Carboxylate Catalysts Research Articles

Unactivated Pd-catalyzed oxidative cross-coupling reactions of arenes with benzene: a theoretical and experimental study

Oxidative C–H bond activated palladium-catalyzed aromatic cross-coupling reactions offer advantages of fewer required synthetic steps but disadvantages of loss of inherent regioselectivity and selectivity. In one proposed catalytic cycle, each arene adds to a palladium carboxylate catalyst according to a concerted metalation deprotonation mechanism, followed by reductive elimination to couple the arenes and the replenishment of the catalyst by an oxidant. Using this proposed mechanism, molecular structures and free energy profiles were calculated for coupling of benzene with a range of heteroarenes containing various substituents and with several Pd-carboxylate catalysts. Reaction kinetics were analysed using the energetic span model, predicting turnover frequencies (TOFs) of heterocoupling (HB and BH) and homocoupling (HH and BB) pathways and assessing degrees of turnover frequency control (XTOFs). More electron-withdrawing carboxylate ligands and more electron-donating heteroarene substituents decrease activation and reaction free energies of addition steps, with C-2 more favourable than C-3. When heteroarene and catalyst choice are particularly favourable, the first association can become exergonic, causing a significant change in catalytic kinetics. When not exergonic, free energy for addition of benzene is almost always higher, leading to large energetic spans and slower TOFs (BB < HB < BH < HH). Non-exergonic pathways are also faster with more electron-withdrawing ligands and more electron-donating heteroarene substituents because of lower energy addition steps and a smaller energetic span. For exergonic associations, the low-energy arene–catalyst intermediate creates a larger energetic span, slowing down the HH and HB pathways. More electron-withdrawing ligands and more electron-donating substituents increase the relative TOF for BH, and the reaction requires only small excesses of benzene to induce heterocoupling.

Aerobic Oxidative Carboxylation of Styrene Over Cobalt Catalysts: Integrated CO2 Capture and Conversion.

The direct synthesis of cyclic carbonates through oxidative carboxylation of alkenes using CO2 and O2 offers a sustainable and carbon-neutral method for CO2 utilization, which is, however, still a largely unexplored field. Here we develop a single-atom catalyst (SAC) Co-N/O-C as the earth-abundant metal catalyst for the oxidative carboxylation of styrene with CO2 and O2. Remarkably, even using the flue gas as an impure CO2 and O2 source, desired cyclic carbonate could be obtained with moderate productivity, which shows the potential for integrated CO2 capture and conversion, leveraging the high CO2 adsorption capacity of Co-N/O-C. In addition, the catalyst can be reused five times without an obvious decline in activity. Detailed characterizations and theoretical calculations elucidate the crucial role of single Co atoms in activating O2 and CO2, as well as controlling selectivity.

Enhanced photoelectric performance of ZnFe2O4 catalysts for oxidative carboxylation of styrene by tuning crystal planes and thermal and electrical conductivity

The photocatalytic oxidative carboxylation to yield cyclic carbonates was a promising approach for value-added utilization of olefins. Herein, various ZnFe2O4 (ZFO) based catalysts with different morphologies and supports were successfully prepared. Layered ZFO exposed highly active (422) crystal planes, which endowed the catalyst with narrow band gap and favorable electron transfer and charge separation, thereby promoting photocatalytic styrene epoxidation. Consequently, a 95 % selectivity for styrene oxide along with the complete conversion of styrene was achieved. Graphene with excellent electrical and thermal conductivity was compounded into ZFO based catalysts. On the one hand, ZFO/Graphene hybrids composed of 50 wt% graphene promoted fast charge transport in oxygen evolution reaction (OER), and the OER performance was comparable to the commercial RuO2 electrocatalyst. On the other, it exhibited significant photothermal effect and used into CO2 cycloaddition, delivering 95 % selectivity for styrene carbonate at a styrene oxide conversion of 95 %. As a result, a coupled photo-electro-heterogeneous catalytic system was proposed to yield styrene carbonate over ZFO based catalysts, including OER, styrene epoxidation and CO2 cycloaddition. This study provided an effective avenue for designing and fabricating ZFO based catalysts by crystal plane and support control.

