Aerobic Baeyer–Villiger Oxidation of Cyclic Ketones over Metalloporphyrins Bridged Periodic Mesoporous Organosilica

Hydroxyl Radical Mediated Heterogeneous Photocatalytic Baeyer-Villiger Oxidation over Covalent Triazine/Heptazine-Based Frameworks

The Baeyer-Villiger (B-V) oxidation of ketones to the corresponding lactones/esters is a classic and essential reaction in the chemical industry. However, this oxidation process has not yet been achieved in ambient conditions with the aid of oxygen and heterogeneous photocatalysts. In this study, we developed an organic photocatalytic system using covalent triazine/heptazine-based frameworks (CTF-TB/CHF-TB) to enable the B-V oxidation reaction under mild conditions through a cascade reaction pathway. Experimental data and theoretical calculations showed that heptazine/triazine units can "chelate" and decompose the in situ generated H2O2 into hydroxyl radicals (⋅OH). Compared to conventional methods that primarily involve metal-activated benzaldehyde at elevated temperatures (e.g., 60 °C), the ⋅OH generated in our study can readily cleave the C-H bond of benzaldehyde, forming an active intermediate that drives subsequent sequential processes: O2→H2O2→⋅OH→Ph-CO⋅→Ph-COOO⋅. By employing this photocatalytic process, a yield of 91 % and a selectivity of over 99 % were obtained in the oxidation of cyclohexanone to caprolactone at room temperature. This performance is comparable to the state-of-the-art catalysts, and our CHF-TB catalyst demonstrates impressive reusability, maintaining a high yield after 5 consecutive runs. This work presents a straightforward approach for C-H cleavage by organocatalysts to produce ϵ-caprolactone in a mild manner by B-V oxidation.

Ru Metalloligands Participate in the Construction of POM@MOF for Enhancing the Visible Photoinduced Baeyer-Villiger Oxidation Reaction

Directed synthesis of high-efficiency visible photoinduced Baeyer-Villiger oxidation catalysts is of primary significance. Here, the isopolymolybdate anion [β-Mo8O26]4- is for the first time encapsulated with the photosensitive metalloligand [Ru(bpy)2(H2dcbpy)]2+ (bpy = 2,2'-bipyridine; H2dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) to synthesize polyoxometalate@metal-organic frameworks, {(CdDMF)2[Ru(bpy)2(dcbpy)]3([β-Mo8O26])}·5DMF (Ru-Mo8). The composite photocatalyst Ru-Mo8 not only has a light absorption of 700 nm but also shortens the photogenerated electron transfer distances and accelerates charge and proton transfer. Ru-Mo8 can perform the Baeyer-Villiger oxidation with high selectivity and up to 96.7% yield under visible light (λ > 400 nm) irradiation. The turnover number and turnover frequency of the reaction were computed to be 967 and 548 h-1, respectively, and the apparent quantum yield was 6.84% by 425 nm. Simultaneously, the radical mechanism of Baeyer-Villiger oxidation of Ru-Mo8 in the O2/benzaldehyde system under visible light (λ > 400 nm) irradiation was proposed.

In Situ Synthesis of Iron-Based Porphyrin Metal-Organic Frameworks for the Baeyer-Villiger Oxidation Using Air as an Oxidant

