Catalyst Ligand Research Articles

Copper-Catalyzed Intramolecular Aldehyde-Ketone Nucleophilic Additions for the Synthesis of Chromans Bearing a Tertiary Alcohol Motif.

The synthesis of chroman-3-ol derivatives via intramolecular nucleophilic additions has been established. Aldehydes can be used as alkyl carbanion equivalents via reductive polarity reversal which is facilitated by a copper catalyst and N-heterocyclic carbene ligand under mild conditions. The key to success is the difference in reaction activity between aldehydes and ketones. Finally, this methodology also can be used to construct other cyclic structures containing tertiary alcohols including tetraline, cyclohexane, indan, and 9,10-dihydrophenanthrene.

Data-Informed Discovery of High-Performance Cu-Ligand Catalysts for Acetylene Hydrochlorination

Understanding the relationship between catalyst structure and performance is essential for the rational design of active sites for high-performance Cu-ligand catalysts for acetylene hydrochlorination. Herein, two descriptors are informed by data to screen efficient ligands and predict high-performance Cu-ligand catalysts: (1) the reduction potential of Cu2+-Cu+ and catalytic performance were correlated in a volcano-like manner; (2) the C≡C bond length of C2H2 and catalytic performance were linearly related within the Cu2+-Cu+ reduction limit. By using the two descriptors, the MPPO (3-methyl-1-phenyl-2-phospholene 1-oxide) ligand was successfully identified and developed into an effective Cu-MPPO catalyst. The conversion of C2H2 of 96.11 % and the selectivity of VCM of 99 % were obtained under the conditions of 180 h−1, 180 °C, and VHCl: VC2H2 = 1.2. The C2H2 conversion was still above 90 % after running for more than 600 h at 50 h−1. The excellent activity and exceptional stability of the Cu-MPPO catalyst validate the developed model for optimal ligand selection. Detailed structural characterization and density function theory calculations of the designed catalysts were performed to reveal the relationship between catalyst structure and activity. The results indicated that the addition of MPPO altered the local microenvironment of Cu. The simultaneous increased electron density around Cu and Cl resulted in strengthened HCl adsorption and weakened acetylene adsorption, inhibiting the generation of carbon deposition. In addition, MPPO contributed to suppressing the reduction of Cu species, thus stabilizing the active species. This research provides valuable insights and reference strategies for screening efficient ligands in Cu-ligand catalysts.

Enantioselective Synthesis of Benzimidazole Atropisomers Featuring a N‐N Axis†

Comprehensive SummaryAtroposelective synthesis of N‐N atropisomers is an emerging area but remains underexplored; in particular, the synthesis of N‐N benzimidazole atropisomers is still unprecedented. Herein, the first enantioselective synthesis of N‐N benzimidazole atropisomers via the palladium‐catalyzed de novo construction of benzimidazole skeleton is presented. With readily available palladium catalyst and biphosphine ligand, a broad range of nonbiaryl benzimidazole and indole‐benzimidazole atropisomers were conveniently accessed in high yields and with excellent enantioselectivities. Significantly, these N‐N benzimidazole atropisomers showed great antitumor activity and selectivity to breast cancer MCF‐7 cells. The simple catalytic system, broad substrate scope, high enantioselectivity, and good bioactivity make this approach highly attractive.

Surface-Modified Pd/CeO2 Single-Atom Catalyst Shows Increased Activity for Suzuki Cross-Coupling.

Single-atom catalysts (SACs) comprise catalytically active atoms dispersed on supports; they combine the high activity and site uniformity of homogeneous catalysts with the ease of separability of heterogeneous catalysts. However, SACs lack fine control over the active site, provided by ligands in homogeneous catalysts. In this work, we demonstrate that modification of the support with an organic monolayer is a viable approach to improving the catalytic performance. The addition of catechol-type monolayers to a Pd/CeO2 SAC increases its catalytic activity for Suzuki cross-coupling, a central reaction in the synthesis of fine chemicals and pharmaceuticals. Kinetic trials reveal that the coating reduces the activation energy from 49 ± 9 to 22 ± 5 kJ/mol and produces a 4-fold rate enhancement at 25 °C, an effect we attribute to π-π interactions between the reactant and the catechol coating. Further development of this approach could vastly increase the utility of SACs in organic synthesis.

