Reagents Lewis acid

Diazoalkanes alkylate acidic and enolic groups rapidly and other groups with replaceable hydrogens slowly. Carboxylic and sulfonic acids, phenols and enols are alkylated virtually instantaneously when treated with this reagent. Lewis acid catalysts (e.g., BF3.Et20) are used to promote the reaction of... 

Marshall, J. A. Preparation and addition reactions of allylic and allenic tin and indium reagents. Lewis Acids in Organic Synthesis 2000,1, 453-522. 

Nakai, T., Tomooka, K. Lanthanide(lll) reagents. Lewis Acid Reagents 1999, 203-223. 

Run Substrate (36 and 37) Reagent Lewis acid/nucleophile Product (38 and 39) Ratio (38 39) ... 

Friedel-Crafts reagents (Lewis acids) represent the most widely used class of cationic polymerization catalysts. 

Organozinc, Organocadmium and Organomercury Reagents Organocerium Reagents Samarium and Ytterbium Reagents Lewis Acid Carbonyl Complexation... 

Cl increases the Lewis acidity of boron making it a more reactive reagent... 

In view of the remarkable effects that water can exert on the uncatalysed Diels-Alder reaction, there might well be a similar effect on the rate and the selectivity of the Lewis-acid catalysed process. At the same time, coordination of a Lewis-acid to a Diels-Alder reagent is likely to overcome the... 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Lewis acid catalysts, such as AlCl or BF, coordinate strongly with non-bonded electron pairs but they iateract only weakly with bonded electron pairs. Therefore, n-donon reagents, such as alkyl haUdes, can react with Lewis acid catalysts even under complete exclusion of moisture or any other proton source ... 

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

The polymerization of ethyleneimine (16,354—357) is started by a catalyticaHy active reagent (H or a Lewis acid), which converts the ethyleneimine into a highly electrophilic initiator molecule. The initiator then reacts with nitrogen nucleophiles, such as the ethyleneimine monomer and the subsequendy formed oligomers, to produce a branched polymer, which contains primary, secondary, and tertiary nitrogen atoms in random ratios. Termination takes place by intramolecular macrocycle formation. 

Often the requisite THF oxonium ion is generated m situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive haUde with a Lewis acid, a reactive hahde with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

Alkylation involving formaldehyde in the presence of hydrogen chloride is known as chloromethylation (eq. 3). The reagent may be a mixture of formalin and hydrochloric acid, paraformaldehyde and hydrochloric acid, a chloromethyl ether, or a formal. Zinc chloride is commonly employed as a catalyst, although many other Lewis acids can be used. Chloromethylation of sahcyhc acids yields primarily the 5-substituted product 5-chlotomethylsahcyhc acid [10192-87-7] (4). 

Chlorination with Other Reagents. Chlorotoluenes can also be obtained in good yields by the reaction of toluene with stoichiometric proportions of certain Lewis acid chlorides such as inon(III) chloride, as the chlorinating agent (51). Generally, the product mixture contains /)-chlorotoluene as the principal component. Several modifications have been proposed to improve product yields (52,53). 

Ethers are weakly basic and are converted to unstable oxonium salts by strong acids such as sulfudc acid, perchlodc acid, and hydrobromic acid relatively stable complexes ate formed between ethers and Lewis acids such as boron trifluodde, aluminum chlodde, and Gtignatd reagents (qv) (9) ... 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

These mechanisms ascribe in jortance to the Lewis acid-Lewis base interaction between the allyl halide and the organolithium reagent. When substitution is complete, the halide ion is incorporated into the lifliium cluster in place of one of the carbon ligands. 

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