Redox Activation of Acyclic (Aryl)(amino)carbene Gold(I) Complexes

Abstract

Activation of halido gold(I) precatalysts Au(L)X to a cationic species [Au(L)]+ with a vacant coordination site for substrate binding typically requires abstraction of the halide X–. The Fischer-type carbene gold(I) precatalysts 2–4 feature redox-active dimethylanilinyl, 2-furyl, and ferrocenyl substituents without (2a–4a) and with (2b–4b), a dangling dimethylamino substituent. After single-electron oxidation, 2–4 catalyze the cyclization of N(2-propyn-1-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline without the presence of a halide scavenger. While all dimethylanilinyl and 2-furyl substituted precatalysts likely form catalytically active nanoparticles after oxidation, the ferrocenyl substituted gold(I) complexes 4a and 4b operate in a homogeneous fashion. The dimethylamino substituted ferrocenyl precatalyst 4b is the most active one. The formation of the catalytically active molecular species after oxidation was probed by stopped-flow experiments, quantitative EPR spectroscopy, and quantum chemical calculations to arrive at a consistent mechanistic picture that involves one-electron oxidation of the ferrocene, valence isomerization to a gold(II) species and anion coordination to the gold(II) center. This oxidation/isomerization/coordination activation mechanism is fundamentally different from the typical activation of gold(I) precatalysts by halide abstraction and opens new avenues in gold catalysis beyond gold(I) and gold(III) catalyses.