Gold(I)-coordination triggered multistep and multiple photochromic reactions in multi-dithienylethene (DTE) systems

The preparation, characterization, and photochromic properties of a mononuclear gold(I) complex (1oo) with two identical DTE-acetylides and a dinuclear gold(I) complex (2ooo) with both DTE-acetylide and DTE-diphosphine are described. Both gold(I) complexes exhibit multistep and multiple photocyclization/cycloreversion reactions. Particularly, four-state and four-color photochromic switch is successfully achieved for the dinuclear gold(I) complex upon irradiation with appropriate wavelengths of light. In contrast, fully ring-closed form is unattained through multiple photocyclization for the two corresponding model organic compounds coupling with the same DTE units as gold(I) complexes but without gold(I)-participation. It is demonstrated that coordination of gold(I) ion to DTE-acetylides exerts indeed a crucial role in achieving stepwise and selective photocyclization and cycloreversion reactions for both gold(I) complexes, in which the coordinated gold(I) atom acts as an effective "barrier" to prohibit intramolecular energy transfer between multi-DTE moieties.     

Gold-catalyzed bis-cyclization of 1,2-diol- or acetonide-tethered alkynes. Synthesis of β-lactam-bridged acetals: a combined experimental and theoretical study

2-Azetidinone-tethered alkyn-1,2-diols or alkynyl acetonides, readily prepared from imines of (R)-2,3-O-isopropylideneglyceraldehyde, were used as starting materials for the regio- and diastereospecific catalytic bis-oxycyclization reaction in the presence of a gold/acid binary system. Interestingly, in contrast to the gold-catalyzed reactions of N-tethered terminal alkynes, which lead to the corresponding 6,8-dioxabicyclo[3.2.1]octane derivatives (proximal adduct), the reactions of substituted alkynic diols and acetonides under identical conditions gave the 7,9-dioxabicyclo[4.2.1]nonane derivatives (distal adducts) as the sole products, through exclusive 7-endo/5-exo bis-oxycyclizations by initial attack of the oxygen atom to the external alkyne carbon. Moreover, the mildness of the method allowed the incorporation of a 1,3-diyne moiety as reactive partner, displaying exquisite chemoselectivity toward the internal alkynic moiety. In order to confirm the mechanistic proposal, labeling studies with deuterium oxide have been performed. Besides, density functional calculations were performed to gain insight into the mechanisms of the bis-oxycyclization reactions.     

氮杂环卡宾双核金络合物催化的胺芳基化反应

联萘胺出发合成了氮杂环卡宾双核和单核金络合物, 通过X射线的单晶衍射确定了它们的结构, 并将其应用于催化胺芳基化反应中, 以高达95%的收率得到吡咯烷类化合物. 综合上述实验结果, 发现氮杂环卡宾双核金络合物4b中存在着Au(I)-Au(I)间相互弱作用力, 而且这种弱相互作用可能对该催化反应起重要的作用, 以高收率得到吡咯烷类化合物.     

Determinants for Tight and Selective Binding of a Medicinal Dicarbene Gold(I) Complex to a Telomeric DNA G‐Quadruplex: a Joint ESI MS and XRD Investigation

The dicarbene gold(I) complex [Au(9‐methylcaffein‐8‐ylidene)₂]BF₄ is an exceptional organometallic compound of profound interest as a prospective anticancer agent. This gold(I) complex was previously reported to be highly cytotoxic toward various cancer cell lines in vitro and behaves as a selective G‐quadruplex stabilizer. Interactions of the gold complex with various telomeric DNA models have been analyzed by a combined ESI MS and X‐ray diffraction (XRD) approach. ESI MS measurements confirmed formation of stable adducts between the intact gold(I) complex and Tel 23 DNA sequence. The crystal structure of the adduct formed between [Au(9‐methylcaffein‐8‐ylidene)₂]⁺ and Tel 23 DNA G‐quadruplex was solved. Tel 23 maintains a characteristic propeller conformation while binding three gold(I) dicarbene moieties at two distinct sites. Stacking interactions appear to drive noncovalent binding of the gold(I) complex. The structural basis for tight gold(I) complex/G‐quadruplex recognition and its selectivity are described.     

