Selective Transformation Research Articles

Aliphatic Amines Unlocked for Selective Transformations through Diazotization.

While aromatic diazonium salts are important reagents in organic synthesis, 'Diazonium ions generated from ordinary aliphatic primary amines are usually useless for preparative purposes, since they lead to a mixture of products giving not only substitution by any nucleophile present, but also elimination and rearrangements if the substrate permits.'[1] In this work, we report that the statement is no longer valid, and it is now possible to control diazotization of aliphatic amines by utilizing isopentyl nitrite in HFIP. This transformation enabled electrophilic aromatic substitution with these highly abundant and commercially available alkyl reagents, as well as transforming them into building blocks typically employed in organic synthesis. The methodology opens an avenue for reactions involving aliphatic amines, even such demanding substrates as amino acids, as a source of carbocations thus expanding the degree of chemical space.

Aliphatic Amines Unlocked for Selective Transformations through Diazotization

While aromatic diazonium salts are important reagents in organic synthesis, ‘Diazonium ions generated from ordinary aliphatic primary amines are usually useless for preparative purposes, since they lead to a mixture of products giving not only substitution by any nucleophile present, but also elimination and rearrangements if the substrate permits.’[1] In this work, we report that the statement is no longer valid, and it is now possible to control diazotization of aliphatic amines by utilizing isopentyl nitrite in HFIP. This transformation enabled electrophilic aromatic substitution with these highly abundant and commercially available alkyl reagents, as well as transforming them into building blocks typically employed in organic synthesis. The methodology opens an avenue for reactions involving aliphatic amines, even such demanding substrates as amino acids, as a source of carbocations thus expanding the degree of chemical space.

Interface Engineering: Enhanced Catalysis Through Precise Control of Metal Nanocluster Transformation.

Atomically precise metal nanoclusters (NCs) have garnered significant interest due to their unique atomic stacking structures, the effect of quantum confinement, and enriched active sites but suffer from thermal- or light-induced poor instability and self-aggregation, together with in situ self-conversion to conventional metal nanoparticles (NPs). How to effectively harness the generic detrimental self-transformation property of metal NCs has so far not garnered immense attention within the realm of catalysis. In this work, we develop a layer-by-layer assembly technology to accurately anchor metal NCs to the metal oxide matrix. Then, the anchoring of metal NCs to metal NPs is triggered by a simple thermal treatment that enables precise control over the interface structure, resulting in a hollow core-shell heterostructure with a metal core (Au, Ag) encapsulated by a metal oxide (CeO2, Fe2O3, SnO2) shell. Benefiting from the synergistic interplay between metal NPs and the metal oxide substrate, such self-assembled metal NPs@metal oxide heterostructures display excellent catalytic activities and stability in the reduction of aromatic nitro compounds. The detailed catalytic mechanism is elucidated. Our work offers fresh impetus for the judicious utilization of the inherent instability of metal NCs for catalytic selective organic transformation.

One‐Pot Cobalt‐ or Copper‐Catalyzed Asymmetric Ring‐Opening Hydrosilylation/Hydroboration of Arylidenecyclopropanes

An operationally convenient cobalt‐catalyzed one‐pot one‐step hydrosilylation/hydroboration reaction of arylidenecyclopropanes is developed to access racemic 1,4‐borylsilylalkanes. In addition, the corresponding asymmetric reaction is developed with a chiral copper catalyst to prepare 1,4‐borylsilylalkanes with high enantioselectivity by a one‐pot two‐step procedure. Mechanistic studies reveal that this difunctionalization process begins with metal‐hydride‐catalyzed ring‐opening hydrosilylation of arylidenecyclopropanes to generate homoallylsilane intermediates, followed by regio‐ or enantioselective metal‐hydride‐catalyzed hydroboration of homoallylsilanes to produce skipped borylsilylalkanes. Selective transformations of C–B and Si–H bonds in skipped borylsilylalkane products are also demonstrated.

