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Recent advances in visible light-mediated chemical transformations of enaminones

Yu Han ,  Liyun Zhou ,  Chengyu Wang ,  Shangti Feng ,  Rong Ma ,  Jie-Ping Wan

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Citation:  Yu Han, Liyun Zhou, Chengyu Wang, Shangti Feng, Rong Ma, Jie-Ping Wan. Recent advances in visible light-mediated chemical transformations of enaminones[J]. Chinese Chemical Letters, 2024, 35(2): 108977. doi: 10.1016/j.cclet.2023.108977 shu

Recent advances in visible light-mediated chemical transformations of enaminones

English

  • 1. Introduction

    Enaminones or enaminoesters, which consist of an amino group linked through a carbon-carbon double bond to a carbonyl or ester group, are important and powerful synthons [1–7]. Owing to the amine-alkene-carbonyl conjugated structural features, they combine the triple nucleophilicity (C-2, amino and carbonyl oxygen) with the ambident electrophilicity of enones (C-1 and C-3) (Fig. 1). Enaminones have been utilized as versatile building blocks in organic the synthesis of carbocyclic [8–11], heterocyclic [12–18] as well as acyclic compounds [19–23] under proper reaction conditions. The known transformations usually take place in the unit of “enamines” or “enones” in the presence of one or more other reaction partners, enabling the construction of heterocycles though tandem cyclization under proper environments [24–27], the functionalization of the internally activated vinyl C–H bond under the catalyst of transition metal [28–30,27], iodide [31–34] or other oxidants [35,36], C–H bond functionalization of aryl ring in the aromatic ketone or aryl amine moiety [37–39], the synthesis of acyclic compounds through the cleavage of the C=C double bond by means of different pathways of transformation [40–43], the functionalization of the C–N bond to different enones [44,20], as well as some other novel reactions in enaminones [45–49]. The results, especially the ones reported over the last decade by us and others, have opened new frontiers in the enaminone-based synthetic chemistry.

    Figure 1

    Figure 1.  Structure of enaminones.
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    Following the daily increasing emphasis on sustainable synthesis, the visible light-mediated reactions has won significant attention and advances [50–52] since the pioneer reports by MacMillan [53–55], Yoon [56,57], Stephenson [58–60] and Nicewicz [61–63]. By employing visible light photocatalysis, a large variety of new synthetic reactions have been established to deliver diverse organic products via mild, easy to handle, and environmentally benign operations. Such reactions are more favorable than conventional heating protocols from the perspective of green chemistry. In this regard, applying photocatalysis to the chemical transformations of enaminones could be valuable approaches for organic synthesis. Actually, during the last decade, a broad range of synthetic works on the functionalization of enaminones under visible light catalysis have been published. However, no review work on the photocatalytic reactions of enaminones is currently available. Thus, we present herein an overview on the progress of visible light-mediated chemical transformations of enaminones, in hope of bring some guides to the development of more environmentally friendly synthesis by the transformation of enaminones.

    2. Direct α-C(sp2)-H bond functionalization

    In recent years, the direct functionalization of the alkenyl α-C(sp2)-H bond in enaminones has gained extensive attention because of its highly versatile reactivity and widespread application in designing the synthesis of biologically active molecules and pharmaceutical substances. Among these approaches, visible light-mediated direct α-C(sp2)-H bond functionalization is definitely one of the simple and ideal methods. Recently, several direct α-functionalization reactions of enaminones such as α-selenylation, α-fluoroalkylation and α-thiocyanation have been developed.

    2.1 Photocatalytic α-C-H selenylation of enaminones

    In 2018, Yang and coworkers reported the direct C(sp2)-H α-selenylation of coumarin derivatives under metal- and photocatalyst-free visible light catalysis [64]. The reactions were run with the conditions of (NH4)2S2O8 as the oxidant, CH3CN as the solvent under air atmosphere as well as the irradiation of 12 W blue LEDs visible light at room temperature. As shown in Scheme 1, aryl selenium could be smoothly incorporated to various coumarin derivatives at α-site. A wide array of diselenides and coumarin derivatives, bearing either electron-withdrawing or electron-donating groups, were tolerated in this reaction, leading to the desired product in generally good to excellent yields. It is worth noting that as to N-substituted 4-(phenylamino)−2H-chromen-2-ones 4, the dual selenylated products were selectively generated in 71%−81% yields (Scheme 2). When the para-site of the 4-phenylamino group was substituted, only C-3 selenylated products were obtained. Two possible mechanisms to rationalize the formation of 3 and 5 were illustrated (Scheme 3). First, the in situ generated active radical anion SO4•− from (NH4)2S2O8 occurred under the irradiation of visible light. Then, a single electron transfer (SET) process between 1 and SO4•− to delivered the radical cation 1A or 1B, which further reacted with diselenides 2 to form the intermediate 1C. Finally, if R2 was H atom, the final product 3 was obtained by the elimination of H+ from 1C. If R2 was methyl, the reaction of intermediate 1C with diselenides 2 led to the C-3 selenylated species 1D. Subsequently, a SET process took place between 1D with SO4•− to give radical cation 1E or 1F, which was then captured by ArSe· to afford the intermediate 1G. The deprotonation of 1G afforded the final product 5.

