Alternative Reaction Research Articles

Mechanism of the Oxidation of Heptafulvenes to Tropones Studied by Online Mass Spectrometry and Density Functional Theory Calculations.

We have identified the most likely reaction mechanism for oxidizing heptafulvenes to the corresponding tropones by experimental and theoretical investigations. The experimental studies were done by coupling a three-dimensional printed miniaturized reactor with an integrated electrospray ionization needle to a mass spectrometer. Using the experimentally observed ions as a basis, nine alternative reaction pathways were investigated with density functional theory calculations. The lowest energy reaction pathway starts with the formation of an epoxide that is opened upon the addition of a second equivalent of the oxidizing species meta-chloroperoxybenzoic acid. The adduct formed then undergoes a Criegee-like rearrangement to yield a positively charged hemiketal, which on deprotonation dissociates into acetone and tropone. Overall, the reaction mechanism resembles a Hock-like rearrangement.

Catalytic Effect on CO2 Electroreduction by Hydroxyl-Terminated Two-Dimensional MXenes

Electrocatalysis represents a promising method to generate renewable fuels and chemical feedstock from the carbon dioxide reduction reaction (CO2RR). However, traditional electrocatalysts based on transition metals are not efficient enough because of the high overpotential and slow turnover. MXenes, a family of two-dimensional metal carbides and nitrides, have been predicted to be effective in catalyzing CO2RR, but a systematic investigation into their catalytic performance is lacking, especially on hydroxyl (-OH)-terminated MXenes relevant in aqueous reaction conditions. In this work, we utilized first-principles simulations to systematically screen and explore the properties of MXenes in catalyzing CO2RR to CH4 from both aspects of thermodynamics and kinetics. Sc2C(OH)2 was found to be the most promising catalyst with the least negative limiting potential of -0.53 V vs RHE. This was achieved through an alternative reaction pathway, where the adsorbed species are stabilized by capturing H atoms from the MXene's OH termination group. New scaling relations, based on the shared H interaction between intermediates and MXenes, were established. Bader charge analyses reveal that catalysts with less electron migration in the *(H)COOH → *CO elementary step exhibit better CO2RR performance. This study provides new insights regarding the effect of surface functionalization on the catalytic performance of MXenes to guide future materials design.

Investigation of the <i>α</i>-pinene photooxidation by OH in the atmospheric simulation chamber SAPHIR

Abstract. The photooxidation of the most abundant monoterpene, α-pinene, by the hydroxyl radical (OH) was investigated at atmospheric concentrations in the atmospheric simulation chamber SAPHIR. Concentrations of nitric oxide (NO) were below 120 pptv. Yields of organic oxidation products are determined from measured time series giving values of 0.11±0.05, 0.19±0.06, and 0.05±0.03 for formaldehyde, acetone, and pinonaldehyde, respectively. The pinonaldehyde yield is at the low side of yields measured in previous laboratory studies, ranging from 0.06 to 0.87. These studies were mostly performed at reactant concentrations much higher than observed in the atmosphere. Time series of measured radical and trace-gas concentrations are compared to results from model calculations applying the Master Chemical Mechanism (MCM) 3.3.1. The model predicts pinonaldehyde mixing ratios that are at least a factor of 4 higher than measured values. At the same time, modeled hydroxyl and hydroperoxy (HO2) radical concentrations are approximately 25 % lower than measured values. Vereecken et al. (2007) suggested a shift of the initial organic peroxy radical (RO2) distribution towards RO2 species that do not yield pinonaldehyde but produce other organic products. Implementing these modifications reduces the model–measurement gap of pinonaldehyde by 20 % and also improves the agreement in modeled and measured radical concentrations by 10 %. However, the chemical oxidation mechanism needs further adjustment to explain observed radical and pinonaldehyde concentrations. This could be achieved by adjusting the initial RO2 distribution, but could also be done by implementing alternative reaction channels of RO2 species that currently lead to the formation of pinonaldehyde in the model.

