Visible-Light-Induced [4+1] Cyclization-Aromatization of Acylsilanes and α,β-Unsaturated Ketones

Abstract

Open AccessCCS ChemistryCOMMUNICATIONS19 Aug 2022Visible-Light-Induced [4+1] Cyclization-Aromatization of Acylsilanes and α,β-Unsaturated Ketones Zhihong Zhu†, Weilu Zhang†, Yizhi Zhang, Shanshan Liu and Xiao Shen Zhihong Zhu† Institute for Advanced Studies, Engineering Research Center of Organosilicon Compounds and Materials, Ministry of Education, Wuhan University, Wuhan 430072 , Weilu Zhang† Institute for Advanced Studies, Engineering Research Center of Organosilicon Compounds and Materials, Ministry of Education, Wuhan University, Wuhan 430072 , Yizhi Zhang Institute for Advanced Studies, Engineering Research Center of Organosilicon Compounds and Materials, Ministry of Education, Wuhan University, Wuhan 430072 , Shanshan Liu Institute for Advanced Studies, Engineering Research Center of Organosilicon Compounds and Materials, Ministry of Education, Wuhan University, Wuhan 430072 and Xiao Shen *Corresponding author: E-mail Address: [email protected] Institute for Advanced Studies, Engineering Research Center of Organosilicon Compounds and Materials, Ministry of Education, Wuhan University, Wuhan 430072 https://doi.org/10.31635/ccschem.022.202202199 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Herein we report the first [4+1] cyclization-aromatization reaction of acylsilanes and α,β-unsaturated ketones. The unprecedented visible-light-induced reaction proceeded through mild conditions without addition of any catalyst or additive, affording a variety of furans with broad substrate scope and good functional-group tolerance. The synthetic utility of the method was demonstrated by various downstream transformations of the otherwise difficult-to-access sulfone-containing silyl furans. The mechanism study reveals that 1,4-diketones are not likely to be the intermediates of the reaction. Download figure Download PowerPoint Introduction Organosilicon compounds are widely used in synthetic chemistry, materials science, and medicinal chemistry because silicon is earth-abundant, and organosilicon compounds are generally stable, easy to handle, and nontoxic.1–5 Among various silicon-containing compounds, acylsilanes are particularly fascinating.6–9 Since both an oxygen and a silicon atom are directly bonded to the same sp2-carbon atom, acylsilanes are considered to be unique carbonyl compounds. Acylsilane has been used as the aldehyde equivalent in synthetic transformations10–14 and can also be employed in the generation of acyl radicals.15 Another classical reaction of acylsilanes is the formation of siloxycarbenes via radical 1,2-silyl transfer16–23 under photolysis or high temperature, and the carbenes have participated in X–H insertion reactions,24–32 addition to alkenes,33–37 alkynes,38–40 aldehydes, ketones and CO2,41–45 cross-coupling with organoboronic esters,46,47 electrocyclization reaction,48 [2+2] cyclization reaction,49 and photodegradation of polymers.50 In particular, in the absence of transition metals, the free siloxycarbenes are generally nucleophilic and can participate in both intermolecular and intramolecular [2+1] cyclization of α,β-unsaturated esters to generate cyclopropanes (Scheme 1a,b).33,34 Although the intermolecular [2+1] cyclization reactions are not always diastereoselective,33 our group reported that the trifluoroacetylsilane-derived carbenes were ambiphilic carbenes and could undergo [2+1] cyclization of both aromatic and aliphatic alkenes as well as electron-deficient alkenes, affording the cyclopropanation products in a highly diastereoselective way (Scheme 1c).35 However, to the best of our knowledge, there has been no [4+1] cyclization of α,β-unsaturated ketones and siloxycarbenes.51 Herein we report our recent success on the direct synthesis of furans through [4+1] cyclization-aromatization of acylsilanes and α,β-unsaturated ketones (Scheme 1d). Scheme 1 | Background and our strategy on the direct synthesis of furans through the [4+1] cyclization-aromatization of α,β-unsaturated ketones with acylsilanes. [Si]: silyl groups. Download figure Download PowerPoint It is important to emphasize that the construction of furans has been a subject of long-standing interest in chemistry.52–58 As a unique and important scaffold, furan is widely found in natural products, functional materials, and pharmaceuticals. For example, natural product Lophotoxin59 and