Oxidation of imines

Peracid oxidation of imines is the most general synthesis of oxaziranes (Section 5.08.4.1.1). Other peroxides and metal catalysis have also been employed. 

The third variant is that of Genies and Lapowski They base their model on the assumption of fundamental differences between the two oxidation steps in PANI. The first step is analogous to that of MacDiarmid. For the second redox process the authors postulate the additional oxidation of imine-N... 

Oxidation oflmines Oxidation of imines with peracids leads to oxazi-ridines, with the possibility of their successive rearrangement into nitrones, depending on their structure and the employed oxidant. 

Also, nitrones can be formed by photochemical oxidation (X350 nm) of aldimines in acetonitrile, in the presence of O2 over a TiC>2 suspension (20, 21). Air oxidation of imines into oxaziridines with their subsequent transformation into nitrones, using cobalt catalysts, provides good yields. Utilization of molecular oxygen in the oxidation process seems highly promising due to its cost-effectiveness, availability, and the possibility of industrial application (22). 

Efficient oxidation of imines into nitrones can be achieved by using methyl (trilluoromethyl)dioxirane as an oxidant. This method provides enantiopure nitrones derived from 2H -pyrrole 1-oxide (23, 24). 

Oxidation of primary amines with DMD or other oxidants leads to the formation of a complex mixture of nitroso, oximes, and nitro compounds (76). Utilization of DMD in acetone affords dimethyl nitrone (22). This is likely to be a result of the initial oxidation of primary amine (19) to hydroxylamine (20) with the subsequent condensation of acetone and oxidation of imine (21) (Scheme 2.9) (77). 

Nonequivalence of Oxaziridines from (+)-MPCA Oxidation of Imines ... 

TRIMETHYL-, /3-LACTONE, 48, 72 Perbenzoic acid, oxidation of imines with, 49,13... 

This procedure is an adaptation of that described by Emmons for the preparation of oxaziranes from imines using peracetic acid. Other procedures which may be more useful for oxazirane preparation in specific instances are the oxidation of imines with iw-chloroperbenzoic acid and the reaction of aldehydes or ketones with hydroxylamine 0-sulfonic acid in alkaline solution. 2-<-Butyl-3-phenyloxazirane has also been prepared by photolysis of a-phenyl-N-f-butylnitrone (a general reaction of considerable theoretical interest since it represents direct conversion of electromagnetic energy to chemical energy) and in low yields by ozonoly-sis of N-f-butylbenzaldimine. ... 

In a study of the chemistry of oxaziranes [75], it was discovered that this class of novel heterocyclic compounds is readily oxidized with peracetic acid to give dimeric nitroso compounds. Since the oxaziranes are, in turn, prepared by a peracetic acid oxidation of imines, the crude imines can, by proper adjust-... 

The most useful preparation of oxaziranes is the very ready oxidation of imines with organic peraoids. This is a remarkably general reaction, and, since imines of widely varying structure can easily be... 

An elegant method to prepare nitrones is the oxidation of imines with permanganate ions under PTC conditions (equation 7)41. 

TABLE 1. Oxidation of imines 1 to nitrones 2 with potassium permanganate under phase transfer conditions"... 

The permanganate ion oxidation of imines to nitrones is very much dependent on the reaction conditions. For the investigation of this dependency, reactions with benzylidene-r-butylamine, la, were carried out (equation 8). la was oxidized with m-chloroperbenzoic acid to give the corresponding oxaziridine 3a. As byproduct traces (<3%) of the nitrone 2a, amine 4a and benzaldehyde were formed. The permanganate ion and m-chloroperbenzoic acid may then oxidize the imine in two different ways. 

The oxidation of imines derived from substituted cyclohexanones occurs predominantly from the equatorial direction. However, the product oxaziridines can undergo subsequent equilibration to favor a more stable conformation which places the bulkier nitrogen substituent in an equatorial conformation (equation 44)219. 

The metal-catalyzed oxidation of imines using molecular oxygen as the final oxidant and aldehydes as co-reductants has been studied223. Various transition metal complexes have been tested as catalysts and it is found that cobalt complexes can catalyze the selective oxidation of imines to oxaziridines in good yield (ca 80%) (Scheme 4). 

Oxidation of imine, amidine and guanidine nitrogens yields oximes, oxidative attack at the nitrogen centers in alkyl-, aryl- or cyclic imines affords nitrones. The oxidation state of nitrogen in these functions11 is defined as —1. Both oximes and nitrones exist as a mixture of two geometric isomers, Z and E, previously termed syn and anti, respectively... 

Toda, F., and Ochi, M. (1996) Enantioselective Oxidation of Imines in Inclusion Complexes with a Chiral Host Compound and Preparation of Optically Pure Oxaziridines by Enantiomer Resolution, Enantiomer, 1, 85-88. 

Jorgensen has studied the oxidation of imines 137 to oxaziridines 138 catalyzed by transition metal complexes using molecular oxygen as the stoichiometric oxidant. Optimized... 

The oxidation of imines continues to be the most common method for the preparation of oxaziridines. Page et al. recently described the preparation of enantiomerically pure chiral NH-oxaziridines derived from camphor and fenchone <2000JOC4204>. Treatment of camphor imine 303 with 1 equiv of MCPBA at —30 to —40°C in dichloromethane gave NH-oxaziridine 50 in 94% yield. The fenchone-derived oxaziridine 305 was obtained similarly... 

The reaction of 2-isopropyl-5,5-dimethyl-5,6-dihydro-2//-l,3-oxazine 306 with MCPBA leads to formation of the corresponding A -alkyl oxaziridine 307 as a single isomer <1995T139>. Due to the preferential conformation of imine 306, the imine double bond is exclusively oxidized anti to the f-propyl moiety. Oxidation of imine 308 with MCPBA gave oxaziridine nitroxide 309 in 90% yield <2000TL8787> and similar oxidation of 310 gave oxaziridine 311 in 74% yield <2005S1496>. 

Oxidation of imines 321 with MCPBA in methylene chloride resulted in the formation of the corresponding oxaziridine 322 along with lactam 323 and nitrone 324 <2002TL2649>. Similar oxidation of imine 325 gave oxaziridine 326 in 51-67% yield along with 20-31% of isomeric nitrone 327 as a by-product <1995JOCl 131>. 

Aldimines are oxidized to oxaziridines by tetrabutylammonium monoperoxysulfate (TBAO) and was investigated by Mohajer et al. <2004TL631>. The reaction gave the corresponding E)- or a mixture of ( )- and (Z)-oxaziridines with high yields (>90%) and good to excellent selectivities (75-100%). The ( /Z)-ratio depends critically on the stereoelectronic nature of the imine substituents, the solvent, and the presence of Lewis acids and bases. For example, oxidation of imine 333 with TBAO afforded ( )-oxaziridine 334 in quantitative yield. 

Oxidation of imine 340 with H2O2 in the presence of tosyl imidazole 341 gave oxaziridine 129 in 61% yield <1996T5773, 1997JOC188>. The oxidation takes place through the tr/ -generated -toluenesulfonic peracid. 

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