Manganese porphyrins concerted intermediate formation

The rhodium complex [CpRh(bipy)Cl2] is reported (162) to act as one-half of a redox couple that, in concert with a manganese porphyrin system, catalyzes the epoxidation of olefins by dioxygen. In this two-phase system, the aqueous phase contains sodium formate, and the organic phase is a trichloroethane solution of [Mnm(tpp)]1+ and the rhodium complex (tpp = meso-tetraphenylporphyrin). Apparently, the rhodium complex catalyzes the reduction of [Mnin(tpp)]1+ by formate, and the manganese(II) species thus formed binds dioxygen and reacts with the substrate olefin to form the epoxide. However, the intermedi-... 

The mechanism in Fig. 19.4 is stepwise, in which the radical intermediate is formed before the concerted one. Step 1 involves formation of the metal-oxo intermediate via oxygen transfer to the manganese center from iodosylbenzene. In Step 2, the alkene reacts with the oxidized porphyrin to form the radical intermediate. From this intermediate, the concerted intermediate is formed (Step 3), and finally styrene oxide desorbs in Step 4. The radical intermediate can also... 

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