Six-membered Ring Transition State Research Articles

Acid-base synergistic effect towards catalytic transfer hydrogenation reactions

Catalytic transfer hydrogenation (CTH) of aldehydes/ketones by using alcohols as hydrogen donors is an attractive and environmentally friendly hydrogenation technology, which has evoked widespread attention in synthesis of fine chemicals from biomass resources. In this work, a Co-Al mixed metal oxide catalyst (Co1Al2-MMO-200) was obtained through structural topological transformation process of layered double hydroxides, which was used in the CTH reaction of furfural with isopropanol. Notably, the reaction rate achieves 0.023 mol g−1 h−1, which is preponderate to acid-base catalysts earlier reported. The joint research based on CO2-TPD, NH3-TPD and poisoning experiment substantiates that the synergy effect of acid-base sites plays an essential role in this catalytic system. Isotopic labelling MS, in-situ FT-IR and DFT studies further verify that the CTH reaction of furfural occurs via the Meerwein-Ponndorf-Verley (MPV) route, in which the basic site promotes the deprotonation of isopropanol whilst the acid site facilitates the formation of a six-membered ring transition state. This work demonstrates an acid-base synergetic catalysis for the CTH reaction by revealing the structure-property correlation and exploring reaction route, which is instructive for designing high-performance heterogenous catalysts.

Visible light-driven selective cleavage of Cα-Cβ or Cβ-O bond in lignin β-O-4 model into high-value aromatic chemicals over surface modified 2D g-C3N4

Selective cleavage of C–C/C–O linkages in lignin is highly pursued to obtain aromatic monomers but remains challenging due to distinct dissociation energies and steric hindrance. Herein, two photocatalysts were reported to respectively scissor the Cα-Cβ and Cβ-O in lignin β-O-4 model (2-phenoxy-1-phenylethanol) based on the reactive oxygen species control. On one hand, 2D g-C3N4 promotes protons extraction from Cβ-H and ·O2– formation, which further scissored Cα-Cβ via six-membered ring transition state produced via the coupling of ·Cβ and ·HO2, providing aromatic aldehydes and esters. When single-atomic Cu+ species were coordinated with N atoms in 2D g-C3N4 during surface modification, however, the ·O2– formation was prohibited because of the improper redox potentials. Under the catalysis of Cu/2D g-C3N4, CαO-H would be preferentially activated, causing the selective Cβ-O cleavage via self-hydrogen transfer hydrogenolysis to provide phenolics and aromatic ketones. The access and inhibition mechanism for ·O2– formation as well as the different catalytic mechanisms for two reaction pathways were discussed. Current work shows a promising way to selectively supply aromatic chemicals using the renewable biomass and photoenergy.

Diastereoselective Petasis-Borono-Mannich Crotylation Reactions of Chiral α-Heteroatom (F, OBz, OH) Aldehydes: Rapid Access to Valuable Mono and Bicyclic Heterocyclic Scaffolds.

The crotylation reactions of chiral α-F, α-OBz and α-OH aldehydes under Petasis-borono-Mannich conditions using (E)- or (Z)-crotylboronates and primary amines resulted in γ-addition products in high dr and high er. α-F and α-OBz aldehydes gave 1,2-anti-2,3-syn and 1,2-anti-2,3-anti, products, respectively while an α-OH aldehyde gave 1,2-syn-2,3-syn products. The stereochemical outcomes of reactions of the former aldehydes can be explained using a six-membered ring transition state (TS) model in which a Cornforth-like conformation around the imine intermediate is favoured resulting in 1,2-anti products. The 2,3-stereochemical outcome is dependent upon the geometry of the crotylboronate. These TS models were also supported by DFT calculations. The stereochemical outcomes of reactions employing an α-OH aldehyde can be rationalised as occurring via an open-TS involving H-bonding in the imine intermediate between the α-OH group and the imine N atom. Representative products were converted to highly functionalized 1,2,3,6-tetrahydropyridines and 3H-oxazolo[3,4-a]pyridine-3-ones which will be valuable scaffolds in synthesis.

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.

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.

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 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.

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.

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.

Rhodium(III)-Catalyzed Asymmetric [4+1] and [5+1] Annulation of Arenes and 1,3-Enynes: A Distinct Mechanism of Allyl Formation and Allyl Functionalization.

We report chiral RhIII cyclopentadienyl-catalyzed enantioselective synthesis of lactams and isochromenes through oxidative [4+1] and [5+1] annulation, respectively, between arenes and 1,3-enynes. The reaction proceeds through a C-H activation, alkenyl-to-allyl rearrangement, and a nucleophilic cyclization cascade. The mechanisms of the [4+1] annulations were elucidated by a combination of experimental and computational methods. DFT studies indicated that, following the C-H activation and alkyne insertion, a RhIII alkenyl intermediate undergoes δ-hydrogen elimination of the allylic C-H via a six-membered ring transition state to produce a RhIII enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare RhIII η4 ene-allyl species with π-agostic interaction undergoes SN2 '-type external attack by the nitrogen nucleophile, instead of C-N reductive elimination, as the stereodetermining step.

