Abstract
Chemoselective autoxidation of 4-imino-1,5-dimethyl-3-(4-nitrophenyl)imidazolidin-2-one (1b) to its 5-hydroxy derivative 2 occurred in solutions of DMSO-d6, acetonitrile-d3 or refluxing ethanol. Also bis(imidazolidin-5-yl) peroxide 5 was isolated as a minor product. It crystallizes as a 1:1 mixture of R*, R* and R*, S* diastereomers, whereas the NMR spectra of the reaction solution in DMSO-d6 showed unequal amounts of the two isomers. Molecular mechanics modeling studies with the MM3 force field indicate the R*,S* diastereomer as the more stable one. The 5-unsubstituted and the 5,5-dimethyl substituted imines 1a and 1c, respectively, were found stable against autoxidation; the difference in reactivity of 1b is attributed to the single 5methyl group enhancing the population of the enamine tautomer. The 5-hydroxy-4-imino-1,5dimethylimidazolidin-2-one (2) underwent acid hydrolysis to form 5-hydroxyhydantoin 4.
Highlights
Apart from causing aging and deterioration of products, autoxidation can be a useful synthetic method because of its simplicity and economy in reactants using molecular oxygen in air.[1,2] Autoxidation can complicate the synthesis of imines or their enamine tautomers, that of imines of simple aldehydes.[3]
We report that the instability of 4-imino-1,5-dimethyl-3-(4-nitrophenyl)imidazolidin-2one (1b) is caused by its autoxidation, which was found to occur in solution by selectively forming the 5-hydroxy derivative 2 (Scheme 2)
While cyanoalkylureas 3a and 3c in refluxing ethanol for 1 hour[4] smoothly cyclized to the corresponding imines 1a and 1c, the imine 1b with one methyl group in position 5 could not be isolated in a pure state due to its instability
Summary
Apart from causing aging and deterioration of products, autoxidation can be a useful synthetic method because of its simplicity and economy in reactants using molecular oxygen in air.[1,2] Autoxidation can complicate the synthesis of imines or their enamine tautomers, that of imines of simple aldehydes.[3] Recently,[4] we prepared 4-imino-3-(4-nitrophenyl)imidazolidin2-ones 1 (Scheme 1). Contrary to the 5-unsubstituted derivative 1a and the 5,5-dimethyl substituted derivative 1c, the 5-methyl derivative 1b was found to be unstable turning yellow in the solid state or upon heating in solution, and a satisfactory elemental analysis could not be obtained. NH R1 R2 NAr Ar = N Me O NO2 R1 R2.
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