Deep eutectic solvent an efficient reaction medium for the synthesis of chromeno pyrazolo and indazolo phthalazine derivatives

AbstractThe volatile, harmful and hazardous solvents have impact on human health, all organisms and environment. To reduce the use of hazardous volatile solvents, there is need to develop the alternative green reaction medium for organic transformations and is the main goal of green chemistry. The green solvent should be nonvolatile, nontoxic, biodegradable and biorenewable. Considering these parameters, herein, we have reported an efficient and environmentally benign green synthesis of chromeno pyrazolo and indazolo phthalazine derivatives in deep eutectic solvent. The present protocol contains excellent yield of the product, short reaction time, operation simplicity, reusability of solvent and no need of further purification.

Hydroxyl Radical-Coupled Electron-Transfer Mechanism of Flavin-Dependent Hydroxylases.

Class A flavin-dependent hydroxylases (FdHs) catalyze the hydroxylation of organic compounds in a site- and stereoselective manner. In stark contrast, conventional synthetic routes require environmentally hazardous reagents and give modest yields. Thus, understanding the detailed mechanism of this class of enzymes is essential to their rational manipulation for applications in green chemistry and pharmaceutical production. Both electrophilic substitution and radical intermediate mechanisms have been proposed as interpretations of FdH hydroxylation rates and optical spectra. While radical mechanistic steps are often difficult to examine directly, modern quantum chemistry calculations combined with statistical mechanical approaches can yield detailed mechanistic models providing insights that can be used to differentiate reaction pathways. In the current work, we report quantum mechanical/molecular mechanical (QM/MM) calculations on the fungal TropB enzyme that shows an alternative reaction pathway in which hydroxylation through a hydroxyl radical-coupled electron-transfer mechanism is significantly favored over electrophilic substitution. Furthermore, QM/MM calculations on several modified flavins provide a more consistent interpretation of the experimental trends in the reaction rates seen experimentally for a related enzyme, para-hydroxybenzoate hydroxylase. These calculations should guide future enzyme and substrate design strategies and broaden the scope of biological spin chemistry.

(Invited) Preserving a High Energy Density of the Lithium-Rich Layered Oxides (LLO) and New Lithium Diffusion Model for Llo

Lithium-rich layered oxides (LLOs) are considered promising cathode materials for lithium-ion batteries because of their high reversible capacity, which is attributed to the exploitation of the novel anionic redox in addition to the conventional cationic redox process. Cobalt-free layered lithium-rich nickel manganese oxides, Li[LixNiyMn1 − x − y]O2, are one of the promising positive electrode materials for lithium rechargeable batteries because of their high energy density and low materials cost. However, substantial voltage decay is inevitable upon electrochemical cycling, which makes this class of materials less practical. It has been proposed that undesirable voltage decay is linked to irreversible structural rearrangement involving irreversible oxygen loss and cation migration. In the first part of my talk, we demonstrate that the voltage decay of the electrode is correlated to Mn4+/Mn3+ redox activation and subsequent cation disordering, which can be remarkably suppressed via simple compositional tuning to induce the formation of Ni3+ in the pristine material. By implementing our new strategy, the Mn4+/Mn3+ reduction is subdued by an alternative redox reaction involving the use of pristine Ni3+ as a redox buffer, which has been designed to be widened from Ni3+/Ni4+ to Ni2+/Ni4+, without compensation for the capacity in principle. Negligible change in the voltage profile of modified LLO is observed upon extended cycling, and manganese migration into the lithium layer is significantly suppressed. In the second part of the talk, I will discuss about transition metal (TM) migration, which is now understood to be the critical factor triggering the anionic redox as well as the phase transition, although the origin is still under debate. A better understanding of the specific TM migration behavior and its effect during charge/discharge would thus enable further development of this class of materials. I will show that the unique TM migration during charge/discharge significantly alters the lithium diffusion mechanism/kinetics of LLO cathodes. I present clear evidence of the much more sluggish lithium diffusion occurring during discharge (lithiation) than during charge (de-lithiation), which contrasts with the traditional lithium diffusion model based on simple topotactic lithium intercalation/deintercalation in the layered framework. The reversible but asymmetric TM migration in the structure, which originates from the non-equivalent local environments around the TM during the charge and discharge processes, is shown to affect the lithium mobility. This correlation between TM migration and lithium mobility led us to propose a new lithium diffusion model for layered structures and suggests the importance of considering TM migration in designing new LLO cathode materials.

