Bond Of Toluene Research Articles

Catalyst-free selective oxidation of C(sp3)-H bonds in toluene on water

The anisotropic water interfaces provide an environment to drive various chemical reactions not seen in bulk solutions. However, catalytic reactions by the aqueous interfaces are still in their infancy, with the emphasis being on the reaction rate acceleration on water. Here, we report that the oil-water interface activates and oxidizes C(sp3)-H bonds in toluene, yielding benzaldehyde with high selectivity (>99%) and conversion (>99%) under mild, catalyst-free conditions. Collision at the interface between oil-dissolved toluene and hydroxyl radicals spontaneously generated near the water-side interfaces is responsible for the unexpectedly high selectivity. Protrusion of free OH groups from interfacial water destabilizes the transition state of the OH-addition by forming π-hydrogen bonds with toluene, while the H-abstraction remains unchanged to effectively activate C(sp3)-H bonds. Moreover, the exposed free OH groups form hydrogen bonds with the produced benzaldehyde, suppressing it from being overoxidized. Our investigation shows that the oil-water interface has considerable promise for chemoselective redox reactions on water without any catalysts.

Modulation of Rh species in Rh/Al2O3/FeCrAl-foam structured catalysts for steam reforming of toluene

Steam reforming of tar is an appealing way for tar elimination during biomass gasification. In this work, structured Rh/Al2O3 supported on FeCrAl-foam catalysts with different Rh species contents (Rh0, RhiOx and Rh(AlO2)y) were modulated through calcination and/or reduction at different temperatures for toluene steam reforming. Among all the structured catalysts and conventional supported catalysts, the structured Rh/Al2O3/FeCrAl-foam catalyst, calcined in air at 300 °C followed by reduction with hydrogen at 600 °C, showed superior activity as well as excellent resistance to sintering and coke deposition. Such high performance can be attributed to the suitable Rh0/RhiOx ratio as well as the enhanced mass transfer capability. The integrated studies of controlled catalytic tests, catalyst characterizations, TPSR, H2O-TPD and in-situ DRIFTS revealed that metallic Rh0 site was responsible for the cleavage of the C-C/C-H bonds in toluene. The activate ability of H2O was governed by the Rh0/RhiOx ratio, while the RhiOx specie played a significant role, which further promotes the reaction activity and eliminate coke deposition. Furthermore, the RhiOx specie enable the catalyst with high sintering resistance, due to the strong affinity to the Al2O3 surface. However, the Rh(AlO2)y specie was inactive. The catalytic performance was controlled by balancing the toluene activation and H2O dissociation. Consequently, rational tuning of these species is crucial for obtaining high-performance Rh/Al2O3 catalysts for steam reforming of toluene.

Triggering photocatalytic performance of La2CoxMn2-xO6 via heat activation.

The La-based perovskite (LaBO3) exhibits excellent optical properties. However, its valence band (VB) potential is not sufficiently positive to reach the oxidation potential required for the cleavage of chemical bonds (such as benzylic C-H), limiting its application in photocatalysis. Herein, we report the unconventional effects of heat activation on the reduction of the dissociation energy of benzylic C-H and aqueous H-O, thereby triggering the photocatalytic activity of La2CoxMn2-xO6 perovskites. Additionally, we demonstrate that photocatalysis is the main contributor to substrate conversion in the selective oxidation of toluene and reduction of CO2. Particularly, La2Co1.5Mn0.5O6 shows excellent performance with a product yield of 550.00 mmol gcat-1 and a toluene conversion of 22,866.67 μmol gcat-1 h-1. To the best of our knowledge, this is the highest reported product yield for the selective oxidation of benzylic C-H bond of toluene. Our findings provide insight into the specific role of heat activation in photocatalysis, which is crucial for breaking and overcoming the VB barrier to realize challenging reactions.

The Dynamic Lewis Acid-Carbene Hybrid: Pushing the Electrophilicity of Carbenes to the Limit.

