Ligand-enabled oxidation of gold(i) complexes with o-quinones

Consolidation of the Oxidant-Free Au(I)/Au(III) Catalysis Enabled by the Hemilabile Ligand Strategy

In this minireview we survey the challenges and strategies in gold redox catalysis. Gold's reluctance to oxidative addition reactions due to its high redox potential limits its applicability. Initial attempts to overcome this problem focused on the use of sacrificial external oxidants in stoichiometric amounts to bring Au(I) compounds to Au(III) reactive species. Recently, innovative approaches focused on employing hemilabile ligands, which are capable of coordinating to Au(I) and stabilizing square-planar Au(III) intermediates, thus facilitating oxidative addition steps and enabling oxidant-free catalysis. Notable examples include the use of the (P^N) bidendate MeDalphos ligand to achieve various cross-coupling reactions via oxidative addition Au(I)/Au(III). Importantly, hemilabile ligand-enabled catalysis allows merging oxidative addition with π-activation, such as oxy- and aminoarylation of alkenols and alkenamines using organohalides, expanding gold's versatility in C-C and C-heteroatom bond formations and unprecedented cyclizations. Moreover, recent advancements in enantioselective catalysis using chiral hemilabile (P^N) ligands are also surveyed. Strikingly, versatile bidentate (C^N) hemilabile ligands as competitors of MeDalphos have appeared recently, by designing scaffolds where phosphine groups are substituted by N-heterocyclic or mesoionic carbenes. Overall, these approaches highlight the evolving landscape of gold redox catalysis and its tremendous potential in a broad scope of transformations.

Ligand-Enabled Oxidative Fluorination of Gold(I) and Light-Induced Aryl-F Coupling at Gold(III)

MeDalphos Au(I) complexes featuring aryl, alkynyl, and alkyl groups readily react with electrophilic fluorinating reagents such as N-fluorobenzenesulfonimide and Selectfluor. The ensuing [(MeDalphos)Au(R)F]+ complexes have been isolated and characterized by multinuclear NMR spectroscopy as well as X-ray diffraction. They adopt a square-planar contra-thermodynamic structure, with F trans to N. DFT/IBO calculations show that the N lone pair of MeDalphos assists and directs the transfer of F+ to gold. The [(MeDalphos)Au(Ar)F]+ (Ar = Mes, 2,6-F2Ph) complexes smoothly engage in C-C cross-coupling with PhCCSiMe3 and Me3SiCN, providing direct evidence for the oxidative fluorination/transmetalation/reductive elimination sequence proposed for F+-promoted gold-catalyzed transformations. Moreover, direct reductive elimination to forge a C-F bond at Au(III) was explored and substantiated. Thermal means proved unsuccessful, leading mostly to decomposition, but irradiation with UV-visible light enabled efficient promotion of aryl-F coupling (up to 90% yield). The light-induced reductive elimination proceeds under mild conditions; it works even with the electron-deprived 2,6-difluorophenyl group, and it is not limited to the contra-thermodynamic form of the aryl Au(III) fluoride complexes.

