Cl Bond Cleavage Research Articles

Mechanistic investigation of iron-catalyzed cross-coupling using sterically encumbered β-diketonate ligands

Iron β-diketonate compounds are reported using the hindered 2,6-dimesitylbenzoyl pinacolone (Aracac), which sterically precludes formation of tris-Aracac complexes. Fe(Aracac)2 (1), Fe(Aracac)2Cl (2) were synthesized and characterized using crystallographic, spectroscopic, elemental analysis, and electrochemical techniques. Iron complexes with only one Aracac ligand form dimers [Fe(Aracac)(THF)2]2(μ-OTf)2 (3) and [(Aracac)Fe(μ-mesityl)]2 (4). Complexes 1–4 were evaluated in a Kumada cross-coupling reaction and compared with the parent FeIII(acac)3 complex. Whereas the bis-acac complexes 1 and 2 show only modest activity (∼50 % yield after 60 min), the mono-acac complexes 3 and 4 are nearly as effective as the parent compound (∼90 % yield). DFT computations, in conjunction with catalytic results and electrochemical observations, suggest a catalytic cycle involving an oxidative addition of an aryl-chloride to Fe(II) to produce a high-spin Fe(IV) complex that could alternately be described as a high-spin Fe(II) stabilizing two coordinated organoradicals. These results suggest that a unique aspect of acac as a ligand for this reaction is its ability to support high-spin organoiron complexes, which can provide low-barrier pathways for C–Cl bond cleavage and CC bond formation reactions.

Enhancing C[sbnd]Cl bond cleavage and ozone conversion: Optimizing surface Co3+ and chemosorbed oxygen through polystyrene and dodecylamine modification

The activation of CCl bonds and the conversion of O3 are crucial for the degradation of chlorinated hydrocarbons through catalytic ozonation. This work constructed the surface active sites of Co3O4 with polystyrene (PS) and dodecylamine to enhance dichloromethane (DCM) removal and O3 conversion in photo-ozone catalysis simultaneously. The dodecylamine created active Co3+ on the catalyst surface, and the PS prevented the elimination of active Co3+ during calcination by forming CCoO. The O3 pre-treatment removed the deposited carbon to expose the active Co3+ and the chemisorbed oxygen. The surface Co3+ played a crucial role in cleaving the CCl bonds of DCM, and the chemosorbed oxygen facilitated the conversion of O3 into O. Moreover, the photogenerated holes oxidized the intermediates formed after the DCM dechlorination. The removal rate of DCM for the sample co-modified by PS and dodecylamine was 80.8 mg/(g‧h) under one sun irradiation, which was 2.2 times higher than that by Co3O4.

Catalytic combustion of 1,2-dichloroethane via C[sbnd]C/C[sbnd]Cl bond cleavage through core-shell structured CeO2@TiO2 catalyst with excellent catalytic performances

A highly representative chlorinated volatile organic compound (Cl-VOCs), including 1,2-dichloroethane (1,2-DCE), which is extremely hazardous to human health and the environment. Improving the catalytic activity is very important for catalytic degradation of 1,2-DCE. In the work, CeO2@TiO2 catalyst which has a core-shell structure were made by a simple two-step hydrothermal synthesis method for 1,2-DCE oxidation. CeO2@TiO2 catalysts show higher activity (T90=275 °C), better carbon dioxide yield (YCO2=89.54 %) and lower by-products production than primitive CeO2 and TiO2. Due to its core-shell structure, CeO2@TiO2 shows the appropriate oxidation and reduction ability and appropriate acidity distribution, the powerful relationship between Ce and Ti elements in CeO2@TiO2 leads to effectively generate activity Ce3+and oxygen, which is greatly great enhance the activity of CeO2@TiO2 in 1,2-DCE catalytic oxidation. Based on the analysis of 1,2-DCE degradation performance and physical and chemical attributes, it proposes a way to catalyze 1,2-DCE catalytic oxidation over CeO2@TiO2. In addition, CeO2@TiO2 has good stability, and adding water vapor over the CeO2@TiO2 during 1,2-DCE degradation to promote 1,2-DCE conversion. The findings of this study might offer some new suggestions for creating catalysts that will destroy Cl-VOCs in their preparation and design.

