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In recent years, metal‐free organic synthesis using triarylboranes as catalysts has become a prevalent research area. Herein we report a comprehensive computational and experimental study for the highly selective synthesis of N‐substituted pyrazoles through the generation of carbenium species from the reaction between aryl esters and vinyl diazoacetates in the presence of catalytic tris(pentafluorophenyl)borane [B(C6F5)3]. DFT studies were undertaken to illuminate the reaction mechanism revealing that the in situ generation of a carbenium species acts as an autocatalyst to prompt the regiospecific formation of N‐substituted pyrazoles in good to excellent yields (up to 81 %)., Tris(pentafluorophenyl)borane‐catalyzed reactions between vinyl diazoacetates and aryl esters afforded highly regioselective N‐alkylated pyrazoles. These reactions involve an autocatalytic process in which in situ formed carbenium species act as a catalyst to regioselectively afford pyrazoles. Detailed DFT studies were carried out to explain the reaction mechanism.

The n-hexane was used as a model compound to study the catalytic cracking behavior of light hydrocarbon in HZSM-5 zeolites, and the law of product selectivity of real acid-catalyzed reaction was investigated by analyzing the product distributions. The results showed that no pyrolysis reaction was found at 300°C. Only the acid catalytic reaction took place by the mechanism of carbocation, whose activity was positively correlated to the amount of Bronsted (B) acid sites. The selectivity of ethane, ethylene and propane was negatively correlated, while that of propylene was positively correlated with the Si/Al ratios and catalyst to oil ratios, suggesting that low acid density might be more favorable for monomolecular cracking reactions. It was worth nothing that the total selectivity of C4 products was much higher than that of C2 products. Combined with the quantum chemistry calculation results, it could be confirmed that the super-stability of C2H+ 5 carbenium ion from the monomolecular cracking of n-hexane made it difficult to produce ethylene and ethane through hydrogen transfer reaction. It’s easier to form a C8 carbenium ion (C8H+ 19) with another n-hexane molecule, and then to generate more C4 products. These results revealed the nature of the low selectivity of ethylene in light hydrocarbon catalytic cracking products. It could be concluded that the product selectivity of catalytic cracking of light hydrocarbons could be modulated by controlling reaction paths depending on the catalyst acid properties and the catalyst to oil ratios. This work will provide important theoretical support for the catalyst design and process development of naphtha catalytic cracking.

Porous aluminosilicates such as zeolites are ubiquitous catalysts for the production of high-value and industrially relevant commodity chemicals, including the conversion of hydrocarbons, amines, alcohols, and others. Bimolecular reactions are an important subclass of reactions that can occur on Brønsted acid sites of a zeolite catalyst. Kinetic modeling of these systems at the process scale requires the interaction energetics of reactants and the active sites to be described accurately. It is generally known that adsorption is a coverage-dependent phenomenon, with lower heats of adsorption observed for molecules at higher coverage. However, few studies have systematically investigated the coadsorption of molecules on a single active site, specifically focusing on the strength of interaction of the second adsorbate after the initial adsorption step. In this work, we quantify the unimolecular and bimolecular adsorption energies of varying adsorbates, including paraffins, olefins, alcohols, amines, and noncondensible gases in the acidic and siliceous ZSM-5 frameworks. As a special case, olefin adsorption was examined for physisorption and chemisorption regimes, characterized by π-complex, framework alkoxide and carbenium ion adsorption, respectively. The effects of functional groups and molecular size were quantified, and correlations that relate the adsorption of the second adsorbate identity to that of the first adsorbate are provided.

Octanes in alkylation products obtained from industrial alkylation were studied by batch experiments. More than eight octane isomers were identified and quantified by gas chromatography-mass spectrometry. Based on a classic carbenium ion mechanism, the carbocation transition states in concentrated sulfuric acid catalyzed alkylation were investigated using quantum-chemical simulations and predicted the concentration and octane isomerization products including trimethylpentane and dimethylhexane as well as the formation of heavier compounds that resulted from the oligomerization of octane and butene. The agreement between model calculations and experimental data was quite satisfactory. Calculation results indicated that composition and content of trimethylpentanes in the alkylation products were 2,2,4-trimethylpentane > 2,3,3-trimethylpentane > 2,3,4-trimethylpentane > 2,2,3-trimethylpentane whether the 2-butene or i-butene acts as olefin. Heavier compounds in the alkylate were primarily formed by the oligomerization of dimethylhexane with 1-butene. Hopefully, the carbocation transition state models developed in this work will be useful for understanding the product distributions of octane in alkylation products.

