Brønsted acid catalysis of photosensitized cycloadditions

Catalysis is central to contemporary synthetic chemistry. There has been a recent recognition that the rates of photochemical reactions can be profoundly impacted by the use of Lewis acid catalysts and co-catalysts. Herein, we show that Brønsted acids can also modulate the reactivity of excited-state organic reactions. Brønsted acids dramatically increase the rate of Ru(bpy)₃²⁺-sensitized [2 + 2] photocycloadditions between C-cinnamoyl imidazoles and a range of electron-rich alkene reaction partners. A combination of experimental and computational studies supports a mechanism in which the Brønsted acid co-catalyst accelerates triplet energy transfer from the excited-state [Ru*(bpy)₃]²⁺ chromophore to the Brønsted acid activated C-cinnamoyl imidazole. Computational evidence further suggests the importance of driving force as well as geometrical reorganization, in which the protonation of the imidazole decreases the reorganization penalty during the energy transfer event.

Brønsted acids can catalyze triplet energy transfer reactions, and DFT computations suggest the unexpected importance of reorganization energy for catalysis.     

Cooperative activating effects of metal ion and Brønsted acid on a metal oxo species

Metal oxo (M O) complexes are common oxidants in chemical and biological systems. The use of Lewis acids to activate metal oxo species has attracted great interest in recent years, especially after the discovery of the CaMn₄O₅ cluster in the oxygen-evolving centre of photosystem II. Strong Lewis acids such as Sc³⁺ and BF₃, as well as strong Brønsted acids such as H₂SO₄ and CF₃SO₃H, are commonly used to activate metal oxo species. In this work, we demonstrate that relatively weak Lewis acids such as Ca²⁺ and other group 2 metal ions, as well as weak Brønsted acids such as CH₃CO₂H, can readily activate the stable RuO₄⁻ complex towards the oxidation of alkanes. Notably, the use of Ca²⁺ and CH₃CO₂H together produces a remarkable cooperative effect on RuO₄⁻, resulting in a much more efficient oxidant. DFT calculations show that Ca²⁺ and CH₃CO₂H can bind to two oxo ligands to form a chelate ring. This results in substantial lowering of the barrier for hydrogen atom abstraction from cyclohexane.

Combining a weak Lewis acid and weak Brønsted acid produces strong cooperative effects for activating metal oxo species towards alkane oxidation.     

Friedel-Crafts-Type Acylation and Amidation Reactions in Strong Brønsted Acid: Taming Superelectrophiles

In this review, we discuss Friedel-Crafts-type aromatic amidation and acylation reactions, not exhaustively, but mainly based on our research results. The electrophilic species involved are isocyanate cation and acylium cation, respectively, and both have a common ⁺C=O structure, which can be generated from carboxylic acid functionalities in a strong Brønsted acid. Carbamates substituted with methyl salicylate can be easily ionized to the isocyanate cation upon (di)protonation of the salicylate. Carboxylic acids can be used directly as a source of acylium cations. However, aminocarboxylic acids are inert in acidic media because two positively charged sites, ammonium and acylium cation, will be generated, resulting in energetically unfavorable charge-charge repulsion. Nevertheless, the aromatic acylation of aminocarboxylic acids can be achieved by using tailored phosphoric acid esters as Lewis bases to abrogate the charge-charge repulsion. Both examples tame the superelectrophilic character.     

Bidentate substrate binding in Brønsted acid catalysis: structural space,hydrogen bonding and dimerization

