Enhanced alkaline hydrolysis of monoesterified 4-tert-butylcalix[4]arenes involving intramolecular electrophilic catalysis by the phenolic hydroxy group

Abstract

Rate enhancements over model systems up to 1600-fold are observed in the alkaline hydrolysis of monobenzoate esters of calix[4]arenes.

Spectrophotometric titration of the mono-benzoate ester over a pH range indicates 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 °C) obey pseudo-first-order kinetics which fit the rate law, k_obs=(k₁K_w/K′_a+k₂[OH])[OH]/(K_w/K′_a+[OH]) where k₁ and k₂ correspond to bimolecular attack of hydroxide ion on monoanion and dianion respectively. The kinetics were measured at pHs at which the calixarene esters are in their monoanionic form. The kinetics of the alkaline hydrolyses (k_OH) of substituted benzoate esters of 4-nitrophenol were measured under the same conditions. The following Hammett equations are obeyed. log k₁= 1.86σ+1.33, log k₂= 2.21σ+ 0.34, pk′_a=–2.90σ+ 11.78, log k_OH= 2.23σ+ 0.26

The large negative Hammett ρ value for the pK′_a of the calixarenes (determined kinetically) is consistent with a strong interaction between the ester and the ionised hydroxy groups, attributed to formation of an intramolecular tetrahedral adduct. The formation of the adduct means that hydrolysis is retarded and the enhancements observed are lower limits.

The alkaline hydrolysis of the calixarene esters is due to hydroxide ion attack on monoanion for k₁ and on the dianion for k₂. The substantial negative Hammett ρ values for water attack on dianion and trianion respectively provide unequivocal evidence to exclude these mechanisms in favour of hydroxide ion attack.