Darensbourg Donald J, Mackiewicz Ryan M, Rodgers Jody L
Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
J Am Chem Soc. 2005 Oct 12;127(40):14026-38. doi: 10.1021/ja053544f.
The mechanism of the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexylene)carbonate catalyzed by (salen)CrN3 (H2salen = N,N,'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylene-diimine) in the presence of a broad range of cocatalysts has been studied. We have previously established the rate of copolymer formation to be very sensitive to both the electron-donating ability of the salen ligand and the [cocatalyst], where N-heterocyclic amines, phosphines, and ionic salts were effective cocatalysts. Significant increases in the rate of copolymerization have been achieved with turnover frequencies of approximately 1200 h(-1), thereby making these catalyst systems some of the most active and robust thus far uncovered. Herein we offer a detailed explanation of the role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide catalyzed by chromium salen derivatives. A salient feature of the N-heterocyclic amine- or phosphine-cocatalyzed processes is the presence of an initiation period prior to reaching the maximum rate of copolymerization. Importantly, this is not observed for comparable processes involving ionic salts as cocatalysts, e.g., PPN+ X-. In these latter cases the copolymerization reaction exhibits ideal kinetic behavior and is proposed to proceed via a reaction pathway involving anionic six-coordinate (salen)Cr(N3)X- derivatives. By way of infrared and 31P NMR spectroscopic studies, coupled with in situ kinetic monitoring of the reactions, a mechanism of copolymerization is proposed where the neutral cocatalysts react with CO2 and/or epoxide to produce inner salts or zwitterions which behave in a manner similar to that of ionic salts.
研究了在多种助催化剂存在下，(salen)CrN3（H2salen = N,N'-双(3,5-二叔丁基水杨醛)-1,2-乙二胺）催化氧化环己烯与二氧化碳共聚生成聚(环己撑)碳酸酯的机理。我们之前已经确定，共聚物的形成速率对salen配体的给电子能力和[助催化剂]都非常敏感，其中N-杂环胺、膦和离子盐是有效的助催化剂。通过约1200 h(-1)的周转频率实现了共聚速率的显著提高，从而使这些催化剂体系成为迄今为止发现的一些最具活性和稳定性的体系。在此，我们详细解释了助催化剂在铬salen衍生物催化二氧化碳与氧化环己烯共聚反应中的作用。N-杂环胺或膦催化过程的一个显著特征是在达到最大共聚速率之前存在一个引发期。重要的是，在涉及离子盐作为助催化剂的类似过程中，例如PPN+ X-，未观察到这种情况。在这些后一种情况下，共聚反应表现出理想的动力学行为，并被认为是通过涉及阴离子六配位(salen)Cr(N3)X-衍生物的反应途径进行的。通过红外和31P NMR光谱研究，结合反应的原位动力学监测，提出了一种共聚机理，其中中性助催化剂与二氧化碳和/或环氧化物反应生成内盐或两性离子，其行为方式与离子盐类似。
