Amines, acid-base equilibria acylation

On the other hand, rare-earth trifluoromethanesulfonates (rare earth triflate, RE(OTf)3) have been found to work efficiently as Lewis acids even in aqueous media or in the presence of amines [4], A catalytic amount of RE(OTf)3 enables several synthetically useful reactions, for example aldol, Michael, allylation, Mannich, Diels-Alder reactions, etc., to proceed. It has also been demonstrated that a small amount of RE(OTf)3 is enough to complete the reactions and that RE(OTf)3 can easily be recovered from the reaction mixture and can be reused. A key to accomplishing the catalytic processes was assumed to be the equilibrium between Lewis acids and Lewis bases, for example water, carbonyl compounds, and amines, etc. A similar equilibrium was expected between Lewis adds and aromatic ketones, and, thus, RE(OTf)3-catalyzed Friedel-Crafts acylation was investigated [5]. 

The analogous reaction of acyl chlorides with alcohols is a highly effective way of producing esters. A base such as an alkali metal hydroxide, pyridine, or a tertiary amine is usually added to neutralize the HCl by-product. Because acyl chlorides are readily made from the corresponding carboxylic acids (Section 19-8), the sequence RCOOH RCOQ RCOOR is a good method for esterification. By maintaining neutral or basic conditions, this preparation avoids Ihe equilibrium problem of acid-catalyzed ester formation (Fischer estoification. Section 19-9). 

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