Thermodynamic Strategies for C-O Bond Formation and Cleavage via Tandem Catalysis

Radical-Friedel-Crafts benzylation of arenes over a metallic-basic bifunctional MoO2 surface

Selective ether bond activation is essential in organic synthesis and natural polymer depolymerization. Herein, we report that MoO2 with negligible acidity but characterized metallic-basic bifunctional properties can efficiently catalyze the benzylation of arenes with benzyl ethers as a benzylation reagent via a radical-Friedel-Crafts mechanism. Through combining catalyst characterizations, control experiments, thermomechanical analysis, intermediates capture, and density functional theory (DFT) calculations, multiple Mo sites on the MoO2 surface facilitate the initial transfer of oxygen-centered groups from adsorbed dibenzyl ether (DBE*). This process involved the homolytic cleavage of Bn-OBn ether bonds, promoted by adjacent-group auxiliary activation of benzyl aromatic rings adsorbed on the MoO2 surface. The generated benzyl radical (Bn˙) attacks the aromatic ring of the arene substrates, while the first-exfoliated oxygen-centered group (BnO*) and its potential decomposition fragment, the oxidative MoO* species, abstract a H atom from the above addition-intermediate to restore aromaticity and achieve benzylation. The remaining alcohol (BnOH) can also participate in the benzylation mediated by MoO2. This study could provide some inspiration on C-O bond activation and ether utilization mediated by a Mo-based catalyst.

A General Deoxygenative Hydrosilylation of Oxo Chemicals Catalyzed by a Bipyridine-Ligated Chromium Complex

We report the general deoxygenative hydrosilylation of oxo chemicals that was catalyzed by an air-stable bipyridine-ligated chromium complex. A broad range of oxygen-containing compounds such as carboxylic esters, ketones, aldehydes, alcohols, and ethylene oxides can be used as precursors, providing a versatile strategy for the formation of methyl- or methylene-bearing motifs by the dissipation of oxygen moieties. The method has been applied to the synthesis of several bioactive molecules.

Ce[N(SiMe3)2]3(THF)3-Catalyzed Hydroboration of CO2, Esters and Epoxides with Pinacolborane: Selective Synthesis of Methanol in Multigram Scale

In this work, we have reduced CO2 with HBpin to afford borylated methanol product selectively in ~99 % yield using Ce[N(SiMe3)2]3(THF)3 as a catalyst. This led to multigram scale isolation of methanol obtained from CO2 reduction via the hydrolysis of borylated methanol, this establishes the potential of Ce[N(SiMe3)2]3(THF)3 as an efficient homogeneous catalyst for the bulk scale methanol synthesis. A practical application of this catalytic system was also shown by reducing CO2-containing motorbike exhaust efficiently and selectively. Further, C-O bond activation of esters and epoxides using HBpin and 1-2 mol % of Ce[N(SiMe3)2]3(THF)3 at 60 °C afforded the borylated alcohols in good to excellent yields, which can easily be hydrolysed to the eco-friendly corresponding alcohol. The stoichiometric experiments were performed to prove the formation of in-situ generated cerium hydride [Ce]-H as an active catalyst.

Low-Temperature Borylation of C(sp3)-O Bonds of Alkyl Ethers by Gold-Metal Oxide Cooperative Catalysis

Since ether moieties are often found not only in petrochemical products but also in natural organic molecules, the development of methods for manipulating C-O bonds of ethers is important for expanding the range of compound libraries synthesized from biomass resources, which should contribute to the goal of carbon neutrality. We report herein that gold nanoparticles supported on Lewis acidic metal oxides, namely α-Fe2O3, showed excellent catalytic activity for the reaction of dialkyl ethers and diborons, which enables the conversion of unactivated C(sp3)-O bonds to C(sp3)-B bonds at around room temperature. Various acyclic and cyclic ethers as well as a series of diborons participated in the heterogeneous gold-catalyzed borylation of unactivated C(sp3)-O bonds, to give a series of alkylboronates in high yields. Mechanistic studies corroborated that the present borylation of C(sp3)-O bonds of dialkyl ethers proceeded at the interface between gold nanoparticles and Lewis acidic metal oxides. Furthermore, adsorption IR measurements supported the notion that strong Lewis acid sites were generated at the boron atom of diborons adsorbed at the interface between Lewis acidic metal oxides and gold nanoparticles, which enabled us to ensure that the cooperation of gold nanoparticles and Lewis acidic metal oxides was responsible for the efficient transformation of unactivated C(sp3)-O bonds in ethers under mild conditions. This novel reaction technology which is specific to heterogeneous catalysts enables the activation of stable C(sp3)-O bonds of oxygenated chemical feedstock, which is beneficial for the sustainable synthesis of value-added organoboron compounds.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

