DFT-Enabled Development of Hemilabile (P∧N) Ligands for Gold(I/III) RedOx Catalysis: Application to the Thiotosylation of Aryl Iodides

Chelating-Group-Assisted C(sp2)-O Reductive Elimination at the Gold(III) Center

Herein, we demonstrate chelating-group-assisted C(sp2)-O reductive elimination at gold(III) centers. Detailed stoichiometric studies highlighted the importance of a chelating group for achieving successful C-O reductive elimination, paving the way for the development of a catalytic version. The mechanistic investigations, including control experiments, 31P NMR, mass spectrometry, and density functional theory (DFT) studies, suggested that the synergistic effect of the ligand and chelating group creates a highly coordinated environment around the Au(III) center to facilitate the C(sp2)-O bond-forming reaction.

Bidentate (P^N) Au(III)-azide complexes: synthesis and reductive elimination studies

Herein, we report the synthesis and characterization of novel (P^N) aryl Au(III)-azide complexes. The reductive elimination from these Au(III) complexes to forge C(sp2)-N3 bonds has also been demonstrated. Considering the feasibility of C(sp2)-N3 reductive elimination in Au(III)-azide complexes, the gold-catalyzed C(sp2)-N3 cross-coupling reaction has been achieved. Furthermore, it is shown that the Au(III)-azide complexes undergo facile azide exchange with cyano and alkyne nucleophiles, facilitating C(sp2)-C(sp) cross-coupling.

Gold-Catalyzed Deallylative C-S Cross-Coupling Reactions

Herein, we report the gold-catalyzed deallylative C-S cross-coupling reaction through ligand-enabled Au(I)/Au(III) redox catalysis. One of the major challenges in gold-catalyzed C-S cross-coupling reactions is to prevent catalyst deactivation caused by the formation of a strong gold-sulfur bond. We discovered that the use of allyl phenyl sulfide as a sulfur surrogate facilitates a dynamic equilibrium between cationic Au(I) and Au(I)-sulfide complexes, overcoming the gold quenching problem. A detailed mechanistic investigation, including 31P NMR studies, mass analysis, and stoichiometric experiments, provided valuable insights into the reaction mechanism, which is further supported by computational studies.

Gold-Catalyzed Cross-Coupling Reactions of Organoiodides with Disulfides: Access to Aryl Sulfides and Vinyl Sulfide Derivatives

Thioethers and their derivatives play important roles in synthetic chemistry, medicinal chemistry, and materials science. Herein, we report a hemilabile P,N-ligand-assisted gold-catalyzed C-S cross-coupling reaction of organoiodides with disulfides. In this reaction, alkyl or aryl disulfides react smoothly with aryl or vinyl iodides to afford a series of aryl sulfide and vinyl sulfide derivatives in good to excellent yields. The robust synthetic capabilities of thioether synthesis are exemplified by the readily available and easily handled reagents as well as the excellent compatibility with a wide range of functional groups.

Exploring the Electronic and Steric Effects of Hemilabile (P^N) Ligands in Redox Gold Catalysis: Application to the Cross-Coupling Reaction of Aliphatic Amines with Aryl Iodides

Herein, we report 17 new (P^N) ligands for redox gold catalysis, featuring various substituents at -C4, -C5, and -C6 of the aryl ring and nitrogen handle. Rate kinetics experiments revealed that electron-rich substituents at -C4 and -C5 positions of the aryl ring enhanced the rate of oxidative addition of Au(I) with C(sp2)-Br bonds compared to electron-poor substituents. Further, we report an unprecedented gold-catalyzed arylation of aliphatic amines using an electronically rich ligand (L6) with an -OMe group at the -C5 position.

Gold-Catalyzed 1,2-Carboxyarylation of Alkenes

Herein, we disclose an unprecedented gold-catalyzed 1,2-carboxyarylation of alkenes through ligand-enabled Au(I)/Au(III) catalysis. Unlike other approaches for the arylative functionalization of C-C multiple bonds, attempts to utilize weak nucleophiles such as carboxylate anions were unsuccessful. The key to achieving this transformation is the use of a 1,3-diketone-appended alkene, which undergoes gold-catalyzed oxyarylation followed by retro-aldol reaction to afford the product. Detailed mechanistic investigations were conducted to support the proposed mechanism.

In silico screening of P,N-ligands facilitates optimization of Au(iii)-mediated S-arylation

Metal-mediated cysteine S-arylation is an emerging bioconjugation technique due to its high chemoselectivity, rapid kinetics, and aqueous compatibility. We have previously demonstrated that by altering the steric profile of the ligand and aryl groups of an Au(iii) oxidative addition complex, one can modulate the kinetics of the bimolecular coordination and induce rate constants up to 16 600 M-1 s-1. To further enhance the rate of coordination, density functional theory (DFT) calculations were performed to investigate the steric properties of the P,N-ligated Au(iii) oxidative addition complex as well as the thermodynamics of the S-arylation reaction. This allowed for the accelerated screening of 13 new Au(iii) oxidative addition complexes. Three of the more sterically available, synthetically accessible P,N-ligands were synthesized, incorporated into Au(i) and Au(iii) complexes, and their rates determined experimentally. The comprehensive mechanistic insights from the DFT calculations led to the development of new reagents with bimolecular coordination rate constants as fast as 20 200 M-1 s-1. Further experimental characterization of these reagents' efficacy as S-arylation reagents led to a proposed switch in selectivity-determining step for the fastest reagent, which was further confirmed by profiling the reductive elimination kinetics. This work provides a concise workflow for the screening of metal-mediated cysteine S-arylation reagents and new fundamental insights into the coordination chemistry behavior of Au(iii) systems.

