Bond Formation Pathway Research Articles

Comparison of the Efficiency of B-O and B-C Bond Formation Pathways in Borane-Catalyzed Carbene Transfer Reactions Using α-Diazocarbonyl Precursors: A Combined Density Functional Theory and Machine Learning Study.

Lewis acidic boranes, especially tris(pentafluorophenyl)borane [B(C6F5)3], have emerged as metal-free catalysts for carbene transfer reactions of α-diazocarbonyl compounds in a variety of functionalization reactions. The established mechanism for how borane facilitates carbene generation for these compounds in the scientific community is based on the formation of a B-O (C=O) intermediate (path O). Herein, we report an extensive DFT study that challenges the notion of a ubiquitous path O, revealing that B-C(=N=N) bond formation (path C) for certain diazocarbonyl substrates proves to be the preferred pathway. This study elucidates, through the introduction of 22 various substituents on each side of the α-diazocarbonyl backbone, how the electron-donating and -withdrawing properties of substituents influence the competition between these B-O and B-C pathways. To elucidate the impact of the electronic features of diazo substrates on the competition between the O and C pathways in the studied dataset, we employed a machine learning approach based on the Random Forest model. This analysis revealed that substrates with higher electron density on the diazo-attached carbon, lower electron density on the carbonyl carbon, and more stable HOMO orbitals tend to proceed via path C. Furthermore, this study not only demonstrates that borane efficiency in facilitating N2 release is greatly affected by the nature of substituents on both sides of the α-diazocarbonyl functionality but also shows that for some substrates, borane is incapable of catalyzing the release of molecular nitrogen.

Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species.

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in RuV=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry. The shifts in geometric orientation altered its O-O bond formation pathway from water nucleophilic attack (WNA) to I2M, resulting in a 70-fold increase in water oxidation activity. This implies that the I2M pathway is concurrently influenced by the spin density on oxo and the geometry organization of the catalysts. The observed mechanistic switch and theoretical studies provide insights into controlling reaction pathways for homogeneous water oxidation catalysis.

CuI/DMAP-Catalyzed Oxidative Alkynylation of 7-Azaindoles: Synthetic Scope and Mechanistic Studies.

An efficient and practical method for the N-alkynylation of 7-azaindoles has been established by using CuI/DMAP catalytic system at room temperature and in open air. This simple protocol has been successfully employed in the synthesis of a wide range of N-alkynylated 7-azaindoles with good yields. Also, this approach is well-suited for large-scale N-alkynylation reactions. The designed N-alkynylated 7-azaindoles were further subjected to Cu-/Ir-catalyzed alkyne-azide cycloaddition (CuAAC/IrAAC) or "click" reaction for the rapid synthesis of 1,4-/1,5 disubstituted 1,2,3-triazole decorated 7-azaindoles. A mechanistic study based on density functional theory (DFT) calculations and ultraviolet-visible (UV) spectroscopic studies revealed that the CuI and DMAP combination formed a [CuII(DMAP)2I2] species, which acts as an active catalyst. The DFT method was used to assess the energetic viability of an organometallic in the C-N bond formation pathway originating from the [CuII(DMAP)2I2] complex. We expect that the newly designed Cu/DMAP/alkyne system will offer valuable insights into the field of Cu-catalyzed transformations.

Carboxamide Fe(III) complex as the electrocatalyst of water oxidation reaction: WNA and I2M O–O bond formation pathways

A single-site Fe(III) carboxamide complex, [Fe(HbpH) (Cl)2] (1), (HbpH‾ = N,N′-Bis(picolinoyl)hydrazine) anion), was synthesized and used as the electrocatalyst of water oxidation reaction. Complex 1 catalyzes the oxidation of water to O2 at the pH of 8 in phosphate buffer solution with the onset of a catalytic wave at about 0.45 V versus RHE with the turnover frequency (TOF) of 5.8 s−1 at room temperature. The stability of the catalyst in water oxidation processes by electrochemical and UV–vis measurements, demonstrated the realistic molecular electrocatalysis of 1. Differential pulse voltammetry (DPV) of 1 in different pH values showed the dependency of oxidation potential to the pH of the environment, establishing the fact that the catalytic cycle involves the proton-coupled electron transfer (PCET) process. Density functional theory (DFT) calculations on the mechanism of water oxidation process of 1 shows that the oxygen evolution reaction is carried out through the water nucleophilic attack (WNA) to the metal center ion, resulted to the formation of FeIV(O) species. Also, DFT calculations showed that the formation of an O–O bond proceeds through the interaction two molecular mechanism (I2M), which compete with the WNA mechanism following from experimental analysis. The electroactivity of the metal centre and carboxamide ligand and the ability of the ligand to stabilize the high oxidation numbers of the metal-ion centre during the oxidation process, provide different oxidation pathways for an OER process.