One-Pot Cobalt- or Copper-Catalyzed Asymmetric Ring-Opening Hydrosilylation/Hydroboration of Arylidenecyclopropanes.

An operationally convenient cobalt-catalyzed one-pot one-step hydrosilylation/hydroboration reaction of arylidenecyclopropanes is developed to access racemic 1,4-borylsilylalkanes. In addition, the corresponding asymmetric reaction is developed with a chiral copper catalyst to prepare 1,4-borylsilylalkanes with high enantioselectivity by a one-pot two-step procedure. Mechanistic studies reveal that this difunctionalization process begins with metal-hydride-catalyzed ring-opening hydrosilylation of arylidenecyclopropanes to generate homoallylsilane intermediates, followed by regio- or enantioselective metal-hydride-catalyzed hydroboration of homoallylsilanes to produce skipped borylsilylalkanes. Selective transformations of C-B and Si-H bonds in skipped borylsilylalkane products are also demonstrated.

A Covalent Triazine Framework for Photocatalytic Anti‐Markovnikov Hydrofunctionalizations

AbstractPorous materials‐based heterogeneous photocatalysts, performing selective organic transformations, are increasing the applicability of photocatalytic reactions due to their ability to merge traditional photocatalysis with structured pores densely decorated with catalytic moiety for efficient mass and charge transfer, as well as added recyclability. We herein disclose a porous crystalline covalent triazine framework (CTF)‐based heterogeneous photocatalyst that exhibits excellent photoredox properties for different hydrofunctionalization reactions such as hydrocarboxylations, hydroamination and hydroazidations. The high oxidizing property of this CTF enables the activation of styrenes, followed by regioselective C−N and C−O bond formation at ambient conditions. A change in the physicochemical and optoelectronic properties of the CTF, upon protonation during catalysis, lies at the basis of its photocatalytic properties. This allows us to obtain hydrocarboxylations, hydroamination, and hydroazidations from a myriad of electron‐donating and ‐withdrawing aromatic and aliphatic substrates. This catalytic approach is further extended to late‐stage functionalization of bio‐active molecules. Finally, detailed characterizations of the CTF and further mechanistic investigations provide mechanistic insights into these reactions.

A Covalent Triazine Framework for Photocatalytic Anti-Markovnikov Hydrofunctionalizations.

Porous materials-based heterogeneous photocatalysts, performing selective organic transformations, are increasing the applicability of photocatalytic reactions due to their ability to merge traditional photocatalysis with structured pores densely decorated with catalytic moiety for efficient mass and charge transfer, as well as added recyclability. We herein disclose a porous crystalline covalent triazine framework (CTF)-based heterogeneous photocatalyst that exhibits excellent photoredox properties for different hydrofunctionalization reactions such as hydrocarboxylations, hydroamination and hydroazidations. The high oxidizing property of this CTF enables the activation of styrenes, followed by regioselective C-N and C-O bond formation at ambient conditions. A change in the physicochemical and optoelectronic properties of the CTF, upon protonation during catalysis, lies at the basis of its photocatalytic properties. This allows us to obtain hydrocarboxylations, hydroamination, and hydroazidations from a myriad of electron-donating and -withdrawing aromatic and aliphatic substrates. This catalytic approach is further extended to late-stage functionalization of bio-active molecules. Finally, detailed characterizations of the CTF and further mechanistic investigations provide mechanistic insights into these reactions.

The investigation of highly efficient chlorinated hydrocarbons removal based on metal nanoparticles carbonaceous synthetic particles: The degradation behavior and selective transformation mechanism

