English

• As one of the fundamental goals of advanced synthesis and catalysis, developing efficient, economical, and modular synthesis of complex value-added molecules in an environmentally friendly manner has always attracted the attention of organic chemists [1–4]. Consequently, green catalytic strategies, including organocatalysis [5–15] and photoredox catalysis [16–18], have become the main pillars of modern organic synthesis. Although significant advancements have been achieved in each catalytic system, the combination of organocatalysis and photoredox catalysis in a synergetic way is still in its infancy, and its synthetic potential remains open-ended [19–26]. Therefore, merging organo- and photoredox catalysis to allow diversity-oriented synthesis of valuable molecules is highly desirable.

  Ketones represent one of the most prevalent structural motifs in various bioactive natural products and pharmaceutical agents [27–31]. They also serve as necessary starting materials for assembling structurally complex molecules. In particular, the 1,4-diketones, frequently used for diverse cyclization reactions, have played a crucial role in advancing heterocyclic chemistry and thus accelerating the discovery of novel agrochemicals, drug molecules, and dye materials. (Scheme 1a) [32–35]. Traditional preparation methods of 1,4-diketones typically involve forming one chemical bond and connecting two functionalized keto-substrates. As illustrated in Scheme 1b, there are two main established strategies, namely as 2,3-disconnection and 3,4-disconnection pathway, which include the classical nucleophilic substitution reaction [36–38], oxidative cross-coupling of metal enolates [39–44], and Stetter-type addition [45–54], respectively. In recent years, radical bis-acylation of simple olefins, based on a more efficient 1,2- and 3,4-disconnection strategy, has emerged as a powerful tool for rapidly synthesizing 1,4-diketones. For instance, in 2020, Xia reported an elegant example of using 4-benzoyl-dihydropyridines (DHP) as bis-acylating reagents to react with styrene in the presence of nickel catalysts under visible light irradiation [55]. Soon afterward, Wu and Xiao's groups independently utilized α-keto acids and olefins to achieve the target bis-acylation reactions by using photoredox/N(n-Bu)₄⁺ catalytic system and photoredox/chromium dual catalysis, respectively [56,57]. Later, Feng and co-workers disclosed that acyl fluorides were also suitable substrates for the bis-acylation process upon visible light-promoted NHC catalysis [58]. Our group has recently developed a conceptually-different oxidative radical NHC catalysis to achieve 1,4-diketone synthesis through bis-acylation of olefins by using various commercially available aldehydes as acylating substrates [59]. Despite this, all of the above works belong to the two-component reaction manifold, which could only allow the synthesis of symmetric 1,4-diketones (Scheme 1c, left). In parallel, the first example of an unsymmetric synthsis of 1,4-diketones was reported by Wu and co-workers, and they realized the three-component reactions of α-keto acids, 4-benzoyl-DHP, and simple styrene under photoredox catalysis [56]. After that, more catalytic synthesis methods were reported; for example, Larionov disclosed an elegant visible light-induced NHC-catalyzed radical coupling of aldehyde, oxime, and styrene to access a spectrum of 1,4-diketone products (Scheme 1c, right) [60]. Inspired by these advancements, we envisaged that the unsymmetric olefin bis-acylation process might be also achieved by merging NHC organocatalysis and photoredox catalysis [61,62]. As part of our continuing interests in NHC radical organocatalytic synthesis [6,63–72], we herein disclose the modular synthesis of 1,4-diketones through regioselective bis-acylation of olefins by using the bench-stable α-keto acids [27,73] and acyl imidazoles upon organo- and photoredox dual catalysis [74–76]. This protocol could offer a straightforward route to diketones featuring excellent functional group tolerance under mild conditions.

  Scheme 1

  

  Scheme 1.  Overview of the 1,4-diketone synthesis and our motivation of this study.

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  We commenced the investigations by selecting α-keto acid 1a, acyl imidazole 2a, and styrene 3a as model substrates, using a blue light-emitting diode (LED) as the visible-light source. To our delight, the corresponding 1,4-diketone 4a was smoothly isolated by using a triazolium salt catalyst C1 in dichloromethane at 60 ℃, albeit with poor yield (Table 1, entry 1). Next, several solvents were screened in the presence of NHC catalyst C1, photocatalyst PC1, and Cs₂CO₃ at 60 ℃. As shown in entries 2−5, dichloroethane (DCE) provided the best result. Subsequently, various NHC catalysts, including triazolium, thiazolium, and imidazolium salts, were tested in this dual catalytic system, but poor results were generally observed (entries 6−9).

  Table 1

  

  Table 1.  Optimization of reaction conditions.^a

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  The catalytic efficiencies of triazolium salts C2−C3 were unsatisfactory and delivered 4a in poor yield (entries 6 and 7). Other catalysts cannot promote the radical target transformation (entries 8 and 9). Further screening of photocatalysts did not improve the reaction outcome (entries 10−13). Then, the effect of various inorganic and organic bases was investigated, but no better results were obtained (entries 14−17). Moreover, we screened the amount of base and photocatalyst (entries 18−20), and the substrate loading (entries 21 and 22). Unfortunately, the yield did not improve. Furthermore, we screened the substrates with different leaving groups, such as benzoyl fluoride, benzoic anhydride. However, no significant promotion in conversion was observed (for more details, see Supporting information).

