Electronic effect in iron catalysed C-H bond oxidation

The search for efficient and selective methods to oxidize C-H bonds has been a longstanding goal in organic chemistry. However, it is less than 5 years that this goal managed to become a useful tool in synthetic chemists hands by the use of nonheme iron catalysts. One of the most efficient catalytic systems prepared so far is White’s complex, reported in Figure 1 (R=H, the first structure on the left). In our work we shed some light on the mechanism by which this system works, and evaluated the substituent effect on the reactivity of the catalyst in C-H bond oxidation. We report the synthesis and the characterization of the three substituted iron (II) PDP complexes shown above, two of which were never prepared before. The chosen substituents let us investigate the entire spectrum of electronic effects, from the EWGs to ERGs. Moreover we studied the reaction of the iron (II) complexes with several oxidants in order to delucidate further the mechanism by which the active form of the catalysts is generated and subsequently oxidizes the substrate. We observed the formation and the subsequent decay of an intermediate, which can be the Fe(III) peroxide complex or the the oxoferryl. To discriminate between these two hypothesis we used a non-peroxide type oxidant, the iodosylbenzene. Finally we studied the kinetics of the formation and the subsequent decay of the intermediate with the three substituted complexes, thus comparing their reactivity. We investigated also the substituent effect on the yield and the product distribution of the reaction. Our work let us understand further the way the Fe(II)PDP system works, and this is a step fundamental for the design of more efficient catalysts. The different oxidation activities exhibited by the substituted complexes also indicate the way to best modify the complex structure in order to improve their reactivity.