Metal-free allylic/benzylic oxidation strategies with molecular oxygen: recent advances and future prospects

Abstract

The selective oxo-functionalization of hydrocarbons under mild conditions with molecular oxygen as the terminal oxidant continues to be a hot topic in organic synthesis and industrial chemistry. Though many oxidation protocols in combination with transition metal salts, enzymes, organometallic catalysts, or organocatalysts have been summarized recently, a review that focuses solely on the metal-free allylic/ benzylic oxidation strategies with molecular oxygen is still unavailable. This critical review will summarize recent significant advances achieved in this important field under the scope of green chemistry, which covers the promising applications and brief mechanistic profiles involving three kinds of efficient catalysts, namely N-hydroxyimides, homogeneous/heterogeneous light-sensitive molecules, and heteroatomdoped carbon materials, and concerns the sustainability of these methods, as well as predicts the potential utilization of available but unreported analogous catalysts or catalytic systems in this field. Special emphasis will also be placed on the burgeoning metal-free strategies with visible light irradiation from the long-term greenness and sustainability of these oxidation processes due to their established appealing performances under ambient conditions.

<i>N</i>-Hydroxyphthalimide as a Catalyst of Cumene Oxidation with Hydroperoxide

“…[9][10][11][12] NHPI-catalyzed oxidation of hydrocarbons, (including cumene oxidation with oxygen), have been performed under mild conditions, and high yields as well as high selectivities of products have been achieved. In oxidation processes, PINO radical which is formed in-situ from NHPI, abstracts hydrogen atom from oxidized hydrocarbon much faster, than alkylperoxy radicals in non-catalytic processes (Figure 2).…”

Enhanced Structural Variety of Nonplanar <i>N</i>-Oxyl Radical Catalysts and Their Application to the Aerobic Oxidation of Benzylic C–H Bonds

“…The residue was purified with flash column chromatography (SiO 2 , n-hexaneEtOAc=2 : 1 1 : 1) to afford 5 (1.02 g, 94%). 2-Benzyloxy-3-(2,2,2-trif luoroethoxy)-3-trif luoromethy lisoindolin-1-one (4) According to the procedure A in which 2,2,2-trifluoroenthanol was used as the alcohol instead, chloride 2 (1.71 g, 5.00 mmol) was converted into ether 4 (1.57 g, 78%). 2-Hydroxy-3-(2,2,2-trif luoroethoxy)-3-trif luoromethy lisoindolin-1-one (6) According to the procedure B, benzyl ether 4 (1.45 g, 3.58 mmol) was converted into 6 (1.07 g, 95%).…”

Theoretical study on the catalytic reactivity of N-hydroxyphthalimide tuned by different heterocyclic substitutions on its phenyl ring for aerobic oxidation

“…Among the proposed strategies with the awareness of environmental issues and sustainability of chemical processes, the catalytic systems comprising hydroxyimide organocatalysts with or without co-catalysts have received substantial attention recently due to their established fascinating high reactivity and selectivity in diverse transformations [2][3][4][5][6][7], such as the production of phenols or hydroperoxides from isopropyl aromatics [8][9][10], aromatic carboxylic acids from toluenes [11], and keto-isophorone from a/b-isophorone [12].…”