This work describes a Lewis-acid-coordination strategy to efficiently enhance the electrophilicity of a carbene beyond structural modification. A hybrid BCF-DAC is formed by the coordination of a Lewis acid, B(C6F5)3 (BCF), to an N,N'-diamidocarbene (DAC), possessing superior low LUMO energy that is indicated by theoretical calculation. This endows the hybridized carbene with a unique reactivity that speeds up the activation of the sp3-hybridized C-H bond of toluene and the [2+1] cycloaddition with C2H2. More strikingly, the hybrid readily undergoes [2+1] cycloaddition with C2H4 under ambient conditions, which is the first example of a stable carbene reacting with ethylene. The Lewis acid approach also features dynamic behavior and electrophilicity tunability.

On the mechanism of intermolecular nitrogen-atom transfer from a lattice-isolated diruthenium nitride intermediate.

Catalyst confinement within microporous media provides the opportunity to site isolate reactive intermediates, enforce intermolecular functionalization chemistry by co-localizing reactive intermediates and substrates in molecular-scale interstices, and harness non-covalent host-guest interactions to achieve selectivities that are complementary to those accessible in solution. As part of an ongoing program to develop synthetically useful nitrogen-atom transfer (NAT) catalysts, we have demonstrated intermolecular benzylic amination of toluene at a Ru2 nitride intermediate confined within the interstices of a Ru2-based metal-organic framework (MOF), Ru3(btc)2X3 (btc = 1,3,5-benzenetricarboxylate, i.e., Ru-HKUST-1 for X = Cl). Nitride confinement within the extended MOF lattice enabled intermolecular C-H functionalization of benzylic C-H bonds in preference to nitride dimerization, which was encountered with soluble molecular analogues. Detailed study of the kinetic isotope effects (KIEs, i.e., kH/kD) of C-H amination, assayed both as intramolecular effects using partially labeled toluene and as intermolecular effects using a mixture of per-labeled and unlabeled toluene, provided evidence for restricted substrate mobility on the time scale of interstitial NAT. Analysis of these KIEs as a function of material mesoporosity provided approximate experimental values for functionalization in the absence of mass transport barriers. Here, we disclose a combined experimental and computational investigation of the mechanism of NAT from a Ru2 nitride to the C-H bond of toluene. Computed kinetic isotope effects for a H-atom abstraction (HAA)/radical rebound (RR) mechanism are in good agreement with experimental data obtained for C-H amination at the rapid diffusion limit. These results provide the first detailed analysis of the mechanism of intermolecular NAT to a C-H bond, bolster the use of KIEs as a probe of confinement effects on NAT within MOF lattices, and provide mechanistic insights unavailable by experiment because rate-determining mass transport obscured the underlying chemical kinetics.

High-efficiency visible-light-driven oxidation of primary C-H bonds in toluene over a CsPbBr3 perovskite supported by hierarchical TiO2 nanoflakes.

Photocatalytic oxidation of toluene to valuable fine chemicals is of great significance, yet faces challenges in the development of advanced catalysts with both high activity and selectivity for the activation of inert C(sp3)-H bonds. Halide perovskites with remarkable optoelectronic properties have shown to be prospective photoactive materials, but the bulky structure with a small surface area and severe recombination of photogenerated electron-hole pairs are obstacles to application. Here, we fabricate a hierarchical nanoflower-shaped CsPbBr3/TiO2 heterojunction by assembling CsPbBr3 nanoparticles on 2D TiO2 nanoflake subunits. The design significantly downsizes the size of CsPbBr3 from micrometers to nanometers, and forms a type II heterojunction with intimate interfacial contact between CsPbBr3 and TiO2 nanoflakes, thereby accelerating the separation and transfer of photogenerated charges. Moreover, the formed hierarchical heterojunction increaseslight absorption by refraction and scattering, offers a large surface area and enhances the adsorption of toluene molecules. Consequently, the optimized CsPbBr3/TiO2 exhibits a high performance (10 200 μmol g-1 h-1) for photocatalytic toluene oxidation with high selectivity (85%) for benzaldehyde generation under visible light. The photoactivity is about 20 times higher than that of blank CsPbBr3, and is among the best photocatalytic performances reported for selective oxidation of toluene under visible light irradiation.

