Abstract
The development of sustainable processes and products through innovative catalytic materials and procedures that allow a better use of resources is undoubtedly one of the most significant issues facing researchers nowadays. Environmental and economically advanced catalytic processes for selective oxidation of alcohols are currently focused on designing new catalysts able to activate green oxidants (dioxygen or peroxides) and applying unconventional conditions of sustainable significance, like the use of microwave irradiation as an alternative energy source. This short review aims to provide an overview of the recently (2015–2020) discovered homogeneous aerobic and peroxidative oxidations of primary and secondary alcohols catalyzed by copper complexes, highlighting new catalysts with potential application in sustainable organic synthesis, with significance in academia and industry.
Highlights
In organic synthesis, the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, aldehydes and ketones, respectively, plays a central role due to the wide use of these commodities as precursors and intermediates for fine chemistry [1,2,3].Classic oxidation methods use stoichiometric quantities of inorganic oxidants, such as manganese dioxide, chromium(VI) or the Swern or Dess—Martin reagents [4], which are often toxic and generate considerable amounts of by-products
2,2,6,6–tetramethylpiperidine-1-oxyl radical (TEMPO), have emerged as one of the most effective catalysts for selective alcohol oxidation, and these catalytic systems are frequently considered the biomimetic functional model of the galactose oxidase enzyme [8]. The efficiency of the latter has led to extensive research to mimic the active sites of this enzyme, leading to a variety of Cu/TEMPO catalytic systems being reported over the years
Repo and coworkers disclosed [32] a new Cu(I) complex bearing N-(4–fluorophenyl)-1(furan-2-yl)methanimine (C11 H8 FNO) [CuBr(C11 H8 FNO)2 ] (35, Figure 8) acting as catalyst for the selective oxidation of several primary alcohols to aldehydes under ambient (r.t.), conditions. 35 exhibited high activity, leading to, e.g., nearly quantitative yields of aldehydes (e.g., 1 h of oxidation of benzyl alcohol at r.t. yielded 99% of benzaldehyde, entry 18, Table 3)
Summary
The oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, aldehydes (or carboxylic acids) and ketones, respectively, plays a central role due to the wide use of these commodities as precursors and intermediates for fine chemistry (mainly drugs, vitamins and fragrances) [1,2,3]. 2,2,6,6–tetramethylpiperidine-1-oxyl radical (TEMPO), have emerged as one of the most effective catalysts for selective alcohol oxidation, and these catalytic systems are frequently considered the biomimetic functional model of the galactose oxidase enzyme [8] The efficiency of the latter has led to extensive research to mimic the active sites of this enzyme, leading to a variety of Cu/TEMPO catalytic systems being reported over the years. Other key challenges are functional group tolerance and the ability to chemoselectively oxidize the alcohol in the presence of other groups susceptible to oxidation In this minireview, the most recent advances (2015–2020 period) in the peroxidative or aerobic oxidation of primary and secondary alcohols, using copper catalysts in homogeneous medium, are addressed
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