Carbenium Ion Intermediates Research Articles

Unveiling the Brønsted acid mechanism for Meerwein–Ponndorf–Verley reduction in methanol conversion over ZSM-5

The conversion of methanol over zeolites offers a sustainable alternative for fuels and chemicals production. However, a complete understanding of the competing reaction pathways, particularly those leading to C-C bond formation, remains elusive. This work presents a novel mechanism for selective methanol conversion in ZSM-5 zeolites, involving a Brønsted acid site (BAS)-mediated Meerwein–Ponndorf–Verley (MPV) reduction pathway. Employing a multidimensional solid-state NMR spectroscopy combined with isotopic labeling and theoretical calculations, we identify this pathway for acetaldehyde reduction with methanol, directly contributing to ethene formation. This mechanism, involving carbenium ion intermediates like 1-hydroxyethane or 1-methoxyethane ions, contrasts with the well-established Lewis acid-catalyzed MPV process. Based on reactant adsorption modes, we propose two distinct reaction routes for BAS-MPV reduction, bridging the gap between direct and hydrocarbon pool mechanisms in methanol conversion. We further demonstrate the applicability of this pathway to acetone, highlighting its broader role in the early stages of the reaction.

Chloride and Hydride Transfer as Keys to Catalytic Upcycling of Polyethylene into Liquid Alkanes

AbstractTransforming polyolefin waste into liquid alkanes through tandem cracking‐alkylation reactions catalyzed by Lewis‐acid chlorides offers an efficient route for single‐step plastic upcycling. Lewis acids in dichloromethane establish a polar environment that stabilizes carbenium ion intermediates and catalyzes hydride transfer, enabling breaking of polyethylene C−C bonds and forming C−C bonds in alkylation. Here, we show that efficient and selective deconstruction of low‐density polyethylene (LDPE) to liquid alkanes is achieved with anhydrous aluminum chloride (AlCl3) and gallium chloride (GaCl3). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline‐range liquid alkanes over 70 %. AlCl3 showed an exceptional conversion rate of 5000 , surpassing other Lewis acid catalysts by two orders of magnitude. Through kinetic and mechanistic studies, we show that the rates of LDPE conversion do not correlate directly with the intrinsic strength of the Lewis acids or steric constraints that may limit the polymer to access the Lewis acid sites. Instead, the rates for the tandem processes of cracking and alkylation are primarily governed by the rates of initiation of carbenium ions and the subsequent intermolecular hydride transfer. Both jointly control the relative rates of cracking and alkylation, thereby determining the overall conversion and selectivity.

Chloride and Hydride Transfer as Keys to Catalytic Upcycling of Polyethylene into Liquid Alkanes.

Transforming polyolefin waste into liquid alkanes through tandem cracking-alkylation reactions catalyzed by Lewis-acid chlorides offers an efficient route for single-step plastic upcycling. Lewis acids in dichloromethane establish a polar environment that stabilizes carbenium ion intermediates and catalyzes hydride transfer, enabling breaking of polyethylene C-C bonds and forming C-C bonds in alkylation. Here, we show that efficient and selective deconstruction of low-density polyethylene (LDPE) to liquid alkanes is achieved with anhydrous aluminum chloride (AlCl3) and gallium chloride (GaCl3). Already at 60 °C, complete LDPE conversion was achieved, while maintaining the selectivity for gasoline-range liquid alkanes over 70 %. AlCl3 showed an exceptional conversion rate of 5000 , surpassing other Lewis acid catalysts by two orders of magnitude. Through kinetic and mechanistic studies, we show that the rates of LDPE conversion do not correlate directly with the intrinsic strength of the Lewis acids or steric constraints that may limit the polymer to access the Lewis acid sites. Instead, the rates for the tandem processes of cracking and alkylation are primarily governed by the rates of initiation of carbenium ions and the subsequent intermolecular hydride transfer. Both jointly control the relative rates of cracking and alkylation, thereby determining the overall conversion and selectivity.

