Spirocyclic Nitroxide Biradicals: Synthesis and Evaluation as Dynamic Nuclear Polarizing Agents

Abstract

A method for the synthesis of rigid nitroxide biradicals with various spatial orientations between the radical centers is reported. Diketones were employed as substrates for tin amine protocol (SnAP) reagents to provide the parent spirocyclic diamines. Oxidation by peroxyacids provided the corresponding nitroxide biradicals. A set of four different biradicals with various interelectron distances and torsion angles between the radical planes was synthesized using this method. The exact geometries were determined by X‐ray crystallography and the biradicals were investigated by EPR spectroscopy and evaluated for their dynamic nuclear polarization (DNP) performance. 1H‐DNP enhancements in the range of 1.2–2.1 at 14.1 Tesla (600 MHz spectrometer) were achieved. This synthetic methodology opens a promising alternative to access nitroxide biradicals with various torsional angles and inter radical distances. © 2020 Wiley‐VHCA AG.