Construction of efficient Pb(II) carboxylate catalysts for Oxygen and Hydrogen Evolution Reactions

The development of cost-effective and efficient electrocatalysts can solve the problem in the production of energy via water splitting reactions. In this work, we focused on two lead-based coordination polymers...

Gold(I)-Organic Frameworks as Catalysts for Carboxylation of Alkynes with CO2.

Construction of gold-based metal-organic frameworks (Au-MOFs) would bring the merits of gold chemistry into MOFs. However, it still remains challenging because gold cations are easily reduced to metallic gold under solvothermal conditions. Herein, we present the first example of Au-MOFs prepared from the networking of cyclic trinuclear gold(I) complexes by formal transimination reaction in a rapid (<15 min) and scalable (up to 1 g) fashion under ambient condition. The Au-MOFs feature uniform porosity, high crystallinity, and superior chemical stability toward base (i.e., 20 M NaOH). With open Au(I) sites in the skeleton, the Au-MOFs as heterogeneous catalysts delivered good performance and substrate tolerance for the carboxylation reactions of alkynes with CO2. This work demonstrates a facile approach to reticularly synthesize Au-MOFs by combining the coordination and dynamic covalent chemistry.

Asymmetric reductive carboxylation of N-(2-bromophenyl)-N-methylmethacrylamide with CO2 catalyzed by nickel complex: A theoretical study

To provide guiding information for developing efficient catalysts for CO2 carboxylation, we investigated the mechanism of asymmetric reductive carboxylation of N-(2-bromophenyl)-N-methylmethacrylamide with CO2 catalyzed by a nickel complex using density functional theory (DFT) calculations. The reaction was found to proceed via a multistep and elaborate mechanism. Additionally, CO2 exhibits a preference for inserting into the Ni(I) intermediate rather than Ni(II) intermediate. The noncovalent interaction (NCI) analysis reveals that the π-π stacking interactions and hydrogen bonding interactions lead to the stereoselectivity.

Amphiphilic polymeric nanoparticles enable homogenous rhodium-catalysed NH insertion reactions in living cells.

Rh-catalysed NH carbene insertion reactions were exported to living cells with help of amphiphilic polymeric nanoparticles. Hereto, hydrophobic dirhodium carboxylate catalysts were efficiently encapsulated in amphiphilic polymeric nanoparticles comprising dodecyl and Jeffamine as side grafts. The developed catalytic nanoparticles promoted NH carbene insertions between α-keto diazocarbenes and 2,3-diaminonaphthalene, followed by intramolecular cyclisation to form fluorescent or biologically active benzoquinoxalines. These reactions were studied in reaction media of varying complexity. The best-performing catalyst was exported to HeLa cells, where fluorescent and cytotoxic benzoquinoxalines were synthesized in situ at low catalyst loading within a short time. Most of the developed bioorthogonal transition metal catalysts reported to date are easily deactivated by the reactive biomolecules in living cells, limiting their applications. The high catalytic efficiency of the Rh-based polymeric nanoparticles reported here opens the door to expanding the repertoire of bioorthogonal reactions and is therefore promising for biomedical applications.

Sequential Polymerization from Complex Monomer Mixtures: Access to Multiblock Copolymers with Adjustable Sequence, Topology, and Gradient Strength

Switchable polymerization shows considerable potential for simulating the molecular precision of natural biopolymers, such as nucleic acids or proteins, to synthesize highly sequence-controlled macromolecules but is mainly limited to three- and four-component systems. To expand the scope to systems with up to five components, we established a reactivity gradient among 12 monomers, including cyclic anhydrides, cyclic esters, and epoxides. Highly selective competitive anhydride/epoxide, self-switchable cyclic anhydride/epoxide/cyclic ester, and competitive cyclic ester/trimethylene carbonate copolymerizations were achieved using a simple alkyl metal carboxylate catalyst. Anhydrides gave access to gradient copolymers with reactivity ratios of 2 < r1 < 5, 0.7 > r2 > 0.3 giving medium-gradient copolymers and r1 > 400, r2 < 0.03 giving block copolymers. Further, the anhydride reactivity was predicted using 13C NMR chemical shifts. This comonomer library will allow more complex copolymer structures with adjustable sequence, topology, and gradient strength to be predicted and prepared.

