Stoichiometric Silver Research Articles

Direct Heterocycle C-H Alkenylation via Dual Catalysis Using a Palladacycle Precatalyst: Multifactor Optimization and Scope Exploration Enabled by High-Throughput Experimentation.

One of the major challenges in developing catalytic methods for C-C bond formation is the identification of generally applicable reaction conditions, particularly if multiple substrate structural classes are involved. Pd-catalyzed direct arylation reactions are powerful transformations that enable direct functionalization of C-H bonds; however, the corresponding direct alkenylation reactions, using vinyl (pseudo) halide electrophiles, are less well developed. Inspired by process development efforts toward GSK3368715, an investigational active pharmaceutical ingredient, we report that a Pd(II) palladacycle derived from tri-tert-butylphosphine and Pd(OAc)2 is an effective single-component precatalyst for a variety of direct alkenylation reactions. High-throughput experimentation identified optimal solvent/base combinations for a variety of HetAr-H substrate classes undergoing C-H activation without the need for cocatalysts or stoichiometric silver bases (e.g., Ag2CO3). We propose this reaction proceeds via a dual cooperative catalytic mechanism, where in situ-generated Pd(0) supports a canonical Pd(0)/(II) cross-coupling cycle and the palladacycle effects C-H activation via CMD in a redox-neutral cycle. In all, 192 substrate combinations were tested with a hit rate of approximately 40% and 24 isolated examples. Importantly, this method was applied to prepare a key intermediate in the synthesis of GSK3368715 on multigram scale.

Nonstoichiometry, structure and properties of nanocrystalline oxides, carbides and sulfides

The results of recent experimental studies of the influence of the size of oxide, carbide and sulfide nanoparticles on changes in their nonstoichiometry are analyzed and generalized. In relation to intrinsically nonstoichiometric titanium oxides and niobium and vanadium carbides, on the one hand, and stoichiometric silver sulfides, on the other hand, it is shown that a decrease in the particle size to the nano scale has a strong impact on the chemical composition of nanocrystalline compounds, viz., it leads to the occurrence of or increase in nonstoichiometry. A conclusion is drawn that interplay between the particle size and nonstoichiometry is common to all solid nanocrystalline substances and should be taken into account in studies on the synthesis, structure and properties of these systems.The bibliography includes 260 references.

Scalable Rhodium(III)-Catalyzed Aryl C-H Phosphorylation Enabled by Anodic Oxidation Induced Reductive Elimination.

Transition metal catalyzed C-H phosphorylation remains an unsolved challenge. Reported methods are generally limited in scope and require stoichiometric silver salts as oxidants. Reported here is an electrochemically driven RhIII -catalyzed aryl C-H phosphorylation reaction that proceeds through H2 evolution, obviating the need for stoichiometric metal oxidants. The method is compatible with a variety of aryl C-H and P-H coupling partners and particularly useful for synthesizing triarylphosphine oxides from diarylphosphine oxides, which are often difficult coupling partners for transition metal catalyzed C-H phosphorylation reactions. Experimental results suggest that the mechanism responsible for the C-P bond formation involves an oxidation-induced reductive elimination process.

Scalable Rhodium(III)‐Catalyzed Aryl C−H Phosphorylation Enabled by Anodic Oxidation Induced Reductive Elimination

AbstractTransition metal catalyzed C−H phosphorylation remains an unsolved challenge. Reported methods are generally limited in scope and require stoichiometric silver salts as oxidants. Reported here is an electrochemically driven RhIII‐catalyzed aryl C−H phosphorylation reaction that proceeds through H2 evolution, obviating the need for stoichiometric metal oxidants. The method is compatible with a variety of aryl C−H and P−H coupling partners and particularly useful for synthesizing triarylphosphine oxides from diarylphosphine oxides, which are often difficult coupling partners for transition metal catalyzed C−H phosphorylation reactions. Experimental results suggest that the mechanism responsible for the C−P bond formation involves an oxidation‐induced reductive elimination process.

