posted on 2006-04-17, 00:00authored byGuochuan Yin, Maria Buchalova, Andrew M. Danby, Chris M. Perkins, David Kitko, John D. Carter, William M. Scheper, Daryle H. Busch
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 [MnIV(Me2EBC)(OH)2](PF6)2 (pKa = 6.86) in either neutral or basic solution confirms earlier observations
that neither the oxo−MnIV nor oxo−MnV 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 MnIV 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, MnIV(Me2EBC)(O)(OOH)+, was detected by mass
spectra in aqueous solutions prepared from MnII(Me2EBC)Cl2 and excess hydrogen peroxide. A Lewis acid pathway,
in which oxygen is transferred to the olefin from that adduct, MnIV(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.