1H nuclear magnetic resonance study of methoxide addition to pyrylium and thiopyrylium cations; heteroatom and substituent effects

Methoxide addition to pyrylium and thiopyrylium salts with various α-substituents (Ph or But) and γ-substituents (H, Me, But, Et3C, Ph, or MeO) in methanol at -40 and 25 °C has been studied by 1H n.m.r. The composition of the product mixtures, kinetically controlled at -40 °C and thermodynamically controlled at 25 °C, gives information on the factors affecting positional selectivity and the relative thermodynamic stability of the addition products (the 2H- and 4H-adduct). The methyl-substituted substrates also undergo deprotonation, to yield the corresponding anhydro-bases. Analysis of heteroatom and substituent effects on the reaction course emphasizes the role of polar and steric interactions, and sheds light on the structures of the transition states.