Synthesis and Catalytic Applications of Advanced Sn- and Zr-Zeolites Materials

The incorporation of isolated Sn (IV) and Zr (IV) ions into silica frameworks is attracting widespread attention, which exhibits remarkable catalytic performance (conversion, selectivity, and stability) in a broad range of reactions, especially in the field of biomass catalytic conversion. As a representative example, the conversion route of carbohydrates into valuable platform and commodity chemicals such as lactic acid and alkyl lactates, has already been established. The zeotype materials also possess water-tolerant ability and are capable to be served as promising heterogeneous catalysts for aqueous reactions. Therefore, dozens of Sn- and Zr-containing silica materials with various channel systems have been prepared successfully in the past decades, containing 8 membered rings (MR) small pore CHA zeolite, 10-MR medium pore zeolites (FER, MCM-56, MEL, MFI, MWW), 12-MR large pore zeolites (Beta, BEC, FAU, MOR, MSE, MTW), and 14-MR extra-large pore UTL zeolite. This review about Sn- and Zr-containing metallosilicate materials focuses on their synthesis strategy, catalytic applications for diverse reactions, and the effect of zeolite characteristics on their catalytic performances.

Construction of Octahydro-4H-cyclopenta[b]pyridin-6-one Skeletons using Pot, Atom, and Step Economy (PASE) Synthesis

Cascade aza-Piancatelli reaction and [3+3]/[4+2] cycloaddition reactions are carried out using the ideality principles of pot, atom, and step economy (PASE) synthesis. The reaction resulted in generation of octahydro-4H-cyclopenta[b]pyridin-6-one scaffolds. Moreover, octahydro-5,7a-epoxycyclopenta[cd]isoindol-4-one frameworks of gracilamine alkaloid and a novel decahydro-1H-dicyclopenta[cd,hi]isoindol-6-one were also realized in good yields with excellent regio- and diastereo-selectivities.

RNA Hydrolysis at Mineral-Water Interfaces

As an essential biomolecule for life, RNA is ubiquitous across environmental systems where it plays a central role in biogeochemical processes and emerging technologies. The persistence of RNA in soils and sediments is thought to be limited by enzymatic or microbial degradation, which occurs on timescales that are orders of magnitude faster than known abiotic pathways. Herein, we unveil a previously unreported abiotic pathway by which RNA rapidly hydrolyzes on the timescale of hours upon adsorption to iron (oxyhydr)oxide minerals such as goethite (α-FeOOH). The hydrolysis products were consistent with iron present in the minerals acting as a Lewis acid to accelerate sequence-independent hydrolysis of phosphodiester bonds comprising the RNA backbone. In contrast to acid- or base-catalyzed RNA hydrolysis in solution, mineral-catalyzed hydrolysis was fastest at circumneutral pH, which allowed for both sufficient RNA adsorption and hydroxide concentration. In addition to goethite, we observed that RNA hydrolysis was also catalyzed by hematite (α-Fe₂O₃) but not by aluminum-containing minerals (e.g., montmorillonite). Given the extensive adsorption of nucleic acids to environmental surfaces, we anticipate previously overlooked mineral-catalyzed hydrolysis of RNA may be prevalent particularly in iron-rich soils and sediments, which must be considered across biogeochemical applications of nucleic acid analysis in environmental systems.

Domino synthesis of 5-aminoimidazoles from Strecker multicomponent adducts via ytterbium-promoted isocyanide insertion/5-exo-dig cyclization

A new Lewis acid promoted domino isocyanide insertion/5-exo-dig cyclization of readily available Strecker 3-component adducts to 4-substituted 5-aminoimidazole derivatives is herein reported. Despite their potential as relevant heterocyclic scaffolds in medicinal chemistry programs, this class of compounds is still underrepresented, with current synthetic strategies poorly efficient in terms of timing and yields. To this end, we show how the exploitation of unconventional reactivities of isocyanides, promoted by ytterbium-triflate, could represent a key resource to enable a fast and easy access to such an unexplored area of the chemical space.

