Do aziridines require Lewis acids for cleavage with ionic nucleophiles?

A variety of activated aziridines were cleaved by sodium azide and sodium cyanide in aqueous acetonitrile at reflux, in the absence of any Lewis acid, to provide ring-opened products in quantitative yields. However, the reaction was sluggish in the ring opening of unactivated aziridines with sodium azide where the yields could be increased by adding 50 mol% CuCl2·2H2O. The reaction was used to synthesize chiral diamines.