Catalysis of acyl group transfer by a double-displacement mechanism: The cleavage of aryl esters catalyzed by calixcrown-Ba2+ complexes

The scope of the barium salt of p-tert-butplcalix[4]arene-crown-5 as a tran sacylation catalyst has been defined by evaluating its efficiency in the me thanolysis of a series of aryl acetates at 25.0 degrees C in MeCN/MeOH 9:1 (v/v) under slightly basic conditions. In this system a phenolic hydroxyl i s the acyl-receiving and -releasing unit in a double-displacement mechanism . The complexed barium ion acts both as a nucleophile carrier and a built-i n Lewis acid in providing electrophilic assistance to the ester carbonyl bo th in the acylation and deacylation step (nucleophilic- electrophilic catal ysis). Turnover capability is ensured by the acylated intermediate reacting with the solvent more rapidly than the original eater, but a serious drawb ack derives from the incursion of back-acylation of the liberated phenol. A gradual shift from rate-determining deacylation (p-nitrophenyl acetate) to rate-determining acylation (phenyl acetate) is observed along the investig ated series. It is shown that the scope of the catalyst is restricted to ac etate eaters whose reactivity lies in the range approximately defined by th e phenyl acetate-p-nitrophenyl acetate pair, with a maximum efficiency for p-chlorophenyl acetate. Moreover, the catalyst effectively promotes ester i nterchange be-tween phenols, showing that its activity is not limited to so lvolysis reactions. The very high sensitivity of the rate of acylation of t he catalyst to leaving group basicity has been interpreted as due to rate-d etermining decomposition of the tetrahedral intermediate, which is believed to arise from the presumably low basicity of the metal ion stabilized nucl eophile. The turnover frequency was in the range of 3.8 x 10(-4) min(-1) fo r phenyl acetate to 7.4 x 10(-3) min(-1) for p-nitrophenyl acetate ([ArOAc] (0) = 4.0 mM]). A first attempt to enhance the rate of acylation of the cat alyst through intramolecular general acid catalysis is also described.