commercially available drugs Nitrofurantoin60 and Ranitidine61 all contain furan scaffold. Therefore, straightforward methods for furan formation are of high value in organic synthesis. The Paal–Knorr reaction, which relies on acid-catalyzed intramolecular cyclization-dehydration of 1,4-dicarbonyl compounds, is among the most important methodologies to access furans.10,11,52–58,62 However, the need to prepare 1,4-diketones still limits its application. The direct synthesis of furans disclosed in this manuscript proceeded through the generation of siloxyl carbenes under simple visible-light-induced conditions without any catalyst or additive. The [4+1] cyclization-aromatization cascade reaction shows broad substrate scope and good functional-group tolerance. More than 10 downstream transformations of the functionalized furan demonstrate the potential of this methodology. Results and Discussion Sulfone is known as the "chemical chameleon" and has been widely used in various transformations.63,64 The sulfone-containing α,β-unsaturated ketones can be easily prepared in one step from readily available acetophenones and sodium sulfonates.65 In addition, the C–Si bond is robust for various transformations.1 Therefore, we commenced our study by investigating the reaction of sulfonyl group-substituted α,β-unsaturated ketone 1a and bis(dimethyl(phenyl)silyl)methanone 2a66–68 under light irradiation, based on the assumption that these two functional groups maintained in the furan product 3a would facilitate downstream transformations. To the best of our knowledge, bissilyl ketones have not been employed in synthetic photochemical reactions. Encouragingly, when the reaction between 1a and 2a was carried out in dichloromethane (DCM) under white light-emitting diodes (LEDs), a yield of 18% was afforded for furan 3a (Table 1, entry 1). Further investigation revealed that green light (519 nm) was better than white light, UV light, and blue light (Table 1, entries 1–4). Under green light, changing the solvent from DCM to EtOAc, MeCN, dioxane, tetrahydrofuran (THF), or toluene resulted in decreased yield (Table 1, entries 5–9). Increasing the amount of 1a to 3.0 equiv and increasing the concentration of 2a to 0.4 M improved the yield to 74% (73% isolated yield, Table 1, entry 10). The control experiment confirmed that light is necessary for the reaction to take place (Table 1, entry 11). Table 1 | Investigation of Reaction Conditions Entry Solvent Light Yield (%)a,b 1 DCM White LEDs 18 2 DCM UV 23 3 DCM Blue LEDs 25 4 DCM Green LEDs 43 5 EtOAc Green LEDs 28 6 MeCN Green LEDs 23 7 Dioxane Green LEDs 20 8 THF Green LEDs 12 9 Toluene Green LEDs 2 10 DCM Green LEDs 74 (73)c 11 DCM — 0 aUnless otherwise noted, the reaction conditions were as follows: 1a (0.1 mmol), 2a (0.1 mmol), solvent (0.1 M), under illumination by LEDs, 12 h. bYield was determined by the analysis of 1H NMR spectroscopy of unpurified reaction mixture using BrCH2CH2Br as an internal standard. c 1a (0.3 mmol), 2a (0.1 mmol), DCM (0.4 M), under illumination by green LEDs (519 nm), 12 h; yield in parentheses referred to the isolated yield. With the optimal reaction conditions in hand, we first investigated the reaction scope with respect to α,β-unsaturated ketones (Scheme 2). The reaction proved to be efficient, and the desired furans were synthesized in 56%–82% yield. Halogen substituents on the benzene ring, including F, Cl, Br, and I, were all tolerated by the photochemical reaction. Electron-donating OMe and electron-withdrawing CF3 groups did not significantly affect the efficiency of the reaction ( 3d, 61% yield, 3e, 66% yield; 3f, 70% yield). Naphthyl-substituted product 3m was also successfully synthesized in 63% yield. It is worth noting that heterocyclic aromatic ring-containing substrate afforded bi-heterocyclic compounds as the products ( 3n, 68% yield; 3o, 70% yield). In addition, alkyl groups such as cyclopropyl group were also tolerated, affording furan 3p in 63% yield. The effect of the substituents on the sulfone moiety was further studied, and aryl sulfones with electron-donating, electron-neutral, and electron-withdrawing groups all afforded the corresponding furans 3q– 3w in synthetically useful yields. Naphthyl- and thienyl-substituted sulfones were both tolerated, affording compounds 3s and 3w in 73% and 76% yield, respectively. The structure of compound 3q was confirmed by X-ray single crystal analysis.a Scheme 2 | Reaction