Ring Transition Research Articles

Mechanistic Insights into the Synergistic Effect of Palladium(0) and Copper(I) on the Selective Transformation of Isocyanate to Indole.

Density functional theory calculations have been utilized to investigate the synergistic effect of palladium(0) and copper(I) catalyzed selective formation of allylated indole from (2-alkynyl)-phenylisocyanate and allyl methyl carbonate. The main competing reaction yielding the N-allyl carbamate was also considered. Calculated results indicate that using the Pd(0)-complex alone, the generation of indole is kinetically much less favored than producing the N-allyl carbamate compound. However, the co-addition of Cu(I) catalyst can considerably decrease the barrier for the intramolecular cyclization step, leading to the formation of the indole product. Analysis of the cyclization process suggests that Cu(I) complex can act as a Lewis acid to activate the linear alkyne group via a π-coordination manner prior to the formation of a 5-membered ring transition state toward indole formation. Altogether, the mechanistic insights revealed in the present study aim at a better understanding of the mechanism and the factors governing the selectivity in synergistic Pd/Cu catalysis.

Efficient removal of safranin from aqueous solution using a new type of metalated highly self-doped polyaniline nanocomposite

We report the chemical synthesis of poly(aniline-co-aniline-2,5-disulfonic acid)) and its composite containing L-hexuronic acid and metallic Ag/SiO2 nanoparticles as a new thermally stable anionic polyelectrolyte for removing safranin dye. The composite was characterized by IR, UV, cyclic voltammetry, SEM, TEM, TGA, DSC, EDXS and elemental analyses. Microscopic images exhibited intensified spherical particles dispersed over almost the entire surface. The XRD exhibited peaks of the partially crystalline material at many 2θ values, and their interatomic spacing and sizes were calculated. The cyclic voltammograms exhibited characteristic redox peaks relative to the quinoid ring transition states. The uptake rates up to 82.5% adsorption were completed within 75 min and the equilibrium time was 45 min. The isotherm of dye adsorption interprets the interaction with the adsorbent and explain the relationship between the dye removal capacity and the initial dye concentration. In the current, the Langmuir isotherm model was the optimum to interpret both the dye/copolymer and the dye/composite interactions. The uptake of safranin by copolymer/SiO2@Ag nanocomposite was well defined by pseudo second order model with rate constant K2 = 0.03 g− 1 mg− 1 min− 1 for 19 mg safranin. A comparison of safranin adsorption efficiency of the synthesized material with other reported material in the same domain suggested that the present composite has a higher adsorption rate and capacity. The ongoing research is devoted to improving the removal percentage of the dye by using 1,3,5-triazine based sulfonated polyaniline/Ag@ SiO2 nanocomposite.

A new facet of amide synthesis by tandem acceptorless dehydrogenation of amines and oxygen transfer of DMSO

We have developed a new catalytic system for preparing amides by tandem acceptorless dehydrogenation of amines and oxygen transfer of DMSO. This system shows very good catalytic activity and selectivity, a variety of primary amines and even diamines are converted into the corresponding amides in good to quantitative yields (>99 %). The unprecedented catalytic performance of this binuclear ruthenium complex was attributed to a unique chelating model wherein one Ru center combines with N atom of RCHNH produced from the dehydrogenation of RCH2NH2 and the other co-ordinates with the S atom of DMSO, thus bringing the DMSO and imine molecules into proximate to enhance the efficiency of the oxygen transfer from DMSO via a six-membered ring transition state. The mechanism study indicated that the oxygen in the product amides came from DMSO and CH bond cleavage in DMSO was the rate determining step.

Theoretical Insight into the Multiple Roles of LiHMDS in Pd-Catalyzed Borylation of Fluorobenzene.

Pd-catalyzed borylation of fluorobenzene was theoretically studied. DFT calculations revealed that the reaction occurs through an unprecedented 3 + 6-membered ring transition state, in which one LiHMDS (HMDS = hexamethyldisilazane) acts as a ligand and another LiHMDS is essential to provide Li···N and Li···F interactions, overcoming the large destabilization of the strong phenyl-F bond distortion. The characteristic feature of LiHMDS was elucidated by comparing it with HMDS and NaHMDS analogues.