Intermolecular 1,2-Difunctionalization of Nitriles via Redox Gold Catalysis: Synthesis of Benzoxazoles and Benzimidazoles

The unprecedent gold-catalyzed intermolecular 1,2-difunctionalization of nitriles with o-iodophenols or o-iodoanilines via Au(I)/Au(III) redox catalysis has been developed, providing an expedient route to the synthesis of benzoxazoles or benzimidazoles with broad substrate scope and high functional compatibility. Mechanistic investigation reveals that the Au(III)-Ar species generated via oxidative addition of o-iodophenol to MeDalphosAu+, serves as a key intermediate. Particularly, this annulation is initiated by oxidative addition, rather than the nucleophilic attack of the phenol moiety in o-iodophenol towards the nitrile. The method was also applied to the synthesis of poly aza-heterocycles via a cascade Au(I)/Au(III) and Au(I) catalysis relay.

Photosensitized Gold-Catalyzed Cross-Couplings of Aryl Bromides

Recently, ligand-promoted Au(I)/Au(III)-catalyzed cross-coupling reactions with aryl iodides have garnered considerable attention. Here, we report the first visible-light-driven gold-catalyzed cross-couplings of challenging aryl bromides. In the presence of a (P, N)-gold(I) catalyst and an acridinium photocatalyst under blue LED irradiation, C-O coupling of aryl bromides with carboxylic acids was achieved, and soon it was found that this photoinduced gold-catalyzed cross-coupling of aryl bromides was appliable for other C-C, C-N, and C-S bond formation. Experimental and computational studies suggest that this visible-light-driven gold-catalyzed cross-couplings of aryl bromides involves two discrete photoinduced energy transfer (EnT) events: first, energy transfer (EnT) from a photosensitizer produces an excited-state gold(I) complex that allows the bottleneck oxidative addition of aryl bromides to form an aryl Au(III) complex and second, the reductive elimination of aryl-Au(III) complex to regenerate Au(I). Collectively, the new synergistic catalytic method developed here highlights the tremendous potential of photochemical gold catalysis via excited-state organogold complexes, as well as its potential to facilitate drug discovery due to the biocompatibility and mildness of the reaction conditions.

Gold(I/III)-Catalyzed Sulfonylation of Aryl/Vinyl Iodides To Synthesize Aryl Sulfones

A gold-catalyzed sulfonylation of aryl/vinyl iodides to synthesize aryl sulfones facilitated by the ligand-enabled Au(I)/Au(III) redox catalysis was developed. In the reaction, aryl sodium sulfinates or sulphinic acids can react smoothly with aryl/vinyl iodides to directly construct various aryl sulfones. The strong synthetic capabilities of sulfone synthesis are demonstrated by its easily available and handled reagents, good functional group compatibility, and late-stage application of complicated biomolecules. Mechanistic studies suggest that the silver salt plays a crucial role in the transmetalation with the Au(I)/Au(III) intermediate, and the gold complex favors Au-S bond formation over Au-O bond formation.

Electronic Effects of Bidentate P,N-Ligands on the Elementary Steps of Au(I)/Au(III) Reactions Relevant to Cross-Coupling Chemistry

Oxidant-free Au(I)/Au(III)-catalyzed cross-coupling has been recently enabled by the use of bidentate P,N-ligands. To further develop these P,N-ligands, computational studies were performed to understand their effects on the oxidative addition of aryl iodide electrophiles with Au(I). Using this mechanistic understanding, six new electron-rich P,N-ligands were synthesized. The ligand exchange equilibrium and reductive elimination were then characterized by using a Au(III)-mediated S-arylation reaction. The results detailed herein provide new fundamental insights in Au(I)/Au(III) ligand design.

Tuning the Steric and Electronic Properties of Hemilabile NHC ligands for Gold(I/III) Catalyzed Oxyarylation of Ethylene: A Computational Study

Mechanistic studies on 1,2-oxyarylation of ethylene promoted by gold catalysts bearing hemilabile N-Heterocyclic Carbene (NHC^X) ligands were conducted by DFT calculations, exploring the whole catalytic cycle. After highest energy transition state (TS) barriers were located for NHC^N gold catalyst, and experimental results with different iodoarenes and alcohols rationalized, the study was extended to modified NHC^X catalysts, to observe how electronic and steric effects could affect the rate determining step TS. Electronic effects were investigated on NHC^X (X=H, N, O, P, and S), whereas steric effects emerged when comparing catalysts with different N-R groups (R=Dipp, Mes, tBu and Me). Finally, we suggest a different catalyst design based on N-aryl N-o-donor-aryl NHC, with different donors and NHC backbones to search for better performing systems.

