Lewis Acid-catalyzed Reactions Research Articles

Intramolecular Hosomi-Sakurai Reaction for the Synthesis of Benzoxasiloles.

A Lewis acid-catalyzed intramolecular Hosomi-Sakurai reaction of o-(allylsilyl)benzaldehyde/ketone has been developed. The reaction proceeds through simultaneous C-Si bond cleavage and C-C bond reconstruction. This protocol provides a rapid approach for the synthesis of allyl-substituted benzoxasiloles under mild conditions.

Lewis Acid-Catalyzed Formal [4 + 2] Reaction of Alkynyl Sulfides and 2-Pyrones To Access Polysubstituted Aryl Sulfides.

A practical and efficient method to access polysubstituted aryl sulfides has been discovered via a Lewis acid-catalyzed reaction between alkynyl sulfide and 2-pyrone, involving a Diels-Alder/retro-Diels-Alder pathway. Alkynyl sulfide as an electron-rich dienophile and 2-pyrones as electron-poor dienes are conjunctively transformed into a series of polysubstituted aryl sulfides with broad functional group compatibility in good to excellent yields (40 examples, 43-88% yield). The robustness and practicality of the protocol has been demonstrated through gram-scale synthesis and the ease of transformation of the resulting products.

Lewis Acid-Catalyzed Tandem Reaction Strategy for the Synthesis of Dihydrophenalene-Fused Lactones.

A Lewis acid-catalyzed tandem reaction strategy for the construction of a dihydrophenalene-lactone tetracyclic skeleton has been disclosed. Starting with 2-naphthol-tethered ketones and active methylene esters, the tandem reaction catalyzed by Sc(OTf)3 proceeded well to afford an array of dihydrophenalene-fused lactones with moderate to high efficiency and diastereoselectivity. Moreover, the synthetic utility of this protocol was demonstrated by easy gram-scale preparation and diverse product transformations.

Hexafluoroisopropanol as a Bioconjugation Medium of Ultrafast, Tryptophan-Selective Catalysis.

The past decade has seen a remarkable growth in the number of bioconjugation techniques in chemistry, biology, material science, and biomedical fields. A core design element in bioconjugation technology is a chemical reaction that can form a covalent bond between the protein of interest and the labeling reagent. Achieving chemoselective protein bioconjugation in aqueous media is challenging, especially for generally less reactive amino acid residues, such as tryptophan. We present here the development of tryptophan-selective bioconjugation methods through ultrafast Lewis acid-catalyzed reactions in hexafluoroisopropanol (HFIP). Structure-reactivity relationship studies have revealed a combination of thiophene and ethanol moieties to give a suitable labeling reagent for this bioconjugation process, which enables modification of peptides and proteins in an extremely rapid reaction unencumbered by noticeable side reactions. The capability of the labeling method also facilitated radiofluorination application as well as antibody functionalization. Enhancement of an α-helix by HFIP leads to its compatibility with a certain protein, and this report also demonstrates a further stabilization strategy achieved by the addition of an ionic liquid to the HFIP medium. The nonaqueous bioconjugation approaches allow access to numerous chemical reactions that are unavailable in traditional aqueous processes and will further advance the chemistry of proteins.

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

Fixing CO2 under Atmospheric Conditions and Dual Functional Heterogeneous Catalysis Employing Cu MOFs: Polymorphism, Single-Crystal-to-Single-Crystal (SCSC) Transformation and Magnetic Studies.

