Be Rate Determining in Anion-Cation Combination Reactions?

The complete set of kinetic and equilibrium constants for the methoxide attachment to a series of 2,6-di-tert-butyl-4-arylthiopyrylium cations (aryl = X-C6H4 with X = p-N02, m-CI, p-C1, H, p-Me, p-OMe, p-NMe2) has been obtained in MeOH at 25 OC. The reaction involves the kinetically controlled formation of both the corresponding 2H- and 4H-thiopyrans which equilibrate to form only the thermodynamically more stable 2H adduct. The observed kinetic patterns show that the rate-determining step is the combination of the nucleophile with the cations to give the adducts, thus disproving the views indicating the ion pair formation as the rate-determing step in anion-cation combination reactions. Moreover the experimental data clearly indicate that the Leffler-Hammond postulate cannot be held valid for this class of reactions. The different selectivity observed for the methoxide attachment to the CY and y positions is explained on the basis of the different transmission efficiency of the electronic substituent effects.