Home About us Contact
|
Home About us Contact | ||
|
|
||
Strong Brønsted Acid (strong + bronst_acid)
Selected AbstractsSynthesis 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] 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] 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] 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] 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] | ||||||||||