Iron-based porphyrin metal-organic frameworks (PMOFs) are a new type of material with good stability, catalytic activities, and reusability, which have broad application prospects. However, the synthesis of iron-based PMOFs with different metalloporphyrin units requires a considerable amount of reagents and time. Herein, an in situ synthesis method was developed to simplify the synthetic process of iron-based PMOFs. A series of highly stable iron-based PMOFs (Msitu-PMOF-3(Fe) (M = Fe, Co, Ni, Cu, Pd)) were synthesized in one pot without the preparation of metalloporphyrin ligands in advance. This synthesis method is simpler and more efficient, saving considerable time and material costs. The resultant Msitu-PMOF-3(Fe) exhibits a high tolerance to aqueous solutions with pH ranging from 0 to 11, as well as various organic solvents. The catalytic experiments show that both Fesitu-PMOF-3(Fe) and Cositu-PMOF-3(Fe) can catalyze the Baeyer-Villiger oxidation of cyclohexanone using air as an oxidant at 35 °C. Moreover, when the mixtures of Fesitu-PMOF-3(Fe) and Cositu-PMOF-3(Fe) were employed as catalyst, they can exert a cooperative catalysis effect, resulting in a significantly enhanced catalytic activity compared to a single component. The mild reaction condition, high catalytic efficiency, and excellent selectivity endow this catalytic system with a good application prospect. This work not only provides a new approach for the synthesis of iron-based PMOFs but also offers guidance for the Baeyer-Villiger oxidation reactions using air under mild conditions.

Interfacial Anion-Induced Dispersion of Active Species for Efficient Electrochemical Baeyer-Villiger Oxidation

Degradable polymers are an effective solution for white plastic pollution. Polycaprolactone is a type of degradable plastic with desirable mechanical and biocompatible properties, and its monomer, ε-caprolactone (ε-CL), is often synthesized by Baeyer-Villiger (B-V) oxidation that demands peroxyacids with low safety and low atom-efficiency. Herein, we devised an electrochemical B-V oxidation system simply driven by H2O2 for the efficient production of ε-CL. This system involves two steps with the direct oxidation of H2O2 into •OOH radicals at the electrode surface and the indirect oxidation of cyclohexanone by the generated reactive oxygen species. The modulation of the interfacial ionic environment by amphipathic sulfonimide anions [e.g., bis(trifluoromethane)sulfonimide (TFSI-)] is highly critical. It enables the efficient B-V oxidation into ε-caprolactone with ∼100% selectivity and 68.4% yield at a potential of 1.28 V vs RHE, much lower than the potentials applied for electrochemical B-V oxidation systems using water as the O sources. On hydrophilic electrodes with the action of sulfonimide anions, hydrophilic H2O2 can be enriched within the double layer for direct oxidation while hydrophobic cyclohexanone can be simultaneously accumulated for rapidly reacting with the reactive oxygen species. This work not only enriches the electrified method of the ancient B-V oxidation by using only H2O2 toward monomer production of biodegradable plastics but also emphasizes the critical role of the interfacial ionic environment for electrosynthesis systems that may extend the scope of activity optimization.

Intensifying Photocatalytic Baeyer-Villiger Oxidation of Ketones with the Introduction of Ru Metalloligands and Bimetallic Units in POM@MOF

The synthesis of visible light-responsive and efficient photocatalysts toward green Baeyer-Villiger oxidation organic synthesis is of extraordinary significance. In this work, we have synthesized two examples of visible light responsive crystalline polyoxometalate@metal-organic framework materials Ru-NiMo and Ru-CoMo by introducing Ru metalloligands and {CdM3O12} bimetallic units (M = Ni or Co). This is the first report of metalloligand-modified polyoxometalate@metal-organic framework materials with bimetallic nodes, and the materials form a three-dimensional framework directly through coordination bonds between {CdM3O12} bimetallic units and metalloligands. In particular, Ru-NiMo can achieve efficient photocatalytic conversion of cyclohexanone to ε-caprolactone in yields as high as 95.5% under visible light excitation in the range of λ > 400 nm, achieving a turnover number and turnover frequency of 955 and 440 h-1, respectively, which are the best known photocatalysts for Baeyer-Villiger oxidation, while apparent quantum yield measured at 485 nm is 4.4%. Moreover, Ru-NiMo exhibited excellent structural stability and recyclability, producing a 90.8% yield after five cycles of recycling.