A complex of gold(I) benzenethiolate with isocyanide: synthesis and crystal and molecular structures

2,6-Dimethylphenyl isocyanide forms complexes with gold( I) chloride (complex 4) and gold(I) benzenethiolate (complex 5) but forms no stable complexes with gold alkanethiolates. A reaction of complex 5 with tetramethylthiuram disulfide yields gold(I) dimethyldithiocar-bamate. Stable gold(III) derivatives cannot be obtained in such a way. Shortened intermolecular Au...Au contacts in complexes 4 and 5 (X-ray diffraction) suggest the presence of “aurophilic” interactions in them.     

Controlling the oxidative addition of aryl halides to Au(I)

By means of density functional theory calculations, we computationally analyze the physical factors governing the oxidative addition of aryl halides to gold(I) complexes. Using the activation strain model of chemical reactivity, it is found that the strain energy associated with the bending of the gold(I) complex plays a key role in controlling the activation barrier of the process. A systematic study on how the reaction barrier depends on the nature of the aryl halide, ligand, and counteranion allows us to identify the best combination of gold(I) complex and aryl halide to achieve a feasible (i.e., low barrier) oxidative addition to gold(I), a process considered as kinetically sluggish so far. © 2014 Wiley Periodicals, Inc.     

Reactions of Cationic Gold(I) with Allenoates: Synthesis of Stable Organogold(I) Complexes and Mechanistic Investigations on Gold‐Catalyzed Cyclizations

The reaction of allenoates with cationic gold(I)—generated in situ from a phosphine gold chloride and a silver salt—formed unusual, room temperature stable vinyl gold(I) lactones under very mild conditions. The scope and limitations for the synthesis of this novel organogold complex was investigated. DFT calculations on the highest occupied molecular orbitals (HOMOs) of allenoates and the natural bond orbital (NBO) charge densities provided an explanation for the limitations. A plausible mechanism for its formation was proposed based on in situ ¹H and ³¹P NMR spectroscopic analyses. Controlled experiments for the cleavage of the gold–carbon bond by electrophiles indicated that this vinyl gold(I) complex is the likely intermediate in the gold‐catalyzed reaction of carbon–carbon multiple bonds.     

A family of alkynylgold(III) complexes [AuI(mu-{CH2}2PPh2)2Au(III)(C triple bond CR)2] (R = Ph, tBu, Me3Si): facile and reversible comproportionation of gold(I)/gold(III) to digold(II)

The symmetric digold(II)dichloride bis(ylide) complex [Au2Cl2(mu-{CH2}2PPh2)2] reacts with acetylides to form the asymmetric heterovalent gold(I)/gold(III) complexes [AuI(mu-{CH2}2PPh2)2AuIII(CCR)2] [R = Ph, tBu, and SiMe3], the phenyl analogue of which was characterized by X-ray crystallography. These compounds represent the first examples of gold(III) complexes containing two acetylide ligands. [AuI(mu-{CH2}2PPh2)2AuIII(CCPh)2] undergoes a reversible comproportionation reaction upon treatment with [Ag(ClO4)tht] to give the symmetric digold(II) cationic complex [Au2(tht)2(mu-{CH2}2PPh2)2](ClO4)2. If this complex is treated with phenylacetylene in the presence of base, the heterovalent gold(I)/gold(III) complex is re-formed. This reversible interconversion between binuclear gold(I)/gold(III) and digold(II) bis(ylide) complexes is unprecedented.     

Dinuclear gold(I)-N-heterocyclic carbene complexes: Synthesis,characterization, and catalytic application for hydrohydrazidation of terminal alkynes

Dinuclear gold(I)-N-heterocyclic carbene complexes were developed for the hydrohydrazidation of terminal alkynes. The gold(I)-N-heterocyclic carbene complexes 2a-2b were synthesized in good yields from silver complexes synthesized in situ, which in turn were obtained from the corresponding imidazolium salts with Ag₂O in dichloromethane as a solvent. The new air-stable gold(I)-NHC complexes, 2a - 2b, were characterized using NMR spectroscopy, elemental analysis, infrared, and mass spectroscopy studies. The gold(I) complex 2a was characterized using X-ray crystallography. Bis-N-heterocyclic carbene–based gold(I) complexes 2a - 2b exhibited excellent catalytic activities for hydrohydrazidation of terminal alkynes yielding acylhydrazone derivatives. The working catalytic system can be used in gram-scale synthesis. In addition, the catalytic reaction mechanism of the hydrohydrazidation of terminal alkynes by gold(I)-NHC complex was studied in detail using density functional theory.     

