Elucidation of transition structures and solvent effects for epoxidation by dimethyldioxirane
Elucidation of transition structures and solvent effects for epoxidation by dimethyldioxirane
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DOI：
10.1021/ja971766a
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发表时间：
1997-12-31
影响因子：
15
通讯作者：
Jorgensen, WL
中科院分区：
文献类型：
--
作者：
Jenson, C;Liu, J;Jorgensen, WL
二环氧乙烷是多用途的氧化剂，特别适用于烯烃的环氧化。1合成和机理研究表明，反应速率和非对映选择性对溶剂的选择具有显著的敏感性。1-6 Baumstark及其同事记录了顺式烯烃比反式异构体更大的反应性，并且在丙酮中向二甲基二环氧乙烷（DMD）反应中添加H2O也提供了增加的反应性。[2]他们提出了一种极化的螺环过渡结构，这种结构将受益于溶剂分子的氢键捐赠。最近，非对映体选择性和溶剂效应的进一步量化已经发生; 3，4可以在过渡结构中提供内部H-键的底物被发现表现出增强的反应速率，4-6和手性二环氧乙烷的对映体选择性环氧化已经被报道。7在本研究中，我们采用从头算密度泛函方法在B3 LYP/6- 31 G * 水平上对DMD与顺式和反式2-丁烯反应的反应物几何构型和过渡态结构进行了优化，详细解释了选择性和溶剂效应的起源。8-11图1沿着示出了由此产生的过渡结构以及计算出的电子活化能。顺式-2-丁烯的反过渡结构（TS）是一个较早的TS比空间拥挤的顺式替代，这是3.4千卡/摩尔的能量高。与反式-2-丁烯加成的活化能比顺式异构体的活化能高1.7千卡/摩尔，这与观察到的顺式烯烃的反应性高8-10倍完全一致。2反TS振动频率的计算证实了稳定点的性质，并提供了298 K下的活化焓和熵为11.8 kcal/
Dioxiranes are versatile oxidizing reagents with particular utility in the epoxidation of olefins. 1 Synthetic and mechanistic investigations have revealed notable sensitivity of reaction rates and diastereoselectivity to solvent selection. 1-6 Baumstark and co-workers documented the greater reactivity of cis alkenes than trans isomers and that addition of H2O to dimethyldioxirane (DMD) reactions in acetone also provides increased reactivity. 2 They invoked a polarized, spiro transition structure that would benefit from H-bond donation from solvent molecules. Recently, further quantification of diastereoselectivity and solvent effects has occurred; 3, 4 substrates that can provide an internal H-bond in the transition structures were found to exhibit enhanced reaction rates, 4-6 and enantioselective epoxidations with chiral dioxiranes have been reported. 7 In the present computational study, the detailed origins of the selectivity and solvent effects are elucidated.Ab initio density functional calculations at the B3LYP/6-31G* level were used to fully optimize geometries for reactants and transition structures for the reactions of DMD with cis-and trans-2-butene. 8-11 The resultant transition structures are illustrated in Figure 1 along with the computed electronic activation energies. The anti transition structure (TS) for cis-2-butene is an earlier TS than the more sterically crowded syn alternative, which is 3.4 kcal/mol higher in energy. The activation energy for addition to trans-2-butene is 1.7 kcal/mol higher in energy than that for the cis isomer, which is fully consistent with the observed 8-10-fold greater reactivity of cis alkenes. 2 Computation of the vibrational frequencies for the anti TS confirmed the nature of the stationary point and provided an enthalpy and entropy of activation at 298 K of 11.8 kcal/