Enantioselective Strecker reactions between aldimines and trimethylsilyl cyanide promoted by chiral N,N′-dioxides

Axially chiral N,N′-dioxide Lewis base promoters have been developed and have for the first time been applied to the asymmetric synthesis of α-amino nitriles through cyanide addition to aldimines. The chiral 3,3′-dimethyl-2,2′-biquinoline N,N′-dioxide 2 exhibited high enantioselectivity for asymmetric Strecker reactions between N-benzhydrylimines and trimethylsilyl cyanide. In the presence of 1 equiv. of chiral promoter 2, the cyanosilylation of aldimines afforded the corresponding α-amino nitriles with ee values of up to 95%. Optically pure products (99% ee) were obtained simply by recrystallization in the cases of some of the products. Moreover, the promoter 2 could be recovered and reused at least four times without any loss of enantioselectivity and reactivity. A putative mechanism for the enantioselective Strecker reactions is also discussed.