Bond Cleavage Process Research Articles

Recent Progress in Asymmetric Ion-Pairing Catalysis with Ammonium Salts.

Quaternary ammonium salts play an important role in asymmetric catalysis. In this Minireview, how asymmetric ion-pairing catalysis with ammonium ions has been utilized in organic synthesis is explained, particularly in the design of novel catalytic cycles. This includes the use of chiral ammonium-based catalysts for the construction of challenging stereogenic centers. Ammonium-derived electrophilic reagents, typically formed in situ and in the context of phase-transfer catalysis (PTC), have also been utilized in asymmetric bond-forming reactions. Furthermore, ammonium salts have been employed as substrates in several stereocontrolled C-N bond cleavage processes, leading to enantioenriched products by using novel asymmetric induction modes. In addition, merging ammonium ion-pairing catalysis with other catalytic approaches has also emerged as a new platform for achieving previously less straightforward reactions, thereby allowing new synthetic applications.

Cu-catalyzed oxygenation of alkene-tethered amides with O2 via unactivated C[double bond, length as m-dash]C bond cleavage: a direct approach to cyclic imides.

The transformations of unactivated alkenes through C[double bond, length as m-dash]C bond double cleavage are always attractive but very challenging. We report herein a chemoselective approach to valuable cyclic imides by a novel Cu-catalyzed geminal amino-oxygenation of unactivated C[double bond, length as m-dash]C bonds. O2 was successfully employed as the oxidant as well as the O-source and was incorporated into alkenyl amides via C[double bond, length as m-dash]C bond cleavage for the efficient preparation of succinimide or glutarimide derivatives. Moreover, the present strategy under simple conditions can be used in the late-stage modification of biologically active compounds and the synthesis of pharmaceuticals, which demonstrated the potential application.

Assays of Thiol Isomerase Enzymatic Activity.

Thiol isomerases are oxidoreductases that mediate disulphide bond formation in nascent proteins of the endoplasmic reticulum to ensure their structural integrity. In addition to its role in protein folding, thiol isomerases can modify allosteric disulphide bonds in both intracellular and extracellular proteins, thereby controlling protein function. The process of disulphide bond formation and cleavage is strictly regulated and responsive to redox conditions. Understanding disulphide bond regulation under different redox environments is critical to understanding physiological and pathological processes related to disulphide bond chemistry. Here we describe protocols for the measurement of disulphide bond modulation by thiol isomerases, including reductase and denitrosylase assays. These methods can be applied to study recombinant thiol isomerases and thiol isomerases in cellular settings.

Visible-light-driven photocatalytic degradation of diclofenac by carbon quantum dots modified porous g-C3N4: Mechanisms, degradation pathway and DFT calculation

Metal-free photocatalysts have attracted growing concern in recent years. In this work, a new class of carbon quantum dots (CQDs) modified porous graphitic carbon nitride (g-C3N4) is synthesized via a facile polymerization method. With the optimal CQDs loading, the CQDs modified g-C3N4 exhibits ∼15 times higher degradation kinetic towards diclofenac (DCF) than that of pure g-C3N4. The enhanced photocatalytic activity can be ascribed to the improved separation of charge carriers as well as the tuned band structure. Moreover, a photosensitation-like mechanism is proposed to elucidate the photo-generated electrons transfer and reactive radicals formation. CQDs are anchored to g-C3N4 surface via C–O bond, which provide channels for the preferential transfer of photo-excited electrons on DCF molecule to the conduction band of g-C3N4. Superoxide radical (·O2−) dominates the degradation of DCF, while holes (h+) show a negligible contribution. Density functional theory (DFT) calculation successfully predicts that the sites on DCF molecule with high Fukui index (f0) are preferable to be attacked by radicals. DCF degradation pathway mainly includes ring hydroxylation, ring closure and C–N bond cleavage processes. Acute toxicity estimation indicates the formation of less toxic intermediates/products compared to DCF after photocatalysis. Moreover, the hybrid photocatalysts exhibit good reusability in five consecutive cycles. This work not only proposes a deep insight into photosensitation-like mechanism in the photocatalysis system by using C3N4-based materials, but also develops new photocatalysts for potential application on removal of emerging organic pollutants from waters and wastewaters.

