ENHANCED ALKALINE-HYDROLYSIS OF MONOESTERIFIED 4-TERT-BUTYLCALIX[4]ARENES INVOLVING INTRAMOLECULAR ELECTROPHILIC CATALYSIS BY THE PHENOLIC HYDROXY GROUP

Rate enhancements over model systems up to 1600-fold are observed in t he alkaline hydrolysis of monobenzoate esters of calix[4]arenes. Spect rophotometric titration of the mono-benzoate ester over a pH range ind icates ionisation of phenolic hydroxy groups at pK 6.84,12.14 and > 14 .02. The kinetics of hydrolysis of substituted monobenzoate esters of 4-tert-butylcalix[4]arene in 50% (v/v) ethanol-water solvent (at 25-de grees-C) obey pseudo-first-order kinetics which fit the rate law, k(ob s) = (k1K(w)/K(a)' + k2[OH])[OH/(K(w)/K(a)' + [OH]) where k, and k2 co rrespond to bimolecular attack of hydroxide ion on monoanion and diani on respectively. The kinetics were measured at pHs at which the calixa rene esters are in their monoanionic form. The kinetics of the alkalin e hydrolyses (k(OH)) of substituted benzoate esters of 4-nitrophenol w ere measured under the same conditions. The following Hammett equation s are obeyed. log k1 = 1.86sigma + 1.33 log k2 = 2.21sigma + 0.34 pK(a )' = -2.90sigma + 11.78 log k(OH) = 2.23sigma + 0.26 The large negativ e Hammett rho value for the pK(a)' of the calixarenes (determined kine tically) is consistent with a strong interaction between the ester and the ionised hydroxy groups, attributed to formation of an intramolecu lar tetrahedral adduct. The formation of the adduct means that hydroly sis is retarded and the enhancements observed are lower limits. The al kaline hydrolysis of the calixarene esters is due to hydroxide ion att ack on monoanion for k1 and on the dianion for k2. The substantial neg ative Hammett rho values for water attack on dianion and trianion resp ectively provide unequivocal evidence to exclude these mechanisms in f avour of hydroxide ion attack.