13C NMR Study of 2,6-Di-t-butyl-4-arylpyrylium and Thiopyrylium Cations. Carbon Shifts versus Reactivities

The carbon shifts of 2,6-di-t-butyl-4-arylpyrylium and thiopyrylium cations (aryl = XC6H4- with X = p-N02 , m-Cl, p-Cl, H, p-Me, p-MeO, p-NMe2 ) have been determined in CD3CN. While the C-4 chemical shifts of the two heteroaromatic rings are largely affected by rr-polarization, as shown by the shielding induced by electron-withdrawing substituents, the C-2 (C-6) chemical shifts are free from such effect. The C-2 (C-6) and C-4 chemical shifts have been compared with the corresponding second-order rate constants for the methoxide attachment to C-2 (C-6) and C-4 positions (k2 and k4 , respectively), obtained in MeOH at 25 °C. For both the cation series the C-2 (C-6) chemical shifts are linearly correlated with the corresponding log k2 values, thus suggesting a charge control for the methoxide attachment to these cations. At variance, the C-4 chemical shifts do not show any correlation with log k4 values. Thus, whenever the chemical shifts are affected by 1t -polarization, they are no longer reliable indicators of reactivity changes, despite the dominant role of charge-charge interactions in the transition state.