Synthesis of diverse aryl‐substituted amino propanes

AbstractAmines bearing diverse aryl substituents for potential utilities as energy and new materials have been synthesized from aziridine‐2‐carboxaldehyde and diverse aryl‐containing substrates. At first two aryl groups were to yield (aziridine‐2‐yl)‐1,1‐diaryl‐methane which was converted to 5‐diarylmethyl oxazolidin‐2‐one as a synthetic intermediate. They were further reacted with another aryl group in the presence of methanesulfonic acid to yield 2,3,3‐triarylpropylamine with the possible carbenium ion intermediate. In addition, this reaction with a second additional phenolic aryl group gave rise to the diversely substituted 3‐(4‐methoxyphenyl)‐(2,3‐dihydrobenzofuran‐2‐yl)methanamine derived from the intramolecular aziridine ring‐opening reaction by hydroxy group at phenol.

Front Cover: α‐Aziridinyl Carbenium Ion Intermediate and Stereoselective Dehydroxylative Diarylation of Aziridin‐2‐yl Carboxaldehyde (Asian J. Org. Chem. 7/2022)

α-Aziridinyl carbenium ion as an intermediate was observed for the first time from the reaction of aziridin-2-yl arylmethanol with Lewis acid BF3⋅OEt2. This intermediate was further reacted with various nucleophiles to afford the adducts in highly stereoselective manner as single-isomer molecules. α-Aziridinyl carbenium ion intermediate was observed spectroscopically as a crucial intermediate, and density functional theory (DFT) calculations show the origin of stereoselectivity based on thermodynamic preference for the reaction. More information can be found in the Research Article by Hyun-Joon Ha, and Hyunwoo Kim et al.

α‐Aziridinyl Carbenium Ion Intermediate and Stereoselective Dehydroxylative Diarylation of Aziridin‐2‐yl Carboxaldehyde

Abstractα‐Aziridinyl carbenium ion as an intermediate was observed for the first time from the reaction of aziridin‐2‐yl arylmethanol with Lewis acid BF3 ⋅ OEt2. This intermediate was further reacted with various nucleophiles including electron‐rich arenes to afford the adducts in highly stereoselective manner as singly isomers. α‐Aziridinyl carbenium ion intermediate was observed spectroscopically as a crucial intermediate, and density functional theory (DFT) calculations show the origin of stereoselectivity based on thermodynamic preference for the reaction.

Multiscale Modeling as a Tool for the Prediction of Catalytic Performances: The Case of n-Heptane Hydroconversion in a Large-Pore Zeolite

The optimization of predictive kinetic models for catalytic processes is a topical challenge. In the present work, a predictive multiscale single-event microkinetic model based on the data obtained by density functional theory (DFT) calculation for n-heptane hydroconversion in large-pore zeolites has been obtained. It was validated by a large set of kinetic data obtained by high-throughput kinetic experiments, performed with a well-balanced Pt/Beta zeolite catalyst. DFT calculations show that secondary cations are much less stable than tertiary cations and adsorbed alkenes. This is of prime importance in the quantification of type B isomerization reaction barriers depending on the type of the carbenium ion. Cracking reaction barriers are also strongly affected by the nature of the cation that cracks and that of the cracking products. The agreement between simulated and experimental kinetic data is satisfactory, showing the reliability of the multiscale kinetic approach. Only a few parameters were adjusted to improve the correspondence with experiments. The analysis of the simulated coverage demonstrates a very low proportion of acidic sites involved in the adsorption and further reactions in the relevant experimental conditions. When these occur, tertiary carbenium ion intermediates appear in a significantly higher concentration with respect to other species. This works opens the route to a better prediction of the catalytic performance of large-pore zeolites in alkane hydroconversion.

Stereo‐Defined Ferrocenyl Glycol Nucleic Acid (Fc‐GNA) Constituents: Synthesis, Electrochemistry, Mechanism of Formation, and Anticancer Activity Studies

