A Study on the Diazo-Transfer Reaction Using o -Nitrobenzenesulfonyl Azide

¹⁵ N-Labeled azides have a great potential as practical and effective tags for vibrational probing and hyperpolarized magnetic resonance imaging of biomolecules. They can be synthesized by reaction of primary amines with a ¹⁵ N-labeled diazo-transfer reagent. TfNN ¹⁵ N, a γ- ¹⁵ N-labeled diazo-transfer reagent, was developed to prepare β- ¹⁵ N-labeled azides; these are vibrational probes devoid of strong spectral interference by Fermi resonance. To overcome the stability and safety problems associated with TfNN ¹⁵ N, there is a strong demand for the development of a novel γ- ¹⁵ N-labeled diazo-transfer reagent. We present a study on the diazo-transfer reaction using o -nitrobenzenesulfonyl azide (o -NsN ₃). o -NsNN ¹⁵ N, a γ- ¹⁵ N-labeled diazo-transfer reagent, was newly developed and found to be better than TfNN ¹⁵ N with respect to its physicochemical properties and ease of synthesis. Unlike TfNN ¹⁵ N, however, o -NsNN ¹⁵ N was found to afford a mixture of β- and γ- ¹⁵ N-labeled azides rather than the β- ¹⁵ N-labeled azide alone. A mechanism for the diazo-transfer reaction of o -NsNN ¹⁵ N with primary amines is proposed to explain the formation of such isotopomeric mixtures.