锰催化剂在烯烃氧化和 H2O2 歧化反应中的可逆失活,ACS Catalysis

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锰催化剂在烯烃氧化和 H2O2 歧化反应中的可逆失活
ACS Catalysis ( IF 11.3 ) Pub Date : 2023-04-25 , DOI: 10.1021/acscatal.3c00866
Johann B. Kasper ₁ , Laia Vicens ₂ , C. Maurits de Roo ₁ , Ronald Hage _{1,

3} , Miquel Costas ₂ , Wesley R. Browne ₁

Affiliation

具有基于氨基吡啶的配体的单核 Mn ^II氧化催化剂实现了烯烃与 H ₂ O ₂的高转换数 (TON) 对映选择性环氧化。结构反应性关系表明对映选择性和最大 TON 对外围配体取代基的电子效应的依赖性。通过在低温下进行反应并缓慢添加 H ₂ O ₂来减少竞争性 H ₂ O ₂歧化，这提高了烯烃氧化的 TON，但掩盖了取代基对周转频率 (TOF) 的影响。在这里，原位拉曼光谱提供了确定最小 TOF 大于 10 秒所需的高时间分辨率^–1在烯烃与配合物 [Mn(OTf) ₂ ( ^R PDP)] 的环氧化反应中 [其中 R = H ( ^H PDP-Mn ) 和 R = OMe ( ^MeO PDP-Mn ) 和^R PDP = N , N ′ -双(2″-(4″-R-吡啶基甲基)-2,2'-联吡咯烷)]。通过拉曼光谱同时进行的顶空监测表明，H ₂ O ₂歧化过程伴随着底物的氧化进行，并且底物氧化和 H ₂ O ₂歧化的反应性比率是配体依赖性的。_{值得注意的是，底物氧化和 H 2} O的速率由于催化剂逐渐失活，在连续添加 H ₂ O _{2的情况下，}₂歧化均随时间减少，这表明同一催化剂负责这两个反应。电化学、UV/vis 吸收和共振拉曼光谱和光谱电化学表明，Mn ^{II络合物在加入 H}₂ O ₂后数秒内氧化态增加，形成 Mn ^III和 Mn ^{IV的动态混合物}物种，其组成取决于温度和烯烃的存在。然而，正是这些复合物（静止状态）的形成，而不是配体降解，导致催化剂失活，尤其是在低温下，因此，催化剂的固有反应性大于观察到的 TOF。这些数据表明，解释配体取代基对氧化剂和最大 TON 的反应效率（和转化率）的影响需要考虑催化剂的可逆失活，尤其是各种反应途径的相对重要性。

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Reversible Deactivation of Manganese Catalysts in Alkene Oxidation and H2O2 Disproportionation

Mononuclear Mn^II oxidation catalysts with aminopyridine-based ligands achieve high turnover-number (TON) enantioselective epoxidation of alkenes with H₂O₂. Structure reactivity relations indicate a dependence of enantioselectivity and maximum TON on the electronic effect of peripheral ligand substituents. Competing H₂O₂ disproportionation is reduced by carrying out reactions at low temperatures and with slow addition of H₂O₂, which improve TONs for alkene oxidation but mask the effect of substituents on turnover frequency (TOF). Here, in situ Raman spectroscopy provides the high time resolution needed to establish that the minimum TOFs are greater than 10 s^–1 in the epoxidation of alkenes with the complexes [Mn(OTf)₂(^RPDP)] [where R = H (^HPDP-Mn) and R = OMe (^MeOPDP-Mn) and ^RPDP = N,N′-bis(2″-(4″-R-pyridylmethyl)-2,2′-bipyrrolidine)]. Simultaneous headspace monitoring by Raman spectroscopy reveals that H₂O₂ disproportionation proceeds concomitant with oxidation of the substrate and that the ratio of reactivity toward substrate oxidation and H₂O₂ disproportionation is ligand-dependent. Notably, the rates of substrate oxidation and H₂O₂ disproportionation both decrease over time under continuous addition of H₂O₂ due to progressive catalyst deactivation, which indicates that the same catalyst is responsible for both reactions. Electrochemistry, UV/vis absorption, and resonance Raman spectroscopy and spectroelectrochemistry establish that the Mn^II complexes undergo an increase in oxidation state within seconds of addition of H₂O₂ to form a dynamic mixture of Mn^III and Mn^IV species, with the composition depending on temperature and the presence of alkene. However, it is the formation of these complexes (resting states), rather than ligand degradation, that is responsible for catalyst deactivation, especially at low temperatures, and hence, the intrinsic reactivity of the catalyst is greater than observed TOFs. These data show that interpretation of effects of ligand substituents on reaction efficiency (and conversion) with respect to the oxidant and maximum TONs needs to consider reversible deactivation of the catalyst and especially the relative importance of various reaction pathways.

更新日期：2023-04-25

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