Lewis Acid Catalyst Research Articles

Highly enantioselective hydroxymethylation of unmodified α-substituted aryl ketones in water.

Catalytic asymmetric direct-type aldol reactions of ketones with aldehydes are a perennial puzzle for organic chemists. Notwithstanding the emergence of a myriad of chiral catalysts to address the inherent reversibility of the aldol products, a general method to access acyclic α-chiral ketones from prochiral aryl ketones has remained an unmet synthetic challenge. The approach outlined herein is fundamentally different to that used in conventional catalysis, which typically commences with an α-proton abstraction by a Brønsted base. The use of a chiral 2,2'-bipyridine scandium complex enabled the hydroxymethylation of propiophenone to be run under base-free conditions, which avails effectual suppression of hydrolytic deactivation of the Lewis acid catalyst. Intriguingly, the use of water as a reaction medium had an overriding effect on the progress of the reaction. The sagacious selection of sodium dodecyl sulfate and lithium dodecyl sulfate as surfactants allowed a variety of propiophenone derivatives to react in a highly enantioselective manner.

Nano-γ-Al2O3/Sb(V) as an effective catalyst for the synthesis of highly substituted tetrahydropyridines under ultrasonic or thermal conditions

In this research work, nano-γ-Al2O3/Sb(V) was synthesized and characterized as a heterogeneous Lewis acid catalyst. Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), Brunauer–Emmett–Teller plot (BET), energy dispersive X-ray spectroscopy (EDS), and thermo gravimetric analysis (TG-DTG) were used to the characterization of nano-γ-Al2O3/Sb(V). Tetrahydropyridine is known as the core moiety in some natural products and drugs. Polysubstituted tetrahydropyridine-3-carboxylates are synthesized through a one-pot multicomponent reaction (MCR) of ethyl acetoacetate, aldehyde, and aniline derivative. This present study introduces the synthesis of these structures in the presence of nano-γ-Al2O3/Sb(V) under ultrasonic or thermal conditions. Compared with previously reported methods, our approach is convenient and offers several advantages such as milder reaction conditions, shorter reaction time, excellent yield, and the use of reusable catalyst.

Reusable SiO2@NiO core-shell nanoparticles catalyzed efficient synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8, 13-dione derivatives

Multicomponent condensation reaction of substituted aromatic aldehydes with 2-naphthol/2,7-dihydroxynaphthalene and 2-hydroxynaphthalene-1,4-dione was catalyzed by nickel oxide supported on silica core-shell nanoparticles (SiO2@NiO) as Lewis acid catalyst in the ethanolic medium at 80 °C. The present catalytic protocol was used to synthesize diverse 14-Aryl-14H-dibenzo[a,i]xanthene-8,13-diones in good yields with a high turnover number within a short reaction time leading to good reusability with almost unchanged catalytic activity. The synthesized compounds were screened for in vitro antimicrobial activity and found to exhibit promising antibacterial and antifungal activities.

A Polystyrene supported Scandium (III) microencapsulated Lewis acid catalyst for hydrothermal carbonization of glucose

Hydrothermal carbonization of glucose in the absence and presence of recoverable solid polystyrene-supported microencapsulated Lewis acid catalyst has been carried out at 180 and 200 °C for 6, 12, and 24 h using either 0.25 g or 0.5 g of catalyst. The effects of temperature, residence time, and catalyst loading on the resulting hydrochars were investigated. At the lowest temperature (180 °C) and the shortest residence time (6 h) without the presence of a catalyst, the glucose conversion was the lowest as expected. No hydrochar was formed at 180 °C for 6 h without using any catalyst. In catalytic runs, the formation of hydrochar was observed under identical conditions although the yields of hydrochar were low. The use of the catalyst increased the yield of acid compounds (acetic, glycolic, lactic and levulinic) at the expense of 5-hydroxymethylfurfural under identical conditions. Except for 180 °C for 6 and 12 h, the use of the catalyst decreased the yield of hydrochar. The use of the catalyst led to increasing the diameter of carbon sphere particles under identical conditions. The presence of the catalyst resulted in the production of a notable amount, approximately 20 wt% of levulinic acid as a by-product.

Computational Study of the Solid-State Incorporation of Sn(II) Acetate into Zeolite β.