Glucose:urea:NH4Cl low melting mixture for the synthesis of symmetric azines

Alternate reaction media have become very important due to the problems created by the highly volatile nature of the solvents. The deep eutectic mixture is a kind of an alternate reaction medium which has emerged in recent years. Low melting mixtures were introduced by making the deep eutectic mixture more cost-effective and renewable by introducing carbohydrates into it. The properties of low melting mixtures include easiness to prepare, usage of low-cost components, biodegradability, solubility in water, easy separation from organic compounds, etc. The low melting mixtures such as glucose:urea:NH4Cl, glucose:ChCl, glucose:urea:ChCl, glycerol:urea:NH4Cl, and ethylene glycol:urea:NH4Cl were used in different ratios for the reactions. The properties such as viscosity, density, acidity, glass transition temperature, and thermal stability were studied. An unusual method for the synthesis of symmetrical azines is introduced wherein benzaldehyde and hydroxylamine are reacted in the presence of glucose:urea:NH4Cl. The method of synthesis needs only less reaction time, temperature and the product was easily separated. The products were confirmed using GC-MS and NMR techniques. The recyclability of glucose:urea:NH4Cl was studied.

Rapid and eco-friendly high yield synthesis of dihydroquinazolinones mediated by urea/zinc chloride eutectic mixture

In this contribution an efficient and environmentally friendly one-pot multicomponent synthesis of dihydroquinazolinones is reported. Several deep eutectic solvents were studied as alternative reaction media for the three-component reaction between isatoic anhydride, amines and aldehydes or ketones. Among the low melting mixtures, the mixture urea/zinc chloride efficiently catalyzed the reaction affording a variety of dihydroquinazolinones in excellent yields with short reaction times. In addition, this alternative solvent was recycled and reused in three cycles showing a good performance in the catalytic activity. The versatility of the eutectic solvent was further explored for the obtention of the biologically active compounds methaqualone, mecloqualone and the natural alkaloid tryptanthrin giving an excellent yield.

Strongly Coupled Redox-Linked Conformational Switching at the Active Site of the Non-Heme Iron-Dependent Dioxygenase, TauD.

2-Oxoglutarate (2OG)-dependent dioxygenases catalyze C-H activation while performing a wide range of chemical transformations. In contrast to their heme analogues, non-heme iron centers afford greater structural flexibility with important implications for their diverse catalytic mechanisms. We characterize an in situ structural model of the putative transient ferric intermediate of 2OG:taurine dioxygenase (TauD) by using a combination of spectroelectrochemical and semiempirical computational methods, demonstrating that the Fe(III/II) transition involves a substantial, fully reversible, redox-linked conformational change at the active site. This rearrangement alters the apparent redox potential of the active site between -127 mV for reduction of the ferric state and +171 mV for oxidation of the ferrous state of the 2OG-Fe-TauD complex. Structural perturbations exhibit limited sensitivity to mediator concentrations and potential pulse duration. Similar changes were observed in the Fe-TauD and taurine-2OG-Fe-TauD complexes, thus attributing the reorganization to the protein moiety rather than the cosubstrates. Redox-difference infrared spectra indicate a reorganization of the protein backbone in addition to the involvement of carboxylate and histidine ligands. Quantitative modeling of the transient redox response using two alternative reaction schemes across a variety of experimental conditions strongly supports the proposal for intrinsic protein reorganization as the origin of the experimental observations.