New copper compounds, [Cu(C14H8O6)(C10H8N2)(H2O)] (1), [Cu(C14H8O6)(C10H8N2)(H2O)]·(C3H7ON)2 (2), [Cu(C14H8O6)(C10H8N2)(H2O)2]·(C3H7ON) (3), [Cu(C14H8O6)(C10H8N4)] (4), and [Cu(C14H8O6)(C10H8N4)]·(H2O) (5), were prepared employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2,5-BPTA) as the primary ligand and 4,4'-bipyridine (1-3) and 4,4'-azopyridine (4-5) as the secondary ligands. Single-crystal studies indicated that compounds 1-4 have two-dimensional layer structures and compound 5 has a three-dimensional structure. Compounds 1-3 were isolated from the same reaction mixture but by varying the time of reaction. The framework structures of compounds 1-3 are similar and may be considered as polymorphic structures. Compounds 4 and 5 can also be considered polymorphic with a change in dimensionality of the structure. Compounds 1-3 can be formed through a single-crystal-to-single-crystal transformation under a suitable solvent mixture. The Cu center was explored for the Lewis acid-catalyzed cycloaddition reaction of epoxide and CO2 under ambient conditions in a solventless condition and also for the synthesis of propargylamine derivatives by three-component coupling reactions (A3 coupling) in a DCM medium. The Lewis basic functionality of the MOF (-N═N- group) has been explored for the Henry reaction (aldol condensation) in a solventless condition. In all of the catalytic reactions, good yields and recyclability were observed. The magnetic studies indicated that compounds 1 and 4 have antiferromagnetic interactions and compound 5 has ferromagnetic interactions. The present studies illustrated the rich diversity that the copper-containing compounds exhibit in extended framework structures.

Lewis Acid-Catalyzed Carbonyl-Ene Reaction: Interplay between Aromaticity, Synchronicity, and Pauli Repulsion.

The physical factors governing the catalysis in Lewis acid-promoted carbonyl-ene reactions have been explored in detail quantum chemically. It is found that the binding of a Lewis acid to the carbonyl group directly involved in the transformation greatly accelerates the reaction by decreasing the corresponding activation barrier up to 25 kcal/mol. The Lewis acid makes the process much more asynchronous and the corresponding transition state less in-plane aromatic. The remarkable acceleration induced by the catalyst is ascribed, by means of the activation strain model and the energy decomposition analysis methods, mainly to a significant reduction of the Pauli repulsion between the key occupied π-molecular orbitals of the reactants and not to the widely accepted stabilization of the LUMO of the enophile.

Synthesis of pyranobenzopyrans from D-glucal using ionic liquids

Imidazolium room temperature ionic liquids (RTILs) were used as solvents in the Lewis acid-catalyzed cycloaddition reactions of D-glucal with substituted salicylaldehydes. These reactions selectively led to various, novel, cis-pyrano [4,3-b]benzopyrans in modest yields, different products than those isolated from analogous solution studies. Furan diol was isolated as the major byproduct from all the reactions. The use of RTILs facilitated the use of unprotected sugars in these reactions.

The Origin of Catalysis and Regioselectivity of Lewis Acid-Catalyzed Diels-Alder Reactions with Tropone.

We have studied the uncatalyzed and Lewis acid (LA)-catalyzed cycloaddition reaction between tropone and 1,1-dimethoxyethene using dispersion-corrected relativistic density functional theory (DFT). The LA catalysts BF3, B(C6H5)3, and B(C6F5)3 efficiently accelerate both the competing [4+2] and [8+2] cycloaddition reactions by lowering the activation barrier up to 12 kcal mol-1 compared to the uncatalyzed reaction. Our study reveals that the LA catalyst promotes both cycloaddition reaction pathways by LUMO-lowering catalysis and demonstrates that Pauli-lowering catalysis is not always the operative catalytic mechanism in cycloaddition reactions. Judicious choice of the LA catalyst can effectively impart regiocontrol of the cycloaddition: B(C6H5)3 furnishes the [8+2] adduct while B(C6F5)3 yields the [4+2] adduct. We discovered that the regioselectivity shift finds its origin in the ability of the LA to absorb distortion by adopting a trigonal pyramidal geometry around the boron atom.