Leveraging Metal and Ligand Reactive Sites for One Pot Reactions: Ligand-Centered Borenium Ions for Tandem Catalysis with Palladium

Tandem catalysts that perform two different organic transformations in a single pot are highly desirable because they enable rapid and efficient assembly of simple organic building blocks into more complex molecules. Many examples of tandem catalysis rely on metal-catalyzed reactions involving one or more metal complexes. Remarkably, despite surging interest in the development of chemically reactive (i. e., non-innocent) ligands, there are few examples of metal complexes that leverage ligand-centered reactivity to perform catalytic reactions in tandem with separate catalytic reactions at the metal. Here we report how multifunctional Pd complexes with triaminoborane-derived diphosphorus ligands, called TBDPhos, appear to facilitate borenium-catalyzed cycloaddition reactions at the ligand, and Pd-catalyzed Stille and Suzuki cross-coupling reactions at the metal. Both transformations can be accessed in one pot to afford rare examples of tandem catalysis using separate metal and ligand catalysis sites in a single complex.

Radical-Friedel-Crafts benzylation of arenes with benzyl ethers over 2H-MoS2: ether cleavage into carbon- and oxygen-centered radicals

The selective activation of C-O ether bonds is an essential tool in organic synthesis and natural polymer depolymerization. However, the direct cleavage of the ether bond is still challenging work, especially breaking this inert and redox-neutral bond to provide one active carbon radical and another oxygen-centered fragment with oxidation capacity that can participate in the controllable radical reaction. We herein report that commercial 2H-MoS₂ with negligible acidity can efficiently catalyze the benzylation of arenes with benzyl ethers, and a new Radical-Friedel-Crafts mechanism is proposed, which is quite different from the strong acid-catalyzed Friedel-Crafts mechanism. With dibenzyl ether as the model benzylation reagent, 2H-MoS₂ can achieve the homolytic cleavage of the Bn-OR bond to generate the benzyl carbon radical and RO˙ species, identified by EPR measurement and radical trap experiments. The following radical-involved benzylation is confirmed by the Hammett results and a plausible pathway is proposed to clarify the Radical-Friedel-Crafts process. Heterogeneous 2H-MoS₂ can be consecutively used four times without regeneration and it offers 94-95% yields of 2-benzyl-1,4-dimethylbenzene from dibenzyl ether and p-xylene in 30 min at 140 °C. Furthermore, this mechanism can provide some inspiration to activate the ether bond and to utilize ether as an oxidant in C-H bond activation.

Mechanistic Studies Yield Improved Protocols for Base-Catalyzed Anti-Markovnikov Alcohol Addition Reactions

The catalytic anti-Markovnikov addition of alcohols to simple alkenes is a longstanding synthetic challenge. We recently disclosed the use of organic superbase catalysis for the nucleophilic addition of alcohols to activated styrene derivatives. This article describes mechanistic studies on this reversible reaction, including thermodynamic and kinetic profiling as well as computational modeling. Our findings show the negative entropy of addition is counterbalanced by an enthalpy that is most favored in nonpolar solvents. However, a large negative alcohol rate order under these conditions indicates excess alcohol sequesters the active alkoxide ion pairs, slowing the reaction rate. These observations led to an unexpected solution to a thermodynamically challenging reaction: use of less alcohol enables faster addition, which in turn allows for lower reaction temperatures to counteract Le Chatelier's principle. Thus, our original method has been improved with new protocols that do not require excess alcohol stoichiometry, enable an expanded alkene substrate scope, and allow for the use of more practical catalyst systems. The generality of this insight for other challenging hydroetherification reactions is also demonstrated through new alkenol cyclization and oxa-Michael addition reactions.

Catalytic conversion of waste corrugated cardboard into lactic acid using lanthanide triflates

The efficient strategy for waste conversion and resource recovery is of great interest in the sustainable bioeconomy context. This work reports on the catalytic upcycling of waste corrugated cardboard (WCC) into lactic acid using lanthanide triflates catalysts. WCC, a primary contributor to municipal solid wastes, has been viewed as a feedstock for producing a wide range of renewable products. Hydrothermal conversion of WCC was carried out in the presence of several lanthanide triflates. The reaction with erbium(III) triflate (Er(OTf)₃) and ytterbium(III) triflate (Yb(OTf)₃) resulted in high lactic acid yields, 65.5 and 64.3 mol%, respectively. In addition, various monomeric phenols were readily obtained as a co-product stream, opening up opportunities in waste management and resource recovery. Finally, technoeconomic analysis was conducted based on the experimental results, which suggests a significant economic benefit of chemocatalytic upcycling of WCC into lactic acid.

