Lewis acids protic solvents

In the case of Lewis acids, protic solvents such as water or alcohol can strongly influence their reactivity, cause it to react via an alternative path to the one desired, or even cause decomposition. Recently, rare earth metal triflates were used to develop water tolerant Lewis acids that can be used in many organic reactions. ... 

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

The Curtius rearrangement can be catalyzed by Lewis acids or protic acids, but good yields are often obtained also without a catalyst. From reaction in an inert solvent (e.g. benzene, chloroform) in the absence of water, the isocyanate can be isolated, while in aqueous solution the amine is formed. Highly reactive acyl azides may suffer loss of nitrogen and rearrange already during preparation in aqueous solution. The isocyanate then cannot be isolated because it immediately reacts with water to yield the corresponding amine. 

The first carbonyl complex of gold, [AuCl(CO)], was prepared in 192 52080,2081 and since then only a few more derivatives have been obtained. The [AuBr(CO)] derivative was prepared later and is unstable in the solid state.2082,2083 The reductive carbonylation of Au(S03F)3 in fluorosulfonic acid leads via [Au(CO)2]+ (solvent) to solid [Au(S03F)(C0)] (Scheme 30).2084 [Au(CO)2]+ salts are produced in strongly ionizing protic acids or in Lewis acids such as SbF5. 

S. Kobayashi, S. Nagayama, T. Busujima, Chiral Lewis Acid Catalysis in Aqueous Media. Catalytic Asymmdric Aldol Readions of Silyl Enol Ethers with Aldehydes in a Protic Solvent Induding Water Chem. Lett. 1999, 71-72. 

The donor number, DN, of a solvent, proposed by Gutmann, is a measure of the Lewis basicity of the solvent, i.e. its ability to donate a pair of electrons [16]. The DN is determined by measuring the negative enthalpy for the reaction of equimolar quantities of the solvent with the standard Lewis acid, SbCls, at room temperature in 1,2-dichloroethane (Scheme 1.1), and reflects the ability of the solvent to solvate Lewis acids. SbCls reacts with protic solvents such as alcohols... 

Hydrogen-bond donors have the ability to enhance the selectivities and rates of organic reactions. Examples of catalytic active hydrogen-bond donor additives are urea derivatives, thiourea derivatives (Scheme 10, Tables 12 and 13) as well as diols (Table 14). The urea derivative 7 (Scheme 9) increases the stereoselectivity in radical allylation reactions of several sulphoxides (Scheme 10)171. The modest increase in selectivity was comparable to the effects exerted by protic solvents (such as CF3CH2OH) or traditional Lewis acids like ZnBr2172. It was mentioned that the major component of the catalytic effect may be the steric shielding of one face of the intermediate radical by the complex-bound urea derivative. 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

Enolate ions, which are usually strong nucleophiles, are more important in preparative applications than are the enols. In additions to carbonyl groups, the carbon end, rather than the oxygen end, attacks but in SA,2 substitutions on alkyl halides, significant amounts of O-alkylation occur. The more acidic compounds, such as those with the j3-dicarbonyl structure, yield enolates with the greater tendency toward O-alkylation. Protic solvents and small cations favor C-alkylation, because the harder oxygen base of the enolate coordinates more strongly than does the carbon with these hard Lewis acids.147... 

In conclusion, SNAr reactions are often facihtated by polar protic solvents such as butanol or by polar solvents in the presence of Brpnsted or Lewis acids. When displacements of halides with amines are used, both polar and protic solvents can solubilize the ammonium halides resulting from the reactions, and lead to the formation of Brpnsted acids. Fluoride or chloride ions are preferred leaving groups for this reaction. Suliinyl and sulfenyl groups require somewhat harsher conditions. 

The solvating ability of solvents depends not only on their general polarity, which is a non-specific property, but in a large part to their ability to interact in a specific manner with the solute. This may take place by the donation of a nonbonding pair of electrons from a donor atom of the solvent towards the formation of a coordinate bond with the solute, therefore exhibiting Lewis basicity, or the acceptance of such a pair from a solute, an exhibition of Lewis acidity of a protic or protogenic solvent towards the formation of a hydrogen bond between it and... 

The neutral A1H3 molecule formed when an A1H4 ion acts as a hydride donor is a Lewis acid that coordinates to the negatively charged oxygen atom in the product of this reaction. When, in a second step, a protic solvent is added to the reaction, an alcohol is formed. 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

Lewis acid that acts as a Bronsted acid in protic solvents. In water the equilibrium... 

Notice that the acid catalyst is regenerated at the end of the reaction. The reaction need not be carried out in an acidic solvent, or even with a protic acid at all. Lewis acids make excellent catalysts for the bromination of ketones. This example with an unsymmetrical ketone gives 100% yield of the bromoketone with catalytic AICI3 in ether as solvent. 

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