Lewis acids and bases, catalyst

Murahashi SI, Naota T (1996) A new way for efficient catalysis by using low valent ruthenium complexes as redox Lewis acid and base catalysts. Bull Chem Soc Jpn 69 1805... 

Murahashi SI, Takaya H (2000) Low-valent ruthenium and iridium hydride complexes as alternatives to Lewis acid and base catalysts. Acc Chem Res 33 225... 

A two-component epoxy system consists of a resin and a hardener, along with possible additives such as accelerators, reactive diluents, resin modifiers, plasticizers, and fillers. Typical hardeners include aliphatic polyamines, which cure at room temperature or at slightly elevated temperatures polyamides, which provide flexibility and are widely used aromatics, which are solid and anhydrides, which require elevated temperature cure and produce thermally stable but brittle adhesives. Low molecular weight epoxies are liquid and are usually cured by amines, carboxylic acid anhydrides, and Lewis acid and base catalyst. Higher molecular weight epoxies are cured through their hydroxyl groups. Cure of epoxies involves an exothermic reaction. 

The late transition metal hydrides may behave like mild redox Lewis acid and base catalysts. This makes them useful for the generation of carbon nucleophiles from protonucleophiles by activation of the a-C-H bond adjacent to electron withdrawing groups (CN, COR). 

In Chapter 6 we survey what has been accomplished and indicate directions for future research. Furthermore, we critically review the influence of water on Lewis acid - Lewis base interactions. This influence has severe implications for catalysis, in particular when hard Lewis acids and bases are involved. We conclude that claims of Lewis-acid catalysis should be accompanied by evidence for a direct interaction between catalyst and substrate. 

The scope of this reaction is similar to that of 10-21. Though anhydrides are somewhat less reactive than acyl halides, they are often used to prepare carboxylic esters. Acids, Lewis acids, and bases are often used as catalysts—most often, pyridine. Catalysis by pyridine is of the nucleophilic type (see 10-9). 4-(A,A-Dimethylamino)pyridine is a better catalyst than pyridine and can be used in cases where pyridine fails. " Nonbasic catalysts are cobalt(II) chloride " and TaCls—Si02. " Formic anhydride is not a stable compound but esters of formic acid can be prepared by treating alcohols " or phenols " with acetic-formic anhydride. Cyclic anhydrides give monoesterified dicarboxylic acids, for example,... 

Thus, the solubility characteristics of the catalyst precursor may provide only a rough guide to those of the rest state. Accordingly, rational design can carry one only so far. However, some classes of catalysts have a higher probability of resting in their isolable form. The most obvious example would be Brousted catalysts. With Lewis acids and bases, the question is whether they prefer to rest in their native form, or as some adduct of the product, or a reactant that may be in excess. There are many additional factors critical for the evaluation and comparison of recyclable catalysts, and these have been reviewed elsewhere [30-32]. 

Several examples have been described on Lewis acid and base catalyzed Michael additions. Cyclopentanone-2-carboxylic acid ethylester was added at room temperature to methyl vinyl ketone using 2 mol% FeCl3 -6 H20 as catalyst yielding > 90% of the addition product (Scheme 11) [31]. Cerium(III) chloride in the presence of sodium iodide [32] and trifluoro-methanesulfonic acid have been used as catalysts as well [33]. 

Solid-phase synthesis is of importance in combinatorial chemistry. As already mentioned RuH2(PPh3)4 catalyst can be used as an alternative to the conventional Lewis acid or base catalyst. When one uses polymer-supported cyanoacetate 37, which can be readily obtained from the commercially available polystyrene Wang resin and cyanoacetic acid, the ruthenium-catalyzed Knoevenagel and Michael reactions can be performed successively [27]. The effectiveness of this reaction is demonstrated by the sequential four-component reaction on solid phase as shown in Scheme 11 [27]. The ruthenium-catalyzed condensation of 37 with propanal and subsequent addition of diethyl malonate and methyl vinyl ketone in TH F at 50 °C gave the adduct 40 diastereoselectively in 40 % yield (de= 90 10). 

