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Brønsted Acid (Brønst + acid)
Kinds of Brønsted Acid Terms modified by Brønsted Acid Selected AbstractsEnantioselective Mannich-Type Reaction Catalyzed by a Chiral Brønsted Acid Derived from TADDOLADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-13 2005Takahiko Akiyama Abstract A novel cyclic dialkyl phosphate was synthesized starting from (+)-diethyl tartrate. Its catalytic activity as a chiral Brønsted acid has been examined in the Mannich-type reaction of a ketene silyl acetal with aldimines as a model reaction. The corresponding ,-amino acid esters were obtained with high enantioselectivity. [source] Cationic Brønsted Acids for the Preparation of SnIV Salts: Synthesis and Characterisation of [Ph3Sn(OEt2)][H2N{B(C6F5)3}2],[Sn(NMe2)3(HNMe2)2][B(C6F5)4] and [Me3Sn(HNMe2)2][B(C6F5)4]EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 16 2006Yann Sarazin Abstract Ph3SnN(SiMe3)2 (1) was prepared in good yields by reaction of [{NaN(SiMe3)2}2·THF] (2) with Ph3SnF. Treatment of 1 with [H(OEt2)2][H2N{B(C6F5)3}2] (4) in dichloromethane afforded the stannylium cation [Ph3Sn(OEt2)][H2N{B(C6F5)3}2] (5), which was characterised by 1H, 13C{1H}, 11B, 19F and 119Sn NMR spectroscopy. The reaction of Sn(NMe2)4 with [Ph2MeNH][B(C6F5)4] (3) gave the amidotin(IV) compound [Sn(NMe2)3(HNMe2)2][B(C6F5)4] (6) which proved very stable towards ligand substitution and resisted treatment with Et2O, THF, TMEDA and pyrazine. Two new Brønsted acid salts [H(NMe2H)2][B(C6F5)4] (7) and [(C4H4N2)H·OEt2][H2N{B(C6F5)3}2] (8) were synthesised. The reaction of 7 with Sn(NMe2)4 in Et2O allowed the preparation of 6 in a much improved yield (83,%). The treatment of 7 with Me3SnN(SiMe3)2 in Et2O yielded [Me3Sn(HNMe2)2][B(C6F5)4] (9) nearly quantitatively. Compounds 1, 2, 6, 8 and 9 were characterised by single-crystal X-ray diffraction analyses; 6 is the first example of a structurally characterised amidotin(IV)cation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Direct Alkynylation of Thiophenes: Cooperative Activation of TIPS,EBX with Gold and Brønsted Acids,ANGEWANDTE CHEMIE, Issue 40 2010Jonathan P. Brand Gemeinsam sind wir stark! Die kooperative Aktivierung des hypervalenten Iodreagens TIPS-EBX mit einem Goldkatalysator und einer Brønsted-Säure ermöglichte die erste direkte Ethinylierung von Thiophenen bei Raumtemperatur (siehe Schema; TFA=Trifluoressigsäure). Die erhaltenen Ethinylthiophene sind wichtige Bausteine für organische Farbstoffe und elektronische Materialien. [source] Synthesis of ,-Amino Nitriles from Carbonyl Compounds, Amines, and Trimethylsilyl Cyanide: Comparison between Catalyst-Free Conditions and the Presence of Tin Ion-Exchanged MontmorilloniteEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2010Jiacheng Wang Abstract In the absence of catalysts, the three-component, one-pot synthesis of ,-amino nitriles proceeded using various aldehydes and ketones together with amines and trimethylsilyl cyanide (TMSCN) in high yields under neat conditions at room temperature. The addition order of the reagents had a significant influence on the yields of the desired ,-amino nitriles. In contrast, when tin ion-exchanged montmorillonite (Sn-Mont), prepared by the ion-exchange of sodium montmorillonite (Na-Mont) with a tin tetrachloride solution, was used as a catalyst, the reaction rates significantly increased compared with those without catalysts, and the range of the applicable carbonyl compounds was also extended: structurally diverse aromatic, aliphatic and heteroatom-containing carbonyl compounds, including sterically hindered ketones as well as aliphatic and aromatic amines, were converted into the desired ,-amino nitriles in good to excellent yields with short reaction times under mild conditions. Sn-Mont showed a better catalytic activity than proton or other metal ion-exchanged montmorillonites, supported SnO2 catalysts and the previously reported homogeneous or heterogeneous catalysts. The recovered catalyst was reused several times without loss of catalytic performance. Along with the expansion of the interlayer space of Sn-Mont, the strong Brønsted acid and Lewis acid nature of Sn-Mont derived from protons and SnO2 nanoparticles present in the interlayers of Sn-Mont likely played important and cooperative roles in the high catalytic activity. [source] Thiiranation of 2,-adamantylidene-9-benzonorbornenylidene using 4,4,-oligothiodimorpholine and brønsted acidHETEROATOM CHEMISTRY, Issue 1 2009Yoshiaki Sugihara On leaving 4,4,-dithiodimorpholine 6 powder undisturbed at room temperature over 10 years, it led to the formation of 4,4,-tetrathiodimorpholine 7. Reactions of 2,-adamantylidene-9-benzonorbornenyidene 1 with 6, 7, and 4,4,-thiodimorpholine 8 and a Brønsted acid in CH2Cl2 at room temperature proceeded to afford the corresponding thiiranes, 2 and 3. The order of reactivity of 4,4,-oligothiodimorpholines combined with a Brønsted acid is 7 > 6 > 8. The thiirane 3 was transformed to 1 and 2 under the reaction conditions. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:12,18, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20505 [source] Enantioselective Organocatalytic Synthesis of Arylglycines via Friedel,Crafts Alkylation of Arenes with a Glyoxylate ImineADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2010Dieter Enders Abstract The enantioselective organocatalytic synthesis of arylglycines has been developed employing 1,mol% of an enantiopure N -triflyl phosphoramide Brønsted acid as organocatalyst. Various differently substituted phenylglycine derivatives can be synthesized in good to excellent yields and enantiomeric excesses based on a Friedel,Crafts alkylation of electron-rich arenes with a glyoxylate imine. A novel protocol for the deprotection of the N - tert -butylsulfonyl (Bus) group has also been developed. [source] Dual Amine- and Brønsted Acid-Catalyzed ,-Allylic Alkylation of AldehydesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2010Li-Wen Xu Abstract A very simple method was developed for the direct, palladium-free catalytic ,-allylic alkylation of aldehydes. The direct organocatalytic intermolecular ,-allylic alkylation reaction was mediated by a simple combination of Brønsted acid and enamine catalysis which furnished ,-allylic alkylated aldehydes and cyclohexanone in high yields and chemoselectivities. The reaction conditions are mild and environmental friendly, the process is conducted under an atmosphere of air without the need for dried solvents, and water is the only side product of the allylic alkylation reaction. [source] Highly Enantioselective Biginelli Reaction Promoted by Chiral Bifunctional Primary Amine-Thiourea Catalysts: Asymmetric Synthesis of DihydropyrimidinesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 18 2009Yangyun Wang Abstract The diastereospecific formation of dihydropyrimidines (DHPMs) has been achieved in moderate to high yields with up to 99% ee by a Biginelli reaction. The reaction was performed by using a combined catalyst consisting of a chiral bifunctional primary amine-thiourea 9f and a Brønsted acid with tert -butylammonium trifluoroacetate (t- BuNH2,TFA) as additive in dichloromethane at room temperature. The possible mechanism for the reaction has been proposed to explain the origin of the activation and the asymmetric induction. [source] Significant Self-Acceleration Effects of Nitrile Additives in the Rhodium-Catalyzed Conversion of Aldoximes to Amides: A New Mechanistic AspectADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2009Min Kim Abstract It was found that the catalytic activity of rhodium complexes was highly sensitive to the type of N-heterocyclic carbene (NHC) ligands in the conversion of aldoximes to amides. Among those species examined, the (cyclooctadiene)rhodium chloride-carbene complex Rh(cod)(IMes)Cl exhibited the highest reactivity when it was employed in combination with a Brønsted acid, thus allowing mild reaction conditions. A significant rate acceleration effect resulting from the addition of nitrile additives was also observed. With the new protocol, the substrate scope of aldoximes has been widely expanded to include sterically congested and electronically varied derivatives. On the basis of detailed mechanistic studies, it is proposed that the reaction proceeds mainly via intramolecular electrophilic addition of aldoxime to rhodium-bound nitrile, which is different from the generally postulated two-step route: dehydration of aldoxime to nitrile followed by hydration of the latter intermediate. [source] Enantioselective Trifunctional Organocatalysts for Rate- Enhanced Aza-Morita,Baylis,Hillman Reactions at Room TemperatureADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2009Jean-Marc Garnier Abstract A Brønsted acid-activated trifunctional organocatalyst, based on the BINAP scaffold, was used for the first time to catalyze aza-Morita-Baylis,Hillman reactions between N -tosylimines and methyl vinyl ketone with fast reaction rates and good enantioselectivity at room temperature. This trifunctional catalyst, containing a Lewis base, a Brønsted base, and a Brønsted acid, required acid activation to confer its enantioselectivity and rate improvement for both electron-rich and electron-deficient imine substrates. The role of the amino Lewis base of 1a was investigated and found to