Tetrazine- and sydnone-based click chemistries have emerged as important bioconjugation strategies with fast kinetics and N2 or CO2 as the only by-product. Mechanistic studies of these reactions have focused on the initial rate-determining cycloaddition steps. The subsequent N2 or CO2 release from the bicyclic intermediates has been approached mainly through computational studies, which have predicted lifetimes of femtoseconds. In the present study, bioorthogonal cycloadditions involving N2 o...