Ultrafast Click Chemistry with Fluorosydnones

We report the synthesis and reactivity of 4-fluorosydnones, a unique class of mesoionic dipoles displaying exquisite reactivity towards both copper-catalyzed and strain-promoted cycloaddition reactions with alkynes. Synthetic access to these new mesoionic compounds was granted by electrophilic fluorination of s-sydnone Pd-II precursors in the presence of Selectfluor. Their reactions with terminal and cyclic alkynes were found to proceed very rapidly and selectively, affording 5-fluoro-1,4-pyrazoles with bimolecular rate constants up to 10(4) M-1 S-1, surpassing those documented in the literature with cycloalkynes. Kinetic studies were carried out to unravel the mechanism of the reaction, and the value of 4-fluorosydnones was further highlighted by successful radio-labeling with [F-18] Selectfluor.