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 π-complexation for arenes bearing strongly electron-withdrawing
substituents (σ > 0.43). Reductive elimination is accelerated
by electron-donating substituents (ρ = −2.0) on one or
both rings, with the individual σ-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 π-complexation of the arene by Ar–AuX2 becoming the turnover-limiting step.