New Cascade Syntheses of Nitronyl Nitroxides and a New Synthetic Approach to Imino Nitroxides

Suspensions of M x O y (MnO 2 , Co 2 O 3 , Ni 2 O 3 ) in protic solvents (MeOH, EtOH) have been found to be suitable systems for use in a cascade transformation of 4,4,5,5‐tetramethyl‐2‐[2‐(trimethylsilyl)ethynyl]imidazolidine‐1,3‐diol ( 1a ) into 2‐ethynyl‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxide ( 3 ). In MnO 2 /MeOH, the products of further transformation of 3 , ( Z )‐ and ( E )‐2‐(2‐methoxyvinyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxides, were obtained by the regiospecific addition of MeOH to the triple bond of 3 . In the reaction in MnO 2 /MeOH+H 2 O (1:1), the Z isomer was the sole product. A multistep one‐pot transformation of 1 into 4,4,5,5‐tetramethyl‐2‐[2‐oxo‐1‐(4,4,5,5‐tetramethylimidazolidin‐2‐ylidene)ethyl]‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxide occurred in MnO 2 /EtOH. For ( E )‐2‐[2‐(diethylamino)vinyl]‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxide, it was shown that use of the MnO 2 /secondary amine system provided a one‐pot transformation of 1a and gave ( E )‐aminovinyl‐substituted nitronyl nitroxide as the sole product. In combined MnO 2 /ROH/K 2 CO 3 systems (R = Me, Et), the imidazolidine‐1,3‐diol 1a was transformed into ( E )‐2‐(2‐ethoxyvinyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxide and the acetals 2‐(2,2‐dimethoxyethyl)‐ and 2‐(2,2‐diethoxyethyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxide. In MnO 2 /MeNO 2 we obtained the products of the transformations of different 2‐substituted 4,4,5,5‐tetramethylimidazolidine‐1,3‐diols (R = C≡C–SiMe 3 , aryl, hetaryl, alkyl) or nitronyl nitroxides into the corresponding imino nitroxides (13 products); this reaction is a new method for the preparation of imino nitroxides and allowed us to synthesize the first α‐acetylenyl‐substituted imino nitroxides. The majority of paramagnetic compounds, including the derivatives of 1a , were obtained as perfect crystals, and their structures were determined by X‐ray structure analysis (14 solved structures). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)