BINOL derived chiral phosphoric acids (CPAs) are a prominent class of catalysts in the field of asymmetric organocatalysis, capable of transforming a wide selection of substrates with high stereoselectivities. Exploiting the Brønsted acidic and basic dual functionality of CPAs, substrates with both a hydrogen bond acceptor and donor functionality are frequently used as the resulting bidentate binding via two hydrogen bonds is expected to strongly confine the possible structural space and thus yield high stereoselectivities. Despite the huge success of CPAs and the popularity of a bidentate binding motif, experimental insights into their organization and origin of stereoinduction are scarce. Therefore, in this work the structural space and hydrogen bonding of CPAs and N-(ortho-hydroxyaryl) imines (19 CPA/imine combinations) was elucidated by low temperature NMR studies and corroborated by computations. The postulated bidentate binding of catalyst and substrate by two hydrogen bonds was experimentally validated by detection of trans-hydrogen bond scalar couplings. Counterintuitively, the resulting CPA/imine complexes showed a broad potential structural space and a strong preference towards the formation of [CPA/imine]₂ dimers. Molecular dynamics simulations showed that in these dimers, the imines form each one hydrogen bond to two CPA molecules, effectively bridging them. By finetuning steric repulsion and noncovalent interactions, rigid and well-defined CPA/imine monomers could be obtained. NOESY studies corroborated by theoretical calculations revealed the structure of that complex, in which the imine is located in between the 3,3′-substituents of the catalyst and one site of the substrate is shielded by the catalyst, pinpointing the origin or stereoselectivity for downstream transformations.

Brønsted acid/substrate complexes with bidentate binding motif were studied by NMR and molecular dynamics. A variety of different arrangements was found, including bridged dimers and monomers were characterised in detail.     

Nickel/Brønsted acid dual-catalyzed regio- and enantioselective hydrophosphinylation of 1,3-dienes: access to chiral allylic phosphine oxides

While chiral allylic organophosphorus compounds are widely utilized in asymmetric catalysis and for accessing bioactive molecules, their synthetic methods are still very limited. We report the development of asymmetric nickel/Brønsted acid dual-catalyzed hydrophosphinylation of 1,3-dienes with phosphine oxides. This reaction is characterized by an inexpensive chiral catalyst, broad substrate scope, and high regio- and enantioselectivity. This study allows the construction of chiral allylic phosphine oxides in a highly economic and efficient manner. Preliminary mechanistic investigations suggest that the 1,3-diene insertion into the chiral Ni–H species is a highly regioselective process and the formation of the chiral C–P bond is an irreversible step.

Asymmetric hydrophosphinylation of 1,3-dienes with phosphine oxides using an inexpensive chiral catalyst has been demonstrated, providing access to chiral allylic phosphine oxides with broad substrate scope and high regio- and enantioselectivity.     

Dynamic parallel kinetic resolution of α-ferrocenyl cation initiated by chiral Brønsted acid catalyst

The dynamic parallel kinetic resolution (DPKR) of an α-ferrocenyl cation intermediate under the influence of a chiral conjugate base of a chiral phosphoric acid catalyst has been demonstrated in an S_N1 type substitution reaction of a racemic ferrocenyl derivative with a nitrogen nucleophile. The present method provides efficient access to a ferrocenylethylamine derivative in a highly enantioselective manner, which is potentially useful as a key precursor of chiral ligands for metal catalysis. The mechanism of the present intriguing resolution system was elucidated by control experiments using the enantio-pure precursor of relevant α-ferrocenyl cation intermediates and the hydroamination of vinylferrocene. Further theoretical studies enabled the elucidation of the origin of the stereochemical outcome as well as the efficient DPKR. The present DPKR, which opens a new frontier for kinetic resolution, involves the racemization process through the formation of vinylferrocene and the chemo-divergent parallel kinetic resolution of the enantiomeric α-ferrocenyl cations generated by the protonation/deprotonation sequence of vinylferrocene.

The dynamic parallel kinetic resolution (DPKR) of an enantiomeric α-ferrocenyl cation using a chiral phosphate anion of an acid catalyst was accomplished by the combination of the PKR and the racemization through the formation of vinylferrocene.     

Tailoring Lewis/Brønsted acid properties of MOF nodes via hydrothermal and solvothermal synthesis: simple approach with exceptional catalytic implications

The Lewis/Brønsted catalytic properties of the Metal–Organic Framework (MOF) nodes can be tuned by simply controlling the solvent employed in the synthetic procedure. In this work, we demonstrate that Hf-MOF-808 can be prepared from a material with a higher amount of Brønsted acid sites, via modulated hydrothermal synthesis, to a material with a higher proportion of unsaturated Hf Lewis acid sites, via modulated solvothermal synthesis. The Lewis/Brønsted acid properties of the resultant metallic clusters have been studied by different characterization techniques, including XAS, FTIR and NMR spectroscopies, combined with a DFT study. The different nature of the Hf-MOF-808 materials allows their application as selective catalysts in different target reactions requiring Lewis, Brønsted or Lewis–Brønsted acid pairs.