Lewis Acid-Catalyzed Formal (4+2)-Cycloaddition between Cross-Conjugated Azatrienes and Styrylmalonates: The Way to Functionalized Quinolizidine Precursors

Quinolizidine and azaphenalene alkaloids are common in nature and exhibit a pharmaceutical activity, which stirs up increased interest in expanding the range of methods for the synthesis of the corresponding derivatives. In this work, we attempted to adapt our previously presented method for the synthesis of tetrahydropyridines to the preparation of potential precursors for these heterocycles as a separate development of a necessary intermediate stage. To this end, we studied the reactions of β-styrylmalonates with N-protected cross-conjugated azatrienes in the presence of Sn(OTf)₂. Moreover, the regioselectivity of the process involving unsymmetrically substituted azatrienes was estimated. The diene character of vinyltetrahydropyridines was studied in detail with the participation of PTAD. Finally, for the Ts-protected highly functionalized vinyltetrahydropyridines synthesized, a detosylation method to give new desired azadiene structures as precursors of the quinolizidine core was suggested.

Synthesis of endo-fused 5-unsubstituted Hexahydro-2H-pyrano[3,2-c]quinolinesvia Sequential Sc(OTf)3-catalyzed Cationic Imino-Diels-Alder Reaction/N-debenzylation using N-benzylanilines, 3,4-dihydro-2H-pyran and Paraformaldehyde under MW Irradiation

BACKGROUND

Hexahydro-2H-pyrano[3,2-c]quinolines are known to have antibacterial, antifungal, and antitumor properties. Great efforts have been made to develop new synthetic methods that lead to the synthesis of valuable libraries. Extensive methodologies, low yields, excessive amounts of catalyst and expensive reactants are some of the limitations of current methodologies.

AIMS AND OBJECTIVE

Developing a useful and efficient method to construct diversely substituted hexahydro-2Hpyrano[ 3,2-c]quinolines into good to excellent yields through a cationic imino-Diels-Alder/N-debenzylation methodology.

METHOD

The cationic imino-Diels-Alder/N-debenzylation methodology was used for the preparation of substituted hexahydro-2H-pyrano[3,2-c]quinolines. It involves the use of Sc(OTf)₃ for activation of cationic imino- Diels-Alder cycloaddition reaction of N-benzylanilines, 3,4-dihydro-2H-pyran and paraformaldehyde in MeCN; and microwave irradiation to shorten reaction time to afford new 6-benzyl-hexahydro-2H-pyrano[3,2- c]quinolines whose catalytic transfer debenzylation reactions with HCO₂NH₄ in the presence of Pd/C (10%) and methanol give the new 5-unsubstituted pyrano[3,2-c]quinolines in excellent yields.

RESULTS

We found that optimal conditions for the preparation of hexahydro-2H-pyrano[3,2-c]quinolines were Sc(OTf)₃ 0.5 % and acetonitrile at 160°C for 15 min; and using paraformaldehyde obtained the 6-benzylhexahydro- 2H-pyrano [3,2-c]quinolines with excellent yields, while the N-debenzylation process using ammonium formate in the presence of Pd/C and methanol resulted in the synthesis of hexahydro-2H-pyrano [3,2-c] quinolines with quantitative yields (95-98%).

CONCLUSION

We describe an efficient method to synthesize hexahydro-2H-pyrano[3,2-c]quinolines via the cationic imino-Diels-Alder/N-debenzylation methodology using Sc(OTf)3 0.5 % as Lewis Acid catalyst. Excellent yields of the products, use of MW irradiation, short times of reactions, and an efficient and highly diversified method are some of the main advantages of this new protocol.

How Lewis Acids Catalyze Diels-Alder Reactions

The Lewis acid(LA)-catalyzed Diels-Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled-cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I2 Collapse

Key Words

• Activation strain model
• Diels-Alder reactions
• Lewis acid catalysis
• Pauli repulsion
• density functional calculations

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MESH Headings Collapse

Grants

• NWO Nederlandse Organisatie voor Wetenschappelijk Onderzoek
• CTQ2016-78205-P and CTQ2016-81797-REDC MINECO
• Nederlandse Organisatie voor Wetenschappelijk Onderzoek

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Affiliation(s)

Pascal Vermeeren Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081HVAmsterdamThe Netherlands
Trevor A. Hamlin Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081HVAmsterdamThe Netherlands
Israel Fernández Departamento de Química Orgánica I and Centro de Innovación en Química Avanzada (ORFEO-CINQA)Facultad de Ciencias QuímicasUniversidad Complutense de Madrid28040MadridSpain
F. Matthias Bickelhaupt Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081HVAmsterdamThe Netherlands Institute for Molecules and Materials (IMM)Radboud UniversityHeyendaalseweg 1356525AJNijmegenThe Netherlands