Sn-doped zeolites are potent Lewis acid catalysts for important reactions in the context of green and sustainable chemistry; however, their synthesis can have long reaction times and harsh chemical requirements, presenting an obstacle to scale-up and industrial application. To incorporate Sn into the β zeolite framework, solid-state incorporation (SSI) has recently been demonstrated as a fast and solvent-free synthetic method, with no impairment to the high activity and selectivity associated with Sn-β for its catalytic applications. Here, we report an ab initio computational study that combines periodic density functional theory with high-level embedded-cluster quantum/molecular mechanical (QM/MM) to elucidate the mechanistic steps in the synthetic process. Initially, once the Sn(II) acetate precursor coordinates to the β framework, acetic acid forms via a facile hydrogen transfer from the β framework onto the monodentate acetate ligand, with low kinetic barriers for subsequent dissociation of the ligand from the framework-bound Sn. Ketonization of the dissociated acetic acid can occur over the Lewis acidic Sn(II) site to produce CO2 and acetone with a low kinetic barrier (1.03 eV) compared to a gas-phase process (3.84 eV), helping to explain product distributions in good accordance with experimental analysis. Furthermore, we consider the oxidation of the Sn(II) species to form the Sn(IV) active site in the material by O2- and H2O-mediated mechanisms. The kinetic barrier for oxidation via H2 release is 3.26 eV, while the H2O-mediated dehydrogenation process has a minimum barrier of 1.38 eV, which indicates the possible role of residual H2O in the experimental observations of SSI synthesis. However, we find that dehydrogenation is facilitated more significantly by the presence of dioxygen (O2), introduced in the compressed air gas feed, via a two-step process oxidation process that forms H2O2 as an intermediate and has greatly reduced kinetic barriers of 0.25 and 0.26 eV. The results provide insight into how Sn insertion into β occurs during SSI and demonstrate the possible mechanism of top-down synthetic procedures for metal insertion into zeolites.

Enantioconvergent 6π Electrocyclization Enabled by Photoredox Racemization.

This study presents a novel photoredox-enabled enantioconvergent catalytic strategy used to construct chiral 2H-1,3-benzoxazines via an unprecedented oxa-6π electrocyclization utilizing racemic α-substituted glycinates as substrates. The approach leverages a cobalt-based chiral Lewis acid catalyst, which promotes the transformation under thermal or photoredox conditions. While the thermal reaction selectively converts only the (S)-configured glycinates into enantioenriched 2H-1,3-benzoxazines (up to 96:4 e.r.), the addition of 0.5 mol % of a commercially available iridium photocatalyst under visible light irradiation transforms the reaction into an enantioconvergent process. Detailed mechanistic and time course studies of optically pure α-deuterated substrates revealed the presence of an enantiospecific kinetic isotope effect, which helped to clarify the role of both the photo- and chiral Lewis acid catalyst in the reaction sequence. In this dual catalytic system, the photocatalyst promotes a dynamic interconversion between the substrate enantiomers─a process not accessible via ground-state chemistry─while the chiral Lewis acid selectively transforms only the (S)-configured substrates. Further mechanistic evidence for the proposed mechanism is provided by linear free energy relationship analysis, which suggests that the stereodetermining step involves a 6π electrocyclization under both thermal and photoredox conditions.

Acetate/Alkoxide/Halide Shuttle Systems Mediated by Lewis Acid Catalysts for Insertion Reaction of a One-Carbon Unit into Carbon–Carbon or Carbon–Halogen Bonds

AbstractIn this account, we describe our research on a Lewis acid-catalyzed insertion reaction of α-diazo esters into a carbon–carbon or carbon–halogen bond. Indium catalysts mediated not only the insertion of α-diazo esters into a carbon–carbon bond of alkyl acetates, alkyl ethers, acetals, and alkyl halides, but also a carbon–halogen bond of alkyl chlorides, bromides, and iodides. BF3 specifically accelerated the insertion of α-diazo esters into a carbon–fluorine bond. The key to this catalysis is acetate, alkoxide, and halide shuttle systems mediated by a Lewis acid, in which the Lewis acid abstracts a leaving group from a starting substrate and releases the leaving group to the appropriate carbocation intermediate in the catalytic cycle.1 Introduction2 Acetate/Alkoxide Shuttle: Insertion Reaction of α-Diazo Esters into a Carbon–Carbon Bond of Alkyl Acetates, Alkyl Ethers, and Acetals3 Halide Shuttle: Insertion Reaction of α-Diazo Esters into a Carbon–Carbon Bond of Alkyl Halides4 Halide Shuttle: Insertion of α-Diazo Esters into a Carbon–Halogen Bond of Alkyl Halides5 Conclusion

A new class of anticancer activity with computational studies for a novel bioactive aminophosphonates based on pyrazole moiety

