Differences in chemoselectivity in olefin oxidation by a series of non-porphyrin manganese(iv)-oxo complexes

Abstract

High valent metal-oxo intermediates are versatile oxidants known to facilitate both oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions in nature. In addition to performing essential yet challenging biological reactions, these intermediates are known for their selectivity in favoring the formation of one oxidation product. To understand the basis for this selectivity, we explore the role of equatorial ligand field perturbations in Mn^IV-oxo complexes on chemoselectivity in cyclohexene oxidation. We also examine reactions of Mn^IV-oxo complexes with cyclohexene-d₁₀, cyclooctene, and styrene. Within this series, the product distribution in olefin oxidation is highly dependent on the coordination environment of the Mn^IV-oxo unit. While Mn^IV-oxo complexes with sterically encumbered, and slightly tilted, MnO units favor CC epoxidation products in cyclohexene oxidation, a less encumbered analogue prefers to cleave allylic C–H bonds, resulting in cyclohexenol and cyclohexenone formation. These conclusions are drawn from GC-MS product analysis of the reaction mixture, changes in the UV-vis absorption spectra, and kinetic analyses. DFT computations establish a trend in thermodynamic properties of the Mn^IV-oxo complexes and their reactivity towards olefin oxidation on the basis of the MnO bond dissociation free energy (BDFE). The most reactive Mn^IV-oxo adduct from this series oxidizes cyclohexene-d₁₀, cyclooctene, and styrene to give corresponding epoxides as the only detected products. Collectively, these results suggest that the chemoselectivity obtained in oxidation of olefins is controlled by both the coordination environment around the MnO unit, which modulates the MnO BDFE, and the BDFEs of the allylic C–H bond of the olefins.