Chandra Bittu, Ahsan Faiza, Sheng Yuan, Swart Marcel, Que Lawrence
Department of Chemistry, University of Minnesota, Minneapolis, MN 55455.
Institut de Química Computacional i Catàlisi and Department of Chemistry, University of Girona, 17003 Girona, Spain.
Proc Natl Acad Sci U S A. 2024 Mar 19;121(12):e2319799121. doi: 10.1073/pnas.2319799121. Epub 2024 Mar 13.
TMC- and TMC- the two topological isomers of [Fe(O)(TMC)(CHCN)] (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Mecyclam), differ in the orientations of their Fe=O units relative to the four methyl groups of the TMC ligand framework. The Fe=O unit of TMC- points away from the four methyl groups, while that of TMC- is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC- reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC-, but the reactivity difference increases dramatically in oxygen-atom transfer reactions. RS substrates are oxidized into RS=O products at rates 2-to-3 orders of magnitude faster by TMC- than TMC-. Even more remarkably, TMC- epoxidizes all the olefin substrates in this study, while TMC- reacts only with -cyclooctene but at a 100-fold slower rate. Comprehensive quantum chemical calculations have uncovered the key factors governing such reactivity differences found between these two topological isomers.
[Fe(O)(TMC)(CHCN)]（TMC = 1,4,8,11 - 四甲基 - 1,4,8,11 - 四氮杂环十四烷，即美西环素）的两种拓扑异构体TMC - 和TMC - ，其Fe = O单元相对于TMC配体框架的四个甲基的取向不同。TMC - 的Fe = O单元远离四个甲基，而TMC - 的Fe = O单元被甲基包围，导致它们的氧化反应活性存在差异。TMC - 与氢原子转移（HAT）底物反应的速率比TMC - 快1.3至3倍，但在氧原子转移反应中，反应活性差异急剧增加。TMC - 将RS底物氧化为RS = O产物的速率比TMC - 快2至3个数量级。更值得注意的是，在本研究中，TMC - 使所有烯烃底物环氧化，而TMC - 仅与环辛烯反应，但其反应速率慢100倍。全面的量子化学计算揭示了控制这两种拓扑异构体之间这种反应活性差异的关键因素。