    Scheme 1

    Scheme 1.  Visible-light-promoted regioselective selenylation of 4-(phenylamino) coumarins.
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    Scheme 2

    Scheme 2.  Visible-light-induced selenylation of N-substituted 4-(phenylamino) coumarins.
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    Scheme 3

    Scheme 3.  Proposed mechanisms for the formation of 3 and 5.
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    2.2 Photocatalytic α-C-H perfluoroalkylation of enaminones

    Incorporation of fluoroalkyl groups into a molecule is important because such elaboration can dramatically alter the chemical and physical properties of the molecules and results in various applications in pharmaceuticals, agrochemicals, and functional materials [65]. In 2018, He et al. reported catalyst-free and visible light promoted trifluoromethylation and perfluoroalkylation of uracils, cytosines and pyridiones in the α-C-H bond of the enaminone fragments (Scheme 4) [66]. The reactions were conducted with 12 W blue LED irradiation at room temperature using Cs2CO3 as base and DMSO as solvent. The negatively ionized uracil 5A was first generated in the presence of Cs2CO3. A fluoroalkyl radical could be formed under the irradiation of blue LEDs via the EDA complex. Then two pathways might be involved in the propagation step: (1) fluoroalkyl radical reacted with 5A to produce intermediate 5B, which accessed to intermediate 5C via a SET process in the influence of fluoroalkyl iodides. Finally, the desired product 8 or 10 were formed by 1,3-hydrogen migration process with the help of Cs2CO3. (2) The Rf radical might add to uracils 6 and generate the carbon radical intermediate 5D, which abstracted an iodine atom from RfI to afford 5E. The final products 8 or 10 were obtained via the fast re-aromatization of 5E via HI elimination (Scheme 5).

    Scheme 4

    Scheme 4.  The synthesis of α-perfluoroalkylated uracils, cytosines and pyridiones.
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    Scheme 5

    Scheme 5.  Proposed mechanisms for the formation of 8 and 10.
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    2.3 Photocatalytic α-C-H thiocyanation of enaminones

    Organic thiocyanates are moieties possessing plentiful biological and pharmaceutical activities in both synthesized and naturally occurring molecules [67]. Among the reported strategies on organic thiocyanate synthesis, the direct C–H thiocyanation is more practical and cost-effective because such a method allows for the synthesis of organic thiocyanates using those prevalently available C–H bond donors. In 2019, our group realized the first vinyl C–H bond thiocyanation reaction of tertiary enaminones under metal-free, photocatalytic conditions in the presence of Rose Bengal, which enables the synthesis of thiocyanated alkene derivatives and chromones using NH4SCN as the thiocyano source under an aerobic atmosphere [68]. Besides, employing Ru(bpy)3Cl2·H2O as the photocatalyst switches the reaction pathway to provide NH2-functionalized thiocyanated enamines. As shown in Scheme 6, the reactions might start from the quenching of NH4SCN to the excited RB* species generated from the visible light irradiation to the RB catalyst, which gave rise to the SCN free radical. Meanwhile, the RB·− formed therein could be regenerated to RB by the oxidation of molecular oxygen. Subsequently, the addition of the SCN· to the C=C double bond in 13 provided the free radical intermediate 6A, which was further oxidized to carbon cation 6B. The deprotonation on 6B then yielded thiocyanated product 14. when the Ru-catalyst is employed, a transamination between the tertiary enaminones and the ammonium was promoted to form products 15, wherein the Ru-catalyst might act also as a Lewis acid (LA) catalyst. In addition, when o-hydroxylphenyl-functionalized enaminones were used as substrates, the successive chromone annulation took place to give thiocyanated chromones 16.

    Scheme 6

    Scheme 6.  Photocatalytic synthesis of polyfunctionalized alkenes and thiocyano chromones.
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    After our work, Yang et al. reported the photocatalyst-free C–H thiocyanation of 4-anilinocoumarins by visible light irradiation (Scheme 7) [69]. The reactions started from ammonium incorporation of thiocyanate and TFA forming thiocyanic acid (HSCN). Under the visible light irradiation, oxygen was converted to singlet oxygen which subsequently abstracting an electron from HSCN to generate HO2· and ·SCN. The selective addition of thiocyano free radical to the coumarin derivatives 17 gave the intermediate 7A. The SET processes from HO2· to the intermediate 7A donated the desired products 18 by releasing H2O2. If R2 was H, the intramolecular nucleophilic addition of NH to CN afforded the cyclization products 19.

    Scheme 7

    Scheme 7.  Photocatalyst-free regioselective C–H thiocyanation of 4-anilinocoumarins.
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    Despite the fact that N,N-disubstituted and N-mono substituted enaminones have been successfully used as the C–H bond donors for C–H thiocyanation by photocatalysis, equivalent photocatalytic C–H thiocyanation reactions of the free NH2-enaminones kept inapplicable in the aforementioned systems. In 2022, our group reported the first α-C-H thiocyanation of NH2-enaminones with air oxygen as the oxidant via visible light photocatalysis [70]. By using NH2-enaminones 20 and NH4SCN as starting materials, the synthesis of α-thiocyanated NH2-enaminones 21 could be accessed by means of blue LEDs irradiation in the presence of Ru(bpy)3Cl2·6H2O photocatalyst. The reactions were initiated by the Ru-based photocatalyst enabling the formation of ·SCN and PC anionic radical. The free radical addition of ·SCN to the enaminone C=C double bond in 20 donated 8A. Meanwhile, the coupling of the PC anionic free radical to O2 leads to the generation of an oxygen anionic free radical accompanied by PC regeneration. Intermediate 8A captured oxygen anionic free radical (observed by HRMS) afforded intermediate 8B in the presence of an ammonium cation. Further, the elimination of hydrogen peroxide in intermediate 8B generated thiocyanated NH2-enaminones 21. When base was used following the photocatalytic process, the tandem cyclization and tautomerization processes took place to give 2-aminothiazoles 22. On the other hand, when NH2-enaminones 21 are employed in aqueous acidic conditions, 2-thiazolinones 23 were obtained via cyclization, hydrolysis, and the elimination of ammonium (Scheme 8).

