Oxidative Hydroxylation Of Arylboronic Acids Research Articles

Pyrazine-based supramolecular photosensitizing assemblies as type I photosensitizers for high-efficiency photooxidation reactions

Donor-acceptor thermally activated delayed fluorescent (TADF) materials have been widely used in photocatalytic reactions as organic photosensitizers. However, the construction of TADF photosensitizers via a supramolecular self-assembly approach with the regulation of reactive oxygen species (ROS) is still a big challenge. In this study, a TADF molecule 4,4'-(6-(pyridin-4-yl)quinoxaline-2,3-diyl)bis(N,N-diphenylaniline) (TPA-PQ) by incorporating triphenylamine (TPA) act as donor and 6-(pyridin-4-yl)quinoxaline (PQ) act as an acceptor was designed and synthesized, which exhibited excellent TADF property and strong intramolecular charge transfer. Meanwhile, the TPA-PQ can not only self-assemble to form nanofiber structure in mixed solvent of H2O and THF, but also can be used as a type I photosensitizer to selectively produce superoxide anion radicals (O2•–). More interestingly, the ROS generated from TPA-PQ can be used as photocatalysts for the oxidative hydroxylation of arylboronic acids and aerobic cross dehydrogenation coupling (CDC) reactions with high yields of 92 % and 88 %, respectively. This study presents an innovative and eco-friendly approach to utilize TADF supramolecular structures for the purpose of conducting photocatalyzed organic reactions.

Improving charge separation capability of CdS/Bi/Ti3C2Tx nanocomposites for efficient photocatalytic oxidative organic transformations

The development of new strategies to improve the separation of photogenerated carriers is crucial for enhancing photocatalytic efficiency of heterogeneous photocatalysts. Here, we have successfully constructed CdS/Bi/Ti3C2Tx nanocomposites with high electron-hole separation efficiency through coupling Bi and Ti3C2Tx on CdS nanorods. The average emission lifetime of the CdS/Bi/Ti3C2Tx nanocomposites can be up to ca. 6.74 ns, which realizes efficient heterogeneous photocatalysis for oxidative hydroxylation of arylboronic acids and coupling of amines with high yields. The photocatalytic rates are ∼15250 μmol·g−1·h−1 for 4-cyanophenol production and ∼12125 μmol·g−1·h−1 for (E)-N-benzyl-1-phenylmethanimine production, which are twice and three-fold higher than those (∼7500 and ∼4000 μmol·g−1·h−1) of the CdS nanorods, respectively. The high photocatalytic efficiency can be ascribed to the strong electron-hole separation capacity by migrating electrons into Bi and Ti3C2Tx from CdS nanorods. In addition, the CdS/Bi/Ti3C2Tx nanocomposites also have satisfactory sustainable recycling and reuse properties. This work provids a new strategy for constructing heterogeneous metal/semiconductor nanophotocatalysts with strong electron-hole separation capacity for high-efficient light-driven organic transformations.

Aerobic Oxidative Hydroxylation of Arylboronic Acids under Visible-Light Irradiation without Metal Catalysts or Additives.

Phenols are versatile synthetic intermediates and key structural motifs in many natural products and biologically active compounds. We herein report a visible-light-induced aerobic oxidative hydroxylation of arylboronic acids/pinacol esters using air as oxidant and without using any catalysts and base, etc., additives, providing a green entry to a variety of phenols in a highly efficient and concise fashion. This novel reaction is enabled by photoactivation of an electron donor-acceptor complex, in which THF serves as both the solvent and electron donor. DFT studies indicated that the oxidation process involves a concerted hydrogen abstraction transfer from THF and dehydroxylation of boronic acid undergoing spin crossover from triplet to singlet to produce an active peroxoboronic acid intermidiate. Salient merits of this chemistry include broad substrate scope and excellent functional group tolerance, gram-scale synthesis, and versatile late-stage functionalizations as well as the use of air, visible light, and catalyst- and additive-free conditions. This strategy introduces a novel photoreaction mode with the aid of a solvent, offering a succinct and environmentally sustainable route for synthesizing phenols. The strong practicability and highly efficient access to modifying complex biorelevant molecules bode well for the potential applications of this chemistry in pharmaceutical chemistry.

Anthraquinone-Modified Silica Nanoparticles as Heterogeneous Photocatalyst for the Oxidative Hydroxylation of Arylboronic Acids.

