Molecular recognition in Mn-catalyzed C-H oxidation. Reaction mechanism and origin of selectivity from a DFT perspective
Molecular recognition in Mn-catalyzed C-H oxidation. Reaction mechanism and origin of selectivity from a DFT perspective
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DOI：
10.1039/b905317d
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发表时间：
2009-01-01
影响因子：
4
通讯作者：
Eisenstein, Odile
中科院分区：
文献类型：
--
作者：
Balcells, David;Moles, Pamela;Eisenstein, Odile
实验研究表明，使用[(terpy‘) Mn(OH2)(mu-O)(2)Mn(OH2)(terpy‘)]（3+）作为催化剂，布洛芬和甲基环己烷乙酸的C-H氧化反应具有很高的选择性，其中terpy’是由苯基连接剂和Kemp’三酸官能化的三吡啶配体，通过h键识别反应物。这里描述的实验表明，硫酸盐反阴离子以化学计量量存在，与锰代替水。以[(terpy') Mn(O)(mu-O)(2)Mn(SO4)(terpy')]（+）为模型催化剂进行了DFT计算，分析了选择性的来源及其与分子识别的关系，以及叔丁基苯甲酸对催化剂的抑制机理。计算表明，许多自旋态都具有自由基氧的特征，在能量上是可接近的。所有这些自旋态都通过反弹机制促进碳氢氧化。催化剂通过双氢键识别底物。这种相互作用使底物定向，诱导高选择性的C-H氧化。双氢键对反应物、过渡态和生成物具有相同程度的稳定作用。因此，与没有分子识别时相比，反应发生的能量更低。催化剂与叔丁基苯甲酸的结合被证明可以屏蔽未结合的底物进入活性氧位点，从而防止非选择性羟基化。结果表明，该催化剂的两个识别位点可以协同使用来控制进入反应中心的途径。
Experimental studies have shown that the C-H oxidation of Ibuprofen and methylcyclohexane acetic acid can be carried out with high selectivities using [(terpy') Mn(OH2)(mu-O)(2)Mn(OH2)(terpy')](3+) as catalyst, where terpy' is a terpyridine ligand functionalized with a phenylene linker and a Kemp's triacid serving to recognize the reactant via H-bonding. Experiments, described here, suggest that the sulfate counter anion, present in stochiometric amounts, coordinates to manganese in place of water. DFT calculations have been carried out using [(terpy') Mn(O)(mu-O)(2)Mn(SO4)(terpy')](+) as a model catalyst, to analyze the origin of selectivity and its relation to molecular recognition, as well as the mechanism of catalyst inhibition by tert-butyl benzoic acid. The calculations show that a number of spin states, all having radical oxygen character, are energetically accessible. All these spin states promote C-H oxidation via a rebound mechanism. The catalyst recognizes the substrate by a double H bond. This interaction orients the substrate inducing highly selective C-H oxidation. The double hydrogen bond stabilizes the reactant, the transition state and the product to the same extent. Consequently, the reaction occurs at lower energy than without molecular recognition. The association of the catalyst with tert-butyl benzoic acid is shown to shield the access of unbound substrate to the reactive oxo site, hence preventing non-selective hydroxylation. It is shown that the two recognition sites of the catalyst can be used in a cooperative manner to control the access to the reactive centre.