    Scheme 8

    Scheme 8.  Photocatalytic C–H thiocyanation of NH2-enaminones.
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    3. Functionalization reactions via C=C double bond cleavage

    Scissoring the C=C bond of enaminones constitutes one of the irreplaceable tools of modern organic synthesis in the form of either partial [71–74] or full bond cleavage [75–77]. Among them, visible-light-mediated C=C bond cleavage of enaminones have been also identified as useful synthetic approaches.

    3.1 Synthesis of 1,2-diketone or its derivatives

    As early as in 1970s, Wasserman and Ives firstly revealed that 1,2-diketones can be obtained from enaminones via C=C double bond cleavage under the conditions of 650 W lamp heating at −78 ℃ to room temperature [78,79]. In these reactions, the C=C bonds of enamines were cleaved through [2 + 2] cycloaddition procedure to give two carbonyl compounds. In 2015, our group developed a facile method for the synthesis of 1,2-diketones via C=C bond cleavage of enaminones under ambient conditions through visible-light photocatalysis in presence of Rose Bengal (RB) [80]. By employing enaminones 24 as the substrate, as shown in Scheme 9, with visible light irradiation, RB was converted into the excited RB*, which further reacted with 3O2 to regenerate the stage of RB and produce singlet oxygen 1O2. The singlet oxygen then incorporated to the enaminone 24, giving peroxide intermediates 9A. The subsequent decomposition of 9A gave the 1,2-diketones 25, which could be captured by diamine to afford quinoxalines 26 via stepwise operation.

    Scheme 9

    Scheme 9.  Synthesis of 1,2-diketones and quinoxalines via C=C bond cleavage of enaminones.
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    Interestingly, by employing enaminones 27 to react with alcohols 28 under the irritation of 20 W green LEDs in the presence of Rose Bengal, our group further developed a facile method for the synthesis of useful α-ketoesters [81]. This method showed general tolerance to the aryl group of enaminones and primary/secondary alcohols. Analogously, the visible light irradiation to RB gave rise to excited RB* species, which further activates 3O2 to the active singlet oxygen 1O2. The highly active singlet oxygen 1O2 then coupled the C=C double bond of enaminone to offer 1,2-dioxetane 10A which was further converted to intermediate 10B through the ring opening and a subsequent N—O bond formation process. Next, alcohol acted as nucleophilic reagent to attack the C–O bond in intermediate 10B, delivering zwitterion intermediate 10C. The successive decomposition afforded the final product α-ketoester 29 and 10D. The hemiaminal 10D might be further oxidized to DMF under the titled oxidative conditions (Scheme 10).

    Scheme 10

    Scheme 10.  Synthesis of α-ketoester via C=C bond cleavage of enaminones.
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    3.2 Synthesis of quaternary amino acid derivatives

    In 2014, Li and co-workers reported a visible-light-promoted transformation involving the ene-type reaction of secondary enamino ketones with singlet oxygen, followed by a 1,2-acyl migration, affording quaternary amino acid derivatives [82]. The reaction pathway was distinctively different from the previous reports of C=C bond cleavage by singlet oxygen. As shown in Scheme 11, under the irradiation of 14 W CFL, ruthenium(Ⅱ) is converted into a high-energy excited singlet 1RuⅡ*, which underwent intersystem crossing (ISC) to triplet 3RuⅡ*. Then the triplet 3RuⅡ* species reacted with 3O2 via intermolecular energy transfer process to regenerate the ground-state RuⅡ and produced the reactive 1O2 species. Next, an ene-type reaction happened between 30 and 1O2 to yield the intermediate 11A. The dehydration of 11A gave 11B, and a nucleophilic addition of alcohol to 11B led to 11C. With subsequent 1,2-acyl migration and protonation, the products 31 were formed (Scheme 11). It should be noted that a fluorescence emission quenching study and the redox potential of the substrate 30 measured by cyclic voltammetry experiments were carried out to support the proposed mechanism.

    Scheme 11

    Scheme 11.  Visible-light-mediated the partial cleavage of enaminone C=C double bond.
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    3.3 Synthesis of α-substituted γ-ketoesters

    In 2022, Jin and co-workers developed a blue visible-light-promoted approach for the synthesis of α-substituted γ-ketoester derivatives via C=C bond partial cleavage of enaminones by using enaminones 32 and diazoesters 33 as the substrate [83]. The blue visible light irradiation induced the selective photolysis of aryl diazoesters 33 to give free carbene species with by releasing nitrogen gas. The in situ formed free carbene species was captured by enaminones 32 to provide the cyclopropane intermediate 12B. Due to the stronger electron-withdrawing effect of arylcarbonyl group than that of ester carbonyl group, path a instead of path b could occur via ring opening driven by nitrogen atom, affording the intermediate 12C which might be further converted into α-formyl aryl γ-ketoeser 34 via hydrolysis. Then, with the help of Al2O3 Lewis acid, the final product α-substituted γ-ketoester 35 could be obtained by decarbonylation of 34 (Scheme 12). This strategy broadened the methods available for accessing the α-formyl aryl γ-ketoester or α-aryl γ-ketoester derivatives.