Multidimensional Control of Repeating Unit/Sequence/Topology for One-Step Synthesis of Block Polymers from Monomer Mixtures.

Synchronously and thoroughly adjusting the chemical structure difference between two blocks of the diblock copolymer is very useful for designing materials but difficult to achieve via self-switchable alternating copolymerization. Here, we report self-switchable alternating copolymerization from a mixture of two different cyclic anhydrides, epoxides, and oxetanes, where a simple alkali metal carboxylate catalyst switches between ring-opening alternating copolymerization (ROCOP) of cyclic anhydrides/epoxides and ROCOP of cyclic anhydrides/oxetanes, resulting in the formation of a perfect block tetrapolymer. By investigating the reactivity ratio of these comonomers, a reactivity gradient was established, enabling the precise synthesis of block copolymers with synchronous adjustment of each unit's chemical structure/sequence/topology. Consequently, a diblock tetrapolymer with two glass transition temperatures (Tg) can be easily produced by adjusting the difference in chemical structures between the two blocks.

Atomically Dispersed Dual Metal Sites Boost the Efficiency of Olefins Epoxidation in Tandem with CO2 Cycloaddition.

Tandem catalysis provides an economical and energy-efficient process for the production of fine chemicals. In this work, we demonstrate that a rationally synthesized carbon-based catalyst with atomically dispersed dual Fe-Al sites (ADD-Fe-Al) achieves superior catalytic activity for the one-pot oxidative carboxylation of olefins (conversion ∼97%, selectivity ∼91%), where the yield of target product over ADD-Fe-Al is at least 62% higher than that of monometallic counterparts. The kinetic results reveal that the excellent catalytic performance arises from the synergistic effect between Fe (oxidation site) and Al sites (cycloaddition site), where the efficient CO2 cycloaddition with epoxides in the presence of Al sites (3.91 wt %) positively shifts the oxidation equilibrium to olefin epoxidation over Fe sites (0.89 wt %). This work not only offers an advanced catalyst for oxidative carboxylation of olefins but also opens up an avenue for the rational design of multifunctional catalysts for tandem catalytic reactions in the future.

Molecular structure models of amorphous bismuth and cerium carboxylate catalyst precursors

As our societal need for materials and energy has grown, so has our need for catalyst processes in hydrogen production. A major function in these applications, for both homogenous and heterogeneous catalysis processes, is the synthesis of an active metal catalyst. It must first be soluble to control the physical properties of the metal being used. Recent work in metal precursors has begun to turn toward these metal carboxylate types of material. Structural models are proposed for bismuth 2-ethylhexanoate and 2,2-dimethyloctanoate and cerium 2-ethylhexanoate. The bismuth compounds have been characterized at different ratios of bismuth to carboxylate as solutions of the free acids. Their structures are most consistent with a Bi4(RCO2)12 motif where the Bi ions are arranged in a flattened tetrahedron with Bi – Bi distances of about 4.3 Å. There is evidence for Bi – O – Bi linkages at low free acid concentrations. The cerium compound is most consistent with a linear tetracerium molecule where the Ce – Ce distances repeat at about 4.3 Å out to 16.4 Å. The models were generated by analogy with known crystal structures and compared to high-energy x-ray scattering data. To further evaluate the models, DFT calculations were made, and the equilibrium geometries were compared. The vibrational spectra calculated from those geometries are presented and compared to the experimental results. Magnetization vs. temperature data was collected on the cerium compound, and its behavior was consistent with the proposed model. A geometrical approach to determining the dimensionality and relative positions of the metal ions in these structures is presented.

