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A highly enantioselective Mannich reaction of biphenyl‐bridged seven‐membered cyclic N‐sulfonylimines with methyl alkyl ketones is disclosed in this study. The reaction was performed under organocatalysis by using a quinine‐derived primary amine as the catalyst in combination with a Brønsted acid as the co‐catalyst. High yields (up to 89 %) and excellent enantioselectivities (up to 97 % ee) were observed. For methyl alkyl ketones containing a larger alkyl substituent, specific regioselective addition to the C=N bond is favored at the methyl group. On the contrary, ketones containing a smaller alkyl substituent or hydroxyacetone substrates gave major syn selective Mannich products at the methylene group. [ABSTRACT FROM AUTHOR]

Here we show that the sp‐hybridized nitrogen cation is strongly stabilized by a peri‐iodine substituent in the tetralone system. The cation is captured by anionic species such as CF3CO2−, affording hypervalent iodine(III) compounds with a short nitrogen‐iodine (N−I) bond, in which the cation serves as a Lewis acid. Notably, the O−I bond of the O‐trifluoroacetate or O‐acetate is intrinsically weaker than the N−I bond due to its more ionic character and is further weakened by protonation in trifluoroacetic acid. As a result, the oxygen ligand can dissociate in the presence of a Brønsted acid, affording a I+ cation intermediate that retains the N−I bond. We isolated the cation as the tetrafluoroborate, and characterized it experimentally by 1H NMR spectroscopy and X‐ray structure analysis, and theoretically by means of DFT calculation. The results suggest that the N−I bonded cation is intrinsically stable, and is weakly coordinated with water and the BF4 counter anion or trifluoroacetate anion. This cation can be employed as a reagent for α‐oxidation of ketones. [ABSTRACT FROM AUTHOR]

γ‐Valerolactone (GVL) is considered as a star biochemical which can be used as a green solvent, fuel additive and versatile organic intermediate. In this study, metal triflate (M(OTf)n) was utilized as the catalyst for one‐pot transformation of furfural (FF) to GVL in alcohol media under microwave irradiation. Alcohol plays multiple functions including solvent, hydrogen donor and alcoholysis reagent in this cascade reaction process. And process efficiency of GVL production from FF upgrading is strongly related to the effective charge density of selected catalyst and the reduction potential of selected alcohol. Complex (OTf)n‐M−O(H)R, presenting both Brønsted acid and Lewis acid, is the real catalytic active species in this cascade reaction process. Among various catalysts, Sc(OTf)3 exhibited the best catalytic activity for GVL production. Various reaction parameters including the Sc(OTf)3 amount, reaction temperature and time were optimized by the response surface methodology with the central composite design (RSM‐CCD). Up to 81.2 % GVL yield and 100 % FF conversion were achieved at 143.9 °C after 8.1 h in the presence of 0.16 mmol catalyst. This catalyst exhibits high reusability and can be regenerated by oxidative degradation of humins. In addition, a plausible cascade reaction network was proposed based on the distribution of product. [ABSTRACT FROM AUTHOR]

Isolation and characterization of highly reactive intermediates are crucial to understand the nature of chemical reactivity. Accordingly, the reactivity of weakly coordinating anions (WCA), usually used to stabilized cationic super electrophiles are of fundamental interest. When a variety of WCA are known to form stable σ‐complexes with a proton, inducing Brønsted super acidity, bis‐coordinated weak‐coordinated anions are much more elusive and considered as long‐sought reactive species. In this work, the chemistry of borylated sulfate, triflimidate and triflate anions were scouted in details with the aim of synthetizing the unique analogs of protonated Brønsted superacids. Those complexes were formed by successive borylation with a 9‐boratriptycene derived Lewis super acid paired with a weak coordinated anion, characterized in solution and in the solid state and exhibit unique structures and reactivities. [ABSTRACT FROM AUTHOR]

A dual catalytic approach combining photocatalyst and selenium‐π‐acid synergy has been used to cyclized of N‐propargylamides. This method offers readily access to oxazole aldehydes under chemical oxidant‐free conditions with low catalyst loadings, where air acts as a terminal and gratuitous oxidant. The reaction is demonstrated with a range of substrates, including aryl and alkyl propargyl amides, and in the late‐stage functionalization of several amide‐containing drug molecules. Mechanistic studies suggest that the acridinium catalyst is able to oxidize diselenide and generate singlet oxygen (1O2), which is responsible for this transformation. [ABSTRACT FROM AUTHOR]

Since the pioneer reports of the groups of Akiyama and Terada on Brønsted acid organocatalysis, this field never stopped growing with the development of ingenious strategies for the activation of challenging poorly reactive substrates. The development of superacidic organocatalysts is an important way to selectively functionalize reluctant electrophiles and other approaches have also emerged such as the combination of Lewis and Brønsted acids as well as the consecutive organocatalysis and superacid activation. This Concept aims to highlight these different strategies and demonstrate their complementarity. [ABSTRACT FROM AUTHOR]