Mechanistic analysis of gold(I)-catalyzed intramolecular allene hydroalkoxylation reveals an off-cycle bis(gold) vinyl species and reversible C-O bond formation

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C-O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex.     

Gold(I)-catalyzed stereoselective olefin cyclopropanation

A triphenylphosphinegold(I)-catalyzed cyclopropanation of olefins using propargyl esters as gold(I)-carbene precursors is reported. This reaction provided the basis for the use of a DTBM-SEGPHOS gold(I) complex as a catalyst in the enantioselective (up to 94% ee) preparation of vinyl cyclopropanes with high cis-selectivity.     

Characterization of the Isochromen-4-yl-gold(I) Intermediate in the Gold(I)-Catalyzed Glycosidation of Glycosyl ortho-Alkynylbenzoates and Enhancement of the Catalytic Efficiency Thereof

Gold standard: The title gold complex was characterized unambiguously as an important intermediate in the title reaction. Protonolysis of this vinyl gold(I) complex was critical for regeneration of the active gold(I) species for the catalytic cycle, and use of a protic acid co-catalyst significantly lowered the required catalyst loading to 0.5?mol?%.     

Inclusion Complexes of Gold(I)-Dithiocarbamates with β-Cyclodextrin: A Journey from Drug Repurposing towards Drug Discovery

The gold(I)-dithiocarbamate (dtc) complex [Au(N,N-diethyl)dtc]₂ was identified as the active cytotoxic agent in the combination treatment of sodium aurothiomalate and disulfiram on a panel of cancer cell lines. In addition to demonstrating pronounced differential cytotoxicity to these cell lines, the gold complex showed no cross-resistance in therapy-surviving cancer cells. In the course of a medicinal chemistry campaign on this class of poorly soluble gold(I)-dtc complexes, >35 derivatives were synthesized and X-ray crystallography was used to examine structural aspects of the dtc moiety. A group of hydroxy-substituted complexes has an improved solubility profile, and it was found that these complexes form 2 : 1 host–guest inclusion complexes with β-cyclodextrin (CD), exhibiting a rarely observed “tail-to-tail” arrangement of the CD cones. Formulation of a hydroxy-substituted gold(I)-dtc complex with excess sulfobutylether-β-CD prevents the induction of mitochondrial reactive oxygen species, which is a major burden in the development of metallodrugs.     

A multipurpose gold(I) precatalyst

[Au(tmbn)(2)](SbF(6)) is the first gold(I) complex supported by two nitrile ligands that is indefinitely stable at room temperature. This is a highly versatile precatalyst that can be used for the preparation of active and robust solid-supported gold(I) catalysts.     

Decarboxylation of aromatic carboxylic acids by gold(I)-N-heterocyclic carbene (NHC) complexes

A recently isolated gold(I) complex, [Au(IPr)(OH)], permits the transformation of carboxylic acids to the corresponding decarboxylated gold(I)-aryl complex without the use of silver co-catalyst under mild reaction conditions.     

Inhibited and enhanced nucleation of gold nanoparticles at the water|1,2-dichloroethane interface

The deposition of gold at the interface between immiscible electrolyte solutions has been investigated using reduction of tetrachloroaurate or tetrabromoaurate in 1,2-dichloroethane, with aqueous phase hexacyanoferrate as reducing agent. In a clean environment without defects present at the interface, the Au(III) complex was reduced to the Au(I) complex, but no solid phase formation could be observed. A deposition process could only be observed through the addition of artificial nucleation sites in the form of palladium nanoparticles at the interface. This process could be associated with the reduction of the Au(I) halide complex to metallic gold, by determining the gold reduction potentials in 1,2-dichloroethane. XANES measurements indicate that tetrachloroaurate ion transfers intact into the organic phase, with the central Au atom retaining its oxidation state of +3 and the overall anion remaining charged at -1.     

