Lewis acids acid catalysis

TABLE 7.5 Solvent Effects and Combined Lewis-Acid/Micellar Catalysis Increasing the Rate Constant of Reaction (1)... 

CHIRAL BRONSTED BASE-LEWIS ACID BIFUNCTIONAL CATALYSIS... 

Electrophilic reagents are Lewis acids (- acid-base theories). Electrophilic catalysis is catalysis by Lewis acids. The term electrophilic is also used to designate the apparent polar character of certain -> radicals, as inferred from their higher relative reactivities with reaction sites of higher -> electron density. See also -> electrophilicity. Ref. [i] Muller P (1994) Pure Appl Chem 66 1077... 

Zeolites are not typically used in Lewis acid type catalysis due to the absence of Lewis acid centers in zeolites. This is due to the coordination of the Al-site to four lattice-oxygens in a perfect zeolite framework. It has, however, been shown for zeolite Beta that the aluminum atom can reversibly move between a framework Brpnsted acid site and a framework-grafted Lewis-acid site.70 Accordingly, Creyghton et al. showed that zeolite Beta is active in the Meerwein-Ponndorf-Verley reduction (MPV) of ketones (scheme 4).71 In this reaction a hydrogen hydride transfer reaction between an alcohol and a ketone takes place. 

Although boron trifluoride, sulphur trioxide and the like may be titrated against bases in inert solvents, there is no unequivocal instance of a Lewis acid causing catalysis in the absence of protons. Thus the concept is... 

Branchadall and co-workers studied the effect of Lewis acid (AlCfO catalysis on the Diels-Alder reactions of methyl (Z)-(5)-4,5-(2,2-propylidenedioxy)pent-2-enoate (10) with cyclopentadiene, which usually exhibits a high level of syn-endo selectivity under the influence of the Lewis acid, at the B-LYP/6-31G level [16]. The most stable conformation of the complex IO-AICI3 revealed the significant difference with the structure of uncomplexed molecule, i.e., s-trans arrangement of the carbonyl group with respect to the carbon-carbon double bond (Fig. 1-3). 

Electrophilic reagents are Lewis acids. Electrophilic catalysis is catalysis by Lewis acids. 

Keywords Diels-Alder reaction. Cycloaddition, Lewis acids, Enantioselective catalysis... 

Together, the biochemical, spectroscopic, and structural data suggest a mechanism for aconitase in which the unique iron site of the [4Fe S] cluster serves as a Lewis acid in catalysis, binding and polarizing substrate to facilitate the dehydration/rehydration reactions. The use of an iron-sulfur cluster in a nonredox role, as well as its function in binding substrates, was novel and unexpected, and was the first indication of the remarkable diversity of these clusters in functions beyond electron transfer. 

Zinc ion is essential for the catalytic activities of both yeast and liver alcohol dehydrogenase. Until recently, model systems have been notably unsuccessful in accounting for the participation of Zn(II) in the enzyme-catalyzed oxidoreductive interconversion of aldehyde and alcohol. The studies of Creighton and Sigman (20) and of Shinkai and Bruice (21, 22) conclusively demonstrate that Lewis (general) acid catalysis by Zn + (and other divalent metal ions) can effectively promote aldehyde reduction by the reduced 1,4-dihydropyridine moiety. 

In 2012, Scheldt and coworkers developed the enantioselective [3 + 2] an-nulation of enals and isatins under NHC/Lewis acid cooperative catalysis. Both electron-withdrawing and electron-donating groups were tolerated, furnishing the spiro-oxindole lactones 56 in good yields and with high enantioselectivities. The spiro-oxindole lactone was converted into maremycin B in five steps and in 17% overall yield (Scheme 20.27). 

Enamine/metal Lewis acid bifunctional catalysis has been used to achieve good yields, des, and excellent ees in hetero-Diels-Alder reactions of six-membered cyclic ketones ... 

The nickel-iminophosphine-catalysed 4- -2-cycloaddition of enones with allenes formed highly substituted dihydropyrans. The enantioselective amine-catalysed 4-I-2-cycloaddition of allenoates with oxo-dienes produced polysubstituted dihydropyrans in high yields and with high enantioselectivities. Novel enam-ine/metal Lewis acid bifunctional catalysis has been used in the asymmetric inverse-electron-demand hetero-Diels—Alder reactions of cyclic ketones with Q ,j9-unsaturated a-ketoesters. The 4- -2-cycloaddition of acylketenes (80) with 2-unsubstituted and 2-monosubstituted 3-aryl-2//-azirines (81) produced 1 1 (82) or 2 1 (83) adducts, being derivatives of 5-oxa-l-azabicyclo[4.1.0]hept-3-ene or 5,7-dioxa-l-azabicyclo[4.4.1]undeca-3,8-diene. The formation of the monoadducts proceeds via a stepwise non-pericyclic mechanism (Scheme 25). A-heterocyclic carbene-catalysed 4- -2-cycloaddition of ketenes with 1-azadienes yielded optically active 3,4-dihydropyrimidin-2-ones (93% ee) ... 

Metalloids-Based Lewis Acids Boron Catalysis. Metalloid compounds, including boron- and silicon-based organic compounds, have moderate... 

Mixtures of polyphosphoric acid (PPA) or K-10 clay and Bi(OTf)3 xH20 have been reported to be efficient systems for the rearrangement of indanone oximes (Equation 36) [73]. The apparent synergy between PPA and Bi(OTf)3 xH20 might be rationalized as Bronsted acid assisted Lewis acid (BLA) catalysis. 

Greeves e( al. reported that lanthanide triflates were effective for allylation of carbonyl compounds (20). Five mole percent of Yb(OTf)3 catalyzed the allylation of a series of aldehydes in good yield (Scheme 13.9) [20a]. Later they found a significant rate of acceleration by the Bronsted add in allylation of aldehydes with allyltributyltin catalyzed by ytterbium triflate (Table 13.9) [20b]. Recently, the concept of combined Lewis acid-Bronsted catalysis has attracted much attention from the chemical community because it often gives rise to significant improvement... 

Titanitun complexes are one kind of the most useful reagents in chemical synthesis, especially as the Lewis acids in catalysis. Owing to the facts of the high abundances, low cost, low toxicity, and diverse chemical reactivity of titanium, the study of titanium derivatives continue to provide various catalytic synthetic applications. A number of review articles on the catalytic organic reactions with titanium compounds as Lewis acids have appeared (1). This chapter will focus on the most recent advances of titanium Lewis acids for organic reactions. 

Hydrosilylations afford a-vinylsilanes when catalyzed by [CpRu (MeCN)3]PFg (eq 20). (Z)-/3-Vinylsilanes are similarly made under [RuCl2(p-cymene)]2 catalysis or by the trans hydrosilylation of 1-alkynes under Lewis acid (AICI3) catalysis. 

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