Effect of magnesium and silver doping on the thermoelectric performance of cast Mg2Sn alloys

High quality undoped and silver (Ag) doped Mg2Sn cast products with relatively less cracking were obtained using radio-frequency (RF) induction method under argon atmosphere. The thermoelectric properties of cast alloys were studied as a function of magnesium added up to ∼10.8 wt. % in the temperature range of 323 K (50 °C) to 723 K (450 °C). It was observed that the electrical and thermal conductivity at 323 K (50 °C) was increased by approximately 12 and 4 times as the magnesium concentration changed from 2 to 10.8 wt.%. The lattice thermal conductivity of the near stoichiometric alloy decreased by at least 2 times due to silver doping. However, such effect is faded with an increase in magnesium concentration. This was attributed to the probable increase in the electronic contribution due to metallic magnesium phase. Interestingly, the electrical (and thermal) conductivity was irreversible during the first cycle of heating and cooling. The irreversibility in the conductivity was attributed to the inhomogeneous distribution of tin (Sn) in the Mg phase prior to heating. The Seebeck coefficient of the Ag doped alloy with 10.8 wt. % Mg was ∼120 μV/K and it was reduced to 55 μV/K after the first cycle of heating. However, the decrease in the Seebeck coefficient following cooling was not systematic across all the alloys investigated here. The inability to obtain a unique Seebeck coefficient in the cast alloys at ∼323 K (50 °C) was primarily due to the coarse grain size of Mg2Sn and Mg phases and their respective grain orientations. Consequently, a maximum ZT of 0.055 was obtained at 323 K. However, a ZT of the order of 0.015 can be readily obtained in near stoichiometric silver doped cast Mg2Sn alloys at high temperatures ∼723 K.

Electrochemical Cobalt-Catalyzed C-H Oxygenation at Room Temperature.

Electrochemical cobalt-catalyzed C-H functionalizations were achieved in terms of C-H oxygenation under mild conditions at 23 °C. The robust electrochemical C-H functionalization was characterized by ample substrate scope, whereas mechanistic studies provided support for a facile C-H cleavage. The electrochemical cobalt-catalyzed C-H oxygenation proved viable on arenes and alkenes with excellent levels of positional and diastereo-selectivity, avoiding the use of stoichiometric silver(I) oxidants under ambient conditions.

Silver-Mediated Base Pairs in DNA Incorporating Purines, 7-Deazapurines, and 8-Aza-7-deazapurines: Impact of Reduced Nucleobase Binding Sites and an Altered Glycosylation Position.

Formation of silver-mediated DNA was studied with oligonucleotides incorporating 8-aza-7-deazapurine, 7-deazapurine, and purine nucleosides. The investigation was performed on non-self-complementary duplexes with one or two modifications and self-complementary duplexes with an alternating dA-dT motif. Homo base pairs as well as base pair mismatches of dA analogues with dC and Watson-Crick pairs with dT were studied by stoichiometric silver ion titration and Tm measurements. N8 -Glycosylated 8-aza-7-deazaadenine forms silver-ion-mediated base pairs capturing two silver ions (low silver content) whereas regularly glycosylated 8-aza-7-deazapurine, 7-deazapurine (c7 Ad ), and dA do not form comparable structures. Stable silver-mediated "dA-dC" base pair mismatches were detected for all nucleosides. Two silver ions per base pair are bound by 8-aza-7-deazapurine whereas c7 Ad binds only one silver ion. The situation is different when the equivalents of silver ions were increased to the number of total base pairs. Surprisingly, in 12-mer duplexes as well as in related 25-mer duplexes every base pair consumed one silver ion.

Palladium(II)-Catalyzed Homocoupling of Oxazole/Thiazole in Absence of Silver Oxidant

Homocoupling of oxazole/thiazole via palladium-catalyzed C-H bond activation using oxone as an oxidant has been achieved in moderate to excellent yields with good functional group tolerance. No other additive or ligand was employed in this efficient reaction. Comparing to our previously reported homocoupling of oxazole/thiazole, the absence of stoichiometric silver oxidant renders this reaction more environment-benign and cost-effective. A preliminary mechanism involving a Pd Ⅱ /Pd Ⅳ catalytic cycle is also proposed. Further exploration of this environmental-benign oxidant on other categories of coupling reactions, as well as applications of this revised homocoupling to structural modification of oxazole/thiazole-containing natural products, are undergoing in our lab.