The usage of hierarchical MFI zeolite enables a boost of the catalytic performance of Mo‐based catalysts for the olefin‐metathesis reaction. The harvest of active catalysts roots in a segmental evolution track between hierarchical zeolite and Al2O3 slices for the fabrication of active sites. The working evolution track requires the indispensable engagements from intracrystalline mesoporous surface, Al2O3 slices, and zeolitic Brønsted acid sites. The infilling of disaggregated Al2O3 slices into the intracrystalline mesopores triggers the creation of localized intrazeolite−Al2O3 interfaces, which enables the subsequent migration and trapping of surface molybdates into the micropores. The insulation of intrazeolite−Al2O3 interface or shielding of zeolitic Brønsted acid sites leads to the break of the evolution track. Our findings disclose the hidden functionality of mesoporosity as intrazeolite interface boundary for the fabrication of active sites and supply a new strategy for the rational design of zeolite catalysts. [ABSTRACT FROM AUTHOR]

The Cloke‐Wilson rearrangement is an important method to construct heterocycles which was conventionally facilitated by strong Lewis acids, Brønsted acids and Lewis bases. A weak interaction catalysis approach to the Cloke‐Wilson rearrangement remains a challenging topic and yet no example is reported. Herein, a chalcogen bonding catalysis method to achieve the Cloke‐Wilson rearrangement is described that involves activation of carbonyl cyclopropanes by double Se⋅⋅⋅O interactions. This chalcogen bonding catalysis approach afforded a wide range of dihydrofurans with good yields, thus establishing an alternative strategy to catalyze the Cloke‐Wilson rearrangement. [ABSTRACT FROM AUTHOR]

A photo‐induced indole 2‐alkylation reaction was developed with unactivated stereo‐congested alkenes used as the Friedel–Crafts alkylation reagent. The neutral, traceless and inexpensive perfluoroalkyl iodide C4F9I was used as the radical initiator in catalytic amount under irradiation by a blue LED light. Brønsted acids are in situ generated from the radical transfer reactions between the indole substrates and the perfluoroiodide catalyst. A variety of 2‐branched alkylated indole molecules could be obtained from this approach in generally good efficiencies, with a broad scope of functional groups well tolerated. Mechanistic studies via both experimental and computational methods indicate that the reaction was accelerated by the protons generated from the crack of the indole‐derived radical cation species. [ABSTRACT FROM AUTHOR]

A Friedel‐Crafts alkylation of electron‐deficient arenes with aldehydes through ′′catalyst activation′′ is presented. Through hydrogen bonding interactions, the solvent 1,1,1,3,3,3, ‐hexafluoroisopropanol (HFIP) interacted with the added Brønsted acid catalyst pTSA•H2O, increasing its acidity. This activated catalyst enabled the Friedel–Crafts alkylation of electron‐neutral as well as electron‐deficient arenes. Strongly electron withdrawing arenes including arenes with multiple halogen atoms, NO2, CHO, CO2R, and CN, groups acted as efficient nucleophiles in this reaction. DFT studies reveal multiple roles of solvent HFIP viz; increasing the Brønsted acidity of the catalyst pTSA•H2O, and stabilization of the transition states through a concerted pathway enabling the challenging reaction. [ABSTRACT FROM AUTHOR]

A pyrazole‐based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO2) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV‐vis spectro‐electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro‐electrochemical studies under CO2 and CO atmosphere are consistent with a reduced CO‐containing dicobalt complex which results from the electroreduction of CO2. In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single‐site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at −1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO‐containing species formed during the electrocatalytic process, which inhibit the reduction of CO2. [ABSTRACT FROM AUTHOR]

The concept of Brønsted‐Lowry acids and bases is long and widely recognized as the most reasonable theory to explain the behavior of H+ ions. Here, we report a Brønsted acid/base pair that does not follow this theory. Two heteronuclear metal complexes, in which Brønsted acid/base sites are sterically isolated, were synthesized and characterized. These sterically isolated sites exhibited anomalous behavior, wherein the H+ species encapsulated in the Brønsted acid site did not undergo a deprotonation reaction, and the corresponding protonation reaction at the Brønsted base site failed to proceed. As a result, two states that are in a relationship of a Brønsted acid/base pair stably exist over a wide pH range without any interconversion, generating a thermodynamically metastable state. Additionally, these two states exhibited distinct electron transfer abilities and reactivities. The system presented in this study is in sharp contrast with the traditional concept of Brønsted–Lowry acids and bases. [ABSTRACT FROM AUTHOR]

