Demystifying the Mechanism of Regio- and Isoselective Epoxide Polymerization Using the Vandenberg Catalyst
Demystifying the Mechanism of Regio- and Isoselective Epoxide Polymerization Using the Vandenberg Catalyst
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DOI：
10.1021/acs.macromol.7b02091
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发表时间：
2018-03-13
期刊：
影响因子：
5.5
通讯作者：
Lynd, Nathaniel A.
中科院分区：
文献类型：
--
作者：
Ferrier, Robert C., Jr.;Pakhira, Srimanta;Lynd, Nathaniel A.
对经典的Vandenberg催化剂的结构和反应机理进行了理论和实验研究，得到了一个一致的反应机理。最可能的反应途径是基于双（μ-氧代）二（铝）（BOD）的静止状态，通过单（μ-氧代）二（铝）（MOD）过渡状态进行。Vandenberg催化剂的等选择性来源于BOD结构的刚性及其沿聚醚主链与最后和倒数第二个氧杂原子沿着的键合。等选择性的能量驱动力是在相反构型的环氧化物的束缚过程中能量上有利的二级Al-O相互作用的损失，提供了一个ca。2 kcal/mol的驱动力为新兴的isoselectivity。实验光谱和动力学证据的基础上模型BOD和MOD复合物支持新的机制框架开发使用密度泛函理论计算。有目的地合成的BOD类似物的建议的范登堡结构产生的特征性的全同立构富集的聚（烯丙基缩水甘油醚），由经典的范登堡催化剂产生。原位H-1 NMR光谱的Vandenberg催化聚合的烯丙基缩水甘油醚揭示了活化焓（Δ H-双刃= 21 kcal/mol）和环氧-铝配位的能量学（Δ H = -4.0 +/- 1.0千卡/摩尔，Δ S = -0.018 +/-0.004kcal/（K mol）），通过观察聚合的H-1 NMR谱中位于Vandenberg催化剂活性位点上的乙酰丙酮化物的位移信号。
A combined theoretical and experimental investigation into the structure and mechanism of the classical Vandenberg catalyst for the isoselective polymerization of epoxides has led to a consistent mechanistic proposal. The most likely reaction pathway was based on a bis(mu-oxo)di(aluminum) (BOD) resting state that proceeded through a mono(mu-oxo)di(aluminum) (MOD) transition state. The isoselectivity of the Vandenberg catalyst was derived from the rigidity of the BOD structure and its bonding to the ultimate and penultimate oxygen heteroatoms along the polyether backbone. The energetic driving force for isoselectivity was the loss of an energetically favorable secondary Al-O interaction during enchainment of oppositely configured epoxides, providing a ca. 2 kcal/mol driving force for the emergent isoselectivity. Experimental spectroscopic and kinetic evidence based on model BOD and MOD complexes support the new mechanistic framework developed using density functional theory calculations. A purposefully synthesized BOD analogue of the proposed Vandenberg structure produced a characteristically isotactically enriched poly(allyl glycidyl ether) as produced by the classical Vandenberg catalyst. In situ H-1 NMR spectroscopy of a Vandenberg-catalyzed polymerization of allyl glycidyl ether revealed the activation enthalpy (Delta H-double dagger = 21 kcal/mol) and energetics of epoxide-aluminum coordination (Delta H = -4.0 +/- 1.0 kcal/mol, Delta S = -0.018 +/- 0.004 kcal/(K mol)) by observation of the shifting acetylacetonate signal located on the active site of the Vandenberg catalyst in the H-1 NMR spectra of polymerization.