A tale of two topological isomers: Uptuning [Fe<sup>IV</sup>(O)(Me<sub>4</sub>cyclam)]<sup>2+</sup> for olefin epoxidation.
basic_science · Level V
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- Record sourced from PubMed, PMID 38478690.
- Also identified by DOI 10.1073/pnas.2319799121 and PMC identifier 10962992.
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Abstract
TMC-<i>anti</i> and TMC-<i>syn,</i> the two topological isomers of [Fe<sup>IV</sup>(O)(TMC)(CH<sub>3</sub>CN)]<sup>2+</sup> (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me<sub>4</sub>cyclam), differ in the orientations of their Fe<sup>IV</sup>=O units relative to the four methyl groups of the TMC ligand framework. The Fe<sup>IV</sup>=O unit of TMC-<i>anti</i> points away from the four methyl groups, while that of TMC-<i>syn</i> is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC-<i>syn</i> reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC-<i>anti</i>, but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R<sub>2</sub>S substrates are oxidized into R<sub>2</sub>S=O products at rates 2-to-3 orders of magnitude faster by TMC-<i>syn</i> than TMC-<i>anti</i>. Even more remarkably, TMC-<i>syn</i> epoxidizes all the olefin substrates in this study, while TMC-<i>anti</i> reacts only with <i>cis</i>-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.