The synthesis of a helically chiral carbenium ion is reported. The new motif is essentially a trityl cation embedded into a [7]helicene-like framework. The key step in its preparation establishes the π-extended fluorenone system in one step by an unprecedented palladium-catalyzed carbonylative annulation of a 4,4′-biphenanthryl-3,3′-diyl precursor. The racemic form of the new carbon Lewis acid was found to catalyze a representative set of reactions typically promoted by the trityl cation.

Since the discovery of carbocations in 1901, the past 120 years have witnessed many marvelous advances in the chemistry of carbocations. The state-of-the-art research in this field is to overcome the intrinsic instability and high reactivity of the prochiral carbocationic intermediates to develop catalytic asymmetric reactions. Such transformations enable the facile synthesis of structurally diverse value-added products from readily available starting materials such as alkenes, alcohols, and carbonyl derivatives, and enjoy high and even perfect atom-economy in most cases. Notably, such allows catalytic stereoconvergent synthesis from racemic substrates and can realize regioselectivity in olefin functionalization reactions complementary to radical processes. With the rapid developments in modern asymmetric organocatalysis, a variety of highly enantioselective protocols evolving prochiral carbocationic intermediates have been achieved by employing three strategies, namely chiral ion-pairing, chiral nucleophile, or chiral carbenium ion strategy. This feature article aims to summarize the exciting advances in this emerging area and briefly showcase the possible mechanisms. The advantages and limitations of each strategy are presented as well as their synthetic applications in the synthesis of natural products or bioactive compounds.

Redox flow batteries (RFBs) represent a promising technology for grid-scale integration of renewable energy. Redox-active molecular pairs with large potential windows have been identified as key components of these systems. However, cross-contamination problems encountered by the use of different catholyte and anolyte species still limits the development of reliable organic RFBs. Herein, we report the first use of a helical carbenium ion, with three stable oxidation states, as electrolyte for the development of symmetric cells. Cyclic voltammo-amperometric studies were conducted in acetonitrile to assess the essential kinetic properties for flow battery performance and cycling stability of this molecule. The selected [4]helicenium ion was then evaluated by using mono- and bi-electronic cycling experiments, resulting in 745 and 80 cycles respectively, with near-perfect capacity retention. This helical carbenium ion based electrolyte achieved a proof-of-principle 2.12 V open circuit potential as an all-organic symmetric RFB.

Red light has the advantages of low energy, less health risk and high penetration depth through various media. Herein, a helical carbenium ion (N,N’-di-n-propyl-1,13-dimethoxyquinacridinium (nPr-DM...

Nonoxidative alkane dehydrogenation is a promising route to produce olefins, commonly used as building blocks in the chemical industry. Metal oxides, including γ-Al2O3 and β-Ga2O3, are attractive dehydrogenation catalysts due to their surface Lewis acid–base properties. In this work, we use density functional theory (DFT) to investigate nonoxidative dehydrogenation of ethane, propane, and isobutane on the Ga-doped and undoped (100) γ-Al2O3via the concerted and stepwise mechanisms. We revealed that doping (100) γ-Al2O3 with Ga atoms has significant improvement in the dehydrogenation activity by decreasing the C–H activation barriers of the kinetically favored concerted mechanism and increasing the overall dehydrogenation turnover frequencies. We identified the dissociated H2 binding energy as an activity descriptor for alkane dehydrogenation, accounting for the strength of the Lewis acidity and basicity of the active sites. We demonstrate linear correlations between the dissociated H2 binding energy and the activation barriers of the rate determining steps for both the concerted and stepwise mechanisms. We further found the carbenium ion stability to be a quantitative reactant-type descriptor, correlating with the C–H activation barriers of the different alkanes. Importantly, we developed an alkane dehydrogenation model that captures the effect of catalyst acid–base surface properties (through dissociated H2 binding energy) and reactant substitution (through carbenium ion stability). Additionally, we show that the dissociated H2 binding energy can be used to predict the overall dehydrogenation turnover frequencies. Taken together, our developed methodology facilitates the screening and discovery of alkane dehydrogenation catalysts and demonstrates doping as an effective route to enhance catalytic activity.