Kinetics of H/D hydrogen exchange between deuterated isobutane‐d10 and Brønsted acid sites (BAS) of three zeolite samples (H‐BEA, ZnO/H‐BEA, Zn2+/H‐BEA) were monitored with 1H MAS NMR in situ at 343–468 K. The regioselective H/D exchange in the methyl groups detected on H‐BEA can be rationalized in terms of the mechanism of indirect exchange, which involves protonation of the intermediate olefin and further hydride abstraction from the other alkane molecule by the formed carbenium ion. Loading of Zn species in the zeolite results in a decrease of the rate and an increase of the activation energy of the exchange. The loaded Zn species provide the tuning effect on the reaction occurrence, changing the mechanism from the indirect one to the mechanism of the direct exchange. [ABSTRACT FROM AUTHOR]

The linking of phosphoric acids via covalent or mechanical bonds has proven to be a successful strategy for the design of novel organocatalysts. Here, we present the first systematic investigation of singly‐linked and macrocyclic bisphosphoric acids, including their synthesis and their application in phase‐transfer and Brønsted acid catalysis. We found that the novel bisphosphoric acids show dramatically increased enantioselectivities in comparison to their monophosphoric acid analogues. However, the nature, length and number of linkers has a profound influence on the enantioselectivities. In the asymmetric dearomative fluorination via phase‐transfer catalysis, bisphosphoric acids with a single, rigid bisalkyne‐linker give the best results with moderate to good enantiomeric excesses. In contrast, bisphosphoric acids with flexible linkers give excellent enantioselectivities in the transfer‐hydrogenation of quinolines via cooperative Brønsted acid catalysis. In the latter case, sufficiently long linkers are needed for high stereoselectivities, as found experimentally and supported by DFT calculations. [ABSTRACT FROM AUTHOR]

The identification of acid and nonacid species at the external surface of zeolites remains a major challenge, in contrast to the extensively‐studied internal acid sites. Here, it is shown that the synthesis of zeolite ZSM‐5 samples with distinct particle sizes, combined with solid‐state NMR and computational studies of trimethylphosphine oxide (TMPO) adsorption, provides insight into the chemical species on the external surface of the zeolite crystals. 1H–31P HETCOR NMR spectra of TMPO‐loaded zeolites exhibit a broad correlation peak at δP ∼35–55 ppm and δH ∼5–12 ppm assigned to external SiOH species. Pore‐mouth Brønsted acid sites exhibit 31P and 1H NMR resonances and adsorption energies close to those reported for internal acid sites interacting with TMPO. The presence of an external tricoordinate Al‐Lewis site interacting strongly with TMPO is suggested, resulting in 31P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites. [ABSTRACT FROM AUTHOR]

We have studied the enantioselective transannular aminohalogenation reaction of unsaturated medium‐sized cyclic benzosulfonamides by using both chiral Brønsted acid and phase‐transfer catalysis. Under optimized conditions, a variety of bicyclic adducts can be obtained with good yields and high enantioselectivities. The mechanism of the reaction was also studied by using computational tools; we observed that the reaction involves the participation of a conformer of the nine‐membered cyclic substrate with planar chirality in which the stereochemical outcome is controlled by the relative reactivity of the two pseudorotational enantiomers when interacting with the chiral catalyst. [ABSTRACT FROM AUTHOR]

The reaction of a cyclic alkyl(amino)carbene (CAAC)‐stabilized thiazaborolo[5,4‐d]thiazaborole (TzbTzb) with strong Brønsted acids, such as HCl, HOTf (Tf=O2SCF3) and [H(OEt2)2][BArF4] (ArF=3,5‐(CF3)2C6H3), results in the protonation of both TzbTzb nitrogen atoms. In each case X‐ray crystallographic data show coordination of the counteranions (Cl−, OTf−, BArF4−) or solvent molecules (OEt2) to the doubly protonated fused heterocycle via hydrogen‐bonding interactions, the strength of which strongly influences the 1H NMR shift of the NH protons, enabling tuning of both the visible (yellow to red) and fluorescence (green to red) colors of these salts. DFT calculations reveal that the hydrogen bonding of the counteranion or solvent to the protonated nitrogen centers affects the intramolecular TzbTzb‐to‐CAAC charge transfer character involved in the S0→S1 transition, ultimately enabling fine‐tuning of their absorption and emission spectral features. [ABSTRACT FROM AUTHOR]

An efficient and mild reaction protocol for the decarbonylation of isocyanates has been developed using catalytic amounts of Lewis acidic boranes. The electronic nature (electron withdrawing, electron neutral, and electron donating) and the position of the substituents (ortho/meta/para) bound to isocyanate controls the chain length and composition of the products formed in the reaction. Detailed DFT studies were undertaken to account for the formation of the mono/di‐carboxamidation products and benzoxazolone compounds. [ABSTRACT FROM AUTHOR]