Invariants for optimal operation of a reactor-separator-recycle process

Globally optimal invariants for the one unconstrained degree of freedom in an A + B → C reactor-separator-recycle process are derived for four alternative reaction kinetic expressions from an analysis of the overall plant material balance. The derivation is performed for the simplified economic objective of minimizing recycle rate (Mode I) at given production and maximizing production (Mode II) with maximum recycle rate as the capacity bottleneck. For power law kinetics, holding the reactor A/B ratio constant is obtained as the invariant while a more complex non-linear expression is obtained for Langmuir-Hinshelwood kinetics. For the more practical economic objective of minimizing separation column boilup or maximum column boilup as the capacity bottleneck, the invariant as the controlled variable (CV) achieves near optimal operation with the loss being <0.1% over the envisaged operating space in all cases. The invariants are thus good self-optimizing CVs (SOCVs). The work provides a physical basis for using reactant ratio as an SOCV in plantwide control.

Integrating Hydrogen Production with Aqueous Selective Semi‐Dehydrogenation of Tetrahydroisoquinolines over a Ni2P Bifunctional Electrode

AbstractExploring an alternative anodic reaction to produce value‐added chemicals with high selectivity, especially integrated with promoted hydrogen generation, is desirable. Herein, a selective semi‐dehydrogenation of tetrahydroisoquinolines (THIQs) is demonstrated to replace the oxygen evolution reaction (OER) for boosting H2 evolution reaction (HER) in water over a Ni2P nanosheet electrode. The value‐added semi‐dehydrogenation products, dihydroisoquinolines (DHIQs), can be selectively obtained with high yields at the anode. The controllable semi‐dehydrogenation is attributed to the in situ formed NiII/NiIII redox active species. Such a strategy can deliver a variety of DHIQs bearing electron‐withdrawing/donating groups in good yields and excellent selectivities, and can be applied to gram‐scale synthesis. A two‐electrode Ni2P bifunctional electrolyzer can produce both H2 and DHIQs with robust stability and high Faradaic efficiencies at a much lower cell voltage than that of overall water splitting.

Integrating Hydrogen Production with Aqueous Selective Semi-Dehydrogenation of Tetrahydroisoquinolines over a Ni2 P Bifunctional Electrode.

Exploring an alternative anodic reaction to produce value-added chemicals with high selectivity, especially integrated with promoted hydrogen generation, is desirable. Herein, a selective semi-dehydrogenation of tetrahydroisoquinolines (THIQs) is demonstrated to replace the oxygen evolution reaction (OER) for boosting H2 evolution reaction (HER) in water over a Ni2 P nanosheet electrode. The value-added semi-dehydrogenation products, dihydroisoquinolines (DHIQs), can be selectively obtained with high yields at the anode. The controllable semi-dehydrogenation is attributed to the in situ formed NiII /NiIII redox active species. Such a strategy can deliver a variety of DHIQs bearing electron-withdrawing/donating groups in good yields and excellent selectivities, and can be applied to gram-scale synthesis. A two-electrode Ni2 P bifunctional electrolyzer can produce both H2 and DHIQs with robust stability and high Faradaic efficiencies at a much lower cell voltage than that of overall water splitting.

Davis-Beirut Reaction: A Photochemical Brønsted Acid Catalyzed Route to N-Aryl 2H-Indazoles.

The Davis-Beirut reaction provides access to 2H-indazoles from aromatic nitro compounds. However, N-aryl targets have been traditionally challenging to access due to competitive alternate reaction pathways. Previously, the key nitroso imine intermediate was generated under alkaline conditions, but as reported here, the photochemistry of o-nitrobenzyl alcohols empowered Brønsted acid catalyzed conditions for accessing N-aryl targets. Anilines and alkyl amines give different outcomes under optimized conditions; the proposed mechanism was studied using quantum chemical calculations.

