Fenton chemistry enables the catalytic oxidative rearrangement of indoles using hydrogen peroxide

Abstract

The discovery of iron(ii) bromide and cerium(iii) bromide as a bifunctional catalyst enables the oxidative rearrangement of indoles with hydrogen peroxide as the terminal oxidant.

From Reactive Oxygen Species to Reactive Brominating Species: Fenton Chemistry for Oxidative Bromination

“…Fenton chemistry (Fe II /H 2 O 2 ) was developed in the 1890s by Henry J. H. Fenton as an analytical reagent and has been frequently used in the environmental sciences to destroy organic compounds through nonselective oxidation by hydroxyl/hydroperoxyl radicals (Figure a). − This nonselective oxidation has hindered its synthetic application in organic synthesis. On the other hand, it is well-known that hydroxyl and hydroperoxyl radicals are able to oxidize bromide ion (Figure b). − We envisioned that Fenton chemistry and bromide oxidation might be combined in a reaction to produce reactive brominating species (RBS, i.e. , Br + ) (Figure c) and that these in situ RBS could be used for various oxidative bromination reactions.…”

Electrochemical Oxidative Rearrangement of Tetrahydro-β-carbolines in Zero-gap Flow Cell

“…Inspired by this general mechanism, we envisioned that the electro-generated RHS could be used as an oxidant for the oxidative rearrangement of THβCs (Figure 2) because RHS generated in situ from chemical oxidation of halide (Oxone/halide or Fenton-halide) was successfully used for this reaction. 19,20 The challenge is the competing nonselective anodic oxidation of THβCs over halide. Notably, direct electro-oxidation of indoles was reported by Oliveira-Brett, 26 Mount, 27 Vincent, 28,29 Lei, 30 Fang, 31 etc.…”

“…[15][16][17][18] However, these chemical oxidants are hazardous and toxic and would generate stoichiometric amount of harmful chemical waste that poses a significant threat to human health and environment. Recently, our group reported two green protocols that exploited oxone-halide 19 and Fenton-halide 20 for the oxidative rearrangement of THβCs, which significantly reduce the negative impact on environment and human health.…”

Rapid Oxidation Indoles into 2‐Oxindoles Mediated by PIFA in Combination with n‐Bu4NCl ⋅ H2O

“…A breakthrough was very recently reported by the Tong's group, [2d,4a] in which different types of oxidized indole were prepared in a mild and green manner that included converting the indole into the 2‐oxindole, [2d,4a] oxidative rearrangement [2d,4a] and Witkop oxidation [2d] using oxone [2d] or H 2 O 2 [4a] as the terminal oxidant. Although often effective, this process suffers from limited benzene‐ring functional‐group tolerance (only Me, Br, and MeO substituents were investigated) and long reaction times (up to 4 h).…”