Tiopronin gold nanoparticle precursor forms aurophilic ring tetramer

In the two step synthesis of thiolate-monolayer protected clusters (MPCs), the first step of the reaction is a mild reduction of gold(III) by thiols that generates gold(I) thiolate complexes as intermediates. Using tiopronin (Tio) as the thiol reductant, the characterization of the intermediate Au(4)Tio(4) complex was accomplished with various analytical and structural techniques. Nuclear magnetic resonance (NMR), elemental analysis, thermogravimetric analysis (TGA), and matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) were all consistent with a cyclic gold(I)-thiol tetramer structure, and final structural analysis was gathered through the use of powder diffraction and pair distribution functions (PDF). Crystallographic data has proved challenging for almost all previous gold(I)-thiolate complexes. Herein, a novel characterization technique when combined with standard analytical assessment to elucidate structure without crystallographic data proved invaluable to the study of these complexes. This in conjunction with other analytical techniques, in particular mass spectrometry, can elucidate a structure when crystallographic data is unavailable. In addition, luminescent properties provided evidence of aurophilicity within the molecule. The concept of aurophilicity has been introduced to describe a select group of gold-thiolate structures, which possess unique characteristics, mainly red photoluminescence and a distinct Au-Au intramolecular distance indicating a weak metal-metal bond as also evidenced by the structural model of the tetramer. Significant features of both the tetrameric and the aurophilic properties of the intermediate gold(I) tiopronin complex are retained after borohydride reduction to form the MPC, including gold(I) tiopronin partial rings as capping motifs, or "staples", and weak red photoluminescence that extends into the Near Infrared region.     

The Impact of Crystal Packing and Aurophilic Interactions on the Luminescence Properties in Polymorphs and Solvate of Aroylacetylide–Gold(I) Complexes

The influence of the chemical substitution, crystal packing, and aurophilic interactions of the gold(I) acetylide complexes of the type (ArCOC≡C)_nAuPEt₃ (n=1,2) on their luminescent properties were examined. All described complexes undergo ligand scrambling in solution, which results in the formation of stable, easily isolated crystals that contain [ArCO(C≡C)_n]₂Au⁻(Et₃P)₂Au⁺ homoleptic species. In particular, we observed that the (benzoylacetylide)gold(I) complex yields three crystal forms with strikingly different luminescence properties. We monitored the conversion pathway for these forms: an orange luminescent form of homoleptic complex upon drying undergoes spontaneous transformation to bright green fluorescent form and finally to the weakly blue emissive one. In addition, we report a rare example of a helical arrangement of Au⋅Au⋅Au chains that are observed for the first time in acetylide gold(I) complexes in the case of heteroleptic (benzoylacetylide)gold(I) complex. This is a very rare case in which crystal structures and ensuing electronic properties of the heteroleptic and Au^I complexes could be directly compared.     

Gold(I)-catalyzed addition of thiols and thioacids to 3,3-disubstituted cyclopropenes

Gold(I)-catalyzed reactions of thiols, thiophenols, and thioacids with 3,3-disubstituted cyclopropenes occur in a regioselective and chemoselective manner to produce either vinyl thioethers or primary allylic thioesters in good yields. A survey of commonly used gold(I) catalysts shows Echavarren's cationic gold(I) catalyst to be most tolerant of deactivation by sulfur. A novel digold with bridging thiolate complex is characterized by X-ray crystallography, shedding light on a possible deactivation pathway.     

Dual catalysis with an Ir(III)-Au(I) heterodimetallic complex: reduction of nitroarenes by transfer hydrogenation using primary alcohols

A triazolyl-di-ylidene ligand has been used for the preparation of a homodimetallic complex of gold, and a heterodimetallic compound of gold and iridium. Both complexes have been fully characterized and their molecular structures have been determined by means of X-ray diffraction. The catalytic properties of these two complexes have been evaluated in the reduction of nitroarenes by transfer hydrogenation using primary alcohols. The two complexes afford different reaction products; whereas the Au(I)-Au(I) catalyst yields a hydroxylamine, the Ir(III)-Au(I) complex facilitates the formation of an imine.