CARBODICATIONS .3. THE 2 DICATIONS DERIVED FROM MESITYL OXIDE

The hydronation of mesityl oxide (4) is best followed by measuring the difference between chemical shifts of signals for C(4) and C(3) (DELTA-delta), linearly extrapolated at infinite dilution. It was found that half-hydronation occurs near H0-4. The acidity dependence of C(4) and DELTA-delta-0 indicates, however, that the amount of positive charge increases monotonically in stronger acids (100% H2SO4, CF3SO3H, 4:1 FSO3H-SbF5), far beyond the acidity level where hydronation should be complete. The existence of a fast second hydronation of 4 is proposed; the same conclusion comes from a study of acidity dependence of the slope of the DELTA-delta vs concentration of 4 line. The fast second hydronation forms a dication with two hydrons attached to oxygen. The other dication, hydronated at C(3) and at oxygen, is the intermediate in conversions of 4 in superacid. Its rate of formation is estimated from the exchange of C(3)-H in FSO3D-SbF5 to an extent of 60% in 20 min at 30-degrees-C. Even though the second hydronation at C(3) is slow, the rate-determining step for the conversion of 4 occurs after it.