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Carbonyl Activation by Selenium- and Tellurium-Based Chalcogen Bonding in a Michael Addition Reaction

In the last years the use of chalcogen bonding—the noncovalent interaction involving electrophilic chalcogen centers—in noncovalent organocatalysis has received increased interest, particularly regarding the use of intermolecular Lewis acids. Herein, we present the first use of tellurium‐based catalysts for the activation of a carbonyl compound (and only the second such activation by chalcogen bonding in general). As benchmark reaction, the Michael‐type addition between trans‐crotonophenone and 1‐methylindole (and its derivatives) was investigated in the presence of various catalyst candidates. Whereas non‐chalcogen‐bonding reference compounds were inactive, strong rate accelerations of up to 1000 could be achieved by bidentate triazolium‐based chalcogen bond donors, with product yields of >90 % within 2 h of reaction time. Organotellurium derivatives were markedly more active than their selenium and sulphur analogues and non‐coordinating counterions like BAr^F₄ provide the strongest dicationic catalysts.

The Intermediates in Lewis Acid Catalysis with Lanthanide Triflates

Lanthanide triflates are effective Lewis acid catalysts in reactions involving carbonyl compounds due to their high oxophilicity and water stability. Despite the growing interest, the identity of the catalytic species formed in lanthanide catalysed reactions is still unknown. We have therefore used mass spectrometry and ion spectroscopy to intercept and characterize the intermediates in a reaction catalysed by ytterbium and dysprosium triflates. We were able to identify a number of lanthanide intermediates formed in a simple condensation reaction between a C‐acid and an aldehyde. Results show correlation between the reactivity of lanthanide complexes and their charge state and suggest that the triply charged complexes play a key role in lanthanide catalysed reactions. Spectroscopic data of the gaseous ions accompanied by theoretical calculations reveal that the difference between catalytic efficiencies of ytterbium and dysprosium ions can be explained by their different electrophilicity.

A three-component, Zn(OTf)2-mediated entry into trisubstituted 2-aminoimidazoles

A three-component reaction involving in situ generation of propargylureas and subsequent Zn(OTf)₂-mediated cyclocondensation with a primary amine yielded trisubstituted 2-aminoimidazoles. These findings are in contrast to the previously reported base-promoted unimolecular cyclization of propargylureas (leading to 2-imidazolones) and extend the range of Lewis acid-catalyzed azole syntheses based on N-carbonyl propargylamines.

Lewis acid-catalyzed carbonyl-olefin metathesis

Catalytic carbonyl-olefin metathesis reactions enable direct carbon-carbon bond formation between carbonyl and olefin substrates relying on carbonyl-activation with a suitable Lewis acid. Based on this reaction design principle, efficient protocols for intermolecular carbonyl-olefin metathesis, as well as ring-closing and ring-opening carbonyl-olefin metathesis have been developed.

Recent advances in the application of ring-closing metathesis for the synthesis of unsaturated nitrogen heterocycles

This short review summarizes recent advances relating to the application of ring-closing olefin-olefin and carbonyl-olefin metathesis reactions towards the synthesis of unsaturated five- and six-membered nitrogen heterocycles. These developments include catalyst modifications and reaction designs that will enable access to more complex nitrogen heterocycles.

Origin of Nb2 O5 Lewis Acid Catalysis for Activation of Carboxylic Acids in the Presence of a Hard Base

The Nb2 O5 surface catalyzes the amidation of carboxylic acids with amines through Nb5+ Lewis acid activation of the C=O group. In this work, DFT calculations were applied to theoretically investigate the C=O bond activation of a model carboxylic acid (acetic acid) on θ-Al2 O3 (110), anatase TiO2 (101), and T-Nb2 O5 (100) surfaces. The adsorption sites, adsorption energies, reaction energy barriers, electronic properties, and vibrational frequency of acetic acid were examined in detail. It was found that the bond activation of the carbonyl group is most efficient on Nb2 O5 , although the adsorption energy is larger on Al2 O3 and TiO2 . The most efficient C=O bond activation on Nb2 O5 results in the lowest energy barrier of C-N bond formation during amidation. The Nb2 O5 surface also shows larger tolerance to methylamine and water molecules than Al2 O3 and TiO2 surfaces. These crucial factors contribute to the highest amidation catalytic reactivity on Nb2 O5 . Furthermore, the position of the mean density of states of the d-conduction band of the active metal site relative to the Fermi energy level correlates well with the efficiency in the C=O bond activation and, consequently, the catalytic activity for amidation. These results suggest that, unlike a classical understanding of strong acid sites of metal oxide surfaces, interaction of a carbonyl HOMO with an unoccupied metal d-orbital, or, in other words, covalent-like interaction between a carbonyl group and metal adsorption site, is relevant to the present system.