A novel metal preloading route is proposed to prepare Pt/SAPO-11 bifunctional catalysts. The addition of platinum brings about an increase in catalyst product yield with a growth rate of 87% attributed to chemical adsorption between Pt and the lone pair of electrons in the nitrogen atom of the structure-directing agent. More interestingly, it can be concluded from the HAADF-STEM, SE-STEM images and DRIFTS data on CO adsorption, there are small Pt nanoparticles confined inside the SAPO-11 molecular sieve, while the big Pt nanoparticles are dispersed on the surface. The synergistic effect between the metal sites and acid sites can be improved by the enhancement of metal–acid balance, by adjusting the nPt/nA ratio. A better metal–acid balance property is achieved and the carbenium ion can quickly go through the hydrogenating reaction on Pt nanoparticles, hence shortening the time it is trapped in the acid sites upon n-dodecane hydroisomerization. Consequently, the cracking reaction can be effectively avoided and liquid yield as high as 99% and isomer yield up to 84% is produced, much superior than the Pt/SAPO-11 catalyst synthesized via the traditional impregnation method of Pt with the maximum isomer yield of 52%. Moreover, more multi-branched isomers are produced and can further help improve the low-temperature rheology when used as lubricant base oil.

Typically substitution of alcohols involves a two-step activation/displacement pathway thus leading to the generation of additional waste. The current chapter considers an alternative reaction manifold with the displacement taking place directly through activation of the alcohol by either a Lewis or Brønsted acid. With particular focus on the literature since 2011, an initial overview of carbenium ion reactivity is provided followed by a survey of displacement reactions grouped by the nature of the nucleophile. Finally, advances in both diastereoselective and enantioselective variants of the reaction are discussed.

Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationic phosphine gold complexes of general formula [(o-Ph2P(C6H4)Carb)Au(tht)]2+ decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold center. While the most acidic examples of such compounds have limited stability, the dicationic complexes with Carb+ = 9-N-methylacridinium and Carb+ = [C(ArN)2]+ (ArN = p-(C6H4)NMe2) are active as catalysts for the cycloisomerization of N-propargyl-4-fluorobenzamide, a substrate chosen to benchmark reactivity. The dicationic complex [(o-Ph2P(C6H4)C(ArN)2)Au(tht)]2+, which also promotes hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts toward the generation and isolation of free γ-cationic phosphines of general formula [(o-Ph2P(C6H4)Carb)]+. While cyclization into phosphonium species is observed for Carb+ = [C(ArN)2]+, [C(Ph)(ArN)]+, and 9-xanthylium, [(o-Ph2P(C6H4)-9-N-methylacridinium)]+ can be isolated as an air stable, biphilic derivative with uncompromised Lewis acidic and basic properties., This work describes the synthesis of carbenium-based, γ-cationic phosphines and their coordination to Au(i) cations , leading to carbophilic catalysts whose activity is enhanced by the ligand-enforced convergence of the positively charged moieties.

Gold carbenes of the general type [LAu=CR2 ]+ are sufficiently long-lived for spectroscopic inspection only if the substituents compensate for the largely missing stabilization of the carbene center by the [LAu]+ fragment. π-Donation by two fluorine substituents (R=F) is insufficient; rather, difluorocarbene complexes are so deprived in electron density that they sequester even "weakly coordinating" anions such as triflate or triflimide. This particular bonding situation translates into unmistakable carbenium ion chemistry upon reaction with stilbene as a model substrate.

Hydride transfer as the key step in the alkylation reaction harbors burgeoning research interest. Herein, the influence of acid strength on hydride transfer was investigated via density functional theory. Based on energetic analysis, increasing acid strength can stabilize the carbenium ion and destabilize alkoxide species, resulting in significant improvement of hydride transfer performance. Notably, the hydride transfer reaction cannot occur at a weak acid site owing to difficulty in forming a stable H-shared intermediate, which perfectly coincides with experiments. More interestingly, the changing extent of energy for intermediates and transition states correlates well with the ionic character at various acid strengths. A stronger ionic character leads to a larger changing extent of energy. Therefore, the changing trends of activation energy barriers to acid strength can be predicated by comparing the strength of ionic character between reactants and transition states. The results are of referential sig...