AbstractIn this paper, we report the Yb(OTf)3‐mediated etherification reaction that allowed obtaining R,R and S,R isomers of ferrocenyl glycol nucleic acid (Fc‐GNA) nucleosides from their corresponding stereo‐defined (S,R) precursor. The R,R absolute configuration of the chiral carbon atoms in one of the Fc‐nucleoside isomers was assigned by single‐crystal X‐ray diffraction. Density functional theory calculations showed that the stereochemical outcome of the reaction can be explained by an exo attack of ethylene glycol on the pro‐R‐ or pro‐S‐configured α‐ferrocenyl carbenium ion intermediate. The R,R isomer was transformed into the corresponding nucleotide diethyl ester which is, to the best of our knowledge, the first Fc‐GNA nucleotide ever reported. The obtained compounds feature reversible one‐electron oxidation of the ferrocenyl portion of their molecular structures. Anticancer activity studies showed that the (S,R) nucleoside was the most active against HeLa and Ishikawa cancer cells, while it did not show any activity against nontumorigenic L929 cells. The compound induced apoptosis in HeLa cells at IC50=66.0 μM concentration upon 72 h of treatment.

Mechanism Underlying Anti-Markovnikov Addition in the Reaction of Pentalenene Synthase

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 atom of the farnesyl diphosphate substrate. The mechanism by which these enzymes stabilize and guide the 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 nonreactive substrate analogue 12,13-difluorofarnesyl diphosphate, and determined 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 perfectly positioned to make a C-H···π interaction with the side chain benzene ring of residue F76; this would enhance hyperconjugation to stabilize a developing cation at C10 and thus support the anti-Markovnikov regioselectivity of the cyclization. The benzene ring is also positioned to catalyze the migration of H to C10 and stabilize a C9 carbocation. On the opposite face of C9, further cation stabilization is possible via interactions with the main chain carbonyl of I177 and the neighboring intramolecular C6═C7 bond. Mutagenesis experiments also support a role for residue 76 in these interactions, but most interesting is the F76W mutant, whose crystal structure clearly shows 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.

Silylium-Ion-Promoted Ring-Opening Hydrosilylation and Disilylation of Unactivated Cyclopropanes.

A silylium-ion-promoted ring-opening hydrosilylation of unactivated cyclopropanes is reported. The reaction is facilitated by the γ-silicon effect, and the regioselectivity is influenced by various stabilizing effects on the carbenium-ion intermediates, including the β-silicon effect. The experimental observations are in accord with the computed reaction mechanism. The work also showcases the ability of silylium ions to isomerize cyclopropyl to allyl groups, and the resulting α-olefins engage in a silylium-ion-mediated disilylation with hexamethyldisilane.

Identification of different carbenium ion intermediates in zeolites with identical chabazite topology via 13C-13C through-bond NMR correlations.

13C–13C through-bond NMR correlation experiments reveal the stabilization of different carbenium ion intermediates in two zeolites possessing identical CHA topology (H-SAPO-34 and H-SSZ-13) during the methanol to olefins reaction.

Enhancement of propene oligomerization and aromatization by proximate protons in zeolites; FTIR study of the reaction pathway in ZSM-5

The enhanced effect of strongly acidic proximate protons (distance 5.0–5.5 Å) in ZSM-5 was presented on complex propene oligomerization up to the aromatization and development of individual carbenium ion intermediates in the zeolite pores.

Deactivation Kinetics of Solid Acid Catalyst with Laterally Interacting Protons.

