Acid-promoted rearrangement carbonyl compounds

Lewis acid-mediated reactions can be classified into two groups (Fig. 4). In the first (type 1) the complex between substrate and Lewis-acid reagent produces the product. Claisen rearrangement promoted by a Lewis-acid catalyst is a typical example of this type. Some complexes formed between Lewis acids and substrates are, however, stable enough to react with a variety of reagents from outside the system to generate the product (type 2). The Diels-Alder reaction between Lewis acid-activated unsaturated carbonyl compounds and dienes is an example of type 2 reactions. 

Zeolites have also been described as efficient catalysts for acylation,11 for the preparation of acetals,12 and proved to be useful for acetal hydrolysis13 or intramolecular lactonization of hydroxyalkanoic acids,14 to name a few examples of their application. A number of isomerizations and skeletal rearrangements promoted by these porous materials have also been reported. From these, we can underline two important industrial processes such as the isomerization of xylenes,2 and the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam,15 which is an intermediate for polyamide manufacture. Other applications include the conversion of n-butane to isobutane,16 Fries rearrangement of phenyl esters,17 or the rearrangement of epoxides to carbonyl compounds.18... 

Epoxides can be isomerized to carbonyl compounds by Lewis acids.104 105 Boron trifluoride is frequently used as the reagent. Carbocation intermediates appear to be involved, and the structure and stereochemistry of the product are determined by the factors which govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. 

The ring contraction of 1,2-cyclobutanediones is promoted either by acids or by bases in protic media". The reaction products are a-hydroxycyclopropanecarboxylic acids or the corresponding esters. Rearrangements of some 2-alkylidenecyclobutanols to cyclopropyl carbonyl compounds are also known. 1,2-Epoxycyclobutanes afford cyclopropanecarbal-dehydes or cyclopropyl ketones under the catalytic action of protic acids, lithium iodide or lithium thiocyanate (equation 107) . Under the latter conditions, the reaction... 

Titanium tetrachloride is a moisture-sensitive, highly flammable liquid reacting violently with water (34). It is a strong Lewis acid capable of promoting Diels-Alder reactions (35) and induces the addition of silyl enol ethers and allyl silanes to carbonyl compounds and derivatives (34r-36). It is a less commonly used catalyst in Friedel-Crafts reactions but very useful for the acylation of activated alkenes and in the Fries rearrangement. 

Abstract The use of organoaluminum-based Lewis acids (A1R X3 R = alkyl, alkynyl, X = halide or pseudohalide) in the period 2000 to mid-2011 is overviewed with a focus on (1) stoichiometric reactions in which one of the organoaluminum substituents is transferred to the substrate (e.g., the opening of epoxides, 1,2-additions to carbonyl compounds, coupling with C-X, and Reissert chemistry) and (2) asymmetric, often catalytic, reactions promoted by Lewis acid catalysts derived from organoaluminum species (e.g., use of auxiliaries with alanes, Diels-Alder, and related cycloaddition reactions, additions to aldehydes and ketones, and skeletal rearrangement reactions). 

Like most Lewis acids, SbFs promotes the rearrangement of epoxides to carbonyl compounds, and SbFs is an efficient catalyst for this reaction. However, the migratory aptitude of sub-stitutent groups in the reaction under SbFs catalysis is much less selective compared to that promoted by weak Lewis acids such as Methylaluminum Bis(4-bromo-2,6-di-t-butylphenoxide) Nevertheless, SbFs is well suited for the rearrangement of perfluoroepoxides. Excellent yields of ketones are obtained from the reaction. When diepoxides are used, diketones are obtained as the reaction products (eq 22). ... 

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