Dynamic combinatorial chemistry is a method widely used for generating responsive libraries of compounds, with applications ranging from chemical biology to materials science. It relies on dynamic covalent bonds that are able to form in a reversible manner in mild conditions, and therefore requires the discovery of new types of these bonds in order to progress. Amides, due to their high stability, have been scarcely used in this field and typically require an external catalyst or harsh conditions for exchange. Compounds able to undergo uncatalysed transamidation at room temperature are still rare exceptions. In this work, we describe reversible amide formation and transamidation in a class of compounds known as maleamic acids. Due to the presence of a carboxylic acid in β‐position, these compounds are in equilibrium with their anhydride and amine precursors in organic solvents at room temperature. First, we show that this equilibrium is responsive to external stimuli: by alternating the additions of a Brønsted acid and a base, we can switch between amide and anhydride several times without side‐reactions. Next, we prove that this equilibrium provides a pathway for reversible transamidation without any added catalyst, leading to thermodynamic distributions of amides at room temperature. Lastly, we use different preparation conditions and concentrations of Brønsted acid to access different library distributions, easily controlling the transition between kinetic and thermodynamic regimes. Our results show that maleamic acids can undergo transamidation in mild conditions in a reversible and tunable way, establishing them as a new addition to the toolbox of dynamic combinatorial chemistry. [ABSTRACT FROM AUTHOR]

Since the global peptide drug market demand has been predicted to increase, highly efficient and inexpensive mass scale peptides are required. However, the production process raises questions about the cost of energy input, scale‐up production, raw materials, and solvents treatment. This paper introduces 2 methods for the 2–4 mer oligopeptides bond formation for batch reaction utilizing 50–100 mol% of a mild Brønsted acid under the mild condition. One of the methods has been capably adapted to flow synthesis at room temperature using organic solvents with boiling points below 100 °C. The method applies the tert‐butoxycarbonyl amino methoxy group, forming the desired dipeptide without solvent at mild temperatures. Furthermore, the conversion of the carboxylic acid leaving the group to phenyl ester promotes peptide bond formation, and the reaction were applied to di, tri, and tetrapeptide bond formation in excellent yield without notable racemization at ambient temperature (up to >99 % yield and 99 : 1 dr). Finally, this study proposes this new production method to overcome the limited scale‐up production by reaction device scale: liquid phase biomimetic catalytic peptide flow synthesis utilizing a mild Brønsted acid. [ABSTRACT FROM AUTHOR]

We report a method for the synthesis of chiral vicinal chloroamines via asymmetric protonation of catalytically generated prochiral chloroenamines using chiral Brønsted acids. The process is highly enantioselective, with the origin of asymmetry and catalyst substituent effects elucidated by DFT calculations. We show the utility of the method as an approach to the synthesis of a broad range of heterocycle‐substituted aziridines by treatment of the chloroamines with base in a one‐pot process, as well as the utility of the process to allow access to vicinal diamines. [ABSTRACT FROM AUTHOR]

Catalytic reduction of oximes represents a direct efficient approach to synthesize valuable hydroxylamine derivatives. However this transformation presents significant challenges: oximes are hard to reduce and, if reactive, reductive cleavage of the weak N−O bond often leads to primary amine side products. The first suitable systems involved the use of platinum‐based heterogeneous catalysts with hydrogen as reductant and stoichiometric amounts of a strong Brønsted acid. More recently metal‐free and transition‐metal‐based homogeneous catalysts have been developed, which display the highest turnovers (up to 4000). In the asymmetric variants, the E/Z‐geometry of the oxime double bond affects significantly the stereoselectivity, sometimes requiring extra synthetic efforts in substrate preparation. This minireview provides an overview of the advances and limitations in catalytic oxime to hydroxylamine reduction. Emphasis is put on highlighting and comparing the practical aspects of the existing methods, such as their reaction conditions and substrate scope. Additionally, future directions for improving this young research area are suggested. [ABSTRACT FROM AUTHOR]

Boronic acids are Lewis acids that exist in equilibrium with boronate forms in aqueous solution. Here we experimentally and computationally investigated the Lewis acidity of 2,6‐diarylphenylboronic acids; specially designed phenylboronic acids that possess two flanking aromatic rings with tunable aromatic character. Hammett analysis of 2,6‐diarylphenylboronic acids reveals that their Lewis acidity remains unchanged upon the introduction of EWG/EDG at the distant para position of the flanking aromatic rings. Structural and computational studies demonstrate that polar‐π interactions and solvation effects contribute to the stabilization of boronic acids and boronate forms by aromatic rings. Our physical‐organic chemistry work highlights that boronic acids and boronates can be stabilized by aromatic systems, leading to an important molecular knowledge for rational design and development of boronic acid‐based catalysts and inhibitors of biomedically important proteins. [ABSTRACT FROM AUTHOR]

