Atom Transfer Pathway Research Articles

Visible‐Light‐Mediated Selective Fluorine Incorporation by Anthraquinone‐Based Covalent Organic Framework as Recyclable Photocatalyst

Covalent organic frameworks (COFs) serve as an excellent foundation for heterogeneous photocatalysis. Herein, we synthesized an anthraquinone‐based COF (TpAQ) on a gram scale via a mechanochemical grinding pathway. This COF was employed as a visible‐light‐harvesting photocatalyst for selective fluorination of benzylic C‐H bonds and perfluoroalkylation of arenes. The carbonyl core in the anthraquinone linker facilitated benzylic fluorination through a hydrogen atom transfer (HAT) pathway. Control experiments and photophysical analysis were conducted to gain deeper insight into the reaction mechanism. The recyclability up to 5th cycle without significant loss of the yield (> 90%) highlighted the robustness of the catalyst. This reaction strategy was also executed on a gram scale to validate the scalability of the protocol.

Triphasic Hydroxysilylation of Alkenes by Mechanically Piezoelectric Catalysis

AbstractThe 1,2‐hydroxysilylation of alkenes is crucial for synthesizing organosilicon compounds which are key intermediates in material science, pharmaceuticals, and organic synthesis. The development of strategies employing hydrogen atom transfer pathways is currently hindered by the existence of various competing reactions. Herein, we reported a novel mechanochemical strategy for the triphasic 1,2‐hydroxysilylation of alkenes through a single‐electron‐transfer pathway. Our approach not only circumvents competitive reactions to enable the first‐ever 1,2‐hydroxysilylation of unactivated alkenes but also pioneers the research in mechanic force‐induced triphasic reactions under ambient conditions. This gentle method offers excellent compatibility with various functional groups, operates under simple and solvent‐free conditions, ensures rapid reaction time. Preliminary mechanistic investigations suggest that silylboronate can be transformed to a silicon radical by highly polarized Li2TiO3 particles and oxygen under ball‐milling condition.

Triphasic Hydroxysilylation of Alkenes by Mechanically Piezoelectric Catalysis.

The 1,2-hydroxysilylation of alkenes is crucial for synthesizing organosilicon compounds which are key intermediates in material science, pharmaceuticals, and organic synthesis. The development of strategies employing hydrogen atom transfer pathways is currently hindered by the existence of various competing reactions. Herein, we reported a novel mechanochemical strategy for the triphasic 1,2-hydroxysilylation of alkenes through a single-electron-transfer pathway. Our approach not only circumvents competitive reactions to enable the first-ever 1,2-hydroxysilylation of unactivated alkenes but also pioneers the research in mechanic force-induced triphasic reactions under ambient conditions. This gentle method offers excellent compatibility with various functional groups, operates under simple and solvent-free conditions, ensures rapid reaction time. Preliminary mechanistic investigations suggest that silylboronate can be transformed to a silicon radical by highly polarized Li2TiO3 particles and oxygen under ball-milling condition.

Red Light-Blue Light Chromoselective C(sp2)-X Bond Activation by Organic Helicenium-Based Photocatalysis.

Chromoselective bond activation has been achieved in organic helicenium (nPr-DMQA+)-based photoredox catalysis. Consequently, control over chromoselective C(sp2)-X bond activation in multihalogenated aromatics has been demonstrated. nPr-DMQA+ can only initiate the halogen atom transfer (XAT) pathway under red light irradiation to activate low-energy-accessible C(sp2)-I bonds. In contrast, blue light irradiation initiates consecutive photoinduced electron transfer (conPET) to activate more challenging C(sp2)-Br bonds. Comparative reaction outcomes have been demonstrated in the α-arylation of cyclic ketones with red and blue lights. Furthermore, red-light-mediated selective C(sp2)-I bonds have been activated in iodobromoarenes to keep the bromo functional handle untouched. Finally, the strength of the chromoselective catalysis has been highlighted with two-fold functionalization using both photo-to-transition metal and photo-to-photocatalyzed transformations.

