Quantitative comparison of the heteroatom effects in the methoxide attachment to pyrylium and thiopyrylium cations. Thermodynamics of the isomerization of pyrans and thiopyrans

The complete set of kinetic and equilibrium constants for the methoxide attachment to a series of 2,6-di-tert-butyl-4-arylpyrylium cations (aryl = XC6H4 with X = p-NO2, m-Cl, p-Cl, H, p-Me, p-OMe, p-NMe2) has been obtained in MeOH at 25°C. These data complement those previously obtained by studying the methoxide attachment to the corresponding thiopyrylium cations. In both series the reaction involves the kinetically controlled formation of both the corresponding 2H and 4H adducts 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 cation to give the adducts. Moreover, the experimental data indicate that the Leffler-Hammond postulate cannot give information on the position of the transition state along the reaction coordinate. Both kinetic and equilibrium constants for the formation of the 2H and 4H adducts are correlated with the σ+ constants. The obtained ρ values show, for the pyrylium series, a greater sensitivity to the substituent effects with respect to the corresponding thiopyrylium series. From the equilibrium data we estimate that, in contrast with quantum mechanical calculations, the unsubstituted 2H-pyran is at least 4.6 kcal/mol more stable than the corresponding 4H isomer. © 1986 American Chemical Society.