(3+2) Annulation of 4‐Acetoxy Allenoate with Aldimine Enabled by AgF‐Assisted P(III)/P(V) Catalysis

AbstractA phosphine‐catalyzed (3+2) annulation of 4‐acetoxy allenoate and aldimine with the assistance of AgF is described. The success of this reaction hinges on the metathesis between the enolate‐phosphonium zwitterion and AgF, leading to a key intermediate comprising of silver enolate and a fluorophosphorane P(V)‐moiety. The former is able to undergo a Mannich reaction with aldimine, whereas the latter initiates a cascade sequence of AcO‐elimination/aza‐addition, thus furnishing the P(III)/P(V) catalysis. By taking advantage of the silver enolate, a preliminary attempt at an asymmetric variant was conducted with the combination of an achiral phosphine catalyst and a chiral bis(oxazolinyl)pyridine ligand (PyBox), giving moderate enantioselectivity.

(3+2) Annulation of 4-Acetoxy Allenoate with Aldimine Enabled by AgF-Assisted P(III)/P(V) Catalysis.

A phosphine-catalyzed (3+2) annulation of 4-acetoxy allenoate and aldimine with the assistance of AgF is described. The success of this reaction hinges on the metathesis between the enolate-phosphonium zwitterion and AgF, leading to a key intermediate comprising of silver enolate and a fluorophosphorane P(V)-moiety. The former is able to undergo a Mannich reaction with aldimine, whereas the latter initiates a cascade sequence of AcO-elimination/aza-addition, thus furnishing the P(III)/P(V) catalysis. By taking advantage of the silver enolate, a preliminary attempt at an asymmetric variant was conducted with the combination of an achiral phosphine catalyst and a chiral bis(oxazolinyl)pyridine ligand (PyBox), giving moderate enantioselectivity.

Environmentally Friendly Synthesis and Self‐Catalytic Hydrolysis of Triazole‐Modified Organosilanes for Polysiloxane Production

AbstractOrganotrialkoxysilanes are unique compounds that play an important role in modern science and industry. Their structure determines their varied use both in production and academic research. The development of the synthesis of organotrialkoxysilanes and polymers based on them, taking into account environmental realities, is an important task. In our work, we present an original approach for the preparation of organotriethoxysilanes, starting from azidoalkyltriethoxysilanes and various types of substrates containing a terminal triple bond, by the azide‐alkyne cycloaddition (CuAAC) mechanism without the use of solvents, catalyst ligands, and amines. We also found the self‐catalyzing effect of the triazole fragment resulting from the azide‐alkyne cycloaddition in the reaction of hydrolysis and condensation of alkoxy groups, which made it possible to obtain a series of silsesquioxane products only by adding water, without the use of catalysts. As a result, a new, original scheme for the preparation of organosilicon organotrialkoxysilanes of a monomeric structure with their subsequent transformation into silsesquioxane polymers under “green” conditions is built.

A soluble Salen-DOPO flame retardant for efficiently improving PBAT/PLLA film

Organic catalyst ligand Salen exhibits sound catalytic activity in ring-opening polymerization, but few flame retardations involve it. In the work, the Salen was synthesized and coupled with 9, 10-dihydro-9-oxa-10-phosphate-10-oxide (DOPO) to make a SD flame retardant that could dissolve in various organic solvents at room temperature. It was integrated into poly (butylene adipate phthalate)/poly(L-lactic acid) (PBAT/PLLA) film by solution casting. When the mass ratio of SD to polymers was 15 wt%, the composite film showed the ability to form carbon, the limiting oxygen index (LOI) was 27.5 %, and the UL-94 test reached the V-0 rating. The total heat release (THR) and total smoke production (TSP) decreased to 26.58 MJ/m2 and 7.57 m2, respectively. The multidimensional and biphasic action contributed to excellent suppression of flume and flame retardations. The tensile strength and elongation at the break increased to 26.95 MPa and 248.64 %, which were higher than those of PBAT/PLLA. SD also enhanced the hydrophobicity of PBAT/PLLA. The interesting work would open up a new scene for the flame retardation sciences and the development of biodegradable polymers.

Mechanistic Study on Steric Activity Interplay of Olefin/Polar Monomers for Industrially Selective Late Transition Metal Catalytic Reactions.