Core taxa, co-occurrence pattern, diversity, and metabolic pathways contributing to robust anaerobic biodegradation of chlorophenol

It is hard to achieve robustness in anaerobic biodegradation of trichlorophenol (TCP). We hypothesized that specific combinations of environmental factors determine phylogenetic diversity and play important roles in the decomposition and stability of TCP-biodegrading bacteria. The anaerobic bioreactor was operated at 35 °C (H condition) or 30 °C (L condition) and mainly fed with TCP (from 28 μM to 180 μM) and organic material. Metagenome sequencing was combined with 16S rRNA gene amplicon sequencing for the microbial community analysis. The results exhibited that the property of robustness occurred in specific conditions. The corresponding co-occurrence and diversity patterns suggest high collectivization, degree and evenness for robust communities. Two types of core functional taxa were recognized: dechlorinators (unclassified Anaerolineae, Thermanaerothrix and Desulfovibrio) and ring-opening members (unclassified Proteobacteria, Methanosarcina, Methanoperedens, and Rubrobacter). The deterministic process of the expansion of niche of syntrophic bacteria at higher temperatures was confirmed. The reductive and hydrolytic dechlorination mechanisms jointly lead to C–Cl bond cleavage. H ultimately adapted to the stress of high TCP loading, with more abundant ring-opening enzyme (EC 3.1.1.45, ∼55%) and hydrolytic dechlorinase (EC 3.8.1.5, 26.5%) genes than L (∼47%, 10.5%). The functional structure (based on KEGG) in H was highly stable despite the high loading of TCP (up to 60 μM), but not in L. Furthermore, an unknown taxon with multiple functions (dechlorinating and ring-opening) was found based on genetic sequencing; its functional contribution of EC 3.8.1.5 in H (26.5%) was higher than that in L (10.5%), and it possessed a new metabolic pathway for biodegradation of halogenated aromatic compounds. This new finding is supplementary to the robust mechanisms underlying organic chlorine biodegradation, which can be used to support the engineering, regulation, and design of synthetic microbiomes.

Degradation and dechlorination of trichloroacetic acid induced by an in situ 222 nm KrCl* excimer radiation

Since the coronavirus disease 2019 (COVID-19) pandemic epidemic, the excessive usage of chlorinated disinfectants raised the substantial risks of disinfection by-products (DBPs) exposure. While several technologies may remove the typical carcinogenic DBPs, trichloroacetic acid (TCAA), their application for continuous treatment is limited due to their complexity and expensive or hazardous inputs. In this study, degradation and dechlorination of TCAA induced by an in situ 222 nm KrCl* excimer radiation as well as role of oxygen in the reaction pathway were investigated. Quantum chemical calculation methods were used to help predict the reaction mechanism. Experimental results showed that UV irradiance increased with increasing input power and decreased when the input power exceeded 60 W. Decomposition and dechlorination were simultaneously achieved, where around 78% of TCAA (0.62 mM) can be eliminated and 78% dechlorination within 200 min. Dissolved oxygen showed little effect on the TCAA degradation but greatly boosted the dechlorination as it can additionally generate hydroxyl radical (•OH) in the reaction process. Computational results showed that under 222 nm irradiation, TCAA was excited from S0 to S1 state and then decayed by internal crossing process to T1 state, and a reaction without potential energy barrier followed, resulting in the breaking of C–Cl bond and finally returning to S0 state. Subsequent C–Cl bond cleavage occurred by a barrierless •OH insertion and HCl elimination (27.9 kcal/mol). Finally, the •OH attacked (14.6 kcal/mol) the intermediate byproducts, leading to complete dechlorination and decomposition. The KrCl* excimer radiation has obvious advantages in terms of energy efficiency compared to other competitive methods. These results provide insight into the mechanisms of TCAA dechlorination and decomposition under KrCl* excimer radiation, as well as important information for guiding research toward direct and indirect photolysis of halogenated DBPs.