Palladium-catalyzed decarboxylative, decarbonylative and dehydrogenative C(sp2)-H acylation at room temperature.

Over the past few decades, an impressive array of C-H activation methodology has been developed for organic synthesis. However, due to the inherent inertness of the C-H bonds (e.g. ∼110 kcal mol-1 for the cleavage of C(aryl)-H bonds) harsh reaction conditions have been realized to overcome high energetic transition states resulting in a limited substrate scope and functional group tolerance. Therefore, the development of mild C-H functionalization protocols is in high demand to exploit the full potential of the C-H activation strategy in the synthesis of a complex molecular framework. Although, electron-rich substrates undergo electrophilic metalation under relatively mild conditions, electron-deficient substrates proceed through a rate-limiting C-H insertion under forcing conditions at high temperature. In addition, a stoichiometric amount of toxic silver salt is frequently used in palladium catalysis to facilitate the C-H activation process which is not acceptable from the environmental and industrial standpoint. We report herein, a Pd(ii)-catalyzed decarboxylative C-H acylation of 2-arylpyridines with α-ketocarboxylic acids under mild conditions. The present protocol does not require stoichiometric silver(i) salts as additives and proceeds smoothly at ambient temperature. A novel decarbonylative C-H acylation reaction has also been accomplished using aryl glyoxals as acyl surrogates. Finally, a practical C-H acylation via a dehydrogenative pathway has been demonstrated using commercially available benzaldehydes and aqueous hydroperoxides. We also disclose that acetonitrile solvent is optimal for the acylation reaction at room temperature and has a prominent role in the reaction outcome. Control experiments suggest that the acylation reaction via decarboxylative, decarbonylative and dehydrogenative proceeds through a radical pathway. Thus we disclose a practical protocol for the sp2 C-H acylation reaction.

Improved cyclization conditions to prepare 6-substituted pyrazolo[1,5-a]pyridines and pyrazolo[1,5-a]pyrazines using catalytic Ag(I) and Au(III) salts

An efficient synthesis of 6-substituted-pyrazolo[1,5-a]pyridines via an intramolecular cyclization of 1-amino-2-triethyl-silanylethynyl-pyridinium salts has been developed. The cyclization reaction proceeds in the presence of catalytic silver(I) and gold(III) salts to give 2-silyl pyrazolo[1,5-a]pyridines; while the use of stoichiometric silver(I) fluoride predominantly gives the des-silyl heterocycles.

Replacing a stoichiometric silver oxidant with air: ligated Pd(ii)-catalysis to β-aryl carbonyl derivatives with improved chemoselectivity

Air was employed as a green reoxidant of Pd(0), replacing stoichiometric and toxic silver salt, in the chelation-controlled Pd(ii)-modulated arylative enolization of prop-2-en-1-ols to acquire synthetically-important β-aryl carbonyl derivatives.

Synthesis, Characterization, Photoluminescence, and Simulations of a CCC-NHC-Supported Pt2Ag2 Mixed-Metal Cluster Containing a PtAg2 Metallacyclopropane

