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12.3.4.5.1.6 Method 6: N-Oxides and N-Hydroxy Species

DOI: 10.1055/sos-SD-012-00398

Grimmett, M. R.Science of Synthesis, (200212508.

Imidazole N-oxides are potentially in equilibrium with their N-hydroxy tautomers. At times they can be sufficiently unstable to be explosive.[‌6‌,‌46‌] The compounds are frequently hygroscopic, and deoxygenate on heating. It is more usual though to remove the oxygen function by a reductive procedure such as the use of zinc and acid,[‌778‌,‌779‌] sodium borohydride, hydriodic acid, sodium dithionite, titanium(III) chloride in methanol,[‌45‌] or catalytic hydrogenation with palladium or Raney nickel.[‌46‌,‌780‌] As with other N-oxides, phosphorus trichloride and phosphoryl chloride are also useful deoxygenating reagents, although such reagents can also introduce a chlorine at the 2-position.[‌4‌,‌185‌] When 1-hydroxy-2-methyl-5-phenyl-1H-imidazole is treated with butyllithium followed by hexachlorodisilane, this also results in deoxygenation.[‌781‌] Dehydrogenation of 1-hydroxyimidazoles and their 3-oxides with lead dioxide or lead(IV) acetate gives short-lived nitroxide radicals.[‌4‌] When N-oxides and N-hydroxyimidazoles react with alkylating agents, such alkylation occurs preferentially on the exocyclic oxygen rather than the annular nitrogen to form alkoxides, for example in the case of 1-hydroxy-1H-imidazole 3-oxide (379) the product is the 3-alkoxyimidazole 380 (Scheme 177);[‌46‌,‌779‌,‌782‌] acylation and aroylation are similar.[‌83‌,‌782‌] The esters formed in the latter reactions can act as acyl-transfer reagents. The N-oxide function confers 1,3-dipole properties on the molecules which can become involved in 1,3-dipolar additions as a consequence.[‌4‌]

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