Chemistry and spectroscopy in strongly acidic solutions. Part XVIII: Novel ring contraction of cyclohexenones via dicarbonium ions

Abstract

A novel quantitative conversion of cyclohex‐2‐enone into 3‐methylcyclopent‐2‐enone in HF‐SbF₅ solution is reported. The acidity dependence of the rate constant shows that the mechanism involves a protonated keto‐carbonium ion (dicarbonium ion) as an intermediate. Rate constants and activation parameters have been determined. The rate in FSO₃H‐SbF₅ is much lower than that in HF‐SbF₅, which is compatible with the lower acidity of the former solvent system.Cyclohex‐3‐enone is also converted into 3‐methylcyclopent‐2‐enone, after initial isomerization to cyclohex‐2‐enone. This constitutes proof of a rapid equilibration between the cyclohexenones prior to the rate‐determining formation of 3‐methylcyclopent‐2‐enone.5‐Methylcyclohex‐2‐enone is converted into 3‐methylcyclohex‐2‐enone, which shows that hydride shifts in the intermediate dication are more rapid than the ring‐contraction steps.