Continuous flow (Sulfated) Zirconia Catalysed Cascade Conversion of Levulinic Acid to γ‐Valerolactone

Abstractγ‐Valerolactone (GVL) is a renewable and versatile platform chemical derived from sustainable carbon feedstocks. The cascade conversion of levulinic acid into GVL requires Brønsted and Lewis acid catalysed reactions. Here, a dual‐catalyst bed configuration is demonstrated that promotes synergy between Brønsted acid sites in sulfated zirconia (SZ) and Lewis acid sites in ZrO2/SBA‐15 for the liquid phase, continuous flow esterification and subsequent catalytic transfer hydrogenation (CTH) of levulinic acid to GVL. A saturated surface sulfate monolayer, possessing a high density of strong Brønsted acid sites, was optimal for levulinic acid esterification to isopropyl levulinate over SZ (>80 % conversion). A conformal ZrO2 bilayer, deposited over a SBA‐15 mesoporous silica and possessing mixed Brønsted:Lewis acidity, catalysed CTH of the levulinate ester and subsequent dealcoholisation/cyclisation to GVL (>60 % selectivity). Maximum stable productivity for the dual‐bed was 2.2 mmolGVL.gcat.h−1 at 150 °C, significantly outperforming either catalyst alone or a physical mixture of both. Flow chemistry is a versatile approach to achieve spatial control over cascade transformations involving distinct catalytically active sites.

A Lewis acid-catalyzed tandem reaction enabling 2-arylglycerol derivative as a versatile 1,3-biselectrophile for the synthesis of 4H-chromenes and 2-pyridinones

Acid-catalyzed tandem reactions were established by employing a novel class of 2-arylglycerol derivative, 5-aryl-1,3-dioxan-5-ol, as versatile 1,3-biselectrophile. In the reactions, 5-aryl-1,3-dioxan-5-ol works like atropaldehydes or 2-aryl malondialdehydes, and can react with 2-naphthols and β-keto amides, allowing the synthesis of 4H-chromenes and 5-aryl-2-pyridinones. High yields, good functional group tolerance, broad substrate scope and simple reaction operation make this protocol attractive.

Synthesis of azepane-fused pyrano[3,2-b]indoles by Lewis acid-catalysed oxa Diels-Alder reactions.

The dihydroazepino[1,2-a]indole diones 3 are tricyclic oxindole-type enones which are readily accessible by catalytic photooxygenation of cyclohepta[b]indoles 1 followed by dehydration. Lewis acid-catalysed oxa Diels-Alder reactions of enones 3 with enol ethers 4 were developed that lead to novel tetracyclic azepane-fused pyrano[3,2-b]indoles 5, with high stereoselectivity and under mild reaction conditions.

Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

Previous studies have shown that the catalytically active species in the Lewis acid-catalyzed addition reactions of allyl silanes or silyl enol ethers to carbonyl compounds or Michael acceptors are often silylium-carbonyl adducts rather than the adducts of the carbonyl group with the Lewis acid used for the induction of the reaction. Indirect evidence for such catalyst variations has so far been derived from double-label crossover experiments and comparisons of absolute reaction rates. We have now performed a detailed investigation on the kinetics and mechanism of the triarylborane-initiated conjugate addition reactions of allylsilanes and silyl enol ethers to α,β-unsaturated carbonyl compounds. NMR spectroscopic monitoring of such reactions gave rise to sigmoidal kinetic profiles, allowing us to directly follow the change of the catalytically active Lewis acid from triarylboranes in the induction period to silylium ions during the main part of the reaction. Crossover experiments and the isolation of four- and five-membered cyclic intramolecular trapping products provided further insight into the mechanism. DFT calculations of various mechanistic variants and kinetic modeling elucidated the operation of a complex reaction network, which rationalizes the experimental observations.

Lewis Acid-Catalyzed Ring-Opening Cross-Coupling Reaction of gem-Difluorinated Cyclopropanes Enabled by C-F Bond Activation.

gem-Difluorinated cyclopropanes have attracted wide research interest in organic synthesis due to their high reactivity. Herein, we report a Lewis acid-catalyzed cross-coupling reaction of mono- and disubstituted gem-difluorinated cyclopropanes with nucleophiles. The formation of a fluoroallyl cation species triggered via the Lewis acid-assisted activation of the C-F bond is proposed in this transformation. The cation species is then trapped by the nucleophiles, including electron-rich arenes and allylsilanes, to deliver a series of fluoroallylic products in good yields. The reaction provides an alternative mode for using gem-difluorinated cyclopropanes as fluoroallyl surrogates.

Lewis acid-catalyzed Pudovik reaction-phospha-Brook rearrangement sequence to access phosphoric esters.

