Substituent effects in the addition of carboxylic acids to arylcarbodiimides

Abstract

Rates of addition in aqueous solution of RCOOH (R = CH₃–, CH₃OCH₂–, ClCH₂–, Cl₂CH–) to ArNCNCH₂CH₂CH₂N⁺(CH₃)₃ [Ar = C₆H₅–, 3-ClC₆H₄–, 4-CH₃OC₆H₄–, 3,4-Cl₂C₆H₃–, 2,4-(CH₃O)₂C₆H₃–] 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 (Brønsted β value of ∼0.2). Ease of prefatory protonation of aryl-attached nitrogen within ArNCNR′ (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.