Chlorine Atom Transfer of Unactivated Alkyl Chlorides Enabled by Zirconocene and Photoredox Catalysis

Alkyl chlorides are robust precursors to carbon radicals; however, their relative inertness has hampered their practical use. Although modern photochemical strategies have greatly enhanced the utility of alkyl chlorides as radical precursors, these methods often depend on strongly reducing conditions leading to unproductive side reactions. Here, we report a catalytic radical generation from 1°, 2°, and 3° unactivated alkyl chlorides with zirconocene and photoredox catalysis, which enables both hydrogenation and borylation on a range of structurally complex molecules. This mild zirconocene-catalyzed protocol shows that zirconium can render the C–Cl bond cleavage more exergonic and can lower the activation energy of the transition state, amplifying the ability of metallocenes toward halogen atom transfer.

Regioselective Transfer Hydrogenative Defluorination of Polyfluoroarenes Catalyzed by Bifunctional Azairidacycle

The catalytic hydrodefluorination (HDF) with a bifunctional azairidacycle using HCOOK was examined for cyano- and chloro-substituted fluoroarenes, including penta- and tetrafluorobenzonitriles, tetrafluoroterephthalonitrile, tetrafluorophthalonitrile, 3-chloro-2,4,5,6-tetrafluoropyridine, and 4-cyano-2,3,5,6-tetrafluoropyridine. The reaction was performed in the presence of a controlled amount of HCOOK with a substrate/catalyst ratio (S/C) of 100 in a 1:1 mixture of 1,2-dimethoxyethane (DME) and H2O at an ambient temperature of 30 °C to obtain partially fluorinated compounds with satisfactory regioselectivities. The C–F bond cleavage proceeded favorably at the para position of substituents other than fluorine, which is in consonance with the nucleophilic aromatic substitution mechanism. In the HDF of tetrafluoroterephthalonitrile and 4-cyano-2,3,5,6-tetrafluoropyridine, which do not contain a fluorine atom at the para position of the cyano group, the double defluorination occurred solely at the 2- and 5-positions, as confirmed by X-ray crystallography. The HDF of 3-chloro-2,4,5,6-tetrafluoropyridine gave preference to the C–F bond cleavage over the C–Cl bond cleavage, unlike the dehalogenation pathway via electron-transfer radical anion fragmentation. In addition, new azairidacycles with an electron-donating methoxy substituent on the C–N chelating ligand were synthesized and served as a catalyst precursor (0.2 mol%) for the transfer hydrogenative defluorination of pentafluoropyridine, leading to 2,3,5,6-tetrafluoropyridine with up to a turnover number (TON) of 418.

Palladium(II)‐Catalyzed Direct Arylations of Quinoxalin‐2(1H)‐ones with Arylsulfonyl Chlorides

AbstractA palladium‐catalyzed desulfitative coupling of quinoxalin‐2(1H)‐ones with arylsulfonyl chlorides is described. C3‐Arylquinoxalin‐2(1H)‐ones are readily accessible by reacting various arylsulfonyl chloride derivatives with quinoxalin‐2(1H)‐ones. The arylsulfonyl chloride undergoes palladium‐catalyzed C−S and S−Cl bond cleavage, which leads to direct desulfitative cross‐coupling with the quinoxalin‐2(H)‐one. Chloro‐ and fluoro‐aryl sulfonyl chlorides are successfully coupled with quinoxalin‐2(1H)‐ones without carbon–halogen bond cleavage.

Mechanochemical Synthesis of Ketones via Chemoselective Suzuki-Miyaura Cross-Coupling of Acyl Chlorides.

The direct synthesis of ketones via acyl Suzuki-Miyaura cross-coupling of widely available acyl chlorides is a central transformation in organic synthesis. Herein, we report the first mechanochemical solvent-free method for highly chemoselective synthesis of ketones from acyl chlorides and boronic acids. This acylation reaction is conducted in the solid state, in the absence of potentially harmful solvents, for a short reaction time and shows excellent selectivity for C(acyl)-Cl bond cleavage.

Photoredox Activation of Inert Alkyl Chlorides for the Reductive Cross-Coupling with Aromatic Alkenes.

The inertness of chloroalkanes has precluded them as coupling partners for cross-coupling reactions. Herein we disclose a general strategy for the activation of inert alkyl chlorides through photoredox catalysis and their use as coupling partners with alkenes. The catalytic system is formed by [Ni(OTf)(Py2 Ts tacn)](OTf) (1Ni ), which is responsible for the Csp3 -Cl bond activation, and [Ir(NMe2 bpy)(ppy)2 ]PF6, (PCIr NMe2 ), which is the photoredox catalyst. Combined experimental and theoretical studies show an in situ photogenerated NiI intermediate ([Ni(Py2 Ts tacn)]+ ) which is catalytically competent for the Csp3 -Cl bond cleavage via a SN 2 mechanism for primary alkyl chlorides, forming carbon-centered free radicals, which react with the olefin leading to the formation of the Csp3 -Csp3 bond. These results suggest inert alkyl chlorides can be electrophiles for developing new intermolecular strategies in which low-valent aminopyridine nickel complexes act as key catalytic species.

