Photocatalytic olefin double bond cleavage acylation

Research groups led by Qing-Yuan Meng from the Institute of Chemistry, Chinese Academy of Sciences, and Xiu-Long Yang from Hebei University recently reported a novel acylation reaction for photocatalytic cleavage of olefin double bonds. Using a metal-free continuous photoredox catalytic strategy, they achieved a tertiary amine-mediated acylation of aromatic olefins via carbon-carbon double bond cleavage under mild conditions, resulting in the synthesis of a series of α-aryl ketones. Through controlled experiments and theoretical calculations, they explored the reaction mechanism, including the cleavage of both the π and σ bonds of the olefins, providing a new strategy for functionalization based on olefin double bond cleavage. This method exhibits excellent functional group compatibility and has potential applications in the synthesis and structural modification of bioactive molecules. These results were published as an open access article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background:

Olefins are a class of organic building blocks characterized by a carbon-carbon double bond (C=C) as a functional group. Due to their unique structural characteristics and reactivity, they occupy a prominent position in organic chemistry. Traditionally, chemists have developed a variety of effective methods for constructing complex carbon skeletons by utilizing strategies for cleaving and recombining σ and π bonds in olefins. However, functionalization reactions involving C=C double bond cleavage have primarily relied on olefin oxidative cracking reactions involving highly reactive ozone and olefin metathesis reactions catalyzed by transition metals.

Photocatalytic technology has attracted widespread attention due to its ability to achieve a variety of organic transformations under mild conditions. However, the application of this technology in olefin cracking reactions is still in the initial exploratory stage and is mainly limited to the photooxidative carbonylation reaction of olefins (Figure 1a). As an excited state reaction, photoreaction can usually achieve chemical transformations that are difficult to complete with traditional thermal catalysis. This provides a unique opportunity to develop new C=C double bond cleavage strategies and has important research significance for expanding olefin cracking functionalization reactions. In view of the problems of traditional α-aryl ketone synthesis methods such as dependence on transition metal catalysts, harsh reaction conditions, and limited functional group compatibility (Figure 1b), this study explored a new route for carbon-carbon double bond cleavage under visible light catalysis, realizing the C=C double bond cleavage acylation reaction of olefins with acid anhydrides or acyl imidazoles, and efficiently synthesizing α-aryl ketone compounds (Figure 1c).

Highlights of this article:

This study builds on the research progress made by Qing-Yuan Meng's group in the photocatalytic C=C double bond cleavage and carboxylation of olefins with CO₂. This strategy utilizes a sequential photoredox strategy, using bulky tertiary amines as N-α-radical precursors, to achieve carbon-carbon double bond cleavage of aromatic olefins and efficiently introduce acyl functional groups, resulting in the synthesis of a series of pharmaceutically active α-aryl ketones. Compared to traditional metal-catalyzed and N-heterocyclic carbene (NHC)-catalyzed acylation strategies, this reaction boasts mild conditions, a metal-free nature, and a broad substrate applicability (Figure 2). Furthermore, mechanistic validation, through radical capture, deuteration labeling, key intermediate transformation experiments, and DFT calculations, reveals the key transformation pathways for photo-promoted olefin π bond cleavage and radical-induced β-scission (Figures 3 and 4).

Summary and Outlook:

This study developed a novel strategy based on photocatalytic carbon-carbon double bond cleavage, enabling the efficient synthesis of α-aryl ketones from readily available anhydrides and aromatic olefins under mild conditions and exhibiting good functional group tolerance. Unlike conventional aromatic olefin oxidation pathways, this strategy avoids the formation of benzylic oxidation products, selectively yielding α-aryl ketones. Mechanistic studies revealed that the reaction proceeds through amine alkylation and acylation of the olefin C=C double bond, followed by photopromoted β-cleavage of the C=C single bond, to yield the target product. This work opens new avenues for the development of more diverse olefin cleavage functionalization reactions.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.