Asymmetric Borylative Coupling of Vinylazaarenes and Ketones Catalyzed by a Copper(I) Complex

Azaarenes are attractive structural units widely found in chiral pharmaceuticals, agrochemicals, and biologically active natural products. An ongoing strategy has been the construction of new chira...

Collision-Induced Dissociation Studies of Protonated Ions of Alkylated Thymidine and 2′-Deoxyguanosine

Mass spectrometry and tandem MS (MS/MS) have been widely employed for the identification and quantification of damaged nucleosides in DNA, including those induced by alkylating agents. Upon collisional activation, protonated ions of alkylated nucleosides frequently undergo facile neutral loss of a 2-deoxyribose in MS/MS, and further cleavage of the resulting protonated nucleobases in MS3 can sometimes be employed for differentiating regioisomeric alkylated DNA lesions. Herein, we investigated systematically the collision-induced dissociation (CID) of the protonated ions of O4-alkylthymidine (O4-alkyldT), O2-alkyldT, O6-alkyl-2'-deoxyguanosine (O6-alkyldG), and N2-alkyldG through MS3 analysis. The MS3 of O2- and O4-MedT exhibit different fragmentation patterns from each other and from other O2- and O4-alkyldT adducts carrying larger alkyl groups. Meanwhile, elimination of alkene via a six-membered ring transition state is the dominant fragmentation pathway for O2-alkyldT, O4-alkyldT, and O6-alkyldG adducts carrying larger alkyl groups, whereas O6-MedG mainly undergoes elimination of ammonia. The breakdown of N2-alkyldG is substantially influenced by the structure of the alkyl group, where the relative ease in eliminating ammonia and alkene is modulated by the chain length and branching of the alkyl groups. We also rationalize our observations with density functional theory (DFT) calculations.

Mechanistic investigations on Pinnick oxidation: a density functional theory study.

A computational study on Pinnick oxidation of aldehydes into carboxylic acids using density functional theory (DFT) calculations has been evaluated with the (SMD)-M06-2X/aug-pVDZ level of theory, leading to an important understanding of the reaction mechanism that agrees with the experimental observations and explaining the substantial role of acid in driving the reaction. The DFT results elucidated that the first reaction step (FRS) proceeds in a manner where chlorous acid reacts with the aldehyde group through a distorted six-membered ring transition state to give a hydroxyallyl chlorite intermediate that undergoes a pericyclic fragmentation to release the carboxylic acid as a second reaction step (SRS). 1H NMR experiments and simulations showed that hydrogen bonding between carbonyl and t-butanol is unlikely to occur. Additionally, it was found that the FRS is a rate-determining and thermoneutral step, whereas SRS is highly exergonic with a low energetic barrier due to the Cl(III) → Cl(II) reduction. Frontier molecular orbital analysis, intrinsic reaction coordinate, molecular dynamics and distortion/interaction analysis further supported the proposed mechanism.

Hydrolytic deamination mechanisms of guanosine monophosphate: A computational study

The computational importance of guanosine monophosphate (GMP) comes from its importance in the synthesis of nucleic acids. Deamination reaction mechanisms, kinetics, and thermodynamics of GMP with 3H2O and 2H2O/OH− to produce xanthosine monophosphate (XMP) have been investigated. The proposed reactions were optimized in the gas-phase and in an aqueous medium. Two key steps were found for the proposed pathways: a tetrahedral intermediate formation and a single water-mediated six-membered ring transition state, yielding the desired product. The deamination of GMP with 2H2O/OH− yield deprotonated XMP− with only one pathway, which is highly exothermic, and the activation energy is significantly lower (33 kJ mol−1 at M11/6-31G(d)) compared to the other pathways. Our results suggest that pathway C is the most kinetically and thermodynamically favorable mechanism for the deamination of GMP with 3H2O with the lowest energy barrier of 123 kJ mol−1, due to multiple functions of water that enhance the reaction profile.

Hydrogen bonding promoted simple and clean photo-induced reduction of C-X bond with isopropanol.

We herein report a simple and clean photo-induced metal-free reduction of C-X bond under an atmosphere of air at room temperature. Isopropanol is used as both the reducing reagent and solvent. Various functional groups (acids, esters, alcohols, anilines, phenols, indoles, pyridines, cyano and trifluoromethyl groups) and other heterocyclic compounds are tolerated. Different organic halides (including C-I, C-Br and C-Cl bonds) can be dehalogenated with moderate to excellent yields. Polyhalides are also reduced chemoselectively and efficiently. DFT calculation suggests a six-membered ring transition state via C-XH-O hydrogen bonding to decrease the activation energy.