Multiplicative enhancement of stereoenrichment by a single catalyst for deracemization of alcohols.

Stereochemical enrichment of a racemic mixture by deracemization must overcome unfavorable entropic effects as well as the principle of microscopic reversibility; recently, photochemical reaction pathways unveiled by the energetic input of light have led to innovations toward this end, most often by ablation of a stereogenic C(sp3)-H bond. We report a photochemically driven deracemization protocol in which a single chiral catalyst effects two mechanistically different steps, C-C bond cleavage and C-C bond formation, to achieve multiplicative enhancement of stereoinduction, which leads to high levels of stereoselectivity. Ligand-to-metal charge transfer excitation of a titanium catalyst coordinated by a chiral phosphoric acid or bisoxazoline efficiently enriches racemic alcohols that feature adjacent and fully substituted stereogenic centers to enantiomeric ratios up to 99:1. Mechanistic investigations support a pathway of sequential radical-mediated bond scission and bond formation through a common prochiral intermediate and reveal that, although the overall stereoenrichment is high, the selectivity in each individual step is moderate.

Copper Complex Catalyzed Two-Electron and Proton Shuttle Mechanism of O-O Bond Formation from DFT-Based Metadynamics Simulations.

We performed first-principles metadynamics simulations to explore the mechanistic pathway of oxygen-oxygen bond formation catalyzed by cis-bis(hydroxo) and cis-(hydroxo)oxo copper complexes. The ligands of considered complexes involve modified bipyridine ligands with oxo and hydroxo groups on 6, 6' positions. The study focuses on the kinetics and thermodynamics of the oxygen-oxygen bond formation. The individual migration of the proton to the hydroxyl group and hydroxide to the oxo and hydroxo moieties of the complexes was examined. The proton transfer requires more kinetic barrier than the hydroxide migration. The nature of the electronic density was analyzed with the help of spin population analysis. The molecular orbitals and natural orbital analysis were carried out to examine the nature of the orbitals involved in the oxygen-oxygen bond formation. The σ*(dx2-y2-px) molecular orbital of the Cu-O or Cu-OH bond overlaps with the pz orbital of the hydroxide ion in forming the oxygen-oxygen bond. The two-electron two-centered (2e--2C) bond is observed in the oxygen-oxygen bond formation. In the oxidation process, these ligands stabilize the electron density from the water or hydroxide ion. These redox-active ligands also help stabilize the formed hydrogen peroxide or peroxide complexes.

Mechanistic Regulation by Oxygen Vacancies in Structural Evolution Promoting Electrocatalytic Water Oxidation

Introduction of oxygen vacancies (Vo) has been considered an effective strategy for promoting the oxygen evolution reaction (OER) performance of electrocatalysts. However, the role of Vo in improving OER activity is unclear. Herein, a Vo-rich spinel Co3O4 was used as a model catalyst to explore the mechanism of Vo in structural evolution during OER. The results suggest that the Vo-rich environment facilitates the reconstruction of Co3O4 to the Co(OH)2 intermediate with proton vacancies (Co(II)Ox(OH)y), which is favorable for the formation of the active species of CoOOH. Correlative operando characterizations and electrokinetic analyses indicate that a moderate Vo density can switch the O–O bond formation pathway, from a water nucleophilic attack to an intramolecular nucleophilic attack pathway, which is more kinetically favorable for water oxidation. However, a high Vo density quenches the formation of highly valent active intermediates. This study provides significant insights into the crucial role of vacancy defects during OER.

Electronic Effect on Phenoxide Migration at a Nickel(II) Center Supported by a Tridentate Bis(phosphinophenyl)phosphido Ligand.

