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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Counterion and Substrate Effects on Barrier Heights of the Hydrolytic Kinetic Resolution of Terminal Epoxides Catalyzed by Co(III)-salen
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Counterion and Substrate Effects on Barrier Heights of the Hydrolytic Kinetic Resolution of Terminal Epoxides Catalyzed by Co(III)-salen

机译：抗衡离子和底物对Co（III）-salen催化末端环氧化合物水解动力学拆分的势垒高度的影响

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摘要

Density functional theory (DFT) has been applied to the proposed rate-limiting step of the hydrolytic kinetic resolution (HKR) of terminal epoxides as catalyzed by Co-salen-X (X = counterion) in order to resolve questions surrounding the mechanism. The present results indicate that the bimetallic mechanism proposed by Jacobsen shows nonadditive, cooperative catalysis with a larger reduction in barrier height than the sum of the barrier height reductions from the two monometallic reaction pathways. We computed barrier heights for the reaction using several counterions (chloride, acetate, tosylate, and hydroxide). For the three counterions that are experimentally active (chloride, acetate, and tosylate) the barrier heights are 35, 38, and 34 kJ mol(-1), respectively, while for hydroxide it is 48 kJ mol(-1). The similarity of the barrier heights for chloride, acetate, and tosylate is in agreement with their similar peak reaction rates. The finding that Co-salen-X with these counterions leads to rather different overall reaction profiles suggests that they have quite different rates of reaction with epoxide to form the activated Co-salen-OH required for the bimetallic mechanism. Co-salen-OH is inactive as the sole catalyst for HKR, and this inactivity is ascribed to its larger barrier height for the ring-opening step, rather than to any inability to activate epoxide. Barrier heights were also computed using propylene oxide, 1-hexene oxide, and epichlorohydrin; propylene oxide and 1-hexene oxide have similar barrier heights, 35.5 and 33.2 kJ mol1, respectively, and epichlorohydrin has a significantly lower barrier height of 18.8 kJ mol(-1), which is qualitatively consistent with experiments showing faster reactions for epicholorohydrin than propylene oxide when catalyzed by Co-salen-OAc.
机译：密度泛函理论（DFT）已应用于Co-salen-X（X =抗衡离子）催化的末端环氧化物水解动力学拆分（HKR）的拟议限速步骤，以解决有关该机理的问题。目前的结果表明，雅各布森提出的双金属机理显示出非加成的，协同的催化作用，其势垒高度的减小比两个单金属反应途径的势垒高度减小的总和更大。我们使用几种抗衡离子（氯离子，乙酸根，甲苯磺酸根和氢氧根）计算了反应的势垒高度。对于具有实验活性的三个抗衡离子（氯离子，乙酸根和甲苯磺酸根），势垒高度分别为35、38和34 kJ mol（-1），而对于氢氧化物，势垒高度为48 kJ mol（-1）。氯化物，乙酸盐和甲苯磺酸盐的势垒高度相似，与它们的峰值反应速率相似。发现Co-salen-X与这些抗衡离子会导致完全不同的整体反应曲线，这一发现表明它们与环氧化物形成双金属机理所需的活化Co-salen-OH的反应速率完全不同。 Co-salen-OH作为HKR的唯一催化剂是无活性的，并且这种无活性归因于其在开环步骤中较大的阻隔高度，而不是由于没有任何活化环氧化物的能力。还使用环氧丙烷，1-己烯氧化物和环氧氯丙烷来计算壁垒高度;环氧丙烷和1-己烯氧化物的阻隔高度分别相似，分别为35.5和33.2 kJ mol1，环氧氯丙烷的阻隔高度明显更低，为18.8 kJ mol（-1），这与实验表明表氯醇比丙烯反应更快的实验定性一致Co-salen-OAc催化时的氧化物。

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