Acylating agents mild, selective

Mild reaction conditions and excellent selectivity provide a large scope of potential acylating agents that include a variety of alkyl and aryl methyl esters [133,136]. As a further advantage over traditional methods, acid sensitive esters readily undergo transesterification in quantitative yield (Table 21, entry 2). 

Under mild conditions (liquid phase, 160°C) HBEA zeolites can catalyse the acylation of phenol with phenyl acetate. High selectivity to p-hydroxyacetophenone is obtained by using sulfolane as a solvent, which can be explained by a better dissociation of phenyl acetate into acylium ions due to a solvation effect. However a competition between sulfolane and phenyl acetate for adsorption on the active acid sites is also demonstrated. A preliminary investigation of the effect of the acylating agent shows that generally, donor groups in aromatic acetates have a positive effect on the rate of acylation provided they do not block the access of the acetate to the acid sites of the zeolite pores. 

In addition to acid chlorides and acid anhydrides, there are a number of other types of compounds that are reactive acylating agents. Many have been developed to facilitate the synthesis of polypeptides, in which mild acylation conditions and high selectivity are required. The role of acylation reactions in polypeptide synthesis will be discussed in Part B, Chapter 11, but since these reagents can also be used effectively in the acylation of simpler compounds, brief consideration here is appropriate. 

Mayr [35] has shown very recently that, for very reactive systems, reaction will proceed without a catalyst. Bromination of phenol is claimed to occur with high / -selectivity [31], but this is unexceptional. A solution of sulphur trioxide in sulphur dioxide is claimed to be an especially mild sul-phonating agent. Sulphonation of toluene gives mainly /7-isomer, with no sulphones. Acylation of hindered alcohols with acid chlorides occurs unusually rapidly at ambient temperature [33] (equation 12.17). 

---