Oxygen compounds Lewis acid catalysis

A regioselective aldol condensation described by Biichi succeeds for sterical reasons (G. Biichi, 1968). If one treats the diaidehyde given below with acid, both possible enols are probably formed in a reversible reaaion. Only compound A, however, is found as a product, since in B the interaction between the enol and ester groups which are in the same plane hinders the cyclization. BOchi used acid catalysis instead of the usual base catalysis. This is often advisable, when sterical hindrance may be important. It works, because the addition of a proton or a Lewis acid to a carbonyl oxygen acidifies the neighbouring CH-bonds. 

Re has recently come to the forefront in liquid phase oxidation catalysis, mainly as a result of the discovery of the catalytic properties of the alkyl compound CH3Re03 [methyltrioxorhenium (MTO)]. MTO forms mono-and diperoxo adducts with H2O2 these species are capable of transferring an oxygen atom to almost any nucleophile, including olefins, allylic alcohols, sulfur compounds, amides, and halide ions (9). Moreover, MTO catalysis can be accelerated by coordination of N ligands such as pyridine (379-381). An additional effect of such bases is that they buffer the strong Lewis acidity of MTO in aqueous solutions and therefore protect epoxides, for example. 

Keto-enol tautomerism of carbonyl compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom (a Lewis base) to give an intermediate cation that can lose H from its a carbon to yield a neutral enol (Figure 22.1, p. 904). This proton loss from the cation intermediate is similar to what occurs during an El reaction when a carbocation loses H" to form an alkene (Section 11.14). 

The Lewis acid mechanism of catalysis is shown in Figure 6.2.8. In this mechanism, the tin compound functions as a classic Lewis acid, coordinating to the isocyanate either through the carbonyl oxygen or nitrogen in the initial step to polarize the carbonyl. This increases the electrophilicity of the isocyanate carbon toward nucleophilic attack by alcohol. It has been shown by cryoscopy that there is a strong association of isocyanate with di-n-butyltin dicarboxylates. ... 

Many additions to a,p-unsaturated carbonyl compounds, take advantage of coordination to the oxygen by a metal cation or a proton, or even just a hydrogen bond. This is true for hydrides and carbon nucleophiles. In such a situation, the LUMO coefficient is largest at the carbonyl carbon, but not at the p carbon. Thus, even soft nucleophiles can be expected to attack directly at the carbonyl carbon when Lewis or protic acid catalysis is involved. It is likely that the difference in the... 

Complexes of carbonyl oxygen with trivalent boron and aluminum compounds tend to adopt a geometry consistent with directional interaction with one of the oxygen lone pairs. Thus the C—O—M bonds tend to be in the trigonal (120°—140°) range and the boron or aluminum is usually close to the carbonyl plane.The structural specificity that is built into Lewis acid complexes can be used to advantage to achieve stereoselectivity in catalysis. For example, use of chiral ligands in conjunction with Lewis acids is frequently the basis for enantioselective catalysts. 

The catalytic effect of Ir4(CO)i2 on the methanation reaction is notoriously enhanced by the presence of strong Lewis acids, probably due to a cooperative activation of the carbon-oxygen bond. Thus the catalysis with Ir4(CO)i2 in a NaCl/AlCl3 melt converts at very reasonable rate carbon monoxide and hydrogen into light hydrocarbons, primarily ethane, without producing oxygenated compounds. 

Ceria is an oxide widely employed in catalysis for its redox properties. It also has weak Lewis acid sites and stronger basic sites, which makes this oxide suitable as a catalyst or a support for many reactions requiring acid-base pairs or basic sites. Combining redox and acid-base properties allows ceria-containing solids to catalyze numerous reactions involved in biomass transformation. The objective of this chapter is to review these reactions stressing the role of ceria-based solid catalysts in the conversion of oxygenated molecules, which are essential compounds in biomass. 

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