Oxidative and non-oxidative degradation of a TDI-based polyurethane foam: Volatile product and condensed phase characterisation by FTIR and solid state 13C NMR spectroscopy

The oxidative and non-oxidative degradation behaviour of a flexible polyurethane foam, synthesised from toluene diisocyanate and a polyether polyol, is reported. Both toluene diisocyanate and diaminotoluene were identified as major products under non-oxidative conditions, which indicates that the urethane linkages are degrading by two competing degradation mechanisms. Degradation of the urethane linkage by a depolymerisation reaction to yield toluene diisocyanate and polyol is proposed to occur initially. In addition, the atmospheric pressure conditions favour the degradation of the urethane linkages via a six-membered ring transition state reaction to form diaminotoluene, carbon dioxide and alkene terminated polyol chains. Solid-state 13C NMR spectroscopy and elemental analysis of the residues indicates that at temperatures above 300 °C ring fusion of the aromatic components within the foam occurs, and this leads to a nitrogen-containing carbonaceous char which has a complex aromatic structure. It is proposed that under the confined conditions of the degradation the aromatic nitrogen-containing species, such as toluene diisocyanate and diaminotoluene, undergo secondary reactions and ring fusion to yield a complex char structure.Under oxidative conditions, degradation, including ring fusion, occurs at a lower temperature than under non-oxidative conditions. Neither toluene diisocyanate nor diaminotoluene were observed as major degradation products. The polyol is observed to undergo thermo-oxidative degradation at much lower temperatures than purely thermal degradation. As a consequence, the depolymerisation reaction via the six-membered ring transition state is limited in extent and diaminotoluene is not evolved. The absence of toluene diisocyanate is proposed to be a result of this species undergoing oxidative degradation reactions which lead to it being incorporated into the char.

Enantioselective Electrophilic Cyanation of Boron Enolates: Scope and Mechanistic Studies.

Chiral β-ketonitriles bearing a stereogenic carbon center at the α-position are an important class of compounds, many of which serve as useful synthetic intermediates for the preparation of chiral 1,3-aminoalcohols, β-hydroxy nitriles, and related derivatives. Although the enantioselective electrophilic cyanation of enolate equivalents is one of the most promising approaches for the synthesis of chiral β-ketonitriles, the available methods are largely limited to reactions of 1,3-dicarbonyl compounds. Herein, we report on enantioselective electrophilic cyanation of boron enolates, which are readily prepared from α,β-unsaturated ketones and diisopinocampheylborane (Ipc2 BH) to afford chiral β-ketonitriles with a high level of enantioselectivity. The present method is scalable and provides facile access to both enantiomers of chiral β-ketonitriles. Analysis of the in situ generated boron enolates by NMR revealed that hydroboration proceeds in a stereospecific manner, providing α,α-disubstituted boron enolates in the form of single isomers. Furthermore, the results of DFT calculations suggest that the cyanation of the boron enolates with p-toluenesulfonyl cyanide (TsCN) proceeds in a highly enantioselective manner through a unique six-membered ring transition state.

T-jump pyrolysis and combustion of diisopropyl methylphosphonate

We monitor the T-jump pyrolysis and combustion reactions of diisopropyl methylphosphonate (DIMP) via FTIR and kinetic FTIR. DIMP is heated at 10 °C/s, ∼1800 °C/s, and ∼18,000 °C/s to temperatures between 500 °C and 1200 °C in nitrogen, air, and an oxygen-rich environment. We observe propene, isopropyl methylphosphonate (IMP), and methylphosphonic acid (MPA) as final decomposition products. In addition, we identify 2-propanol in the early phase of the reaction. The results indicate that DIMP has two possible decomposition pathways: (1) A two-step decomposition process in which DIMP first transfers an H atom from the OCH(CH3)2 group to the double-bonded oxygen through a six-membered ring transition state and forms IMP and propene. Subsequently, IMP decomposes into either propene and MPA or 2-propanol and methyl(oxo)-phosphoniumolate (MOPO). (2) DIMP transfers an H atom from the OCH(CH3)2 group to the O atom of the other OCH(CH3)2 group through a different six-membered ring and simultaneously forms MOPO, propene, and 2-propanol. Our theoretical calculation indicates that the energy barriers for the two reaction pathways are 36.5 kcal/mol (pathway 1, 1st step), 49.9 kcal/mol (pathway 1, 2nd step), and 51.2 kcal/mol (pathway 2), respectively. When using a heating rate of 10 °C/s, pathway (1) is the major reaction in the system; using a heating rate of ∼18,000 °C/s, pathway (2) is the most probable reaction; and when the heating rate is ∼1800 °C/s, both pathways are observed during the early phase of the reaction. When heated in air and an oxygen-rich environment, we observe the same decomposition products plus CO and CO2.

Catalytic Regio- and Enantioselective Oxytrifluoromethylthiolation of Aliphatic Internal Alkenes by Neighboring Group Assistance.

Chiral selenide-catalyzed oxytrifluoromethylthiolation of aliphatic internal alkenes by a formally intermolecular strategy is disclosed, affording CF3S 1,3-amino alcohol and 1,3-diol derivatives with high regio-, enantio-, and diastereoselectivities. The reactions are promoted by a neighboring imide or ester group on substrates via a six-membered ring transition state. This assistance strategy is also successfully applied to the regio- and diastereoselective oxyhalofunctionalization of internal alkenes and the conversion of alkynes.