NH3 temperature‐programmed desorption (NH3‐TPD) is frequently used for probing the nature of the active sites in CuSSZ‐13 zeolite for selective catalytic reduction (SCR) of NOx. Herein, we propose an interpretation of NH3‐TPD results, which takes into account the temperature‐induced dynamics of NH3 interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first‐principles thermodynamic (FPT) modeling of NH3 adsorption on single Cu2+, Cu+, [CuOH]+ centers, dimeric [Cu‐O‐Cu]2+, [Cu‐O22−‐Cu]2 species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH3 desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low‐, medium‐ and high‐temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH3‐TPD experiment. Thus, a rigorous interpretation of the NH3‐TPD profiles of CuSSZ‐13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers. [ABSTRACT FROM AUTHOR]

The Brønsted acidity of the perfluorinated trialkoxysilanol {(F3C)3CO}3SiOH is more than 13 orders of magnitude higher than that of orthosilicic acid, Si(OH)4, and even more for most previously known silanols. It is easily deprotonated by simple amines and pyridines to give the conjugate silanolates [OSi{OC(CF3)3}3]−, which possess extremely short Si−O bonds, comparable to those of silanones. [ABSTRACT FROM AUTHOR]

The aryltellurenyl cation [2‐(tBuNCH)C6H4Te]+, a Lewis super acid, and the weakly coordinating carborane anion [CB11H12]−, an extremely weak Brønsted acid (pKa=131.0 in MeCN), form an isolable ion pair complex [2‐(tBuNCH)C6H4Te][CB11H12], in which the Brønsted acidity (pKa 7.4 in MeCN) of the formally hydridic B−H bonds is dramatically increased by more than 120 orders of magnitude. The electrophilic activation of B−H bonds in the carborane moiety gives rise to a proton transfer from boron to nitrogen at slightly elevated temperatures, as rationalized by the isolation of a mixture of the zwitterionic isomers 12‐ and 7‐[2‐(tBuN{H}CH)C6H4Te(CB11H11)] in ratios ranging from 62 : 38 to 80 : 20. [ABSTRACT FROM AUTHOR]

Over the past decades, the development of enantioselective catalysis using organocatalysts has evolved into an active research field and a number of enantioselective transformations have been established. However, despite their being a highly desirable process for the synthesis of organic molecules in an enantioenriched form, the enantioconvergent substitution reactions of racemic electrophiles using organocatalysts still present several challenges. Although intrinsic difficulties in the catalytic stereocontrol abound due to the initial chiral information of racemic electrophiles, in recent years, mechanistically diverse enantioconvergent processes have been intensively investigated in organocatalysis. This Minireview focuses on recent achievements in the development of enantioconvergent substitution reactions of racemic electrophiles using organocatalysts. The contents are classified on the basis of the mechanistic types of enantioconvergent processes. [ABSTRACT FROM AUTHOR]

Intermediary radical cations, generated through single‐electron oxidation of enol silyl ethers by excited Ir‐based photocatalysts, can be exploited as Brønsted acids for the activation of heteroarylcyanides. This strategy enables the direct allylic C−H heteroarylation of enol silyl ethers under visible‐light irradiation. [ABSTRACT FROM AUTHOR]

The different reactivity of trienones under Lewis and Brønsted acids catalysis was investigated, resulting in distinct cyclization products and carbon backbones that originated either from a conjugate Prins cyclization or an interrupted Nazarov cyclization. In particular, an unprecedented Nazarov cyclization tandem reaction is presented, terminating the oxyallyl cation by an ene‐type reaction, and leading stereoselectively to bicyclic spiro compounds. The terminal olefin of this motif represents a useful handle for further functionalization, making it a strategic intermediate in total syntheses. The tandem Nazarov/ene cyclization was shown to be preferred over a Nazarov/[3+2] tandem reaction for all our substrates, independent of chain length. Deuteration studies further support the mechanistic hypothesis of the terminating ene reaction. [ABSTRACT FROM AUTHOR]

Strain relief of oxetanes offers a plethora of opportunities for the synthesis of chiral alcohols and ethers. In this context, enantioselective desymmetrization has been identified as a powerful tool to construct molecular complexity and this has led to the development of elegant strategies on the basis of transition metal, Lewis acid, and Brønsted acid catalysis. This review highlights recent examples that harness the inherent reactivity of prochiral oxetanes and offers an outlook on the immense possibilities for synthetic application. [ABSTRACT FROM AUTHOR]

The direct, catalytic dehydrative substitution of alcohols is a challenging, yet highly desirable process in the development of more sustainable approaches to organic chemistry. This review outlines recent advances in Brønsted acid‐catalysed dehydrative substitution reactions for C−C, C−O, C−N and C−S bond formation. The wide range of processes that are now accessible using simple alcohols as the formal electrophile are highlighted, while current limitations and therefore possible future directions for research are also discussed. [ABSTRACT FROM AUTHOR]

