Cyclisations Catalysed by Bismuth(III) Triflate

Synthesis of bismuthanyl-substituted monomeric triel hydrides

The syntheses and characterizations of the first bismuthanylborane monomers stabilized only by a donor in D·BH2Bi(SiMe3)2 (D = DMAP 1a, IDipp 1b, IMe41c; DMAP = 4-dimethylaminopyridine, IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene, IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) are presented. All compounds were synthesized by salt metathesis reactions between D·BH2I and KBi(SiMe3)2(THF)0.3 and represent some of the extremely rare compounds featuring a 2c-2e B-Bi bond in a molecular compound. The products display high sensitivity towards air and light and slowly decompose in solution even at -80 °C. By the reaction of IDipp·GaH2(SO3CF3) with KBi(SiMe3)2(THF)0.3, the synthesis of the first bismuthanylgallane IDipp·GaH2Bi(SiMe3)2 (2) stabilized only by a 2-electron donor was possible, as evident from single crystal X-ray structure determination, NMR spectroscopy and mass spectrometry. Computational studies shed light on the stability of the products and the electronic nature of the compounds.

What Determines the Lewis Acidity of a Bismuthane? Towards Bi-Based FLPs

Aiming at intramolecular frustrated Lewis pairs (FLPs) based on soft Lewis acidic bismuth centers, a phosphine function was combined with a dichloridobismuthane unit on a phenylene backbone utilizing a scrambling approach. The reaction between two equivalents of BiCl3 and (o-(Ph2P)C6H4)3Bi yielded (o-(Ph2P)C6H4)BiCl2(THF), the structure of which indicated Bi…P interactions and thus a pronounced Lewis acidity at the bismuth center that was confirmed by the Gutmann-Beckett method. However, the system turned out to be insufficient to be utilized for FLP reactivity. Hence, the chloride ligands were exchanged by iodide and C2F5 substituents, respectively. Despite a lower electronegativity the iodide compound exhibits a shorter Bi…P contact, while the C2F5 substituents led to a further decrease of the Lewis acidity, despite their high group electronegativity. DFT calculations rationalized this by a quenching of the Lewis acidity inherent to the σ*(Bi-C) orbital by negative hyperconjugation from occupied p-orbitals at the F atoms. Furthermore, it turned out that the strength of the covalent Bi-X σ-bond is a more important factor than the charge at Bi in determining the energetic accessibility and thus Lewis acidity of the antibonding σ*(Bi-C) orbital.

Bismuth(III)-Catalyzed Regioselective Selenation of Indoles with Diaryl Diselenides: Synthesis of 3-Selanylindoles

Heterocyclic aryl selenides have recently attracted considerable research interest owing to their applications in biological and pharmaceutical fields. Herein, we describe a simple and general synthesis of 3-selanylindoles via a novel regioselective C-H selenation of indoles using a bismuth reagent as a catalyst. The reactions of indoles with diselenides in the presence of 10 mol% BiI3 at 100 °C in DMF afforded the corresponding 3-selanylindoles in moderate-to-excellent yields. The reaction proceeded efficiently under aerobic conditions by adding only a catalytic amount of BiI3, which was non-hygroscopic and less toxic, and both selanyl groups of the diselenide were transferred to the desired products.

Bismuth(III) triflate: an economical and environmentally friendly catalyst for the Nazarov reaction

We describe the use of bismuth(III) triflate as an efficient and environmentally friendly catalyst for the Nazarov reaction of aryl vinyl ketones, leading to the synthesis of 3-aryl-2-ethoxycarbonyl-1-indanones and 3-aryl-1-indanones. By changing the temperature and reaction time, it was possible to modulate the reactivity, allowing the synthesis of two distinct product classes (3-aryl-2-ethoxycarbonyl-1-indanones and 3-aryl-1-indanones) in good to excellent yield. The reaction did not require additives and was insensitive to both air and moisture. Preliminary biological evaluation of some indanones showed a promising profile against some human cancer line cells.