Overconsumption in industrial manufacturing has led to critical emissions of chloroalkanes, posing a significant environmental threat. In this study, the innovative biochar (BC) particles containing nanocrystalline: NZVI, Fe/Pd, Fe/Ni were prepared by impregnation-liquid phase reduction using the pyrolysis of coconut husk as precursors to explore the degradation capabilities of chloroform (CF) via orthogonal experiment. The results indicated that when the pH value was 3, the temperature was 20 ℃, and the reaction time was 60 min, the CF removal efficiency of Fe/Ni-BC reached 80.38 ± 2.08 %, outperforming all other tested synthetic particles. Advanced characterization techniques showed that the abundant dispersed metallic nanoparticle sites would refresh the surface crystal morphology, improve the adsorption enrichment ability of biochar, enhance electron transfer capacity of synthetic particles, and further participate in reductive dechlorination by inducing an electrophilic attack of active hydrogen. Theoretical calculations verified that hydrogen activation and CF facile adsorption would be easily achieved after the introduction of metal nanoparticles, profiting from the multiple synergistic effects between special π-delocalized bond of biochar in carbon layer and multiple nanoparticles docking sites, and thus enhancing the specific CF catalytic selectivity of Fe/Ni as the active center. Collectively, the CF degradation on Fe/Ni-BC was a complex chemisorption process along with reductive hydrogenation decomposition, which achieved the parent CF reduction and promoted the conversion of the low-chlorinated benign products. These findings offer new insights into the developed multifunctional BC and their comprehensive assessment of CF sewage remediation.

Unlocking Photocycloaddition Reactivity of Tropolone by Cage-Confined Visible-Light Photocatalysis for Multilevel Selective Transformation.

The precise asymmetric photochemical transformation of organic compounds containing multiple reactive sites presents significant progress in synthetic chemistry. Herein, we report an unprecedented visible-light-induced cascade transformation of tropolone cyclic triene derivatives by using chiral photoactive metal-organic cages (cPMOCs) as enzyme-mimicking multipocket photocatalysts. The cage-confined photocatalysis promotes three successive elementary steps, i.e., enantioselective [2 + 2] photocycloaddition with chalcone, regio-, and diastereoselective α-ketol rearrangement, and a stereoselective 1,3-acyl shift, resulting in bicyclo[3.2.2]nonane skeleton with multichiral-centers unattainable by other methods. This study demonstrates how complex synthetic challenges of peri-, chemo-, and stereoselectivities could be subtly manipulated by cage-confined supramolecular catalysis for exploration of new reactivities.

Borane‐Catalyzed Selective Transformation of Levulinic Acid and Hydrazines into Hexahydropyridazines and Tetrahydropyridazin‐3(2H)‐ones Using Hydrosilane

AbstractBorane‐catalyzed one‐pot tandem cyclization/hydrosilylation of levulinic acid and other γ‐keto acids with hydrazines for selective access to hexahydropyridazines and tetrahydropyridazin‐3(2H)‐ones has been developed. The combination of B(C6F5)3 and Et3SiH allows for synthesis of various hexahydropyridazines and tetrahydropyridazin‐3(2H)‐ones in 67–97% yields with tolerance towards various functional groups and easy scale‐up. This reaction can be rendered enantioselective by using a chiral borane catalyst generated in situ from HB(C6F5)2 and a binaphthyl‐based chiral diene and PhMe2SiH, affording the target products in 70–92% yields with up to >99% ee. The judicious choice of borane catalyst and hydrosilane reductant proves to be critical for catalysis. Mechanistic investigations elucidate the catalytic pathway.

Non-noble Co supported on beta framework for hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to renewable biofuel 2,5-dimethylfuran

Selective transformation of biomass platform molecule 5-hydroxymethylfurfural (HMF) into biofuel 2,5-dimethylfuran (DMF) through hydrogenolysis path has attracted significant attention in the field of biomass catalytic conversion. In general, this process required supported noble catalysts under elevated temperature (423–533 K) and enough H2 pressure (1–4 MPa). Herein, Co/Beta-DA catalysts with various Co loadings of 5–20 wt% were post-synthesized through wet impregnation. Benefiting from the relatively open channel systems and strong metal-support interactions, the obtained 10Co/Beta-DA with ∼10 wt% Co contents facilitated the hydrogenation of C=O bonds and the cleavage of C-O bonds, which was efficient for the selective hydrogenolysis from HMF to DMF (Conv.HMF ≥ 99.9 %, Sel.DMF ≥ 99.9 %) under mild reaction conditions (H2 pressure, 1.0 MPa; temp., 423 K; time, 3 h). The innovative strategy of designing and preparing rational non-noble impregnated zeolite provided brand-new perspectives to solve the problems of high cost and harsh reaction conditions in the HMF hydrogenolysis process, which has the potential to convert biomass into renewable liquid fuels.