A phosphide nickel(II) phenoxide pincer complex (2) reacts with CO(g) to give a pseudo-tetrahedral nickel(0) monocarbonyl complex (3) possessing a phosphinite moiety. This metal-ligand cooperative (MLC) transformation occurs with a (PPP)Ni scaffold (PPP- = P[2-PiPr2-C6H4]2-), which can accommodate both square planar and tetrahedral geometries. The 2-electron reduction of a nickel(II) species induced by CO coordination involves group transfer to generate a P-O bond. For better mechanistic understanding, a series of nickel(II) phenolate complexes (2a-2e, XC6H4O- (X = OMe, Me, H, and CF3) and pentafluorophenolate) were prepared. Kinetic experimental data reveal that a phenolate species with an electron-withdrawing group reacts faster than those with electron-donating groups. The reaction kinetic experiments were conducted in pseudo-first order conditions at room temperature monitored by UV-vis spectroscopy. A pentafluorophenolate nickel(II) complex (2e) reveals instantaneous reactions even at -40 °C to give a nickel(0) monocarbonyl species (3e) and the reverse reaction is also possible. According to kinetic experiments, the rate determining step (RDS) would be the formation of a 5-coordinate intermediate 4 with a negative entropy value (ΔS‡ < 0), and a positive ρ value based on the Hammett plot indicates that the electron-deficient phenolate leads to a faster CO association. Furthermore, scramble experiments suggest that phenolate de-coordinates from the intermediate 4, which gives a (PPP)Ni-CO species 6. The cationic nickel monocarbonyl intermediate can possess a P--Ni(II), P•-Ni(I), or even a P+-Ni(0) character. Such an inner-sphere electron transfer is suggested when a π-acidic ligand such as CO coordinates to a metal ion. Another possible reaction is homolysis of a Ni-O bond to give P--Ni(I) or P•-Ni(0), when a phenoxyl radical is liberated. Considering the P-O bond formation, closed-shell nucleophilic and open-shell radical pathways are suggested. A phenolate pathway reveals a lower energy state for 2e relative to other complexes (2c and 2d), while its radical pathway undergoes via a higher energy state. Therefore, the formation of a P-O bond may occur with the binding of a closed-shell phenolate to the electron-deficient P center.

Effects of Amylopectins from Five Different Sources on Disulfide Bond Formation in Alkali-Soluble Glutenin.

Wheat, maize, cassava, mung bean and sweet potato starches have often been added to dough systems to improve their hardness. However, inconsistent effects of these starches on the dough quality have been reported, especially in refrigerated dough. The disulfide bond contents of alkali-soluble glutenin (ASG) have direct effects on the hardness of dough. In this paper, the disulfide bond contents of ASG were determined. ASG was mixed and retrograded with five kinds of amylopectins from the above-mentioned botanical sources, and a possible pathway of disulfide bond formation in ASGs by amylopectin addition was proposed through molecular weight, chain length distribution, FT-IR, 13C solid-state NMR and XRD analyses. The results showed that when wheat, maize, cassava, mung bean and sweet potato amylopectins were mixed with ASG, the disulfide bond contents of alkali-soluble glutenin increased from 0.04 to 0.31, 0.24, 0.08, 0.18 and 0.29 μmol/g, respectively. However, after cold storage, they changed to 0.55, 0.16, 0.26, 0.07 and 0.19 μmol/g, respectively. The addition of wheat amylopectin promoted the most significant disulfide bond formation of ASG. Hydroxyproline only existed in the wheat amylopectin, indicating that it had an important effect on the disulfide bond formation of ASG. Glutathione disulfides were present, as mung bean and sweet potato amylopectin were mixed with ASG, and they were reduced during cold storage. Positive/negative correlations between the peak intensity of the angles at 2θ = 20°/23° and the disulfide bond contents of ASG existed. The high content of hydroxyproline could be used as a marker for breeding high-quality wheat.

Three-Electron Two-Centered Bond and Single-Electron Transfer Mechanism of Water Splitting via a Copper–Bipyridine Complex

We report the atomistic and electronic details of the mechanistic pathway of the oxygen-oxygen bond formation catalyzed by a copper-2,2'-bipyridine complex. Density functional theory-based molecular dynamics simulations and enhanced sampling methods were employed for this study. The thermodynamics and electronic structure of the oxygen-oxygen bond formation are presented in this study by considering the cis-bishydroxo, [CuIII(bpy)(OH)2]+, and cis-(hydroxo)oxo, [CuIV(bpy)(OH)(═O)]+, complexes as active catalysts. In the cis-bishydroxo complex, the hydroxide transfer requires a higher kinetic barrier than the proton transfer process. In the case of [CuIV(bpy)(OH)(═O)]+, the proton transfer requires a higher free energy than the hydroxide one. The peroxide bond formation is thermodynamically favorable for the [CuIV(bpy)(OH)(═O)]+ complex compared with the other. The hydroxide ion is transferred to one of the Cu-OH moieties, and the proton is transferred to the solvent. The free energy barrier for this migration is higher than that for the former transfer. From the analysis of molecular orbitals, it is found that the electron density is primarily present on the water molecules near the active sites in the highest occupied molecular orbital (HOMO) state and lowest unoccupied molecular orbital (LUMO) of the ligands. Natural bond orbital (NBO) analysis reveals the electron transfer process during the oxygen-oxygen bond formation. The σ*Cu(dxz)-O(p) orbitals are involved in the oxygen-oxygen bond formation. During the bond formation, three-electron two-centered (3e--2C) bonds are observed in [CuIII(bpy)(OH)2]+ during the transfer of the hydroxide before the formation of the oxygen-oxygen bond.

