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 =kb[BuNH2]2[Ester]+kc[BuNH2][Crown][Ester]. The individual rate constants fit Brønsted-type relationships: log kb=–0.75 pKa+ 4.21, log kc=–0.58 pKa+ 3.41 [18-crown-6], log kc=–0.61 pKa+ 3.18 [12-crown-4] where pKa refers to the ionization of the phenol in aqueous solution.
The Brønsted β1g values for kb and kc are calibrated with the value of βeq recently determined for acetyl transfer between phenolate ions in chlorobenzene. The sensitivity, β1g, of kc 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 kb when calibrated by βeq 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.