Arylgold(I) Complexes from Base-Assisted Transmetalation: Structures, NMR Properties, and Density-Functional Theory Calculations

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Gold(I)-Catalyzed Nucleophilic Allylation of Azinium Ions with Allylboronates

Gold(I)-catalyzed nucleophilic allylations of pyridinium and quinolinium ions with various allyl pinacolboronates are reported. The reactions are completely selective with respect to the site of the azinium ion that is attacked, to give various functionalized 1,4-dihydropyridines and 1,4-dihydroquinolines. Evidence suggests that the reactions proceed through nucleophilic allylgold(I) intermediates formed by transmetalation from allylboronates. Density functional theory (DFT) calculations provided mechanistic insight.

Digold(I) Thianthrenyl Complexes. Effect of Diphosphine Ligands on Molecular Structures in the Solid State and in Solution

A series of digold complexes possessing two thianthrenyl ligands, Au₂(Thi)₂(Ph₂P(CH₂) _n PPh₂) (Thi: 1-thianthrenyl; 1: n = 1, 2: n = 2, 3: n = 3, 4: n = 4), were prepared and characterized by crystallographic and spectroscopic measurements. X-ray crystallography of complexes 1 and 3 revealed U-shaped structures with short Au-Au distances [3.2171(3) Å and 3.0735(2) Å]. Complex 2 and three of the four structure-determined molecules of complex 4 showed structures without Au-Au contacts. UV-vis spectroscopic measurements of 1-4 and TD-DFT calculations of the two conformers of 1 revealed that complexes 1 and 3 in the solution phase contained conformers with Au(I)-Au(I) interactions in a much higher proportion than complexes 2 and 4. As a result, complexes with diphosphine ligands containing an odd number of methylene groups preferred structures with Au-Au interactions in the solid state and in solution. Oxidation of 1 with 2 equiv of PhICl₂ yielded a mixture of monomeric and dimeric thianthrenes and its dimer via ligand elimination and C-C coupling, respectively.

Non-conventional hydrogen bonding and dispersion forces that support embedding mesitylgold into a tailored bis(amidine) framework

A bis(amidine) ligand operates as a molecular lock for two AuMes fragments. The resulting complex retains a flexible double macrocycle with two non-conventional N-H⋯C_ipso hydrogen bonds and distinct intramolecular dispersion forces. Instead of unfolding of the double-ring structure through bond rupture in solution, a conformational ring inversion is observed.

Hydrogen bonds and dispersion forces serving as molecular locks for tailored Group 11 bis(amidine) complexes

A flexible polydentate bis(amidine) ligand LH2 operates as a molecular lock for various coinage metal fragments and forms the dinuclear complexes [LH2(MCl)2], M = Cu (1), Au (2), the coordination...

Transmetalation of boronic acids and their derivatives: mechanistic elucidation and relevance to catalysis

The Suzuki-Miyaura reaction (the cross-coupling reaction of boronic acids with organic halides catalysed by Pd complexes) has been recognised as a useful synthetic organic reaction that forms a C(sp²)-C(sp²) bond. The catalytic cycle of the reaction involves the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. It migrates the aryl and alkenyl groups of boronic acid to Pd and produces a Pd-C bond. Many studies have investigated the mechanism of transmetalation. They elucidated the mechanism of the organometallic reaction and its role as a fundamental step in catalytic reactions. This perspective reviews studies on the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. Emphasis was laid on the structures and chemical properties of the intermediate Pd complexes and the effects of OH^- on the pathways of the catalytic Suzuki-Miyaura reaction. The reactions of arylboronic acids with Rh(I)-OH complexes were investigated, which are relevant to the mechanism of Rh-catalysed addition of aryl boronic acids to enones and aldehydes. Recent studies on the transmetalation of boronic acids with other late transition metals such as Fe(II), Co(I), Pt(II), Au(III), and Au(I) are presented with the related catalytic reactions and their utilisation in the synthesis of aromatic molecules and π-conjugated materials.