The first example of an enantioselective carbocyclization of an alkyne‐containing substrate catalyzed by chiral Brønsted acids was achieved. The use of the 2‐hydroxynaphthyl substituent on the alkyne as a directing group constituted the key parameter enabling both efficient regioselective protonation of the carbon–carbon triple bond and chiral induction. The key cationic intermediate could be depicted either as a cationic vinylidene ortho‐quinone methide or a stabilized vinyl cation. Atropoisomeric phenanthrenes derivatives were produced in high yields and good enantioselectivities under mild, metal‐free reaction conditions in the presence of chiral N‐triflylphosphoramide catalysts. The carbenic nature of the cationic intermediate was also exploited to describe an example of alkyne/alkane cycloisomerization. [ABSTRACT FROM AUTHOR]

We have prepared Zn and free‐base porphyrins appended with a fac‐Re(phen)(CO)3Br (where phen is 1,10‐phenanthroline) at the meso position of the porphyrin, and performed photocatalytic CO2 reduction using porphyrin–Re dyads in the presence of either triethylamine (TEA) or 1,3‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1H‐benzo[d]imidazole (BIH) as an electron donor. The Zn porphyrin dyad showed a high turnover number for CO production compared with the free‐base porphyrin dyad, suggesting that the central Zn ion of porphyrin plays an important role in suppressing electron accumulation on the porphyrin part and achieving high durability of the photocatalytic CO2 reduction using both TEA and BIH. The effect of acids on the CO2 reduction was investigated using the Zn porphyrin–Re dyad and BIH. Acetic acid, a relatively strong Brønsted acid, rapidly causes the porphyrin's color to fade upon irradiation and dramatically decreases CO production, whereas proper weak Brønsted acids such as 2,2,2‐trifluoroethanol and phenol enhance the CO2 reduction. [ABSTRACT FROM AUTHOR]

Metal‐free hydrocarboxylation of allenamides with various functionalized carboxylic acids were achieved with complete regio‐ and stereocontrol (>49:1). This environmentally compatible transformation affords γ‐acyloxyenamides with exclusive E‐selectivity. Electron rich, electron poor, aliphatic, aryl, and heterocyclic carboxylic acids all gave excellent yields (avg. 89 %, 47 examples). We demonstrate the synthetic potential of this transformation in the late‐stage modification of complex natural carboxylic acids and simple modification of the products to three‐carbon synthons with ample opportunity for further diversification. DFT studies revealed that the reaction occurs in a stepwise manner through the intermediacy of a conjugated iminum species, which is rapidly captured by the carboxylate ion, resulting in the observed linear selectivity. [ABSTRACT FROM AUTHOR]

A catalytic enantioselective synthesis of heterocyclic vicinal fluoroamines is reported. A chiral Brønsted acid promotes aza‐Michael addition to fluoroalkenyl heterocycles to give a prochiral enamine intermediate that undergoes asymmetric protonation upon rearomatization. The reaction accommodates a range of azaheterocycles and nucleophiles, generating the C−F stereocentre in high enantioselectivity, and is also amenable to stereogenic C−CF3 bonds. Extensive DFT calculations provided evidence for stereocontrolled proton transfer from catalyst to substrate as the rate‐determining step, and showed the importance of steric interactions from the catalyst's alkyl groups in enforcing the high enantioselectivity. Crystal structure data show the dominance of noncovalent interactions in the core structure conformation, enabling modulation of the conformational landscape. Ramachandran‐type analysis of conformer distribution and Protein Data Bank mining indicated that benzylic fluorination by this approach has the potential to improve the potency of several marketed drugs. [ABSTRACT FROM AUTHOR]

Despite the significant progress of the enantioselective reaction using chiral catalysts, the enantioselective nucleophilic substitution reaction at the chiral sp3‐hybridized carbon atom of a racemic electrophile has not been largely explored. Herein, we report the enantioconvergent propargylic substitution reaction of racemic propargylic alcohols with thiols using chiral bis‐phosphoric acid as the chiral Brønsted acid catalyst. The substitution products were formed in high yields with high enantioselectivities in most cases. The cation‐stabilizing effect of the sulfur functional group introduced at the alkynyl terminus is the key to achieving the efficient enantioconvergent process, in which chiral information originating from not only the racemic stereogenic center but also the formed contact ion pair is completely eliminated from the present system. [ABSTRACT FROM AUTHOR]

Starting from readily available o‐diazoacyl‐substituted arene carboxylates, scaffolds with the 5,9‐epoxycyclohepta[b]pyran‐2(3H)‐one core were obtained by cooperative RhII, Lewis and Brønsted acid catalysis. Four new bonds, three functional groups (lactone, ketal, and alcohol) and four contiguous stereocenters are formed during this regio‐ and diastereoselective process in a single synthetic step. Intensive optimization and mechanistic studies, including the trapping, isolation, and elucidation of reaction intermediates, led to a plausible mechanistic scenario. The reaction is proposed to involve carbonyl ylides but also transient species of the ketocarbene equilibrium that undergo a cascade of cycloaddition and skeletal rearrangements. [ABSTRACT FROM AUTHOR]

