Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts:: Two faces of an FeIII-OOH coin
Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts:: Two faces of an FeIII-OOH coin
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DOI：
10.1021/ja0120025
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发表时间：
2002-03-27
影响因子：
15
通讯作者：
Que, L
中科院分区：
文献类型：
--
作者：
Chen, K;Costas, M;Que, L
铁酶对碳-碳双键的氧化通常会导致环氧化物的形成，但在里斯克双加氧酶的情况下除外，其中产生顺式二醇。在此，我们报告了一组非血红素铁配合物，即 [Fe-II(BPMEN)(CH3CN)(2)](2+) (1, BPMEN = N,N'-二甲基-N,N'-双(2-吡啶基甲基)-1,2-二氨基乙烷) 催化 H2O2 烯烃氧化的系统研究，以及[Fe-II(TPA)(CH3CN)(2)](2+) (4, TPA = 三(2-吡啶基甲基)胺) 及其 6- 和 5- 甲基取代的衍生物。我们证明，烯烃环氧化和顺式二羟基化是常见 Fe-III-OOH 中间体反应性的不同方面，其自旋态可以通过配体环境的电子和空间性质进行调节。不超过 1 个 6-甲基取代基的催化剂有利于高度立体选择性环氧化，从而产生低自旋 Fe-III-OOH 物质（A 类）。另一方面，具有多个 6-甲基取代基的催化剂有利于顺式二羟基化，其提供高自旋 Fe-III-OOH 物质（B 类）。对于 A 类催化剂，环氧化物和顺式二醇产物均包含 (H2O)-O-18 中的 O-18，结果表明顺式 -(HO)-O-18-Fe-V=O 物种源自顺式 -(H2O)-O-18-Fe-III-OOH 中间体的 O-O 键杂解。相比之下，B 类催化剂通过假定的 Fe-III-eta(2)-OOH 物质将 (H2O2)-O-18 中的两个氧原子结合到主要的顺式二醇产物中。因此，该系列催化剂的一个关键特征是具有过氧化物活化所需的两个顺式不稳定位点。这里描述的烯烃环氧化和顺式二羟基化研究不仅证实了我们早期通过同一系列催化剂进行烷烃羟基化研究得出的机理方案（Chen, K.; Que, L, Jr. J. Am. Chem. Soc. 2001, 123, 6327），而且还进一步增强了其可信度。总而言之，这些反应证明了这些配合物的催化多功能性，并为 Nature 选择进行烯烃氧化的生物催化剂中的配体环境提供了理论依据。
The oxygenation of carbon-carbon double bonds by iron enzymes generally results in the formation of epoxides, except in the case of the Rieske dioxygenases, where cis-diols are produced. Herein we report a systematic study of olefin oxidations with H2O2 catalyzed by a group of non-heme iron complexes, i.e., [Fe-II(BPMEN)(CH3CN)(2)](2+) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [Fe-II(TPA)(CH3CN)(2)](2+) (4, TPA = tris(2-pyridylmethyl)amine) and their 6- and 5-methyl-substituted derivatives. We demonstrate that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe-III-OOH intermediate, whose spin state can be modulated by the electronic and steric properties of the ligand environment, Highly stereoselective epoxidation is favored by catalysts with no more than one 6-methyl substituent, which give rise to low-spin Fe-III-OOH species (category A). On the other hand, cis-dihydroxylation is favored by catalysts with more than one 6-methyl substituent, which afford high-spin Fe-III-OOH species (category B). For catalysts in category A, both the epoxide and the cis-diol product incorporate O-18 from (H2O)-O-18, results that implicate a cis-(HO)-O-18-Fe-V=O species derived from O-O bond heterolysis of a cis-(H2O)-O-18-Fe-III-OOH intermediate. In contrast, catalysts in category B incorporate both oxygen atoms from (H2O2)-O-18 into the dominant cis-diol product, via a putative Fe-III-eta(2)-OOH species. Thus, a key feature of the catalysts in this family is the availability of two cis labile sites, required for peroxide activation. The olefin epoxidation and cis-dihydroxylation studies described here not only corroborate the mechanistic scheme derived from our earlier studies on alkane hydroxylation by this same family of catalysts (Chen, K.; Que, L, Jr. J. Am. Chem. Soc. 2001, 123, 6327) but also further enhance its credibility. Taken together, these reactions demonstrate the catalytic versatility of these complexes and provide a rationale for Nature's choice of ligand environments in biocatalysts that carry out olefin oxidations.