Thiopyrylium cations as appropriate compounds for the quantitative evaluation of ipso-substituent effects

The kinetic and equilibrium constants for the reaction of 2,6-diphenyl-4-X-thiopyrylium cations (X = H, Ph, But, or OMe) with methoxide ion to yield the corresponding 2H- and 4H-adducts have been determined in MeOH at 25 °C. The completion of an analogous study for 2,4,6-triphenylpyrylium ion is also reported. The equilibrium data allow the quantitative evaluation, in free energy terms, of /pso-effects relative to H (OMe, ca. -12; But ca. 0; Ph ca. 10 kJ mol-1). The estimate of the ^em-dimethoxy stabilisation is in accordance with a previous estimate based on data referring to the formation of negatively charged Meisenheimer adducts. Although steric interactions between the geminal groups seem to be negligible in the final adducts, the kinetics data suggest that they occur in the transition states. This observation further supports our previous hypothesis of an ion-pair-like transition state for this class of reactions.