Nitro compounds are known to change reaction rates and kinetic concentration dependence of Brønsted‐acid‐catalyzed reactions. Yet, no mechanistic model exists to account for these observations. In this work, an atomistic model for the catalytically active form for an alcohol dehydroazidation reaction is presented, which is generated by DFT calculations and consists of an H‐bonded aggregate of two molecules of Brønsted acid and two molecules of nitro compound. The computed O−H stretching frequencies for the aggregate indicate they are stronger acids than the individual acid molecules and serve as predictors for experimental reaction rates. By applying the model to a chemically diverse set of potential promoters, it was predicted and verified experimentally that sulfate esters induce a similar co‐catalytic effect. The important implication is that Brønsted‐acid catalysis must be viewed from a supramolecular perspective that accounts for not only the pKa of the acid and the bulk properties of a solvent, but also the weak interactions between all molecules in solution. [ABSTRACT FROM AUTHOR]

Boronic acid catalysis has emerged as a mild method for promoting a wide variety of reactions. It has been proposed that the mode of catalysis involves Lewis acid or covalent activation of hydroxyl groups by boron, but limited mechanistic evidence exists. In this work, representative boronic acid catalyzed reactions of alcohols and oximes have been reinvestigated. A series of control experiments with boronic and Brønsted acids were interpreted along with correlations between their reactivity and their acidity measured by the Gutmann–Beckett method. Overall, it was concluded that the major modes of catalysis involve either dual H‐bond catalysis or Brønsted acid catalysis. Strong Brønsted acids were shown to be generated in situ from covalent assembly of the boronic acids with hexafluoroisopropanol, explaining why the solvent had such a major impact on the reactivity. This new insight should guide the future development of boronic acid catalysis, where the diverse and solvent‐specific nature of catalytic modes has been overlooked. [ABSTRACT FROM AUTHOR]

A catalytic enantioselective synthesis of β‐amino secondary amides was achieved using vinyl azides as the enamine‐type nucleophile and chiral N‐Tf phosphoramide as the chiral Brønsted acid catalyst through a five‐step sequential transformation in one pot. The established sequential transformation involves an enantioselective [4+2] cycloaddition reaction of vinyl azides with N‐acyl imines as the key stereo‐determining step that is efficiently accelerated by a chiral N‐Tf phosphoramide catalyst in a highly enantioselective manner in most cases. Further generation of the iminodiazonium ion intermediate through ring opening of the cycloaddition product and subsequent skeletal rearrangement involving Schmidt‐type 1,2‐aryl group migration followed by recyclization of the resulting nitrilium ion were also initiated by the same acid catalyst. Final acid hydrolysis of the recyclized products in the same pot gave rise to enantioenriched β‐amino amides through C−C bond formation at the α‐position of the secondary amides. [ABSTRACT FROM AUTHOR]

Catalytic thiolation and azidation of a single C−F bond of trifluoromethylarenes were achieved assisted by an ortho‐silyl group with all‐in‐one reagents to generate a trityl cation and nucleophiles. The reactions catalyzed by ytterbium triflate efficiently afforded a wide variety of difluoromethylenes avoiding the further C−F bond cleavage, by virtue of the mild conditions without the generation of a Brønsted acid. [ABSTRACT FROM AUTHOR]

The luminescence of the inorganic–organic hybrid nanoparticles ZrO(MFP) (MFP=methylfluorescein phosphate) and ZrO(RP) (RP=resorufin phosphate) was modified by addition of different rare earth halides LnCl3. The resulting composite materials form dispersible nanoparticles that exhibit modified nanoparticle fluorescence depending on the rare earth ion. The resulting chromaticity of the luminescence is further variable by the employment of different solvents for ZrO(MFP)‐based composite systems. The strong solvatochromic effect of the MFP chromophore leads to different luminescence chromaticities of the composite materials between green, yellow, and blue in THF, toluene, and dichloromethane, respectively. The luminescence of ZrO(RP)‐based composite particles can be modified between the red and blue spectral regions in dependence on the applied reaction temperature. Beside a luminescence shift that is derived from nanoparticle modification by LnCl3, a strong turn‐on effect of ZrO(RP) particles results after contact with different Brønsted acids and bases in combination with a respective chromaticity shift. Both effects enable the potential employment of such particles as highly sensitive optical pH sensors. [ABSTRACT FROM AUTHOR]