Copper(II)-catalyzed synthesis of benzoxazoles from inactive 2-chloroanilides

Aim and Objective:

Benzoxazoles are of great importance in natural products, pharmaceutical agents as well as synthetic intermediates. Although many works reported on construction of benzoxazoles by Cu-catalyzed intramolecular O-arylation of ortho-haloanilides have been improved, there are very few reports about transition metal-catalyzed synthesis of benzoxazoles from inactive 2-chloroanilides so far. This work is to explore a green and cheap protocol for intramolecular O-arylation of inactive 2-chloroanilides to prepare 2-arylbenzoxazoles.

Materials and Methods:

We have found Cu(acac)2/1,10-Phen complex was beneficial to intramolecular O-arylation of 2-chloroanilides using K2CO3 as base in EtOH at 90 oC to prepare benzoxazoles.

Results:

An efficient and green method was developed for Cu(II)-catalyzed intramolecular O-arylation of inactive 2-chloroanilides.

Conclusion:

In this way, many 2-arylbenzoxazoles were prepared in good yields.

An efficient catalytic method for the borohydride reaction of esters using diethylzinc as precatalyst

A cheap and easily available ZnEt2 is an effective precatalyst, which can be used for the hydroboration reaction of various organic carbonates and esters with HBpin.

Efficient Polyester Hydrogenolytic Deconstruction via Tandem Catalysis

Using a mechanism-based solvent-free tandem catalytic approach, commodity polyester plastics such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN) are rapidly and selectively deconstructed by combining the two air- and moisture-stable catalysts, Hf(OTf)₄ and Pd/C, under 1 atm H₂ , affording terephthalic acid (or naphthalene dicarboxylic acid for PEN) and ethane (or butane for PBT) in essentially quantitative yield. This process is effective for both laboratory grade and waste plastics, and comingled polypropylene remains unchanged. Combined experimental and DFT mechanistic analyses indicate that Hf(OTf)₄ catalyzes a mildly exergonic retro-hydroalkoxylation reaction in which an alkoxy C-O bond is first cleaved, yielding a carboxylic acid and alkene, and this process is closely coupled to an exergonic olefin hydrogenation step, driving the overall reaction forward.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Development of highly efficient platinum catalysts for hydroalkoxylation and hydroamination of unactivated alkenes

Hydrofunctionalization, the direct addition of an X-H (e.g., X=O, N) bond across an alkene, is a desirable strategy to make heterocycles that are important structural components of naturally occurring molecules. Described here is the design and discovery of "donor-acceptor"-type platinum catalysts that are highly effective in both hydroalkoxylation and hydroamination of unactivated alkenes over a broad range of substrates under mild conditions. A number of alkene substitution patterns are accommodated, including tri-substituted, 1,1-disubstituted, (E)-disubstituted, (Z)-disubstituted and even mono-substituted double bonds. Detailed mechanistic investigations suggest a plausible pathway that includes an unexpected dissociation/re-association of the electron-deficient ligand to form an alkene-bound "donor-acceptor"-type intermediate. These mechanistic studies help understand the origins of the high reactivity exhibited by the catalytic system, and provide a foundation for the rational design of chiral catalysts towards asymmetric hydrofunctionalization reactions.

Catalytic One-Pot Conversion of Renewable Platform Chemicals to Hydrocarbon and Ether Biofuels through Tandem Hf(OTf)4 +Pd/C Catalysis

Efficient conversion of renewable biomass platform chemicals into high-quality fuels remains challenging. A one-pot catalytic approach has been developed to synthesize various structurally defined biofuels by using Hf(OTf)₄ and Pd/C for selective tandem catalysis and 2-methylfuran (2-MF) as a renewable feedstock. 2-MF first undergoes Lewis acid-catalyzed hydroxyalkylation/alkylation (HAA) condensation with carbonyl compounds to afford intermediates containing the targeted carbon skeletons of hydrocarbon or ether products, and these intermediates then undergo hydrogenation or hydrodeoxygenation to afford the target products, catalyzed by metal triflate+Pd/C in the same pot. The present process can produce structurally defined alkanes and cyclic ethers under mild conditions.

Tracking Electrical Fields at the Pt/H2O Interface during Hydrogen Catalysis

We quantify changes in the magnitude of the interfacial electric field under the conditions of H₂/H⁺ catalysis at a Pt surface. We track the product distribution of a local pH-sensitive, surface-catalyzed nonfaradaic reaction, H₂ addition to cis-2-butene-1,4-diol to form n-butanol and 1,4-butanediol, to quantify the concentration of solvated H⁺ at a Pt surface that is constantly held at the reversible hydrogen electrode potential. By tracking the surface H⁺ concentration across a wide range of pH and ionic strengths, we directly quantify the magnitude of the electrostatic potential drop at the Pt/solution interface and establish that it increases by ∼60 mV per unit increase in pH. These results provide direct insight into the electric field environment at the Pt surface and highlight the dramatically amplified field existent under alkaline vs acidic conditions.