Electron-donating N−–Ti3+–Ov interfacial sites with high selectivity for the oxidation of primary C–H bonds

Element doping can fabricate different types of oxygen vacancies (O v ) on TiO 2 , but the effects of O v coordination and configuration on the oxidation reaction remain unclear. Here, we modify the local geometric ligand environment of O v by doping N and B into anatase TiO 2 (N-TiO 2 and B-TiO 2 ). N − –Ti 3+ –O v is induced in N-TiO 2 by substituting O with N, which has similar ionic size and electronegativity, and Ti 3+ –O v is found in TiO 2 and B-TiO 2 . Density functional theory calculations indicate that N − –Ti 3+ –O v is more reactive than Ti 3+ –O v toward O 2 activation. The resultant N-TiO 2 -0.25 enriched in N − –Ti 3+ –O v contributes to a rapid formation of superoxygen species (•O 2 − ), which increase the oxidation rate of primary C–H bonds in toluene, and thus exhibits much higher selectivity and yield. The fabrication of N − –Ti 3+ –O v develops doped catalysts with improved O v reactivity and enhanced primary C–H bond oxidation selectivity. • N − –Ti 3+ –O v is more reactive than Ti 3+ –O v toward O 2 activation to generate ⋅O 2 − • The generation of ⋅O 2 − is the rate-determining step in selective oxidation of toluene • The introduced photo energy does not change the thermal selective oxidation process • A higher yield of products is attributed to the increased selective oxidation rate Oxygen vacancies in TiO 2 influence catalytic activity differently depending on their coordination environment. Chen et al. explore N and B doping and show that N − –Ti 3+ –O v is more reactive than Ti 3+ –O v toward O 2 activation, enhancing the primary C–H bond oxidation of toluene.

Design of a Polyoxometalate-Based Metal-Organic Framework for Photocatalytic C(sp3)-H Oxidation of Toluene.

A powerful and promising route for developing novel photocatalysts for light-driven toluene oxidation in water under mild conditions is presented. Herein, a novel polyoxometalate-based metal-organic framework (POMOF), {Co4W22-DPNDI}, is prepared by incorporating the unusual Co4-sandwiched POM anion [Co4(μ-OH)2(SiW11O39)2]10- ({Co4W22}) and the photoactive organic bridging link N,N'-bis(4-pyridylmethyl)naphthalene diimide (DPNDI) into a framework. {Co4W22} is a good candidate for photocatalytic water oxidation. DPNDI is easily excited to form the radical species DPNDI* in the presence of an electron donor, which is beneficial for activation of the inert O2. Anion···π interactions and covalent bonds between {Co4W22} and DPNDI facilitate electron-hole separation and electron transfer. {Co4W22-DPNDI} displays high catalytic activity for the activation of the C(sp3)-H bond of toluene using light as a driving force and inexpensive water as an oxygen source under mild conditions. In particular, the yield and selectivity are improved by replacing oxygen with water, which may be ascribed to the release of protons during the water oxidation process that facilitate the generation of •OH.

Boosting Toluene Combustion by Tuning Electronic Metal-Support Interactions in In Situ Grown Pt@Co3O4 Catalysts.

Electronic metal-support interaction (EMSI) has attracted great attention in volatile organic compound (VOC) abatement. Herein, Pt@Co3O4 catalysts were prepared via a metal-organic framework (MOF) in situ growth approach to boost toluene degradation. The partial electron transfer from Co3O4 to Pt species was induced by the EMSI effect to generate the electron-rich Pt and Co3+ species. The electrophilic O2 molecules could be activated by picking up the electrons from electron-rich Pt species to form nucleophilic oxygen species, which is conducive to attack C-H bonds in toluene. The redox ability and surface oxygen species activity of catalysts were improved due to strong EMSI. As expected, the excellent toluene activity was achieved, meanwhile exhibiting satisfactory water resistance and long-term stability for toluene combustion. In situ diffuse reflectance infrared Fourier transform spectroscopy results elucidated that surface lattice oxygen species should deeply participate in toluene degradation, which could be efficiently replenished by gaseous oxygen. This work may provide a new idea for exploring the relationship between the electron transfer effect and efficient catalytic performance of VOCs.

