Active acylating agent

Appaiendy a molai equivalent of catalyst (AlCl ) combines with the acyl halide, giving a 1 1 addition compound, which then acts as the active acylating agent. Reaction with aromatics gives the AlCl complex of the product ketone hberating HX ... 

In these reactions the active acylating agent is the carbamyl chloride, formed by the reaction of the isocyanate with hydrogen chloride (137) ... 

Ca.rboxyhc Acids. Esters are the main products in the reaction of chloroformates with carboxyHc acids. The intermediate mixed carboxyhc—carbonic anhydrides are very active acylating agents (25—27) but these agents may be isolated in cold temperatures for producing useful products (28). 

Under more forcing conditions with acid anhydrides, EDA can form tetraacyl derivatives (29). However, much milder conditions or less active acylating agents are needed to obtain the monoamide essentially free of the diamide (30—32). 

The active acylating agent may be the protonated mixed anhydride, or, alternatively, the anhydride m dissociate to the acylium and trifluoroacetate ions ... 

The active acylating agent is presumably the C-protonated enol ester... 

Hydroxyenones have also been used in catalytic amide formation, although 1,2,4-triazole is required as a co-catalyst. Assumed protonation of the Breslow intermediate and tautomerisation generates an acylazolium intermediate, which is trapped by triazole, releasing the NHC and generating the acyltriazole 60 that is the active acylating agent for the amine (Scheme 12.11) [16]. 

Scheme 3.4. Preparation and Reactions of Active Acylating Agents...

Acyl triflamides are excellent acylating agents for alcohols and amines [52], Classical methods for carboxyl activation include formation of anhydrides, either homo- or mixed (e.g. phosphoryl, sulfonyl, etc.). Further activation is possible by adding 4-dimethylaminopyridine as catalyst [53], The active acylating agents are throught to be... 

Dimethylaminopyridine (DMAP) as a Highly Active Acylation Agent. Hofle, G. Steglich, W. Vorbrtlggen, H. Angew. Chem. fnt. Ed. Engl. 1978, 17, 569. 

Other Activated Acylating Agents for Ketone Synthesis... 

Effenberger, R and Epple, G. 1972. Electrophilic substitution of aromatic compounds. 1. Trifluoromethanesulfonic-carboxylic anhydrides, highly active acylating agents. Angew. Chem., Int. Ed. Engl. 11 299-300. 

Yb(OTf)3. More elaborate structures have also been cyclized. The active acylating agents are presumably ketenes [396]. 

An important group of reagents for converting carboxylic acids to active acylating agents are the carbodiimides. The most frequently used compound is dicyclohexylcarbodiimide. The mechanism is shown below ... 

Scheme 3.4 gives some typical examples of preparation and use of active acylating agents from carboxylic acids. 

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