Photoredox Activation of Inert Alkyl Chlorides for the Reductive Cross‐Coupling with Aromatic Alkenes

AbstractThe inertness of chloroalkanes has precluded them as coupling partners for cross‐coupling reactions. Herein we disclose a general strategy for the activation of inert alkyl chlorides through photoredox catalysis and their use as coupling partners with alkenes. The catalytic system is formed by [Ni(OTf)(Py2Tstacn)](OTf) (1Ni), which is responsible for the Csp3−Cl bond activation, and [Ir(NMe2bpy)(ppy)2]PF6, (PCIrNMe2), which is the photoredox catalyst. Combined experimental and theoretical studies show an in situ photogenerated NiI intermediate ([Ni(Py2Tstacn)]+) which is catalytically competent for the Csp3−Cl bond cleavage via a SN2 mechanism for primary alkyl chlorides, forming carbon‐centered free radicals, which react with the olefin leading to the formation of the Csp3−Csp3 bond. These results suggest inert alkyl chlorides can be electrophiles for developing new intermolecular strategies in which low‐valent aminopyridine nickel complexes act as key catalytic species.

Insight into the Surface-Tuned Activity and Cl2/HCl Selectivity in the Catalytic Oxidation of Vinyl Chloride over Co3O4(110) versus (001): A DFT Study

Co3O4 has received tremendous attention in the oxidation of chlorinated volatile organic compounds (CVOCs) for its general catalytic oxidation ability. However, to rationally optimize the Co3O4-based catalysts, ascertaining the reaction mechanism and active-site-specific activity variation at the molecular level remains an open issue. Herein, we performed the first-principles calculations to systematically explore the catalytic oxidation process of vinyl chloride (VC) on two representative (110) and (001) surfaces of Co3O4. The reaction pathways of VC oxidation are thoroughly calculated, mainly consisting of three subprocesses, namely, the C–Cl bond scission, CH2CH oxidation, and the elimination of Cl species. Results show that the oxygen vacancy is indispensable to break the C–Cl bond on Co3O4(110), whereas the C–Cl bond cleavage can be achieved by the collaboration of two adjacent Co5c sites at a lower barrier on Co3O4(001). Both surfaces are reactive for the oxidation of the CH2CH group, indicative of the excellent oxidative ability of the Co3O4 catalyst. More importantly, we identify the rate-determining step in the overall VC oxidation as the elimination of Cl species, and HCl is the preferential chlorine-containing product rather than Cl2. In comparison with Co3O4(110), the Co3O4(001) surface gives a lower energy barrier for Cl elimination and indicates its potentially superior performance for VC oxidation. Finally, some propositions are made for how to improve the VC oxidation on Co3O4 catalysts. These fundamental insights identified in this work may provide a theoretical basis for the design and synthesis of Co3O4-based catalysts in CVOC oxidation.

Synthesis, Structure, and Bonding Properties of Hypercoordinate Triorganotin Compounds Featuring Three O→Sn Interactions

AbstractHere, heptacoordinate and hexacoordinate triorganotin compounds [o‐{(Ph2)(O)P}C6H4]3SnX (X=F (2 a), Cl (2 b), Me (2 f)) featuring three O→Sn interactions were synthesized. Chloro (2 b) and methyl (2 f) compounds were obtained from the oxygenation of the heptacoordinate triorganotin compounds {o‐(Ph2P)C6H4}3SnX (X=Cl (1 b), Me (1 f)) bearing three P donors. The fluorination of 2 b using tetrabutylammonium fluoride (TBAF) yielded the fluoro analogue 2 a. Structural studies were performed on a series of hypercoordinate triorganotin compounds, 2. The chloro analogue (2 b) exhibited a hexacoordinate geometry attributed to the Sn−Cl bond cleavage, where the highly electrophilic stannyl cation was stabilized by three O donors. In contrast, both the fluoro (2 a) and methyl (2 f) analogues exhibited heptacoordinate structures. Although 2 f adopted a tricapped tetrahedral geometry, 2 b is classified to adopt a distinctly rare bicapped trigonal pyramidal geometry featuring one strong LP(O)→σ*(Sn−F) interaction and two weak LP(O)→σ*(Sn−C) interactions (LP: lone pair electrons). The origin of the structural alternations depending on the substituent is discussed using density functional theory (DFT) calculations.

