Mechanochemical Halogenations of Sydnones and Rearrangement to 1,3,4-Oxadiazolin-2-ones

Herein are presented new mechanochemical methods for sydnone halogenation and insights into an efficient ring rearrangement yielding 1,3,4-oxadiazolin-2-ones by heated ball-milling. Using equimolar amounts of N chlorosuccinimide (NCS), oxone and sodium chloride, 3-phenylsydnone was quantitatively chlorinated in short reaction time and high yields. Moreover, a more efficient method for the bromination of sydnones has been developed both in solution and by ball-milling. Either by using 1 equiv. of NBS in Ac2O, or 1 equiv. of Ac2O in a vibratory ball-mill, a quantitative bromination occurs with excellent yields and a reduced environmental impact. When the reaction was heated while milling, a fast and efficient ring rearrangement yielded 1,3,4 oxadiazolin 2 ones. Insights concerning the mechanism of the reaction under solvent-less conditions are discussed and the scope of this reaction, including solid anhydrides, is presented.