Ligand-Enabled Gold-Catalyzed Cyanation of Organohalides

Herein, we disclose the first report on gold-catalyzed C(sp2)-CN cross-coupling reaction by employing a ligand-enabled Au(I)/Au(III) redox catalysis. This transformation utilizes acetone cyanohydrin as a nucleophilic cyanide source to convert simple aryl and alkenyl iodides into the corresponding nitriles. Combined experimental and computational studies highlighted the crucial role of cationic silver salts in activating the stable (P,N)-AuCN complex towards the oxidative addition of aryl iodides to subsequently generate key aryl-Au(III) cyanide complexes.

Synthesis of Aryl Thiocyanates, Aryl Dithiocarbamates, Aryl Sulfones, and Aryl Thiobenzoates via Au-Catalyzed C-S Cross-Couplings

We have developed an efficient gold-catalyzed C-S cross-coupling of various silver-based thionucleophiles with aryl iodides. Our method offers a one-stop-shop solution for synthesizing diverse sulfur-containing aromatics, including aryl thiocyanic acids, aryl diethyldithiocarbamates, aryl sulfones, and aryl thiobenzoates. Our protocol gives moderate to excellent yields and compatibility with various functional groups. In comparison to other transition-metal-catalyzed C-S cross-couplings, our method has advantages in broad applicability and high tolerance toward air and moisture.

Au(I)/Au(III)-Catalyzed Sulfonylation of Aryl Iodides for the Synthesis of Various Functionalized Aryl Sulfones

A gold-catalyzed sulfonylation reaction of aryl iodides with different sodium sulfinates facilitated by the ligand-enabled Au(I)/Au(III) redox catalysis was developed. In the reaction of gold-catalyzed C-S coupling, a variety of functionalized sodium sulfinates, such as CF3, CHF2, CH3, and alkyl groups, can react smoothly with aryl iodides to directly construct diversely functionalized aryl sulfones. This gold-catalyzed sulfonylation offers a complementary method for synthesizing functionalized aryl sulfones with electron-donating groups.

Gold-Catalyzed Alkoxy-Carbonylation of Aryl and Vinyl Iodides

Herein, for the first time, we disclose the gold-catalyzed alkoxy-carbonylation of aryl and vinyl iodides utilizing ligand-enabled Au(I)/Au(III) redox catalysis. The present methodology is found to be general, efficient, employs mild reaction conditions and showcases a broad substrate scope even with structurally complex molecules. Density functional theory (DFT) calculations revealed mechanistic pathways distinct from those of conventional transition metal-catalyzed carbonylation reactions.

Gold-Catalyzed Arylative Cope Rearrangement

Cope rearrangements have garnered significant attention owing to their ability to undergo structural reorganization in stereoselective manner. While substantial advances have been achieved over decades, these rearrangements remained applicable exclusively to parent 1,5-hexadienes. Herein, we disclose the gold-catalyzed arylative Cope rearrangement of 1,6-heptadienes via a cyclization-induced [3,3]-rearrangement employing ligand-enabled gold redox catalysis. Detailed mechanistic investigations including several control experiments, cross-over experiment, HRMS analysis, 31P NMR and DFT studies have been performed to underpin the mechanism.

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.

Comparison of Cyclic and Linear PEG Conjugates

Bioconjugation of polymers to proteins is a method to impart improved stability and pharmacokinetic properties to biologic systems. However, the precise effects of polymer architecture on the resulting bioconjugates are not well understood. Particularly, cyclic polymers are known to possess unique features such as a decreased hydrodynamic radius when compared to their linear counterparts of the same molecular weight, but have not yet been studied. Here, we report the first bioconjugation of a cyclic polymer, poly(ethylene glycol) (PEG), to a model protein, T4 lysozyme, containing a single engineered cysteine residue (V131C). We compare the stability and activity of this conjugate with those of a linear PEG-T4 lysozyme analogue of similar molecular weight. Furthermore, we used molecular dynamics (MD) simulations to determine the behavior of the polymer-protein conjugates in solution. We introduce cyclic polymer-protein conjugates as potential candidates for the improvement of biologic therapeutics.

Ultrafast Au(III)-Mediated Arylation of Cysteine

Through mechanistic work and rational design, we have developed the fastest organometallic abiotic Cys bioconjugation. As a result, the developed organometallic Au(III) bioconjugation reagents enable selective labeling of Cys moieties down to picomolar concentrations and allow for the rapid construction of complex heterostructures from peptides, proteins, and oligonucleotides. This work showcases how organometallic chemistry can be interfaced with biomolecules and lead to a range of reactivities that are largely unmatched by classical organic chemistry tools.

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.