Radical and migratory insertion reaction mechanisms in Schiff base zirconium alkyls
作者:Paul D. Knight、Guy Clarkson、Max L. Hammond、Brian S. Kimberley、Peter Scott
DOI:10.1016/j.jorganchem.2005.03.043
日期:2005.11
nevertheless transforms an inactive precatalyst into a stable system for the polymerisation of ethene. Closely related unbridged salicylaldimine catalysts are known to be highly active catalysts, but in most cases they appear to suffer from high temperature instability. The first examples of zirconium alkyls of this class are isolated, and it is found that they are inherently much more resistant to decomposition
2,2'-二氨基-6,6'-二甲基联苯的四个水杨醛亚胺衍生物H 2 L 4–7,其中CN键在苯酚环上的取代基在空间上受到保护,形成锆的烷基,顺式-α-[Zr L 4–7(CH 2 Ph)2 ]。它们不是通过已确定的烷基1,2-迁移插入亚胺的途径分解,而是经历自由基机理。观察到的大量产品,动力学和热力学数据(Rice-Herfeld,3/2阶,正ΔS ‡)证明了这一点。),对空间因素的反应以及切换为不稳定的自由基离去基团会抑制反应的事实。相反,1,2-迁移插入是干净的一阶分子内过程,ΔS ‡为负。然而,配体的空间修饰将惰性的预催化剂转化为用于乙烯聚合的稳定体系。紧密相关的未桥水杨醛亚胺催化剂已知是高活性催化剂,但是在大多数情况下,它们似乎遭受高温不稳定性的困扰。分离了这类锆烷基的第一个实例,发现它们固有地对通过任一途径(迁移插入或自由基)分解的抵抗力强得多。结构研究被用来解释这种行为上的差异