Metal-catalysed reactions of imines with ethyl diazoacetate leading to aziridines

Abstract

The metal-catalysed aziridination of imines with ethyl diazoacetate as the carbene fragment donor using various Lewis acids as the catalyst has been investigated. The catalytic properties of different Lewis acid complexes have been tested and it has been found that both main-group complexes, such as BF₃·OEt₂, early- and late-transition metal complexes, such as TiCl₂(O-Prⁱ)₂, Cu(OTf)₂ and Zn(OTf)₂ and rare-earth metal complexes, such as Yb(OTf)₃, can catalyse the formation of aziridines. The aziridination gives mainly the cis-aziridines as the major diastereoisomer, but the selectivity is dependent on the substrate, catalyst and solvent. Zn(OTf)₂ and Yb(OTf)₃ have been shown to be general catalysts for the formation of various aziridines using different imines and a variety of reaction conditions. Both Zn(OTf)₂ and Yb(OTf)₃, as well as some of the other Lewis acids, in combination with various chiral ligands, have been tested as catalysts for the formation of optically active aziridines, but only low ees are obtained. The Zn(OTf)₂- and Yb(OTf)₃-catalysed reactions have been investigated for imines having both electron-donating and -withdrawing substituents, and in reactions containing diethyl fumarate as a trapping reagent, in attempts to obtain insight into the mechanism of the aziridination.