Kinetics of Amide Formation through Carbodiimide/N-Hydroxybenzotriazole (HOBt) Couplings

Abstract

The kinetics of formation of amide, 4, from the corresponding carboxylic acid by reaction with the isopropyl ester of methionine (MIPE), mediated by carbodiimide EDCI, 1, and HOBt, 2, have been studied in 1-methyl-2-pyrrolidinone (NMP) using reaction calorimetry. The reaction rates have been found to be independent of the concentration of HOBt, showing that the rate-determining step is the reaction between the carboxylic acid and EDCI to give the corresponding O-acylisourea. The pH dependence of the observed rate constants for O-acylisourea formation is consistent with a second-order reaction between doubly protonated EDCI (EDCIH2(2+), 6) and the carboxylate group. The observed rate constants fall sharply at high pH, as the fraction of EDCI as EDCIH2(2+) continues to fall strongly, whereas the carboxylic acid group is already fully ionized. The rate constant, kP, for reaction between the carboxylate group of acid, 3, and EDCIH2(2+) has a value of kP = 4.1 x 10(4) M(-1) s(-1) at 20 degrees C, some 10(5) times higher than similar rate constants measured in water. The subsequent catalytic cycle, involving reaction of O-acylisourea with HOBt to give HOBt ester, which then reacts with the amine to give the amide with regeneration of HOBt, determines the product distribution. In the case of the amino acid, 3, reaction of the O-acylisourea with MIPE to give amide, 4, is increasingly favored at higher pH values over that with the less basic internal aromatic amine of 3 to give the diamide 5.