Effect of Mixed-Solvent Environments on the Selectivity of Acid-Catalyzed Dehydration Reactions
Effect of Mixed-Solvent Environments on the Selectivity of Acid-Catalyzed Dehydration Reactions
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DOI：
10.1021/acscatal.9b03460
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发表时间：
2020-02-07
期刊：
影响因子：
12.9
通讯作者：
Van Lehn, Reid C.
中科院分区：
文献类型：
--
作者：
Chew, Alex K.;Walker, Theodore W.;Van Lehn, Reid C.
液相的组成可以改变各个反应步骤的速率，从而改变酸催化反应的选择性，但这些溶剂效应难以用于设计目的。本文报道了在433 K下，在纯水和极性非质子助溶剂γ-戊内酯、1，4-二氧六环、四氢呋喃、N-甲基-2-吡咯烷酮、四亚甲基亚砜和二甲基亚砜的水溶液中，布朗斯台德酸催化1，2-丙二醇脱水反应的动力学和选择性。我们发现，1，2-丙二醇脱水的主要产品是丙醛在大多数混合溶剂的环境与选择性之间的1和68摩尔%。相比之下，1，2-丙二醇在二甲基亚砜的含水混合物中脱水得到丙酮作为主要产物，具有高达48%的选择性和最小的丙醛形成。我们使用经典的分子动力学模拟探测这些溶剂的影响，通过计算极性非质子助溶剂的水溶液混合物中的1，2-丙二醇和丙醛的溶剂化自由能之间的差异，并在纯水中。我们发现，溶剂化自由能的差异与在所有混合溶剂环境中的丙醛形成的速率相关，这表明溶剂介导的稳定化的产物状态相对于反应物状态转化为对相同产物的选择性增加。模拟溶剂化自由能和实验反应速率/选择性之间的类似协议建立了酸催化脱水的顺式和反式-1，2-环己二醇和1，3-环己二醇。最后，1，2-丙二醇周围的溶剂化环境的分析表明，二甲基亚砜独特地与水竞争溶剂化反应性羟基，这导致在该溶剂系统中的反应机制的变化，导致丙酮而不是丙醛的形成。这些结果代表了一个步骤，对酸催化，液相过程的溶剂系统的计算有效的筛选。
The composition of the liquid phase can alter the rates of individual reaction steps and thus alter the selectivity of acid-catalyzed reactions, but these solvent effects are difficult to anticipate for design purposes. Herein, we report the kinetics and selectivity of Bronsted acid-catalyzed 1,2-propanediol dehydration in pure water and in aqueous mixtures of the polar aprotic cosolvents gamma-valerolactone, 1,4-dioxane, tetrahydrofuran, N-methyl-2-pyrrolidone, tetramethylene sulfoxide, and dimethyl sulfoxide at 433 K. We find that the major product of 1,2-propanediol dehydration is propanal in most mixed-solvent environments with selectivities between 1 and 68 mol %. In contrast, 1,2-propanediol dehydration in aqueous mixtures of dimethyl sulfoxide affords acetone as the major product with up to 48% selectivity with minimal propanal formation. We use classical molecular dynamics simulations to probe these solvent effects by computing the difference between the solvation free energies of 1,2-propanediol and propanal in aqueous mixtures of polar aprotic cosolvents and in pure water. We find that the difference in the solvation free energies is correlated with the rates of propanal formation in all mixed-solvent environments, indicating that the solvent-mediated stabilization of the product state relative to the reactant state translates to increased selectivity toward the same product. Similar agreement between simulated solvation free energies and experimental reaction rates/selectivities is established for the acid-catalyzed dehydration of cis- and trans-1,2-cyclohexanediol and 1,3-cyclohexanediol. Finally, analysis of the solvation environment around 1,2-propanediol shows that dimethyl sulfoxide uniquely competes against water to solvate reactive hydroxyl groups, which causes a change in reaction mechanism in this solvent system that leads to the formation of acetone rather than propanal. These results represent a step toward the computationally efficient screening of solvent systems for acid-catalyzed, liquid-phase processes.