Catalytic amidation of natural and synthetic polyol esters with sulfonamides

Triacylglycerides are naturally abundant and renewable feedstock for biofuels and chemicals. In this report, these seemingly stable compounds are shown to be reactive toward a variety of sulfonamides under Lewis acid catalysis. In these reactions, alkyl C(sp³)–O bonds are cleaved and C–N bonds constructed, providing functionalized value-added products directly from renewables. Mechanistic and scope study demonstrate that the origin of the reactivity could be the synergy of Lewis acid catalysis and neighboring group participation by the 2- or 3-acyloxy or acylamido group with respect to the reactive site. Since poly(ethylene terephthalate) (PET), a widely available consumer polyester, also contains 1,2-diol diester group as the repeating unit in the main chain, this chemistry can also be applied to efficient depolymerization of PET.

Triacylglycerides are naturally abundant and renewable feedstock, but their chemical transformation is hindered by their stability. Here, under Lewis acid catalysis, the authors report the selective alkyl C–O bond conversion of triglycerides into C–N bonds and even apply this efficient method to PET depolymerization.

Reversible chemoselective transetherification of vinylogous esters using Fe-catalyst under additive free conditions

An additive/Brønsted acid/base free, highly efficient and chemoselective transetherification of electron deficient vinylogous esters and water mediated de-alkylation using an earth-abundant Fe-catalyst under very mild reaction conditions is described. This reaction is highly selective to primary alcohols over secondary alcohols, has good functional group tolerance, is scalable to gram scale and a purification free sequential transetherification in a continuous flow mode is demonstrated.

Ru(ii)-Catalyzed C7-acyloxylation of indolines with carboxylic acids

Ruthenium(ii)-catalyzed site-selective C7-acyloxylation of indolines with carboxylic acids is presented. The substrate scope and functional group tolerance are important practical features. The kinetic isotope studies suggest that C-H bond activation may be the rate-determining step.

Catalytic regeneration of a Th-H bond from a Th-O bond through a mild and chemoselective carbonyl hydroboration

Here we present an unprecedented chemoselective hydroboration for aldehydes and ketones catalysed by actinides. The reaction features a very low catalyst loading (0.1-0.004 mol%) and quantitative product formation in less than 15 minutes, at room temperature. Thermodynamic and kinetic studies including stoichiometric and labeling studies with deuterated pinacolborane allow us to propose a plausible mechanism for this remarkable catalytic regeneration of a Th-H bond via carbonyl hydroboration.

Production of Jet Fuel-Range Hydrocarbons from Hydrodeoxygenation of Lignin over Super Lewis Acid Combined with Metal Catalysts

Super Lewis acids containing the triflate anion [e.g., Hf(OTf)₄ , Ln(OTf)₃ , In(OTf)₃ , Al(OTf)₃ ] and noble metal catalysts (e.g., Ru/C, Ru/Al₂ O₃ ) formed efficient catalytic systems to generate saturated hydrocarbons from lignin in high yields. In such catalytic systems, the metal triflates mediated rapid ether bond cleavage through selective bonding to etheric oxygens while the noble metal catalyzed subsequent hydrodeoxygenation (HDO) reactions. Near theoretical yields of hydrocarbons were produced from lignin model compounds by the combined catalysis of Hf(OTf)₄ and ruthenium-based catalysts. When a technical lignin derived from a pilot-scale biorefinery was used, more than 30 wt % of the hydrocarbons produced with this catalytic system were cyclohexane and alkylcyclohexanes in the jet fuel range. Super Lewis acids are postulated to strongly interact with lignin substrates by protonating hydroxyl groups and ether linkages, forming intermediate species that enhance hydrogenation catalysis by supported noble metal catalysts. Meanwhile, the hydrogenation of aromatic rings by the noble metal catalysts can promote deoxygenation reactions catalyzed by super Lewis acids.

Graphene-enhanced platinum-catalysed hydrosilylation of amides and chalcones: a sustainable strategy allocated with in situ heterogenization and multitask application of H2PtCl6

This work developed a new sustainable strategy with comprehensive utilization of recovered catalyst, which the organosilicon/graphene-supported platinum catalyst prepared from reduction of amides could be further used in the 1,4-hydrosilylation of chalcones.

t-BuOK-mediated reductive addition of P(O)–H compounds to terminal alkynes forming β-arylphosphine oxides

A novel and efficient t-BuOK-mediated reductive addition of P(O)–H compounds to terminal alkynes forming β-arylphosphine oxides was developed. This reaction may proceed via a tandem process involving regio-selective addition and subsequent transfer hydrogenation.