Microporous Nanotubes and Nanospheres with Iron-Catechol Sites: Efficient Lewis Acid Catalyst and Support for Ag Nanoparticles in CO2 Fixation Reaction

Fe^III -containing hyper-crosslinked microporous nanotubes (FeNTs) and nanospheres (FeNSs) are synthesized through the reaction of catechol and dimethoxymethane in the presence of FeCl₃ or CF₃ SO₃ H. Both FeNTs and FeNSs demonstrate excellent catalytic activity in Lewis acid catalysis (hydrolysis and regioselective methanolysis of styrene oxide) and tandem catalysis involving a sequential oxidation-cyclization process, which selectively converts benzyl alcohol to 2-phenyl benzimidazole. Apart from Lewis acidity, the FeNTs and FeNSs also showed CO₂ uptake capacities of 2.6 and 2.2 mmol g^-1 , respectively, at a pressure of 1 atm and temperature of 273 K. Furthermore, Ag nanoparticles are immobilized successfully on the surfaces of FeNTs and FeNSs by the liquid-phase impregnation method to prepare Ag@FeNT and Ag@FeNS nanocomposites, which show high catalytic activity for the selective fixation of CO₂ to phenylacetylene to yield phenylpropiolic acid at 60 °C and 1 atm CO₂ pressure. Hence, Fe^III -catechol-containing hyper-crosslinked nanotubes and nanospheres have huge potential not only as Lewis acid catalysts, but also as excellent supports for immobilizing Ag nanoparticles in the design of a robust catalyst for the carboxylation of terminal alkynes, which has wide scope in catalysis and environmental research.

A Novel Sc(OTf)3 -Catalyzed (2+2+1)-Cycloannulation/Aza-Friedel-Crafts Alkylation Sequence toward Multicyclic 2-Pyrrolines

The rapid assembly of molecular complexity continues to be at the forefront of novel reaction development. In the pursuit of that goal, we herein report a novel Sc(OTf)₃ -catalyzed, one-pot multicomponent reaction that furnishes complex multicyclic 2-pyrrolines with excellent overall yields and perfect diastereocontrol. This process is based on our previously established (2+2+1)-cycloannulation of in situ generated 1-azaallyl cations, 1,3-dicarbonyls and primary amines. The newly formed and highly reactive aminal moiety is readily substituted with indoles and pyrroles both as external and internal π-nucleophiles to provide densely functionalized N-heterocycles with four new σ-bonds and two vicinal quaternary stereogenic centers. In addition, DFT calculations have been conducted to further characterize the intermediate 1-azaallyl cations.

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Functionalized Cyclopentanes via Sc(III)-Catalyzed Intramolecular Enolate Alkylation

We report herein the intramolecular α-tert-alkylation of unsaturated β-ketoesters which gives rise to highly functionalized cyclopentanes. This strategy is characterized by its operational simplicity, mild reaction conditions and the use of scandium(III) triflate as a Lewis acid catalyst. Of interest, cyclopentanes bearing heterocycles, sites for post reaction functionalization and spirocyclic architectures are accessible with this strategy.

3-Aryl-2,5-Dihydropyrroles via Catalytic Carbonyl-Olefin Metathesis

Herein, we describe the development of a synthetic strategy towards chiral 3-pyrrolines based on the design principle of iron(III)-catalyzed carbonyl-olefin metathesis. This approach takes advantage of commercially available amino acids as chiral pool reagents and FeCl3 as a Lewis acid catalyst. Our strategy is characterized by its operational simplicity, mild reaction conditions and functional group tolerance. Investigations show that an electron-deficient nitrogen protecting group overcomes limitations arising from competitive binding of the Lewis acid catalyst to unfavorable Lewis basic sites, which ultimately enables catalytic turnover.