The Brønsted/Lewis acid properties of Hf-MOF-808 can be tuned by simply controlling the solvent employed in its synthesis, with direct catalytic implications on the activity and selectivity of organic reactions sensitive to the active site nature.     

An induced-fit model for asymmetric organocatalytic reactions: a case study of the activation of olefins via chiral Brønsted acid catalysts

We elucidate the stereo-controlling factors of the asymmetric intramolecular hydroalkoxylation of terminal olefins catalyzed by bulky Brønsted acids [Science2018, 359 (6383), 1501–1505] using high-level electronic structure methods. The catalyst–substrate interaction is described using a dispersion-driven induced-fit model, in which the conformational changes of the catalyst and of the substrate in the transition states are governed to a large extent by London dispersion forces. The distortion energy of the catalyst is dominated by the change in the intramolecular dispersion interactions, while intermolecular catalyst–substrate dispersion interactions are the major stabilizing contribution in the transition state. This model provides a new general framework in which to discuss the stereoselectivity of transformations catalyzed by such confined organocatalysts.

We elucidate the stereo-controlling factors of the asymmetric intramolecular hydroalkoxylation of terminal olefins catalyzed by bulky Brønsted acids [Science2018, 359 (6383), 1501–1505] using high-level electronic structure methods.     

Synergistic Brønsted/Lewis acid catalyzed aromatic alkylation with unactivated tertiary alcohols or di-tert-butylperoxide to synthesize quaternary carbon centers

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-butylation of electron-rich arenes using di-tert-butylperoxide. This transformation inspired the development of a synergistic Brønsted/Lewis acid catalyzed aromatic alkylation that fills a gap in the Friedel–Crafts reaction literature by employing unactivated tertiary alcohols as alkylating agents, leading to new quaternary carbon centers. Corroborated by DFT calculations, the Lewis acid serves a role in enhancing the acidity of the Brønsted acid. The use of non-allylic, non-benzylic, and non-propargylic tertiary alcohols represents an underexplored area in Friedel–Crafts reactivity.

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-alkylation of arenes using di-tert-butylperoxide and tertiary alcohols.     

Synthesis of Cyano-Benzylidene Xanthene Synthons Using a Diprotic Brønsted Acid Catalyst,and Their Application as Efficient Inhibitors of Aluminum Corrosion in Alkaline Solutions

Novel cyano-benzylidene xanthene derivatives were synthesized using one-pot and condensation reactions. A diprotic Brønsted acid (i.e., oxalic acid) was used as an effective catalyst for the promotion of the synthesis process of the new starting xanthene–aldehyde compound. Different xanthene concentrations (ca. 0.1–2.0 mM) were applied as corrosion inhibitors to control the alkaline uniform corrosion of aluminum. Measurements were conducted in 1.0 M NaOH solution using Tafel extrapolation and linear polarization resistance (LPR) methods. The investigated xanthenes acted as mixed-type inhibitors that primarily affect the anodic process. Their inhibition efficiency values were enhanced with inhibitor concentration, and varied according to their chemical structures. At a concentration of 2.0 mM, the best-performing studied xanthene derivative recorded maximum inhibition efficiency values of 98.9% (calculated via the Tafel extrapolation method) and 98.4% (estimated via the LPR method). Scanning electron microscopy (SEM) was used to examine the morphology of the corroded and inhibited aluminum surfaces, revealing strong inhibitory action of each studied compound. High-resolution X-ray photoelectron spectroscopy (XPS) profiles validated the inhibitor compounds’ adsorption on the Al surface. Density functional theory (DFT) and Monte Carlo simulations were applied to investigate the distinction of the anticorrosive behavior among the studied xanthenes toward the Al (111) surface. The non-planarity of xanthenes and the presence of the nitrile group were the key players in the adsorption process. A match between the experimental and theoretical findings was evidenced.     