WITHDRAWN: Visible-light-driven photocatalytic degradation of diclofenac by carbon quantum dots modified porous g-C3N4: Mechanisms, degradation pathway and DFT calculation

Metal-free photocatalysts have attracted growing concern in recent years. In this work, a new class of carbon quantum dots (CQDs) modified porous graphitic carbon nitride (g-C3N4) is synthesized via a facile polymerization method. With the optimal CQDs loading, the CQDs modified g-C3N4 exhibits ∼15 times higher degradation kinetic towards diclofenac (DCF) than that of pure g-C3N4. The enhanced photocatalytic activity can be ascribed to the improved separation of charge carriers as well as the tuned band structure. Moreover, a photosensitation-like mechanism is proposed to elucidate the photo-generated electrons transfer and reactive radicals formation. CQDs are anchored to g-C3N4 surface via CO bond, which provide channels for the preferential transfer of photo-excited electrons on DCF molecule to the conduction band of g-C3N4. Superoxide radical (·O2-) dominates the degradation of DCF, while holes (h+) show a negligible contribution. Density functional theory (DFT) calculation successfully predicts that the sites on DCF molecule with high Fukui index (f0) are preferable to be attacked by radicals. DCF degradation pathway mainly includes ring hydroxylation, ring closure and CN bond cleavage processes. Acute toxicity estimation indicates the formation of less toxic intermediates/products compared to DCF after photocatalysis. Moreover, the hybrid photocatalysts exhibit good reusability in five consecutive cycles. This work not only proposes a deep insight into photosensitation-like mechanism in the photocatalysis system by using C3N4-based materials, but also develops new photocatalysts for potential application on removal of emerging organic pollutants from waters and wastewaters.

Selective hydrogenation of 1,3-butadiene over single Pt1/Cu(1 1 1) model catalysts: A DFT study

Single atom alloy (SAA) catalyst usually shows high selective hydrogenation properties as compared to that of traditional metal catalyst, but the mechanism is still needed to be understood at the molecular level. The first-principles density functional theory (DFT) calculations were performed to study the selective hydrogenation of 1,3-butadiene into 1-butene catalyzed by sing Pt catalyst [Pt1/Cu(1 1 1) ] as well as high Pt content catalyst [Pt4-line/Cu(1 1 1), i.e. four Pt atoms in a linear type]. The calculation results indicated that the activity of H2 dissociation as well as the H diffusion is facile on both Pt1/Cu(1 1 1) and Pt4-line/Cu(1 1 1) catalysts, which means that the production of H species is not the key factor controlling the selectivity of 1,3-butadiene hydrogenation towards the 1-butene formation. The reaction path calculation results indicated that the kinetic behavior of hydrogenation of 1,3-butadiene to 1-butene is similar for both Pt1/Cu(1 1 1) and Pt4-line/Cu(1 1 1) models, whereas the CH bond scission of 1,3-butadiene displayed a significant difference on these two models, namely it is facile to broke CH bond on Pt4-line/Cu(1 1 1) model, and thus leading to the lower selectivity for the 1-butene formation over Pt4-line/Cu(1 1 1). The reason behind the high activity of CH bond scission on Pt4-line/Cu(1 1 1) comes from the fact that a small deformation energy of 1,3-butadiene molecule occurs on Pt4-line/Cu(1 1 1) surface during the process of CH bond cleavage due to the relatively short CH bond length at transition states as compared with that on Pt1/Cu(1 1 1), and thus resulting in a relatively low CH bond cleavage barrier on Pt4-line/Cu(1 1 1) surface. The present study may help people to design an efficient catalyst for selective hydrogenation of 1,3-butadiene by controlling the activity of CH and/or CC bond cleavage.

Brønsted Acid Mediated Direct α-Hydroxylation of Cyclic α-Branched Ketones

We report a Brønsted acid mediated direct α-hydroxylation of cyclic α-branched ketones via a tandem aminoxylation/N–O bond-cleavage process. Nitrosobenzene is used as the oxidant and subsequently promotes the liberation of the free alcohol. The desired products could be isolated in moderate to good yields at a maximum tested scale of 10 mmol. Derivatizations of the obtained products are presented.