Enones from aldehydes and alkenes by carbene-catalyzed dehydrogenative couplings

Enones are widely explored in synthetic chemistry as fundamental building blocks for a wide range of reactions and exhibit intriguing biological activities that are pivotal for drug discovery. The development of synthetic strategies for highly efficient preparation of enones thereby receives intense attention, in particular through the transition metal-catalyzed coupling reactions. Here, we describe a carbene-catalyzed cross dehydrogenative coupling (CDC) reaction that enables effective assembly of simple aldehydes and alkenes to afford a diverse set of enone derivatives. Mechanistically, the in situ generated aryl radical is pivotal to “activate” the alkene by forming an allyl radical through intermolecular hydrogen atom transfer (HAT) pathway and thus forging the carbon-carbon bond formation with aldehyde as the acyl synthon. Notably, our method represents the first example on the enone synthesis through coupling of “non-functionalized” aldehydes and alkenes as coupling partners, and offers a distinct organocatalytic pathway to the transition metal-catalyzed coupling transformations.

Photoredox chromium and cobalt dual catalysis for carbonyl allylation with butadiene via allyl radical intermediates

The allylation of aldehydes with dienes was achieved using cobalt and chromium catalysts through a metal-hydride hydrogen atom transfer (MHAT) pathway. The novel reaction resulted in the formation of various homo-allyl alcohols with excellent diastereoselectivity.

On the mechanism and selectivity of a novel iodine/peracetic acid process for the efficient and rapid elimination of micropollutants

In recent years, halogen radical-based advanced oxidation processes (AOPs) have shown promise for metal-free, highly selective elimination of phenolic pollutants with high mineralization. This study explored a novel approach combing peracetic acid (PAA) and naturally available I− to efficiently remove bisphenol A (BPA) over a broad pH range (2–8). A dose of 500 μM PAA and 100 μM I− at a pH of 3 completely eliminated 43.8 µM of BPA within 10 min. Aside from I−, two more halogen ions, Cl− and Br−, could also enhance BPA elimination in the presence of PAA. Based on quenching experiments and density functional theory (DFT) thermodynamic calculations, only I− among the three halogen ions can react with PAA (primarily through the single electron transfer pathway) to generate •I, which efficiently removes BPA. The Cl− and Br− only contribute partially to the BPA removal, mainly through HOCl and HOBr following the oxygen atom transfer pathway. The water matrices had no noticeable influence on the I−/PAA process, and eliminating micropollutants with electron-rich groups from actual water samples remained effective. This work proposes an innovative and feasible technique for PAA-based AOPs and showcases its promising applicability for the treatment of phenolic contaminants in wastewater.

Repurposing Iron‐ and 2‐Oxoglutarate‐Dependent Oxygenases to Catalyze Olefin Hydration

AbstractMononuclear nonheme iron(II) and 2‐oxoglutarate (Fe/2OG)‐dependent oxygenases and halogenases are known to catalyze a diverse set of oxidative reactions, including hydroxylation, halogenation, epoxidation, and desaturation in primary metabolism and natural product maturation. However, their use in abiotic transformations has mainly been limited to C−H oxidation. Herein, we show that various enzymes of this family, when reconstituted with Fe(II) or Fe(III), can catalyze Mukaiyama hydration—a redox neutral transformation. Distinct from the native reactions of the Fe/2OG enzymes, wherein oxygen atom transfer (OAT) catalyzed by an iron‐oxo species is involved, this nonnative transformation proceeds through a hydrogen atom transfer (HAT) pathway in a 2OG‐independent manner. Additionally, in contrast to conventional inorganic catalysts, wherein a dinuclear iron species is responsible for HAT, the Fe/2OG enzymes exploit a mononuclear iron center to support this reaction. Collectively, our work demonstrates that Fe/2OG enzymes have utility in catalysis beyond the current scope of catalytic oxidation.

Repurposing Iron- and 2-Oxoglutarate-Dependent Oxygenases to Catalyze Olefin Hydration.

Mononuclear nonheme iron(II) and 2-oxoglutarate (Fe/2OG)-dependent oxygenases and halogenases are known to catalyze a diverse set of oxidative reactions, including hydroxylation, halogenation, epoxidation, and desaturation in primary metabolism and natural product maturation. However, their use in abiotic transformations has mainly been limited to C-H oxidation. Herein, we show that various enzymes of this family, when reconstituted with Fe(II) or Fe(III), can catalyze Mukaiyama hydration-a redox neutral transformation. Distinct from the native reactions of the Fe/2OG enzymes, wherein oxygen atom transfer (OAT) catalyzed by an iron-oxo species is involved, this nonnative transformation proceeds through a hydrogen atom transfer (HAT) pathway in a 2OG-independent manner. Additionally, in contrast to conventional inorganic catalysts, wherein a dinuclear iron species is responsible for HAT, the Fe/2OG enzymes exploit a mononuclear iron center to support this reaction. Collectively, our work demonstrates that Fe/2OG enzymes have utility in catalysis beyond the current scope of catalytic oxidation.

