Ester Cleavage in Superacid Media Involving Diprotonated Gitonic Carboxonium Dications

Article abstract of DOI:10.1021/ja00129a001

The reactivity of protonated and methylated methyl ester in superacidic media was investigated by experiment and theory.Protonated methyl acetate was found to undergo slow acyl oxygen cleavage at -78 deg C in FSO3H/SbF5/SO2 solution to give acetyl cation and methyloxonium ion. 1,1-Dimethoxyethyl cation (methylated methyl acetate) was found to undergo slow methyl exchange in CD3SO3F/SbF5 solution.The reaction of 1,1-dimethoxyethyl cation with toluene in the presence of trifluoromethanesulfonic acid at -78 deg C gave acylation in 4percent yield.Theoretical calculations at the MP4(SDTQ)/6-31G^*//MP2/6-31G^* level of theory were performed to find stationary points on the potential energy surface of the mono- and diactivated ester system.Based on the available evidence a new mechanism for the acid-catalyzed ester cleavage in superacidic media is proposed.