Effects of α and γ substituents on the relative reactivity of pyrylium and thiopyrylium cations

The rates of methoxide attachment to the γ position of nine pyrylium cations, symmetrically substituted at the α position with Ph and Bu t, and at the γ position with H, Me, But, Ph, and OMe, have been compared with those of the corresponding thiopyrylium cations. The results suggest that the effect of the α substituent depends on the nature of the ring heteroatom. Steric inhibition to solvation of the ring heteroatom is taken into account to explain such features. By contrast the effect of the γ substituent, with the exception of the OMe group, is independent of the ring heteroatom. It is suggested that such invariance is only apparent due to the equality of the steric effect in the two series which tends to conceal small differences in sensitivity to electronic effects.