Steric Effects on Rates and Equilibria of a Cation-Anion Combination Reaction: The Methoxide Attachment to 4-Substituted 2,6-Di-tert-butylpyrylium Cations

Article abstract of DOI:10.1021/jo00243a024

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 deg 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 2H 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.