Competitive gold/nickel transmetalation

Transmetalation is a key method for the construction of element-element bonds. Here, we disclose the reactivity of [Ni^II(Ar)(I)(diphosphine)] compounds with arylgold(I) transmetalating agents, which is directly relevant to cross-coupling catalysis. Both aryl-for-iodide and unexpected aryl-for-aryl transmetalation are witnessed. Despite the strong driving force expected for Au-I bond formation, aryl scrambling can occur during transmetalation and may complicate the outcomes of attempted catalytic cross-coupling reactions.

Base-assisted synthesis of 4-pyridinate gold(I) metallaligands: a study of their use in self-assembly reactions

The synthesis of di- and tritopic gold(i) metallaligands of the type [(Au4-py)2(μ2-diphosphane)] (diphosphane = bis(diphenylphosphanyl)isopropane or dppip (1), 1,2-bis(diphenylphosphanyl)ethane or dppe (2), 1,3-bis(diphenylphosphanyl)propane or dppp (3) and 1,4-bis(diphenylphosphanyl)butane or dppb (4)) and [(Au4-py)3(μ3-triphosphane)] (triphosphane = 1,1,1-tris(diphenylphosphanylmethyl)ethane or triphos (5) and 1,3,5-tris(diphenylphosphanyl)benzene or triphosph (6)) from [(AuCl)2(μ2-diphosphane)] or [(AuCl)3(μ3-triphosphane)] and 4-pyridylboronic acid in the presence of Cs2CO3 has been conducted. Interestingly, when [(AuCl)2(μ2-dppm)] (dppm = bis(diphenylphosphanyl)methane) was used as a starting material, the cyclic tetranuclear gold(i) compound [(Au4-py)2(CH)2{μ2-Au(PPh2)2}2] (I) was obtained instead. All the products have been characterized by IR and multinuclear NMR spectroscopy, mass spectrometry and elemental analysis and in the case of 1, 3, 5 and I by X-ray crystallography, which showed the presence of aurophilic interactions in all of them. The obtained metallaligands have been used as building blocks in self-assembly reactions with cis-blocked palladium or platinum acceptor moieties producing [2 + 2] metallamacrocycles or trigonal bipyramidal (TBP) [2 + 3] metallacages in good yields. The photophysical properties of both the metallaligands and the corresponding assemblies have been investigated.

Simple Synthetic Routes to N-Heterocyclic Carbene Gold(I)-Aryl Complexes: Expanded Scope and Reactivity

The discovery of sustainable and scalable synthetic protocols leading to gold-aryl compounds bearing N-heterocyclic carbene (NHC) ligands sparked an investigation of their reactivity and potential utility as organometallic synthons. The use of a mild base and green solvents provide access to these compounds, starting from widely available boronic acids and various [Au(NHC)Cl] complexes, with reactions taking place under air, at room temperature and leading to high yields with unprecedented ease. One compound, (N,N'-bis[2,6-(di-isopropyl)phenyl]imidazol-2-ylidene)(4-methoxyphenyl)gold, ([Au(IPr)(4-MeOC₆ H₄ )]), was synthesized on a multigram scale and used to gauge the reactivity of this class of compounds towards C-H/N-H bonds and with various acids, revealing simple pathways to gold-based species that possess attractive properties as materials, reagents and/or catalysts.

Photophysical properties of organogold(i) complexes bearing a benzothiazole-2,7-fluorenyl moiety: selection of ancillary ligand influences white light emission

Herein we report three new gold(i) complexes with a benzothiazole-2,7-fluorenyl moiety bound through a gold–carbon σ-bond and either an N-heterocyclic carbene or organophosphine as ancillary ligands.

Bioactive and luminescent indole and isatin based gold(i) derivatives

Combination of bioactive indole and isatin derivatives with Au(i) affords highly cytotoxic metallic species even for cisplatin resistant leukemia cells (Jurkat-shBak).