The present study involves synthesis a new series of α-aminophosphonates 2a-f and 4a-d derivatives in good yield with a simple workup via Kabachnik-Fields reaction in the presence of lithium perchlorate as Lewis acid catalyst. All the newly synthesized compounds were confirmed using various physical, spectroscopic, and analytical data. The in vitro anticancer activities of each compound were evaluated against colorectal carcinoma Colon cancer (HCT-116) and Epdermoid carcinoma (HEP2) and also Human lung fibroblast normal cell line (WI38) compared with Doxorubicin. The results showed that Compounds 2a, 4b and 4d exhibited more potent inhibitory activity for Epdermoid Carcinoma (HEP2) compared with doxorubicin. For colon carcinoma cells (HCT-116) Compounds 2a, 2d and 4b gave the strongest activity among all compounds compared with doxorubicin. Moreover, all designed structures were docked into the active site of VEGFR2 and FGFR1 proteins. The result reveals that compound 2b and have the strongest inhibitory activity of the VEGFR2 and FGFR1 proteins indicating that these substances might conceivably operate as VEGFR2 and FGFR1 inhibitors and hence might take role in anticancer activities with various binding interactions. The 3D-QSAR models produced strong statistical results since they were defined by PLS factors 4 and confirmed by parameters as R2, R2 CV, Stability, F-value, P-value, RMSE, Q2, and Pearson-r.

Photoinduced Superacceleration of Metal-Free Living Cationic Polymerization Using Diaryliodonium Salts as Organic Lewis Acid Catalysts

Photoinduced Superacceleration of Metal-Free Living Cationic Polymerization Using Diaryliodonium Salts as Organic Lewis Acid Catalysts

Chiral Lewis Acid-Catalyzed Asymmetric Multicomponent Michael Reaction through [1,2]-Phospha-Brook Rearrangement.

The multicomponent catalytic asymmetric Pudovik addition/[1,2]-phospha-Brook rearrangement/Michael reaction sequence of isatins, phosphites, and 4-oxobutenoates was realized. A series of oxindole derivatives containing two contiguous stereocenters was obtained in high yields and excellent stereoselectivities (up to >99% yield, >95:5 dr, >99% ee) using a chiral Lewis acid catalyst. A possible catalytic model is presented to illustrate the stereocontrol.

A robust and efficiently reprocessable poly(dimethylsiloxane) elastomers enhanced by self‐catalysis Zn(II)‐ligand bonds and silyl ethers

AbstractIn recent years, as an emerging dynamic bonding, silyl ether linkages have been concerned and applied. However, the reprocessability of vitrimers based on it is far from satisfactory because of the inactive dynamic exchange reaction. At the same time, the mechanical properties of polysiloxane elastomers are poor because of the high flexibility of polysiloxane molecular chains and weak intermolecular forces. Herein, we have successfully synthesized a PDMS elastomer incorporating Zn(II)‐amine coordination bonds as sacrificial units, which are crosslinked through dynamic silyl ether linkages. Importantly, the presence of Zn ions promotes the exchange between the silyl ether linkages and hydroxyl groups. The elastomers exhibited excellent mechanical properties (35 times improvement in toughness) and outstanding reprocessability. The mechanical property recovery of the PDMS elastomers reached approximately 90% after four reprocessing cycles. Meanwhile, small‐molecule simulation experiments were conducted to verify the significant catalytic effect of Zn (II) ions as Lewis acid catalysts on the exchange reaction of silyl ether linkages and hydroxyl groups. In a word, this work provides a facile strategy to simultaneously enhance the mechanical properties and reprocessing performance of silyl ether‐linked polysiloxane elastomers.

Three‐Component Aza‐Diels‐Alder Reaction for the Syntheses of Tetrahydroisoquinoline Derivatives

AbstractReported herein is the development of a normal electron‐demand, three‐component aza‐Diels‐Alder reaction between sultines, aldehydes, and amines. In the presence of Cu(OTf)2 as a Lewis acid catalyst, the use of sultines as o‐quinodimethane precursors in conjunction with the in situ generation of imines enables the facile preparation of various polysubstituted tetrahydro‐isoquinoline derivatives from readily available substrates. The synthetic utility of these products was further explored through a number of product transformations, including the syntheses of tricyclic N‐heterocycles.

