Substituent effects in the addition of carboxylic acids to arylcarbodiimidesElectronic supplementary information (ESI) available: details on the preparation of substrates and carbodiimide characterization data. See http://www.rsc.org/suppdata/p2/b1/b105867n/
作者:William L. Mock、Krzysztof J. Ochwat
DOI:10.1039/b105867n
日期:2002.3.25
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.
在水溶液中,RCOOH(R = CH3–, CH3OCH2–, ClCH2–, Cl2CH–)与ArNCNCH2CH2CH2N+(CH3)3 [Ar = C6H5–, 3-ClC6H4–, 4-CH3OC6H4–, 3,4-Cl2C6H3–, 2,4-(CH3O)2C6H3–]反应生成瞬态O-酰基异脲的加成速率,已通过测量pH值的变化来进行评估。相对活性表明反应机制涉及羧酸盐阴离子与少量存在的一或二质子化芳基碳二亚胺的加成。注意到反应速率与羧酸盐亲核试剂的碱性之间仅存在微弱依赖关系(Brønsted β值约为0.2)。芳基连接氮原子的初步质子化容易性(根据相应取代喹啉的碱度估算)似乎主导了反应活性,因此,芳环取代基的电子供体性质使得这种芳基碳二亚胺对羧酸盐的加成更为敏感。