Water-soluble gold(I)–NHC complexes of sulfonated IMes and SIMes and their catalytic activity in hydration of alkynes

The water-soluble carbene ligand precursors sIMesH+Cl− and sSIMesH+Cl− were synthesized in high yields by direct sulfonation of IMesH+Cl− (1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride) and SIMesH+Cl− (1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride). Gold(I)-N-heterocyclic carbene complexes [AuCl(sIMes)] and [AuCl(sSIMes)] were prepared by carbene transfer from the zwitterionic [Ag(sIMes)2] and [Ag(sSIMes)2] to [AuCl(tht)] (tht = tetrahydrothiophene). In methanol–water mixtures or in neat water, the new gold(I)–NHC complexes showed high catalytic activity in Markovnikov type hydration of terminal alkynes (up to a turnover frequency 1990 h−1; ethynyltoluene, 0.1 mol% catalyst) but were markedly less active in case of internal alkynes (TOF = 3.6 h−1; diphenylethyne, 1 mol% catalyst). These new Au(I)-NHC catalysts do not require acid co-catalysts or activation by Ag(I)-additives.

Precursors of the water-soluble N-heterocyclic carbene ligands sIMes and sSIMes were prepared by direct sulfonation of IMesH+Cl− and SIMesH+Cl−, respectively. Gold(I)-complexes of these ligands catalyzed the hydration of alkynes in methanol–water mixtures or in neat water with turnover frequencies up to 1990 h−1.Figure optionsDownload high-quality image (29 K)Download as PowerPoint slideHighlights
► Direct sulfonation of IMesH+Cl− and SIMesH+Cl− in fuming sulfuric acid.
► Water-soluble [AuCl(sIMes)], 3a and [AuCl(sSIMes)], 3b synthesized.
► Catalytic hydration of alkynes with 3a and 3b in H2O/MeOH 1/1 or in neat water.
► High catalytic activities: TOF up to 1980 h−1, catalyst concentration 100–740 ppm.
► No need for Brønsted acid co-catalysts or activation by silver(I) salts.