Cationic β‐Scission of C−H and C−C Bonds for Selective Dimerization and Subsequent Sulfur‐Free RAFT Polymerization of α‐Methylstyrene and Isobutylene

AbstractA series of exo‐olefin compounds ((CH3)2C(PhY)−CH2C(=CH2)PhY) were prepared by selective cationic dimerization of α‐methylstyrene (αMS) derivatives (CH2=C(CH3)PhY) with p‐toluenesulfonic acid (TsOH) via β‐C−H scission. They were subsequently used as reversible chain transfer agents for sulfur‐free cationic RAFT polymerization of αMS via β‐C−C scission in the presence of Lewis acid catalysts such as SnCl4. In particular, exo‐olefin compounds with electron‐donating substituents, such as a 4‐MeO group (Y) on the aromatic ring, worked as efficient cationic RAFT agents for αMS to produce poly(αMS) with controlled molecular weights and exo‐olefin terminals. Other exo‐olefin compounds (R−CH2C(=CH2)(4‐MeOPh)) with various R groups were prepared by different methods to examine the effects of R groups on the cationic RAFT polymerization. A sulfur‐free cationic RAFT polymerization also proceeded for isobutylene (IB) with the exo‐olefin αMS dimer ((CH3)2C(Ph)−CH2C(=CH2)Ph). Furthermore, telechelic poly(IB) with exo‐olefins at both terminals was obtained with a bifunctional RAFT agent containing two exo‐olefins. Finally, block copolymers of αMS and methyl methacrylate (MMA) were prepared via mechanistic transformation from cationic to radical RAFT polymerization using exo‐olefin terminals containing 4‐MeOPh groups as common sulfur‐free RAFT groups for both cationic and radical polymerizations.

Cationic β-Scission of C-H and C--C Bonds for Selective Dimerization and Subsequent Sulfur-Free RAFT Polymerization of α-Methylstyrene and Isobutylene.

A series of exo-olefin compounds ((CH3)2C(PhY)-CH2C(=CH2)PhY) were prepared by selective cationic dimerization of α-methylstyrene (αMS) derivatives (CH2=C(CH3)PhY) with p-toluenesulfonic acid (TsOH) via β-C-H scision. They were subsequently used as reversible chain transfer agents for sulfur-free cationic RAFT polymerization of αMS via β-C-C scission in the presence of Lewis acid catalysts such as SnCl4. In particular, exo-olefin compounds with electron-donating substituents, such as a 4-MeO group (Y) on the aromatic ring, worked as efficient cationic RAFT agents for αMS to produce poly(αMS) with controlled molecular weights and exo-olefin terminals. Other exo-olefin compounds (R-CH2C(=CH2)(4-MeOPh)) with various R groups were prepared by different methods to examine the effects of R groups on the cationic RAFT polymerization. A sulfur-free cationic RAFT polymerization also proceeded for isobutylene (IB) with the exo-olefin αMS dimer ((CH3)2C(Ph)-CH2C(=CH2)Ph). Furthermore, telechelic poly(IB) with exo-olefins at both terminals was obtained with a bifunctional RAFT agent containing two exo-olefins. Finally, block copolymers of αMS and MMA were prepared via mechanistic transformation from cationic to radical RAFT polymerization using exo-olefin terminals containing 4-MeOPh groups as common sulfur-free RAFT groups for both cationic and radical polymerizations.

N-Heterocyclic Nitrenium-Catalyzed Photohomolysis of CF3SO2Cl for Alkene Trifluoromethylation.

N-Heterocyclic nitreniums (NHNs) have been utilized as Lewis acid catalysts to activate substrates with lone pairs. Alternative to their conventional applications, we have discovered that NHNs can also serve as charge transfer complex catalysts. Herein, we present another potential of NHNs by utilizing a weak interaction between NHNs and CF3SO2Cl. The method promotes CF3SO2Cl to undergo photohomolysis, resulting in the CF3 radical. Mechanistic studies suggested that the weak interaction could be due to the π-hole effect of NHNs.

Approaching High-Performance TS-1 Zeolites in the Presence of Alkali Metal Ions via Combination of Adjusting pH Value and Modulating Crystal Size.

Lewis acid zeolites play an important role in industrially important green reactions closely related to fine chemical and biomass conversion. Titanium-doped TS-1 zeolite is a milestone Lewis acid zeolite widely used in industrially significant green oxidation processes with hydrogen peroxide as an oxidant under mild conditions. TS-1 zeolites are normally synthesized in basic conditions under hydrothermal treatment. Up to now, there has still been no success in synthesizing active TS-1 Lewis acid zeolites by using inorganic alkali, e.g., NaOH or KOH as base, which is cheaper and more stable compared to the quaternary ammonium hydroxide or organic amines used in traditional synthesis. Here, an inorganic base of NaOH was employed in synthesizing active TS-1 zeolites for the first time. The crucial factor was the control of adverse effects of sodium cations on the incorporation of active titanium cations. Higher catalytic activity was achieved by further reducing the size of the TS-1 crystal by using the seed-added strategy, which uses the catalytic activity of a commercial catalyst, the production cost being much lower than commercial TS-1 catalysts, indicating great commercial potential and the possibility of preparing other cheap Lewis acid catalysts by using inorganic alkali.

