Synthesis of Secondary Amines via Self-Limiting Alkylation.

-centered nucleophilicity increases upon alkylation, and thus selective partial alkylation of ammonia and primary amines can be challenging: Poor selectivity and overalkylation are often observed. Here we introduce -aminopyridinium salts as ammonia surrogates for the synthesis of secondary amines via self-limiting alkylation chemistry. Readily available -aryl--aminopyridinium salts engage in -alkylation and depyridylation to afford secondary aryl-alkyl amines without any overalkylation products. The method overcomes classical challenges in selective amine alkylation by accomplishing alkylation via transient, highly nucleophilic pyridinium ylide intermediates and can be applied in the context of complex molecular scaffolds. These findings establish -aminopyridinium salts as ammonia synthons in synthetic chemistry and a strategy to control the extent of amine alkylation.