Insights on the Anion Effect in N‐heterocyclic Carbene Based Dinuclear Gold(I) Catalysts

The catalytic activity of dinuclear gold(I) complexes depends on the nature of the counterion and on the linker length. Both factors influence two main important steps in hydromethoxylation of 3-hexyne and cycloisomerization of N-(prop-2-yn-1-yl)benzamide: deactivation and pre-equilibrium (formation of active intimate OSIP ion pairs)

Abstract

Dinuclear bisNHC (bis(N-heterocyclic carbene)) gold(I) complexes 3 a and 4 a of general formula [Au₂Br₂(bisNHC)] were tested as catalysts in the cycloisomerization of N-(prop-2-yn-1-yl)benzamide and in the hydromethoxylation of 3-hexyne in the presence of silver(I) activators bearing different counteranions. The catalytic performance of mononuclear NHC complexes (1 a, 2 a) in the same reactions was also studied. The results highlighted the fundamental role of both NHC ligand and counterion in the catalytic cycles and activation process: dinuclear catalysts exhibit higher initial activity even under milder conditions but suffer in terms of stability with respect to mono NHCs. Furthermore, a new dinuclear bisNHC gold(I) complex 4 b of general formula [Au₂(OTs)₂(bisNHC)] (OTs=p-toluenesulfonate) was successfully synthesized and characterized by means of NMR and ESI-MS analyses.

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