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

It was found that reactions of 2‐Me3Si‐R‐substituted (Si–Csp or Si–C sp 2) 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)

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

“…Experiments performed under normal conditions showed that after the addition of excess MnO 2 , Co 2 O 3 , or Ni 2 O 3 to a solution of 1a in MeOH or EtOH, the reaction mixtures became deep dark violet after a minute , as was also the case in the previously described reaction of 1a in MeOH in the presence of PbO 2 . Spin‐labeled acetylene 3 was then quickly8 isolated from the reaction mixture (Scheme ). According to TLC data, other products formed in relatively small amounts over this time (Entries 1–3, Table 1), and 3 was obtained in a high yield (ca.…”

“…Until recently, the α‐acetylene derivatives of nitronyl and imino nitroxides were inaccessible compounds; their precursors each contained a combination of an N–OH group and a triple bond, rearrangement of which led to the aminoenone fragment,5 also formed in reactions between 4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐3‐oxides and activated alkynes 6,7. Thanks to the discovery of the cascade reaction between 1a and PbO 2 in MeOH, leading to 2‐ethynyl‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H ‐imidazole‐1‐oxyl 3‐oxide ( 3 ), however, compound 3 became accessible and could be used in various transformations 8. To study the synthetic potential of the cascade reaction here, we concentrated on how the heterophase oxidation of 1a could be changed by varying the oxidant, reaction time, and protic and aprotic polar and low‐polar solvents.…”

Silicon- and germanium-substituted propynals as ambident electrophiles for the design of new heterocycles and polyfunctional acetylenes

“…158 The efficient synthetic method includes a cascade process of oxidation-desilylation of imidazolidine 1,3 diol (32a), obtained by the reaction of formyltriazole 30a with N,N´ (2,3 dimethylbutane 2,3 diyl)bis(hydroxylamine) (МеОН, 25 °С, 80% yield) followed by the subsequent oxidation with PbO 2 under mild conditions (МеОН, 25 °С). For the first time found readiness of desilylation of Me 3 Si-C sp 2 upon treatment with Pb(OH) 2 , formed in the course of the reaction, allowed one to find a new approach to nitroxide 33.…”

Highly efficient desilylation of 3-trimethylsilylprop-2-ynamides by the action of KF-Al2O3

“…We recently showed that successful desilylation may be achieved with the aid of even weak bases generated in protic medium from metal oxides (PbO 2 , MnO 2 , Co 2 O 3 , Ni 2 O 3 ) [11].…”