Au-Catalyzed Biaryl Coupling To Generate 5-to 9-Membered Rings: Turnover-Limiting Reductive Elimination versus π-Complexation

The intramolecular gold-catalyzed arylation of arenes by aryl-trimethylsilanes has been investigated from both mechanistic and preparative aspects. The reaction generates 5- to 9-membered rings, and of the 44 examples studied, 10 include a heteroatom (N, O). Tethering of the arene to the arylsilane provides not only a tool to probe the impact of the conformational flexibility of Ar-Au-Ar intermediates, via systematic modulation of the length of aryl-aryl linkage, but also the ability to arylate neutral and electron-poor arenes-substrates that do not react at all in the intermolecular process. Rendering the arylation intramolecular also results in phenomenologically simpler reaction kinetics, and overall these features have facilitated a detailed study of linear free energy relationships, kinetic isotope effects, and the first quantitative experimental data on the effects of aryl electron demand and conformational freedom on the rate of reductive elimination from diaryl-gold(III) species. The turnover-limiting step for the formation of a series of fluorene derivatives is sensitive to the reactivity of the arene and changes from reductive elimination to pi-complexation for arenes bearing strongly electron-withdrawing substituents (sigma > 0.43). Reductive elimination is accelerated by electron-donating substituents (rho = 2.0) on one or both rings, with the individual sigma-values being additive in nature. Longer and more flexible tethers between the two aryl rings result in faster reductive elimination from Ar-Au(X)-Ar and lead to the g-complexation of the arene by Ar-AuX2 becoming the turnover-limiting step.