    Scheme 12

    Scheme 12.  Synthesis of α-substituted γ-ketoesters via C=C bond cleavage of enaminones.
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    4. Annulation reactions for the synthesis of heterocyclic and carbocyclic scaffolds

    4.1 Synthesis of five-membered N-heterocycles

    As a typical five-membered N-heterocycles, the indole motif is a well-documented core structure of numerous natural products and bioactive compounds. In addition to the Fischer indole syntheses, many different synthetic approaches using modern transition-metal catalysis are known. The enaminone annulations leading to indoles via intramolecular C–H activation by transition metal catalysis have been known for long period, but the reliance on stoichiometric chemical oxidant and heating at elevated temperature are limits of such methods. In 2014, Rueping et al. reported the enaminone-based indole synthesis using a new combination of palladium and photoredox catalysis [84]. With catalytic amount of the photoredox catalyst in the presence of visible light, the typical high loadings of external oxidants could be avoided. In the first step, the C–H bond activation of the olefin of 36 took place via Pd(Ⅱ) insertion, giving the intermediate 13A which further underwent the arene C–H bond to form the six-membered cyclopalladium intermediate 13B. After reductive elimination, indoles 37 were produced and the resulting Pd(0) was re-oxidized to Pd(Ⅱ) by either the photoredox catalyst or the in situ formed superoxide anion (Scheme 13).

    Scheme 13

    Scheme 13.  Synthesis of indoles via intramolecular annulation reactions of enamine ester.
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    Very recently, Baell and Huang et al. established a protocol for the synthesis of polysubstituted oxazoles through photocatalytic benzylic C–H oxidation/cyclization of enaminones with molecular oxygen as an ideal and green oxidant [85]. This strategy featured broad substrate scope and good functional group tolerance. Based on the results of control experiments and the DFT computations, the authors proposed a plausible mechanism (Scheme 14). Firstly, the Ru(bpy)2(PF6)2 was excited by 495 nm LEDs to form Ru(bpy)2(PF6)2* which underwent a SET with the benzylic C–H bond of B-O complex 14A to give Ru(bpy)2(PF6)2•− and benzyl radical 14B. The Ru(bpy)2(PF6)2•− is subsequently oxidized by molecular oxygen to regenerate Ru(bpy)2(PF6)2 and release superoxide radical anion O2•−. The superoxide anion coupled proton to generate ·OOH. The benzyl radical 14B coupled with ·OOH to access adduct 14C which could be further transferred into oxygen radical 14D by losing PhBpin and hydroxyl radical. Then, two possible reaction pathways could occur. On the one hand, the 2,3-dihydrooxazole skeleton 14G was formed via intramolecular cyclization of oxygen radical intermediate 14D and a deprotonation process (path a, Scheme 14). On the other hand, diradical species 14F could be obtained via deprotonation, and the subsequent intramolecular radical-radical coupling gave 14G (path b, Scheme 14). Finally, the desired oxazole product 39 was afforded through further oxidation and deprotonation process under O2. Due to the high electro-negativity of oxygen in the hydroxyl radical, it is relatively unstable and tends to react with the C–H bond to form a more stable carbon radical, path a was hypothesized as the more favorable route.

    Scheme 14

    Scheme 14.  Synthesis of polysubstituted oxazoles from enaminones.
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    4.2 Synthesis of six-membered N-heterocycles and O-heterocycles

    In 2018, Yu and co-workers achieved the synthesis of quinoxalines via tandem azidation/intramolecular C–H amination under visible light irradiation by using 1-azidyl-1,2-benziodoxole as the azidating agent with N-arylenamines [86]. Under the irradiation of visible light, the I-N bond of 15A cleaved to yield the azidyl radical 15B, which subsequently added to the C=C double bond in N-arylenamines to form the intermediate 15C. The 15C was further oxidized to carbocation intermediate 15D, which was then converted into vinyl azide 15E via oxidative deprotonation. Vinyl azide 15E was more prone to be oxidized to imine radical 15G because the azide group would render the C=C double bond in 15E electronically richer than the one in 40. Subsequent cyclization of 15G followed by oxidation and deprotonation afforded the final products 41 (Scheme 15). It was worth mentioning that the substituent at β-site in N-arylenamines 40 exhibited crucial influence on the reaction, and the conditions were needed to be modified for the reactions of different substrates. For example, Cu(OAc)2 was required for the preparation of 3-(trifluoromethyl)quinoxalines, whereas Ru(bpy)3Cl2 along with Cu(OAc)2 combination was found to be more favorable for the synthesis of 2,3-diarylquinoxalines (Scheme 15).

