通过使用紫外可见光谱法测量相应的前体在DMSO中的平衡酸度,建立了包含9种代表性N-杂环烯烃(NHO)的布朗斯台德碱度标度。基本性(p K aH被调查的NHO的s)范围从14.7到24.1。不饱和NHO的碱性比其N-杂环卡宾(NHC)类似物的碱性强。但是,饱和盐的碱度比其NHC类似物的碱度弱得多,这在很大程度上是由于芳香化作用本质上影响NHC和NHO前体的酸离解。通过监测这些NHO与常用参比亲电试剂的反应动力学来光度法测量四个NHO的亲核性,以定量亲核反应性。通常,NHO的亲核性比常用的路易斯碱(例如Ph 3)强得多P或DMAP [4-(二甲氨基)吡啶],但比其NHC类似物弱;然而,将这一结论推广到具有明显电子和结构特性的各种亲电子试剂时,应谨慎行事。
N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)
作者:Stefan Naumann、Anthony W. Thomas、Andrew P. Dove
DOI:10.1002/anie.201504175
日期:2015.8.10
polymerization of propylene oxide (PO) using a special class of alkene—N‐heterocyclic olefins (NHOs)—as catalysts is described. Manipulation of the chemical structure of the NHO organocatalyst allows for the preparation of the poly(propylene oxide) in high yields with high turnover (TON>2000), which renders this the most active metal‐free system for the polymerization of PO reported to date. The resulting polyether