Catalyst-Counterion Controlled, Regioselective C–C Bond Cleavage in 1-Azabiphenylene: Synthesis of Selectively Substituted Benzoisoquinolines

Catalytic C–C bond cleavage processes followed by further transformations are some of the most fascinating reactions in chemistry and valuable organic synthesis tools. Herein, we demonstrate that t...

Cu-Catalyzed Aerobic Oxidative Sulfuration/Annulation Approach to Thiazoles via Multiple Csp3-H Bond Cleavage.

A novel and practical Cu-catalyzed aerobic oxidative synthesis of thiazoles was developed. This chemistry for the first time achieved thiazole construction from simple aldehydes, amines, and element sulfur through multiple Csp3-H bond cleavage processes. Molecular oxygen was used as a green oxidant in this oxidative protocol. The substrate scope is broad with the tolerance of aliphatic amines. The mechanistic study might promote the reaction design for a new sulfuration/annulation reaction with readily available element sulfur.

Critical Factors in Determining the Heterolytic versus Homolytic Bond Cleavage of Terminal Oxidants by Iron(III) Porphyrin Complexes

Heterolytic versus homolytic cleavage of the metal-bound terminal oxidant is the key for determining the nature of reactive intermediates in metalloenzymes and metal catalyzed oxygenation reactions. Here, we study the bond cleavage process of hypochlorite by iron(III) porphyrin complexes having 4-methoxy-2,6-dimethylphenyl (1), 2,4,6-trimethylphenyl (2), 4-fluoro-2,6-dimethylphenyl (3), 2-chloro-6-methylphenyl (4), 2,6-dichlorophenyl (5), and 2,4,6-trichlorophenyl (6) groups at the meso position. Oxoiron(IV) porphyrin π-cation radical complexes (CompI) are characterized from the reactions of 1-4 with tetra- n-butylammonium hypochlorite (TBA-OCl) in dichloromethane at -80 °C, while oxoiron(IV) porphyrin complexes (CompII) are characterized for 5 and 6 under the same conditions. For all of 1-6, we find the formation of an epoxidation product in good yields from the catalytic reactions with TBA-OCl, suggesting heterolytic cleavages of the O-Cl bonds. CompI of 5 and 6 are reduced to the corresponding CompII by both chloride and hypochlorite, while CompI of 1-4 are not. The reduction reactions with hypochlorite are much faster than those with chloride. These results provide a mechanism where the O-Cl bond of the iron-bound hypochlorite is cleaved heterolytically to form CompI for all of 1-6, but the subsequent reduction reaction with remaining hypochlorite affords CompII for 5 and 6. The E(OCl•/OCl-) value is the boundary to discriminate the identity of the final product: CompI or CompII. Thermodynamic analysis based on the redox potential is successfully applied for explaining the bond cleavage processes of the hypochlorite, hydroperoxide, and tert-butyl peroxide complexes.

Uranyl to Uranium(IV) Conversion through Manipulation of Axial and Equatorial Ligands.

The controlled manipulation of the axial oxo and equatorial halide ligands in the uranyl dipyrrin complex, UO2Cl(L), allows the uranyl reduction potential to be shifted by 1.53 V into the range accessible to naturally occurring reductants that are present during uranium remediation and storage processes. Abstraction of the equatorial halide ligand to form the uranyl cation causes a 780 mV positive shift in the UV/UIV reduction potential. Borane functionalization of the axial oxo groups causes the spontaneous homolysis of the equatorial U-Cl bond and a further 750 mV shift of this potential. The combined effect of chloride loss and borane coordination to the oxo groups allows reduction of UVI to UIV by H2 or other very mild reductants such as Cp*2Fe. The reduction with H2 is accompanied by a B-C bond cleavage process in the oxo-coordinated borane.

A fluorescent probe for differentiating Cys, Hcy and GSH via a stepwise interaction

It is still challenging to simultaneously distinguish general biothiols (cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) etc.) in complex redox environments, due to the lack of reliable and convenient method. Although a number of fluorescence and/or UV–vis absorption based measurements have been employed for single or multiple biothiols detection, few of them is capable of recognizing each biothiol with individual signal at the same time. We present a one-step obtained selenadiazole derivative (1) to distinguish Cys, Hcy and GSH, which not only can produce three new kind of fluorescent compounds, but also simultaneously induce different “turn-on” fluorescence emission and red-shifted absorption signals for discriminating each biothiol. With a series of systematically analyses elucidate that probe 1 possesses high sensitivity and good selectivity for the three biothiols. Furthermore, the detection mechanism was also verified via a stepwise selenium–nitrogen (SeN) bond cleavage process of probe 1 interacting with each biothiol. In the application of tracking in vitro and in vivo biothiols, probe 1 can be acted as a dual-mode fluorescent and colorimetric platform.