Radiometals for imaging and theranostics, current production, and future perspectives.

The aim of this review is to make the reader familiar with currently available radiometals, their production modes, capacities, and quality concerns related to their medical use, as well as new emerging radiometals and irradiation technologies from the perspective of their diagnostic and theranostic applications. Production methods of 177 Lu serve as an example of various issues related to the production yield, specific activity, radionuclidic and chemical purity, and production economy. Other radiometals that are currently used or explored for potential medical applications, with particular focus on their theranostic value, are discussed. Using radiometals for diagnostic imaging and therapy is on the rise. The high demand for radiometals for medical use prompts investigations towards using alternative irradiation reactions, while using existing nuclear reactors and accelerator facilities. This review discusses these production capacities and what is necessary to cover the growing demand for theranostic nuclides.

Synthesis and properties of polyurea/malonamide dendritic co-adsorbents for dye-sensitized solar cells

A series polyurea/malonamide dendron (G0.5-G2.5) containing C10 alkyl chain located at the peripheral were prepared as dendritic building blocks via alternative addition and ring opening reactions. Afterward, G0.5, G1.5 and G2.5 consisting of azetidine-2,4-dione functional group was selected and reacted with 6-aminohexanoic to generate the corresponding acid anchor containing dendritic co-adsorbents (GA6-0.5, GA6-1.5 and GA6-2.5). The dye-sensitized solar cells (DSSCs) were then fabricated using phenothiazine-based dyes (HL5) as sensitizer with different generations and concentrations of dendritic co-adsorbents to explore the effect on cell performances and interfacial charge properties. Upon addition 0.3 mM of GA6-2.5 co-adsorbent, the best efficiency was 6.66 ± 0.07%, with a current density (Jsc) of 12.88 ± 0.11 mA cm−2, an open circuit voltage (Voc) of 0.72 ± 0.00 V and a fill factor of 0.72 ± 0.00. This cell exhibits superior efficiency than the HL5 dye only (PCE = 5.67 ± 0.17%) and slightly higher than the standard 3 mM CDCA co-adsorbent based device (PCE = 6.33 ± 0.04%). In addition, this best efficiency of this cell reached 83% of the N719 standard cell. This study may pave a new way for achieving high-performance with low cost on DSSCs applications.

Davis-Beirut Reaction: Diverse Chemistries of Highly Reactive Nitroso Intermediates in Heterocycle Synthesis.

Indazoles are an important class of nitrogen heterocycles because of their excellent performance in biologically relevant applications, such as in chemical biology and medicinal chemistry. In these applications, convenient synthesis using commercially available and diverse building blocks is highly desirable. Within this broad class, 2H-indazoles are relatively underexploited when compared to 1H-indazole, perhaps because of regioselectivity issues associated with the synthesis of 2H-indazoles. This Account describes our unfolding of the synthetic utility of the Davis-Beirut reaction (DBR) for the construction of 2H-indazoles and their derivatives; parallel unfoldings of mechanistic models for these interrelated N-N bond forming reactions are also summarized. The Davis-Beirut reaction is a robust method that exploits the diverse chemistries of a key nitroso imine or nitroso benzaldehyde intermediate generated in situ under redox neutral conditions. The resulting N-N bond-forming heterocyclization between nucleophilic and electrophilic nitrogens can be leveraged for the synthesis of multiple classes of indazoles and their derivatives, such as simple or fused indazolones, thiazolo-indazoles, 3-alkoxy-2H-indazoles, 2H-indazole N-oxides, and 2H-indazoles with various substitutions on the ring system or the nitrogens. These diverse products can all be synthesized under alkaline conditions and the various strategies for accessing these heterocycles are discussed. Alternatively, we have also developed methods involving mild photochemical conditions for the nitrobenzyl → aci-nitro → nitroso imine sequence. Solvent consideration is especially important for modulating the chemistry of the reactive intermediates in these reactions; the presence of water is critically important in some cases, but water's beneficial effect has a ceiling because of the alternative reaction pathways it enables. Fused 2H-indazoles readily undergo ring opening reactions to give indazolones when treated with nucleophiles or electrophiles. Furthermore, palladium-catalyzed cross coupling, the Sonagashira reaction, EDC amide coupling, 1,3-dipolar cycloadditions with nitrile oxides, copper-catalyzed alkyne-azide cycloadditions (click reaction), as well as copper-free click reactions, can all be used late-stage to modify 2H-indazoles and indazolones. The continued development and applications of the Davis-Beirut reaction has provided many insights for taming the reactivity of highly reactive nitro and nitroso groups, which still has a plethora of underexplored chemistries and challenges. For example, there is currently a limited number of nonfused 2H-indazole examples containing an aryl substitution at nitrogen. This is caused by relatively slow N-N bond formation between N-aryl imine and nitroso reactants, which allows water to add to the key nitroso imine intermediate causing imine bond cleavage to be a competitive reaction pathway rather than proceeding through the desired N-N bond-forming heterocyclization.

