Aldimines oxaziridination

If, however, hydrogen is present in the a-position of the iV-alkyl substituent, 2-alkyl-oxaziridines are easily decomposed by alkali. Base attack on this H atom effects 1,2-elimination at the C—N bond. From (86) and aldimine (87) forms, and a mixture of ammonia and two carbonyl compounds is finally obtained, one of them containing the carbon atom of the original oxaziridine ring, the other originating from the IV-alkyl group (57JA5739). 

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). 

Epoxidation of both aldimines and ketimines is possible. Most oxaziridines formed are stable compounds, especially aldimines containing aromatic substituents, and 2-sulfonyl-and 2-sulfamyl oxaziridines5. Generally, /V-sulfonyloxaziridines are isolated as stable crystalline solids. Certain compounds are widely used in synthetic organic chemistry as oxygen-transfer reagents (15-17). 

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. 

Enantiomeric purities ranging from 20 to 80% have been reported for the acid-promoted asymmetric oxidation of sulfides to sulfoxides by binaphthyl-derived oxaziridines has been described <2007T6232>. A novel amino hydroxylation of olefins has been developed using /ra t-2-phenylsulfonyl-3-phenyloxaziridine 33 <2007JA1866>. The reaction, which is catalyzed by copper(ll) salts, affords good yields of the product. Oxidation of aldimines to oxaziridines using alumina-supported M0O3 catalyst and anhydrous /-butyl hydroperoxide (TBHP) has been described. Yields are excellent. 

Further studies of asymmetric induction in the synthesis of optically active oxaziridines via oxidation of imines with chiral acids and the degree of stereoselectivity of these reactions have been reported. It was found that the degree of stereoselectivity in the conversion of aldimines and ketimines to oxaziridines by (+)-monopercamphoric acid (MPCA) is dependent on the solvent and the reaction temperature (Tables 4 and 5). The stereoselectivity of the reaction does not seem to depend on the nature of the alkyl group attached to nitrogen. ... 

Perfluoro-c -2,3-dialkyloxaziridines (64) are prepared in good yield (68-77%) by MCPBA oxidation of the corresponding perfluoroazaalkenes (188) (Equation (46)) <93JOC4754>. To be successful, the peracid needs to be > 80% pure, well-dried, and the solvent needs to be acetonitrile. Interestingly, oxidation does not proceed in CH2C12. A series of 3-/-butyl-3-( 1 -haloalkyl)oxaziridine were prepared by oxidation of the corresponding a-chloro, a-bromo, a,a-dichloro, and a,a-dibromo aldimines with MCPBA <92T7345>. 

An efficient, high-yield synthesis of A-alkyl and A-aryl oxaziridine by oxidation of aldimines with buffered Oxone (potassium peroxymonosulfate) has been introduced by Hajipour and Pyne (Equation (47)) <92JCR(S)388>. Oxidation of the aldimine is accomplished in aqueous NaHC03/ acetonitrile or acetone affording the oxaziridine within 15-30 minutes in excellent yield (95-98%). The active oxidizing species in acetone and acetonitrile are thought to be dimethyldioxirane and peroxyimidic acid [MeC(OOH)=NH), respectively. 

The oxidation of N-alkyl or N-aryl aldimines or ketimines with f-amyl hydroperoxide in the presence of either M0CI5 or Mo(CO)6 leads to oxaziridine formation, equation (137) [184]. In benzene solution the reaction proceeds rapidly and yields are high (80-95%). The catalytic procedure is considerably more convenient than conventional methods of oxaziridine preparation. 

Primary amines are converted via oxidation of their aldimines or ketimines (5) into oxaziridines (6), which on base treatment yield an iV-unsubstituted aldimine (7) hydrolysis of this aldimine yields the corresponding aldehyde. The scope of the route is at present under investigation (Scheme 3)." ... 

Although the intermediacy of unstable N-unsubstituted imines has been proposed during the acid- or base-catalysed decomposition of oxaziridines bearing an a-hydrogen atom on the iV-alkyl group, the reaction conditions employed have not permitted their detection. A recent report has now shown that addition of D ABCO or DBN to solutions of oxaziridines (37) gives aldimines (38) which could be isolated or detected, depending upon the nature of R. ... 

A new method for enantioselective oxaziridination of aryl aldimines uses meta-chloroperbenzoic acid and a cinchona alkaloid derivative. ( ... 

The derivative (72) (TBS = t-butyldimethylsilyl) from a series of synthesized cinchona alkaloid derivatives is used as a catalyst for the asymmetric oxaziridination of aryl aldimines with m-CPBA in toluene to give tra 5 -(R,R)-oxaziridines with up to 95% ee.438... 

For the transformation of imines to oxaziridines, in general peroxy adds (e.g., Ph-C03H) can be used. For the selective oxygenation of aryl aldimines tert-BuOOH and alumina-supported M0O3 (as a recyclable heterogeneous catalyst) are recommended... 

Oxidations of nitrogen-containing compounds [213] derived from aldehydes and ketones have been reviewed. The most common products from aldimine oxidations are nitriles [214]. However, other compounds such as nitrile oxides [215], oxaziridines [205, 216] and formamides or amides stemming from a Beckmarm rearrangement (see Beckmann reaction, under Section 9.3.2.2.) can also be formed. Furthermore, the initial aldehyde can be regenerated by oxidative deprotection [190, 217]. 

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