Heterogeneous and supramolecular catalysis are fundamental processes in chemistry. To understand the synergistic role between heterogeneous and supramolecular catalysis, the catalytic mechanism of cucurbituril solid for epoxide alcoholysis was investigated by performing density functional theory (DFT) calculations. The results reveal that styrene oxide (StyOx) ethanolysis has an inherent regioselectivity, which results from different groups linked to the epoxy group. The hydronium ion can catalyze the ring-opening of StyOx ethanolysis and lead to the formation of a planar carbenium ion. StyOx can also be hydrolyzed via the homogenous catalysis of acid to produce 1, 2-diol. Cucurbituril solid can catalyze epoxide alcoholysis because of its acidic property. Its unique cavity can lead to a favorable frontside-attack of the alcohol on the carbenium ion. The product from the heterogeneous catalysis of cucurbituril solid is pure β-alkoxy alcohol. The results are important to understand heterogeneous and supramolecular catalysis and the design of new and effective supramolecular catalysts.

In order to explore the roles of the free radical and carbenium ion mechanisms in alkane cracking to produce light olefins, pentane cracking tests have been carried out over a zeolite catalyst at 600–800 °C, and a particular attention has been paid to the measurement of the product distribution. The cracking modes for pentane included “uncatalytic pyrolysis” following the free radical mechanism, “catalytic cracking” following the carbenium ion mechanism, and “catalytic pyrolysis” following the dual mechanisms (involving the concurrence of the free radical and carbenium ion mechanisms): (1) The free radical mechanism did not occur until 700 °C under the studied conditions, and it led to an ethylene-rich product distribution. (2) Zeolite catalyst initiated the carbenium ion mechanism that led to a propene-rich product distribution. (3) The dual mechanisms exhibited a wide distribution of ethylene and propene, which approximated to that of the free radical mechanism from that of the carbenium ion mechanism with increasing reaction temperature. The kc/kt and kcp/(kt+kc) parameters have been proposed to determine the relationship between the free radical and carbenium ion mechanisms. It was found that the carbenium ion mechanism greatly contributed to pentane cracking at the low temperatures, and the free radical mechanism took over the dominant position at the high temperatures. A proper proportion between the carbenium ion and free radical mechanisms promoted pentane cracking. In addition, yield of propene plus ethylene (P + E) and ratio of propene to ethylene (P/E) have been employed to evaluate the light olefins production. In the research scope, pentane cracking at 740 °C over 200 mg K-ZSM-22 zeolites seemed the optimal process, which has taken the consumption of energy and catalyst as well as the production and distribution of light olefins into consideration.

Zeolite Y is one of the most important solid acid catalysts that find immense applications in catalytic processes such as fluid catalytic cracking (FCC), acylation and cumene cracking. The technical grades of FCC catalyst require specifications that are met by dispersing the zeolite in amorphous binders. On contact with binders, zeolite properties in composite catalysts alter. This article reports an insight into structural and surface modifications of Y zeolites caused by binders’ incorporation. Experiments employing various characterization techniques, including solid state MAS-NMR and FTIR reveal alumina incorporation adjusts zeolite framework in comparison with silica binder. Incorporation of alumina binder into zeolite Y polarizes the framework by extra-framework alumina species (such as Al3+, AlO+ and Al(OH)2+) coming from the binder. The polarization, apparent with USY and REY, is considered as the operating mechanism for distortion of zeolite framework, involving the attraction of aluminum species from the binder to the framework of zeolite and its subsequent framework structure reorientation. Compensating framework Na+ cations restrain the polarization of NaY framework due to its deficiency of protons, revealing Bronsted acid sites as active centers for the zeolite framework polarization. The resulting alumina bound catalysts are amorphized by layers of alumina materials that implicate the Bronsted acidity as a function of its weight in the catalyst. Catalytic reaction reveals that polarization of the zeolite framework by extra-framework alumina species decreases the transformation of paraffin to carbenium ion, increasing the hydrogen transfer reaction pathway.