    Scheme 15

    Scheme 15.  Synthesis of quinoxalines via tandem azidation/cyclization of N-arylenamines.
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    Currently, the annulation reactions of N,N-disubstituted 2-hydorxyphenyl enaminones has been widely used in the construction of diversely functionalized chromones, especially for the synthesis of 3-substituted chromones. Among these methods, photoredox catalysis has won particular attention as options of the enhanced sustainability. In 2017, Yang and Chen reported a visible-light-driven, radical-triggered tandem cyclization of o-hydroxyaryl enaminones for the synthesis of 3-CF2/CF3-chromones [87]. At first, the irradiation of the catalyst [Ir(ppy)3] under visible light generated the excited state Ir(ppy)3* which was then oxidized by BrCF2COOEt or Ph2SCF3OTf to generate [Ir(Ⅳ)(ppy)3]+ complex and RF free radical species. Subsequently, the RF radical added to the C=C double bonds of substrate 42 regioselectively to give radical intermediate 16A. Further oxidation to this intermediate by [Ir(Ⅳ)(ppy)3]+ provided 16B with the concurrent regeneration of [Ir(ppy)3]. Next, the hydroxyl group on benzene ring attacked the iminium cation to give intermediate 16C. Ultimately, the N,N-dimethyl group was eliminated to furnish the desired product 45/46 (Scheme 16). Ten 3-CF2COOEt and ten 3-CF3 substituted chromones were obtained in acetone at room temperature in good to excellent yields.

    Scheme 16

    Scheme 16.  Synthesis of 3-CF2/CF3-containing chromones via tandem cyclization of o-hydroxyaryl enaminones.
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    In the same year, Zhang et al. developed an efficient method for the synthesis of 3-CF2-chromones via a ruthenium reduction cycle from the similar substrates [88]. The catalyst [Ru(bpy)3]2+ was firstly converted into the excited state [Ru(bpy)3]2+* under visible light, and enabled the oxidation of Et3N to generate the [Ru(bpy)3]1+ species. Then the electron-rich metal complex [Ru(bpy)3]1+ underwent a SET process with the cleavage of C-Br bond in CF2BrCOOEt to afford an electron-deficient radical 17A and regenerate [Ru(bpy)3]2+. Intermediate 17A added to the C=C double bond in substrate 42 to give radical 17B. The SET between 17B and [Ru(bpy)3]2+* provided iminium cation intermediate 17C and the metal complex [Ru(bpy)3]1+. Finally, the chromones 47 were obtained via intramolecular annulation and a subsequent elimination of dimethylamine (Scheme 17).

    Scheme 17

    Scheme 17.  Synthesis of 3-CF2-containing chromones via radical cascade reaction of o-hydroxyaryl enaminones.
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    In 2020, Iaroshenko and Mkrtchyan developed two facile routes to synthesis diverse 3-arylchromones through photo-arylation reactions of ortho-hydroxyarylenaminones by using aryldiazonium tetrafluoroborates and diaryliodonium triflates as aryl sources, respectively [89]. Initially, the irradiation of Eosin Y and Ru(Ⅱ) generates their excited states by an appropriate light source. Aryl radicals 18A could be formed from aryldiazonium tetrafluoroborates and diaryliodonium triflates catalyzed by the corresponding excited *Eosin Y and *Ru(Ⅱ) species. The aryl radicals 18A formed thereby attacked promptly on the enaminone to afford the radical intermediate 18B which was oxidized afterwards to carbocation 18C. The annulation on this intermediate then gave intermediate 18D. Finally, the 3-arylchromones 49 were furnished by Et2NH elimination (Scheme 18).

    Scheme 18

    Scheme 18.  Visible-light-mediated arylation of o-hydroxyaryl enaminones by BF4N2Ar and TfOIAr2.
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    In 2021, the same group disclosed different methods for the synthesis of 3-arylchromones by arylation of ortho-hydroxyarylenaminones by photocatalysis by employing sulfonium salts and arenesulfonyl chlorides as the aryl radical precursors [90]. These two routes showed good efficiency and were feasible for the preparation of 3-arylchromones in good to excellent yields. Analogously, the pathways were commenced by a SET oxidation initiated by the excited state Ru(Ⅱ)*, which coupled with triarylsulfonium and arenesulfonyl chlorides to afford aryl radical (Ar·). The addition of the free radical to the enaminone moiety gave carbon-centered radical 19B which could be further be converted into the final products 49 via nucleophilic annulation and amine elimination (Scheme 19).

    Scheme 19

    Scheme 19.  Visible-light-mediated arylation of o–hydroxyl aryl enaminones by sulfonium salts and arenesulfonyl chlorides.
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    In 2021, Xu et al. developed a visible light-promoted, metal-free selenylation/cyclization cascade between ortho-hydroxyaryl enaminones and diselenides for the synthesis of 3-selanylchromones under mild conditions [91]. Notably, no transition metal catalyst, photocatalyst or additional oxidant were required in the reactions. The final products could be easily converted into selenyl-functionalized pyrimidines by reacting with benzamidines. As for mechanism, the light irradiation to diphenyl diselenide 53 produced phenylselenyl radicals which could be further oxidized to phenylselenyl cation 20A by air. Subsequently, the phenylselenyl cation 20A coupled the C=C bond in the enaminone 42 to afford cycloselenium ion 20B on which a ring-opening driven by lone pair electrons of NMe2 took place to give rise to imine cation intermediate 20C. Then the intramolecular cyclization of 20C and a following elimination of dimethylamine delivered the final products 54 (Scheme 20).