Heterogeneous catalytic carboxylation of terminal alkynes with CO2 over a copper(II)-based metal-organic framework catalyst

Developing efficient, inexpensive and robust heterogeneous catalysts for CO2 conversion is greatly important. In this study, a new stable copper(II)-based metal-organic framework [Cu(Fbtx)2Br2]n (denoted as CZU-7, CCDC-2165700, Fbtx = 1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenzene) was synthesized via a facile and fast microwave heating method. CZU-7 exhibits a two-dimensional layer structure with the 44-sql topology. This material was demonstrated to be an efficient heterogeneous catalyst for the direct carboxylation of 1-ethynylbenzene with CO2, and various propiolic acids were synthesized in moderate to good yields under optimized reaction conditions. Moreover, the catalyst could be easily recovered and reused five times without significant loss of catalytic activity.

Carboxylate Catalyzed Isomerization of β,γ-Unsaturated N-Acetylcysteamine Thioesters.

We demonstrate herein the capacity of simple carboxylate salts – tetrametylammonium and tetramethylguanidinium pivalate – to act as catalysts in the isomerization of β,γ‐unsaturated thioesters to α,β‐unsaturated thioesters. The carboxylate catalysts gave reaction rates comparable to those obtained with DBU, but with fewer side reactions. The reaction exhibits a normal secondary kinetic isotope effect (k 1H/k 1D=1.065±0.026) with a β,γ‐deuterated substrate. Computational analysis of the mechanism provides a similar value (k 1H/k 1D=1.05) with a mechanism where γ‐reprotonation of the enolate intermediate is rate determining.

Synthesis, characterization, ion-exchange, and catalytic properties of three isostructural copper(II) coordination polymers with a flexible bis(triazole) ligand

Ion-exchange processes are shown to occur in several coordination polymers (CPs) and their unique subclass of metal-organic frameworks (MOFs). The development of fast and economical routes to stable ion-exchange CP materials is important to broaden the range of applications for these materials. Herein, we report a greatly improved synthesis and coordinated anion exchange of three new copper(II) coordination polymers [Cu(Fbtx)2(NO3)2]n (1), [Cu(Fbtx)2(BF4)2]n (2) and [Cu(Fbtx)2(SO4)]n (3) (Fbtx ​= ​1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenzene) under the microwave-assisted hydrothermal conditions. The three Cu-CPs are crystallographically isostructural, exhibiting two-dimensional layered structures with the 44-sql topology. Complexes 1–3 display good thermal stability and excellent chemical stability in organic solvents and aqueous solutions with wide pH range of 1–13. The anion-exchange functions of the series of sql-type coordination networks were monitored by using powder X-ray diffraction, infrared spectroscopy and ion chromatography. The results revealed that these complexes can undergo anion-exchange under microwave irradiation, and the reversible exchange process depends on the size of the anions. These materials were demonstrated to be efficient heterogeneous catalysts for the direct carboxylation of 1-ethynylbenzene with CO2 under mild conditions, and various propiolic acids were synthesized in moderate to good yields under optimized reaction conditions. Moreover, the catalyst could be recycled up to five times with the retention of both catalytic activity and crystal structure.

One-step synthesis of sequence-controlled multiblock polymers with up to 11 segments from monomer mixture

Switchable polymerization holds considerable potential for the synthesis of highly sequence-controlled multiblock. To date, this method has been limited to three-component systems, which enables the straightforward synthesis of multiblock polymers with less than five blocks. Herein, we report a self-switchable polymerization enabled by simple alkali metal carboxylate catalysts that directly polymerize six-component mixtures into multiblock polymers consisting of up to 11 blocks. Without an external trigger, the catalyst polymerization spontaneously connects five catalytic cycles in an orderly manner, involving four anhydride/epoxide ring-opening copolymerizations and one L-lactide ring-opening polymerization, creating a one-step synthetic pathway. Following this autotandem catalysis, reasonable combinations of different catalytic cycles allow the direct preparation of diverse, sequence-controlled, multiblock copolymers even containing various hyperbranched architectures. This method shows considerable promise in the synthesis of sequentially and architecturally complex polymers, with high monomer sequence control that provides the potential for designing materials.

