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Lewis catalysis

Table 1 Effect of Lewis Catalysis on Molecular Weight and Quantity of the Obtained Products... Table 1 Effect of Lewis Catalysis on Molecular Weight and Quantity of the Obtained Products...
The chemical modification of PS with diene hydrocarbons in the presence of Lewis catalysis are important for synthesizing of higher resistance, elasticity, and adhesion-capable polymers. When polybutadiene or polyi-... [Pg.266]

The acylation reaction of PS with MA by using model compounds in the presence of Lewis catalysis can be explained as follows. [Pg.266]

Transesterification reactions of 2-alkoxycarbonylethyltin trichlorides and their adducts with neutral donors with alcohols proceed readily. This is attributed to the intramolecular Lewis catalysis by the electrophilic SnCl group owing to the coordination of the ester carbonyl group to the tin atom, C = O —Sn, which decreases the electronic density at the carbonyl carbon atom727,741 745 754. [Pg.1104]

Several new methods of preparing pyran-4-ones have appeared. Silyloxydienes [such as (52)] undergo Diels-Alder reactions under Lewis catalysis with a0-unsaturated aldehydes in which the CO group takes part. This provides good yields of the dihydropyran-4-ones under mild conditions.64 65 In a related method, dichloroketene cyclizes the silyloxydiene to give good yields of the pyran-4-one (53). ... [Pg.386]

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. [Pg.1]

Lewis-acid catalysis of Diels-Alder reactions... [Pg.11]

By far the most effective method, however, is catalysis by Lewis-acids. In organic solvents. [Pg.12]

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. [Pg.12]

Unfortunately, the number of mechanistic studies in this field stands in no proportion to its versatility" . Thermodynamic analysis revealed that the beneficial effect of Lewis-acids on the rate of the Diels-Alder reaction can be primarily ascribed to a reduction of the enthalpy of activation ( AAH = 30-50 kJ/mole) leaving the activation entropy essentially unchanged (TAAS = 0-10 kJ/mol)" . Solvent effects on Lewis-acid catalysed Diels-Alder reactions have received very little attention. A change in solvent affects mainly the coordination step rather than the actual Diels-Alder reaction. Donating solvents severely impede catalysis . This observation justifies the widespread use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis-acid catalysed Diels-Alder reactions. [Pg.13]

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). [Pg.25]

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. [Pg.30]

In summary, water is clearly an extremely bad solvent for coordination of a hard Lewis acid to a hard Lewis base. Hence, catalysis of Diels-Alder reactions in water is expected to be difficult due to the relative inefficiency of the interactions between the Diels-Alder reactants and the Lewis-acid catalyst in this medium. [Pg.31]

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. [Pg.31]

What is the scope of Lewis-acid catalysis of Diels-Alder reactions in water An approach of extending the scope by making use of a temporary secondary coordination site is described in Chapter 4. [Pg.32]

What is the effect of micelles on the aqueous Diels-Alder reaction Can micellar catalysis be combined with Lewis-acid catalysis In Chapter 5 these aspects will discussed. [Pg.32]

In order to be able to provide answers to these questions, a Diels-Alder reaction is required that is subject to Lewis-acid catalysis in aqueous media. Finding such a reaction was not an easy task. Fortunately the literature on other Lewis-acid catalysed organic reactions in water was helpful to some extent... [Pg.44]

Lewis-acid catalysis of organic reactions in aqueous solutions ... [Pg.44]

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... [Pg.46]

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. [Pg.48]

In organic solvents Lewis-acid catalysis also leads to large accelerations of the Diels-Alder reaction. Table 2.2 shows the rate constants for the Cu -catalysed Diels-Alder reaction between 2.4a and 2.5 in different solvents. [Pg.54]

Surprisingly, the highest catalytic activity is observed in TFE. One mi t envisage this to be a result of the poor interaction between TFE and the copper(II) cation, so that the cation will retain most of its Lewis-acidity. In the other solvents the interaction between their electron-rich hetero atoms and the cation is likely to be stronger, thus diminishing the efficiency of the Lewis-acid catalysis. The observation that Cu(N03)2 is only poorly soluble in TFE and much better in the other solvents used, is in line with this reasoning. [Pg.54]


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1-Octene Lewis acid catalysis