be the activity switch in response to an acid additive. The counterion of the acid additive was found to influence not only the excess ratio but also the sense of asymmetric induction. [source] Enantioselective Mannich-Type Reaction Catalyzed by a Chiral Brønsted Acid Derived from TADDOLADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-13 2005Takahiko Akiyama Abstract A novel cyclic dialkyl phosphate was synthesized starting from (+)-diethyl tartrate. Its catalytic activity as a chiral Brønsted acid has been examined in the Mannich-type reaction of a ketene silyl acetal with aldimines as a model reaction. The corresponding ,-amino acid esters were obtained with high enantioselectivity. [source] Silane reduction of onium saltsAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 3 2010James V. Crivello Abstract Novel redox initiators for cationic polymerizations were developed consisting of an onium salt together with a SiH functional silane or siloxane. The reduction of the onium salt by the silane is catalyzed by noble metal complexes or certain transition metal compounds and takes place spontaneously at room temperature. The redox reaction of the onium salt with the silane results in the liberation of a strong Brønsted acid that can be subsequently used to initiate cationic polymerizations. Typical onium salts that have been employed in these redox initiator systems are diaryliodonium salts, triarylsulfonium salts and S,S -dialkyl- S -phenacylsulfonium salts. Studies of the effects of variations in the structures of the onium salt, the silane and the type of noble metal catalyst were carried out. In principle, the redox initiator systems are applicable to all types of cationically polymerizable monomers and oligomers, including the ring-opening polymerizations of such heterocyclic monomers as epoxides and oxetanes and, in addition, the polymerization of vinyl monomers such as vinyl ethers, N -vinylcarbazole and styrenic monomers. The use of these novel initiator systems for carrying out commercially attractive crosslinking polymerizations for coatings, composites and encapsulations is discussed. Copyright © 2009 John Wiley & Sons, Ltd. [source] Gold(I) and Brønsted Acid Catalyzed Intramolecular Rearrangements of VinylidenecyclopropanesCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2010Bei-Li Lu Spot the difference! Different intramolecular rearrangements of vinylidenecyclopropanes containing a secondary alcohol chain were observed depending on whether the reaction was catalyzed by a gold(I) complex or a Brønsted acid (HOTf; OTf=trifluoromethanesulfonate). The corresponding adducts were obtained in moderate to good yields (see scheme). The scope and limitations of these reactions, as well as plausible mechanisms, have been discussed. [source] High-Speed Living Polymerization of Polar Vinyl Monomers by Self-Healing Silylium CatalystsCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2010Dr. Yuetao Zhang Abstract This contribution describes the development and demonstration of the ambient-temperature, high-speed living polymerization of polar vinyl monomers (M) with a low silylium catalyst loading (, 0.05,mol,% relative to M). The catalyst is generated in situ by protonation of a trialkylsilyl ketene acetal (RSKA) initiator (I) with a strong Brønsted acid. The living character of the polymerization system has been demonstrated by several key lines of evidence, including the observed linear growth of the chain length as a function of monomer conversion at a given [M]/[I] ratio, near-precise polymer number-average molecular weight (Mn, controlled by the [M]/[I] ratio) with narrow molecular weight distributions (MWD), absence of an induction period and chain-termination reactions (as revealed by kinetics), readily achievable chain extension, and the successful synthesis of well-defined block copolymers. Fundamental steps of activation, initiation, propagation, and catalyst "self-repair" involved in this living polymerization system have been elucidated, chiefly featuring a propagation "catalysis" cycle consisting of a rate-limiting CC bond formation step and fast release of the silylium catalyst to the incoming monomer. Effects of acid activator, catalyst and monomer structure, and reaction temperature on polymerization characteristics have also been examined. Among the three strong acids incorporating a weakly coordinating borate or a chiral disulfonimide anion, the oxonium acid [H(Et2O)2]+[B(C6F5)4], is the most effective activator, which spontaneously delivers the most active R3Si+, reaching a high catalyst turn-over frequency (TOF) of 6.0×103,h,1 for methyl methacrylate polymerization by Me3Si+ or an exceptionally