    Scheme 20

    Scheme 20.  The photocatalytic synthesis of 3-selenochomones.
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    In 2022, Le et al. disclosed a method for the synthesis of 3-aminoalkyl chromones from o-hydroxyphenyl enaminone and N-aryl glycines 55 under visible light catalysis [92]. This synthetic method required no photocatalyst or additive, and proceeded to products with good functional group tolerance. The reactions were proposed to be initiated by the light irradiation to enaminones 42, affording the excited state 21A (42*), which acted as the photocatalyst to provide singlet oxygen (1O2) from molecular oxygen through energy transfer process. The singlet oxygen (1O2) interacted with ammonium-carboxylate salt 21B, an isomeric form of N-arylglycine 55, to give the α-amino radical intermediate 21C and superoxide radicals (O2−·). Intermediate 21C was further oxidized and deprotonated to form the imine specie 21D which was subsequently captured by substrate 42 via nucleophilic addition to provide iminium intermediate 21E. The intramolecular cyclization of 21E by intramolecular nucleophilic addition of OH group to imine cation gives intermediate 21F. The elimination of dimethylamine then led to the desired products 56 (Scheme 21).

    Scheme 21

    Scheme 21.  The photocatalytic synthesis of 3-aminoalkyl chromones.
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    In the same year, the group reported another approach for the synthesis of 3-aminoalkyl chromones 56 using N-arylglycine esters as the alkylating agents by the cooperative catalysis of visible light and an enzyme. By utilizing Methylene Blue (MB) as the photocatalyst [93]. This approach avoided the synthesis of acids from esters in a separate step, since N-aryl glycines were derived from N-aryl glycine salts under strong basic conditions. As shown in Scheme 22, Firstly, N-aryl glycine ester 57 was hydrolyzed into N-arylglycine 22A with the help of Lipase B Candida antarctica (CALB). Then radical cation 22B could be formed from N-aryl glycine 22A via SET oxidation by the excited photocatalyst MB+*. The 3-aminoakyl chromones 56 could be obtained by similar photoredox decarboxylation of N-arylglycine 22A, oxidation of aminoalkyl radicals 22D, Mannich reaction, intramolecular nucleophilic cyclization and subsequent amine elimination.

    Scheme 22

    Scheme 22.  The combined enzyme and photoredox catalysis for the synthesis of 3-aminoalkyl chromones.
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    In 2022, Yang and Xiang et al. established a new cascade protocol for the assembly of 3-aminochromones 42 with o-hydroxyaryl enaminones and O-perhalopyridin-4-yl hydroxyl amine 58, a versatile amidyl-radical precursor [94]. This protocol featured with mild reaction conditions, synthetic simplicity, and scalability. In the reactions, the photocatalyst fac-Ir(ppy)3 was first excited to generate its active species fac-Ir(ppy)3* under the blue LEDs irradiation. The subsequent quenching of the exited photocatalyst by enaminone 42 generated the nitrogen-centered radical 23A and Ir2+ via SET process. The highly reactive Ir2+ was immediately oxidized by 58 to furnish amido radical ·NHBoc along with regeneration of the Ir-photocatalyst. A facile radical transposition of 23A delivered the isomeric carbon-center radical 23B. The species could be quickly trapped by amido radical ·NHBoc to give intermediate 23C prior to the cyclization process (path a). Alternatively, the cyclic radical 23D might be formed via intramolecular cyclization of 23B under in the presence of base (path b). Radical cross-coupling of 23D with ·NHBoc led to the intermediate 23E. The final products 59 were provided via the elimination of dimethylamine (Scheme 23).

    Scheme 23

    Scheme 23.  The synthesis of 3-aminated chromones from o-hydroxyarylenaminones.
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    4.3 Synthesis of seven-membered O-heterocycles and carbocycles

    In 2018, Yang and Chen developed a practical protocol accessing benzoxepines 61/62 by using functionalized enaminones 60 and ethyl bromodifluoroacetate 43 as starting materials with white LEDs irradiation in the presence of Ir(dtbbpy)(bpy)2PF6 [95]. In the formation of these fused seven-membered scaffolds, the excited state Ir(Ⅲ)* occurred from Ir(Ⅲ) under visible-light irradiation. The further reaction with BrCF2COOEt 43 gave Ir(Ⅳ)+ species and RF radicals 24A. Subsequently, the free radical addition of 24A to the olefin unit in substrate 60 and a sequential radical addition cascade process generated radical intermediate 24C. The quick oxidation by Ir(Ⅳ)+ yielded iminium intermediate 24D and regenerated Ir(Ⅲ). When N,N-dimethyl enaminones (R2/R3 = Me) were used, the intermediate 24D was quickly hydrolyzed by water to furnish products 61 (path a). In the case of the monosubstituted iminium ion (R3 = H), the deprotonation and tautomerization processes took place to afford the corresponding enamine 62 (path b) (Scheme 24).

    Scheme 24

    Scheme 24.  The photocatalytic synthesis of CF2-containing benzoxepine derivatives.
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    In 2021, Yang et al. developed a method for the synthesis of cyclohepta[b]indoles 64 via highly functionalized enaminones 63 [96]. This approach involves a visible-light-induced cascade of [2 + 2] cycloaddition and retro-Mannich-type reaction of enaminones. As shown in Scheme 25, radical cation intermediate 25A was first formed by single electron oxidation of substrate 63 in the presence of excited-state FCNIrpic ([IrⅢ]*). The oxidized enaminone moiety in intermediate 25A readily underwent radical addition to the indole ring to furnish benzylic radical intermediate 25B which then added to the iminium moiety to deliver radical cation 25C. The resulting intermediate 25C was unstable and tended to either go back to 25B (breaking bond a) or forward to 25D (breaking bond b). The species 25D would subsequently be reduced by previously generated [IrⅡ] species to zwitterion 25E. The final products 64 were afforded via an intermolecular proton transfer in 25E.