Synthesis of (±)‐Physovenine through Electrochemical C−O Bond Formation

AbstractWe investigated an electrochemical C−O bond formation approach to (±)‐physovenine. The reaction conditions are mild compared to canonical routes, which typically utilize strong hydride reagents such as lithium aluminum hydride. This enabled extension of the electrochemical cyclization to a substrate bearing the fully functionalized phenyl ring of the natural product. The electrochemical cyclization is diastereoselective and starting from enantiopure substrate would provide a single enantiomer of physovenine. In the course of an initial formal synthesis, we identified conditions to afford modest enantioselective substrate synthesis that demonstrate the feasibility of an enantioselective synthesis. Among the substrates investigated, acetyl‐, tosyl‐ or Boc‐nitrogen substitution led to an alternative electrochemical C−O bond formation pathway where the hydroxyl nucleophile cyclized onto the aromatic ring vs. the 2‐position of the indoline. This work expands the utility of electrochemical C−O bond formation and provides a foundation for further applications to functionalized cyclic ether construction.

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Computational Investigations of the Reactivity of Metalloporphyrins for Ammonia Oxidation

Density functional theory and molecular dynamics simulations were used to assess the ability of tetraphenylporphyrin (TPP)M complexes (where M = Cr, Mn, Fe, Co, Ni, Mo, Ru, W, and Os) to coordinate and weaken the N–H bonds of ammonia, as well as their reactivity towards N–N bond formation for N2 generation. Compared to other metalloporphyrins, bis-ammonia complexes (TPP)Mo(NH3)2 and (TPP)W(NH3)2 exhibit low and level N–H BDFEs due to a stabilized (TPP)M(NH3)(NH) intermediate by multiple metal–ligand bonding. These results resemble those previously obtained for polypyridyl metal complexes, suggesting that broad trends in reactivity towards N–H bond cleavage are more metal-dependent rather than ligand-dependent for a metal in a pseudo-octahedral environment with nitrogen-based ligands. We investigated N–N bond formation via NH3 nucleophilic attack on M–NH and M–N intermediates, compared to bimolecular coupling of M-NHx intermediates. We evaluated the reactivity of (TPP)Fe(NH3)2 towards N–N bond formation through a hydrazine pathway, and found amide-amide coupling to form a bridged hydrazido complex to be the most favorable pathway for N–N bond formation. Further investigation of potential N–N bond formation pathways by reaction with NH3 led us to identify a possible FeIII-•NH species with significant aminyl character that bypasses the nucleophilic attack of NH3 and that promotes homolytic N–H bond cleavage of ammonia. This reaction forms a Fe-NH2 moiety and a transient •NH2 radical that subsequently forms an N–N bond with the Fe-NH2 moiety to form a (TPP)Fe(NH3)(N2H4) species. These results indicate the need to evaluate the radical character of imido species and their reactivity towards N–H bond cleavage of ammonia.

Construction of aziridine, azetidine, indole and quinoline-like heterocycles via Pd-mediated C-H activation/annulation strategies.

N-Heterocycles can be found in natural products and drug molecules and are indispensable components in the area of organic synthesis, medicinal chemistry and materials science. The construction of these N-containing heterocycles by traditional methods usually requires the preparation of reactive intermediates. In the past decades, with the rapid growth of transition metal catalysed coupling reactions, syntheses of heterocycles from precursors with inert chemical bonds have become a challenge. More recently, in the field of transition metal associated C-H direct functionalization, efficient methods have been developed for the syntheses of N-heterocyclic compounds such as aziridines, azetidines, indoles and quinolines under the click type of reaction mode. In this review, representative synthetic methodologies developed in the recent 10 years for the preparation of this small class of N-heterocycles via the Pd-catalysed C-H activation and C-N bond formation pathway are discussed. We hope this article will provide new insights from the strategies highlighted into future molecular design, synthesis and applications in medical and materials sciences.