Microporous cobaltporphyrin covalent polymer mediated Co3O4@PNC nanocomposites for efficient catalytic C-H bond activation

A Cobalt porphyrin conjugated porous polymer (Co/PCP) was obtained by hybrid polymerization of a mixture of Co(II)(5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin) (CoTBPP) and 5,10,15,20-tetrakis(4-bromophenyl)-21H,23H-porphyrin (TBPP) (3:7 M ratio) with the coupling reagent of p-phenyldiboric acid. This Co/PCP was carbonized at 400 °C and the obtained catalyst (denoted as Co3O4@PNC-400) was composed of Co3O4 nanoclusters wrapped in the nitrogen-carbon materials. This catalyst has an extremely high catalytic activity for the aerobic oxidation of C–H bonds. It can efficiently catalyze the oxidation of the C–H bonds in toluene and cyclohexane with high conversion and high selectivity of (alcohol + aldehyde), (alcohol + ketone) compounds, respectively. When heat-treated at 400 °C, the porous framework structure of the porphyrin polymer was partially retained, and the highly active Co3O4 nanoclusters were generated in situ. The reaction of CN to CN and connection with the Co3O4 nanoclusters also occurred. Co3O4@PNC-400 has obvious synergistic catalytic effect between Co3O4 and nitrogen-carbon framework during the catalytic reaction. The defective structure caused by the migration of cobalt also greatly improved the catalytic activity. Co/PCPs with different cobalt contents were also synthesized and heat treated at different temperature. These catalysts were a new type of efficient C–H oxidation catalysts with excellent potential application value.

Special-selective C–H oxidation of toluene to benzaldehyde by a hybrid polyoxometalate photocatalyst including a rare [P6W48Fe6O180]30– anion

It is a challenge to develop new photocatalysts for high selective oxidation of primary carbon-hydrogen (C–H) bonds with O2 under mild condition. Herein, a novel hybrid K6(H2O)8H24(C26H16N4O4)8[P6W48Fe6O180]·6H2O (FeW–DPNDI) was firstly synthesized by incorporating a rare Fe-substituted trimeric Dawson derivative and photosensitizer N,N'-bis(4-pyridylmethyl)naphthalene diimide (DPNDI) via solvothermal method. In FeW–DPNDI, [P6W48Fe6O180]30– and DPNDI are orderly arranged in the three-dimensional framework by the anion···π interaction and C–H···O hydrogen bonds, that is beneficial to promote the electron transfer among components. FeW–DPNDI is a typical n-type semiconductor with the calculated energy level at 2.22 V vs. NHE of the highest occupied molecular orbital (HOMO) and – 0.57 vs. NHE of the lowest unoccupied molecular orbital (LUMO), which is theoretically feasible for the reduction O2 to O2•– because of the more negative potential of CB than the standard redox potential (E(O2•–/O2) = – 0.33 V vs. NHE). The proton conductivity of FeW–DPNDI is 2.0 × 10–4 S cm−1 at 98% relative humidity and 358 K. FeW–DPNDI exhibits special selectivity for generating benzaldehyde in the photocatalytic oxidation of stubborn C–H bond of toluene under mild condition. Moreover, FeW–DPNDI can be easily isolated from the products and reused. FeW–DPNDI is the first example of the trimmer of Fe-substituted Dawson derivative, but also the first hybrid POM explored in the activating C–H bond of toluene under white light irradiation.