In this work, the synthesis and characterization of a heterogeneous photocatalyst based on spherical silica nanoparticles superficially modified with anthraquinone 2-carboxylic acid (AQ-COOH) are presented. The nanomaterial was characterized by TEM, SEM, FT-IR, diffuse reflectance, fluorescence, NMR, DLS, XRD and XPS. These analyses confirm the covalent linking of AQ-COOH with the NH2 functionality in the nanomaterial and, more importantly, the photocatalyst retains its photophysical properties once bound. The heterogeneous photocatalyst was successfully employed in the aerobic hydroxylation of arylboronic acids to phenols under sustainable reaction conditions. Phenols were obtained in high yields (up to 100 %) with low catalyst loading (3.5 mol %), reaching TOF values of 3.7 h-1 . Using 2-propanol as solvent at room temperature, the visible light photocatalysis produced H2 O2 as a key intermediate to promote the aerobic hydroxylation of arylboronic acids. The heterogeneous photocatalyst was reused at least 5 times, without modification of the nanomaterial structure and morphology. This simple heterogeneous system showed great catalytic activity under sustainable reaction conditions.

Construction of covalent organic frameworks via the Mannich reaction at room temperature for light-driven oxidative hydroxylation of arylboronic acids.

An increasing variety of organic reactions have been developed for the synthesis of more structurally stable and multifunctional COFs. Here, we report a class of β-ketamine linked covalent organic frameworks that were constructed through the CeCl3-catalyzed multi-component Mannich reaction at room temperature. And the TAD-COF obtained based on this method could significantly promote the light-driven oxidative hydroxylation of arylboronic acids.

Concentration-dependent aggregation of methylene blue acting as a photoredox catalyst.

Hydroxylation reactions are important in biological processes and synthetic schemes. Many challenging hydroxylation reactions have been achieved using photoredox catalytic methods. For the oxidative hydroxylation of arylboronic acids, methylene blue has been used successfully as a photoredox catalyst to produce phenyl groups. Here we use broadband transient absorption spectroscopy to determine the mechanism of the photoredox catalytic reaction of methylene blue with phenylboronic acid in the presence of N,N-diisopropylethylamine. Our results show that the reaction proceeds through the triplet state of methylene blue in the presence of oxygen, generating superoxide radical anions. In addition, we observe dimerization of the methylene blue at typical catalytic loadings. As these dimers do not participate in the reaction, increasing the concentration of methylene blue is potentially detrimental to the overall yield.

Superoxide radical generator based on triphenylamine-based supramolecular organic framework for green light photocatalysis

Supramolecular organic frameworks (SOFs) mostly require high-energy purple or blue light for photocatalytic reactions, while highly abundant and low-energy light systems have rarely been explored. Therefore, it is necessary to construct 2D SOFs for low-energy light-induced photocatalysis. This study describes the design and synthesis of a water-soluble two-dimensional (2D) supramolecular organic framework (TP-SOF) using the host–guest interaction between a triphenylamine derivative (TP-3Py) and cucurbit[8]uril (CB[8]). The formation of the 2D SOF can be attributed to the synergistic impact resulting from the orientated head-to-tail superposition mode between the vinylpyridine arms of TP-3Py and CB[8], which results in a significant redshift in the UV–vis absorption spectrum, especially displaying a strong absorption band in the green light region. The monomeric TP-3Py can effectively produce singlet oxygen (1O2) and realize the photocatalytic oxidation of thioanisole in the aqueous solution. In comparison to monomeric TP-3Py, the confinement effect of CB[8] results in a notable enhancement in the production efficiency of superoxide anion radicals (O2•−), exhibiting promising prospects in the field of photocatalytic oxidation reaction, which facilitates the application of TP-SOF as a very efficient photosensitizer for the promotion of the oxidative hydroxylation of arylboronic acids under green light in the aqueous solution, giving a high yield of 91%. The present study not only presents a compelling illustration of photocatalysis utilizing a 2D SOF derived from triphenylamine, but also unveils promising avenues for the photocatalytic oxidation of SOF employing low-energy light systems.

