Efficient Aerobic Oxidation of Organic Molecules by Multistep Electron Transfer

Abstract

This Minireview presents recent important homogenous aerobic oxidative reactions which are assisted by electron transfer mediators (ETMs). Compared with direct oxidation by molecular oxygen (O2), the use of a coupled catalyst system with ETMs leads to a lower overall energy barrier via stepwise electron transfer. This cooperative catalytic process significantly facilitates the transport of electrons from the reduced form of the substrate‐selective redox catalyst (SSRCred) to O2, thereby increasing the efficiency of the aerobic oxidation. In this Minireview, we have summarized the advances accomplished in recent years in transition‐metal‐catalyzed as well as metal‐free aerobic oxidations of organic molecules in the presence of ETMs. In addition, the recent progress of photochemical and electrochemical oxidative functionalization using ETMs and O2 as the terminal oxidant is also highlighted. Furthermore, the mechanisms of these transformations are showcased.

Iron(II)‐Catalyzed Aerobic Biomimetic Oxidation of Amines using a Hybrid Hydroquinone/Cobalt Catalyst as Electron Transfer Mediator

“…Although numerous oxidation reactions have been developed over the years, the demand for milder, more efficient, and sustainable methods has increased in recent times with the growing interest in green chemical procedures [1] . With regard to green methods, of particular interest are those inspired by biological processes, [2, 3] where environmentally friendly and inexpensive oxidants such as molecular oxygen (O 2 ) or hydrogen peroxide (H 2 O 2 ) are often used. However, direct selective oxidation of an organic substrate by H 2 O 2 or O 2 remains an unmet challenge because of the large energy barriers and low selectivity of such direct oxidations.…”

“…SSRC red ) is re‐oxidized by H 2 O 2 or O 2 . However, direct re‐oxidation of the SSRC red to SSRC by O 2 or H 2 O 2 may still be too slow and there are only a limited number of examples known in the literature of direct reoxidation of an SSRC red by O 2 or H 2 O 2 [2, 3] . In Nature this oxidation problem is solved by enlisting multiple enzymes and co‐enzymes as electron transfer mediators (ETMs), which lower the overall barrier for electron transfer from the SSRC red to H 2 O 2 or O 2 as shown in Scheme 1.…”

Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

“…Benzoquinone (BQ) is commonly used in cocatalytic quantities with a stoichiometric oxidant, such as MnO 2 − or a peroxide. ,, Molecular oxygen (O 2 ) has been used effectively, typically in combination with BQ and a transition-metal (TM) complex, such as iron phthalocyanine or Co­(salophen), as a cocatalyst to support catalytic turnover (Scheme ). ,,− In rare cases, O 2 is an effective oxidant for Pd-catalyzed allylic acetoxylation without any cocatalysts. , …”

Iron(II)‐Catalyzed Aerobic Biomimetic Oxidation of N‐Heterocycles

“…[4] A substrate‐selective redox catalyst (SSRC) can be used to solve this problem, however, direct re‐oxidation of the reduced form of the SSRC (i. e., SSRC red ) by H 2 O 2 or O 2 only works well in a limited number of cases. [ 3c , 3g ] By using electron transfer mediators (ETMs) the energy barrier for electron transfer from the SSRC red to H 2 O 2 or O 2 may be dramatically lowered (Scheme 1 ). [ 3c , 3g ]…”

“…[ 3c , 3g ] By using electron transfer mediators (ETMs) the energy barrier for electron transfer from the SSRC red to H 2 O 2 or O 2 may be dramatically lowered (Scheme 1 ). [ 3c , 3g ]…”