Lewis acids acylating agent activation

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

Depending on the strength of the Lewis acid and on the structure of the R substituent, the actual acylating agent may be the acid-base complex 1 or the acyl cation 2. The active reagents 1 or 2 or even a mixture of both undergo the electrophilic aromatic substitution affording the final ketone products [1]. 

One of the salient features of the original Friedel-Crafts acylation reaction is the requirement of one or more than one equivalent of Lewis acid catalyst that cannot be recovered and reused [1]. The possibility to carry out the reaction with only catalytic amounts of Lewis acids (such as iron trichloride, zinc chloride, iron, and iodine, or, better, with metal triflates such as lanthanide triflates) represents a significant development [113]. Metal triflates, developed in particular by Kobayashi, are very active (1-5% mol with respect to the acylating agent) and insensitive to the water content of the reaction medium and can be reused [126]. 

Initiators which have been used for the cationic polymerization of 8-capro-lactone include protic acids, Lewis acids, stabilized carbocations and acylating agents. The mechanism involves the formation of charged species, followed by the ring opening of the activated lactone by the attack of another monomer and insertion into the chain. This type of polymerization is not very significant for the production of PCL. The reaction is difficult to control and transesterification reactions, which occur, limit the molecular weight of the obtained... 

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