Olefin Epoxidation by the Hydrogen Peroxide Adduct of a Novel Non-heme Mangangese(IV) Complex:  Demonstration of Oxygen Transfer by Multiple Mechanisms

Olefin epoxidations are a class of reactions appropriate for the investigation of oxygenation processes in general. Here, we report the catalytic epoxidation of various olefins with a novel, cross-bridged cyclam manganese complex, Mn(Me₂EBC)Cl₂ (Me₂EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), using hydrogen peroxide as the terminal oxidant, in acetone/water (ratio 4:1) as the solvent medium. Catalytic epoxidation studies with this system have disclosed reactions that proceed by a nonradical pathway other than the expected oxygen-rebound mechanism that is characteristic of high-valent, late-transition-metal catalysts. Direct treatment of olefins with freshly synthesized [Mn^IV(Me₂EBC)(OH)₂](PF₆)₂ (p<i>K</i>_a = 6.86) in either neutral or basic solution confirms earlier observations that neither the oxo−Mn^IV nor oxo−Mn^V species is responsible for olefin epoxidization in this case. Catalytic epoxidation experiments using the ¹⁸O labels in an acetone/water (H₂¹⁸O) solvent demonstrate that no ¹⁸O from water (H₂¹⁸O) is incorporated into epoxide products even though oxygen exchange was observed between the Mn^IV species and H₂¹⁸O, which leads to the conclusion that oxygen transfer does not proceed by the well-known oxygen-rebound mechanism. Experiments using labeled dioxygen, ¹⁸O₂, and hydrogen peroxide, H₂¹⁸O₂, confirm that an oxygen atom is transferred directly from the H₂¹⁸O₂ oxidant to the olefin substrate in the predominant pathway. The hydrogen peroxide adduct of this high-oxidation-state manganese complex, Mn^IV(Me₂EBC)(O)(OOH)⁺, was detected by mass spectra in aqueous solutions prepared from Mn^II(Me₂EBC)Cl₂ and excess hydrogen peroxide. A Lewis acid pathway, in which oxygen is transferred to the olefin from that adduct, Mn^IV(Me₂EBC)(O)(OOH)⁺, is proposed for epoxidation reactions mediated by this novel, non-heme manganese complex. A minor radical pathway is also apparent in these systems.