Insights on the site-selective fragmentation of CF2Cl2 and CH2Cl2 at the chlorine K-edge from ab initio calculations

A theoretical study of CF 2 Cl 2 and CH 2 Cl 2 photoabsorption spectra and photofragmentation profiles at the chlorine K-edge is presented. We simulated the NEXAFS spectra at the Core-Valence Separated Coupled Cluster Singles and Doubles (CVS-CCSD) level and interpreted the photofragmentation profile in terms of the potential energy surfaces (PES) computed at the complete active space self-consistent field (CASSCF) level along the C–Cl bond cleavage for the 1 s Cl → σ C - Cl ∗ , 1 s Cl → 7 b 2 , and 1 s Cl → ∞ states. The PES of CF 2 Cl 2 shows that the chlorine’s 1 s → σ ∗ transition tends to fragment into the CF 2 Cl ∗ radical and the Cl (1s 1 2s 2 2p 6 3s 2 3p 6 ) excited atom, whereas the 1 s Cl → 7 b 2 and 1 s Cl → ∞ states are less prone to undergo this fragmentation channel. The same tendency was observed for the CH 2 Cl 2 molecule, suggesting that the F atom has little influence on the PES.

Controlling Radical Relay Processes with Visible Light

Harnessing free radical intermediates for selective functionalization of organic compounds has been widely demonstrated under photocatalytic conditions requiring a distinct photocatalyst. In this issue of Chem, Seo, Chang, and co-workers demonstrate an alternative photocatalyst-free light-mediated approach that allows efficient amidation of aldehydes via N-centered radical intermediate.

Platinum Nanoparticle Based Dip‐Catalyst for Facile Hydrogenation of Quinoline, Unfunctionalized Olefins, and Imines

AbstractDeveloping a bio‐inspired catalyst with the efficiency of homogeneous catalysts and recyclability similar to heterogeneous catalysts is quite desirable but challenging. Incorporation of metal nanoparticles (<10 nm) on abundant, low‐cost supports from agricultural waste can yield highly active and product‐selective pseudo‐homogeneous catalysts, with attributes such as ease of retrieval for next use, reusability, etc. In this work, the fabrication of an efficient and reusable ‘dip‐catalyst’ by anchoring platinum nanoparticles on white jute plant (Corchorus capsularis) stems (JPS) and its use for the hydrogenation of N‐heteroarenes, unfunctionalized olefins, and imines are described. The catalyst was characterized using XRD, SEM, EDS, TEM, HRTEM, FTIR, and XPS, and TEM shows spherical (average diameter 4–5 nm) non‐agglomerated metal nanoparticles. Catalyst was used for the chemoselective (>99 % selectivity) hydrogenation of quinoline with a quantitative (>99 %) conversion to 1,2,3,4‐tetrahydroquinoline (py‐THQ) under hydrogen at a pressure <30 bar. Also, functional group tolerance is indicated by the quantitative hydrogenation of 6‐chloroquinoline to 6‐chloro‐1,2,3,4‐tetrahydroquinoline, which is a longstanding challenge owing to C−Cl bond cleavage. In the hydrogenation of structurally‐challenging trisubstituted trans‐2‐methyl‐3‐phenyl‐2‐propen‐1‐ol olefins, 64 % conversion and >99 % selectivity was achieved. A series of imines with different chain lengths was also hydrogenated selectively in methanol with >99 % conversion. Density functional theory (DFT) calculations reveal the efficient adsorption of 6‐chloroquinoline on the surface of Pt nanoparticles on Pt@JPS strips in a tilted orientation placing the C−Cl bond away from the metal and allowing facile desorption of 6‐chloro‐1,2,3,4‐tetrahydroquinoline leading to higher chemoselectivity. The spent catalyst can be reused for 12 consecutive cycles without significant damage to the cellulosic surface.

