Catalytic synthesis of phenols with nitrous oxide

Abstract

The development of catalytic chemical processes that enable the revalorization of nitrous oxide (N2O) is an attractive strategy to alleviate the environmental threat posed by its emissions1–6. Traditionally, N2O has been considered an inert molecule, intractable for organic chemists as an oxidant or O-atom transfer reagent, owing to the harsh conditions required for its activation (>150 °C, 50‒200 bar)7–11. Here we report an insertion of N2O into a Ni‒C bond under mild conditions (room temperature, 1.5–2 bar N2O), thus delivering valuable phenols and releasing benign N2. This fundamentally distinct organometallic C‒O bond-forming step differs from the current strategies based on reductive elimination and enables an alternative catalytic approach for the conversion of aryl halides to phenols. The process was rendered catalytic by means of a bipyridine-based ligands for the Ni centre. The method is robust, mild and highly selective, able to accommodate base-sensitive functionalities as well as permitting phenol synthesis from densely functionalized aryl halides. Although this protocol does not provide a solution to the mitigation of N2O emissions, it represents a reactivity blueprint for the mild revalorization of abundant N2O as an O source.

Simple and Practical Conversion of Benzoic Acids to Phenols at Room Temperature

“…A wide array of benzoic acids with various substituents and heterocycles were transformed into phenols under optimized conditions. When they carry weak electron-donating groups including alkyl substituents (12−15), strong electron-donating groups including −OMe at the meta position (16) and −OPh at the para position (17), and two alkyl ether substituents at the ortho and meta positions (18), these benzoic acids could be converted into the corresponding phenols efficiently (Figure 3a). To reveal the practicality, the reactions can be scaled up to the gram scale (12 mmol scale) without affecting the reaction efficiency (12)(13)(14)17).…”

“…Using GC-MS, the formation of CO 2 was confirmed in the model reaction (Figure 7d). When 18 O-labeled benzoic acid was employed as the starting material, careful analysis of the isotope distribution in the products revealed that the 18 O isotope only exists in CO 2 and the ester product only contains the regular 16 O atom (Figure 7e). This isotope labeling experiment suggests that the oxygen atom in the final phenol product is from m-CPBA instead of benzoic acid.…”

“…Phenols are ubiquitous in natural products, pharmaceuticals, agrochemicals, and functional polymers and can also serve as versatile synthetic intermediates in organic synthesis. − Therefore, developing simple, efficient, and practical approaches to prepare phenols is an important research topic in organic chemistry . Current strategies to synthesize phenols from various aryl precursors include the following: (1) direct oxidation of the C–H bond to the C–O bond; − (2) conversion of the C–X bond to the C–O bond in a redox-neutral manner assisted by transition metals; − and (3) oxidation of the C–M bond to the C–O bond , (Figure left). On the other hand, employing benzoic acids as the aryl precursors to prepare phenols is advantageous because (1) unlike direct C-H oxidation of arenes, benzoic acids’ oxidation requires no directing groups and can avoid the regioselectivity issue since the reactive sites of benzoic acids have been fixed without any bias; (2) unlike aryl (pseudo)­halides and aryl metal species, benzoic acids are stable, not moisture- and air-sensitive, and readily available with a wide range of structural diversity and can also be extracted from the biomass platform .…”

Green chemical pathway of plasma synthesis of ammonia from nitrogen and water: a comparative kinetic study with a N2/H2 system

“…38,39 As recent important progress, it is reported that the insertion of N 2 O into a Ni–C bond of the Ni catalyst enabled to deliver phenols with benign N 2 even at room temperature and 1.5–2 bar of N 2 O conditions. 40…”

“…38,39 As recent important progress, it is reported that the insertion of N 2 O into a Ni-C bond of the Ni catalyst enabled to deliver phenols with benign N 2 even at room temperature and 1.5-2 bar of N 2 O conditions. 40 N 2 O has also been found to be equally important in homogeneous solution chemistry, including selective oxidation of highly reactive compounds such as disilenes or low-valent metallocenes and the synthesis of complex organometallic molecules as nitrogen donors. 41 These potential revenues from N 2 O and the possible high conversion rate of N 2 O into NO x seem to be positive evidence for continuing to develop this ammonia synthesis based on N 2 /H 2 O plasma to a further matured level of a green alternative and/or a complement process to the H-B process in the near future.…”