Lewis acid catalytic

Complexes of other metals such as gallium, indium, lead, and antimony have also been used as Lewis acids. Catalytic enantioselective meso-epoxide ring-opening reactions using a chiral gallium(III) catalyst (Ga-Li-linked-BINOL) have been reported (Scheme 84).348 The chemical yields are much improved by linking two BINOL units. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

Whereas methyl 2-siloxycyclopropanecarboxylates are thermally stable up to temperatures as high as 170 °C, they readily rearrange at low temperatures under the influence of appropriate Lewis acids. Catalytic amounts (0.05-0.4 equiv.) of iodo-trimethylsilane within minutes to days promote a quantitative ring opening of cyclopropanes 755 to the corresponding silyl enol ethers 156 (Eq. 68, Table 4)88). 

CONCEPTUAL BACKGROUND GENERATING A MORE LEWIS ACIDIC CATALYTIC CENTER... 

Because of the extensive amount of waste generated in traditional Friedel-Crafts reactions, it is not surprising that this reaction has been studied in RTIL. Early examples included the use of catalytic chloroaluminate ionic liquids. However, the moisture sensitivity of such systems was a drawback. Therefore, water-stable rare-earth Lewis acids, such as Sc(CF3S03)3, have come to be used for these reactions.The same Lewis acid has also been used to catalyse Diels-Alder reactions in RTILs.Interestingly, in this example, the RTIL not only provided a means for recycling the catalyst but also accelerated the rate and improved selectivity. It has also been demonstrated that a moisture stable, Lewis acidic, catalytic ionic liquid could be prepared from choline chloride and zinc dichloride, and that this was an excellent medium for the Diels-Alder reaction. Yields of 90% or more were achieved in reaction times of between 8 min and 5h for a range of dienes and dienophiles. 

Likewise, the competitive reaction of aldehydes 23 and 24 with allyltributylstannane gives homoallylic alcohol (25) exclusively (Scheme 1.18). In contrast, nonfluorinated cycloadduct is produced mainly under the catalysis of BF3 etherate in methylene chloride. These results clearly demonstrate that fluorinated carbonyl compounds are more reactive however, there is a turnabout in the reactivity under the Lewis acid catalytic conditions because of the lower basicity of the carbonyl group than that of the nonfluorinated one. 

To make Lewis acids catalytic for the Freidel-Crafts reaction various solutions can be considered ... 

It is widely known that catalytic cracking of hydrocarbons takes place on the sites of the catalyst that possess the properties of Bronsted or Lewis acids catalytic activities for cracking and acidity run in parallel. 

Evans, D.A., Tregey, S.W., Burgey, C.S. er a/. (2000) C2-Symmetric copper(II) complexes as chiral Lewis acids. Catalytic enantioselective carbonyl-ene reactions with glyoxylate and pyruvate esters. Journal of the American Chemical Society, 122, 7936-7943. 

The X-ray structural studies offer strong evidence in support of a Lewis acid catalytic role for the active site zinc ion in peptide hydrolysis. Since the carbonyl group is no doubt a much weaker dipole than HaO, the initial (ground-state) interaction between the zinc ion and the substrate carbonyl oxygen must be stabilized by the summation of the weak bonding forces between enz5mie and substrate. The result is to displace the transition-state of the enzyme-catalyzed reaction (relative to its hypothetical nonenzymatic cormterpart) along the reaction coordinate toward the enzyme-substrate complex. 

The above principle is amply illustrated by the small molecule systems discussed in Section III. The roles proposed for zinc ion in the three enzyme systems discussed in Section IV also adhere to this principle. The accumulated experimental evidence makes it highly probable that zinc ion has a Lewis acid catalytic function both in the horse liver alcohol dehydrogenase-catalyzed reduction of aldehydes, and in the carboxypeptidase A-catalyzed hydrolysis of peptides. In contrast, the accumulated experimental evidence supports a role for zinc ion involving the enhancement of water nucleophilicity via inner sphere coordination in the carbonic anhydrase-catalyzed hydration of CO 2. The substrates for... 

Lewis acids Catalytic cure (homopolymerization) Modified amine complex. Anchor 1040 (Air Products) 5-12 phr, fast gel time... 

Tovrog BS, Diamond SE, Mares F, SzaUdewicz A (1981) Activation of cobalt-nitro complexes by lewis-acids - catalytic-oxidation of alcohols by molecular-oxygen. J Am Chem Soc 103 3522-3526... 

Evans DA, Murry JA, Kozlowski MC (1996) C2-Symmettic Copper(II) Cranplexes as Chiral Lewis Acids. Catalytic Enantioselective Aldol Additimts of SUylketeaie Acetals to (Benzyloxy)acetaldehyde. J Am Chem Soc 118 5814... 

Evans DA, Kozlowski MC, Bnrgey CS, MacMillan DWC (1997) C2-Symmetric Copper(II) Complexes as Chiral Lewis Acids. Catalytic Enantioselective Aldol Additions of Enolsilanes to Pyruvate Esters. J Am Chem Soc 119 7893... 

Gutmann, V. (1998). The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York. Denmark, S. E., and Wynn, T. "Lewis Base Activation of Lewis Acids Catalytic Enantioselective Allylation and Pro-pargylation of Aldehydes."/. Am. Chem. Soc., 123,6199-6200(2001). 

The tailored dinuclear aluminium Lewis acid displayed a high Lewis-acid catalytic activity due to a double electrophilic activation of a substrate s carbonyl group. This skilful ligand design turned a standard alkoxide into a real multitool for carbonyl chemistry. This type of catalyst was employed for instance in Mukaiyama aldol reactions, in MPV reductions and Oppenauer oxidations and related Tischchenko coupling reactions. 

Denmark SE, Wynn T (2001) Lewis base activation of lewis acids catalytic enantioselective allylation and propargylation of aldehydes. J Am Chem Soc 123 6199-6200... 

Denmark SE, Heemstra JR (2003) Lewis base activation of lewis acids. Catalytic enantioselective addition of silyl enol ethers of achiral methyl ketones to aldehydes. Org Lett 5 2303-2306... 

Denmark SE, Chung W-J (2008) Lewis base activation of lewis acids catalytic, enantioselective addition of glycolate-derived silyl ketene acetals to aldehydes. J Org Chem 73 4582-4595... 

Denmark SE, Heemstra JR (2007) Lewis based activatimi of lewis acids catalytic, enantioselective vinylogous aldol addition reactions. J Oig Chem 72 5668-5688... 

Since then, researchers have continuously refined the scope of the reaction. In the 1970 s and 1980 s the focus was to understand the stereochemical issues associated with the reaction. Researchers then gradually turned their attention to asymmetric Lewis acids, catalytic reactions, catalytic and enantioselective reactions, reactions catalyzed by Lewis bases, and reactions that can be performed in aqueous solvents. 

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