Metal Cocatalyst Directing Photocatalytic Acetonylation of Toluene via Dehydrogenative Cross-Coupling with Acetone

A heterogeneous metal-loaded titanium oxide photocatalyst provided an efficient route to bring out direct dehydrogenative cross-coupling between toluene and acetone without consuming any additional oxidizing agent. The nature of the metal nanoparticle cocatalyst deposited on TiO2 photocatalyst dictated the product selectivity for the cross-coupling. Pd nanoparticles on TiO2 photocatalyst allowed a C–C bond formation between the aromatic ring of toluene and acetone to give 1-(o-tolyl)propan-2-one (1a1) with high regioselectivity, while Pt nanoparticles on TiO2 photocatalyst promoted the cross-coupling between the methyl group of toluene and acetone to give 4-phenylbutan-2-one (1b) as the acetonylated product. These results demonstrated that the selection of the metal cocatalyst on TiO2 photocatalyst could determine which C–H bonds in toluene, aromatic or aliphatic, can react with acetone. Two kinds of reaction mechanisms were proposed for the photocatalytic dehydrogenative cross-coupling reaction, depending on the property of the metal nanoparticles, i.e., only Pd nanoparticles can catalyze the reaction between aromatic ring and the acetonyl radical species.

Photocatalytic Activation of Saturated C-H Bond Over the CdS Mixed-Phase Under Visible Light Irradiation.

Selective activation of saturated C–H bond in hydrocarbons to produce high-value-added chemicals is of great significance for chemical synthesis and transformation. Herein, we present a facile procedure to achieve Ni-doped CdS nanoparticles with mixed (cubic and hexagonal) phases, as well as its application to the photocatalytic activation of saturated primary C–H bond of toluene and its derivatives. The photocatalytic oxidation rate of toluene into benzaldehyde of formation reached up to 216.7 μmolh−1g−1 under visible light irradiation. The excellent photocatalytic performance of Ni(II)-doped CdS [Ni(II)/CdS] can be attributed to its unique structural assembly with cubic and hexagonal phases and also the addition of Ni ions, together taking effect in promoting the separation of photogenerated charge carriers. The possible reaction mechanism for the photocatalytic selective oxidation is illustrated in this work. The band width of the as-prepared mixed phase CdS is reduced, which can effectively expand the response range and improve photocatalytic performance.

A photochromic composite with enhanced carrier separation for the photocatalytic activation of benzylic C–H bonds in toluene

Photocatalysis based on the use of semiconducting materials is an emerging alternative to conventional thermochemical catalysis, and it has the potential to promote chemical synthesis under greener and milder conditions. However, heterogeneous photocatalytic organic reactions are still in their infancy, limited by the low-efficiency carrier separation of the currently available photocatalytic materials. Here, we report photochromic Bi2WO6–x/amorphous BiOCl (p-BWO) nanosheets, which, distinct from pristine Bi2WO6, show blue colouration upon visible light irradiation and are bleached by atmospheric oxygen. Studies on the microscopic structure of the material reveal the existence of abundant W(vi)O6–x units, which serve as the sites for the fast and continuous consumption of photogenerated electrons, thereby effectively facilitating the separation of electron–hole pairs. The prepared composite features a remarkable enhancement in performance for the photocatalytic oxidation of toluene with a conversion rate 166-fold higher compared with that of pristine Bi2WO6. Despite its potential, the visible light-triggered photocatalytic oxidation of toluene remains difficult due to the lack of efficient and scalable catalytic strategies. Now, a photochromic Bi2WO6–x/amorphous BiOCl composite is reported with the ability to oxidize toluene into benzaldehyde and benzoic acid with outstanding rates and quantum efficiencies.

Selective Activation of Methyl C–H Bonds of Toluene by Oxygen on Metallic Gold

In accord with its gas phase acidity, toluene is activated by atomic oxygen adsorbed on Au(111) at the methyl C–H bonds. Titration of this species from the surface with HCOOD yields C6H5CHD2, indicating that much of the toluene has been doubly deprotonated at the methyl carbon. Even at low surface coverages of atomic oxygen the thusly-activated species yields only CO, CO2 and water as gas phase products when heated above room temperature, indicating the facile formation of an adsorbed benzoate, not simple oxygen insertion into the carbene to form benzyl benzyloxy. At higher O pre-coverages benzoic acid forms between 400 and 500 K.

Palladium-Catalyzed Removable 8-Aminoquinoline Assisted Chemo- and Regioselective Oxidative sp2-C-H/sp3-C-H Cross-Coupling of Ferrocene with Toluene Derivatives.

