Oxaziridines nitrogen stability

The photochemical synthesis of optically active oxaziridines from prochiral nitrones was driven by an interest in examining the configurational stability of the oxaziridin nitrogen atom. Nitrone 7 was irradiated at — 78°C in a 1 1 mixture of ( + )- or (— )-2,2,2-trifluorophenylethanol (8) and fhiorotrichloromethane yielding nitrone 9lent-9 in 30% ee (absolute stereochemistry unknown) [17,18]. At room temperature this selectivity decreased to 5% ee. Less bulky residues also reduced the selectivity. An exchange of f-butyl by /-propyl led to 20% ee at — 78°C (Scheme 5). 

A high inversion barrier at nitrogen was found for diaziridines, which parallels the configurational stability at oxaziridine nitrogen. It was first discovered from NMR data and was proved by separation into E and Z isomers and finally by obtaining optically active diaziridines. 

S.08.2.3.1 NMR investigations on oxaziridines and diaziridines, configurational stability at nitrogen... 

NMR investigations in the diaziridine field also led to the problem of inversion stability at nitrogen. Further investigations paralleled those of oxaziridines NMR investigation in solution (67CB1178) was followed by preparative separation of invertomers and finally preparation of optically active individuals. 

Incorporation of d-functions on the inverting N site is especially important. Such functions contribute more to the pyramidal GS than to the TS and thus stabilize the GS with respect to the TS. In the absence of -functions on N, the inversion barrier in NH3 is found to be much too small and H2N—CN and H2N—SiHs are found to be more stable in the planar form inclusion of -functions (and p on H) leads to the correct NH3 barrier 158> and to pyramidal nitrogen sites in HaN—CN and H2N— SiH3 (Table 8 144>). Of course, the relative error introduced by the absence of -functions is smaller the higher the barrier. This may explain in part why the barriers calculated for aziridine and oxaziridine without including -functions are in satisfactory agreement with experimental values. In PH3, the phosphorus inversion barrier changes from 30.9 to 37.2 kcal/mole on inclusion of two sets of -functions 159>. 

Oxaziridines For oxaziridines the N-inversion barrier is considerably higher than that for similar aziridines. A-Alkyloxaziridines show a high degree of configurational stability of the pyramidal nitrogen with inversion barriers... 

Oxaziridines with no substituent on the ring nitrogen may be stabilized as the fV-acyl derivatives. Indeed, the presence of an acylating agent, that is, benzoyl chloride, is necessary for the trapping of such oxaziridines derived from aliphatic aldehydes (Table 2). ... 

The addition of oxygen across the C—N double bond is a common and synthetically useful process. The product oxaziridines are of interest for theoretical reasons (largely due to the high configurational stability of the chiral nitrogen), as reagents (such as oxygen transfer moieties), and as synthetic intermediates. The stereochemical aspects of oxaziridine synthesis and reactivity have been reviewed. ... 

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