A Brønsted acid catalyzed, operationally simple, scalable route to several functionalized α‐(3‐indolyl) ketones has been developed and the long‐standing regioisomeric issue has been eliminated by choosing appropriate carbonyls. A readily available and cheap bottle reagent was used as the catalyst. This protocol was also applicable to the synthesis of densely functionalized α‐(3‐pyrrolyl) ketones. A detailed mechanistic study confirmed the involvement of enolether as a reaction intermediate. Several postsynthetic modifications along with easy access to β‐carboline, tryptamines, tryptophols, and spiro‐indolenine proclaim the synthetic utility of this powerful building block. On the basis of this concept, functionalized carbazoles were constructed by a cascade annulation strategy. [ABSTRACT FROM AUTHOR]

Soft templating with siliceous surfactant is an established protocol for the synthesis of hierarchically porous silicoaluminophosphates (HP SAPOs) with improved mass transport properties. Motivated by the enhanced performance of HP SAPOs in the Beckmann rearrangement of cyclohexanone oxime to the nylon 6 precursor ϵ‐caprolactam, an integrated theoretical and empirical study was carried out to investigate the catalytic potential of the siliceous mesopore network. Inelastic neutron scattering (INS) studies, in particular, provided unique insight into the substrate‐framework interactions in HP (Si)AlPOs and allowed reactive species to be studied independent of the catalyst matrix. The spectroscopic (INS, FTIR spectroscopy, MAS NMR spectroscopy) and computational analyses revealed that in the organosilane‐templated SAPO, the interconnectivity of micro‐ and mesopores permits cooperativity between their respective silanol and Brønsted acid sites that facilitates the protonation of cyclohexanone oxime in a physical mixture at ambient temperature. [ABSTRACT FROM AUTHOR]

The development and use of a multiple‐activation catalyst with ion‐paired Lewis acid and Brønsted acid in an asymmetric aza‐Diels–Alder reaction of simple dienes (non‐Danishefsky‐type electron‐rich dienes) was achieved by utilizing the [FeBr2]+[FeBr4]− combination prepared in situ from FeBr3 and chiral phosphoric acid. Synergistic effects of the highly active ion‐paired Lewis acid [FeBr2]+[FeBr4]− and a chiral Brønsted acid are important for promoting the reaction with high turnover frequency and high enantioselectivity. The multiple‐activation catalyst system was confirmed using synchrotron‐based X‐ray absorption fine structure measurements, and theoretical studies. This study reveals that the developed catalyst promoted the reaction not only by the interaction offered by the ion‐paired Lewis acid and the Brønsted acid but also noncovalent interactions. [ABSTRACT FROM AUTHOR]

Highly position selective alkylations of N‐alkylindoles at C7‐positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7‐positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold. [ABSTRACT FROM AUTHOR]

Abstract: The potential of β‐alkoxy α‐keto amides as pronucleophiles in the enantioselective Mannich type reaction with p‐nosyl imines is presented. The proper combination of β‐alkoxy α‐keto amides and a squaramide‐based Brønsted base catalyst produced highly enantioenriched Mannich adducts, which may be transformed into functionalized amino diols. [ABSTRACT FROM AUTHOR]

Abstract: An acid‐catalyzed aza‐Prins‐type endo cyclization of 2‐alkenylbenzaldehydes with BocNH2 or aniline derivatives through in situ generation of iminium intermediates has been developed, and various 1‐aminoindene derivatives were readily obtained. The first catalytic asymmetric variant of the present reaction has also been demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: Molecular iodine has been used for more than 100 years as a remarkable catalyst for many organic transformations such as cycloadditions, Michael and aldol reactions, or esterifications. Different explanations for the origin of its catalytic effect have been proposed in the last decades including a “hidden” Brønsted acid catalysis by HI, a Lewis‐acid (or halogen‐bond) activation, or catalysis by an iodonium(I) species. Recently, iodine catalysis again gained more interest due to the latest developments in halogen‐bond catalysis. In this Minireview, we first summarize the experimental basis for the proposed modes of activation. Subsequently, we analyze typical iodine‐catalyzed reactions to gain more insights into the underlying reaction mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: An efficient protocol for the asymmetric synthesis of chiral tetrahydroquinolines bearing multiple stereogenic centers by means of asymmetric Brønsted acid catalysis was developed. A chiral 1,1′‐spirobiindane‐7,7′‐diol (SPINOL)‐based N‐triflylphosphoramide (NTPA) proved to be an effective Brønsted acid catalyst for the in situ generation of aza‐ortho‐quinone methides (aza‐o‐QMs) and their subsequent cycloaddition reaction with unactivated alkenes to provide the products with excellent diastereo‐ and enantioselectivities. In addition, DFT calculations provided insight into the activation mode and nature of the interactions between the N‐triflylphosphoramide catalyst and the generated aza‐o‐QMs. [ABSTRACT FROM AUTHOR]