Brønsted base-catalyzed annulation of allyl ketones and alkynyl 1,2-diketones.

The discovery of new reaction modes mediated by easily available substrates is an important research topic in organic synthesis. Allyl ketones and related compounds have been demonstrated to undergo γ- or α-selective additions to different electrophiles. We disclose here the Brønsted base-catalyzed reaction of allyl ketones and alkynyl 1,2-diketones, which undergo a unique α-selective addition/intramolecular aldol-type annulation/C-C bond cleavage process, and a series of 2-acyloxycyclopent-3-enones can be obtained under very mild conditions.

Copper-Catalyzed Redox-Neutral Cyanoalkylarylation of Activated Alkenes with Cyclobutanone Oxime Esters.

The copper-catalyzed cyclization of activated alkenes with cyclobutanone O-acyl oximes under redox-neutral conditions has been reported. This facile protocol provided an efficient approach to a variety of cyanoalkylated oxindoles and dihydroquinolin-2(1H)-ones with a broad substrate scope and excellent functional group tolerance. In this reaction, sequential C-C bond cleavage, radical addition, and cyclization processes were involved, wherein multiple bonds were constructed in a one-pot reaction. Mechanistic studies suggest that the reaction probably proceeded via a radical pathway.

Investigation of Ketone C═O Bond Activation Processes by Heterobimetallic Zr/Co and Ti/Co Tris(phosphinoamide) Complexes

The reactivity of the reduced Zr/Co and Ti/Co complexes (THF)Zr(MesNPiPr2)3CoN2 (1, Mes = 2,4,6-trimethylphenyl) and (THF)Ti(XylNPiPr2)3CoN2 (7, Xyl = 3,5-dimethylphenyl) toward diaryl ketones is explored in an effort to gain mechanistic insight into C═O bond cleavage processes. Complex 1 reacts with 4,4′-dimethoxybenzophenone to generate ((p-OMeC6H4)2CO)Zr(MesNPiPr2)3CoN2 (2), which exists as a mixture of valence tautomers in solution that interconvert via electron transfer from Co–I to the Zr-bound ketone in 2S to form a Zr-bound ketyl radical in 2T. The geometry of 2 in the solid state is most consistent with the singlet ketone adduct tautomer 2S. Upon removal of the Co-bound N2 under vacuum, complex 2 cleanly coverts to the μ-oxo carbene product (η2-MesNPiPr2)Zr(MesNPiPr2)2(μ-O)Co═C(C6H4p-OMe)2 (5) at room temperature in solution. A diamagnetic intermediate, tentatively assigned as ketone-bridged species (η2-MesNPiPr2)Zr(MesNPiPr2)2Co(μ2,η1η2-OC(p-OMeC6H4)2) (6), is observed spectroscopically during t...

A Regioselective Approach to Trisubstituted Pyrazoles via Palladium-Catalyzed Oxidative Sonogashira-Carbonylation of Arylhydrazines.

A palladium-catalyzed oxidative carbonylation of arylhydrazines and alkynes with balloon pressure CO/O2 to afford trisubstituted pyrazoles in a one-pot manner has been developed. The formation of trisubstituted pyrazoles involves a sequential C-N bond cleavage, carbonylation, Sonogashira coupling, Michael addition, and intramolecular condensation cyclization tandem process. An unprecedented oxidative Sonogashira-carbonylation reaction of arylhydrazine plays a key role for such a facile approach to pyrazoles.

Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C-N Bond Cleavage Reactions Induced by C-H Bond Activations.

