Monocyclic biradicals in the imidazolidine series

Spirocyclic Nitroxides as Versatile Tools in Modern Natural Sciences: From Synthesis to Applications. Part I. Old and New Synthetic Approaches to Spirocyclic Nitroxyl Radicals

“…Another technique for the synthesis of imidazolidine SNRs, proposed by Reznikov, involves the addition of organolithium compounds to cyclic nitrones of the 2,5-dihydroimidazole series and allows for the introduction of substituents other than the methyl group at the C-4 atom of the imidazolidine ring. Thus, by alkylation of 5,5-dimethyl-2-spirocyclohexyl-4-phenyl- 3-imidazoline-3-oxide 283 [ 167 ] with a mixture of formic acid and formaldehyde, N -methyl derivative 312 is obtained, whose subsequent reaction with PhLi and oxidation of intermediate hydroxylamine 313 (without isolation) by MnO 2 lead to nitroxide 314 with a quantitative yield ( Scheme 56 ) [ 178 ]. On the other hand, to synthesize spirocyclic diradical 315 , which cannot be obtained by direct oxidation of mono-SNR 314 , a different starting substrate 280 is utilized.…”

“…Treatment with phenyllithium results in a crystalline N,N′ -dihydroxy derivative 316 , which is oxidized by the original technique of slurrying in pentane and aging over manganese dioxide for a short period to prevent overoxidation. In contrast to tetramethyl analogs [ 178 ], evaporation or several-minute ambient-temperature incubation of a solution of diradical 315 causes its complete decomposition, which is accompanied by a release of nitrogen oxides and benzophenon formation. Apparently, the presence of two phenyl substituents and a spirocycle, which are larger than the methyl groups, creates serious steric hindrances that significantly reduce the stability of diradical 315 .…”