Relativistic Functional Groups: Aryl Carbon–Gold Bond Formation by Selective Transmetalation of Boronic Acids

Suzuki-Miyaura coupling of arylboronic acids to gold(iii)

Cyclometalated gold(iii) aryls are prepared through palladium catalysis. Mono- and diarylation are demonstrated. A wide range of functional groups is tolerated.

Gold(iii) is prominent in catalysis, but its organometallic chemistry continues to be restricted by synthesis. Metal–carbon bond formation often relies on organometallic complexes of electropositive elements, including lithium and magnesium. The redox potential of gold(iii) interferes with reactions of these classic reagents. Resort to toxic metals is common, including reagents based on mercury and thallium. We report that the palladium-catalyzed Suzuki–Miyaura coupling of arylboronic acids extends to cyclometalated gold(iii) chlorides. Both monoarylation and diarylation are achieved. We propose a mechanism where oxidative addition to palladium with rearrangement at gold(iii) fixes the stereochemistry of monoarylated intermediates. Singly arylated species form as thermodynamic isomers. These entities then go on to form diarylated complexes. Reactions proceed at room temperature, and the products are stable to air, moisture, and chromatography.

Photoinitiated oxidative addition of CF3I to gold(I) and facile aryl-CF3 reductive elimination

Herein we report the mechanism of oxidative addition of CF₃I to Au(I), and remarkably fast C_aryl–CF₃ bond reductive elimination from Au(III) cations. CF₃I undergoes a fast, formal oxidative addition to R₃PAuR′ (R = Cy, R′ = 3,5-F₂-C₆H₄, 4-F-C₆H₄, C₆H₅, 4-Me-C₆H₄, 4-MeO-C₆H₄, Me; R = Ph, R′ = 4-F-C₆H₄, 4-Me-C₆H₄). When R′ = aryl, complexes of the type R₃PAu(aryl)(CF₃)I can be isolated and characterized. Mechanistic studies suggest that near-ultraviolet light (λ_max = 313 nm) photoinitiates a radical chain reaction by exciting CF₃I. Complexes supported by PPh₃ undergo reversible phosphine dissociation at 110 °C to generate a three-coordinate intermediate that undergoes slow reductive elimination. These processes are quantitative and heavily favor C_aryl–I reductive elimination over C_aryl–CF₃ reductive elimination. Silver-mediated halide abstraction from all complexes of the type R₃PAu(aryl)(CF₃)I results in quantitative formation of Ar–CF₃ in less than 1 min at temperatures as low as −10 °C.

Gold-catalyzed allylation of aryl boronic acids: accessing cross-coupling reactivity with gold

A sp(3)-sp(2) C-C cross-coupling reaction catalyzed by gold in the absence of a sacrificial oxidant is described. Vital to the success of this method is the implementation of a bimetallic catalyst bearing a bis(phosphino)amine ligand. A mechanistic hypothesis is presented, and observable transmetalation, C-Br oxidative addition, and C-C reductive elimination in a model gold complex are shown. We expect that this method will serve as a platform for the development of novel transformations involving redox-active gold catalysts.

Gold(iii) compounds containing a chelating, dicarbanionic ligand derived from 4,4′-di-tert-butylbiphenyl

Various gold(iii)heterocycles were prepared and their structures and photophysical properties were examined.

Gold-catalyzed oxidative coupling of arylsilanes and arenes: origin of selectivity and improved precatalyst

The mechanism of gold-catalyzed coupling of arenes with aryltrimethylsilanes has been investigated, employing an improved precatalyst (thtAuBr3) to facilitate kinetic analysis. In combination with linear free-energy relationships, kinetic isotope effects, and stoichiometric experiments, the data support a mechanism involving an Au(I)/Au(III) redox cycle in which sequential electrophilic aromatic substitution of the arylsilane and the arene by Au(III) precedes product-forming reductive elimination and subsequent cycle-closing reoxidation of the metal. Despite the fundamental mechanistic similarities between the two auration events, high selectivity is observed for heterocoupling (C-Si then C-H auration) over homocoupling of either the arylsilane or the arene (C-Si then C-Si, or C-H then C-H auration); this chemoselectivity originates from differences in the product-determining elementary steps of each electrophilic substitution. The turnover-limiting step of the reaction involves associative substitution en route to an arene π-complex. The ramifications of this insight for implementation of the methodology are discussed.

A cationic gold complex cleaves BArF24

A sterically shielded cationic NHC gold complex IPr**Au-BArF(24) without solvent coordination has been prepared in situ in CH(2)Cl(2). The monovalent transition metal electrophile, a "soft proton", heterolytically activates the C-B bond of the weakly coordinating counterion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate at room temperature.

Highly stable electrochemical immunosensor for carcinoembryonic antigen

The long-term stability of sensing interfaces is an important issue in biosensor fabrication. A novel stable gold nanoparticle (AuNP)-modified glassy carbon (GC) electrode interface (GC-Ph-AuNP)-based biosensor for detecting carcinoembryonic antigen (CEA) was developed. GC electrodes were modified with 1,4-phenylenediamine to form a stable layer, and then AuNPs were bound onto the GC electrodes through CAu bonds. Anti-CEA was directly adsorbed on AuNPs fixed on the GC electrode. The linear range of the immunosensor was from 10 fg to 100 ng mL(-1) with a detection limit of 3 fg mL(-1) (S/N=3). The current of the immunosensor was increased by 4% after one month. The GC-Ph-AuNP immunosensor showed high sensitivity, a wide linear range, low detection limit, and good selectivity and stability. The immobilization method of the immunosensor could be widely applied to construct other immunosensors.

Gold-containing indoles as anticancer agents that potentiate the cytotoxic effects of ionizing radiation

This report describes the design and application of several distinct gold-containing indoles as anticancer agents. When used individually, all gold-bearing compounds display cytostatic effects against leukemia and adherent cancer cell lines. However, two gold-bearing indoles show unique behavior by increasing the cytotoxic effects of clinically relevant levels of ionizing radiation. Quantifying the amount of DNA damage demonstrates that each gold-indole enhances apoptosis by inhibiting DNA repair. Both Au(I)-indoles were tested for inhibitory effects against various cellular targets including thioredoxin reductase, a known target of several gold compounds, and various ATP-dependent kinases. While neither compound significantly inhibits the activity of thioreoxin reductase, both showed inhibitory effects against several kinases associated with cancer initiation and progression. The inhibition of these kinases provides a possible mechanism for the ability of these Au(I)-indoles to potentiate the cytotoxic effects of ionizing radiation. Clinical applications of combining Au(I)-indoles with ionizing radiation are discussed as a new strategy to achieve chemosensitization of cancer cells.