A green approach for A3-coupling reactions: an experimental and theoretical study on NHC silver and gold catalysts

Abstract

The synthesis of propargylamines by traditional methods is limited by several drawbacks such as anhydrous conditions, low temperatures and by-products. A³-coupling reactions (aldehydes, amines, and alkynes), that involve a catalytic amount of metal complexes, have been recently recognized as a powerful alternative for the synthesis of these important scaffolds. In this paper, four new silver and gold complexes (3a, 4a, 3b and 4b), bearing N-heterocyclic carbene (NHC) ligands, were tested in A³-coupling reactions, combining the use of harmless metals as catalysts with sustainable reaction conditions (neat conditions or water as solvent). NHC silver(I) and gold(I) complexes were synthesized and characterized by ¹H and ¹³C NMR spectroscopy, ESI-MS and elemental analysis. The molecular structure of the NHC gold(I) complex 4b was determined by single crystal X-ray diffraction. A comparison of the catalytic behavior in A³-coupling reactions among the synthesized complexes disclosed the higher efficiency of gold catalysts. Computational studies shed light on species possibly involved in the catalytic mechanism.