Cross-Coupling through Ag(I)/Ag(III) Redox Manifold.

Abstract

In ample variety of transformations, the presence of silver as an additive or co-catalyst is believed to be innocuous for the efficiency of the operating metal catalyst. Even though Ag additives are required often as coupling partners, oxidants or halide scavengers, its role as a catalytically competent species is widely neglected in cross-coupling reactions. Most likely, this is due to the erroneously assumed incapacity of Ag to undergo 2e- redox steps. Definite proof is herein provided for the required elementary steps to accomplish the oxidative trifluoromethylation of arenes through AgI /AgIII redox catalysis (i. e. CEL coupling), namely: i) easy AgI /AgIII 2e- oxidation mediated by air; ii) bpy/phen ligation to AgIII ; iii) boron-to-AgIII aryl transfer; and iv) ulterior reductive elimination of benzotrifluorides from an [aryl-AgIII -CF3 ] fragment. More precisely, an ultimate entry and full characterization of organosilver(III) compounds [K]+ [AgIII (CF3 )4 ]- (K-1), [(bpy)AgIII (CF3 )3 ] (2) and [(phen)AgIII (CF3 )3 ] (3), is described. The utility of 3 in cross-coupling has been showcased unambiguously, and a large variety of arylboron compounds was trifluoromethylated via [AgIII (aryl)(CF3 )3 ]- intermediates. This work breaks with old stereotypes and misconceptions regarding the inability of Ag to undergo cross-coupling by itself.