Divergent access to polycyclic spiro- and fused-N,O-ketals through Bi(OTf)3-catalyzed [4+2]-annulation of cyclic N-sulfonyl ketimines and alkynols

Bismuth(III) triflate-catalyzed [4+2]-annulation of cyclic N-sulfonyl ketimines (derived from saccharin) and alkynyl alcohols (4-pentyn-1-ols and 5-hexyn-1-ols) has been reported. This cascade annulation provides a diverse array of polycyclic spiro-and-fused N,O-ketals with excellent substrate scope, good isolated yields, and diastereoselectivities under mild reaction conditions.

Urea-rich porous organic polymer as a hydrogen bond catalyst for Knoevenagel condensation reaction and synthesis of 2,3-dihydroquinazolin-4(1H)-ones

In this research, a new urea-rich porous organic polymer (urea-rich POP) as a hydrogen bond catalyst was synthesized via a solvothermal method. The physiochemical properties of the synthesized urea-rich POP were investigated by using different analyses like Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), energy-dispersive X-ray spectroscopy (EDS), elemental mapping analysis, X-ray diffraction analysis (XRD) and Brunauer-Emmett-Teller (BET) techniques. The preparation of urea-rich POP provides an efficacious platform for designing unique hydrogen bond catalytic systems. Accordingly, urea-rich POP, due to the existence of several urea moieties as hydrogen bond sites, has excellent performance as a catalyst for the Knoevenagel condensation reaction and multi-component synthesis of 2,3-dihydroquinazolin-4(1H)-ones.

Bismuth(III)-Catalyzed Oxidative Cross-Coupling of 3-Hydroxycarbazoles with Arenols under an Oxygen Atmosphere

A Bi(OTf)3-catalyzed oxidative cross-coupling reaction of 3-hydroxycarbazoles with arenols was developed under mild conditions. Both substrates were used in a 1:1 molar ratio in the presence of a catalytic amount of Bi(OTf)3. The reaction was carried out under an oxygen atmosphere at 30 °C to afford C1-symmetric hydroxybiaryls in good yields (up to 94%) with high chemo- and regioselectivity.

Semisynthetic Sesquiterpene Lactones Generated by the Sensibility of Glaucolide B to Lewis and Brønsted-Lowry Acids and Bases: Cytotoxicity and Anti-Inflammatory Activities

Sesquiterpene lactone (SL) subtypes including hirsutinolide and cadinanolide have a controversial genesis. Metabolites of these classes are either described as natural products or as artifacts produced via the influence of solvents, chromatographic mobile phases, and adsorbents used in phytochemical studies. Based on this divergence, and to better understand the sensibility of these metabolites, different pH conditions were used to prepare semisynthetic SLs and evaluate the anti-inflammatory and antiproliferative activities. Therefore, glaucolide B (1) was treated with various Brønsted-Lowry and Lewis acids and bases-the same approach was applied to some of its derivatives-allowing us to obtain 14 semisynthetic SL derivatives, 10 of which are hereby reported for the first time. Hirsutinolide derivatives 7a (CC₅₀ = 5.0 µM; SI = 2.5) and 7b (CC₅₀ = 11.2 µM; SI = 2.5) and the germacranolide derivative 8a (CC₅₀ = 3.1 µM; SI = 3.0) revealed significant cytotoxic activity and selectivity against human melanoma SK-MEL-28 cells when compared with that against non-tumoral HUVEC cells. Additionally, compounds 7a and 7c.1 showed strongly reduced interleukin-6 (IL-6) and nitrite (NOx) release in pre-treated M1 macrophages J774A.1 when stimulated with lipopolysaccharide. Despite the fact that hirsutinolide and cadinanolide SLs may be produced via plant metabolism, this study shows that acidic and alkaline extraction and solid-phase purification processes can promote their formation.