Insights into Catalytic Oxidative Reaction Mechanisms of Pentane on the Ru(0001) Surface.

Developing efficient and selective oxidative transformations of light alkanes into alkenes or oxygenates is vital for advancing to cleaner and more efficient chemical processes. A suitable selective catalyst is required to ease reaction conditions and ensure the formation of desired oxygenated compounds. Here, using periodic density functional theory, we have investigated the suitability of a ruthenium catalyst for the partial oxidation of n-pentane using molecular oxygen. The first step of the process involves the dehydrogenation of primary or secondary carbons in the aliphatic chain, resulting in an adsorbed hydride structure on the metal surface. The intermediate may proceed through different reaction pathways, leading to various products. The successive dehydrogenation, a faster process than the first oxidative dehydrogenation, produces pentene and a water molecule. Alternatively, the direct interaction of the hydroxyl group with the pentyl hydride produces alcohol. The atomistic simulations reveal that Ru is a suitable candidate for catalyzing the conversion of alkanes into alkenes and oxygenates. As a significant outcome, we have observed that catalytic oxidative dehydrogenation is more feasible than direct catalytic dehydrogenation for yielding olefins from alkanes.

Fundamentals of Flexoelectricity, Materials and Emerging Opportunities Toward Strain-Driven Nanocatalysts.

Flexoelectricity, an intrinsic property observed in materials under nonuniform deformation, entails a coupling between polarization and strain gradients. Recent catalyst advancements have reignited interest in flexoelectricity, particularly at the nanoscale, where pronounced strain gradients promote robust flexoelectric effects. This paper comprehensively examines flexoelectricity, encompassing methodologies for precise measurement, elucidating its distinctions from related phenomena, and exploring its potential applications in augmenting catalytic properties. So far, the greatest potentials are based on lead strontium titanate (PST) and other metallic titanates such as titania (TiO2), strontium titanate (STO), barium strontium titanate (BST) sulfates (MoS2, ZnS)and halide perovskites (with archetype XPbI3). This review explores the promise of flexoelectric properties in addressing material and photocatalytic challenges, such as charge carrier recombination and ineffective surface charge separation. Additionally, it sheds light on the synergy with emerging paradigms like photo-flexo catalysis and synergistic flexo-piezo catalysis, specifically focusing on selective chemical transformations like green hydrogen production. Current limitations related to the usage of photoflexoelectricity for photocatalysis are mostly the stability of the used substance (susceptibility to photodegradation) or the voltage values, which represent the inferior potential for specific practical applications. This work underscores the indispensable role of flexoelectricity in catalysisand its capacity to steer future researchand technological advancement.

Platinum-Catalyzed Regio- and Enantioselective Diboration of Unactivated Alkenes with (pin)B-B(dan).

Asymmetric diboration of terminal alkenes is well established, and subsequent selective functionalization of the less hindered primary boronic ester is commonly achieved. Conversely, selective functionalization of the sterically less accessible secondary boronic ester remains challenging. An alternative way to control chemoselective functionalization of bis(boron) compounds is by engendering different Lewis acidity to the two boryl moieties, since reactivity would then be dictated by Lewis acidity instead of sterics. We report herein the regio- and enantioselective Pt-catalyzed diboration of unactivated alkenes with (pin)B-B(dan). A broad range of terminal and cyclic alkenes undergo diboration to furnish the differentiable 1,2-bis(boron) compounds with high levels of regio- and enantiocontrol, giving access to a wide variety of novel building blocks from a common intermediate. The reaction places the less Lewis acidic B(dan) group at the less hindered position and the resulting 1,2-bisboryl alkanes undergo selective transformations of the B(pin) group located at the more hindered position. The regioselectivity of the diboration has been studied by DFT calculations and is believed to originate from the trans influence, which lowers the activation barrier for formation of the regioisomer that places the weaker electron donor [B(pin) vs B(dan)] opposite the strong electron donor (alkyl group) in the platinum complex.

