Synthesis of Alkynyl-Substituted Nitronyl Nitroxides through an Organosilicon Derivative

4,4,5,5‐Tetramethyl‐2‐[2‐(trimethylsilyl)ethynyl]imidazolidine‐1,3‐diol has been synthesized and oxidized to the corresponding nitronyl nitroxide (1). Further removal of the trimethylsilyl group gave the 2‐ethynyl‐substituted nitronyl nitroxide 2. Both 1 and 2 are labile compounds. With certain precautions, however, they may be purified from admixtures and be obtained as single crystals, thanks to which X‐ray diffraction study confirmed that molecules 1 and 2 each contain a triple bond. The accessibility of these spin‐labeled acetylenes makes it possible to synthesize other derivatives of nitroxides, as was demonstrated in the case of 1,3‐dipolar cycloaddition of CH2N2 to 1 and 2, which gave the corresponding pyrazolyl‐substituted nitronyl nitroxides. These were synthesized in a one‐pot synthesis strategy, since the solutions of spin‐labeled alkynes 1 and 2 formed in the course of synthesis are stable for a long time. The reaction between CH2N2 and 1, containing the (CH3)3Si substituent, was much more sluggish and mainly gave the 4‐pyrazolyl‐substituted nitronyl nitroxide, isolated as an individual compound and also identified by X‐ray crystallography. In the same reaction with 2, the product was the 3‐pyrazolyl‐substituted nitronyl nitroxide. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)