Sustainable sonochemical synthesis of N‐protected piperazine‐propargylamines using ionic liquids and CuI.

We present a simple method to prepare diverse benzimidazolium and imidazolium ionic liquids (ILs) from common starting materials imidazole and benzimidazole. These green ILs promote the synthesis of N‐substituted piperazinyl propargylamines via a versatile CuI‐catalyzed coupling of aldehydes, secondary amines, and alkynes under ultrasound. Remarkably, the method works across a broad range of substrates, including diversely substituted aldehydes, heterocycles, cyclic amines, and phenylacetylenes. To understand this versatility, we investigated the influence of IL structure (N‐substitution, anion) on reactivity. Symmetrical long‐chain substituents on the benzimidazolium cation and bromide (Br−) anions led to the most efficient propargylamine formation. This approach offers a sustainable and efficient route to propargylamines with minimal waste and catalyst/IL recyclability over five cycles. [ABSTRACT FROM AUTHOR]