A significant issue in developing metal-catalyzed plastic polymer materials is obtaining distinctive catalytic characteristics to compete with current plastics in industrial commodities. We performed first-principle DFT calculations on the key insertion steps for industrially important monomers, vinyl fluoride (VF) and 3,3,3-trifluoropropene (TFP), to explain how the ligand substitution patterns affect the complex's polymerization behaviors. Our results indicate that the favorable 2,1-insertion of TFP is caused by less deformation in the catalyst moiety of the complexes in contrast to the 1,2-insertion mode. In contrast to the VF monomer, the additional interaction between the fluorine atoms of 3,3,3-trifluoropropene and the carbons of the catalyst ligands also contributed to favor the 2,1-insertion. It was found that the regioselectivity of the monomer was predominated by the progressive alteration of the catalytic geometry caused by small dihedral angles that were developed after the ligand-monomer interaction. Based on the distribution of the 1,2- and 2,1-insertion products, the activity and selectivity were influenced by the steric environment surrounding the palladium center; thus, an increased steric bulk visibly improved the selectivity of the bulkier polar monomer (TFP) during the copolymerization mechanism. In contrast, better activity was maintained through a sterically less hindered Pd metal center; the calculated moderate energy barriers showed that a catalyst with less steric hindrance might provide an opportunity for a wide range of prospective industrial applications.

Half-Sandwich d6-Metal (CoIII, RhIII, IrIII, RuII)-Catalyzed Enantioselective C–H Activation

AbstractTransition-metal-catalyzed enantioselective C–H activation provides a straightforward strategy to synthesize chiral molecules from readily available sources. In this graphical review, we summarize the progress on half-sandwich d6-metal (CoIII, RhIII, IrIII, RuII)-catalyzed enantioselective C–H functionalization reactions. The review is categorized according to the type of metal catalyst and chiral ligand employed. Representative enantio-determining models and catalytic cycles are presented.

Synergistic effect between monophosphine species for regioselective hydroformylation of olefin with CO2

CO2 as a substitute for CO in hydroformylation is promising for alleviating greenhouse effect. Phosphine ligand has a significant impact on the regioselectivity of olefin hydroformylation with CO2. Compared with complex and expensive bisphosphine ligands, easily available and inexpensive monophosphine ligands are potential for widespread application especially in industrial hydroformylation. However, the regioselectivity of monophosphine ligand catalysts is still unsatisfactory. Herein, we proposed synergistic effect existing in two types of monophosphine species, which was copolymerized in a concise manner. The single-atom catalysts (Rh-POL-PPO&PPh3) obtained through such a strategy, are efficient for olefins/CO2 hydroformylation with high regioselectivity (l/b = 18.4) and conversion (94.9 %), which is comparable to diphosphine ligand. The characterization and DFT calculations reveal that high hydroformylation regioselectivity is attributed to the synergistic effect between two monophosphine species, which endows the catalyst with better configuration of active sites, and can promote the association of Rh with the terminal carbon of olefin.

Synthesis of β-Enaminones via Palladium-catalyzed Dehydrogenative β-Amination of Saturated Ketones

Abstract A palladium-catalyzed method was developed for the synthesis of β-enaminones from saturated ketones using allyl acetate. In this reaction, allyl acetate acts as a hydrogen scavenger to α,β-dehydrogenate saturated ketones in the presence of a palladium catalyst and diamine ligand. This reaction achieved internal β-carbon–nitrogen bond formation for the first time via the dehydrogenation/conjugate addition protocol, eliminating the need to isolate unstable α,β-unsaturated ketones generated in the reaction.

Electronic Structures of Nickel(II)-Bis(indanyloxazoline)-dihalide Catalysts: Understanding Ligand Field Contributions That Promote C(sp2)-C(sp3) Cross-Coupling.

NiII(IB) dihalide [IB = (3aR,3a'R,8aS,8a'S)-2,2'-(cyclopropane-1,1-diyl)bis(3a,8a-dihydro-8H-indeno[1,2-d]-oxazole)] complexes are representative of a growing class of first-row transition-metal catalysts for the enantioselective reductive cross-coupling of C(sp2) and C(sp3) electrophiles. Recent mechanistic studies highlight the complexity of these ground-state cross-couplings but also illuminate new reactivity pathways stemming from one-electron redox and their significant sensitivities to reaction conditions. For the first time, a diverse array of spectroscopic methods coupled to electrochemistry have been applied to NiII-based precatalysts to evaluate specific ligand field effects governing key Ni-based redox potentials. We also experimentally demonstrate DMA solvent coordination to catalytically relevant Ni complexes. Coordination is shown to favorably influence key redox-based reaction steps and prevent other deleterious Ni-based equilibria. Combined with electronic structure calculations, we further provide a direct correlation between reaction intermediate frontier molecular orbital energies and cross-coupling yields. Considerations developed herein demonstrate the use of synergic spectroscopic and electrochemical methods to provide concepts for catalyst ligand design and rationalization of reaction condition optimization.

