Mechanochemical Halogenations and Rearrangement of Sydnones: Insight into Reaction Conditions and Mechanism

New mechanochemical methods for sydnone halogenation and insights into an efficient ring rearrangement yielding 1,3,4-oxadiazolin-2-ones by heated ball-milling are reported herein. Using equimolar amounts of N-chlorosuccinimide (NCS), oxone and sodium chloride, 3-phenylsydnone was quantitatively chlorinated in short reaction time and high yields. Moreover, an efficient method for the bromination of sydnones was developed by mixing equimolar amount of N-bromosuccinimide (NBS), Ac₂O and a sydnone in a vibratory ball-mill, in the absence of solvent, with excellent yields and a reduced environmental impact. When the reaction was heated while milling, a fast and efficient ring rearrangement into 1,3,4-oxadiazolin-2-ones was observed. Insights concerning the mechanism of the reaction under solvent-less conditions, supported by DFT calculations, are discussed. The scope of this reaction, including solid anhydrides, is presented.