Gold(I)-catalyzed Benzylation of (Hetero)aryl Boronic Acids with (Hetero)benzyl Bromides by the Strategy of a SN 2-type Reaction

Herein, the first example of gold-catalyzed benzylation of (hetero)aryl boronic acids with (hetero)benzyl bromides to give the corresponding cross-coupling products in moderate to good yields is reported. The reaction proceeds through a possible intermolecular S_N 2-type reaction pathway to give a wide variety of di(hetero)arylmethanes as the desired products. An intriguing reaction mechanism has been proposed on the basis of control experiments, ³¹ P-NMR spectroscopic detection and DFT calculations.

Mechanistic and asymmetric investigations of the Au-catalysed cross-coupling between aryldiazonium salts and arylboronic acids using (P,N) gold complexes

Aryldiazonium salts and arylboronic acids were coupled via three different pathways from (P,N)–AuCl complexes, with enantiomeric excesses up to 26%.

Kinetic Study of Carbophilic Lewis Acid Catalyzed Oxyboration and the Noninnocent Role of Sodium Chloride

In the present study, the oxyboration reaction catalyzed by IPrAuTFA in the presence and absence of NaTFA has been examined with kinetic studies, mass spectrometry, and ¹H NMR and ¹¹B NMR spectroscopy. Data from monitoring the reactions over the temperature range from 30 to 70 °C, the catalyst range from 1.3 to 7.5 mol %, and the NaTFA additive range from 2.5 to 30 mol % suggests a mechanism that involves rate-determining catalyst generation. Data from additive studies that replaced NaTFA with NaBARF (BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) or Bu₄NTFA as an alternative additive suggest that catalyst quenching from residual NaCl remaining from a one-pot substrate synthesis/reaction method is the cause of this effect, despite the low solubility of this NaCl byproduct in toluene. Material produced through an alternative, sodium chloride free substrate synthesis exhibited faster reaction rates, consistent with a change in rate-determining step that depended on the substrate synthesis route.

Triazole-gold promoted intermolecular propargyl alcohol addition to alkyne: the reaction cascade toward substituted allenes

Gold-catalyzed intermolecular propargyl alcohol addition to alkyne was achieved. The reaction of propargyl alcohol with a typical gold catalyst gave the hydration product almost exclusively. The triazole gold catalyst (TA-Au) successfully prevented the hydration, giving the vinyl ether followed by a 3,3-rearrangement. Synthetically useful substituted allenes were formed with high efficiency, substrate tolerability and chemoselectivity. Also, more than 95% chirality transfer from propargyl alcohol to allene was obtained.

NMR spectroscopy studies of electronic effects and equilibrium in the organogold-to-boron transmetalation reaction and studies towards its application to the alkoxyboration addition of boron-oxygen σ bonds to alkynes

Electronic effects in the transmetalation of an aryl group from gold to boron were investigated by NMR spectroscopy. The transmetalation reaction is more facile for increasingly electrophilic boron reagents and is in equilibrium under certain conditions. Observed tetracoordinate boronate compounds suggest a two-step, associative transmetalation reaction mechanism in which the organogold complex first delivers a nucleophilic phenyl group to the empty p orbital of boron. For certain substrates, this tetracoordinate intermediate decomposes to the tricordinate, final transmetallation product, and in others this tricordinate species remains in equilibrium with a tetracoordinate anionic boron compound. Experimental and theoretical investigations into the extension of this transmetalation reaction from a mechanistic step in our previously reported intramolecular gold-catalyzed addition of boron-oxygen σ bonds across alkynes to an intermolecular variant are discussed.2014 Elsevier Ltd. All rights reserved.

Cyclometalated (boroxinato)gold(iii) complexes from arrested transmetalation

Luminescent metallaboroxines of gold(iii) form in self-assembly reactions from trifluoroacetato precursors and alkyl or arylboronic 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.

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.