Acid-base catalysis acyl group transfer

The chemical reaction involved in the catalytic process of these reactions is, however, not a proton transfer but an acyl group transfer so that in all these studies the mechanism is one of nucleophilic catalysis rather than of general base catalysis. Some of the experimental methods and theoretical considerations are nevertheless related to some of those relevant to the study of general acid-base catalysis with polycarboxylic acids. 

Acyl Transfer Blfunctlcfnal Catalysis- Many mechanisms for enzjrme cataly-sls postulate cooperative ("push-pull") catalysis by an acid-base pair in the active site. The analogous bifunctional intramolecular catalysis has been widely sought the evidence presented generally consists of a maximum rate at a pH sufficiently acidic for the acid-catalytic function to be pro-tonated yet sufficiently basic for the base-catalytic function to be free. The hydrolysis of hexachlorophene monosuccinate (14). for example, exhibits such a rate maximum at pH 6.8 (between pK 5.20 for Che carboxyl group and... 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

Acid-catalyzed addition of aliphatic, aromatic or heteroaromatic cyanohydrins to ethyl vinyl ether, n-butyl vinyl ether or dihydro-4//-pyran provides base stable, protected cyanohydrin derivatives. Phase transfer catalyzed alkylation of aliphatic cyanohydrins with allylic bromides gave a-substituted a-allyl-oxyacetonitrile. Carbonyl compounds react wiA cyanide under phase transfer catalysis to give cyanohydrin anions, which are trapped by an acyl chloride or ethyl chloroformate to give acyl- or alkoxycarbonyl-protected cyanohydrins respectively. The reduction of the carbonyl group of an acyl cyanide by NaBH4 under phase transfer conditions followed by esterification serves as an alternative route to aldehyde-derived cyanohydrin esters. ... 

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