从铜离子、酸中心与铝分布的关系分析不同模板剂制备Cu-SSZ-13的NH3-SCR性能

铜离子改性的SSZ-13沸石是以氨气为还原剂选择催化还原柴油发动机尾气中氮氧化物反应(NH₃-SCR)的优良催化剂。本文综述并具体分析了酸中心位点对于Cu-SSZ-13中铜离子落位、迁移的影响,以及骨架铝分布对其决定性的作用,强调了“成对”酸中心,“强铝对”对于催化剂水热稳定性的重要作用,并总结了目前控制“铝对”形成的方法。以此为基础分析了不同有机模板剂、共模板剂法制备的Cu-SSZ-13在催化NH₃-SCR反应中的表现,为使用廉价模板剂或共模板剂替代TMADaOH合成具有良好NH₃-SCR催化活性和水热稳定性的Cu-SSZ-13提供参考。     

Switch From Pauli‐Lowering to LUMO‐Lowering Catalysis in Brønsted Acid‐Catalyzed Aza‐Diels‐Alder Reactions

Brønsted acid‐catalyzed inverse‐electron demand (IED) aza‐Diels‐Alder reactions between 2‐aza‐dienes and ethylene were studied using quantum chemical calculations. The computed activation energy systematically decreases as the basic sites of the diene progressively become protonated. Our activation strain and Kohn‐Sham molecular orbital analyses traced the origin of this enhanced reactivity to i) “Pauli‐lowering catalysis” for mono‐protonated 2‐aza‐dienes due to the induction of an asynchronous, but still concerted, reaction pathway that reduces the Pauli repulsion between the reactants; and ii) “LUMO‐lowering catalysis” for multi‐protonated 2‐aza‐dienes due to their highly stabilized LUMO(s) and more concerted synchronous reaction path that facilitates more efficient orbital overlaps in IED interactions. In all, we illustrate how the novel concept of “Pauli‐lowering catalysis” can be overruled by the traditional concept of “LUMO‐lowering catalysis” when the degree of LUMO stabilization is extreme as in the case of multi‐protonated 2‐aza‐dienes.     

On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Lewis acidic aluminum in zeolites, particularly acidity that is inherent to the framework, is an indeterminate concept. A fraction of framework aluminum changes geometry to octahedral coordination in the proton form of zeolite mordenite. Such octahedrally coordinated aluminum is the precursor of a Lewis acid site and its formation is accompanied by a loss in Brønsted acidity. Herein, we show that such Lewis acid sites have a preferred location in the pore structure of mordenite. A greater proportion of these Lewis acid sites resides in the side-pockets than in the main channel. By reverting the octahedrally coordinated aluminum back to a tetrahedral geometry, the corresponding Brønsted acid sites are restored with a concomitant loss in the ability to form Lewis acid sites. Thereby, reversible octahedral–tetrahedral aluminum coordination provides a means to indirectly switch between Lewis and Brønsted acidity. This phenomenon is unique to Lewis acidity that is inherent to the framework, thereby distinguishing it from Lewis acidity originating from extra-framework species. Furthermore, the transformation of framework aluminum into octahedral coordination is decoupled from the generation of distorted tetrahedrally coordinated aluminum, where the latter gives rise to the IR band at 3660 cm⁻¹ in the OH stretching region.

Framework-associated aluminum is demonstrated to facilitate a reversible switch between Lewis and Brønsted acidity in zeolites with the Lewis acid sites preferentially populating the side-pockets in the case of mordenite.     

Acid Leaching Vermiculite: A Multi-Functional Solid Catalyst with a Strongly Electrostatic Field and Brönsted Acid for Depolymerization of Cellulose in Water

Vermiculite is a natural mineral. In this study, vermiculite and acid-activated vermiculite was used as a solid acid catalyst for the hydrolysis of cellulose in water. The catalysts were characterized by XRD, FT-IR, and BET. The effects of time, temperature, mass ratio and water amount on the reaction were investigated in the batch reactor. The results showed that the highest total reducing sugars (TRS) yield of 40.1% could be obtained on the vermiculite activated by 35 (wt)% H₂SO₄ with the mass ratio of catalyst to cellulose of 0.18 and water to cellulose of 16 at 478 K for 3.5 h. The acid-activated vermiculite was a stable catalyst through calcination at 628 K and the yield of TRS decreased to 36.2% after three times reuse. The results showed that the crystal structure of vermiculite was destroyed and the surface -OH groups increased after the acid treatment. However, the synergistic effect of a strongly electrostatic polarization and Brönsted acid was responsible for the efficient conversion of cellulose. The mechanism of cellulose hydrolysis on the acid-activated vermiculite was suggested. This work provides a promising strategy to design an efficient solid catalyst for the cellulose hydrolysis, and expands the use of vermiculite in a new field.     

Formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloaddition reactions of donor–acceptor cyclobutenes,cyclopropenes and siloxyalkynes induced by Brønsted acid catalysis

Brønsted acid catalyzed formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloadditions of donor–acceptor cyclobutenes, cyclopropenes, and siloxyalkynes with benzopyrylium ions are reported. [4 + 2]-cyclization/deMayo-type ring-extension cascade processes produce highly functionalized benzocyclooctatrienes, benzocycloheptatrienes, and 2-naphthols in good to excellent yields and selectivities. Moreover, the optical purity of reactant donor–acceptor cyclobutenes is fully retained during the cascade. The 1,3-dicarbonyl product framework of the reaction products provides opportunities for salen-type ligand syntheses and the construction of fused pyrazoles and isoxazoles that reveal a novel rotamer-diastereoisomerism.

Brønsted acid catalysis realizes formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloadditions of donor–acceptor cyclobutenes, cyclopropanes, and siloxyalkynes with benzopyrylium ions.

Medium-sized rings are the core skeletons of many natural products and bioactive molecules,¹ and a growing number of strategies have been developed for their synthesis.² Because of their enthalpic and entropic advantages, ring expansion is a highly efficient methodology for these constructions.^3–6 For example, Sun and co-workers have developed acid promoted ring extensions of oxetenium and azetidinium species formed from siloxyalkynes with cyclic acetals and hemiaminals (Scheme 1a).⁴ Takasu and co-workers have reported an elegant ring expansion with a palladium(ii) catalyzed 4π-electrocyclic ring-opening/Heck arylation cascade with fused cyclobutenes (Scheme 1b).⁵ Each transformation is initiated by the formation of fused bicyclic units followed by ring expansion or rearrangement to give medium-sized rings.Open in a separate windowScheme 1Cycloaddition/ring expansion background and this work.Strategies for the formation of fused bicyclic compounds rely on cycloaddition of dienes or dipoles with unsaturated cyclic compounds⁷ and, if the reactant cyclic compound is strained and chiral, the resulting bicyclic compound is activated toward ring opening that results in retention of chirality. We have recently reported access to donor–acceptor cycloalkenes by [3 + n] cycloaddition that have the prerequisites of unsaturated cyclic compounds suitable for cycloaddition.⁸ Donor–acceptor (D–A) cyclopropenes⁹ and cyclobutenes¹⁰ have sufficient strain in the resulting bicyclic compounds to undergo ring opening. We envision that the selection of a diene or dipolar reactant and suitable reaction conditions could realize cycloaddition and subsequent ring expansion. Benzopyrylium species,^11,12 which are generated by metal or acid catalysis, have attracted our attention. We anticipated that their high reactivity would overcome the conventional unfavorable kinetic and/or thermodynamic factors that typically impede medium-sized ring formation. From a mechanistic perspective, benzopyrylium species are often formed by transition metal catalyzed reactions with 2-alkynylbenzaldehydes.¹¹ Recently, the reaction between 1H-isochromene acetal and Brønsted acid catalyst forms the 2-benzopyrylium salts that could react with functional alkenes to give same cycloaddition products.¹² Consequently, we believed that the [4 + 2]-cyclization between benzopyrylium species and donor–acceptor cyclobutenes, cyclopropenes, or siloxyalkynes would give bridged oxetenium intermediates, which contain high strain energy that should provide the driving force for ring expansion. The “push and pull” electronic effect of donor–acceptor functional groups facilitates deMayo-type ring-opening of the cyclobutane or cyclopropane skeletons (Scheme 1c).¹³Here we report bis(trifluoromethanesulfonyl)imide (HNTf₂) catalyzed formal [4 + 4]-, [4 + 3]- and [4 + 2]-cycloaddition reactions of D–A cyclobutenes, cyclopropenes, and siloxyalkynes with benzopyrylium salts. Polysubstituted benzo-cyclooctatrienes, benzocycloheptatrienes and 2-naphthols, are produced in good to excellent yields and selectivities. Complete retention of configuration occurs using chiral cyclobutenes, and opportunities for further functionalization are built into these constructions.Initially, we conducted transition metal catalyzed reactions with 2-alkynylbenzaldehyde intending to produce the corresponding benzopyrylium ion and explore the possibility of cycloaddition/ring opening with donor–acceptor cyclobutene 2a. Use of Ph₃PAuCl/AgSbF₆, Pd(OAc)₂ and Cu(OTf)₂, which were efficient catalysts in previous transformations,¹¹ gave only a trace amount of cycloaddition product (Fig. 1). Spectral analysis showed that mostly starting material remained (Fig. 1-i and ii). Increasing the reaction temperature led only to decomposition of 2-alkynylbenzaldehyde (Fig. 1-iv) or donor–acceptor cyclobutene 2a (Fig. 1-iv).Open in a separate windowFig. 1Reaction of 2-alkynylbenzaldehyde with donor–acceptor cyclobutene 2a.From these disappointments we turned our attention to 1H-isochromene acetal 1a as the benzopyrylium ion precursor. Various Lewis acid catalysts were employed with limited success, but we were pleased to observe the formation of the desired formal [4 + 4]-cycloaddition benzocyclooctatriene product 3aa, albeit in low yields (Table 1, entries 1–6). Selection of the Brønsted super acid HNTf₂ (ref. 14) proved to be the most promising, producing 3aa in 40% yield (Table 1, entry 7). Increasing the amount of D–A cyclobutene 2a by 30% gave a much higher yield of 3aa (Table 1, entry 8 vs. 7). Optimization of the stoichiometric reaction between 1a and 2a by increasing the reaction temperature from rt to 35 °C led to the formation of the desired product in 76% isolated yield (Table 1, entry 9 vs. 8). However, a further increase in the reaction temperature (Table 1, entry 10) or reducing the catalyst loading to 5 mol% did not improve the yield of 3aa (Table 1, entry 11).Optimization of reaction conditions^a