Triangulenium Ions: Versatile Organic Photoredox Catalysts for Green-Light-Mediated Reactions

AbstractThe development of tunable organic photoredox catalysts remains important in the field of photoredox catalysis. A highly modular and tunable family of trianguleniums (azadioxatriangulenium, diazaoxatriangulenium, and triazatriangulenium), and the related [4]helicene quinacridinium have been used as organic photoredox catalysts for photoreductions and photooxidations under visible light irradiation (λ = 518–640 nm). A highlight of this family of photoredox catalysts is their readily tunable redox properties, leading to different reactivities. We report their use as photocatalysts for the aerobic oxidative hydroxylation of arylboronic acids and the aerobic cross-dehydrogenative coupling reaction of N-phenyl-1,2,3,5-tetrahydroisoquinoline with nitromethane through reductive quenching. Furthermore, their potential as photoreduction catalysts has been demonstrated through the catalysis of an intermolecular atom-transfer radical addition via oxidative quenching. These transformations serve as benchmarks to highlight that the easily synthesized trianguleniums, congeners of the acridiniums, are versatile organic photoredox catalysts with applications in both photooxidations and photoreductions.

Hydroxyquinolinate diarylboron complexes for photocatalytic activity

In this paper, readily obtainable hydroxyquinolinate diarylboron complexes were characterized by UV–vis measurements, computational calculations and they were tested as photocatalysts in perfluoroalkylation of unsaturated bonds through a photocatalytic ATRA process. Amongst the studied compounds the respective diphenyl derivative was selected as the most economic photocatalyst. It was next utilized in a visible-light driven aerobic oxidative hydroxylation of arylboronic acids using air as the oxygen source.

Photocatalytic Oxidative Hydroxylation of Arylboronic Acids to Phenols by a CdS-TiO2 Nanohybrid

Photocatalytic Oxidative Hydroxylation of Arylboronic Acids to Phenols by a CdS-TiO2 Nanohybrid

Sodium Hydroxide (NaOH)‐Mediated Oxidative Hydroxylation of Arylboronic Acids and Its Applications in C‐O Bond Formation

AbstractWe have developed a straightforward methodology for converting arylboronic acids into phenols using NaOH‐mediated air oxidation. This reaction offers several advantages, including simple operation, excellent tolerance towards various functional groups, and scalability. The transmetalation of the resulting borate ester with NaOH occurs readily, enabling convenient derivatization of the borate ester into aryl ethers and esters using alkyl and acyl halides.

Bis(2‐pyridinyl)‐oxalamide copper complexes: Scope and limitation in CuAAC and oxidative hydroxylation of arylboronic acids

AbstractA versatile catalytic platform consisting of pyridine‐containing oxalamide and copper salt was developed. The resulting complexes were fully characterized by spectroscopic analyses, showing a successful complexation of copper metal. The applicability of the newly prepared platforms was evaluated in a one‐pot three‐components system for the preparation of 1,4‐disubstituted 1,2,3‐triazole via 1,3‐dipolar cycloaddition of terminal acetylenes, benzyl surrogates, and sodium azide in an aqueous environment. The corresponding cyclization proceeded smoothly affording the expected triazole products in moderate to good yields. In addition, the transformation of arylboronic acids to the corresponding phenols via ipso‐hydroxylation was also explored using the same platform in an aqueous basic condition. In the course of the study, unfortunately, some drawbacks that could limit the applicability of the catalytic platform were also observed especially in terms of recyclability of the platforms.

Linker-Assisted CdS-TiO2 Nanohybrids as Reusable Visible Light Photocatalysts for the Oxidative Hydroxylation of Arylboronic Acids

A variety of phenols have been obtained in aqueous media with moderate to excellent chemical yields (≤100%) by using arylboronic acids and esters as substrates, a robust CdS-TiO2 nanohybrid as a heterogeneous photocatalyst, visible light irradiation (467 nm), and an O2-saturated atmosphere. The nanohybrid was prepared through a linker-assisted methodology that uses mercapto alkanoic acids as the organic linkers. The nanohybrid showed improved photocatalytic activity in the hydroxylation of substituted arylboronic acids and phenyl boronic esters compared with that of pristine CdS quantum dots. The nanohybrid can be reused in up to five photocatalytic cycles with ∼90% of its outstanding activity preserved.

Construction of Covalent Organic Frameworks via a Visible-Light-Activated Photocatalytic Multicomponent Reaction.

Multicomponent reactions (MCRs), as a powerful one-pot combinatorial synthesis tool, have been recently applied to the synthesis of covalent organic frameworks (COFs). Compared with the thermally driven MCRs, the photocatalytic MCR-based COF synthesis has not yet been investigated. Herein, we first report the construction of COFs by a photocatalytic multicomponent reaction. Upon visible-light irradiation, a series of COFs with excellent crystallinity, stability, and permanent porosity are successfully synthesized via photoredox-catalyzed multicomponent Petasis reaction under ambient conditions. Additionally, the obtained Cy-N3-COF exhibits excellent photoactivity and recyclability for the visible-light-driven oxidative hydroxylation of arylboronic acids. The concept of photocatalytic multicomponent polymerization not only enriches the methodology for COF synthesis but also opens a new avenue for the construction of COFs that might not be possible with the existing synthetic methods based on thermally driven MCRs.