  With the optimized conditions in hand, we investigated the substrate scope by testing various α-keto acids 1 and acyl imidazoles 2 (see Supporting information for details). As exhibited in Scheme 2A, a range of α-keto acid substrates bearing either electron-deficient or electron-rich aryl substituents could afford the desired 1,4-diketones 4a−4j in 25%−65% yields. The thienyl and cyclopropyl substituted α-keto acids were also suitable substrates for this target reaction (4k−4l).

  Scheme 2

  

  Scheme 2.  Substrate scope of α-keto acids 1 and acyl imidazoles 2 for the synthesis of 1,4-diones. Reactions condition: 1 (0.10 mmol), 2 (0.20 mmol), 3a (0.20 mmol), Cs₂CO₃ (0.12 mmol), PC1 (0.002 mmol) and NHC C1 (0.02 mmol) in 1.0 mL DCE, irradiation with blue LEDs at 60 ℃ for 2 h. ^a I, intensity; T, temperature; Big scale, 2.0 mmol scale; c, concentration.

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  In addition, the transformation also tolerated a broad range of acyl imidazoles 2 with various electronic and steric properties, offering the 1,4-diones in moderate yields (Scheme 2B). In general, the steric and electronic nature of substituents on the aryl ring did not significantly affect the reaction outcome (4m−4u). Furthermore, 2-naphthyl and heteroaryl acyl imidazoles were also suitable for the transformation to afford products 4v−4z, respectively. Gratifyingly, a collection of the challenging alkyl acyl imidazoles also proved compatible with this catalytic system (4aa−4ad). Moreover, this organocatalytic system was successfully applied to the late-stage functionalization of acyl imidazole derived from probenecid, delivering the target product 4ae in acceptable yields. Furthermore, we applied a condition-based sensitivity screening approach (Scheme 2C) [77], and this reaction exhibited medium sensitivity to water, oxygen, temperature, and light intensity. Hence, an appropriate performance of this reaction requires the assurance of suitable light intensity and an inert atmosphere.

  Then, the reactions with various styrenes 3 were examined under the established conditions. As shown in Scheme 3, the styrenes bearing electron-withdrawing or electron-donating substituents at para- or meta-position could all participate in this reaction and offer the products 4af−4aq in acceptable yields. Moreover, the structure of 4ah was unambiguously confirmed by X-ray analysis. It is noteworthy that the ortho-substituted styrenes could smoothly participate in this reaction, affording the desired products respectively (4ar−4at). The disubstituted styrene could also participate in the reaction affording 4au in moderate yield. In addition, the catalytic system was suitable for naphthyl substrate and afforded the corresponding product 4av. The cyclic 1H-inene could also participate in this reaction, delivering product 4aw in 26% yield with 8:1 diastereoselectivity.

  Scheme 3

  

  Scheme 3.  Substrate scope of the styrenes 3 used for the synthesis of 1,4-diones. Reactions condition: 1a (0.10 mmol), 2a (0.20 mmol), 3 (0.20 mmol), Cs₂CO₃ (0.12 mmol), PC1 (0.002 mmol) and NHC C1 (0.02 mmol) in 1.0 mL DCE, irradiation with blue LEDs at 60 ℃ for 2 h. ^a The representative product 4ah was determined by X-ray diffraction analysis, and other products were tentatively assigned by analogy.

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  To showcase the synthetic value of these 1,4-diketones, we investigated various function group transformations. The products of five-membered heteroarenes 5−8 were obtained by treating the 1,4-diketone 4a under acidic conditions. Treating 4a with N₂H₄·H₂O and EtOH, a pyridazine 9 was obtained in good isolated yield. In addition, the oxidative dehydrogenation of 4a can quickly occur with a catalytic amount of Cs₂CO₃, delivering product 10 as a mixture of 7.5:1 trans/cis isomers in moderate yield (Scheme 4).

  Scheme 4

  

  Scheme 4.  Further synthetic applications.

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  In order to gain insights into the dual catalytic system, we performed a series of mechanistic investigations to elucidate the possible reaction mechanism (Scheme 5) [78–81]. First, control experiments indicated that blue LEDs, NHC, and PC were indispensable for this radical organocatalytic reaction. Then, in the presence of α-keto acid 1a and styrene 3a, the direct use of acyl azolium 11 could also provide desired product 4m in 33% yield (Scheme 5A). This result indicated that the acyl azolium sepeices generated from an imidazolide and NHC is the reactive intermediate. Furthermore, when 3,3-dimethyl-2-oxobutanoic acid 12 was used, alkylative acylation product 13 was obtained (Scheme 5A). Moreover, the catalytic process could be completely suppressed by adding TEMPO under standard condition (Scheme 5A). Then, a radical-clock experiment with the vinylcyclopropane substrate 14 afforded ring-opening product 15 (Scheme 5B). These results indicated that the corresponding catalytic reaction might proceed through a radical process. Next, a light on-off experiment embodied the products only formed during the constant irradiation (Scheme 5C). This result indicated that the reaction underwent a photo-catalytic process.