The catalytic curing agents commonly used include tertiary amines, Lewis acids and bases, and dicyandiamide. Since their function is truly catalytic, the catalyst is added at relatively low concentrations (0 to 5% by weight) to the epoxy formulation. Homopolymerization generally requires both the presence of catalysts and elevated temperatures for the reaction to proceed. Like the polyaddition reaction, the homopolymerization reaction is accelerated by hydroxyl groups or tertiary amines. 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

Finally, it should be noted that Lewis acids and bases can also be used in other non-conventional media, as described in Chapter 7, e.g. fluorous solvents, supercritical carbon dioxide and ionic liquids by designing the catalyst, e.g. for solubility in a fluorous solvent or an ionic liquid, to facilitate its recovery and reuse. For example, the use of the ionic liquid butylmethylimidazolium hydroxide, [bmim][OH], as both a catalyst and reaction medium for Michael additions (Fig. 2.45) has been recently reported [151]. 

Non-catalyzed aldol reactions via hypervalent silicon species have also been studied. An aldol reaction between aldehydes and silyl enol ethers of amides was reported by Myers [105]. The reaction can be conducted under mild conditions to produce anti aldol without Lewis acid or base catalysts (Sch. 62). Asymmetric induction was particularly high when the (Z)-silyl ketene A/,0-acetal derived from prolinol was used. 

Lederer-Manasse reaction, 439 Levorotation, 70 Lewis acids and bases, 44 Lewis structural formulas, 2 Ligands, 71 Lindlar s catalyst, 145... 

Since Cu ions on the zeolite surface exist in an isolated environment, they may interact with the sulfate species on the catalysts deactivated by SOj. The sulfate groups might partially surround the copper ions, as previously supested by Choi et al. [37] and Hamada et al. [38]. The sulfate may also have some characteristics of a coordinate covdent bond, where Cu ions and sulfate species may act as a Lewis acid and base, respectively [39], Ligands such as HjO, NHj, (C2Hj)3P, CO molecules and Cf, CN, OH , NOj, and C204 ions should at least contain a lone pair of electron to form a coordinate covalent bond between metal ions [40]. It should be noted that the sulfate catalyst species formed on the catalysts deactivated by SO2 contains lone electron pairs on O atoms which surround S atom of SO4 groups. Therefore, it is expected that the electrostatic interaction between Cu ions and sulfate species probably influences the local structure of Cu ions on the zeolite catalyst surface. 

Described in this paper is a model system - one in which well-characterized lanthanide complexes exhibit high catalyst activities for ethylene polymerization but where the corresponding oligomerization of propene is sufficiently slowed so that stepwise insertion of the olefin can be studied quantitatively and all important intermediates observed or isolated. Emphasized in this paper is the effect of added Lewis acids and bases on the rate of olefin insertions, and comparison between ethylene and propene reactions. The catalysts, of general structure M(ri -Cp )2CH3 L (M = Yb, Lu ... 

It was later found that Lewis acids and bases were catalysts for the epoxide-anhydride reaction. The preferred stoichiometry with respect to anhydride and epoxy proved to be 1 1. This relationship led Fischer (3) to propose a mechanism wherein the first step was a reaction between the anhydride and the accelerator to form a carboxyl anion. This anion in turn reacts with an epoxide, and the reaction repeats itself. 

Reaction mixtures involving one of six different derivatives of 2-aminophenol and eight separate derivatives of benzoyl chloride were heated in dioxane or xylene at temperatures between 210 °C and 250 °C to give a library of 48 benzoxazoles in good to excellent yields (Scheme 3.7) [27]. This simple method precluded requirements for Lewis acid or base catalysts. 

The enantioselective conjugate addition of nitrogen-based nucleophiles provides products useful in the synthesis of 3-amino acids, and this reaction proceeds with high ee using both enantiomerically pure Lewis acidic metal-based catalysts and organocatalysts. 

Needless to say, solid acid and base catalysts play a key role in the transformation of biomass-derived materials to value-added compounds such as carbonyl compounds [21-33]. For example, lactic acid could be obtained from cellulose using tungstated alumina as a Lewis add catalyst [137], and from glucose using HT as a solid base catalyst [138]. In this parL selective conversions of biomass-sourced materials using solid acid and base catalysts are surveyed alongside mechanistic considerations. 

Owing to its unique acid-base and structural properties, aliiminum oxide, first of all Y-Al2Q3, remains the most popular catalyst and catalyst support. The analysis of catalytic reactions usually deals with Lewis acid and base sites of AI2Q3. However, in the catalyst synthesis, adsorption properties of the surface during its interaction with aqueous solutions strongly determine the composition of surface hydroxyl cover of alumina. It should be noted that modem concepts of the surface structure of aluminum oxides, which were developed in recent 50 years, are based mainly on the vibrational spectroscopy data. Various structural models of the aluminum oxide surface were suggested to explain the experimental data... 

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