Relative reactivity of peracids versus dioxiranes (DMDO and TFDO) in the epoxidation of alkenes. A combined experimental and theoretical analysis
Relative reactivity of peracids versus dioxiranes (DMDO and TFDO) in the epoxidation of alkenes. A combined experimental and theoretical analysis
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DOI：
10.1021/ja026882e
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发表时间：
2003-01-29
影响因子：
15
通讯作者：
Schambony, S
中科院分区：
文献类型：
--
作者：
Bach, RD;Dmitrenko, O;Schambony, S
对计算的乙烯与二甲基二氧六环(DMDO)和过氧甲酸(PFA)环氧化反应的气相活化势垒(DeltaE双匕首)[在QCISD(T)//QCISD/6-31+G(d，p)]和E-2-丁烯[在QCISID(T)/6-31G(D)HB3LYP/6-311+G(3df，2p)]的给氧能力相似。在CH2Cl2溶剂中的竞争实验表明，在严格干燥的条件下，DMDO与环己烯的反应速度比过氧乙酸/醋酸快得多。DMDO的环氧化反应是由冰醋酸催化的，其经典活化势垒降低了8kcal/mol。在许多情况下，观察到的DMDO在溶液中环氧化速率的增加可能归因于良好的溶剂和氢键效应。这种预测的DMDO的环氧化反应活性与通常认为的高度紧张的环状过氧化氢并不一致。重新评估了DMDO的应变能(SE)，其中和值(约11千卡/摩尔)现在与其在环氧化反应中对烯烃的固有气相反应性更一致。DMDO的不同寻常的热力学稳定性在很大程度上是双甲基和二氧六环取代效应以及异常强烈的C-H和C-CH3键的结果。在环氧化反应中，甲基(三氟甲基)二氧六环(TFDO)的计算活化势垒比DMDO低得多(平均DeltaDeltaE(双匕首)值约为7.5kcal/mol)。相对于DMDO的速率增加类似于10(5)，而与TFDO较高的应变能(SE类似于19kcal/mol)是一致的，这主要归因于CF3基团的诱导效应。我们还考察了烯烃菌株对PFA环氧化反应速率的影响。环丙烯(DeltaE(双匕首)=14.5kcal/mol)和环丁烯(DeltaE(双匕首)=13.7kcal/mol)的环氧化势垒仅略高于环戊烯(DeltaE(双匕首)=12.1kcal/mol)，反映了过渡态的应变几乎没有缓解。被扭曲或pi键扭曲的烯烃确实表现出低得多的活化势垒。
Comparative analysis of the calculated gas-phase activation barriers (DeltaEdouble dagger) for the epoxidation of ethylene with dimethyldioxirane (DMDO) and peroxyformic acid (PFA) [15.2 and 16.4 kcal/mol at QCISD(T)//QCISD/6-31+G(d,p)] and E-2-butene [14.3 and 13.2 kcal/mol at QCISID(T)/6-31G(d)HB3LYP/6-311+G(3df,2p)] suggests similar oxygen atom donor capacities for both oxidants. Competition experiments in CH2Cl2 solvent reveal that DMDO reacts with cyclohexene much faster than peracetic acid/acetic acid under scrupulously dried conditions. The rate of DMDO epoxidation is catalyzed by acetic acid with a reduction in the classical activation barrier of 8 kcal/mol. In many cases, the observed increase in the rate for DMDO epoxidation in solution may be attributed to well-established solvent and hydrogen-bonding effects. This predicted epoxidative reactivity for DMDO is not consistent with what has generally been presumed for a highly strained cyclic peroxide. The strain energy (SE) of DMDO has been reassessed and its moderated value (about 11 kcal/mol) is now more consistent with its inherent gas-phase reactivity toward alkenes in the epoxiclation reaction. The unusual thermodynamic stability of DMDO is largely a consequence of the combined geminal dimethyl- and dioxa-substitution effects and unusually strong C-H and C-CH3 bonds. Methyl(trifluoromethyl)dioxirane (TFDO) exhibits much lower calculated activation barriers than DMDO in the epoxiclation reaction (the average DeltaDeltaE(double dagger) values are about 7.5 kcal/mol). The rate increase relative to DMDO of similar to 10(5), while consistent with the higher strain energy for TFDO (SE similar to 19 kcal/mol) is attributed largely to the inductive effect of the CF3 group. We have also examined the effect of alkene strain on the rate of epoxiclation with PFA. The epoxiclation barriers are only slightly higher for the strained alkenes cyclopropene (DeltaE(double dagger) = 14.5 kcal/mol) and cyclobutene (DeltaE(double dagger) = 13.7 kcal/mol) than for cyclopentene (DeltaE(double dagger) = 12.1 kcal/mol), reflecting the fact there is little relief of strain in the transition state. Alkenes strained by twist or pi-bond torsion do exhibit much lower activation barriers.