Gas phase basicity of dihydropyran and dihydro-1,4-dioxin

The gas phase basicity (GB) of dihydropyran 1 and dihydro-1,4-dioxin 2 is measured in equilibrium proton transfer reactions conducted in an ion cyclotron resonance spectrometer. GB(1) is found to be greater than GB(2) by 37 kJ mol−1, this difference parallels the lower reactivity of 2 observed in solution under acidic condition. Conclusion as to the favoured protonation of the C—C double bond, giving rise for both 1 and 2 to oxycarbonium cations, is drawn from comparison with analogous compounds and substantiated by molecular orbital calculations (MNDO) on the protonated structures.