Synthesis and reactivity of 1-sulfonylcyclooctatriazoles.

Strained eight-membered cyclic alkynes undergo rapid inverse electron demand cycloaddition with sulfonyl azides to give the corresponding 1-sulfonylcyclooctatriazoles in excellent yield. Treatment of these sulfonyltriazoles with a chiral rhodium(II) carboxylate catalyst prompted transannular C-H bond insertion in good yield and with excellent ee, or 1,2-H shift.

Covalent Organic Framework Catalysts for Carboxylation of Terminal Alkynes

Covalent Organic Framework Catalysts for Carboxylation of Terminal Alkynes

Controlling Selectivity in the Synthesis of Z‐α,β‐Unsaturated Amidines by Tuning the N‐Sulfonyl Group in a Rhodium(II) Catalyzed 1,2‐H Shift

AbstractN2‐Sulfonyl‐α‐diazo amidines can be synthesized by the reaction of electron rich alkynyl amines with electron poor sulfonyl azides through 1,3‐dipolar cycloaddition that proceeds with perfect regioselectivity. In the presence of rhodium(II) carboxylate catalysts, denitrogenation occurs to give the corresponding metallocarbene but there are then two competing processes: 1,2–H shift and O‐transfer from the sulfonyl group to the metallocarbene center. The outcome can be controlled using an electron poor nitrobenzenesulfonyl group and large carboxylate rhodium ligands to select for 1,2–H shift, forming α,β‐unsaturated amidines in high yield and with excellent Z‐selectivity.

Rh2(II)-Catalyzed Intermolecular N-Aryl Aziridination of Olefins Using Nonactivated N Atom Precursors.

The development of the first intermolecular Rh2(II)-catalyzed aziridination of olefins using anilines as nonactivated N atom precursors and an iodine(III) reagent as the stoichiometric oxidant is reported. This reaction requires the transfer of an N-aryl nitrene fragment from the iminoiodinane intermediate to a Rh2(II) carboxylate catalyst; in the absence of a catalyst only diaryldiazene formation was observed. This N-aryl aziridination is general and can be successfully realized by using as little as 1 equiv of the olefin. Di-, tri-, and tetrasubstituted cyclic or acylic olefins can be employed as substrates, and a range of aniline and heteroarylamine N atom precursors are tolerated. The Rh2(II)-catalyzed N atom transfer to the olefin is stereospecific as well as chemo- and diastereoselective to produce the N-aryl aziridine as the only amination product. Because the chemistry of nonactivated N-aryl aziridines is underexplored, the reactivity of N-aryl aziridines was explored toward a range of nucleophiles to stereoselectively access privileged 1,2-stereodiads unavailable from epoxides, and removal of the N-2,4-dinitrophenyl group was demonstrated to show that functionalized primary amines can be constructed.

Dirhodium Carboxylate Catalysts from 2-Fenchyloxy or 2-Menthyloxy Arylacetic Acids: Enantioselective C-H Insertion, Aromatic Addition and Oxonium Ylide Formation/Rearrangement.

A new class of dirhodium carboxylate catalysts have been designed and synthesized from 2‐fenchyloxy or 2‐menthyloxy arylacetic acids which display excellent enantioselectivity across a range of transformations of α‐diazocarbonyl compounds. The catalysts were successfully applied to enantioselective C−H insertion reactions of aryldiazoacetates and α‐diazo‐β‐oxosulfones affording the respective products in up to 93 % ee with excellent trans diastereoselectivity in most cases. Furthermore, efficient desymmetrization in an intramolecular C−H insertion was achieved. In addition, these catalysts prove highly enantioselective for intramolecular aromatic addition with up to 88 % ee, and oxonium ylide formation and rearrangement with up to 74 % ee.