Identifying Intermediates in Electrocatalytic Water Oxidation with a Manganese Corrole Complex.

Water nucleophilic attack (WNA) on high-valent terminal Mn-oxo species is proposed for O-O bond formation in natural and artificial water oxidation. Herein, we report an electrocatalytic water oxidation reaction with MnIII tris(pentafluorophenyl)corrole (1) in propylene carbonate (PC). O2 was generated at the MnV/IV potential with hydroxide, but a more anodic potential was required to evolve O2 with only water. With a synthetic MnV(O) complex of 1, a second-order rate constant, k2(OH-), of 7.4 × 103 M-1 s-1 was determined in the reaction of the MnV(O) complex of 1 with hydroxide, whereas its reaction with water occurred much more slowly with a k2(H2O) value of 4.4 × 10-3 M-1 s-1. This large reactivity difference of MnV(O) with hydroxide and water is consistent with different electrocatalytic behaviors of 1 with these two substrates. Significantly, during the electrolysis of 1 with water, a MnIV-peroxo species was identified with various spectroscopic methods, including UV-vis, electron paramagnetic resonance, and infrared spectroscopy. Isotope-labeling experiments confirmed that both O atoms of this peroxo species are derived from water, suggesting the involvement of the WNA mechanism in water oxidation by a Mn complex. Density functional theory calculations suggested that the nucleophilic attack of hydroxide on MnV(O) and also WNA to 1e--oxidized MnV(O) are feasibly involved in the catalytic cycles but that direct WNA to MnV(O) is not likely to be the main O-O bond formation pathway in the electrocatalytic water oxidation by 1.

Distinctive O-O bond formation pathways at different electrode potentials

The mechanism of the O–O bond formation at different electrode potentials remains elusive. Recently, Lang et al. reported their observation on the potential-dependent switch of water oxidation mechanism on heterogeneous Co-based catalysts in neutral electrolyte.

Theoretical Insights into Potential-Dependent C-C Bond Formation Mechanisms during CO2 Electroreduction into C2 Products on Cu(100) at Simulated Electrochemical Interfaces.

An improved CO coverage-dependent electrochemical interface model with an explicit solvent effect on Cu(100) is presented in this paper, by which theoretical insights into the potential-dependent C–C bond formation pathways occurring in CO2 electrochemical reduction to C2 products can be obtained. Our present studies indicate that CHO is a crucial intermediate toward C1 products on Cu(111), and dimer OCCO is found to not be a viable species for the production of C2 products on Cu(100). The reaction pathway of CHO with CO and CHO dimerization into dimers COCHO and CHOCHO may be C–C bond formation mechanisms at low overpotential. However, at medium overpotential, C–C bond coupling takes place preferentially through the reaction of COH with CO species and COH dimerization into dimers COCOH and COHCOH. The formed dimers COCHO, CHOCOH, and CHOCHO via reactions of CHO with CO, COH, and CHO species may lead to C2 products, which are regarded as C–C bond formation mechanisms at high overpotential. The difference of obtained adsorption isotherms of CO on Cu(100) with that of Cu(111) may be able to explain the effect of the crystal face of Cu on product selectivity. The excellent consistencies between our present obtained conclusions and the available experimental reports and partial theoretical studies validate the reasonability of the present employed methodology, which can be also used to systematically study potential-dependent CO2 electroreduction pathways toward C2 products on Cu(100) or other metal catalysts.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Summary O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.

Thermodynamic and Kinetic Competition between C–H and O–H Bond Formation Pathways during Electrochemical Reduction of CO on Copper Electrodes

Carbon monoxide is the key intermediate in the electrochemical CO2 reduction reaction and determines the overpotentials and selectivities for C1 and C2 products on copper electrodes. The kinetic mo...

Strategy for Selective Csp2-F and Csp2-Csp2 Formations from Organoplatinum Complexes.

By changing the parameters of fluorination reaction of bisaryl-platinum(II) complexes, each possible competitive pathway of Ar-Ar and Ar-F formation can be selectively controlled. It was discovered that steric hindrance, type of fluorinating reagent, and temperature of reaction are determinants for Ar-F vs Ar-Ar bond formation pathway from bisaryl-fluoro-platinum(IV) complexes. The combination of bulky ligands such as mesityl with Selectfluor at RT leads to Ar-F bond formation in the presence of possible Ar-Ar formation.