International audience; Phenylnitrene radical cations m/z 91, C6H5N, 8a•+ are observed in the mass spectra of 1-, 2-, and 5-phenyltetrazoles, even though no C–N bond is present in 5-phenyltetrazole. Calculations at the B3LYP/6-311G(d,p) level of theory indicate that initial formation of the C-phenylimidoylnitrene 13•+ and/or benzonitrile imine radical cation 19•+ from 1H- and 2H-5-phenyltetrazoles 11 and 12 is followed by isomerizations of 13•+ to the phenylcyanamide ion 15•+ over a low barrier. A cyclization of imidoylnitrene ion 13•+ onto the benzene ring offers alternate, very facile routes to the phenylnitrene ion 8a•+ and the phenylcarbodiimide ion 14•+ via the azabicyclooctadienimine 16•+. Eliminations of HNC or HCN from 14•+ and 15•+ again yield the phenylnitrene radical cation 8a•+. A direct 1,3-H shift isomerizing phenylcarbodiimide ion 14•+ to the phenylcyanamide ion 15•+ requires a very high activation energy of 114 kcal/mol, and this reaction needs not be involved. The benzonitrile imine −3-phenyl-1H-diazirine–phenylimidoylnitrene–phenylcarbodiimide/phenylcyanamide rearrangement has parallels in thermal and photochemical processes, but the facile cyclization of imidoylnitrene 13•+ to azabicyclooctadienimine 16•+ is facilitated by the positive charge making the nitrene more electrophilic. Furthermore, the benzonitrile imine radical cation 19•+ can cyclize to indazole 24•+, and a series of intramolecular rearrangements via hydrogen shifts, ring-openings and ring closures allow the interconversion of numerous ions of composition C7H6N2•+, including 19•+, 24•+, the benzimidazole ion 38•+ and o-aminobenzonitrile ion 40•+, all of which can eliminate either HCN or HNC to yield the C6H5N•+ ions of phenylnitrene, 8a•+, and/or iminocyclohexadienylidene, 34•+. Moreover, benzonitrile imine 19•+ can behave like a benzylic carbenium ion, undergoing a novel ring expansion to cycloheptatetraenyldiazene 45•+. The N-phenylnitrile imine ion 2d•+ derived from 2-phenyltetrazole 1d cleaves efficiently to the phenylnitrene ion 8a•+ but may also cyclize to the indazole ion 24•+. The N-phenylimidoylnitrene 59•+ derived from 1-phenyltetrazole 5d undergoes facile isomerization to the phenylcyanamide ion 15•+ and hence phenylnitrene radical cation 8a•+.

A robust method for preparing controlled poly(vinyl ethers) utilizing commercially available reagents under visible light is reported. Pentacarbonylbromomanganese (Mn(CO)5Br), generated from the conventional photolysis of manganese carbonyl and halide abstraction, is considered as a catalyst to oxidize carbon radical to carbenium ion in this polymerization. Polymerization behavior including the effects of solvents and monomers have been investigated.

The enhanced effect of strongly acidic proximate protons (distance 5.0–5.5 A) in ZSM-5 was presented on complex propene oligomerization up to the aromatization and development of individual carbenium ion intermediates in the zeolite pores. H-ZSM-5 samples were hydrothermally synthesized in order to possess similar contents of framework Al (Si/Al 23.8 and 24.5) located at channel intersections; they also had greatly predominated population of proximate protons defined by their bonding to two AlO4− ions located in one 6MR or possessed big fraction of far distant protons attached to single AlO4− in different rings. The locations of the Al(H+) sites were obtained from the analysis of 27Al (3Q) and 29Si MAS NMR and Co(II) ion-exchange and d–d transitions of bare Co(II) ions. The turnover rates for the conversion of propene to C4–C9 olefins were 5–8 times higher and that to BTX aromatics were up to 20 times higher over proximate Bronsted protons compared with that of single far distant sites. The reaction progress monitored by in situ time-resolved FTIR spectroscopy showed the step-wise formation of saturated (Cn+), alkenyl (Cnm=+) and aromatic carbenium ions formed by olefins' protonation and intermolecular hydride ion transfers, respectively, developing faster over the proximate protons. The similar apparent activation energies and enthalpies for propene conversion over the proximate and single protons but significantly less negative apparent entropy found with proximate sites suggested a later transition state closer to products. Both the polarization of reactants and steric constraints for the carbenium ion intermediates bound on the proximate sites, causing less freedom in a transition state and faster deprotonation of carbenium ions, were suggested to contribute to the enhanced reaction rate.