Description of Si and Al Release from Aluminosilicate in the Acidic Condition Using Density Functional Theory: Protonated Terminal Oxygen

The molecular clusters ((HO)3Si-O-Si(OH)3 and (HO)3Al-O-Si(OH)3) representative of aluminosilicate mineral surface were employed to study the dissolution of aluminosilicate in acidic condition via density functional theory (DFT) with the M06-2X+G(d,p) methodology. The surface termination sites (Si and Al) were both tetra-coordinated and the terminal oxygen was protonated in an acidic condition. In the dissolution reaction, the calculated barrier height of the six-membered ring transition state complex containing two water molecules was predicted to be 76.13 kJ/mol, lower than that of the four-membered ring transition state complex containing one water molecule. The barrier height of the reaction decreased to 6.17 kJ/mol and was 91.90% lower than that for the Siter-O-Si without protonation. In addition, the calculated barrier heights for Al-terminated sites were predicted to be 22.23 kJ/mol, lower than those for the Si-terminated sites, suggesting that breaking the Al-O bond is easier than the Si-O bond in the aluminosilicate mineral surface. With the fracture of Si-O and Al-O bonds, the Si and Al release from the aluminosilicate. The results indicate that the acidic condition facilitates the release of Si and Al from the aluminosilicate, and the concentration of Al leaching from the aluminosilicate is higher than the Si.

Neighboring Group Participation of Nitrogen Cation to Form an Unusual Six-Membered Ring Intermediate During Oxime Rearrangement.

Neighboring group participation involving a 6-membered ring structure is rare, despite the privilege of 6-membered ring transition structures in organic chemistry. We examined the putative role of a 6-membered cyclic intermediate with neighboring group participation of nitrogen cation in syn-migration of peri-ester indanone oximes. Direct observation of a peri-methyl ester-iminylium intermediate in solution by means of 1H NMR supported the existence of the 6-membered cation intermediate. Density functional theory (DFT) calculations also supported the intervention of this intermediate in the rearrangement and indicated that it has a planar structure stabilized by electron delocalization.

Gold-activated luminol chemiluminescence for the selective determination of cysteine over homocysteine and glutathione

This work reports a chemiluminescence assay for the highly selective determination of cysteine in biological fluids without separation techniques. The method is based on the ability of cysteine to selectively enhance the metal-catalyzed chemiluminescence generated by the oxidation of luminol from gold tetrachloride anions under alkaline conditions. The selectivity of the method stems from the fact that, under strongly alkaline conditions, the formation of the four-membered ring transition state of cysteine is less favorable as compared to the formation of the respective 5- and 9- membered ring transition states of homocysteine and glutathione, respectively. These transition states exert stronger hindrance and hydrophobic interactions repelling the negatively charged luminol dianion and possibly exhibit lower reducing ability for dissolved oxygen, towards the formation of superoxide radicals, thus reducing the oxidation of luminol. Under the optimum experimental conditions, the linear range of the method extended from 0.5 to 20 μΜ while cysteine could be determined at concentrations as low as 0.5 μM, with good reproducibility (<3.5%) and recoveries between 80 and 93% in artificial and real biological fluids.

Anomeric Stereoauxiliary Cleavage of the C-N Bond of d-Glucosamine for the Preparation of Imidazo[1,5-a]pyridines.

The targeted cleavage of the C−N bonds of alkyl primary amines in sustainable compounds of biomass according to a metal‐free pathway and the conjunction of nitrogen in the synthesis of imidazo[1,5‐a]pyridines are still highly challenging. Despite tremendous progress in the synthesis of imidazo[1,5‐a]pyridines over the past decade, many of them can still not be efficiently prepared. Herein, we report an anomeric stereoauxiliary approach for the synthesis of a wide range of imidazo[1,5‐a]pyridines after cleaving the C−N bond of d‐glucosamine (α‐2° amine) from biobased resources. This new approach expands the scope of readily accessible imidazo[1,5‐a]pyridines relative to existing state‐of‐the‐art methods. A key strategic advantage of this approach is that the α‐anomer of d‐glucosamine enables C−N bond cleavage via a seven‐membered ring transition state. By using this novel method, a series of imidazo[1,5‐a]pyridine derivatives (>80 examples) was synthesized from pyridine ketones (including para‐dipyridine ketone) and aldehydes (including para‐dialdehyde). Imidazo[1,5‐a]pyridine derivatives containing diverse important deuterated C(sp2)−H and C(sp3)−H bonds were also efficiently achieved.