Mechanistic insight into ligand‐promoted C–H alkenylation of arenes with alkynes: A computational study

AbstractThe ligand‐promoted palladium‐catalyzed arylation of alkynes with arenes without directing group is able to furnish alkenyl chlorides via a 1,4‐chlorine migration or trisubstituted alkenes. This reaction is challenging due to bidentate N, N ligand and electron‐neutral arenes have rarely been reported to afford good yields. Intrigued by the novel strategy, we carried out density functional theory calculations to unravel the ligand effects and origins of substituent‐controlled chemoselectivity of the C–H functionalization reactions. For the n‐propyl‐substituted alkyne system, CMD process is identified as the rate‐determining step. And the chemoselectivity is controlled by oxidative addition with the C–Cl bond cleavage and protonation process. While for 3,5‐dimethylphenyl‐substituented alkyne system, the dominant pathway turns to the protonation process. The electrostatic attractions, repulsive force and aryl substituent effects jointly result in reverse chemoselectivity. Compared with the L2 ligand, the bidentate ligand L1 reacts with palladium acetate to form a different stable square‐planer species. The steric repulsion are found to be mainly responsible for the absence of products using the L2 ligand, which is different from previous reports.

Characterizing the biotransformation of hexachlorocyclohexanes in wheat using compound-specific stable isotope analysis and enantiomer fraction analysis

Hexachlorocyclohexane isomers (HCHs) are persistent organic pollutants being responsible for environmental contamination worldwide. In order to characterize transformation of HCHs in different plant compartments during uptake, a hydroponic experimental setup was designed using wheat as the test plant. The extent of transformation was determined by using compound-specific isotope analysis (CSIA) and enantiomer fraction (EF) analysis. In nutrient solutions, no change of carbon (δ13C) and chlorine isotope ratios (δ37Cl) of α-HCH and β-HCH was detected throughout the experiment indicating no transformation there. In wheat leaves, stems and roots, however, transformation of α-HCH due to a C‒Cl bond cleavage was indicated by increasing δ13C and δ37Cl compared to the nutrient solution. In addition, 1,3,4,5,6-pentachlorocyclohexene (PCCH) was identified as the major metabolite of α-HCH transformation. For β-HCH, in contrast, no transformation was detected. The evaluation of enantiomer fraction analysis revealed no change of the EF(-) in the nutrient solution or on root surface but a decrease in the wheat compartments, providing an evidence for the preferential biological transformation of (-)α-HCH in wheat. The current study provides the first experimental evidence for biotransformation of α-HCH in wheat using CSIA and EF and provides a concept to evaluate processes during phytoremediation.

Photo-aging of polyvinyl chloride microplastic in the presence of natural organic acids

In this study, a new photo-aging pathway in the aquatic environments and the underlying transformation mechanism were described for polyvinyl chloride microplastic (PVC-MP). Our results indicated that the photo-aging of PVC-MP was strongly dependent on particle size and the aging reaction could be facilitated in the presence of low-molecular-weight organic acid (LMWOA) and LMWOA-Fe(III) complex under simulated and natural sunlight irradiation and ambient conditions. The hydroxyl radical (OH•) generated from the photolysis of LMWOA or its ferric complexes played a dominant role in enhancing PVC-MP degradation. In situ Fourier transform infrared and Raman spectroscopic techniques and theoretical calculations further confirmed that C–Cl bond cleavage and formation of polyene and carbonyl underwent on the PVC-MP surface, especially in the presence of LMWOA and LMWOA-Fe(III). Moreover, PVC-MP surface oxidation also led to the increase of the specific surface area and affinity towards water as indicated by the results of scanning electron microscopy, Brunauer-Emmett-Teller tests and contact angles for water, which would further enhance the adsorption of polar contaminants on PVC-MP and thus increase the health risk of PVC-MP on aquatic organisms.

Theoretical studies on the N–X (X = Cl, O) bond activation mechanism in catalytic C–H amination

A favorable SN2-type N–Cl bond cleavage mechanism are proposed for Rh-catalysed C–H amination, which also works for N–O bond cleavage in Rh, Ru, and Pd analogous systems. These results could provide new understanding of C–H amination.