Sugarcane contains various anthocyanins, which are responsible for the colors present in sugarcane. In this study, the color intensification of the major anthocyanin, cyanidin-3-O-glucoside, by phenolic acids/aldehydes (ferulic acid, vanillic acid, p-coumaric acid, syringic aldehyde and vanillic aldehyde) was investigated. The color enhancement of cyanidin-3-O-glucoside (hyperchromic effect and bathochromic shift) was affected by the temperature and concentration of phenolic acids/aldehydes present. Reactions were spontaneous and exothermic, as determined using different thermodynamic parameters (ΔG0, ΔH0, ΔS0). Quantum chemical calculations demonstrated their intermolecular interaction differences, and AIM analysis indicated that hydrogen bonds and van der Waals force interactions contributed to color. Pyranoanthocyanins derived from cyanidin-3-O-glucoside and ferulic/p-coumaric acids during storage were recognized as cyanidin-3-O-glucoside-vinylphenol and cyanidin-3-O-glucoside-vinylguaiacol, respectively, by UPLC-ESI-QTOF-MS/MS. The electron-donating substituents on the aromatic ring of ferulic/p-coumaric acids stabilized the intermediately formed carbenium ion. Decarboxylation and further oxidation of the pyran moieties to the aromatic heterocycles resulted in the final products.

The catalytic hydrocracking reaction pathways, kinetics and mechanisms of 1-methyl naphthalene and phenanthrene were investigated in experiments at 350 °C and 68.1 atm H2 partial pressure (190.6 atm total pressure), using a presulfided NiAV on USY zeolite catalyst. 1-methyl naphthalene hydrocracking led to 2-methyl naphthalene, methyl tetralins, methyl decalins, pentyl benzene and tetralin. Phenanthrene hydrocracking led to dihydro, tetrahydro and octahydro phenanthrene, butyl naphthalene, tetralin to butyl 422tetralin and dibutyl benzene. The rate constants for the dealkylation of butyl tetralins produced in the phenanthrene hydrocracking network conform to a linear free energy relationship (LFER), with the heat of formation of the leaving alkyl carbenium ion as the reactivity index.

Most terpene synthase reactions follow Markovnikov rules for formation of high energy carbenium ion intermediates. However, there are notable exceptions. For example, pentalenene synthase (PS) undergoes an initial anti-Markovnikov cyclization reaction followed by a 1,2-hydride shift to form an intermediate humulyl cation with positive charge on the secondary carbon C9 of the farnesyl diphosphate substrate. The mechanism by which these enzymes stabilize and guide regioselectivity of secondary carbocations has not heretofore been elucidated. In an effort to better understand these reactions, we grew crystals of apo-PS, soaked them with the non-reactive substrate analog 12,13-difluorofarnesyl diphosphate, and solved the x-ray structure of the resulting complex at 2.2 Å resolution. The most striking feature of the active site structure is that C9 is positioned 3.5 Å above the center of the side chain benzene ring of residue F76, perfectly poised for stabilization of the charge through a cation-π interaction. In addition, the main chain carbonyl of I177 and neighboring intramolecular C6,C7-double bond are positioned to stabilize the carbocation by interaction with the face opposite that of F76. Mutagenesis experiments also support a role for residue 76 in cation-π interactions. Most interesting is the F76W mutant which gives a mixture of products that likely result from stabilizing a positive charge on the adjacent secondary carbon C10 in addition to C9 as in the wild-type enzyme. The crystal structure of the F76W mutant clearly shows carbons C9 and C10 centered above the fused benzene and pyrrole rings of the indole side chain, respectively, such that a carbocation at either position could be stabilized in this complex, and two anti-Markovnikov products, pentalenene and humulene, are formed. Finally, we show that there is a rough correlation (although not absolute) of an aromatic side chain (F or Y) at position 76 in related terpene synthases from Streptomyces that catalyze similar anti-Markovnikov addition reactions.

