C–H Bond Oxidation Catalyzed by an Imine-Based Iron Complex: A Mechanistic Insight

Abstract

A family of imine-based nonheme iron(II) complexes (LX)2Fe(OTf)2 has been prepared, characterized, and employed as C-H oxidation catalysts. Ligands LX (X = 1, 2, 3, and 4) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe(OTf)2 in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (L1)2Fe(OTf)(ClO4) and (L3)2Fe(OTf)2 are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. (1)H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (LX)2Fe(OTf)2 efficiently catalyze the oxidation of alkyl C-H bonds employing H2O2 as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C-H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C-H bonds. Isotopic labeling analyses show that H2O2 is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O-O cleavage, without the assistance of a cis-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H2O2 at (LX)2Fe(OTf)2 differs from that occurring in biomimetic iron catalysts described to date.

Oxidation chemistry of C–H bond by mononuclear iron complexes derived from tridentate ligands containing phenolato function

“…However our data is higher than the value obtained in radical based oxidation reactions. This also supports that it is a metal based oxidation reaction [61][62][63][64][65][66]. It is interesting to note that the copper complex described in our previous report [33] the substrate and radical species convert the reactant into the corresponding product [79].…”

Selective oxidation of benzyl alcohol to benzaldehyde, 1‐phenylethanol to acetophenone and fluorene to fluorenol catalysed by iron (II) complexes supported by pincer‐type ligands: Studies on rapid degradation of organic dyes

“…The percentage conversion and TON has been shown in Table . On the basis of A/K ratio, it was found that our complexes provided good catalytic activity as compared to the results reported by Stefano and coworkers however less efficient than the catalysts reported by Que and coworkers . (As shown in Table ).…”

“…Stefano and co‐workers recently reported oxidation of hydrocarbons by using mononuclear iron complexes derived from tridentate meridional ligands. In the mechanism of the reaction hexacoordinated iron was found to be pentacoordinated with the dissociation of one of the Fe‐N bonds during the reaction with hydrogen peroxide was reported and an intermediate iron complex having Fe IV =O moiety was generated …”

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

“…The so-called Shul'pin method [257][258][259] of analysis of oxygenates discussed above in this section and based on GC and PPh3 has been used by other authors (selected examples are given in Refs. [260][261][262][263][264][265][266][267][268][269][270][271][272][273][274][275][276][277][278]).…”