Modulating the Aggregation States of a Pd6L4 Cage for Selectivity Flipping during the Stereo‐Divergent Semi‐Hydrogenation of Alkynes

An enzyme‐mimicking catalytic system has been established using a singular palladium‐based octahedral cage as the supramolecular reactor, deftly unlocking off‐on‐off selectivity in the semi‐hydrogenation of alkynes. Water serves as a critical regulator, modulating the catalyst states, reaction rates, and endpoints. The choice of solvent system influences the activity of host‐guest binding and the reaction types of homogeneous and heterogeneous catalysis, effectively modifying the reaction steps involved in the Z→E isomerization during the semi‐hydrogenation of alkynes. Kinetic and inhibition experiments indicate that the catalyst mimics the binding and activation characteristics of enzymes towards substrates, enabling selective transformations within the confined enzyme‐mimicking environment. The utility of this switchable cage‐confined catalysis has been demonstrated in the synthesis and modification of complex biologically active molecules with controllable E/Z selectivity. This work sheds light on the design and control of artificial supramolecular counterparts of enzymes, offering fundamental insights into the factors influencing the activity and catalytic selectivity of biological macromolecules.

Modulating the Aggregation States of a Pd6L4 Cage for Selectivity Flipping during the Stereo-Divergent Semi-Hydrogenation of Alkynes.

An enzyme-mimicking catalytic system has been established using a singular palladium-based octahedral cage as the supramolecular reactor, deftly unlocking off-on-off selectivity in the semi-hydrogenation of alkynes. Water serves as a critical regulator, modulating the catalyst states, reaction rates, and endpoints. The choice of solvent system influences the activity of host-guest binding and the reaction types of homogeneous and heterogeneous catalysis, effectively modifying the reaction steps involved in the Z→E isomerization during the semi-hydrogenation of alkynes. Kinetic and inhibition experiments indicate that the catalyst mimics the binding and activation characteristics of enzymes towards substrates, enabling selective transformations within the confined enzyme-mimicking environment. The utility of this switchable cage-confined catalysis has been demonstrated in the synthesis and modification of complex biologically active molecules with controllable E/Z selectivity. This work sheds light on the design and control of artificial supramolecular counterparts of enzymes, offering fundamental insights into the factors influencing the activity and catalytic selectivity of biological macromolecules.

Platinum‐Catalyzed Regio‐ and Enantioselective Diboration of Unactivated Alkenes with (pin)B−B(dan)

AbstractAsymmetric diboration of terminal alkenes is well established, and subsequent selective functionalization of the less hindered primary boronic ester is commonly achieved. Conversely, selective functionalization of the sterically less accessible secondary boronic ester remains challenging. An alternative way to control chemoselective functionalization of bis(boron) compounds is by engendering different Lewis acidity to the two boryl moieties, since reactivity would then be dictated by Lewis acidity instead of sterics. We report herein the regio‐ and enantioselective Pt‐catalyzed diboration of unactivated alkenes with (pin)B−B(dan). A broad range of terminal and cyclic alkenes undergo diboration to furnish the differentiable 1,2‐bis(boron) compounds with high levels of regio‐ and enantiocontrol, giving access to a wide variety of novel building blocks from a common intermediate. The reaction places the less Lewis acidic B(dan) group at the less hindered position and the resulting 1,2‐bisboryl alkanes undergo selective transformations of the B(pin) group located at the more hindered position. The regioselectivity of the diboration has been studied by DFT calculations and is believed to originate from the trans influence, which lowers the activation barrier for formation of the regioisomer that places the weaker electron donor [B(pin) vs B(dan)] opposite the strong electron donor (alkyl group) in the platinum complex.