Significance Secondary organic aerosol (SOA) from photochemical oxidation of volatile organic compounds represents one of the most dominant constituents of fine particulate matter in the troposphere, with profound implications for air quality and climate. However, the fundamental chemical mechanisms leading to SOA formation remain highly uncertain. Here, we show oligomer formation from methylglyoxal with the carbenium ions as the key intermediate using quantum chemical calculations. This cationic oligomerization is demonstrated to proceed via barrierless pathways and occurs at fast rates on weakly acidic aqueous aerosols and/or cloud droplets under typical tropospheric conditions. In contrast to a previously proposed hydration mechanism, out results reveal that the carbenium ion-mediated oligomerization of methylglyoxal provides a major SOA source from anthropogenic and biogenic emissions., Secondary organic aerosol (SOA) represents a major constituent of tropospheric fine particulate matter, with profound implications for human health and climate. However, the chemical mechanisms leading to SOA formation remain uncertain, and atmospheric models consistently underpredict the global SOA budget. Small α-dicarbonyls, such as methylglyoxal, are ubiquitous in the atmosphere because of their significant production from photooxidation of aromatic hydrocarbons from traffic and industrial sources as well as from biogenic isoprene. Current experimental and theoretical results on the roles of methylglyoxal in SOA formation are conflicting. Using quantum chemical calculations, we show cationic oligomerization of methylglyoxal in aqueous media. Initial protonation and hydration of methylglyoxal lead to formation of diols/tetrol, and subsequent protonation and dehydration of diols/tetrol yield carbenium ions, which represent the key intermediates for formation and propagation of oligomerization. On the other hand, our results reveal that the previously proposed oligomerization via hydration for methylglyoxal is kinetically and thermodynamically implausible. The carbenium ion-mediated mechanism occurs barrierlessly on weakly acidic aerosols and cloud/fog droplets and likely provides a key pathway for SOA formation from biogenic and anthropogenic emissions.

Red light has the advantages of low energy, less health risk and high penetration depth through various media. Herein, a helical carbenium ion (N,N’-di-n-propyl-1,13-dimethoxyquinacridinium (nPr-DMQA+) tetrafluoroborate) has been used as an organic photoredox catalyst for photoreductions and photooxidations in the presence of red light (λmax = 640 nm). It has catalyzed red-light-mediated dual transition-metal/photoredox-catalyzed C-H arylation and intermolecular atom transfer radical addition through oxidative quenching, affording products in 57-93% yields. Moreover, its potential in photooxidation catalysis has also been demonstrated by successful applications in red-light-induced aerobic oxidative hydroxylation of arylboronic acids and benzylic C(sp3)-H oxygenation through reductive quenching, delivering products in up to 92% yield. Thus, a versatile organic photoredox catalyst (helical carbenium ion) for red-light-mediated photoredox reactions has been developed.

DFT calculations combined with MD simulations have been used to investigate the complicated reaction mechanism of isobutane-isobutylene alkylation catalyzed by the neat chloroaluminate ionic liquid (NIL) and the Cu-containing chloroaluminate ionic liquid (CIL). Transition states of three key elementary reactions were obtained to address the selectivity of the alkylate formation versus the by-product formation in different ionic liquids. The DFT calculations indicate that the Cu species would significantly inhibit the polymerization of C4= olefins and C8+ carbenium ions. Compared to the competitive H-transfer reaction, the reaction rate of polymerization in the NIL was very fast resulting in poor product selectivity. The Mayer bond order (MBO) and electron localization function (ELF) maps reveal that H-transfer from isobutane to C8+ carbenium ion occurs via a concerted H-Cu bond formation between a C8+ and an isobutane to generate the desired isooctane. The Cu complex did engage in the H-transfer of isobutane/C8+, but it did not weaken the catalytic activity in comparison to the neat chloroaluminate anions. The MD simulations show that a combination of high isobutane concentration and low isobutylene concentration at the CIL-hydrocarbon interface can be attributed to the effects of Cu complex, which also leads to the improvement of the alkylation selectivity. An essential role of Cu species in the CIL alkylation is to impede the formation of C12+ carbenium ions and promote the H-transfer between isobutane and C8+ carbenium ion.

