Catalytic formation of aziridines from imines. Characterisation of an intermediate in SnCl4-catalysed aziridination and mechanistic considerations

Abstract

The tin(IV) catalysed aziridination of imines using ethyl diazoacetate as the carbene fragment donor has been investigated from a synthetic and mechanistic point of view. For imines substituted with aromatic substituents, an intermediate in the aziridination reaction has been isolated and characterised by X-ray diffraction. This intermediate is an octahedral complex with a trans coordination of two imines and in which the imines have isomerised from trans to cis by the reaction with/coordination to tin(IV). Tin(IV) is a very effective catalyst for the aziridination of various imines giving cis-aziridines as the major product. The aziridination can proceed with a very low catalyst load as 0.05 mol% of SnCl₄ is sufficient to achieve a high conversion. For the formation of aziridines a linear plot of the experimental data is obtained by means of the Hammett equation for a series of competition experiments. Based on the results the mechanism for the aziridination is discussed in terms of a Lewis acid activation of the imine for a nucleophilic attack of ethyl diazoacetate.