2-Silylated Dihydroquinazolinone as Photocatalytic Energy Transfer Enabled Radical Hydrosilylation Reagent.

The hydrosilylation of alkenes is one of the most important methods for the synthesis of organosilicon compounds. In addition to the platinum-catalyzed hydrosilylation, silyl radical addition reactions are notable as economic reactions. We have developed an efficient and widely applicable silyl radical addition reaction by using 2-silylated dihydroquinazolinone derivatives under photocatalytic conditions. Electron-deficient alkenes and styrene derivatives underwent hydrosilylation to give addition products in good to high yields. Mechanistic studies indicated that the photocatalyst functioned not as a photoredox catalyst but as an energy transfer catalyst. DFT calculations clarified that the triplet excited state of 2-silylated dihydroquinazolinone derivatives released a silyl radical through the homolytic cleavage of a carbon-silicon bond, and this was followed by the hydrogen atom transfer pathway, not the redox pathway.

Copper-Mediated Dehydrogenative C(sp3)-H Borylation of Alkanes.

Borylations of inert carbon-hydrogen bonds are highly useful for transforming feedstock chemicals into versatile organoboron reagents. Catalysis of these reactions has historically relied on precious-metal complexes, which promote dehydrogenative borylations with diboron reagents under oxidant-free conditions. Recently, photoinduced radical-mediated borylations involving hydrogen atom transfer pathways have emerged as attractive alternatives because they provide complimentary regioselectivities and proceed under metal-free conditions. However, these net oxidative processes require stoichiometric oxidants and therefore cannot compete with the high atom economy of their precious-metal-catalyzed counterparts. Herein, we report that CuCl2 catalyzes radical-mediated, dehydrogenative C(sp3)-H borylations of alkanes with bis(catecholato)diboron under oxidant-free conditions. This is a result of an unexpected dual role of the copper catalyst, which promotes oxidation of the diboron reagent to generate an electrophilic bis-boryloxide that acts as an effective borylating agent in subsequent redox-neutral photocatalytic C-H borylations.

Carbene‐Catalyzed Intermolecular Dehydrogenative Coupling of Aldehydes with C(sp3)−H Bonds

AbstractThe development of catalyst‐controlled methods for direct functionalization of two distinct C−H bonds represents an appealing approach for C−C formations in synthetic chemistry. Herein, we describe an organocatalytic approach for straightforward acylation of C(sp3)−H bonds employing readily available aldehyde as “acyl source” involving dehydrogenative coupling of aldehydes with ether, amine, or benzylic C(sp3)−H bonds. The developed method affords a broad range of ketones under mild conditions. Mechanistically, simple ortho‐cyanoiodobenzene is essential in the oxidative radical N‐heterocyclic carbene catalysis to give a ketyl radical and C(sp3) radical through a rarely explored intermolecular hydrogen atom transfer pathway, rendering the acylative C−C formations in high efficiency under a metal‐ and light‐free catalytic conditions. Moreover, the prepared products show promising anti‐bacterial activities that shall encourage further investigations on novel agrochemical development.

Carbene-Catalyzed Intermolecular Dehydrogenative Coupling of Aldehydes with C(sp3)-H Bonds.

The development of catalyst-controlled methods for direct functionalization of two distinct C-H bonds represents an appealing approach for C-C formations in synthetic chemistry. Herein, we describe an organocatalytic approach for straightforward acylation of C(sp3)-H bonds employing readily available aldehyde as "acyl source" involving dehydrogenative coupling of aldehydes with ether, amine, or benzylic C(sp3)-H bonds. The developed method affords a broad range of ketones under mild conditions. Mechanistically, simple ortho-cyanoiodobenzene is essential in the oxidative radical N-heterocyclic carbene catalysis to give a ketyl radical and C(sp3) radical through a rarely explored intermolecular hydrogen atom transfer pathway, rendering the acylative C-C formations in high efficiency under a metal- and light-free catalytic conditions. Moreover, the prepared products show promising anti-bacterial activities that shall encourage further investigations on novel agrochemical development.

