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

Abstract

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[BuNH₂]²[Ester]+k_c[BuNH₂][Crown][Ester]. The individual rate constants fit Brønsted-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 Brønsted β_1g values for k_b and k_c are calibrated with the value of β_eq recently determined for acetyl transfer between phenolate ions in chlorobenzene. The sensitivity, β_1g, of k_c is consistent with the ratelimiting formation of a crown ether–zwitterion adduct with subsequent fast (non-rate-limiting) ArO–C bond fission. The Brønsted data for k_b when calibrated by β_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: BuNH₂+ ArOH + Crown ⇌ Bu–NH₃⁺·Crown + ArO^–. The equilibrium constant (K) for the above reaction with a series of substituted phenols has a Brønsted selectivity (β) of 2.1 compared with that for the ionization of phenols in water.