Mechanism of Friedel-Crafts Acylation Using Metal Triflate in Deep Eutectic Solvents: An Experimental and Computational Study

In this paper, we develop a method for Friedel-Crafts acylation using metal triflate in deep eutectic solvents. Various metal triflates were tested and provided good to excellent yields of corresponding ketone products. The density functional theory calculation revealed the metal effects on the formation of active intermediate acylium triflate as well as the acidic condition. The metal triflate in the deep eutectic solvent can be recovered and reused with a little loss in the catalytic activity.

Bi-Catalyzed 1,2-Reactivity of Spirocyclopropyl Oxindoles with Dithianediol: Access to Spiroheterocycles

The efficient Bi-catalyzed 1,2-reactivity of spirocyclopropyl oxindoles has been disclosed with dithianediols as the sulfur surrogate to furnish spiroheterocycles at moderate temperature. The procedure provides a potential approach for the construction of spirotetrahydrothiophene scaffolds with functional group diversity. The catalytic 1,2-reactivity of cyclopropanes, mechanistic studies using density functional theory studies, diastereoselectivity, and additive-free mild conditions are the important practical features.

Bismuth Subnitrate-Catalyzed Markovnikov-Type Alkyne Hydrations under Batch and Continuous Flow Conditions

Bismuth subnitrate is reported herein as a simple and efficient catalyst for the atom-economical synthesis of methyl ketones via Markovnikov-type alkyne hydration. Besides an effective batch process under reasonably mild conditions, a chemically intensified continuous flow protocol was also developed in a packed-bed system. The applicability of the methodologies was demonstrated through hydration of a diverse set of terminal acetylenes. By simply switching the reaction medium from methanol to methanol-d₄, valuable trideuteromethyl ketones were also prepared. Due to the ready availability and nontoxicity of the heterogeneous catalyst, which eliminated the need for any special additives and/or harmful reagents, the presented processes display significant advances in terms of practicality and sustainability.

Indium-Catalyzed Cycloisomerization of 1,6-Cyclohexenylalkynes

Efficient four- and five-step routes to access functionalized bicyclo[3.2.1]oct-2-ene and bicyclo[3.3.1]nonadiene via indium-mediated cycloisomerization of 1,6-enynes has been developed. This atom-economical catalytic process was optimized and relied on the efficiency of InCl3 leading to the preparation of functionalized bicyclic adducts in up to 99% isolated yield. The cyclization occurred on two different processes (5-exo versus 6-endo pathway) and were influenced by the substitution of the alkynyl moiety. The exo process was favored for non-substituted alkynes whereas the endo pathway was generally observed for substituted alkynes. Then, the presence of electron-withdrawing groups on the aryl substituted alkyne increased the ratio of the exo isomer. DFT calculations were performed on stability of intermediates and corroborated the intervention of InCl3.

A novel one-pot synthesis of flavones

In this paper, a one-pot facile route for the BiCl₃/RuCl₃-mediated synthesis of functionalized flavones is described, including: (i) intermolecular ortho-acylation of substituted phenols with cinnamoyl chlorides, and (ii) intramolecular cyclodehydrogenation of the resulting o-hydroxychalcones. The reaction conditions are discussed herein.

A one-pot facile route for the BiCl₃/RuCl₃-mediated synthesis of functionalized flavones is described, including: (i) ortho-acylation of phenols with cinnamoyl chlorides, and (ii) cyclodehydrogenation of the resulting o-hydroxychalcones.

Bi(OAc)3/chiral phosphoric acid catalyzed enantioselective allylation of seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines

An efficient asymmetric allylation reaction of allylboronates with seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines, is described. The reaction, which is catalyzed by a Bi(OAc)₃/CPA system, gives a range of chiral nitrogen-containing heterocycle structures in high yields and with good enantioselectivities. The conversion of these products to nitrogen-containing heterocycles is also demonstrated.

Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations

This study marks chemical space available for pnictogen-bonding catalysis, and demonstrates that reactivity accessible in this space is unique.