Double shelled titanium dioxide@mesoporous organosilica nanotube as an amphiphilic photoactive nanoreactor for efficient photocatalytic oxidation of styrene

In this work, we have proposed a strategy to fabricate double-shell nanotubes as amphiphilic photoactive nanoreactors (HTTBPC) through the ordered hybridization of mesoporous organosilicon (PMO) and titanium dioxide (TiO2) nanotubes. Unlike the previous rough composite, the heterogeneous structure established between cobalt-porphyrin functionalized PMO and conventional TiO2 has a staggered matching band gap, which makes it have excellent light harvesting and high carrier separation ability. This is still unexplored. Interestingly, the prepared photocatalysts exhibited superior activity (99%) and benzaldehyde selectivity (94%) in the oxidation of styrene in water at room temperature, which was 3.8 and 2.8 times higher than that of TiO2 nanotubes and PMO functionalized with cobalt porphyrin, respectively. It was demonstrated that the strong interaction between cobalt porphyrin PMO and TiO2 improved the separation of photogenerated carriers and the amphiphilic properties of mesoporous organosilica boosted the adsorption of substrate molecules in water, contributing to the significantly enhanced photocatalytic activity. This work provides a design of high-performance photocatalysts for alkene oxidation under green conditions.

The Al-Containing Silicates Modified with Organic Ligands and SnO2 Nanoparticles for Catalytic Baeyer-Villiger Oxidation and Aerobic Carboxylation of Carbonyl Compounds

The Baeyer-Villiger Oxidation (BVO) of ketones and aldehydes produce lactones and formates, while aerobic carboxylation of aldehydes manufactures carboxylic acids, both having high added value. This work prepared a series of Al-containing silicates modified with organic ligands and SnO2 nanoparticles, which were then employed as catalyst in BVO and carboxylation. Characterizations revealed the morphology of the synthesized catalyst was changed from micron-sized thin sheets to smaller blocks, and then to uniform nanoparticles (size of 50 nm) having the doped SnO2 nanoparticles with a size of 29 nm. All catalysts showed high BET surface areas featuring silt-like mesopores. In determining the priority of BVO and carboxylation, an influence evaluation of the parameters showed the order to be substrate > oxidant > solvent > catalyst. Cyclic aliphatic ketones were suitable for BVO, but linear aliphatic and aromatic aldehydes for carboxylation. Coordination of (S)-binaphthol or doping of Sn into catalyst showed little influence on BVO under m-CPBA, but the Sn-doped catalyst largely increased BVO under (NH4)2S2O8 and H2O2. Calculations revealed that the catalyst containing both Al and Sn could give BVO intermediates lower energies than the Sn-beta zeolite model. The present system exhibited merits including wider substrate scope, innocuous catalytic metal, greener oxidant, as well as lower catalyst cost.

A Redox-active Microporous Organosiloxane Containing a Stable Neutral Radical, Trioxotriangulene

A new silyl-substituted trioxotriangulene ( TOT ) neutral radical and corresponding porous organosiloxanes (POSs) were synthesized. The neutral radical exhibited a peculiarly high stability and formed a diamagnetic π-dimer characteristic to TOT neutral radicals stabilized by the strong multiple SOMO-SOMO interaction in both solution and solid states. POSs including TOT units within the organosiloxane-wall were prepared by polycondensation of the silyl groups, and formed microporous structures with ~1 nm-size diameters. Redox ability of TOT units in the POS was demonstrated by the treatment of oxidant/reductant in heterogeneous suspension condition, where the TOT units were reversibly converted between reduced and neutral radical species. Furthermore, the solid-state electrochemical measurements of the POS revealed the reversible multi-stage redox ability of TOT units involving polyanionic species within the organosiloxane-wall.