Br−/BrO−-mediated highly efficient photoelectrochemical epoxidation of alkenes on α-Fe2O3

Epoxides are significant intermediates for the manufacture of pharmaceuticals and epoxy resins. In this study, we develop a Br−/BrO− mediated photoelectrochemical epoxidation system on α-Fe2O3. High selectivity (up to >99%) and faradaic efficiency (up to 82 ± 4%) for the epoxidation of a wide range of alkenes are achieved, with water as oxygen source, which are far beyond the most reported electrochemical and photoelectrochemical epoxidation performances. Further, we can verify that the epoxidation reaction is mediated by Br−/BrO− route, in which Br− is oxidized non-radically to BrO− by an oxygen atom transfer pathway on α-Fe2O3, and the formed BrO− in turn transfers its oxygen atom to the alkenes. The non-radical mediated characteristic and the favorable thermodynamics of the oxygen atom transfer process make the epoxidation reactions very efficient. We believe that this photoelectrochemical Br−/BrO−-mediated epoxidation provides a promising strategy for value-added production of epoxides and hydrogen.

Hydrogen anode/cathode co-productions-coupled anode alcohol selective oxidation and distinctive H/e transfer pathways

Co-electrolysis, which can produce more than one kind of high value products concurrently at lowered energy cost, has attracted increasing attention. However, the development of novel co-electrolysis systems and efficient electro-catalysts, and the investigation of corresponding reaction mechanism, are urgently needed. Anodic biomass oxidation-coupled electrocatalytic anode/cathode hydrogen co-production has been attracting intense attentions most recently. Herein, a co-electrolysis system of α-H-armed alcohols and water has been established by introducing an efficient PdBi/NF electrocatalyst, in which hydrogen can be obtained from both anode and cathode, and alcohols can be selectively oxidized to corresponding high-value chemicals. In particular, valuable 3-Hydroxypropionic acid (3-Hp) at the anode, together with hydrogen gas at both cathode and anode, can be obtained by the co-electrolysis of water and 1, 3-propanediol (1, 3-P). More importantly, the reaction mechanism, especially the hydrogen atom and electron transfer pathway have been investigated in detail, which has been overlooked in previous literatures.

A TBADT photocatalyst-enabled radical-induced cyclization pathway to access functionalized dihydrobenzofurans.

Herein, this work describes a photoinduced visible light-mediated radical cyclization for constructing dihydrobenzofuran (DHB) scaffolds. Notably, this cascade photochemical process is tolerable with various aromatic aldehydes and diverse alkynyl aryl ethers and proceeds via an intramolecular 1,5-hydrogen atom transfer (HAT) pathway. Significantly, acyl C-H activation under mild conditions has been achieved without the use of additives or reagents. The photocatalyst, tetrabutylammonium decatungstate (TBADT), plays an important role in the present strategy by facilitating the well-known hydrogen atom transfer during the course of the reaction.

Reevaluation of radical-induced differentiation in UV-based advanced oxidation processes (UV/hydrogen peroxide, UV/peroxydisulfate, and UV/chlorine) for metronidazole removal: Kinetics, mechanism, toxicity variation, and DFT studies