Herein, we report a Lewis acid-catalyzed Pudovik reaction–phospha-Brook rearrangement sequence between diarylphosphonates or -phosphinates and α-pyridinealdehydes to access valuable phosphoric ester compounds. This transformation provides an extended substrate scope that is complementary to similar previously reported base-catalyzed transformations.

De Novo Diastereoselective Synthesis of 1-Hydroxyl Allogibberic Methyl Ester en Route to Diverse Bioactive Molecules.

The first de novo synthesis of 1-hydroxyl allogibberic methyl ester, en route to pharbinilic acid and other bioactive molecules, is accomplished in diastereoselective manner. Key reactions of the synthesis include a Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, a Lewis acid-catalyzed reductive Prins cyclization reaction, and a SmI2-mediated transannular pinacol coupling reaction. The synthesis provides a new avenue to access diverse relevant bioactive molecules.

How Ionization Catalyzes Diels-Alder Reactions.

The catalytic effect of ionization on the Diels‐Alder reaction between 1,3‐butadiene and acrylaldehyde has been studied using relativistic density functional theory (DFT). Removal of an electron from the dienophile, acrylaldehyde, significantly accelerates the Diels‐Alder reaction and shifts the reaction mechanism from concerted asynchronous for the neutral Diels‐Alder reaction to stepwise for the radical‐cation Diels‐Alder reaction. Our detailed activation strain and Kohn‐Sham molecular orbital analyses reveal how ionization of the dienophile enhances the Diels‐Alder reactivity via two mechanisms: (i) by amplifying the asymmetry in the dienophile's occupied π‐orbitals to such an extent that the reaction goes from concerted asynchronous to stepwise and thus with substantially less steric (Pauli) repulsion per reaction step; (ii) by enhancing the stabilizing orbital interactions that result from the ability of the singly occupied molecular orbital of the radical‐cation dienophile to engage in an additional three‐electron bonding interaction with the highest occupied molecular orbital of the diene.

Lewis Acid-catalyzed Decarboxylative Cyanation of Cyclic Enol Carbonates — Access to Multi-substituted β-Ketonitriles —

Abstract A Lewis acid-catalyzed decarboxylative substitution reaction of cyclic enol carbonates with a cyanation reagent was developed to give β-ketonitriles in good yield. It is proposed that the reaction proceeds through an oxyallyl cation intermediate or its equivalent. Interestingly, cyanation selectively took place at the more hindered site of the intermediate to afford a multi-substituted β-ketonitrile. This regioselectivity was observed by the cyanation of a wide range of substrates and is discussed in terms of the reaction mechanism.

Mechanistic Insight into Hydroboration of Imines from Combined Computational and Experimental Studies.

Boron Lewis acid-catalyzed and catalyst-free hydroboration reactions of imines are attractive due to the mild reaction conditions. In this work, the mechanistic details of the hydroboration reactions of two different kinds of imines with pinacolborane (HBpin) are investigated by combining density functional theory calculations and some experimental studies. For the hydroboration reaction of N-(α-methylbenzylidene)aniline catalyzed by tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF 3 ), our calculations show that the reaction proceeds through a boron Lewis acid-promoted hydride transfer mechanism rather than the classical Lewis acid activation mechanism. For the catalyst- and solvent-free hydroboration reaction of imine, N-benzylideneaniline, our calculations and experimental studies indicate that this reaction is difficult to occur under the reaction conditions reported previously. With a combination of computational and experimental studies, we have established that the commercially available BH3 ⋅ SMe2 can serve as an efficient catalyst for the hydroboration reactions of N-benzylideneaniline and similar imines. The hydroboration reactions catalyzed by BH3 ⋅ SMe2 are most likely to proceed through a hydroboration/B-H/B-N σ-bond metathesis pathway, which is very different from that of the reaction catalyzed by BArF 3 .

Bio-inspired construction of a tetracyclic ring system with an avarane skeleton: total synthesis of dactyloquinone A

We report the asymmetric construction of an avarane skeleton. The strategy involves a Lewis acid-catalyzed cyclization reaction, which drives the methyl groups of two different configurations at the C-4 site to migrate by 1, 2-rearrangement.