Steric effects on rates and equilibria of a cation-anion combination reaction: The methoxide attachment to 4-substituted 2,6-di-tert-butylpyrylium cations

The kinetic and equilibrium constants for the reaction of 2,6-di-tert-butyl-4-R-pyrylium cations (R = H, Me, t-Bu, Et3C, Ph) with methoxide ion, to yield the corresponding 2H and 4H adducts, have been determined in MeOH at 25°C. The reaction involves the kinetically controlled formation of the 4H adduct only when R = H or Me, whereas in the other cases a mixture of both the ZH and 4H adducts is formed. The 2H adducts are the thermodynamically favored products, though in the case of the methyl-substituted cation a comparable amount of the anhydro base is also formed. The rate constants for the formation of the 4H adducts follow a regular trend showing a low sensitivity to steric effects, whereas the corresponding equilibrium constants are not affected by steric interactions until a certain value of the steric hindrance of the γ-substituent is reached. Above this value steric effects are greater on equilibria than on rates. These observations are interpreted in terms of an ion pair-like transition state in which the nucleophile specifically interacts with the electrophilic center. © 1988 American Chemical Society.