3D-Network porous polymer bonded metalloporphyrin: An efficient and reusable catalyst for the Baeyer-Villiger oxidation

A new, green catalyst was prepared through immobilization of metalloporphyrin on the surface of 3D-network polymer based on calix[4]resorcinarene (PC4RA), which efficiently catalyze B-V oxidation reaction using O2/benzaldehyde. The catalyst demonstrated excellent activity, which is highly potential for cyclic aliphatic ketones oxidation under mild conditions. IR spectroscopy, UV-Vis spectroscopy, thermal gravimetric analysis, energy dispersive spectroscopy and scanning electron microscopy are some of the spectroscopic methods used to characterize the new synthesized solid support.

Cobalt-Tetraamide-Phthalocyanine Immobilized on Fe3O4/Chitosan Microspheres as an Efficient Catalyst for Baeyer-Villiger Oxidation

Cobalt-Tetraamide-Phthalocyanine (CoTaPc) immobilized onto magnetic Fe₃O₄ chitosan microspheres (Fe₃O₄/CTO) was synthesized via a simple immersion method, which is an efficient catalyst for the oxidation of cyclic ketones to lactones with O₂/benzaldehyde as the oxidant. The CoTaPc-Fe₃O₄/CTO catalyst was applied for the first time in the Baeyer-Villiger (B-V) oxidation reaction. Characterization results obtained from X-ray diffraction, UV-vis, Fourier transform infrared, and scanning electron microscopy showed that the combination of CoTaPc and magnetic Fe₃O₄/CTO microspheres was achieved. The catalyst could be easily separated from the reaction system with an external magnet and reused several times without the remarkable loss of activity. In addition, a possible radical mechanism for the B-V oxidation in this catalytic system is proposed and verified by controlled experiments.

Gold/Periodic Mesoporous Organosilicas with Controllable Mesostructure by Using Compressed CO2

Gold nanoparticles confined into the walls of periodic mesoporous organosilicas (PMOs) with controllable morphology have been successfully fabricated through a one-pot method by using different CO₂ pressures. The synthesis can be easily conducted in a mixed aqueous solution by using HAuCl₄ as gold source and bis[3-(triethoxysilyl)propyl] tetrasulfide and tetramethoxysilane as the organosilica precursor. P123 and compressed CO₂ served as the template and catalytic/regulative agent, respectively. Transmission electron microscopy, N₂ adsorption, and X-ray diffraction were employed to characterize the structure of the obtained composite materials. To further investigate the formation mechanism, a series of ordered PMOs with one-dimensional nanotube, two-dimensional hexagonal, vesicle-like, and cellular foam structures were obtained by using different CO₂ pressures without the gold source. The mechanism for mesostructure evolution of PMOs with different CO₂ pressures was proposed and discussed in detail. The catalytic performance of Au-based PMOs was evaluated for the reduction of 4-nitrophenol (4-NP). These obtained composites with different mesostructures not only exhibit excellent catalytic activity, high conversion rate, and remarkable thermal stability, but they also exhibit morphology-dependent reaction properties in the reduction of 4-NP. The possible reaction pathway of the reactants to embedded Au active sites was proposed and schemed.

Biomolding Technique to Fabricate the Hierarchical Topographical Scaffold of POMA To Enhance the Differentiation of Neural Stem Cells

In this paper, a biomolding technique was first used to fabricate a scaffold of hierarchical topography with biomimetic morphology for tissue engineering. First, poly(ortho-methoxyaniline) (POMA) was synthesized by conventional oxidative polymerization, followed by characterizations with Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). Moreover, the POMA scaffold with 3D biomimetic morphology was fabricated using poly(dimethylsiloxane) (PDMS) as negative soft template from natural leaf surfaces of Xanthosoma sagittifolium, followed by transferring the pattern of PDMS template to POMA. The as-fabricated POMA scaffold with biomimetic morphology was investigated by scanning electron microscopy (SEM). Subsequently, cell-scaffold interactions were carried out by culturing rat neural stem cells (rNSCs) on biomimetic and nonbiomimetic, or flat, POMA scaffolds, as well as on poly(d-lysine) (PDL)-coated substrate, and evaluating the corresponding adhesion, cell viability, and differentiation of rNSCs. Results showed that there was no significant difference in the attachment of rNSCs on the three surface types, however, both the biomimetic and flat POMA scaffolds induced growth arrest relative to the PDL-coated substrate. In addition, the percentage of cells with elongated neurites after 19 days of culture was higher on the biomimetic POMA scaffold relative to flat POMA and PDL. In summary, the POMA scaffold with biomimetic morphology shows promise in promoting rNSCs differentiation and neurite outgrowth for long-term studies on nerve regenerative medicine.