Brönsted酸性离子液体催化芳香醛和2-甲基喹啉反应合成1,3-二(2-喹啉基)丙烷化合物

以Brönsted酸性离子液体1-乙基-3-丁基咪唑对甲苯磺酸盐为催化剂, 将芳香醛和2-甲基喹啉类化合物在无溶剂或甲苯中于120 ℃反应48 h, 制备了一系列的1,3-二(2-喹啉基)丙烷化合物, 产率56%~92%, 产物结构经核磁共振波谱和高分辨质谱确证. 该方法具有简便易行、 产率较高等特点.     

Assessing the Brønsted Basicity of Diaminoboryl Anions: Reactivity toward Methylated Benzenes and Dihydrogen

Treatment of toluene or p‐xylene with diaminoboryllithium results in consecutive reactions, involving boryl‐anion‐mediated deprotonation at the benzylic position followed by nucleophilic substitution at the boron center, producing benzylborane species and LiH. Diaminoboryllithium also cleaves H₂ heterolytically affording diaminohydroborane and LiH, while the reaction of lithium diaminoboryl(bromo)cuprate with H₂ takes place accompanied by reduction of Cu^I to give diaminohydroborane, LiH, and Cu⁰.     

5,5,5-Trichloropent-3-en-one as a Precursor of 1,3-Bi-centered Electrophile in Reactions with Arenes in Brønsted Superacid CF3SO3H. Synthesis of 3-Methyl-1-trichloromethylindenes

Reactions of 5,5,5-trichloropent-3-en-2-one Cl₃CCH=CHC(=O)Me with arenes in Brønsted superacid CF₃SO₃H at room temperature for 2 h–5 days afford 3-methyl-1-trichloromethylindenes, a novel class of indene derivatives. The key reactive intermediate, O-protonated form of starting compound Cl₃CCH=CHC(=OH⁺)Me, has been studied experimentally by NMR in CF₃SO₃H and theoretically by DFT calculations. The reaction proceeds through initial hydroarylation of the carbon-carbon double bond of starting CCl₃-enone, followed by cyclization onto the O-protonated carbonyl group, leading to target indenes. In general, 5,5,5-trichloropent-3-en-2-one in CF₃SO₃H acts as a 1,3-bi-centered electrophile.