high TOF of 2.4×105,h,1 for n -butyl acrylate polymerization by iBu3Si+, in addition to its high (>90,%) to quantitative efficiencies and a high degree of control over Mn and MWD (1.07,1.12). An intriguing catalyst "self-repair" feature has also been demonstrated for the current living polymerization system. [source] Organocatalytic Asymmetric Synthesis of trans -1,3-Disubstituted Tetrahydroisoquinolines via a Reductive Amination/Aza-Michael SequenceCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2010Dieter Enders Prof. Benzothiazoline better than Hantzsch: A stereoselective Brønsted acid catalyzed reductive amination/aza-Michael approach towards the important class of tetrahydroisoquinolines is presented (see scheme). A biphenyl-substituted benzothiazoline is used as the reducing agent and leads to superior yields and enantiomeric excesses compared with the frequently used Hantzsch ester. The cyclization of the amine intermediate occurs smoothly with potassium tert -butoxide as the base and affords the trans -1,3-disubstituted tetrahydroisoquinolines. [source] Unifying Metal and Brønsted Acid Catalysis,Concepts, Mechanisms, and ClassificationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2010Magnus Rueping Prof. Abstract Asymmetric catalysis is a key feature of modern synthetic organic chemistry. Traditionally, different combinations of ligands and metals are used to perform highly enantioselective reactions. Since the renaissance of organocatalysis in the early 2000s, tremendous improvement in the field of metal-free catalysis has been achieved. Recently, the combination of transition metals and organocatalysts has allowed the development of new protocols enabling transformations that could not previously be realized. This article aims to present the latest contributions in the field of combined chiral Brønsted acid and metal catalyzed reactions, highlighting the advantages of these catalytic systems as well as describing the uncertainties regarding the molecular structure of the catalytically active species and the reaction mechanisms. [source] Byproduct-Catalyzed Four-Component Reactions of Aldehydes with Hexamethyldisilazane, Chloroformates, and Nucleophiles in Acetonitrile Leading to Protected Primary Amines, ,-Amino Esters, and ,-Amino KetonesCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2010Bai-Ling Yang Abstract Multicomponent reactions are a very powerful tool for the construction of complex organic molecules by using readily available starting materials. While most of the multicomponent reactions discovered so far consist of three components, the reactions with four or more components remain sparse. We have successfully developed several four-component reactions using a catalytic amount of water as a hydrolyzing agent to decompose byproduct chlorotrimethylsilane (TMSCl) to yield secondary byproduct HCl that serves as a catalyst. In the presence of 40,mol,% of water, the four-component reaction of aldehydes with hexamethyldisilazane (HMDS), chloroformates, and silylated nucleophiles proceeds smoothly at room temperature to give a range of protected primary amines in moderate to excellent yields. Importantly, a wide variety of protic carbon nucleophiles, such as ,-keto esters, ,-diketones, and ketones, have further been explored as suitable substrates for the synthesis of protected ,-amino esters and ,-amino ketones that are useful building blocks for various pharmaceuticals and natural products. These four-component reactions proceed through a pathway of tandem nitrogen protection/imine formation/imine addition, and the decomposition of byproduct TMSCl, generated in the first step of nitrogen protection, with water results in the formation of secondary byproduct HCl, a strong Brønsted acid that catalyzes the following imine formation/imine addition. Taking advantage of the fact that alcohols or phenols are also able to decompose byproduct TMSCl to yield secondary byproduct HCl, no catalyst is needed at all for the four-component reactions with aldehydes bearing hydroxy groups. [source] An Organocatalytic Asymmetric Tandem Reaction for the Construction of Bicyclic SkeletonsCHEMISTRY - A EUROPEAN JOURNAL, Issue 42 2009Chun-Li Cao Dr. Abstract Cyclic ketones react with (E)-2-nitroallylic acetates in the presence of catalytic pyrrolidine-thiourea, which affords bicyclic skeletons with four or five stereocenters in one single reaction with up to 98,%,ee in moderate to high yields. The cooperative effects of both enamine and the Brønsted acid are found to be crucial for the high reactivity and enantioselectivity of this cascade reaction, which is demonstrated by both theoretical calculation and