The coupling of the C(sp2)-H bond of ferrocene with toluene avoiding in situ functionalization of both coupling partners has been achieved using palladium and iron dual catalysis. This cross oxidative coupling features regioselectivity to primary and secondary C(sp3)-H bonds and chemoselectivity toward the -C(sp3)-H over C-I bond of toluene with gram-scale production. It seems Fe(II) catalyst not only stabilizes the generated benzyl radical but also tames its oxidizing behavior, and consequently transfers it to the palladacycle for C-C coupling.

Visible Light-Induced Oxidative Chlorination of Alkyl sp3 C-H Bonds with NaCl/Oxone at Room Temperature.

A visible light-induced monochlorination of cyclohexane with sodium chloride (5:1) has been successfully accomplished to afford chlorocyclohexane in excellent yield by using Oxone as the oxidant in H2O/CF3CH2OH at room temperature. Other secondary and primary alkyl sp3 C-H bonds of cycloalkanes and functional branch/linear alkanes can also be chlorinated, respectively, under similar conditions. The selection of a suitable organic solvent is crucial in these efficient radical chlorinations of alkanes in two-phase solutions. It is studied further by the achievement of high chemoselectivity in the chlorination of the benzyl sp3 C-H bond or the aryl sp2 C-H bond of toluene.

Ruthenium-Sulfonamide-Catalyzed Direct Dehydrative Condensation of Benzylic C-H Bonds with Aromatic Aldehydes.

The first catalytic dehydrative condensation of the benzylic C-H bonds of toluene and p-xylene with aromatic aldehydes is reported herein. This protocol provides highly atom-economical access to stilbene and p-distyrylbenzene derivatives, whereby water is the sole byproduct. The reaction is based on the deprotonation-functionalization of benzylic C-H bonds through η6-complexation of the arenes, which is realized for the first time using a catalytic amount of a transition metal activator. The key to the success of this method is the use of a sulfonamide anion as a catalyst component, which appears to facilitate not only the deprotonation of the benzylic C-H bonds but also the formation of a C-C bonds via an electrophilic tosylimine intermediate.

Direct C(sp3)-H Cross Coupling Enabled by Catalytic Generation of Chlorine Radicals.

Here we report the development of a C(sp3)-H cross-coupling platform enabled by the catalytic generation of chlorine radicals by nickel and photoredox catalysis. Aryl chlorides serve as both cross-coupling partners and the chlorine radical source for the α-oxy C(sp3)-H arylation of cyclic and acyclic ethers. Mechanistic studies suggest that photolysis of a Ni(III) aryl chloride intermediate, generated by photoredox-mediated single-electron oxidation, leads to elimination of a chlorine radical in what amounts to the sequential capture of two photons. Arylations of a benzylic C(sp3)-H bond of toluene and a completely unactivated C(sp3)-H bond of cyclohexane demonstrate the broad implications of this manifold for accomplishing numerous C(sp3)-H bond functionalizations under exceptionally mild conditions.

Remote para-C-H Functionalization of Arenes by a D-Shaped Biphenyl Template-Based Assembly.

Site-selective C-H functionalization has emerged as an efficient tool in simplifying the synthesis of complex molecules. Most often, directing group (DG)-assisted metallacycle formation serves as an efficient strategy to ensure promising regioselectivity. A wide variety of ortho- and meta-C-H functionalizations stand as examples in this regard. Yet despite this significant progress, DG-assisted selective para-C-H functionalization in arenes has remained unexplored, mainly because it involves the formation of a geometrically constrained metallacyclic transition state. Here we report an easily recyclable, novel Si-containing biphenyl-based template that directs efficient functionalization of the distal p-C-H bond of toluene by forming a D-shaped assembly. This DG allows the required flexibility to support the formation of an oversized pre-transition state. By overcoming electronic and steric bias, para-olefination and acetoxylation were successfully performed while undermining o- and m-C-H activation. The applicability of this D-shaped biphenyl template-based strategy is demonstrated by synthesizing various complex molecules.