The pincer CCC-bis(NHC)-supported Pt(II) complex 2, (1,3-bis(N-((trimethylsilyl)methyl)imidazol-2-ylidene)-2-phenylene)chloroplatinum(II), was prepared and was reacted with excess or stoichiometric silver(I) trifluoroacetate. The reaction with excess yielded the Pt2Ag2 cluster 3, which to our knowledge contains the first unsymmetrically substituted example of a PtAg2 metallacyclopropane, as revealed by X-ray crystallography. A distorted-octahedral configuration was also found in the coordination sphere of one of the silver atoms. Multiple weak hydrogen bonds (WHB) including C–H···O, C–H···F, and C–F···π interactions were identified in the lattice of cluster 3. The unique structure of cluster 3 was simulated using density functional theory (DFT), which revealed that it was thermodynamically favored over the more expected configurations. The stoichiometric reaction with Ag(OCCF3) afforded the simple ligand exchange product (1,3-bis(N-((trimethylsilyl)methyl)imidazol-2-ylidene)-2-phenylene)(trifluoroacetato)platinum(II) (4). The reaction of stoichiometric or excess AgOAc afforded only the monomeric product of simple ligand metathesis, (1,3-bis(N-((trimethylsilyl)methyl)imidazol-2-ylidene)-2-phenylene)(acetato)platinum(II) (5). This observation is consistent with the hypothesis that the weak hydrogen bond network and the increased Lewis acidity of Ag(OCCF3) are significant contributors to the structure of cluster 3. The photophysical properties of complexes 2–5 have been investigated. The emission color of the solution of cluster 3 was found to be concentration dependent.

From α-Arylation of Olefins to Acylation with Aldehydes: A Journey in Regiocontrol of the Heck Reaction

The Pd-catalyzed Mizoroki-Heck reaction of olefins with aryl halides, more often simply called the Heck reaction, was recently recognized with the 2010 Nobel Prize in chemistry. Although highly selective with electron-deficient olefins, which generally yield the linear β-arylated product exclusively, the Heck reaction is less satisfactory with electron-rich olefins. This substrate typically generates a mixture of both α- and β-arylated regioisomeric products, hampering wider application of the reaction in chemical synthesis. Pioneering studies by a number of researchers revealed that high α-regioselectivity could be obtained under Pd-diphosphine catalysis either through (i) the substitution of aryl triflates for halides or (ii) the addition of stoichiometric silver or thallium salts when aryl halides are used. Under these conditions, the arylation is believed to proceed via an ionic pathway. However, silver introduces added cost, thallium salts are toxic, and triflates are generally commercially unavailable, base sensitive, and thermally labile. Believing that the ionic pathway would be promoted in an ionic medium, in the early 2000s, we attempted the Pd-catalyzed arylation of the benchmark electron-rich olefin butyl vinyl ether with aryl bromides in an imidazolium ionic liquid. We were delighted to observe that highly regioselective α-arylation could readily be accomplished, with no need for silver additives, thallium additives, or aryl triflates. A range of other electron-rich olefins has since been shown to be viable as well. The high α-selectivity probably results from the high ionic strength of the medium, which facilitates the dissociation of halide anions from the [L(2)Pd(Ar)X] intermediate, channeling the arylation into the ionic pathway. Hydrogen bonding interactions may also play a role, however. We subsequently discovered that the α-arylation can indeed be significantly accelerated by a hydrogen bond donor salt, in both ionic liquids and common solvents. Evidence shows that the concentration of the cationic Pd(II)-olefin species along the ionic pathway is increased as a result of hydrogen bonding between the hydrogen bond donor and the halide anion. More recently, we reported that cheaper and greener alcohols allow the Heck arylation of electron-rich olefins to proceed in a much faster, productive, and totally α-regioselective manner, circumventing the need for an ionic medium or hydrogen bond donor salt. In particular, aryl chlorides with diverse properties have been demonstrated to be viable substrates for the first time. Significantly, it appears that ethylene glycol facilitates both the oxidative addition of ArCl to Pd(0) and the subsequent dissociation of chloride from Pd(II). A closely related reaction, acylation of aryl halides with aldehydes, was also developed. Proceeding via the intermediacy of an electron-rich enamine, this Pd-pyrrolidine cooperative catalysis affords alkyl aryl ketones in a straightforward manner, extending the Heck reaction from olefins to aldehydes.

Decarboxylative Cross-Coupling of Azoyl Carboxylic Acids with Aryl Halides

Decarboxylative cross-coupling of thiazole and oxazole-5-carboxylic acids with aryl halides is reported. Under a bimetallic system of catalytic palladium and a stoichiometric silver carbonate, a variety of (hetero)arylated azoles can be prepared in excellent yield.

