Efficient and Inexpensive Synthesis of 15N‑Labeled 2‑Azido-1,3-dimethylimidazolinium Salts Using Na15NO2 Instead of Na15NNN

15N-Labeled azides are important probes for infrared and magnetic resonance spectroscopy and imaging. They can be synthesized by reaction of primary amines with a 15N-labeled diazo-transfer reagent. We present the synthesis of 15N-labeled 2-azido-1,3-dimethylimidazolinium salts 1 as a 15N-labeled diazo-transfer reagent. Nitrosation of 1,3-dimethylimidazolinium-2-yl hydrazine (2) with Na15NO2 under acidic conditions gave 1 as a 1:1 mixture of α- and γ-15N-labeled azides, α- and γ-1, rather than γ-1 alone. The isotopomeric mixture thus obtained was then subjected to the diazo-transfer reaction with primary amines 3 to afford azides 4 as a 1:1 mixture of β-15N-labeled azides β-4 and unlabeled ones 4′. The efficient and inexpensive synthesis of 1 as a 1:1 mixture of α- and γ-1 using Na15NO2 instead of Na15NNN facilitates their wide use as a 15N-labeled diazo-transfer reagent for preparing 15N-labeled azides as molecular probes.