Pnictogen-bond donors are attractive for use in catalysis because of deep σ holes, high multivalency, rich hypervalency, and chiral binding pockets. We here report natural product inspired epoxide-opening polyether cyclizations catalyzed by fluoroarylated Sb(v) > Sb(iii) > Bi > Sn > Ge. The distinctive characteristic found for pnictogen-bonding catalysis is the breaking of the Baldwin rules, that is selective endo cyclization into the trans-fused ladder oligomers known from the brevetoxins. Moreover, tris(3,4,5-trifluorophenyl)stibines and their hypervalent stiborane catecholates afford different anti-Baldwin stereoselectivity. Lewis (SbCl₃), Brønsted (AcOH) and π acids fail to provide similar access to these forbidden rings. Like hydrogen-bonding catalysis differs from Brønsted acid catalysis, pnictogen-bonding catalysis thus emerges as the supramolecular counterpart of covalent Lewis acid catalysis.

Bi(iii)-catalyzed aminooxygenation of propargyl amidines to synthesize 2-fluoroalkyl imidazole-5-carbaldehydes and their decarbonylations

The first example of main group metal Bi(iii)-catalyzed aminooxygenation of fluorinated propargyl amidines was developed to produce 2-fluoroalkyl imidazole-5-carbaldehydes in moderate to excellent yields, in which phenol played a critical role and could be recovered and recycled. In the presence of KOt-Bu, an unconventional decarbonylation occurred on the 2-fluoroalkyl imidazole-5-carbaldehydes.

Enantioselective and regioselective aza-Friedel–Crafts reaction of electron-rich phenols with isatin-derived ketimines

An efficient asymmetric aza-Friedel–Crafts reaction of phenols with isatin-derived ketimines is described.

Bi(III)-Catalyzed Enantioselective Allylation Reactions of Ketimines

Chiral homoallylic amines not only are found in pharmaceutically relevant compounds but also serve as versatile building blocks for chemical synthesis. However, catalytic allylation of ketimines with allylboronates, an attractive approach to synthesize chiral homoallylic amine scaffolds remain scarce. Herein, we develop a highly enantioselective allylation of isatin-derived ketimines with boron allylation reagents catalyzed by a Bi(OAc)₃-chiral phosphoric acid catalyst system. The reactions are remarkably efficient and mild, most of which were completed in less than an hour at room temperature with only 1/2 mol% (Bi(OAc)₃/CPA) catalyst loading. A wide range of chiral 3-allyl 3-aminooxindoles were obtained in excellent yields and enantioselectivities. The synthetic utility was demonstrated by efficient formal synthesis of (+)-AG-041R and (−)-psychotriasine. Preliminary mechanism was studied by control experiments and theoretical calculations.

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Asymmetric allylation of ketimines

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Bi(OAc)₃-chiral phosphoric acid catalyst

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Downstream synthetic transformations

Bi(OTf)3-Catalyzed Diastereoselective One-Pot Synthesis of 1,3-Diamines with Three Continuous Stereogenic Centers

A highly modular, diastereoselective one-pot-synthesis of 1,3-diamines with three contiguous stereogenic centers is reported. Our method provides a fast and efficient access to 1,2- anti-2,3- anti-1,3-diamines from three readily available building blocks. This Bi(OTf)₃-catalyzed reaction is insensitive to air and moisture and can be performed on a multigram scale.

Catalytic, Tunable, One-Step Bismuth(III) Triflate Reaction with Alcohols: Dehydration Versus Dimerization

Bi(OTf)₃·xH₂O is a powerful catalyst for the dehydration of tertiary alcohols into alkenes in apolar solvents. The reaction proceeds smoothly and selectively, with amounts as low as 0.01 mol % catalyst, in yields up to 93%. Moreover, in polar solvents, Bi(OTf)₃·xH₂O (0.1-1 mol %) selectively catalyzes the dimerization of the alcohols instead, forming new C-C bonds, in yields up to 96%. This mild, efficient, economic, and eco-friendly method is applicable across different chemical classes and amenable to several functional groups.