  Scheme 5

  

  Scheme 5.  Experimental studies for mechanistic investigation.

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  Moreover, there was a significant absorption of the photocatalyst [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ in the UV–vis absorption spectrum. By contrast, when the substrates, NHC catalyst, or their combination were tested, no absorption was observed in the spectrum of visible light (Scheme 5D). This result indicated that the visible light excitation of the photocatalyst initiated the photocatalytic process. Besides, we performed Stern-Volmer fluorescence quenching experiments. As illustrated in Scheme 5E, the excited photocatalyst PC1 could be readily quenched by Cesium phenylglyoxylate rather than acyl azolium. According to the cyclic voltammetry experiments (Scheme 5F), we proposed a tentative mechanism in which the excited state of Ir(Ⅲ)^* photocatalyst (E_1/2^Ⅲ*/Ⅱ = +1.21 V vs. SCE) reacted with α-keto acid 1a [E_ox1 (PhCOCOO⁻) = + 0.78 V vs. SCE] to form the PhCOCOO· and Ir(Ⅱ) photocatalyst. Then, the acyl azolium 11 (E_red = −1.31 V vs. SCE) oxidized Ir(Ⅱ) (E_1/2^Ⅲ/Ⅱ = −1.37 V vs. SCE) to regenerate the ground state of Ir(Ⅲ) photocatalyst.

  Based on the above experimental studies, a plausible mechanism for the catalytic transformation is proposed in Fig. 1. Initially, the base-mediated deprotonation of α-keto acid 1a provided α-oxocarboxylate. Meanwhile, the ground-state Ir(Ⅲ) catalyst was excited as a single-electron oxidant Ir(Ⅲ)^* under blue LEDs irradiation. Then, the α-oxocarboxylate was oxidized through a single electron transfer, and give acyl radical species through decarboxylation. Subsequently, acyl radical added to styrene 3 to give a benzyl radical. Meanwhile, the ketyl radical and a ground-state Ir(Ⅲ) catalyst was generated through the single-electron transfer from Ir(Ⅱ) photocatalyst to acyl azolium species. Finally, the cross-coupling of these two radical intermediates afforded 1,4-diketone product and released the free carbene catalyst.

  Figure 1

  

  Figure 1.  Proposed reaction mechanism.

  DownLoad: Full-Size Img PowerPoint

  In summary, we have developed a multi-component, regioselective bis-acylation of olefins by merging NHC organocatalysis and photoredox catalysis. With the reported synthetic approach, a broad range of 1,4-diketones was easily prepared using readily accessible starting materials under mild conditions. Interestingly, the diketone products could be readily converted into multi-substituted furan, pyrrole, and pyridazine. Moreover, some mechanistic experiments shed light on understanding this NHC and photoredox dual catalytic radical process. Further investigations on the development of new chiral NHC catalysts and their applications in asymmetric radical transformations are underway in our laboratory, and the results will be reported in due course.

  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.

  Acknowledgments

  Financial support from the National Natural Science Foundation of China (NSFC, Nos. 22071011, 21871031, 22271028 and 82073998), Longquan Talents Program, the Science & Technology Department of Sichuan Province (Nos. 2021YJ0404, 2022JDRC0045 and 2023NSFSC1081), and the innovative project of Chengdu University is gratefully acknowledged.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108271.

  
  
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  Scheme 1  Overview of the 1,4-diketone synthesis and our motivation of this study.

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  Scheme 2  Substrate scope of α-keto acids 1 and acyl imidazoles 2 for the synthesis of 1,4-diones. Reactions condition: 1 (0.10 mmol), 2 (0.20 mmol), 3a (0.20 mmol), Cs₂CO₃ (0.12 mmol), PC1 (0.002 mmol) and NHC C1 (0.02 mmol) in 1.0 mL DCE, irradiation with blue LEDs at 60 ℃ for 2 h. ^a I, intensity; T, temperature; Big scale, 2.0 mmol scale; c, concentration.

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  Scheme 3  Substrate scope of the styrenes 3 used for the synthesis of 1,4-diones. Reactions condition: 1a (0.10 mmol), 2a (0.20 mmol), 3 (0.20 mmol), Cs₂CO₃ (0.12 mmol), PC1 (0.002 mmol) and NHC C1 (0.02 mmol) in 1.0 mL DCE, irradiation with blue LEDs at 60 ℃ for 2 h. ^a The representative product 4ah was determined by X-ray diffraction analysis, and other products were tentatively assigned by analogy.

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  Scheme 4  Further synthetic applications.

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  Scheme 5  Experimental studies for mechanistic investigation.

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  Figure 1  Proposed reaction mechanism.

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  Table 1.  Optimization of reaction conditions.^a

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