Fe-Porphyrin functionalized microporous organic nanotube networks and their application for the catalytic olefination of aldehydes and carbene insertion into N–H bonds

Fe-Porphyrin functionalized microporous organic nanotubes networks were synthesized by an in situ hyper-crosslinking reaction between bottlebrush copolymers and meso-tetraphenylporphyrin iron(iii) chloride.

Nickel containing ionic liquid based ordered nanoporous organosilica: a powerful and recoverable catalyst for synthesis of polyhydroquinolines

The synthesis, characterization and catalytic application of a novel nickel containing ionic liquid based ordered mesoporous organosilica are demonstrated.

Synthesis of meso-tetraarylporphyrins using hafnium (IV) bis(perfluorooctanesulfonyl)imide complex in perfluorodecalin medium

This letter presents a non-conventional synthesis of meso-porphyrins in moderate yields, using catalytic quantities of hafnium (IV) bis(perfluorooctanesulfonyl)imide complex in a perfluorodecalin biphasic system. Various meso-tetraarylporphyrins were obtained in good yields. The catalyst is selectively dissolved in perfluorodecalin, and can be recovered simply by phase separation and reused several times without any obvious decrease of activity.

Chemistry of Mesoporous Organosilica in Nanotechnology: Molecularly Organic-Inorganic Hybridization into Frameworks

Organic-inorganic hybrid materials aiming to combine the individual advantages of organic and inorganic components while overcoming their intrinsic drawbacks have shown great potential for future applications in broad fields. In particular, the integration of functional organic fragments into the framework of mesoporous silica to fabricate mesoporous organosilica materials has attracted great attention in the scientific community for decades. The development of such mesoporous organosilica materials has shifted from bulk materials to nanosized mesoporous organosilica nanoparticles (designated as MONs, in comparison with traditional mesoporous silica nanoparticles (MSNs)) and corresponding applications in nanoscience and nanotechnology. In this comprehensive review, the state-of-art progress of this important hybrid nanomaterial family is summarized, focusing on the structure/composition-performance relationship of MONs of well-defined morphology, nanostructure, and nanoparticulate dimension. The synthetic strategies and the corresponding mechanisms for the design and construction of MONs with varied morphologies, compositions, nanostructures, and functionalities are overviewed initially. Then, the following part specifically concentrates on their broad spectrum of applications in nanotechnology, mainly in nanomedicine, nanocatalysis, and nanofabrication. Finally, some critical issues, presenting challenges and the future development of MONs regarding the rational synthesis and applications in nanotechnology are summarized and discussed. It is highly expected that such a unique molecularly organic-inorganic nanohybrid family will find practical applications in nanotechnology, and promote the advances of this discipline regarding hybrid chemistry and materials.

Preparation and application of surface activated Si-MCM-41 and SBA-16 as reusable supports for reduction of cyclic ketones with preferential stereoselectivity

Surface activated calcined Si-MCM-41 and SBA-16 materials as activators in the reduction of cyclic ketones.

Iron-catalyzed aerobic oxidative cleavage of the C–C σ-bond using air as the oxidant: chemoselective synthesis of carbon chain-shortened aldehydes, ketones and 1,2-dicarbonyl compounds

An efficient iron-catalyzed aerobic oxidative cleavage of the C–C bond to generate a number of carbon-shortened carbonyl compounds.