Enantioselective Synthesis of N-H-Free 1,5-Benzothiazepines

An enantioselective sulfa-Michael-cyclization reaction was developed for the synthesis of 1,5-benzothiazepines with versatile pharmacological activities. The reaction between 2-aminothiophenol and α,β-unsaturated pyrazoleamides gave direct access to N-H-free 1,5-benzothiazepines in the presence of a chiral N,N'-dioxide/Yb(OTf)₃ complex. Excellent enantioselectivities (up to 96 % ee) and high yields (up to 99 %) were obtained for a broad range of substrates under mild reaction conditions. This method provided a facile approach to the antidepressant drug (R)-(-)-Thiazesim.

Theoretical study of the regioselective cyclization of enaminones in the construction of benzofurans and indoles

A theoretical study was undertaken regarding the regioselective Lewis acid-promoted intramolecular cyclization of novel enaminones 1-3 leading to the corresponding benzofurans 4-5 and indoles 6. The density functional theory (DFT) and hard and soft acids and bases (HSAB) principle provided data to describe the electronic effects of the substituents in the reactivity of the benzene ring and the enaminone moiety. The condensed and local Fukui functions for nucleophilic and electrophilic attacks of the reactants accounted for the experimentally observed preference, in regard to precursors 1-3, of the cyclization between the C6' carbon (rather than the C2' carbon) of the benzene ring and the C3 center of the enaminone moiety.

FeCl3-Catalyzed Combinatorial Synthesis of Functionalized Spiro[Indolo-3,10'-indeno [1,2-b]quinolin]-trione Derivatives

An efficient, inexpensive, environmentally friendly and high yield one-pot route to new spiro[indolo-3,10'-indeno [1,2-b]quinolin]-trione derivatives has been developed, involving three-component reaction of enaminones, N-substituted isatins and Indane-1,3-dione catalyzed by FeCl3. The approach to this spiro-heterocycle is noteworthy because it results in the formation of three new σ (two C-C and one C-N) bonds in a single operation, leading to the construction of novel spiro skeleton. This method works on a large scale in excellent yields.

Experimental and computational study of the catalytic asymmetric 4π-electrocyclization of N-heterocycles

The first asymmetric metal-catalyzed Nazarov cyclization of N-heterocycles has been developed. The use of a chiral catalyst allows the enantioselective electrocyclization of N-heterocycles under mild conditions and the corresponding products are obtained in good yields with excellent enantio- and diastereoselectivity. The reaction mechanism and the absolute configuration of the obtained products are explained by means of computational studies.

Facile and efficient Lewis acid catalyzed synthesis of an asymmetric tetrazine useful for bio-orthogonal click chemistry applications

Bio-orthogonal tetrazine click reactions have recently attracted significant interest for applications spanning biological imaging, cancer targeting, and biomaterials science. Here, we report a simple and efficient two-step scheme for the synthesis of an asymmetric tetrazine molecule containing a carboxylic acid handle for subsequent macromolecular conjugation. Yields as high as 75% were achieved using as little as 0.005 equivalents of nickel triflate catalyst, which is a significant improvement over previous methodologies.

Gallium-containing polymer brush film as efficient supported Lewis acid catalyst in a glass microreactor

Polystyrene sulfonate polymer brushes, grown on the interior of the microchannels in a microreactor, have been used for the anchoring of gallium as a Lewis acid catalyst. Initially, gallium-containing polymer brushes were grown on a flat silicon oxide surface and were characterized by FTIR, ellipsometry, and X-ray photoelectron spectroscopy (XPS). XPS revealed the presence of one gallium per 2-3 styrene sulfonate groups of the polymer brushes. The catalytic activity of the Lewis acid-functionalized brushes in a microreactor was demonstrated for the dehydration of oximes, using cinnamaldehyde oxime as a model substrate, and for the formation of oxazoles by ring closure of ortho-hydroxy oximes. The catalytic activity of the microreactor could be maintained by periodic reactivation by treatment with GaCl3.

Two-directional synthesis as a tool for diversity-oriented synthesis: Synthesis of alkaloid scaffolds

Two-directional synthesis represents an ideal strategy for the rapid elaboration of simple starting materials and their subsequent transformation into complex molecular architectures. As such, it is becoming recognised as an enabling technology for diversity-oriented synthesis. Herein, we provide a thorough account of our work combining two-directional synthesis with diversity-oriented synthesis, with particular reference to the synthesis of polycyclic alkaloid scaffolds.