Differences in catalyst deactivation kinetics in solid acid catalysis are studied with catalyst models that allow for lateral interaction between protons. Deactivation of a solid acid catalyst with laterally interacting protons induces inhomogeneity of proton reactivity that develops with time. As a consequence, product selectivity changes and deactivation will accelerate. This is demonstrated by simulations of the deactivation kinetics of the alkylation reaction of propylene with isobutane. The effect of lateral interactions between protons arises because initial catalyst deactivation is not caused by pore blocking or coke deposition but by a molecular mechanism where protons are consumed due to the formation of stable nonreactive carbenium ions. High selectivity to alkylate requires a catalyst with protons of high reactivity. When protons become consumed by formation of stable deactivating carbenium ions, initially reactive protons are converted into protons of lower reactivity. The latter only catalyze deactivating oligomerization reactions. Simulations that compare the deactivation kinetics of a catalyst model with laterally interacting protons and a catalyst model that contains protons of similar but different reactivity, but that do not laterally interact, are compared. These simulations demonstrate that the lateral interaction catalyst model is initially more selective but also has a lower stability. Catalyst deactivation of the alkylation reaction occurs through two reaction channels. One reaction channel is due to oligomerization of reactant propylene. The other deactivation reaction channel is initiated by deprotonation of intermediate carbenium ions that increase alkene concentration. By consecutive reactions, this also leads to deactivation. The hydride transfer reaction competes with oligomerization reactions. It is favored by strongly acid sites that also suppress the deprotonation reaction. Within the laterally interacting proton catalyst model, when reactive protons become deactivated, weakly reactive protons are generated that only catalyze the deactivating alkene oligomerization and consecutive reactions. This rapid formation of the weakly reactive protons is the cause of decreasing selectivity with reaction time and increased rate of deactivation. Solutions of the mean field kinetic equations as well as stochastic simulations are presented. Comparative simulations with a reduced number of neighbors of the protons illustrate decreased deactivation rates when the proton density decreases. Island formation of adsorbed reaction intermediates on the catalyst surface is observed in stochastic kinetics simulations. When alkylation selectivity is high, this island formation increases the rate of catalyst deactivation in comparison to the rate of deactivation according to the mean field studies. A nonlinear dynamics model of proton dynamics is provided, which shows that the differences between stochastic and mean field simulations are due to frustrated proton state percolation.

Synthesis of novel 4-ferrocenyl-1,2,3,4-tetrahydroquinolines and 4-ferrocenylquinolines via α-ferrocenyl carbenium ions as key intermediates

A series of novel 4-ferrocenyl-1,2,3,4-tetrahydroquinolines were synthesized in high-to-excellent yields (up to 99%) starting from the corresponding ferrocenoylethyl aryl amines. These Mannich bases were reduced (NaBH4) to the corresponding 3-(arylamino)-1-ferrocenylpropan-1-ols and submitted to an intramolecular cyclization prompted by acetic acid, proceeding via the corresponding α-ferrocenyl carbenium ion intermediates. Subsequently, the obtained tetrahydroquinolines were smoothly oxidized (aromatized) by means of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to provide the corresponding 4-ferrocenylquinolines (up to 93%).

Direct structural identification of carbenium ions and investigation of host–guest interaction in the methanol to olefins reaction obtained by multinuclear NMR correlations

Probing and determining the intermediates formed during catalytic reactions in heterogeneous catalysis are strong challenges. Using 13C labelling and two dimensional 13C-13C through-bond NMR correlations, we directly reveal the structures of a range of carbenium ion species formed during the conversion of methanol to olefins on acidic H-ZSM-5 zeolite by mapping the carbon-carbon bond connectivities. Additionally, we use 13C-27Al and 29Si-13C through-space NMR experiments to probe the interactions between the confined carbon species (including carbenium ions) and the framework of the zeolite, which quantitatively provide an estimate for the carbon-aluminium and carbon-silicon distances, respectively.

Effect of temperature and branching on the nature and stability of alkene cracking intermediates in H-ZSM-5

Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4 C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773K.

Origin of Zeolite Confinement Revisited by Energy Decomposition Analysis

Our previous work demonstrated that hydrocarbon species can be stabilized in the confined zeolite in the form of an ion pair, π complex, and alkoxy species. Nevertheless, the interaction mechanism between the different reactants/intermediates and the zeolite framework remains undetermined, and thus, the origin of the zeolite confinement effect has not been thoroughly revealed. In this work, a recently developed energy decomposition analysis (EDA) method was applied to theoretically investigate the energy parameters of a series of hydrocarbon species confined in the zeolitic catalysts with different pore diameters. It is demonstrated that for the carbenium ion intermediates, the electrostatic interaction plays a key role in their stabilization; for the alkoxy species, both orbital and electrostatic interactions are the key factors, while for the neutral hydrocarbons, the dispersion interaction favors their stabilization. In addition, the principal components analysis (PCA) reveals that the dispersion interaction does not play a crucial role in improving the reaction activity due to the same extent of stabilization effect for different reaction species (e.g., reactant, transition state, intermediate, or product), and thus, the dispersion contribution would be counteracted in a specific zeolite catalytic reaction. In contrast, the difference in electrostatic interaction caused by the variations of charge characteristics of the various confined species considerably contributes to the decrease of the activation barrier and the increase of the reaction energy, which in turn largely promotes the catalytic performance of zeolite catalysts.

