Nitrone reactions oxaziridines

Photochemical Rearrangement Isomerization of nitrones to oxaziri-dines is a general reaction of various cyclic and acyclic nitrones (447-449). When this reaction is reversible, many transformations of nitrone to oxaziridine and back to nitrone can be carried out without decomposition. This reaction is of special interest in view of light energy accumulation (450, 451). 

The investigations of the UV spectra, the rate constants of hydrolysis and additional investigations of deuterium isotope and temperature effects of TV-t-butylbenzaldoxime and 2-J-butyl-3-phenyloxaziridine57 suggest a mechanism which is shown in Scheme 3. If the protonated nitrone is the intermediate in both reactions, the formation from either nitrone or oxaziridine should have no effect on the subsequent behaviour under identical conditions. 

Oxaziridines are structural isomers of oximes and nitrones. Trialkyl oxaziridines are colourless liquids, sparingly soluble in water. The following reactions are typical for oxaziridine. 

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... 

The reaction of oxaziridines 378 with isothiocyanates has been established as a route to l,2,4-oxadiazolidin-5-thiones 379 (Scheme 64). Two mechanistic possibilities exist via which the products can form, each of which relies upon C-O cleavage, shown as paths a and b in Scheme 64. Interestingly, one of these paths involves the generation of nitrone 380, a species which could be identified as a reaction product <1998H(48)1935>. The nitrone could be generated separately and reacted with isothiocyanates to form identical l,2,4-oxadiazolidin-5-thiones 379. 

For the first time, the primary nitrone (formaldonitrone) generation and the comparative quantum chemical analysis of its relative stability by comparison with isomers (formaldoxime, nitrosomethane and oxaziridine) has been described (357). Both, experimental and theoretical data clearly show that the formal-donitrones, formed in the course of collision by electronic transfer, can hardly be molecularly isomerized into other [C,H3,N,0] molecules. Methods of quantum chemistry and molecular dynamics have made it possible to study the reactions of nitrone rearrangement into amides through the formation of oxaziridines (358). 

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 structure of nitrones is hidden in that of the oxaziridines (Figure 4)57. For both compounds the preparations and properties have been reviewed58-63. The main reactions... 

Hydrolysis and rate constants of oxaziridine compounds and nitrones under variation of the pH value and reaction medium... 

Diarylnitrone (31) formation from N-substituted, diaromatic imines has been recognized to require the presence of NADPH/O2, and has been proposed to proceed via the intermediacy of an oxaziridine3 possibly arising from reaction of the parent imines with the putative P-450 [FeO]3+ species in analogy to the oxidation of olefins118. Ring cleavage of the oxaziridine then yields nitrone or amide (equation 10). 

Other relevant described reactions, which affect the stability [72] and are related to some adverse effects of these compounds (see below) [73], have been the photochemical transformations. In fact, QDO undergoes rearrangements in the presence of UV-irradiation, passing through an oxaziridine intermediate, like an acyclic nitrone (Fig. 8). The final products depend on the QDO 2,3-substitutions and reaction conditions [72,74,75]. 

Details are now available of the reactions of peracids and of hydrogen peroxide with the imines (10) and (11). The reactions of the product oxaziridines and nitrones with the same two oxidants were also discussed. The reactions of sodium borohydride, benzoyl chloride, and toluene-p-sulphonic acid with the hydroxy-nitrones (12) and (13) have been reported.12 The c.d. spectra of 2-substituted pyrrolidines have been compared with that of conanine (14).13... 

Some 3,5-diarylisoxazolidines were prepared hy [3+2]-cycloaddition of C-aryloxaziridines with vinyl aromatic compounds. For example, isoxazolidine 82 was obtained in high yield from oxaziridine 81 and l-methyl-4-vinylbenzene in refluxing toluene. The stereoselectivity of the reaction was greatly affected by the nature of the substituents on the reagents, and in some cases mixtures of cis- and trans-adducts were formed. Experimental data suggested that oxaziridines react directly with alkenes without the intermediate formation of the isomeric nitrones <07T12896>. 

In some cases, the reaction of peracids with imineshas produced nitrones along with oxaziridines. It has been found that product selectivity depends on the structure of the imines and the reaction conditions, especially acidity. The formation of oxaziridines is predominant in the presence of alcohols or carboxylic acids, whereas the yield of nitrones is increased in aprotic media. 

The photochemical isomerization reaction has been shown" to be stereospecific following the disrotatory photochemically allowed cyclization path. Thus the stereochemistry of the nitrone is preserved in the oxaziridine. However, since some nitrones undergo trans-cis isomerization under the conditions of photolysis, mixtures of oxaziridines often result. 

The nitrone-oxaziridine ring-chain isomerization has attracted theoretical attention.The quantum yield for the reaction has been measured and the reaction has been examined in a rigid polymer matrix. ... 

There are a number of thermal and photochemical reactions for which oxaziridine intermediates have been proposed but never isolated. These include, among others, the photochemical Beckmann rearrangement of oximes, many photochemical reactions of aromatic A -oxides, and the thermal rearrangement of nitrones to amides. A brief discussion of the first two seems warranted in this review because they have been studied extensively and some strong inferential evidence for oxaziridine intermediates has been obtained. 

Agawa and his co-workers have reported a series of investigations of the reactions of cumulenes with oxaziridines. Reaction with isocyanates led to oxadiazolidinones 86 45,70 same product obtained from nitrones and isocyanates. Detailed mechanistic information is not available and the classification of these reactions as cyclo-... 

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