Silica Surface Modification and Its Application in Permanent Link with Nucleic Acids

In this paper, the Pt-catalyzed hydrosilylation of hydroxyl ethers is described. Various bifunctional alkoxysilanes were obtained and applied in O-silylation of free hydroxyl groups on the silica surface. These modified solid materials have been used as excellent supports for linking synthetic nucleic acids. Nucleic acids permanently attached to the solid surface were tested in hybridization with complementary fluorescence-labeled sequences. Detection of nucleic acids anchored to the solid support was performed by fluorescence microscopy after hybridization.

Modular synthesis of new C-aryl-nucleosides and their anti-CML activity

The C-aryl-ribosyles are of utmost interest for the development of antiviral and anticancer agents. Even if several synthetic pathways have been disclosed for the preparation of these nucleosides, a direct, few steps and modular approaches are still lacking. In line with our previous efforts, we report herein a one step - eco-friendly β-ribosylation of aryles and heteroaryles through a direct Friedel-Craft ribosylation mediated by bismuth triflate, Bi(OTf)₃. The resulting carbohydrates have been functionalized by cross-coupling reactions, leading to a series of new C-aryl-nucleosides (32 compounds). Among them, we observed that 5d exerts promising anti-proliferative effects against two human Chronic Myeloid Leukemia (CML) cell lines, both sensitive (K562-S) or resistant (K562-R) to imatinib, the "gold standard of care" used in this pathology. Moreover, we demonstrated that 5d kills CML cells by a non-conventional mechanism of cell death.

α-Alkylidene-γ-butyrolactone Formation via Bi(OTf)3-Catalyzed, Dehydrative, Ring-Opening Cyclizations of Cyclopropyl Carbinols: Understanding Substituent Effects and Predicting E/Z Selectivity

A Bi(OTf)₃-catalyzed ring-opening cyclization of (hetero)aryl cyclopropyl carbinols to form α-alkylidene-γ-butyrolactones (ABLs) is reported. This transformation represents different chemoselectivity from previous reports that demonstrated formation of (hetero)aryl-fused cyclohexa-1,3-dienes upon acid-promoted cyclopropyl carbinol ring opening. ABLs are obtained in up to 89% yield with a general preference for the E-isomers. Mechanistically, Bi(OTf)₃ serves as a stable and easy to handle precursor to TfOH. TfOH then catalyzes the formation of cyclopropyl carbinyl cations, which undergo ring opening, intramolecular trapping by the neighboring ester group, subsequent hydrolysis, and loss of methanol resulting in the formation of the ABLs. The nature and relative positioning of the substituents on both the carbinol and the cyclopropane determine both chemo- and stereoselective outcomes. Carbinol substituents determine the extent of cyclopropyl carbinyl cation formation. The cyclopropane donor substituents determine the overall reaction chemoselectivity. Weakly stabilizing or electron-poor donor groups provide better yields of the ABL products. In contrast, copious amounts of competing products are observed with highly stabilizing cyclopropane donor substituents. Finally, a predictive model for E/Z selectivity was developed using DFT calculations.

Synthesis of ent-[3]-Ladderanol: Development and Application of Intramolecular Chirality Transfer [2+2] Cycloadditions of Allenic Ketones and Alkenes

An enantioselective synthesis of ent-[3]-ladderanol is presented. The ladderanes are an interesting class of molecules for their unique structure of fused cyclobutane rings as well as their perceived biological function of organism protection. The route hinges on the development and application of a chirality transfer [2+2] cycloaddition of an allenic ketone and alkene. Further stereocontrolled transformations allowed for completion of the synthesis. The scope of the chirality transfer [2+2] cycloaddition is also presented.

Niobium-Catalyzed Intramolecular Addition of O-H and N-H Bonds to Alkenes: A Tool for Hydrofunctionalization

A convenient, versatile, and easy to handle intramolecular hydrofunctionalization of alkenes (C-O and C-N bonds formation) is reported using a novel niobium-based catalytic system. This atom economic and eco-friendly methodology provides an additional synthetic tool for the straightforward formation of valuable building blocks enabling molecular complexity. Various pyran, furan, pyrrolidine, piperidine, lactone, and lactam derivatives as well as spirocyclic compounds are produced in high yields and selectivities.