ESTER CLEAVAGE IN SUPERACID MEDIA INVOLVING DIPROTONATED GITONIC CARBOXONIUM DICATIONS

The reactivity of protonated and methylated methyl ester in superacidi c media was investigated by experiment and theory. Protonated methyl a cetate was found to undergo slow acyl oxygen cleavage even at -78 degr ees C in FSO3H/SbF5/SO2 solution to give acetyl cation and methyloxoni um ion, 1,1-Dimethoxyethyl cation (methylated methyl acetate) was foun d to undergo slow methyl exchange in CD3SO3F/SbF5 solution. The reacti on of 1,1-dimethoxyethyl cation with toluene in the presence of triflu oromethanesulfonic acid at -78 degrees C gave acylation in 4% yield. T heoretical calculations at the MP4(SDTQ)/6-31G()/MP2/6-31G(*) level o f theory were performed to find stationary points on the potential ene rgy surface of the mono- and diactivated ester system. Based on the av ailable evidence a new mechanism for the acid-catalyzed ester cleavage in superacidic media is proposed.