Cu‐Promoted Sydnone Cycloadditions of Alkynes: Scope and Mechanism Studies

Cu salts have been found to promote the cycloaddition reaction of sydnones and terminal alkynes, providing significant reduction in reaction times. Specifically, the use of Cu(OTf) 2 is found to provide 1,3‐disubstituted pyrazoles, whereas simply switching the promoter system to Cu(OAc) 2 allows the corresponding 1,4‐isomers to be produced. The mechanism of the Cu‐effect in each case has been investigated by experimental and theoretical studies, and they suggest that Cu(OTf) 2 functions by Lewis acid activation of the sydnone, whereas Cu(OAc) 2 promotes formation of reactive Cu I acetylides.