Catalytic Ammonia Oxidation Using Ammonia Solution under Electrochemical Conditions: Investigation on Axial Ligand of Ruthenium Catalysts

Abstract We have investigated catalytic ammonia oxidation using ruthenium complexes as catalysts under electrochemical conditions. Cyclic voltammetry and bulk electrolysis with an ammonia solution in MeCN are conducted in the presence of a catalytic amount of ruthenium complexes bearing a 2,2′-bipyridine-6,6′-dicarboxylate ligand with various 6-substituted isoquinolines and phthalazine as axial ligands. As a result, the ruthenium complex bearing phthalazines shows the highest catalytic activity at low applied potentials, where up to 319 equivalents of dinitrogen per catalyst are generated.

In situ selective deuterated (hydrogenated) aryl alkynes to alkenes on Pd/Ni(OH)2-nickel foam

The preparation of deuterated alkenes via electrochemistry from a readily available deuterated source has the advantages of mild reaction conditions, sustainability, and cost-effectiveness. Hydrogen isotopic exchange is a simple method to introduce deuterium atoms into molecules without prior functionalization. However, this process typically requires complex ligand catalysts, costly sources of deuterium, and strong bases and acids to activate the robust C-H bonds. The deuterated incorporation sites and their corresponding numbers are difficult to control. In this study, an in-situ electrochemical deuteration system is reported that is based on a Pd/Ni(OH)2 nanoflake catalyst grown on a commercial nickel foam conductive substrate was used as the cathodic working electrode. A room-temperature, one-pot, two-step transformation of 4-ethynylaniline to deuterated 4-vinylanilin was achieved with a conversion rate of up to 96%, selectivity of 98%, and a deuterium ratio of 99% using D2O as the deuterium source. The interaction between Pd and Ni(OH)2-NF promoted the rapid cleavage of D2O during the semi-deuteration of aryl alkynes. The key deuterium and carbon radical intermediates were confirmed using electron paramagnetic resonance spectroscopy. A wide range of substrates and easy access to hydrogenated alkenes highlight the promising applications.

Ion-imprinted magnetic adsorbents for the selective capture of Cu(II) and their cascade application as heterogeneous recyclable catalysts for Ullmann and Glaser coupling reactions

The selective capture and efficient reuse of Cu(II) from contaminated seawater requires a multifunctional adsorbent with a high Cu(II) selectivity and magnetic properties to facilitate recycling. Moreover, the cascade application of the recycled adsorbent is important but has not been extensively investigated. Accordingly, in this study, ion-imprinted magnetic adsorbents, particularly, copper ion-imprinted polymers (Fe3O4@mSiO2@CIIP) were prepared using magnetic mesoporous silica microspheres (Fe3O4@mSiO2) as carriers, with organosilicon containing hydroxyl and Schiff bases as the polymer backbone to selectively collect Cu(II) from aqueous media. The adsorption isotherms, kinetics, selectivity, and mechanisms were examined under various adsorption conditions. Fe3O4@mSiO2@CIIP-2 shows high selectivity and fast adsorption kinetics, and the equilibrium adsorption amount of Cu(II) is 263.2 mg/g within 30 min. The pseudo-second-order model and the Langmuir model are supported by the adsorption kinetics and isotherms, respectively. Particularly, the adsorption amount of Fe3O4@mSiO2@CIIP-2 could be reused eight times without substantial loss. Additionally, specific functional monomers bound to Fe3O4@mSiO2@CIIP-2 could both selectively adsorb Cu and serve as functional ligands for catalysts that can heterogeneously catalyze the Ullmann and Glaser coupling reactions after adsorbing Cu(II).

Ferrocene-based catalysts for in-situ hydrothermal upgrading of heavy crude oil: Synthesis and application

