A tale of two topological isomers: Uptuning [Fe(IV)(O)(Me(4)cyclam)](2+) for olefin epoxidation.

TMC-anti and TMC-syn, the two topological isomers of [Fe(IV)(O)(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, or Me(4)cyclam), differ in the orientations of their Fe(IV)=O units relative to the four methyl groups of the TMC ligand framework. The Fe(IV)=O unit of TMC-anti points away from the four methyl groups, while that of TMC-syn is surrounded by the methyl groups, resulting in differences in their oxidative reactivities. TMC-syn reacts with HAT (hydrogen atom transfer) substrates at 1.3- to 3-fold faster rates than TMC-anti, but the reactivity difference increases dramatically in oxygen-atom transfer reactions. R(2)S substrates are oxidized into R(2)S=O products at rates 2-to-3 orders of magnitude faster by TMC-syn than TMC-anti. Even more remarkably, TMC-syn epoxidizes all the olefin substrates in this study, while TMC-anti reacts only with cis-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.