Theoretical investigations on the unimolecular decomposition mechanisms of isopropyl acetate

In this work, a comprehensive theoretical study was performed on the pyrolysis mechanism of Isopropyl acetate (IPA) using density functional methods (M06–2X, ωB97XD) and ab initio complete basis set−quadratic Becke3 (CBS-QB3) methods under combustion conditions over temperature ranges 600−1700 K and pressure ranging from 0.001 to 100 atm. The enthalpies of formation (AE), bond dissociation energies (BDEs), ionization energies (IE), and electron affinities (EA) of IPA and its main dissociation were calculated at the CBS-QB3 method and compared with available experimental results. The IPA pyrolysis mechanism was extended to include fourteen channels, seven complex fission reactions, and seven simple bond fission reactions. Among the investigated reactions, the formation of acetic acid and propene can occur through two different pathways; six-membered ring transition state (TS1SMR) or four-membered ring transition state (TS1FMR), with TS1SMR being 17.40 kcal mol−1 more preferable. The effect of temperature and pressure change was investigated using the transition state theory (TST) in conjunction with Wigner (W) tunneling corrections and the Statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory, respectively. The branching ratios (BR) of all chemical reactions involved were estimated. The results indicate that acetic acid and propene dominate up to 1400 K, but at high temperatures, simple bond fission reactions lead IPA pyrolysis. The BR analysis results are excellent agreement with the obtained BDEs and previous literature on similar biodiesel esters.

A theoretical study on the mechanism and kinetics of the Dimeric TrimethylAluminum + O2 reaction in the gas phase: A potential chain-initiation in the hypergolic combustion of TMA in air

The mechanism and kinetics of the (CH3)6Al2 + O2 reaction and some associated reactions were studied at the CCSD(T)//B3LYP/6–311++(3df,2p) level of theory as well as VTST and RRKM Master Equation calculations. The predicted potential energy surface reveals that the reaction of O2 with TMA dimer has two pathways: one proceeds via a tight transition state with the energy barrier of 27.1 kcal.mol−1 and the other occurs via a loose transition state above the reactants by 9.5 kcal.mol−1, resulting in the formation of the major products, (CH3)2AlOAl(CH3)2OCH3 + CH3. The former, larger radical product was found to be unstable under the ambient condition; it readily fragments to give (CH3)2AlOAl(CH3)OCH3 + CH3 barrierlessly with a small endothermicity, 3.3 kcal.mol−1. (CH3)2AlOAl(CH3)OCH3 was, however, found to be quite stable; its most favorable reaction path taking place by a four-member ring transition state producing CH3-cyc-Al(O)(CH2)Al-CH3 and CH3OH has an energy barrier of 48 kcal.mol−1. The equilibrium constants for the (TMA)2 ⇌ 2TMA reaction at temperatures 100.3, 115, and 155.7 °C were predicted to be 33.3, 92.2, and 1023.7 atm, respectively, agreeing satisfactorily with experimental values, 28.3, 76.7, and 931 atm, respectively, measured by Laubengayer et al. (J. Am. Chem. Soc. 1941, 63, 477). The temperature-dependent rate constants for three investigated reactions have been computed in the range of 300–2000 K and 760 Torr N2. A least-squares analysis of the result for the (TMA)2 + O2 reaction, presented with the modified Arrhenius form, can be written by the expression:k(T) = 2.64 × 10-21T3.08 exp(-9.50 kcal.mol−1/RT) cm3 molecule-1 s−1; T = 300 – 2000 K.

Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate.

A novel allylic 1,6 hydrogen-atom-transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products. A new precursor, Z/E-1,3-diiodobut-1-ene, is synthesized and photolyzed in the presence of oxygen to generate a new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups that facilitates rapid allylic 1,6 H-atom transfer. A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower-energy (tZZ) conformer to a higher-energy (cZZ) conformer followed by 1,6 hydrogen transfer via a seven-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying (tZZ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm-1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate keff(E) on the order of 108 s-1 at ca. 3000 cm-1 in good accord with the experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 106 s-1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the nonphotolytic production of OH radicals upon alkene ozonolysis in the troposphere.

Direct gem-Difluoroalkenylation of X-H Bonds with Trifluoromethyl Ketone N-Triftosylhydrazones for Synthesis of Tetrasubstituted Heteroatomic gem-Difluoroalkenes.