Five bis(quinolylmethyl)-(1H-indolylmethyl)amine (BQIA) compounds, that is, {(quinol-8-yl-CH2 )2 NCH2 (3-Br-1H-indol-2-yl)} (L1 H) and {[(8-R3 -quinol-2-yl)CH2 ]2 NCH(R2 )[3-R1 -1H-indol-2-yl]} (L2-5 H) (L2 H: R1 =Br, R2 =H, R3 =H; L3 H: R1 =Br, R2 =H, R3 =iPr; L4 H: R1 =H, R2 =CH3 , R3 =iPr; L5 H: R1 =H, R2 =nBu, R3 =iPr) were synthesized and used to prepare calcium complexes. The reactions of L1-5 H with silylamido calcium precursors (Ca[N(SiMe2 R)2 ]2 (THF)2 , R=Me or H) at room temperature gave heteroleptic products (L1, 2 )CaN(SiMe3 )2 (1, 2), (L3, 4 )CaN(SiHMe2 )2 (3 a, 4 a) and homoleptic complexes (L3, 5 )2 Ca (D3, D5). NMR and X-ray analyses proved that these calcium complexes were stabilized through Ca⋅⋅⋅C-Si, Ca⋅⋅⋅H-Si or Ca⋅⋅⋅H-C agostic interactions. Unexpectedly, calcium complexes ((L3-5 )CaN(SiMe3 )2 ) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C-N bond cleavage processes as a consequence of intramolecular C-H bond activation, leading to the exclusive formation of (E)-1,2-bis(8-isopropylquinol-2-yl)ethane.

Nickel‐Catalyzed Esterification of Aliphatic Amides

AbstractRecent studies have demonstrated that amides can be used in nickel‐catalyzed reactions that lead to cleavage of the amide C−N bond, with formation of a C−C or C−heteroatom bond. However, the general scope of these methodologies has been restricted to amides where the carbonyl is directly attached to an arene or heteroarene. We now report the nickel‐catalyzed esterification of amides derived from aliphatic carboxylic acids. The transformation requires only a slight excess of the alcohol nucleophile and is tolerant of heterocycles, substrates with epimerizable stereocenters, and sterically congested coupling partners. Moreover, a series of amide competition experiments establish selectivity principles that will aid future synthetic design. These studies overcome a critical limitation of current Ni‐catalyzed amide couplings and are expected to further stimulate the use of amides as synthetic building blocks in C−N bond cleavage processes.

Nickel-Catalyzed Esterification of Aliphatic Amides.

Recent studies have demonstrated that amides can be used in nickel-catalyzed reactions that lead to cleavage of the amide C-N bond, with formation of a C-C or C-heteroatom bond. However, the general scope of these methodologies has been restricted to amides where the carbonyl is directly attached to an arene or heteroarene. We now report the nickel-catalyzed esterification of amides derived from aliphatic carboxylic acids. The transformation requires only a slight excess of the alcohol nucleophile and is tolerant of heterocycles, substrates with epimerizable stereocenters, and sterically congested coupling partners. Moreover, a series of amide competition experiments establish selectivity principles that will aid future synthetic design. These studies overcome a critical limitation of current Ni-catalyzed amide couplings and are expected to further stimulate the use of amides as synthetic building blocks in C-N bond cleavage processes.

Surface Chemistry of Ga(CH3)3 on Pd(111) and Effect of Pre-covered H and O

The adsorption and decomposition of trimethylgallium (Ga(CH3)3, TMG) on Pd(111) and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectroscopy. TMG adsorbs dissociatively at 140 K and the surface is covered by a mixture of Ga(CH3)x (x=1, 2 or 3) and CHx(a) (x=1, 2 or 3) species. During the heating process, the decomposition of Ga(CH3)3 on clean Pd(111) follows a progressive Ga−C bond cleavage process with CH4 and H2 as the desorption products. The desorption of Ga-containing molecules (probably GaCH3) is also identified in the temperature range of 275–325 K. At higher annealing temperature, carbon deposits and metallic Ga are left on the surface and start to diffuse into the bulk of the substrate. The presence of precovered H(a) and O(a) has a significant effect on the adsorption and decomposition behavior of TMG. When the surface is pre-covered by saturated H2, CH4, and H2 desorptions are mainly observed at ∼315 K, which is ascribed to the dissociation of GaCH3 intermediate. In the case of O-precovered surface, the dissociation mostly occurs at ∼258 K, of which a Pd-O-Ga(CH3)2 structure is assumed to be the precusor. The presented results may provide some insights into the mechanism of surface reaction during the film deposition by using trimethylgallium as precursor.