Review article
Recent advances in photochemical transformations using water as an oxygen source

https://doi.org/10.1016/j.cogsc.2023.100759Get rights and content

Abstract

Nowadays, as a consequence of the mushrooming growth of photoinduced electron transfer (PET), the water-involved photooxygenation reactions based on unsaturated bonds are becoming manifold and predominant. In this mini-review, we have summarized the recent advances in photochemical transformations using water as an oxygen source since 2020, and highlighted the sustainable feature brought by harnessing the green properties of water in photoreactions. Relevant reports are comprehensively organized by the type of O-containing product targets, including silanols, alcohols, carbonyls, phenols, diaryl ethers, and oxazolines. We hope this review would provide a prospective overview of this topic and stimulate significant research interest.

Introduction

The onset of Green Chemistry is historically marked by the emergence of concepts, such as atom economy, E-factor, aqueous synthesis, and so on, since the early 1990s [1, 2, 3, 4]. The publication of the 12 Principles of Green Chemistry in 1998 by Anastas and Warneris is a milestone in the cohesive area, which incrementally expand the existing definition of “performance” beyond chemical function to safety and sustainability [5, 6, 7]. Over recent years, the “Principles” offer extensive guidance to derive significant and profound results for studying the general system evolution toward greenness [8, 9, 10, 11]. Nature, as the source of inspiration, introduces optimization paradigms for chemists when it comes to sustainability. For instance, plants utilize photosynthesis to convert raw materials (CO2 and H2O) into fine chemicals (carbohydrates) by exploiting the renewable energy of solar photons. From this standpoint of view, photosynthesis is the quintessence of sustainable chemical manufacturing and the pinnacle that Green Chemistry aims to reach. Perhaps, one step forward in this nature-inspired direction is made by the state-of-the-art photochemical transformations using water as an oxygen source. By virtue of the possibility of generating open-shell reactive species under light irradiation and allowing versatile oxygenation reactions that are not achievable in the ground-state reactivities, this promising branch in photochemistry would predictably provide greener opportunities for industry and academia [12, 13, 14, 15, 16, 17, 18, 19, 20, 21].

Nowadays, as a consequence of the mushrooming growth of photoinduced electron transfer (PET) [17,22, 23, 24, 25, 26], photooxygenation reactions based on unsaturated bonds are becoming manifold and predominant in organic synthesis. Among these, dioxygen (O2) is commonly used as the oxygen source, since superoxide radical O2•− generated by photoreduction can successively participate in follow–up reactions via radical-type cascade [27,28]. In contrast to those using oxygen or peroxide as oxygen sources, H2O ensures the performance of such conversions with high functional group tolerance to fragile substrates. However, one fundamental impediment to confront in water-involved photooxygenation is its inability to directly react as a hydroxyl radical HO due to the excessively high bond dissociation energy of O–H bonds (118 kJ/mol). Therefore, the corresponding investigations focus on the compatible reactivities of water in PET reactions to serve as an oxygen source with a variety of radicophiles. Notably, most mechanisms proposed for the established protocols point to an in-situ production of a carbocation center, where water normally acts as a nucleophile for the installation of O-containing functionalities. This is exemplified by the well-developed photoredox catalytic synthesis of alcohols (Figure 1-A). As illustrated, initially, a photocatalyst (PC) couples an oxidative or reductive quenching cycle with a single electron transfer (SET) event that promotes the generation of reactive open-shell radicals from closed-shell precursors. The photogenerated radicals then add to olefins to afford the electrophilic β-carbon-centered radical intermediates, which lose an electron either to PC∗ or to PC•+ to afford the corresponding carbocation adducts. Upon nucleophilic addition to the carbocation species by the lone pair electron of H2O, various simple alkenes are converted into value-added alcohols [29, 30, 31, 32, 33]. On the other hand, the advent of (hetero)alkynes, including alkynes, isonitriles, and nitriles, as highly manipulable radical acceptors for the synthesis of complex carbonyl molecules has accelerated the active investigation (Figure 1-B). Strategically, trapping of the electrophilic sites by H2O followed by rearrangement or hydrolysis enables the construction of ketones and amides in a photochemical fashion [34, 35, 36, 37, 38, 39].

