Complexation catalysis: effective charge development in the aminolysis of phenyl esters in chlorobenzene catalysed by crown ethers

Kinetics of the butylaminolysis of substituted phenyl acetates in chlorobenzene in the presence of a variety of crown ethers obey the following rate law. Rate = k(b)[BuNH(2)](2)[Ester] + k(c)[BuNH(2)][Crown][Ester] The individual rate constants fit Bronsted-type relationships: log k(b) = -0.75 pK(a) + 4.21 log k(c) = -0.58 pK(a) + 3.41 [18-crown-6] log k(c) = -0.61 pK(a) + 3.18 [12-crown-4] where pK(a) refers to the ionization of the phenol in aqueous solution. The Bronsted beta(1g) values for k(b) and k(c) are calibrated with the value of beta(eq) recently determined for acetyl transfer between phenolate ions in chlorobenzene. The sensitivity, beta(1g), of k(c) is consistent with the rate-limiting formation of a crown ether-zwitterion adduct with subsequent fast (non-rate-limiting) ArO-C bond fission. The Bronsted data for k(b) when calibrated by beta(eq) is consistent with rate-limiting proton transfer from zwitterion to base. 18-Crown-6 enables proton transfer to occur between phenol and butylamine in chlorobenzene according to the equation: BuNH(2) + ArOH + Crown reversible arrow Bu-NH3+. Crown + ArO- equilibrium constant (K) for the above reaction with a series of substituted phenols has a Bronsted selectivity (beta) of 2.1 compared with that for the ionization of phenols in water.