Syntheses of complex sulfides AgCuS and Ag3CuS2 from the elements under hydrothermal conditions

The hydrothermal technique was applied to the syntheses of ternary metal sulfides, AgCuS and Ag3CuS2, using silver, copper, and sulfur powders as starting materials. AgCuS was obtained as a single phase when the starting ratio of Ag:Cu:S was 0.9:1.1:1 in water above 120°C for 10 h except for synthesis condition at 120°C for 60 h and inductively coupled plasma (ICP) analysis revealed that experimental silver and copper contents in products were Ag0.89Cu1.10S. At 120°C for 60 h, some unidentified peaks were observed in the powder X-ray diffraction (XRD) pattern and plate crystals of new compound were found in addition to AgCuS particles from the field emission scanning electron microscopy (FE-SEM) image. Pure Ag3CuS2 was also prepared by a simple reaction of stoichiometric silver, copper, and sulfur powders above 90°C, and the composition of products was Ag2.99Cu1.00S.

Palladium-catalysed cross-coupling of 2-trimethylsilylpyridine with aryl halides

Palladium-catalysed cross-coupling of 2-trimethylsilylpyridine with aryl halides in the presence of stoichiometric silver(I) oxide, and catalytic TBAF allows the rapid preparation of the corresponding pyridin-2-ylaryl compounds in moderate to good yields under mild thermal conditions.

A robust method for the Hiyama-type coupling of arylsiloxanes and disiloxanes with aryl halides

Palladium catalysed Hiyama-type coupling of aryl disiloxanes or aryl silanols with aryl halides in the presence of stoichiometric silver(I) oxide and catalytic TBAF allows the rapid preparation of biaryls in moderate to high yield under mild thermal or microwave irradiation conditions.

Effect of doping with transition and rare-earth metals on the electrical and optical properties of AgGaGe3Se8 single crystals

We have studied the electrical, optical, and thermoelectric properties of AgGaGe3Se8 single crystals grown by the Bridgman-Stockbarger method and doped with transition and rare-earth metals during growth. AgGaGe3Se8 is an anisotropic p-type semiconductor with a band gap of ∼2.25 eV (T ∼ 290 K), which decreases slightly, to ∼2.20 eV, on doping with Nd, Gd, and Er (N ∼ 5 × 1018 cm−3). The high concentration of stoichiometric silver vacancies (N ∼ 1.8 × 1020 cm−3) and random distribution of the Ga and Ge atoms over the cation sites give rise to static disorder, so that the structure of AgGaGe3Se8 approaches that of disordered systems. This leads to the formation of electronic states in the band gap, intrinsic edge broadening, and, as a consequence, a drop in transmission. Depending on the concentration and nature of the dopant, doping of AgGaGe3Se8 may lead to structural ordering and bleaching or may increase the destructive factor at high doping levels (N > 1018 cm−3). 60Co gamma irradiation to a dose of 1.2–1.3 kGy reduces the absorption coefficient of AgGaGe3Se8 crystals doped with high concentrations of Mn and Cu (N > 5 × 1018 cm−3). A model is proposed which explains the observed effects.

Two Methods for Directortho-Arylation of Benzoic Acids

Two new palladium-catalyzed methods for the direct ortho-arylation of free benzoic acids have been developed. The first method employs stoichiometric silver acetate for iodide removal, aryl iodide as the coupling partner, and acetic acid solvent. This method is applicable to the arylation of electron-rich to moderately electron-poor benzoic acids and tolerates chloride and bromide substituents on both coupling partners. The second method involves the use of aryl chloride, cesium carbonate base, n-butyl-di-1-adamantylphosphine ligand, and DMF solvent and is suitable for both electron-rich and electron-poor benzoic acids. Mechanistic studies of the second method point to the heterolytic C-H bond cleavage as the rate-determining step.

Ortho‐Arylation of Benzamides.

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