Darensbourg Donald J, Yarbrough Jason C
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842, USA.
J Am Chem Soc. 2002 Jun 5;124(22):6335-42. doi: 10.1021/ja012714v.
The air-stable, chiral (salen)Cr(III)Cl complex (3), where H(2)salen = N,N'-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a M(n) value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 degrees C and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by (13)C NMR spectroscopy. Whereas the mechanism for the (salen)Cr(III)-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst]. This was demonstrated herein by way of in situ measurements at 80 degrees C and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)Cr(III)-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation.
空气稳定的手性（水杨醛缩邻苯二胺）铬（III）氯配合物（3），其中H(2)salen = N,N'-双（3,5-二叔丁基水杨基亚甲基）-1,2-环己二胺，已被证明是环氧环己烷与二氧化碳偶联生成聚（环己烯碳酸酯）以及少量反式环状碳酸酯的有效催化剂。通过凝胶渗透色谱法测定，如此制得的聚碳酸酯含有>99%的碳酸酯键，M(n)值为8900 g/mol，多分散指数为1.2。在80℃和58.5巴压力下进行的反应中，加入5当量的路易斯碱助催化剂N-甲基咪唑后，每克铬的周转数（TON）和周转频率（TOF）值分别从683克聚合物/克铬和28.5克聚合物/克铬/小时增加了3倍以上。尽管这种手性催化剂在环氧化物的不对称开环（ARO）方面有充分的文献记载，但在这种情况下，如通过(13)C NMR光谱所示，生成的共聚物完全是无规立构的。虽然（水杨醛缩邻苯二胺）铬（III）催化的环氧化物ARO机理显示出对[催化剂]的平方依赖性，这可能对于共聚反应的引发步骤是正确的，但导致共聚物或环状碳酸酯形成的碳酸酯链增长速率与[催化剂]呈线性依赖关系。本文通过在80℃和58.5巴压力下的原位测量证明了这一点。因此，存在一种用于生产共聚物的替代机理，这表明涉及环氧化物在铬（III）配合物的轴向位点的协同进攻，在该位点存在用于环氧化物开环的增长聚合物链。对这种（水杨醛缩邻苯二胺）铬（III）催化体系用于环氧丙烷与二氧化碳偶联的初步研究表明，尽管在高温下环状碳酸酯是主要产物，但在室温下聚碳酸酯的形成占主导。脂环族（环氧环己烷）和脂肪族（环氧丙烷）二氧化碳偶联的常见反应途径被认为是有效的，在后一种情况下，与共聚物形成相比，环状碳酸酯的生成对温度的依赖性更大。