    Scheme 25

    Scheme 25.  The photocatalytic synthesis of cyclohepta[b]indoles.
    DownLoad: Full-Size Img PowerPoint

    5. Multicomponent reaction for heterocycle construction

    Multicomponent reactions (MCRs) are powerful tools in current organic synthesis due to their irreplaceable advantages in simple starting materials, step economy, product diversity and fast construction of molecular diversity [97,98]. Designing enaminone-based MCRs with photocatalyst would inarguably benefit the synthetic chemistry by providing highly sustainable routes toward highly diversified organic products.

    5.1 Synthesis of pyrimido[1,2–b]indazole derivatives

    In 2021, Wang and co-workers reported a visible-light-driven three-component cyclization for the synthesis of pyrimido[1,2-b]indazole derivatives 66 from enaminones 27, 3-aminoindazoles 65 and ethyl bromodifluoroacetate 43 [99]. The reactions were proposed to start from a metal-to-ligand charge transfer (MLCT) process from the photocatalyst fac-[Ir3+(ppy)3] to generate excited state IrⅢ* under blue LEDs irradiation, enabling the C-Br bond cleavage of in 43 via SET reduction process to give alkyl radicals (·CF2COOEt). The alkyl radicals subsequently added to the electron-rich olefin in the enaminone moiety and led to carbon radicals intermediate 26A. The further oxidation on this intermediate by IrⅣ accessed to imine cation 26B Subsequently, the intermediate 26B underwent an elimination of HF to afford 1,3-vinylimine ion intermediate 26C which was trapped by 3-aminoindazoles via addition-elimination process (SNV reaction) to form intermediate 26D or its isomer 26E. Afterwards, the intramolecular addition of the nucleophilic NH group to the iminium site gave tricyclic intermediate 26F. The elimination of dimethyl amine from the amino dihydropyrimidine ring provided products 66 (Scheme 26).

    Scheme 26

    Scheme 26.  Visible-light-driven three-component cyclization for the synthesis of pyrimido[1,2-b]indazole derivatives.
    DownLoad: Full-Size Img PowerPoint

    5.2 Synthesis of 2,3-difunctionalized quinolines

    In 2022, Wang’s group reported another three-component protocol for the synthesis of 2,3-difunctionalized quinolines 69 via the reactions of enaminones 67, aryl amines 68 and ethyl bromodifluoroacetate 43 with the irradiation of blue LEDs in the presence of fac-Ir(ppy)3 [100]. The reaction mechanism was familiar with the former one in Scheme 26. The difference was that when 1,3-vinyl imine ion intermediate 27C was formed, it was rapidly trapped by aryl amine to give intermediate 27D which underwent an intramolecular cyclization and amine elimination to provide products 69 (Scheme 27).

    Scheme 27

    Scheme 27.  Photoinduced three-component cyclization for the synthesis of 2,3-difunctionalized quinolines.
    DownLoad: Full-Size Img PowerPoint

    5.3 Synthesis of trisubstituted pyrazoles

    Recently, Wang and Geng et al. again developed a multicomponent method for the tunable synthesis of pyrazoles 73 and 74 by employing bromodifluoroalkyl acetates 43, enaminones, and hydrazines as starting materials with blue LEDs photocatalysis [101]. In these reactions, 1-methylindazol-3-amine acted as a traceless mediator in switching the regionselectivity of 1,3,4-trisubstituted and 1,4,5-trisubstituted pyrazole formation. Mechanically, the active 1,3-vinylimine ions intermediate 28C was first formed through similar transformation processes as in the above works. This intermediate was then trapped by 1-methylindazol-3-amine 72 to form 28E. Intermediate 28E might further undergo hydrolysis to deliver intermediate 28F, and subsequently reacted with hydrazine to provide 28G. The sequential intramolecular cyclization/deamination processes then took place to afforded 1,4,5-trisubstituted pyrazoles 73. On the other hand, when the traceless mediator 72 was not added, R3NHNH2 was directly captured 1,3-vinylimine ions 28C, giving intermediate 28D via an SNV process. The similar intramolecular cyclization/deamination process then led to the formation of 1,3,4-trisubstituted pyrazoles 74 (Scheme 28).

    Scheme 28

    Scheme 28.  Photoinduced switchable multicomponent cyclization for the synthesis of adjustable multi-substituted pyrazoles.
    DownLoad: Full-Size Img PowerPoint

    5.4 Synthesis of spiro[indoline-3,4′-quinoline] derivatives

    Very recently, Singh et al. reported the visible light-mediated three-component synthesis of spiro[indoline-3,4′-quinoline] derivatives 78 via the reactions of indoles 75, methylene nitriles 76 and enaminones 77 under the irradiation of a 22 W LED lamp [102]. In the formation of these spiroheterocyclic products, the initially irradiation of light to Eosin Y led to the excited state EY* which promoted the transformation of molecular oxygen to singlet oxygen. Indoles captured the singlet oxygen to afford peroxo species 29A, and a subsequent oxidation by EY* led to 29B By the O—O homolytic cleavage in 29B, the cation 29C and hydroxyl radical were created. The C2-site hydrogen atom in 29C could be abstracted by the peroxyl free radical to deliver intermediate 29D. The rapid coupling of 29D with hydroxyl radical gave 29E, and 29E could further isomerize into 29F. Meanwhile, the light irradiation to substrate 76 led to intermediate 29G. The reaction of 29F with 29G provided 29H. On the other hand, the free radical intermediate 29I was formed via the promotion of malononitrile and visible light irradiation. The addition of 29I to 29H led to the formation of 29J. After a sequential hydrogen radical transfer and intramolecular cyclization process, the final products 78 were provided (Scheme 29).

