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(Me<sub>2</sub>EBC)Cl<sub>2</sub> (Me<sub>2</sub>EBC 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<sup>IV</sup>(Me<sub>2</sub>EBC)(OH)<sub>2</sub>](PF<sub>6</sub>)<sub>2</sub> (p<i>K</i><sub>a</sub> = 6.86) in either neutral or basic solution confirms earlier observations that neither the oxo−Mn<sup>IV</sup> nor oxo−Mn<sup>V</sup> species is responsible for olefin epoxidization in this case. Catalytic epoxidation experiments using the <sup>18</sup>O labels in an acetone/water (H<sub>2</sub><sup>18</sup>O) solvent demonstrate that no <sup>18</sup>O from water (H<sub>2</sub><sup>18</sup>O) is incorporated into epoxide products even though oxygen exchange was observed between the Mn<sup>IV</sup> species and H<sub>2</sub><sup>18</sup>O, which leads to the conclusion that oxygen transfer does not proceed by the well-known oxygen-rebound mechanism. Experiments using labeled dioxygen, <sup>18</sup>O<sub>2</sub>, and hydrogen peroxide, H<sub>2</sub><sup>18</sup>O<sub>2</sub>, confirm that an oxygen atom is transferred directly from the H<sub>2</sub><sup>18</sup>O<sub>2</sub> oxidant to the olefin substrate in the predominant pathway. The hydrogen peroxide adduct of this high-oxidation-state manganese complex, Mn<sup>IV</sup>(Me<sub>2</sub>EBC)(O)(OOH)<sup>+</sup>, was detected by mass spectra in aqueous solutions prepared from Mn<sup>II</sup>(Me<sub>2</sub>EBC)Cl<sub>2</sub> and excess hydrogen peroxide. A Lewis acid pathway, in which oxygen is transferred to the olefin from that adduct, Mn<sup>IV</sup>(Me<sub>2</sub>EBC)(O)(OOH)<sup>+</sup>, is proposed for epoxidation reactions mediated by this novel, non-heme manganese complex. A minor radical pathway is also apparent in these systems.