Cascade Reactions of Me3Si‐Substituted Imidazolidine‐1,3‐Diols with PbO2, Including Oxidation of the Corresponding Diol and Subsequent Elimination of the Trimethylsilyl Fragment

Abstract

AbstractIt was found that reactions of 2‐Me3Si‐R‐substituted (Si–Csp or Si–C) imidazolidine‐1,3‐diols with PbO2 in MeOH are cascade reactions, which involve the oxidation of imidazolidine‐1,3‐diols into nitronyl nitroxide (NN–R–SiMe3) and further elimination of the Me3Si group to give NN–R–H. The efficiency of a one‐pot approach in the synthesis of nitronyl nitroxides is demonstrated by reference to syntheses of2‐ethynyl‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (2), 2‐(4‐ethynylphenyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (8), (E)‐4,4,5,5‐tetramethyl‐2‐[2‐(pyrrolidin‐1‐yl)vinyl]‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (3a), (E)‐2‐[2‐(diisopropylamino)vinyl]‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (3b), and 4,4,5,5‐tetramethyl‐2‐(1H‐1,2,3‐triazol‐5‐yl)‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (H11). Interaction of 2 with secondary amines formed only trans isomers 3a and 3b. X‐ray investigation of 3a and 3b confirmed that the reaction is regiospecific. Static magnetochemical measurements detected weak exchange interactions between the odd electrons of the paramagnetic centers in the solid nitronyl nitroxide products, including H11 whose crystals consist of helices formed by numerous intermolecular H bonds. The ESR spectra of 3a, 3b, and H11 show substantial delocalization of spin density to the substituent. Modeling reproducibly gave equal hyperfine coupling constants (HFC) for the imidazoline nitrogen atoms of 3b and different HFC for the nitrogen atoms of 3a and H11, indicating that rotation of the two cyclic substituents is hindered. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)