Bench-stable oxidant sodium percarbonate for functional group transformation of arylboronic acids

Sodium percarbonate (2Na2CO3·3H2O2, sodium carbonate-hydrogen peroxide, SPC), an easily accessible and bench-stable hydrogen peroxide-solid adduct based on inorganic salt, was efficiently used as a solid hydroxylating reagent for the ipso-hydroxylation of arylboronic acids under mild conditions, affording the corresponding phenols in an excellent manner at a fast rate. Moreover, a solvent-free solid-state reaction system using SPC for the oxidative hydroxylation of arylboronic acids was also demonstrated. Furthermore, subsequent treatment with benzyl halide was successfully performed, providing an alternative synthetic route for preparing benzyl phenyl ethers in a one-pot tandem reaction system.

Photoactive Covalent Organic Frameworks for Catalyzing Organic Reactions.

Because of their large surface areas, long-range order, π-π stacking interactions and hierarchically integrated building blocks, covalent organic frameworks (COFs) have attracted increasing attention. Recently, COFs have been regarded as promising heterogeneous photocatalysts, owing to their remarkable light-harvesting and energy transition capabilities. The research progress of COFs in photocatalytic organic synthesis is summarized, such as preparation of imines through the oxidation of amines, selective oxidation of sulfides, oxidative hydroxylation of arylboronic acids, C-H bond activation, cyclization, asymmetric organic synthesis and so on. Finally, a perspective of the present challenges of photoactive COFs in organic transformations is given.

Ru(N^N)3‐Metalloligand Pillared Zr6‐Organic Layers for Aerobic Photooxidation

AbstractThe stable Zr6‐based coordination framework Ru‐1 was assembled via the linkage of Zr6‐organic layers and photosensitized Ru(N^N)3‐metalloligand. Based on its 1D open channel and the incorporation of Ru(N^N)3‐photosensitizer, it can serve as a highly efficient heterogeneous catalyst in the aerobic photooxidation reactions, including photooxidation of sulfides, coupling of amines and oxidative hydroxylation of arylboronic acids. The stability and catalytic performance of recycled Ru‐1 was also explored.

Bare Magnetite-Promoted Oxidative Hydroxylation of Arylboronic Acids and Subsequent Conversion into Phenolic Compounds

AbstractThe simple combination of readily available, recoverable, and recyclable magnetite (Fe3O4) nanoparticles and an environmentally friendly oxidant (H2O2) induced the effective functional group transformation of arylboronic acids into their corresponding phenols under mild conditions. Moreover, subsequent treatment of the reaction intermediate with appropriate electrophiles was accomplished in a one-pot system, leading to the formation of halophenols and phenolic derivatives.

Carbocatalyst-promoted oxidative hydroxylation of arylboronic acids

Acidified multi-walled carbon nanotubes (CNT-COOH) are easily produced and highly compatible with an eco-friendly oxidant, hydrogen peroxide (H2O2), for ipso-hydroxylation of various arylboronic acids under aerobic conditions. The conversion proceeded smoothly at room temperature in a short reaction time, providing the corresponding phenolic derivatives in good to excellent yields. Owing to the easy recovery and reusability of the promoter, the present protocol represents a valuable alternative for functional group transformation of arylboronic acids.

A stack-guiding unit constructed 2D COF with improved charge carrier transport and versatile photocatalytic functions

Two-dimensional COFs have shown great potential in photocatalysis, and the ordered layer arrangement is vital for the efficient utilization of photogenerated charge carriers. Herein, with a twisted knot of hexabenzocoronene, an ultrastable dioxin-linked 2D COF with undulated lattice was synthesized. As evidenced by N2 sorption, optoelectrochemical and transient absorption experiments and theoretical calculation, the concave-convex complementarity between layers effectively prevents stacking dislocation, prolongs the donor/acceptor π-columnar arrays and thus facilitates the transport of charge carriers. This COF was successfully applied to a series of photocatalytic reactions, such as the degradation of pollutants, the oxidative hydroxylation of arylboronic acids and the reduction of CO2. The activity exhibited by this COF in all the photocatalytic systems was obviously superior to the planar counterpart. The results of this work illustrate that the incorporation of stack-guiding building blocks is an effective strategy to improve the photocatalytic performance of COF materials.