The direct gem-difluoroalkenylation of X-H bonds represents the most straightforward approach to access heteroatomic gem-difluoroalkenes that, as the isostere of the carbonyl group, have great potency in drug discovery. However, the construction of tetrasubstituted heteroatomic gem-difluoroalkenes by this strategy is still an unsolved problem. Here, we report the first direct X-H bond gem-difluoroalkenylation of amines and alcohols with trifluoromethyl ketone N-triftosylhydrazones under silver (for (hetero)aryl hydrazones) or rhodium (for alkyl hydrazones), thereby providing a most powerful method for the synthesis of tetrasubstituted heteroatomic gem-difluoroalkenes. This method features a broad substrate scope, high product yield, excellent functional group tolerance, and operational simplicity (open air conditions). Moreover, the site-specific replacement of the carbonyl group with a gem-difluorovinyl ether bioisostere in drug Trimebutine and the post-modification of bioactive molecules demonstrates potential use in medicinal research. Finally, the reaction mechanism was investigated by combining experiments and DFT calculations, and disclosed that the key step of HF elimination occurred via five-membered ring transition state, and the difference in the electrophilicity of Ag- and Rh-carbenes as well as the multiple intermolecular interactions rendered the effectiveness of Rh catalyst selectively for alkyl hydrazones.

A novel and efficient N-doping carbon supported cobalt catalyst derived from the fermentation broth solid waste for the hydrogenation of ketones via Meerwein–Ponndorf–Verley reaction

Most of the non-noble metal catalysts used for the Meerwein–Ponndorf–Verley (MPV) reaction of carbonyl compounds rely on the additional alkaline additives during preparation to achieve high efficiency. To solve this problem, in this work, we prepared a novel N-doped carbon supported cobalt catalyst (Co@CN), in which the carriers were derived from the nitrogen-rich organic waste, i.e., oxytetracycline fermentation residue (OFR, obtained from oxytetracycline refining workshop). No additional nitrogen sources were used during preparation. The results showed that inherent nitrogen in OFR could provide N-containing basic sites, and formed Co-N structures via coordinating with cobalt. The Co-N sites together with the coexisting Co(0) cooperated to catalyze the conversion of ethyl levulinate (EL) to γ-valerolactone (GVL) by MPV reaction. Co(0) dominated the activation of H in isopropanol, while Co-N dominated the formation of the six-membered ring transition state.

A Bifunctional B,N‐Based Asymmetric Catalytic Nitrostyrene‐Michael Addition Acting through a 10‐Membered Ring Cyclic Transition State

AbstractThe B,N‐bifunctional catalyst homoboroproline has been applied to a catalytic asymmetric nitroalkene‐Michael addition to β‐nitrostyrene analogues, showing broad substrate tolerance, high conversions and moderate to good asymmetric induction. The ability of homoboroproline to act as an efficient catalyst based on enamine‐formation of the secondary amine, coupled with intramolecular Lewis‐acid chelation of the nitro function, in a non‐FLP manner, to effect efficient and enantioselective catalysis via a proposed large 10‐membered ring transition state is remarkable and reinforced by theoretical calculations.

Stereoselective Dehydroxyboration of Allylic Alcohols to Access (E)-Allylboronates by a Combination of C-OH Cleavage and Boron Transfer under Iron Catalysis.

Iron-catalyzed direct SN2' dehydroxyboration of allylic alcohols has been developed to access (E)-stereoselective allylboronates. Allylic alcohols with diverse structures and functional groups, especially derived from natural products, underwent smooth transformation. The six-membered ring transition state formed by allylic alcohols and iron-boron intermediate was indicated to be the key component involved in transfer of the boron group, activation of the C-OH bond, and control of the stereoselectivity.

Bound states in the continuum in strong-coupling and weak-coupling regimes under the cylinder – ring transition

Abstract Bound states in the continuum (BIC) have been at the forefront of research in optics and photonics over the past decade. It is of great interest to study the effects associated with quasi-BICs in the simplest structures, where quasi-BICs are very pronounced. An example is a dielectric cylinder, and in a number of works, quasi-BICs have been studied both in single cylinders and in structures composed of cylinders. In this work, we studied the properties of quasi-BICs during the transition from a homogeneous dielectric cylinder in an air environment to a ring with narrow walls while increasing the diameter of the inner air cylinder gradually. The results demonstrate the quasi-BIC crossover from the strong-coupling to the weak-coupling regime, which manifests itself in the transition from the avoided crossing of branches to their intersection with the quasi-BIC being preserved on only one straight branch. In the regime of strong-coupling and quasi-BIC, three waves interfere in the far-field zone: two waves corresponding to the resonant modes of the structure and the wave scattered by the structure as a whole. The validity of the Fano resonance concept is discussed since it describes the interference of only two waves under weak coupling conditions.

