Controlling Iron‐Catalyzed Oxidation Reactions: From Non‐Selective Radical to Selective Non‐Radical Reactions

Abstract

ΔpH important for chemoselectivity: The relationship between absolute proton concentration (pH value), the change in the pH value (ΔpH) and catalytic performance in important iron‐catalyzed selective oxidation reactions is demonstrated (see figure). It is clearly shown that an increased proton concentration led to higher catalyst activity and a small ΔpH during the reaction is responsible for improved selectivity.

Iron(II) α‐Aminopyridine Complexes and Their Catalytic Activity in Oxidation Reactions: A Comparative Study of Activity and Ligand Decomposition

“…The further course of the oxidation reaction depends on the “fate” of species 11 ; it decays by either homolytic or heterolytic bond cleavage of the FeO or OO bond, which has been reported to depend on the spin state of the iron complex51 and on the pH value 52. Homolytic cleavage of the OO or FeO bond yields RO .…”

“…Notably, complex [Fe II ( 6 ) 2 ]OTf 2 exhibits an NH unit; it may become significantly acidic upon coordination to the iron center. A nonradical reaction pathway through an iron–oxo species might be promoted at low pH values 52. Furthermore, for complex [Fe II ( 6 ) 2 ]OTf 2 , the alcohol/ketone ratio does not significantly change when performed in the presence of 2,4,6‐tri‐ tert ‐butylphenol (TBPH), which is an efficient scavenger for oxygen‐centered radicals 59…”

Fe-Catalyzed Oxidation Reactions of Olefins, Alkanes, and Alcohols: Involvement of Oxo- and Peroxo Complexes

“…Despite these examples, controlling iron-catalyzed oxidation reactions of alcohols with air or hydrogen peroxide remain difficult. In 2008, we demonstrated the possibility to switch between nonselective radical pathways and selective nonradical reactions by tuning the absolute pH of the reaction system [141]. As a benchmark reaction, the oxidation of benzyl alcohol was studied, which is catalyzed by various iron salts (mainly Fe(NO 3 ) 3 ) to give benzaldehyde.…”

Iron‐Catalyzed Direct Arylation of Unactivated Arenes with Aryl Halides

“…[28][29][30] The application of inexpensive, non-toxic, commercially available, and environmentally benign iron complexes as catalysts in chemical syntheses has attracted much attention. [31,32] Recently, iron has been utilized extensively as a catalyst to promote the "traditional" cross-coupling between R 1 X and R 2 M. [33][34][35][36][37][38][39][40][41] Iron catalysts are also involved in many important transformations, such as Friedel-Crafts benzylation, [42,43] carbonylation, [44] oxidation [45,46] and other processes. [47][48][49][50][51][52][53][54] Of great interest are the recently developed oxidative coupling reactions of Ar 1 M with Ar 2 H to generate Ar products by employing Fe complexes as the catalysts.…”