Methanol to Olefins Reaction Route Based on Methylcyclopentadienes as Critical Intermediates

Starting from C1 raw material, methanol conversion to hydrocarbons has been realized via a rather complicated pathway. In this contribution, we proposed an alternative methanol reaction route and p...

Exploring the Scope of an α/β-Aminomutase for the Amination of Cinnamate Epoxides to Arylserines and Arylisoserines

Biocatalytic process-development continues to advance toward discovering alternative transformation reactions to synthesize fine chemicals. Here, a 5-methylidene-3,5-dihydro-4H-imidazol-4-one (MIO)...

Alkali metal trifluoroacetates for the nucleophilic trifluoromethylation of fullerenes

Fullerene C60 readily reacts with potassium and cesium trifluoroacetates yielding C60(CF3)–M+ salts, and subsequent acid hydrolysis gives ortho-C60(CF3)H. The reaction rate and the probability of the alternative reaction pathways strongly depend on the particular metal cation. Thus, the reactivity increases in the order Li < Na < K≈Cs, in correlation with the decreasing thermal stability. Furthermore, the sodium and, especially, lithium salts tend to effect CF2- rather than CF3-functionalization of the fullerene, in good accordance with the hard/soft acids and bases theory. The nucleophilic trifluoromethylation is found to be applicable to other pristine fullerenes like C70 as well as to fullerene derivatives like p7mp-C70(CF3)10. It enables selective preparation of low trifluoromethylated fullerenes via regioselective consecutive trifluoromethylation under accurately controlled solution-phase conditions.

NH3 -Driven Benzene C-H Activation with O2 that Opens a New Way for Selective Phenol Synthesis.

Catalytic benzene C-H activation toward selective phenol synthesis with O2 remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three-steps cumene process in liquid phase, which is energy-intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas-phase single-path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O2 under coexisting NH3 by the new inter-ligand concerted mechanism. Among alkali and alkaline-earth metal ions and transition and precious metal ions, single Cs+ and Rb+ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas-phase single-path reaction process. The single Cs+ and Rb+ sites that possess neither significant Lewis acidic-basic property nor redox property, cannot activate benzene, O2 , and NH3 , respectively, whereas when they coadsorbed together, the reaction of the inter-coadsorbates on the single alkali-metal ion site proceeds concertedly (the inter-ligand concerted mechanism), bringing about the benzene C-H activation toward phenol synthesis. The NH3 -driven benzene C-H activation with O2 was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH3 on Pt6 metallic cluster/β and Ni4 O4 oxide cluster/β. The NH3 -driven selective oxidation mechanism observed with the Cs+ /β and Rb+ /β differs from the traditional redox catalysis (Mars-van Krevelen) mechanism, simple Langmuir-Hinshelwood mechanism, and acid-base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.