Visible-Light-Promoted AuIto AuIIIOxidation in Triazol-5-ylidene Complexes

Accessing Bioactive Hydrazones by the Hydrohydrazination of Terminal Alkynes Catalyzed by Gold(I) Acyclic Aminooxy Carbene Complexes and Their Gold(I) Arylthiolato and Gold(III) Tribromo Derivatives: A Combined Experimental and Computational Study

Hydrohydrazination of terminal alkynes with hydrazides yielding hydrazones 5-14 were successfully catalyzed by a series of gold(I) acyclic aminooxy carbene complexes of the type [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuCl, where R² = H, R¹ = Me (1b); R² = H, R¹ = Cy (2b); R² = t-Bu, R¹ = Me (3b); R² = t-Bu, R¹ = Cy (4b). The mass spectrometric evidence corroborated the existence of the catalytically active solvent-coordinated [(AAOC)Au(CH₃CN)]SbF₆ (1-4)A species and the acetylene-bound [(AAOC)Au(HC≡CPhMe)]SbF₆ (3B) species of the proposed catalysis cycle. The hydrohydrazination reaction was successfully employed in synthesizing several bioactive hydrazone compounds (15-18) with anticonvulsant properties using a representative precatalyst (2b). The DFT studies favored the 4-ethynyltoluene (HC≡CPhMe) coordination pathway over the p-toluenesulfonyl hydrazide (NH₂NHSO₂C₆H₄CH₃) coordination pathway, and that proceeded by a crucial intermolecular hydrazide-assisted proton transfer step. The gold(I) complexes (1-4)b were synthesized from the {[(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)]CH}⁺OTf^- (1-4)a by treatment with (Me₂S)AuCl in the presence of NaH as a base. The reactivity studies of (1-4)b yielded the gold(III) [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuBr₃ (1-4)c complexes upon reaction with molecular bromine and the gold(I) perfluorophenylthiolato derivatives, [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuSC₆F₅ (1-4)d, upon treatment with C₆F₅SH.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes

Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.

Hemilabile MIC^N ligands allow oxidant-free Au(I)/Au(III) arylation-lactonization of γ-alkenoic acids

Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Still, a deeper mechanistic understanding is needed for a rational design of these processes. Here we describe the synthesis of two Au(i) complexes bearing bidentated hemilabile MIC^N ligands, [Au^I(MIC^N)Cl], and their ability to stabilize square-planar Au(iii) species (MIC = mesoionic carbene). The presence of the hemilabile N-ligand contributed to stabilize the ensuing Au(iii) species acting as a five-membered ring chelate upon its coordination to the metal center. The Au(iii) complexes can be obtained either by using external oxidants or, alternatively, by means of feasible oxidative addition with strained biphenylene C_sp²–C_sp² bonds as well as with aryl iodides. Based on the fundamental knowledge gained on the redox properties on these Au(i)/Au(iii) systems, we successfully develop a novel Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid. The oxidative addition of the aryl iodide, which in turn is allowed by the hemilabile nature of the MIC^N ligand, is an essential step for this transformation.

A novel hemilabile MIC^N ligand-based Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid is presented.

Silica-Supported Phosphine-Gold Complexes as an Efficient Catalytic System for a Dearomative Spirocyclization

The combination of metal catalyst and inorganic silica frameworks provides a greener approach to recyclable catalysis. In this study, three phosphine-gold chloride complexes have been successfully covalently grafted onto chiral silica nanohelices. The resulting 3D ensembles showed chiroptical properties that allowed the monitoring of the supported ligands. The heterogeneous gold chloride catalysts in cooperation with silver triflate exhibited high reactivity in various reactions, especially in the spirocyclization of aryl alkynoate esters, for which a catalytic loading of 0.05 mol % could be employed. The heterogeneous catalysts could be easily recovered and recycled seven or eight times without any loss of efficiency. By adding more silver triflate, 25 cycles with full conversion were achieved owing to a complex catalytic system based on silica and metallic species.

Cu(ii)-alginate-based superporous hydrogel catalyst for click chemistry azide-alkyne cycloaddition type reactions in water

A novel sustainable hydrogel catalyst based on the reaction of sodium alginate naturally extracted from brown algae Laminaria digitata residue with copper(ii) was prepared as spherical beads, namely Cu(ii)-alginate hydrogel (Cu(ii)-AHG). The morphology and structural characteristics of these beads were elucidated by different techniques such as SEM, EDX, BET, FTIR and TGA analysis. Cu(ii)-AHG and its dried form, namely Cu(ii)-alginate (Cu(ii)-AD), are relatively uniform with an average pore ranging from 200 nm to more than 20 μm. These superporous structure beads were employed for the copper catalyzed [3 + 2] cycloaddition reaction of aryl azides and terminal aryl alkynes (CuAAC) via click chemistry at low catalyst loading, using water as a solvent at room temperature and pressure. The catalytic active copper(i) species was generated by the reduction of copper(ii) by terminal alkyne via the oxidative alkyne homocoupling reaction. The prepared catalysts were found to be efficient (85-92%) and regioselective by affording only 1,4-disubstituted-1,2,3-triazoles. They were also recoverable and reused in their dried form for at least four consecutive times without a clear loss of efficiency. A mechanistic study was performed through density functional theory (DFT) calculations in order to explain the regioselectivity outcome of Cu(ii)-alginate in CuAAC reactions. The analysis of the local electrophilicity (ω k) at the electrophilic reagent and the local nucleophilicity (N k) at the nucleophilic confirms the polar character of CuAAC. This catalyst has the main advantage of being sustainably ligand-free and recyclable.

