Substituent effects in the addition of carboxylic acids to arylcarbodiimides

Rates of addition in aqueous solution of RCOOH (R = CH3-, CH3OCH2-, ClCH2-, Cl2CH-) to ArN=C=NCH2-CH2CH2N+(CH3)(3) [Ar = C6H5-, 3-ClC6H4-, 4-CH3OC6H4-, 3,4-Cl2C6H3-, 2,4-(CH3O)(2)C6H3-] yielding a transient O-acylisourea, have been measured as a function of pH. Relative activities indicate a reaction mechanism in which a carboxylate anion adds to a mono- or di-protonated arylcarbodiimide, available in minor amounts. Only a weak dependence of reaction velocity upon basicity of carboxylate nucleophile is noted (Bronsted beta value of similar to0.2). Ease of prefatory protonation of aryl-attached nitrogen within ArN=C=NR' (as estimated from the basicity of correspondingly substituted quinolines) appears to dominate reactivity, so that the presence of electron-donating ring substituents renders such an arylcarbodiimide significantly more susceptible to addition by carboxylates.