Catalytic enantioselective formation of aziridines from α-imino esters
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.