    Scheme 29

    Scheme 29.  Visible light triggered three-component cyclization for the synthesis of spiro[indoline-3,4′-quinoline].
    DownLoad: Full-Size Img PowerPoint

    6. Conclusions and perspectives

    Herein, we reviewed for the first time the advances in the visible light-mediated chemical transformations of enaminones. The modes of transformations, including the direct C(sp2)-H α-functionalization, C=C double bond functionalization, annulation toward heterocyclic and carbocyclic scaffolds, as well as enaminone-based multicomponent reactions have been covered. These interesting results illustrate the power of combining enaminone chemistry and photocatalysis in the designation and application of energy economic, mild and diversity-oriented organic reactions. However, challenges remain yet in this research area. For example, the frequent application of noble-metal photocatalyst, limited generality in the application of different free radical reaction designation with most of the currently known model, and switchable control of the site selectivity in analogous sites such as the two vinyl C–H bonds are yet non-negligible restrictions. Therefore, in developing more novel and applicable photocatalytic enaminone-based synthesis, extensive efforts are still highly desirable. The discovery on more novel and different transformation and catalytic modes, tuning the chemo-selectivity, application in other important synthesis such as asymmetric synthesis, target-oriented synthesis, are directions worthy of more attention.

    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

    The authors thank the National Natural Science Foundation of China (No. 21702088) and the Natural Science Foundation of Shandong Province (No. ZR2022MB130) for the financial support.


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  • Figure 1  Structure of enaminones.

    下载: 全尺寸图片 幻灯片

    Scheme 1  Visible-light-promoted regioselective selenylation of 4-(phenylamino) coumarins.

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    Scheme 2  Visible-light-induced selenylation of N-substituted 4-(phenylamino) coumarins.

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    Scheme 3  Proposed mechanisms for the formation of 3 and 5.

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    Scheme 4  The synthesis of α-perfluoroalkylated uracils, cytosines and pyridiones.

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    Scheme 5  Proposed mechanisms for the formation of 8 and 10.

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    Scheme 6  Photocatalytic synthesis of polyfunctionalized alkenes and thiocyano chromones.

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    Scheme 7  Photocatalyst-free regioselective C–H thiocyanation of 4-anilinocoumarins.

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    Scheme 8  Photocatalytic C–H thiocyanation of NH2-enaminones.

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    Scheme 9  Synthesis of 1,2-diketones and quinoxalines via C=C bond cleavage of enaminones.

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    Scheme 10  Synthesis of α-ketoester via C=C bond cleavage of enaminones.

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    Scheme 11  Visible-light-mediated the partial cleavage of enaminone C=C double bond.

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    Scheme 12  Synthesis of α-substituted γ-ketoesters via C=C bond cleavage of enaminones.

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    Scheme 13  Synthesis of indoles via intramolecular annulation reactions of enamine ester.

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    Scheme 14  Synthesis of polysubstituted oxazoles from enaminones.

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    Scheme 15  Synthesis of quinoxalines via tandem azidation/cyclization of N-arylenamines.

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    Scheme 16  Synthesis of 3-CF2/CF3-containing chromones via tandem cyclization of o-hydroxyaryl enaminones.

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    Scheme 17  Synthesis of 3-CF2-containing chromones via radical cascade reaction of o-hydroxyaryl enaminones.

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    Scheme 18  Visible-light-mediated arylation of o-hydroxyaryl enaminones by BF4N2Ar and TfOIAr2.

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    Scheme 19  Visible-light-mediated arylation of o–hydroxyl aryl enaminones by sulfonium salts and arenesulfonyl chlorides.

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    Scheme 20  The photocatalytic synthesis of 3-selenochomones.

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    Scheme 21  The photocatalytic synthesis of 3-aminoalkyl chromones.

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    Scheme 22  The combined enzyme and photoredox catalysis for the synthesis of 3-aminoalkyl chromones.

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    Scheme 23  The synthesis of 3-aminated chromones from o-hydroxyarylenaminones.

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    Scheme 24  The photocatalytic synthesis of CF2-containing benzoxepine derivatives.

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    Scheme 25  The photocatalytic synthesis of cyclohepta[b]indoles.

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    Scheme 26  Visible-light-driven three-component cyclization for the synthesis of pyrimido[1,2-b]indazole derivatives.

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    Scheme 27  Photoinduced three-component cyclization for the synthesis of 2,3-difunctionalized quinolines.

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    Scheme 28  Photoinduced switchable multicomponent cyclization for the synthesis of adjustable multi-substituted pyrazoles.

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    Scheme 29  Visible light triggered three-component cyclization for the synthesis of spiro[indoline-3,4′-quinoline].

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  • 发布日期:  2024-02-15
  • 收稿日期:  2023-06-05
  • 接受日期:  2023-08-24
  • 修回日期:  2023-08-21
  • 网络出版日期:  2023-08-26
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