Oxidative access via aqua regia to an electrophilic, mesoionic dicobaltoceniumyltriazolylidene gold(III) catalyst

A gold(III) complex with the hitherto most electron poor mesoionic carbene ligand is presented. Aqua regia was the oxidizing agent of choice for the synthesis of this unusual organometallic compound. The Au^III complex is redox-rich, and also acts as a catalyst for oxazole formation, delivering selectively a completely different isomer compared to its Au^I congener.

Hydroxyl-functionalized triazolylidene-based PEPPSI complexes: metallacycle formation effect on the Suzuki coupling reaction

We report the preparation and full characterization of a series of hydroxyl functionalized 1,2,3-triazolylidene-based PEPPSI complexes 2a-c and their catalytic application in the Suzuki cross coupling reaction of aryl chlorides/amides with boronic acids. Under basic reaction conditions, complexes 2a-c show a notable increase in their catalytic efficiency compared with two ether-wingtip functionalized PEPPSI analogues (3 and 4) and a commercially available NHC-Pd complex (5). The catalytic results suggest that deprotonation of the hydroxyl group in complexes 2a-c plays an important role in the overall process. Deprotonation of the alcohol moiety of complexes 2a-b with sodium tert-butoxide allows for the isolation of metallacycles 6a-b, which are proposed as the active species of cross coupling reactions.

Copper(ii) accelerated azide–alkyne cycloaddition reaction using mercaptopyridine-based triazole ligands

We report the synthesis, characterization and catalytic performance of a series of copper(ii) complexes supported by mercaptopyridine based traizole ligands.

1 H-1,2,3-Triazol-5-ylidenes: Readily Available Mesoionic Carbenes

Classical carbenes are usually described as neutral compounds featuring a divalent carbon with only six electrons in their valence shell. It was only in 1988 that our group prepared the first isolable example, in which the carbene center was stabilized by a push-pull effect, using a phosphino and a silyl substituent. In the last 30 years, a myriad of acyclic and cyclic push-pull and push-push carbenes, bearing different heteroatom substituents, have been isolated. Among them, the so-called N-heterocyclic carbenes (NHCs), which include cyclic (alkyl)(amino)carbenes (CAACs), are arguably the most popular. They have found a vast number of applications ranging from catalysis to material science, and even in medicine. In this Account, we focus on the synthesis, structure, electronic properties, coordination, and applications of a different class of stable cyclic carbenes, namely, 1 H-1,2,3-triazol-5-ylidenes. In contrast with NHCs and CAACs, these compounds have no reasonable canonical resonance forms that can be drawn showing a carbene without additional charges. According to the IUPAC, they belong to the family of mesoionic compounds and thus they are named mesoionic carbenes (MICs). In 2010, we prepared the first stable 1,2,3-triazol-5-ylidene, via a CuAAC reaction, followed by alkylation of the resulting 1,2,3-triazole, and deprotonation. Later, we synthesized more robust N3-arylated counterparts from 1,3-diarylated-1 H-1,2,3-triazolium salts. Both synthetic routes can be carried out in multigram scales, making these MICS readily available. Importantly, MICs do not dimerize which contrasts with NHCs that can give the corresponding Wanzlick-type olefin. This property leads to relaxed steric requirements for their isolation; even C-unsubstituted MICs can be stored for months in the solid state at room temperature. The practicality and easily scalable syntheses of MICs allow for the preparation of polycarbenes, such as bis(1,2,3-triazol-5-ylidenes) (i-bitz), the analogues of the well-known 2,2'-bipyridines (bpy). MIC-transition metal complexes are excellent precatalysts for variety of chemical transformations, which include hydrohydrazination of alkynes, olefin metathesis, reductive formylation of amines with carbon dioxide and diphenylsilane, hydrogenation and dehydrogenation of N-heteroarenes in water, cycloisomerization of enynes, asymmetric Suzuki-Miyaura cross-coupling, and water oxidation (WO) reactions. Besides their catalytic applications, MIC-transition metal complexes have found applications in material sciences as exemplified by the preparation of the first iron(III) complex that is luminescent at room temperature. The peculiar properties of mesoionic triazolylidenes, combined with their enhanced stability, position them as excellent candidates to address some current challenges such as access to high-oxidation-state 3d metal complexes, the stabilization of highly reactive main group elements, the stabilization of nanoparticles, the preparation of efficient catalysts and photosensitizers based on earth-abundant transition metals, and the functionalization of self-assembled monolayers (SAMs) on gold.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Synthesis and characterization of a gold(i) bis(triazolylidene) complex featuring a large [(TpMe2)2K] anion

We report the synthesis, charaterization and catalytic performance of a unique bis(triazolylidene) gold(i) complex featuring a large [(Tp^Me2)₂K] anion.