Reactions of oxaziridines with electrophiles

Like isomerization of oxaziridines to nitrones, acid hydrolysis of oxaziridines proceeds with conservation of the N—O bond. Both reactions are related mechanistically and are 

According to a kinetic study which included (56), (56a) and some oxaziridines derived from aliphatic aldehydes, hydrolysis follows exactly first order kinetics in 4M HCIO4. Proton catalysis was observed, and there is a linear correlation with Hammett s Ho function. Since only protonated molecules are hydrolyzed, basicities of oxaziridines ranging from pii A = +0.13 to -1.81 were found from the acidity rate profile. Hydrolysis rates were 1.49X 10 min for (56) and 43.4x 10 min for (56a) (7UCS(B)778). O-Protonation is assumed to occur, followed by polar C—O bond cleavage. The question of the place of protonation is independent of the predominant IV-protonation observed spectroscopically under equilibrium conditions all protonated species are thermodynamically equivalent. 

The reason for the slow hydrolysis compared to that of structurally similar compounds like nitrones or 0,lV-acetals might be the following (b-67MI50800) in the protonated species (77) assistance of the lone pair of electrons at nitrogen is sterically hindered due to the large angle of its orbital to the plane of the ring. 

The small tendency of oxaziridines to undergo hydrolytic C—O cleavage is underlined by the observation that oxaziridine (52) in a mixture of acid and 0-labeled water yields up to 50% labeled hydroxylamine. NH transfer to water successfully competes with hydrolysis (B-67MI50800). 

Migration to the developing electron sextet at nitrogen is not restricted to hydrogen. In (79) there is methyl migration with formation of methylamine and acetone in the acid-catalyzed decomposition of (80), phenyl migration leads to aniline and acetaldehyde. 

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Fluoroaziridinones have also been synthesized. The highly electrophilic example 161 was formed by the reaction of oxaziridine 160 with bis(trifluoromethyl)ketene. 

In the following years, studies conducted by Sharpless,6 7 Bach,8,9 Curci,10 and others11 relied on reaction kinetics to formulate support of a SN2-type displacement by the nucleophilic substrate on the electrophilic oxygen atom of the three-membered ring. Similarly, the deoxygenation of oxaziridines, such as 1, is kinetically consistent with the aforementioned Sn2 mechanism. 

A Wittig reaction of the THF-swollen polystyrene resin (48) with the P-ketomethylene ylide (49) has been used to functionalise the resin for further transformations. " The aza-Wittig reaction of N-Boc-triphenyliminophosphorane (50) with 4-cyanobenzaldehyde followed by oxone oxidation provides the stable oxaziridine (51) (Scheme 4) which acts as an electrophilic aminating agent towards amines and enolates. ... 

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