Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

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

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, MnO units favor CC 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 MnO 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 MnO unit, which modulates the MnO BDFE, and the BDFEs of the allylic C-H bond of the olefins.

Directing transition metal-based oxygen-functionalization catalysis

This review presents the recent progress of oxygen functionalization reactions based on non-electrochemical (conventional organic synthesis) and electrochemical methods. Although both methods have their advantages and limitations, the former approach has been used to synthesize a broader range of organic substances as the latter is limited by several factors, such as poor selectivity and high energy cost. However, because electrochemical methods can replace harmful terminal oxidizers with external voltage, organic electrosynthesis has emerged as greener and more eco-friendly compared to conventional organic synthesis. The progress of electrochemical methods toward oxygen functionalization is presented by an in-depth discussion of different types of electrically driven-chemical organic synthesis, with particular attention to recently developed electrochemical systems and catalyst designs. We hope to direct the attention of readers to the latest breakthroughs of traditional oxygen functionalization reactions and to the potential of electrochemistry for the transformation of organic substrates to useful end products.

A Mononuclear Non-Heme Manganese(III)-Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer

Metal-oxygen complexes, such as metal-oxo [M(O^2-)], -hydroxo [M(OH^-)], -peroxo [M(O₂^2-)], -hydroperoxo [M(OOH^-)], and -superoxo [M(O₂^•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph₃P). However, OAT of metal-aqua complexes, [M(OH₂)]ⁿ⁺, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)Mn^III(OH₂)]²⁺ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph₃P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)Mn^III(OH)]⁺ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)Mn^II(OH₂)]⁺, followed by nucleophilic attack of H₂O in [(dpaq)Mn^II(OH₂)]⁺ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)₂C₆H₃S^•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph₃P occurs via electron transfer from Ph₃P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph₃P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.