Mechanistic insight on the inhibition of D, D-carboxypeptidase from Mycobacterium tuberculosis by β-lactam antibiotics: an ONIOM acylation study

Mycobacterium tuberculosis cell wall is intricate and impermeable to many agents. A D, D-carboxypeptidase (DacB1) is one of the enzymes involved in the biosynthesis of cell wall peptidoglycan and catalyzes the terminal D-alanine cleavage from pentapeptide precursors. Catalytic activity and mechanism by which DacB1 functions is poorly understood. Herein, we investigated the acylation mechanism of DacB1 by β-lactams using a 6-membered ring transition state model that involves a catalytic water molecule in the reaction pathway. The full transition states (TS) optimization plus frequency were achieved using the ONIOM (B3LYP/6-31 + G(d): AMBER) method. Subsequently, the activation free energies were computed via single-point calculations on fully optimized structures using B3LYP/6-311++(d,p): AMBER and M06-2X/6-311++(d,p): AMBER with an electronic embedding scheme. The 6-membered ring transition state is an effective model to examine the inactivation of DacB1 via acylation by β-lactams antibiotics (imipenem, meropenem, and faropenem) in the presence of the catalytic water. The ΔG# values obtained suggest that the nucleophilic attack on the carbonyl carbon is the rate-limiting step with 13.62, 19.60 and 30.29 kcal mol−1 for Imi–DacB1, Mero–DacB1 and Faro–DacB1, respectively. The electrostatic potential (ESP) and natural bond orbital (NBO) analysis provided significant electronic details of the electron-rich region and charge delocalization, respectively, based on the concerted 6-membered ring transition state. The stabilization energies of charge transfer within the catalytic reaction pathway concurred with the obtained activation free energies. The outcomes of this study provide important molecular insight into the inactivation of D, D-carboxypeptidase by β-lactams. Communicated by Ramaswamy H. Sarma

Hf-MOF catalyzed Meerwein−Ponndorf−Verley (MPV) reduction reaction: Insight into reaction mechanism

Hf-MOF-808 exhibits excellent activity and specific selectivity on the hydrogenation of carbonyl compounds via a hydrogen transfer strategy. Its superior activity than other Hf-MOFs is attributed to its poor crystallinity, defects and large specific surface area, thereby containing more Lewis acid-base sites which promote this reaction. Density functional theory (DFT) computations are performed to explore the catalytic mechanism. The results indicate that alcohol and ketone fill the defects of Hf-MOF to form a six-membered ring transition state (TS) complex, in which Hf as the center of Lewis stearic acid coordinates with the oxygen of the substrate molecule, thus effectively promoting hydrogen transfer process. Other reactive groups, such as –NO2, C = C, -CN, of inadequate hardness or large steric hindrance are difficult to coordinate with Hf, thus weakening their catalytic effect, which explains the specific selectivity Hf-MOF-808 for reducing the carbonyl group.

Peptide Hydrolysis by Metal (Oxa)cyclen Complexes: Revisiting the Mechanism and Assessing Ligand Effects.

The mechanism responsible for peptide bond hydrolysis by Co(III) and Cu(II) complexes with (oxa)cyclen ligands has been revisited by means of computational tools. We propose that the mechanism starts by substrate coordination and an outer-sphere attack on the amide C atom of a solvent water molecule assisted by the metal hydroxo moiety as a general base, which occurs through six-membered ring transition states. This new mechanism represents a more likely scenario than the previously proposed mechanisms that involved an inner-sphere nucleophilic attack through more strained four-membered rings transition states. The corresponding computed overall free-energy barrier of 25.2 kcal mol-1 for hydrolysis of the peptide bond in Phe-Ala by a cobalt(III) oxacyclen catalyst (1) is consistent with the experimental values obtained from rate constants. Also, we assessed the influence of the nature of the ligand throughout a systematic replacement of N by O atoms in the (oxa)cyclen ligand. Increasing the number of coordinating O atoms accelerates the reaction by increasing the Lewis acidity of the metal ion. On the other hand, the higher reactivity observed for the copper(II) oxacyclen catalyst with respect to the analogous Co(III) complex can be attributed to the larger Brönsted basicity of the copper(II) hydroxo ligand. Ultimately, the detailed understanding of the ligand and metal nature effects allowed us to identify the double role of the metal hydroxo complexes as Lewis acids and Brönsted bases and to rationalize the observed reactivity trends.

Dramatic influence of ester steric hindrance on the diastereoselectivity of a Michael addition towards the synthesis of the ABC tricycle of hexacyclinic acid

During our studies toward the synthesis of the ABC ring system of hexacyclinic acid, we observed an unexpected influence of the steric bulk of the ester group of the Michael acceptor in a key conjugate addition. We propose an eight-membered ring transition state to explain the formation of the undesired diastereomer in the case of unhindered esters.