Mechanism of Alkyne Hydroarylation Catalyzed by (P,C)-Cyclometalated Au(III) Complexes

Over the last 5–10 years, gold(III) catalysis
has developed
rapidly. It often shows complementary if not unique features compared
to gold(I) catalysis. While recent work has enabled major synthetic
progress in terms of scope and efficiency, very little is yet known
about the mechanism of Au(III)-catalyzed transformations and the relevant
key intermediates have rarely been authenticated. Here, we report
a detailed experimental/computational mechanistic study of the recently
reported intermolecular hydroarylation of alkynes catalyzed by (P,C)-cyclometalated
Au(III) complexes. The cationic (P,C)Au(OAcF)+ complex (OAcF = OCOCF3) was authenticated
by mass spectrometry (MS) in the gas phase and multi-nuclear NMR spectroscopy
in solution at low temperatures. According to density functional theory
(DFT) calculations, the OAcF moiety is κ2-coordinated to gold in the ground state, but the corresponding κ1-forms featuring a vacant coordination site sit only slightly
higher in energy. Side-on coordination of the alkyne to Au(III) then
promotes nucleophilic addition of the arene. The energy profiles for
the reaction between trimethoxybenzene (TMB) and diphenylacetylene
(DPA) were computed by DFT. The activation barrier is significantly
lower for the outer-sphere pathway than for the alternative inner-sphere
mechanism involving C–H activation of the arene followed by
migratory insertion. The π-complex of DPA was characterized
by MS. An unprecedented σ-arene Au(III) complex with TMB was
also authenticated both in the gas phase and in solution. The cationic
complexes [(P,C)Au(OAcF)]+ and [(P,C)Au(OAcF)(σ-TMB)]+ stand as active species and off-cycle
resting state during catalysis, respectively. This study provides
a rational basis for the further development of Au(III) catalysis
based on π-activation.