In order to elucidate the reaction mechanism, we performed reaction tracking by 1 H NMR analysis using 1M (Figure ). When Selectfluor ® was added to 1M in an NMR tube without stirring, 1M was immediately consumed to produce tetrafluoroborate 3 as the intermediate, which is thought to be the result of electrophilic fluorination at the C3 position. After 2 to 4 hours of reaction time, 1M almost entirely disappeared, and NMR peaks showed a mixture of 2M and 3; finally, practically only 2M resulted in the 1 H NMR spectrum. This experiment indicated that the fluorination and the cleavage of the N-N bond proceeds in a stepwise fashion. When the reaction was stirred in a flask, 1M disappeared after 10 min at room temperature, giving intermediate 3 and the residue 4 of Selectfluor ® (Figure ). Upon removal of 4 from the resulting mixture, further reaction did not procced by heating at 80 °C for 24 h (see Supporting Information for experimental details). Therefore, triethylamine (1.0 equiv) was added, and the reaction proceeded quickly to give the desired 2M quantitatively. This supports the role of Selectfluor ® as the fluorinating agent in the reaction and the residue 4 as the base that promotes the N-N bond cleavage.