experimental data. [source] Water-Accelerated Cationic ,-(7- endo) Cyclisation: Application to Indole-Based Peri-Annulated Polyheterocycles,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 26 2010Mohammad Saifuddin Abstract An efficient and versatile method for the synthesis of indole-based polycyclic indolo-benzazepine and its derivatives through water-accelerated cationic ,-cyclisation is described. The strategy involves condensation of arylamine moieties linked to C-4 in indole/azaindole systems with arylaldehydes in water containing catalytic amount of Brønsted acids. The C,C bond formation in water is complete within 10,30 min, furnishing the title compounds in excellent yields and purities, whereas in organic solvents 10,12 h are required. Furthermore, aldehydes both with electron-donating and-withdrawing substituents facilitate the ,-cyclisation equally. [source] Synthesis and Application of Polymer-Supported Chiral Brønsted Acid OrganocatalystsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Magnus Rueping Abstract A new methodology for the immobilization of chiral Brønsted acids has been developed. The resulting heterogeneous organocatalysts have been employed in multiple consecutive catalysis cycles in the asymmetric organocatalytic transfer hydrogenation. The new catalyst system can not only be easily recovered from the reaction mixture in a tea-bag approach but it can also be easily reused in several catalytic cycles without loss of reactivity and selectivity. [source] Copper(II) Triflate as a Source of Triflic Acid: Effective, Green Catalysis of Hydroalkoxylation ReactionsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Mathieu J.-L. Abstract The hydroalkoxylation of dicyclopentadiene (DCPD) and norbornene (NB) with 2-hydroxyethyl methacrylate (HEMA) for the synthesis of industrially relevant monomers has been investigated with various metal-based Lewis acids and strong Brønsted acids. In the absence of other additives, copper(II) triflate is the most efficient catalyst system. Kinetics, electron spin resonance (ESR), catalyst poisoning and cross experiments indicate that triflic acid (TfOH) is the true active catalyst in these reactions. This in situ generation of TfOH occurs via reduction of Cu(OTf)2 by the olefin reagent (DCPD, NB). The copper ions present in the reaction mixture act as radical polymerization retardants, preventing polymerization of HEMA (which is observed with most other metal salts and strong Brønsted acids investigated), thus improving the selectivity and yield (up to 95%) for the desired products. These observations have led to the development of a highly effective green process, using bulk reagents (no solvent) and a cheap, metal-free catalyst system, based on TfOH and a phenolic radical inhibitor (2,5-di- tert -butylhydroxytoluene, BHT). [source] Feasibility of the spontaneous gas-phase proton transfer equilibria between neutral Brønsted acids and Brønsted basesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7-8 2008Peeter Burk Abstract The computational investigation of interactions of different acid,base pairs regarding the nature and extent of spontaneous proton transfer was carried out at B3LYP/6-311,+,G** level. The selected acid,base pairs include the interactions of strong base (K2O) with acids of different strength (HClO4, HCl, and HF), and strong acid (HClO4) with bases ranging from K2O (GB,=,322.8,kcal/mol) to H2O (GB,=,157.6,kcal/mol). It was shown that spontaneus, unassisted proton transfer can take place in the gas-phase reactions of strong neutral Brønsted acids and bases. The reaction might be barrierless as in case of interactions between strong acids and bases, for example perchloric acid and alkali metal oxides or potassium oxide and halogen hydrides, or involve the encounter complex (hydrogen bonded acid,base cluster), which is separated from ion-pair by the transition state. Copyright © 2008 John Wiley & Sons, Ltd. [source] Interaction of Zinc Oxide Clusters with Molecules Related to the Sulfur Vulcanization of Polyolefins ("Rubber")CHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2006Ralf Steudel Prof. Dr. Abstract The vulcanization of rubber by sulfur is a large-scale industrial process that is only poorly understood, especially the role of zinc oxide, which is added as an activator. We used the highly symmetrical cluster Zn4O4 (Td) as a model species to study the thermodynamics of the initial interaction of various vulcanization-related molecules with ZnO by DFT methods, mostly at the B3LYP/6-31+G* level. The interaction energy of Lewis bases with Zn4O4 increases in the following order: CO
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