Metronidazole (MNZ) is widely employed as an antibiotic, but it is recalcitrant in the water environment. The residual of MNZ has sparked significant concern because to its detrimental impacts on environmental safety and human health. In this study, the elimination of metronidazole (MNZ) was comparatively examined by UV/hydrogen peroxide (UV/H2O2), UV/peroxydisulfate (UV/PDS), and UV/chlorine processes. The best degradation rate constant was demonstrated by the UV/PDS (2.4 × 10−3 s−1), followed by UV/H2O2 AOP (1.0 × 10−3 s−1) and UV/chlorine AOP (3.7 × 10−4 s−1). The hydroxyl radicals (HO) played significant roles in UV-AOPs eliminating MNZ by 87.7 %, 30.2 %, and 43.0 % in UV/H2O2, UV/PDS, and UV/chlorine treatments, respectively. Additionally, the sulfate radical (SO4−) and chlorine radical (Cl) were essential for the elimination of MNZ, contributing 63.0 % and 36.2 %, respectively, in UV/PDS and UV/chlorine. The first-order rate constants (kobs) increase linearly with the increasing oxidant dosage in various UV-AOPs. The higher degradation efficiency of MNZ was observed in acid conditions due to the transition of oxygen active species. The presence of chloride (Cl−), bicarbonate (HCO3–), nitrate (NO3–), and humid acid (HA) remarkably inhabited the degradation of MNZ in UV/H2O2 and UV/PDS, whereas NO3– showed a promotion effect in UV/chlorine. In diverse water matrixes, UV/PDS usually consumed less electrical energy than UV/H2O2 and UV/chlorine. Combining density functional theory calculations and transformation products identification, it was revealed that HO was conducive to attack the nitrogenous heterocycle of MNZ via hydrogen atom transfer (HAT) and radical adduct formation (RAF) pathways, whereas SO4− and Cl oxidation favored RAF route. Some chlorine- and hydroxylation-intermediates, notably those produced in UV/chlorine AOP, displayed higher toxicity than the parent compounds. Overall, the findings unequivocally prove that UV-AOPs are effective, economical, and practical for the remediation of antibiotics-contaminated water.

Difluoromethylation of Unactivated Alkenes Using Freon-22 through Tertiary Amine-Borane-Triggered Halogen Atom Transfer.

The application of abundant and inexpensive fluorine feedstock sources to synthesize fluorinated compounds is an appealing yet underexplored strategy. Here, we report a photocatalytic radical hydrodifluoromethylation of unactivated alkenes with an inexpensive industrial chemical, chlorodifluoromethane (ClCF2H, Freon-22). This protocol is realized by merging tertiary amine-ligated boryl radical-induced halogen atom transfer (XAT) with organophotoredox catalysis under blue light irradiation. A broad scope of readily accessible alkenes featuring a variety of functional groups and drug and natural product moieties could be selectively difluoromethylated with good efficiency in a metal-free manner. Combined experimental and computational studies suggest that the key XAT process of ClCF2H is both thermodynamically and kinetically favored over the hydrogen atom transfer pathway owing to the formation of a strong boron-chlorine (B-Cl) bond and the low-lying antibonding orbital of the carbon-chlorine (C-Cl) bond.

Mo2C as Pre-Catalyst for the C-H Allylic Oxygenation of Alkenes and Terpenoids in the Presence of H2O2

In this study, commercially available molybdenum carbide (Mo2C) was used, in the presence of H2O2, as an efficient pre-catalyst for the selective C-H allylic oxygenation of several unsaturated molecules into the corresponding allylic alcohols. Under these basic conditions, an air-stable, molybdenum-based polyoxometalate cluster (Mo-POM) was formed in situ, leading to the generation of singlet oxygen (1O2), which is responsible for the oxygenation reactions. X-ray diffraction, SEM/EDX and HRMS analyses support the formation mainly of the Mo6O192− cluster. Following the proposed procedure, a series of cycloalkenes, styrenes, terpenoids and methyl oleate were successfully transformed into hydroperoxides. After subsequent reduction, the corresponding allylic alcohols were produced with good yields and in lab-scale quantities. A mechanistic study excluded a hydrogen atom transfer pathway and supported the twix-selective oxygenation of cycloalkenes on the more sterically hindered side via the 1O2 generation.

Site-selective amination towards tertiary aliphatic allylamines

Aliphatic allylamines are widely used for the synthesis of diverse building blocks for agrochemicals and pharmaceuticals; there is therefore considerable interest in developing versatile and direct routes to aliphatic allylamines using common chemical feedstocks—olefins and amines. However, examples of such coupling reactions remain limited. It is even more challenging to achieve this goal with precise site control. Here we report that the combination of a photocatalyst and cobaloxime enables site-selective amination of olefins with secondary alkyl amines to afford allylic amines, eliminating the need for oxidants. This method is compatible with a broad scope of olefins and can be extended to achieve a site- and diastereoselective amination of terpenes. Mechanistic studies disclose that the reaction proceeds via a cobaloxime-promoted hydrogen atom transfer pathway to afford the product that results from cleavage of the stronger, primary allylic C–H bonds over other weaker allylic C–H bond options.