In this work, the effect of synthesized Ferrocene-based ligand catalysts, including Mono-, Di- and Tri-Ferrocene, as additives for promoting the catalytic in-situ hydrothermal upgrading of Tatarstan (Russia) heavy crude oil was studied. In addition, the changes in composition of upgraded oil samples as well as the in-situ transformation and characteristics of used catalysts were evaluated in detail using comprehensive analyzing techniques. All of the catalytic and non-catalytic hydrothermal upgrading experiments were carried out under the steam stimulation conditions at 300 °C, 72 bar (1044.27 psi) and 24 h of reaction time. The results show that introducing Ferrocene-based ligand catalysts to the upgrading system promotes the in-situ upgrading of heavy crude oil and improves the physical and chemical properties of upgraded oil samples. The maximum viscosity reduction was observed in presence Tri-Ferrocene and reached around 40% compared to non-catalytic hydrothermal upgrading. In addition, the upgrading performance also reflects in increase of H/C ratio, reduction of sulfur content, increase of saturates and aromatics, and decrease of the content of resins and asphaltenes as well as a noticeable change (increase) in the light fractions of C10-C20 are observed for all upgraded oil samples. Mono-Ferrocene as an oil-soluble catalyst mainly transformed to the particles of Fe3O4 (Magnetite) and FeS, which can act as active forms of catalysts and accelerate the hydrothermal conversion of heavy crude oil and its heavy fractions including resins and asphaltenes. These transformations were confirmed using XRD, SEM-EDX and Mössbauer analyzing techniques. Generally, the good catalytic performance of used ferrocene-based ligand catalysts as well as their low cost and commercial availability makes them great potential catalysts for improving the efficiency of steam injection for heavy oil production and in-situ upgrading.

Reducing O2 sensitivity in electrochemical nitric oxide releasing catheters: An O2-tolerant copper(II)-ligand nitrite reduction catalyst and a glucose oxidase catheter coating

Electrocatalytic nitric oxide (NO) generation from nitrite (NO2–) within a single lumen of a dual-lumen catheter using CuII-ligand (CuII-L) mediators have been successful at demonstrating NO’s potent antimicrobial and antithrombotic properties to reduce bacterial counts and mitigate clotting under low oxygen conditions (e.g., venous blood). Under more aerobic conditions, the O2 sensitivity of the Cu(II)-ligand catalysts and the reaction of O2 (highly soluble in the catheter material) with the NO diffusing through the outer walls of the catheters results in a large decreases in NO fluxes from the surfaces of the catheters, reducing the utility of this approach. Herein, we describe a new more O2-tolerant CuII-L catalyst, [Cu(BEPA-EtSO3)(OTf)], as well as a potentially useful immobilized glucose oxidase enzyme-coating approach that greatly reduces the NO reactivity with oxygen as the NO partitions and diffuses through the catheter material. Results from this work demonstrate that very effective NO fluxes (>1*10−10 mol min−1 cm−2) from a single-lumen silicone rubber catheter can be achieved in the presence of up to 10% O2 saturated solutions.

Recent Advances in N-Heterocyclic Carbene Coinage Metal Complexes in A3-Coupling and Carboxylation Reaction

Owing of their accessibility and wide range of reactivities, alkynes make for fascinating building blocks. Either a selective alkyne carbon-carbon triple bond reaction or activation of the terminal alkyne C-H bond may be employed to functionalize them. Monocationic coinage metal complexes with a d10 electronic configuration are effective catalysts for alkyne activation. Silver(I) and gold(I) N-heterocyclic (NHC) systems are emerging as promising catalysts in multicomponent alkyne activation reactions; this review paper focuses on A3 (aldehyde-amine-alkyne)-coupling reaction and carbon dioxide fixation, furnishing a systematic overview of the scientific advances achieved during the last two decades. This study will carefully compare the corresponding silver and gold complexes employed in the two processes. The differences in reaction routes brought about by the catalyst ligand structure will be investigated with an emphasis on evaluating the benefits provided by the easily tuneable NHC backbone, in terms of chemo- and stereo-selectivity.

Asymmetric hydrogenation of 1,1-diarylethylenes and benzophenones through a relay strategy

Homogenous transition-metal catalysts bearing a chiral ligand are widely used for asymmetric hydrogenation of unsaturated compounds such as olefins and ketones, providing efficient concise access to products with chiral carbon centers. However, distinguishing the re and si prochiral faces of a double bond bearing two substituents that are sterically and electronically similar is challenging for these catalysts. Herein, we report a relay strategy for constructing compounds with a chiral gem-diaryl carbon center by means of a combination of selective arene exchange between 1,1-diarylethylenes or benzophenones with (naphthalene)Cr(CO)3 and subsequent asymmetric hydrogenation. During the hydrogenation, the Cr(CO)3 unit facilitate differentiation of the two prochiral faces of the substrate double bond via formation of a three-dimensional complex with one of the aromatic rings by selective arene exchange. Density functional theory calculations reveal that during the hydrogenation, chromium coordination affected π–π stacking of the substrate and the catalyst ligand, leading to differentiation of the prochiral faces.