S-Se oxidative addition to auranofin derivatives: a DFT study

Oxidative addition of the S-Se bond to Au(I) complexes is discussed for a series of 26 auranofin (AF) derivatives. AF and its analogues are Au(I) complexes with recognized anticancer activity that act by binding and inhibiting the thioredoxin reductase (TrxR) enzyme. Generally, the oxidative addition to Au(I) is a sluggish reaction under mild conditions (i.e., a high activation barrier - ΔH‡), which is also verified here for AF, ΔH‡ = 33.0 kcal mol-1. However, we predicted that subtle changes in the AF ligands can make the process feasible under standard conditions. For instance, the exchange of -PEt3 by -P(Et2)(OEt), which is a weaker electron σ-donor, reduced the activation barrier to 17.1 kcal mol-1. Furthermore, substitution of the -SAtg ligand by -Cl- leads to a ΔH‡ value of 22.5 kcal mol-1. Overall, the reaction is driven by the nucleophilic attack of the S-Se bond on the Au(I) center, attributed mainly to the charge transfer (4p)Se → (6p)Au, which characterizes the addition step. At the transition state (TS) point, the (5d)Au → σ*(S-Se) charge transfer becomes relevant, facilitating the S-Se bond breakage and the oxidation step. In addition to the electron transfers, the strain energy to deform the linear Au(I) geometry to the tetracoordinated Au(III) arrangement in the TS structure plays a primary role in explaining the trends in the activation barriers. Finally, the activation barrier (ΔH‡) and reaction energy (ΔH°) were correlated for most of the complexes studied, which suggests that the reaction passes through a late or product-like TS and, therefore, the steric and electronic factors affecting ΔH‡ also act on ΔH°. Overall, the results presented here might open up a new field of investigation for interactions between AF derivatives and TrxR, which contributes to a full understanding of the biological mechanism of action of these species.

Pnictogen bonding at the service of gold catalysis: the case of a phosphinostiborane gold complex

The search for alternative gold catalyst activators has led us to consider the design of platforms in which a phosphine gold chloride moiety could be activated via formation of a pnictogen bond with a neighboring antimony unit. Here, we describe that such a system can be accessed from 4-(diphenylphosphino)-5-(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene, by oxidation of the stibine with 3,5-di-tert-butyl-o-benzoquinone and by coordination of an AuCl unit to the phosphine. This strategy affords a complex in which a Lewis acidic or pnictogen-bond donor catecholatostiborane unit flanks the adjacent gold chloride moiety. This design impacts the catalytic reactivity of the gold center, as reflected by the ability of this complex to catalyze propargyl amide cyclization reactions. Comparisons with a phosphinostiborane ferrocene analog and computations point to the formation of an intramolecular Au-Cl → Sb(V) interaction as responsible for the observed catalytic activity.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, Au^I /Au^III catalytic cycle remains much less explored due to the reluctance of Au^I to undergo oxidative addition and the stability of the Au^III intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for Au^I /Au^III redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

Reductive Elimination Reactions in Gold(III) Complexes Leading to C(sp3)-X (X = C, N, P, O, Halogen) Bond Formation: Inner-Sphere vs SN2 Pathways

The reactions leading to the formation of C-heteroatom bonds in the coordination sphere of Au(III) complexes are uncommon, and their mechanisms are not well known. This work reports on the synthesis and reductive elimination reactions of a series of Au(III) methyl complexes containing different Au-heteroatom bonds. Complexes [Au(CF₃)(Me)(X)(PR₃)] (R = Ph, X = OTf, OClO₃, ONO₂, OC(O)CF₃, F, Cl, Br; R = Cy, X = Me, OTf, Br) were obtained by the reaction of trans-[Au(CF₃)(Me)₂(PR₃)] (R = Ph, Cy) with HX. The cationic complex cis-[Au(CF₃)(Me)(PPh₃)₂]OTf was obtained by the reaction of [Au(CF₃)(Me)(OTf)(PPh₃)] with PPh₃. Heating these complexes led to the reductive elimination of MeX (X = Me, Ph₃P⁺, OTf, OClO₃, ONO₂, OC(O)CF₃, F, Cl, Br). Mechanistic studies indicate that these reductive elimination reactions occur either through (a) the formation of tricoordinate intermediates by phosphine dissociation, followed by reductive elimination of MeX, or (b) the attack of weakly coordinating anionic (TfO^- or ClO₄^-) or neutral nucleophiles (PPh₃ or NEt₃) to the Au-bound methyl carbon. The obtained results show for the first time that the nucleophilic substitution should be considered as a likely reductive elimination pathway in Au(III) alkyl complexes.

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(OAc^F)⁺ complex (OAc^F = 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 OAc^F 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(OAc^F)]⁺ and [(P,C)Au(OAc^F)(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.