Previous relevant reviews on this field frequently focus on the innate green properties of photochemistry “in water” or “on water” where water is only used as a reaction medium [40, 41, 42, 43, 44, 45]. Thereby, this mini-review aims to shed light on the opportunities and challenges offered by photochemical transformations “with water” [45, 46, 47], highlighting the exclusive capacity of water to unravel new chemical behaviors and to drive unconventional reaction mechanisms for future advancements. Close attention was paid to the creative reaction modes, substrates, and techniques beyond the typical patterns (Figure 1-C), which covers the related reports in a period since 2020, and an outlook was taken into the possible future direction of this rapidly developing area.

Section snippets

Photocatalytic dehydrogenative cross-coupling of silanes with water to access silanols

Silanols belong to a family of compounds with significant importance in organic synthesis and pharmaceutical chemistry. However, known methods to access silanols suffer from the limitations such as the production of a stoichiometric amount of waste and the inevitable use of transition-metal-based proton reduction cocatalysts [48,49]. In 2021, Zhou and coworkers [50] disclosed a hydrogen evolution cross-coupling of silanes with water enabled by a synergistic combination of photoredox and

Photochemical transformations toward alcohols using water as an oxygen source

The synthesis of alcohols is one of the most classic and eternal themes in organic chemistry, which is well-exemplified by the numerous synthetic methods named after its luminaries, such as Grignard reaction, Nozaki-Hiyama-Kishi coupling, Baylis–Hillman reaction, Cannizzaro reaction, Meerwein-Ponndorf-Verley reduction, Prins reaction, Wittig rearrangement and so on. In contrast to traditional methodologies, photo-mediated alcohol synthesis with water not only provides a green option to overcome

Photochemical transformations toward carbonyls using water as an oxygen source

Similar to alcohols, the classic synthetic methods toward carbonyls are manifold, among which the selective oxidation reactions represent the primary technique for accessing ubiquitous C=O bonds, such as Corey-Kim oxidation, Rubottom oxidation, Wacker oxidation, Criegee oxidation, Swern oxidation, and so on. The water-involved photochemical transformations can effectively prevent the use of stoichiometric oxidants by the means of in situ photooxygenation, providing a sustainable direction into

Photochemical transformations toward phenols using water as an oxygen source

Phenols, as a common chemical, are widely used in the field of material science and pharmaceutical chemistry, with global production reaching more than 10 million tons per year. However, the current industrial approaches to phenols suffer from various drawbacks, such as low efficiency, high energy consumption, and significant waste production. The growing hydroxylation of aryl halides or arenes is considered one of the most promising alternatives. Accordingly, from an academic as well as

Miscellaneous oxygenation reactions using water as an oxygen source

In 2021, Nevado [73] used a merging photoredox and nickel catalysis to synthesize symmetrically substituted diaryl ether skeletons from aryl bromides in the presence of water. The reaction proceeds through a SET-aided process, in which the unreactive ArNi(II)OAr species are transformed into ArNi(III)OAr. ArNi(III)OAr undergoes C(sp2)−OAr reductive elimination, resulting in the final diaryl ethers (Figure 6-23). In 2022, Lambert [74] took advantage of the amino-oxygenation of aryl olefins under

Conclusion and outlook

In this mini-review, we have summarized the recent advances in photochemical transformations using water as an oxygen source, and highlighted the sustainable feature brought by harnessing the green properties of water in photoreactions. Given the successful developments of such methodologies as powerful tools in organic synthesis, the “with water” strategy is becoming a rapidly growing area. The desired products span a diverse array of O-containing compounds, including silanols, alcohols,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (21971224, 22171249), the Key Research Projects of Universities in Henan Province (23A150054, 23A180010), and the Natural Science Foundation of Henan Province (202300410375).

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