In the present work, we studied the protonation reaction of styrene inside the cavity of acidic H-Y zeolite. Density functional theory calculation using M06-2X functional and analysis of quantum theory of atoms in molecules are used to investigate the confinement effects of zeolite framework on species involved on the reaction. A detailed analysis of the topology of the electron density of interactions among reactants, transition state, and intermediate products with the cavity of H-Y zeolite is performed, extracting conclusions about adsorption, catalysis, and confinement effects. Identification and quantification of host–guest interactions between zeolite framework and styryl cation support the larger contribution of weak closed-shell interactions in stabilization of the formed carbenium ion. Our results clearly show that reaction energies for all formed species inside a zeolite with large void structure are also significantly governed by the confinement effects related to weak host–guest interactions. ...

Nonoxidative dehydrogenation of alkanes is an important chemical reaction, as it can be used for production of olefins, commonly used as building blocks for a range of plastics and chemicals. Several metal oxides, including γ-Al2O3, have exhibited dehydrogenation activity, showing promise as candidates for the upgrade of alkanes. In this work we use computational chemistry methods to investigate the mechanism of ethane, propane, n-butane, and i-butane dehydrogenation pathways on strong Lewis acid–base sites of γ-Al2O3. On the basis of our calculations, it was shown that a concerted dehydrogenation mechanism is energetically preferred with the formation of a carbenium-ion-like transition state. An alkane dehydrogenation model was developed on the basis of methodology previously applied in structure–activity relations (SAR) for alcohol dehydration on metal oxides. The carbenium ion stability (CIS), shown to be a descriptor in alcohol dehydration, was used as a quantitative descriptor in alkane dehydrogenati...

The ring-opening and decarboxylation reaction of γ-valerolactone (GVL), yielding a mixture of butene isomers, constitutes an important transformative step for the upgradation of biomass-derived precursors into high value fuels and chemicals. In this study, density functional theory (DFT) simulations were utilized to gain insights into the ring-opening of GVL in presence of Bronsted acid promoted γ-Al2O3 catalyst. DFT simulations suggested towards the formation of stable oxocarbenium ion of GVL on the γ- Al2O3 surface via a proton transfer to the carbonyl oxygen in presence of the Bronsted acid. The surface oxygen atoms of γ-Al2O3 were observed to show high affinity for H-abstraction from the oxocarbenium ion to yield the carbenium ion intermediate, which subsequently gave the product butene via a combination of hydride transfer and decarboxylation steps. Higher reactivity of the Bronsted acid promoted γ-Al2O3 surface, was attributed to the direct ring-opening (Ea = 13 kJ/mol) of the surface adsorbed oxocarbenium ion of GVL, leading to the formation of the γ-carbenium ion, which was unlikely to form with the adsorbed GVL structure. In addition, H-abstraction by the surface oxygen from the C5 of adsorbed oxocarbenium ion (Ea = 13 kJ/mol) was calculated to be of lower activation barrier as compared to the H-abstraction from C5 of the adsorbed GVL (Ea = 40 kJ/mol), which may also lead to facile ring-opening in presence of a Bronsted acid on γ-Al2O3.

Alkylation of phenolics is of great importance in synthetic chemistry and the valorization of lignocellulosic-biomass-derived streams. Here, we unravel how alkylating reactants and solvents significantly alter the reaction pathways of zeolite-catalysed alkylation of phenol in the liquid phase. The carbenium ion formed from the dehydration of cyclohexanol or from the adsorption and protonation of cyclohexene acts as the electrophile, inducing carbon–carbon bond formation. Cyclohexanol at Bronsted acid sites (BAS) forms hydrogen-bonded monomers and protonated dimers in apolar solvents. The dimer appears to generate a much lower concentration of carbenium ions compared with the monomer. Higher alkylation rates in apolar solvents than in water are caused by the energetically more-favourable carbenium ion formation from either alcohol or olefin on non-hydrated zeolite BAS than on hydronium ions produced by BAS in pores filled with water. Zeolite-catalysed alkylations of phenolic compounds offer unique possibilities for the valorization of renewable aromatics into substituted arenes. Now, a mechanistic study reveals that the course of the reaction can be dramatically altered by changing the polarity of the solvent, which affects the nature of surface species and the pathway for the generation of the alkylating electrophile.