Visible light-driven selective oxidative transformation of vicinal diols using ZnS-based photocatalyst in the presence of molecular oxygen

The heterogeneous photocatalysis for the oxidative cleavage of C–C bond is significant to the transformation of biomass feedstock. In this work, a heterojunction photocatalyst based on the conductor ZnS and C3N4 (CN) material is prepared and employed in the aerobic oxidative cleavage reaction of vicinal diol under visible light irradiation. As a result, it is found that 3ZnS/CN catalyst obtained by a mechanical grinding method shows a high photocatalytic activity. In the photocatalytic oxidative cleavage process of 1-phenyl-1,2-glycol, more than 98.7% conversion of substrate with a 96.2% selectivity of benzaldehyde was attained using O2 as oxidant. In addition, the photocatalyst recycling experiments exhibited that the 3ZnS/CN catalyst still kept a good activity and stability even after being recycled for 5 times. Finally, the active reaction intermediates were investigated by the control experiments and the relative electron paramagnetic resonance (EPR) detection. According to the obtained results and photocatalytic principle, the mechanism for the selective oxidative transformatioin of 1-phenyl-1,2-glycol has been proposed. It gives a promising approach for the catalytic utilization of biomass-based lignin and cellulose.

Taming CO2•- via Synergistic Triple Catalysis in Anti-Markovnikov Hydrocarboxylation of Alkenes.

The direct utilization of carbon dioxide as an ideal one-carbon source in value-added chemical synthesis has garnered significant attention from the standpoint of global sustainability. In this regard, the photo/electrochemical reduction of CO2 into useful fuels and chemical feedstocks could offer a great promise for the transition to a carbon-neutral economy. However, challenges in product selectivity continue to limit the practical application of these systems. A robust and general method for the conversion of CO2 to the polarity-reversed carbon dioxide radical anion, a C1 synthon, is critical for the successful valorization of CO2 to selective carboxylation reactions. We demonstrate herein a hydride and hydrogen atom transfer synergy driven general catalytic platform involving CO2•- for highly selective anti-Markovnikov hydrocarboxylation of alkenes via triple photoredox, hydride, and hydrogen atom transfer catalysis. Mechanistic studies suggest that the synergistic operation of the triple catalytic cycle ensures a low-steady-state concentration of CO2•- in the reaction medium. This method using a renewable light energy source is mild, robust, selective, and capable of accommodating a wide range of activated and unactivated alkenes. The highly selective nature of the transformation has been revealed through the synthesis of hydrocarboxylic acids from the substrates bearing a hydrogen atom available for intramolecular 1,n-HAT process as well as diastereoselective synthesis. This technology represents a general strategy for the merger of in situ formate generation with a synergistic photoredox and HAA catalytic cycle to provide CO2•- for selective chemical transformations.

Cobalt-Catalyzed Regio-, Diastereo- and Enantioselective Reductive Coupling of 1,3-Dienes and Aldehydes.

Catalytic regio-, diastereo- and enantioselective reductive coupling of 1,3-dienes and aldehydes through regio- and enantioselective oxidative cyclization followed by regio- and diastereoselective protonation promoted by a chiral phosphine-cobalt complex is presented. Such processes represent an unprecedented reaction pathway for cobalt catalysis that enable selective transformation of the more substituted alkene in 1,3-dienes, affording a broad scope of bishomoallylic alcohols without the need of pre-formation of stoichiometric amounts of sensitive organometallic reagents in up to 98 % yield, >98 : 2 regioselectivity, >98 : 2 dr and 98 : 2 er. Application of this method to construction of axial stereogenicity and deuterated stereogenic center provided a wide range of multifunctional chiral building blocks that are otherwise difficult to access. DFT calculations revealed the origin of regio- and stereoselectivity as well as a unique oxidative cyclization mechanism for cobalt catalysis.