Kinetics of reaction of substituted pyridines and oxygen anions with methyl chloroformate in aqueous solution

Abstract

The rate constants for nucleophilic reaction of nine substituted pyridines with methyl chloroformate gives a sharply curved Brönsted plot, the slope changing from β 0.93 to 0.15 with increasing reactivity. This contrasts with the straight line (β 0.93) observed with p-nitrophenyl acetate, and is shown to be quantitatively consistent with a change in rate-determining step. It is argued that this is from breakdown to formation of a zwitterionic tetrahedral intermediate which is only just stable enough to exist. The hydrolytic stabilities of some of the methoxycarbonylpyridinium ions produced by this reaction, and the nucleophilic reactivities towards methyl cloroformate of imidazole, of phenolate, p-nitrophenolate, and acetate anions, and of phosphate dianion are reported and discussed.