The use of lanthanide triflates in the preparation of poly(thiourethane) covalent adaptable networks

Covalent adaptable networks (CANs) are new polymeric materials with the mechanical properties of thermosets and the possibility of being recycled like thermoplastics. Poly(thiourethane) networks have demonstrated vitrimeric-like behavior at high temperatures due to the trans-thiocarbamoylation process, which Lewis acids and bases can accelerate. In this study, we report the use of lanthanide triflates (La, Sm, Dy, Er, and Yb) as Lewis acid catalysts, a greener alternative to other metallic catalysts as dibutyltin dilaurate (DBTDL) widely used in poly(urethane) and poly(thiourethane) networks. Moreover, they are not as reactive as DBTDL, and the curing mixture can be manipulated for a longer time at room temperature. As monomers, trimethylolpropane tris(3-mercapto propionate) (S3), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI) have been used. We have demonstrated that the materials prepared with lanthanum triflate present the lowest relaxation times than those prepared with other lanthanide triflates or DBTDL. Calorimetry (DSC) and infrared spectroscopy (FTIR) were applied to study the curing process. The materials obtained were fully characterized by thermogravimetric analysis (TGA) and thermomechanical tests (DMA).

Theoretical insight into dimethyl carbonate carboxymethylation of alcohols assisted by Lewis acid proton carrier catalyst FeCl3

To extend the understanding of role of Lewis acid catalysts in dimethyl carbonate (DMC) chemistry, density functional theory (DFT) calculation is conducted to demonstrate the mechanism of DMC mediated carboxymethylation of ethanol catalysed by FeCl3. In parallel with the investigation based on transition state theory (TST), the nature of bonding of all studied species and bonding evolution along the optimal reaction pathways is explored via the topological analysis of electron localization function (ELF) and atoms in molecules (AIM) theory. Results indicate that hydrogen bond among alcohols promotes the molecules to form cyclic transition state, which facilitates reaction in the absence of catalyst. Additionally, FeCl3, acts as a Lewis acid proton carrier catalyst, is proved to reduce the reaction activation energy significantly. Finally, the calculated turnover frequency (TOF) of FeCl3 pathway relative to catalyst-free is 4.06 × 105, providing evidence that FeCl3 is an efficient catalyst for DMC chemistry.

Nucleophilic Allylation of Acylsilanes in Water: An Effective Alternative to Functionalized Tertiary α-Silylalcohols.

A nucleophilic allylation of acylsilanes in water was developed, generating versatile functionalized tertiary α-silyl alcohols in high yields. With the assistance of hydrogen bonding, a reaction model of less reactive acylsilane was achieved. Unlike the conventional strategy, transition metals and an additional Lewis acid catalyst were not required, and rate acceleration was observed in water.

Novel synthesis of aluminum hydroxyfluorides AlFx(OH)3-x and application in E-1-chloro-3,3,3-trifluoropropene isomerization

A facile, safe, and environmentally friendly preparation method is described for the synthesis of AlFx(OH)3-x by the activation of γ-Al2O3 using HCFO-1233zd(E) under various temperatures. Comprehensive characterization techniques including XRD, XPS, N2 sorption, SEM, TEM, 27Al MAS NMR, FTIR, and py-IR were employed, revealing that at lower temperatures such as 275 °C and 300 °C, the synthesis yielded AlF(OH)2, while at higher temperatures like 350 °C and 375 °C, pyrochlore AlF1.5(OH)1.5 and metastable hexagonal tungsten bronze-type (HTB) β-AlF3 were formed, with α-AlF3 detected at 325 °C. Notably, a significant quantity of AlF(OH)2 can be obtained at 325 °C within a specific time frame. This amorphous phase exhibits weak Lewis acid sites and a lower acid density, rendering it significantly more effective in the isomerization of HCFO-1233zd(E) compared to strong Lewis acid catalysts such as AlF1.5(OH)1.5 and β-AlF3, which could promote coke formation, leading to catalyst deactivation. Moreover, due to its limited further fluorination at 290 °C, the optimal catalyst exhibits prolonged lifespan and high efficiency in HCFO-1233zd(E) isomerization.