Catalytic enantioselective formation of aziridines from α-imino esters

Abstract

A new catalytic enantioselective aziridination reaction of α-imino esters with diazo compounds catalyzed by chiral Lewis acid complexes is presented. A series of N-substituted α-imino esters has been tested as substrates for the aziridination reaction using trimethylsilyldiazomethane as the carbenoid fragment donor catalyzed by various chiral complexes. Both chiral BINAP and bisoxazoline † ligands, in combination with copper(I) salts in particular can catalyze the aziridination reaction leading to the cis-aziridine with up to 72% ee applying BINAP–copper(I) complexes as the catalyst, while the bisoxazoline–copper(I) catalysts give an aziridination reaction with lower diastereoselectivity, however, the trans-aziridine was formed in 69% ee. The influence of diazo compounds, Lewis acids, chiral ligands, solvents and counterion on the reaction course has been investigated and a mechanism for the reaction is discussed in which the α-imino ester coordinates to the chiral catalyst. For trimethylsilyldiazomethane, nucleophilic attack on the coordinated α-imino ester probably takes place in the case of copper(I) as the Lewis acid, while for ethyl diazoacetate the reaction course is dependent on the Lewis acid; for silver(I) a nucleophilic attack is probably also operating, while a metal–carbene intermediate is involved when copper(I) is used.

References

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