Britovsek George J P, Cohen Steven A, Gibson Vernon C, Van Meurs Martin
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AY, United Kingdom.
J Am Chem Soc. 2004 Sep 1;126(34):10701-12. doi: 10.1021/ja0485560.
The bis(imino)pyridine iron complex, [[2,6-(MeC=N-2,6-iPr2C6H3)2C5H)N]FeCl2] (1), in combination with MAO and ZnEt2 (> 500 equiv.), is shown to catalyze polyethylene chain growth on zinc. The catalyzed chain growth process is characterized by an exceptionally fast and reversible exchange of the growing polymer chains between the iron and zinc centers. Upon hydrolysis of the resultant ZnR2 product, a Poisson distribution of linear alkanes is obtained; linear alpha-olefins with a Poisson distribution can be generated via a nickel-catalyzed displacement reaction. Other dialkylzinc reagents such as ZnMe2 and ZniPr2 also show catalyzed chain growth; in the case of ZnMe2 a slight broadening of the product distribution is observed. The products obtained from Zn(CH2Ph)2 show evidence for chain transfer but not catalyzed chain growth, whereas ZnPh2 shows no evidence for chain transfer. The Group 13 metal alkyl reagents AlR3 (R = Me, Et, octyl, IBu) and GaR3 (R = Et, nBu) act as highly efficient chain transfer agents, whereas GaMe3 exhibits behavior close to catalyzed chain growth. LinBu, MgnBu2 and BEt3 result in very low activity catalyst systems. SnMe4 and PbEt4 give active catalysts, but with very little chain transfer to Sn or Pb. The remarkably efficient iron catalyzed chain growth reaction for ZnEt2 compared to other metal alkyls can be rationalized on the basis of: (1) relatively low steric hindrance around the zinc center, (2) their monomeric nature in solution, (3) the relatively weak Zn-C bond, and (4) a reasonably close match in Zn-C and Fe-C bond strengths.
双（亚氨基）吡啶铁配合物[[2,6 - （MeC=N - 2,6 - iPr₂C₆H₃）₂C₅H）N]FeCl₂]（1）与MAO和ZnEt₂（> 500当量）组合，被证明可催化锌上的聚乙烯链增长。催化链增长过程的特征是增长的聚合物链在铁和锌中心之间进行异常快速且可逆的交换。水解所得的ZnR₂产物后，可得到线性烷烃的泊松分布；通过镍催化的置换反应可生成具有泊松分布的线性α - 烯烃。其他二烷基锌试剂如ZnMe₂和ZniPr₂也显示出催化链增长；对于ZnMe₂，观察到产物分布略有变宽。从Zn（CH₂Ph）₂得到的产物显示出链转移的证据，但没有催化链增长，而ZnPh₂没有显示出链转移的证据。第13族金属烷基试剂AlR₃（R = Me、Et、辛基、异丁基）和GaR₃（R = Et、正丁基）作为高效的链转移剂，而GaMe₃表现出接近催化链增长的行为。正丁基锂、丁基镁和三乙基硼导致催化剂体系活性非常低。四甲基锡和四乙基铅可得到活性催化剂，但链转移到锡或铅的量很少。与其他金属烷基相比，ZnEt₂的铁催化链增长反应效率显著，这可以基于以下几点来解释：（1）锌中心周围相对较低的空间位阻，（2）它们在溶液中的单体性质，（3）相对较弱的Zn - C键，以及（4）Zn - C和Fe - C键强度相当接近的匹配。
