Substituent effects in the addition of carboxylic acids to arylcarbodiimides

摘要

Rates of addition in aqueous solution of RCOOH (R = CH3–, CH3OCH2–, ClCH2–, Cl2CH–) to ArNu0001 u0001Cu0001 u0001NCH2-nCH2CH2Nu0002(CH3)3 [Ar = C6H5–, 3-ClC6H4–, 4-CH3OC6H4–, 3,4-Cl2C6H3–, 2,4-(CH3O)2C6H3–] yielding a transientnO-acylisourea, have been measured as a function of pH. Relative activities indicate a reaction mechanism in whichna carboxylate anion adds to a mono- or di-protonated arylcarbodiimide, available in minor amounts. Only a weakndependence of reaction velocity upon basicity of carboxylate nucleophile is noted (Brønsted β value of ∼0.2).nEase of prefatory protonation of aryl-attached nitrogen within ArNu0001 u0001Cu0001 u0001NRu0003 (as estimated from the basicity ofncorrespondingly substituted quinolines) appears to dominate reactivity, so that the presence of electron-donatingnring substituents renders such an arylcarbodiimide significantly more susceptible to addition by carboxylates.