Olefin Upgrading over Ir/ZSM-5 catalysts under methane environment

Upgrading olefin in the synthetic oil to alkane is highly desired due to its high volatility and thermal unstability as well as low energy density. Unlike conventional hydrotreating, methane (CH4) was used in this study as the novel hydrogen donor for olefin saturation. The significant increase of H/C atomic ratio of product oil from 1.69±0.02 (over pure ZSM-5) to 2.04±0.02 (over Ir/ZSM-5 (10.0)) and the alkane content up to 83.8±2.1% in the upgraded oil indicated that methane could exhibit comparable catalytic performance to what hydrogen (H2) did for olefin (1-Decene) upgrading over the developed bifunctional catalysts with low Ir loadings. The HRTEM and XPS data revealed that the highly dispersed metallic Ir particles with average size of 1.32nm was coexisting with IrO2 species. The synergic effects of Ir/IrO2 obviously promoted the activation of methane, which supplied sufficient hydrogen for the saturation and stabilization of olefin. The results from BET indicated that the pore size and volume of the ZSM-5 support were increased upon Ir introduction, which provided more active sites for cracking olefin (1-decene). NH3-TPD results suggested that the presence of highly dispersed Ir increased the amount of surface acidity, which enhanced the formation and stabilization of carbenium ion intermediates. As a result, the produced alkanes were mainly composed of cyclopentane-derived compounds, like propylcyclopentane, 3-methylbutyl-cyclopentane and 1,2,4-trimethyl-cyclopentane, which has great application potential as immersion fluid and additive in the field of optics and petroleum.

Refinery oil upgrading under methane environment over PdOx/H-ZSM-5: Highly selective olefin cyclization

Olefin content minimization is critical for the long-term storage and long-distance transportation of oil product, which is commonly achieved through hydrogen saturation at high pressure to form paraffin. In this study, a novel catalytic process is reported to convert olefin to cycloalkane under methane environment with high selectivity. The main components of the product oil collected over Pd/ZSM-5 catalysts during olefin model compound (i.e., 1-decene) upgrading are cyclopentane, 1-methylbutyl- and cyclopentane, hexyl- while there are more than 46 species identified in the liquid product from the run with H-ZSM-5 support engaged. The total selectivity of the aforementioned cycloalkane products is up to 91.3% when 0.27wt% Pd/ZSM-5 catalyst is charged, thus making the product oil more profitable. The HRTEM and XPS data reveal that the highly dispersed palladium particles are in the form of PdOx. The results from pyridine FT-IR and NH3-TPD indicate that these PdOx species anchored onto the surface acidic sites exhibit the moderate acidic feature of Lewis acidic sites, which enhances the formation and stabilization of carbenium ion intermediates generated from 1-decene cracking. The strong interaction between PdOx species and surface acidic sites could not only stabilize the particle size and modify the electronic structure of Pd species but also modulate the acidic properties of ZSM-5 support, leading to promoted methane activation which would not only release sufficient hydrogen donors for preventing olefin aromatization, but also inhibit the over-cracking of carbenium ions. As a result, the selectivity of catalyst assisted olefin cyclization under methane environment is greatly enhanced.

B(C6F5)3 -catalyzed transfer of dihydrogen from one unsaturated hydrocarbon to another.

A transition-metal-free transfer hydrogenation of 1,1-disubstituted alkenes with cyclohexa-1,4-dienes as the formal source of dihydrogen is reported. The process is initiated by B(C6 F5 )3 -mediated hydride abstraction from the dihydrogen surrogate, forming a Brønsted acidic Wheland complex and [HB(C6 F5 )3 ](-) . A sequence of proton and hydride transfers onto the alkene substrate then yields the alkane. Although several carbenium ion intermediates are involved, competing reaction channels, such as dihydrogen release and cationic dimerization of reactants, are largely suppressed by the use of a cyclohexa-1,4-diene with methyl groups at the C1 and C5 as well as at the C3 position, the site of hydride abstraction. The alkene concentration is another crucial factor. The various reaction pathways were computationally analyzed, leading to a mechanistic picture that is in full agreement with the experimental observations.