1.3- dipolar cycloadditions Lewis acid catalysis

2- Butene, 2,3-dimethylene reactions Lewis acid catalysis

2- Butene, 2-methylene reactions Lewis acid catalysis

2-Cyclohexenone Lewis acid catalysis

Acrolein Lewis acid catalysis

Acrylates Lewis acid catalysis

Alkenes Lewis acid catalysis

Alkylation, Lewis acid catalysis

Amides, a-aminosynthesis Lewis acid catalysis

Amino acids via Lewis acid catalysis

Catalysis (cont Lewis acid

Catalysis Lewis acid catalysts

Catalysis by Chiral Lewis Acids

Catalysis by Lewis acids

Catalysis with Other Lewis Acids

Chiral Bronsted Base-Lewis Acid Bifunctional Catalysis

Chiral Lanthanide Lewis Acid Catalysis

Chiral Lewis Base Catalysis

Chiral Lewis acid catalysis

Cobalt complexes Lewis acid catalysis

Combination of Enamine Catalysis and Lewis Acids in SN1-Type Reactions

Copper complexes Lewis acid catalysis

Crotonaldehyde Lewis acid catalysis

Cyclohexane, alkylideneene reactions Lewis acid catalysis

Cyclohexane, methyleneene reactions Lewis acid catalysis

Cyclohexen-2-one Lewis acid catalysis

Cyclopropanes Lewis acid catalysis

Diels-Alder Lewis acid catalysis

Diels-Alder reaction catalysis by Lewis acids

Diels-Alder reactions Lewis acid catalysis

Diels-Alder reactions non-Lewis acid catalysis

Emulsion Catalysis in Lewis Acid-Catalyzed Organic Reactions

Enamine catalysis Lewis bases

Enantioselective Lewis-acid catalysis

Enantioselective reduction Lewis-acid catalysis

Ene reactions Lewis acid catalysis

Enones Lewis base catalysis

Ester hydrolysis Lewis acid catalysis

Frustrated Lewis acid-base pair catalysis

Halogenation Lewis acid catalysis

Heterogeneous catalysis Lewis acids

Hydrolysis Lewis acid catalysis

Hydrosilylation Lewis acid catalysis

Intermolecular Diels-Alder reactions Lewis acid catalysis

Isobutene Lewis acid catalysis

Ketones, methyl vinyl Lewis acid catalysis

Lewis Acid Catalysis of Allyltin Additions

Lewis acid catalysis 1,3-dipolar

Lewis acid catalysis 1,3-dipolar cycloaddition

Lewis acid catalysis 3 4- 2-cycloaddition

Lewis acid catalysis Claisen rearrangement

Lewis acid catalysis Friedel-Crafts acylation

Lewis acid catalysis Friedel-Crafts alkylation

Lewis acid catalysis Friedel-Crafts reaction

Lewis acid catalysis Michael addition

Lewis acid catalysis Mukaiyama aldol reaction

Lewis acid catalysis acylation

Lewis acid catalysis alcohol acylation

Lewis acid catalysis asymmetric

Lewis acid catalysis compounds

Lewis acid catalysis epoxidation

Lewis acid catalysis epoxide ring opening

Lewis acid catalysis for

Lewis acid catalysis in Alder ene reaction

Lewis acid catalysis in aldol reactions

Lewis acid catalysis in reactions of silyl enol ethers

Lewis acid catalysis ligand acceleration

Lewis acid catalysis of electrophilic substitution reaction

Lewis acid catalysis polymer supported

Lewis acid catalysis water compatibility

Lewis acid catalysis, sulfonyl

Lewis acid-Bronsted catalysis

Lewis acid-base catalysis

Lewis acid-surfactant-combined catalysis

Lewis acids acid catalysis

Lewis acids catalysis of Diels-Alder reactions

Lewis acids highly selective catalysis

Lewis acids, catalysis

Lewis acids, catalysis in Diels—Alder reaction

Lewis base-promoted ruthenium catalysis

Lewis surfactant-combined catalysis

Lewis-base catalysis

Methyl a-acetamidoacrylate Lewis acid catalysis

Methyl a-bromomethacrylate Lewis acid catalysis

Methyl a-cyanoacrylate Lewis acid catalysis

Nickel complexes, Lewis acid catalysis

Nitriles, a-aminoacyl anion equivalents via Lewis acid catalysis

Nitrogen compounds Lewis acid catalysis

Oxygen compounds Lewis acid catalysis

Radical reactions Lewis acid catalysis

Selectivity Lewis base-promoted catalysis

Solid Bronsted acid-Lewis base catalysis

Subject Lewis acid catalysis

Sulfone, ethynyl p-tolyl Lewis acid catalysis

Trimethyl a-phosphonoacrylate Lewis acid catalysis

Water-stable rare earth Lewis Acid catalysis

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