scope. aReaction conditions: 1 (0.3 mmol), 2 (0.1 mmol), DCM (0.4 M), under room temperature and illumination by green LEDs (519 nm) 12–24 h, [Si] = SiMe2Ph; b[Si] = SiMe3; cBlue LEDs (460 nm) were used. dBlue LEDs (420 nm) were used. Download figure Download PowerPoint Further study confirmed that the sulfonyl group was not necessary for the furan formation reaction to occur. When R1 and R2 were aryl groups, compounds 3x–3ae were also prepared in 60%–67% yield. Remarkably, this visible-light-induced photochemical reaction was also compatible with α-chlorinated α,β-unsaturated ketones, and the corresponding 3-chlorofurans 3af was prepared in 64% yield. Moreover, an ester-containing furan 3ag was also made in 56% yield. Unfortunately, when R2 was n-C3H7, no furan product was obtained, probably because of the instability of 2-methylene-1-phenylpentan-1-one under the photochemical conditions. The scope of acylsilanes was then studied with α,β-unsaturated ketone 1a as the reaction partner. We found that both green light and blue light were suitable for the photochemical reaction. The reactions were generally efficient, affording the furan products 3ah– 3ar in 60%–78% yield. Functional groups such as F, Cl, Br, CF3, and OMe were tolerated. Moreover, the substitution pattern of methyl on the aromatic ring did not significantly affect the efficiency of the reaction ( 3aj, 71%; 3ak, 71%; 3al, 60%). In addition, biphenyl-, naphthyl-, and thienyl-substituted furans were also prepared in 62%–78% yield. But alkyl acylsilane 1-(dimethyl(phenyl)silyl)octan-1-one was not a suitable substrate since it afforded no furan product in its reaction with α,β-unsaturated ketone 1a. To show the general applicability and large-scale productivity of this protocol, the reaction was carried out on the 5 mmol scale, and 1.4 g of 3a was isolated (69% yield, Scheme 2). Taking advantage of the silyl and sulfonyl groups, we then investigated the downstream transformation of silyl furan 3a (Scheme 3). Upon treatment with tetrabutylammonium fluoride (TBAF) in THF at room temperature, 3a was protodesilylated in 30 min, affording compound 4 in 85% yield. We found that treating 3a with N-chlorosuccinimide (NCS), N-bromosuccinimide (NBS), and N-iodosuccinimide (NIS) in the presence of potassium fluoride (KF) could generate halogenated furans in good yields ( 5, 89%; 6, 92%; 7, 73%). Friedel–Crafts acylation of 3a with acetyl chloride in the presence of ZnCl2 resulted in the formation of ketone 8 in 64% yield. Bisfuran 9 was prepared in 71% yield through a Pd-catalyzed oxidative homocoupling reaction. With an aryl iodide as the reaction partner, Hiyama coupling reaction was also achieved, affording compound 10 in 79% yield. In the presence of CsF, we achieved the addition reaction between silyl furan 3a and benzaldehyde in dimethylformamide (DMF), and alcohol 11 was isolated in 60% yield. Morever, carboxylation of 3a was also successful under CO2 atmosphere, affording ester 12 in 64% yield after methylation of the carboxylate intermediate. The sulfones enabled various downstream transformations, but these sulfonyl group-substituted furans are not easy to make by other methods. In addition to these C–Si bond transformations, we investigated several synthetic applications based on the sulfone group. The α–C–H bond of the sulfone group is acidic. Therefore, treatment of 3a with n-butyllithium or lithium diisopropylamide (LDA) led to smooth deprotonation. The corresponding carbanion reacted with benzyl bromide to form compound 13 in 74% yield. The nucleophilic addition to benzaldehyde was also successful, affording alcohol 14 in 71% yield. Moreover, olefin 15 has been synthesized in 85% yield through a nucleophilic substitution-elimination reaction between furan 3a and ethyl bromoacetate, which further highlights the synthetic application of the functionalized furan. Scheme 3 | Transformations of 3a. Reaction conditions: aTBAF (2.0 equiv, 1.0 M in THF), rt, 30 min; bNCS (4.0 equiv), KF (1.5 equiv), MeCN; cNBS (1.1 equiv), KF (1.1 equiv), MeCN; dNIS (4.0 equiv), KF (1.5 equiv), MeCN; eAcCl (3.0 equiv), ZnCl2 (3.0 equiv); fPdCl2(PhCN)2 (5.0 mol %), AgNO3/KF (4.0 equiv), THF; gPdCl2(PhCN)2 (5.0 mol %), MeO2CC6H4I (3.0 equiv), AgNO3/KF (6.0 equiv), THF; hPhCHO, CsF, DMF; iCsF, CO2 balloon, DMF, then MeI; jn-BuLi (1.2 equiv), benzyl bromide (1.2 equiv), THF; kLDA (2.5 equiv), benzaldehyde (1.5 equiv), THF; lethyl bromoacetate (4.0 equiv), 18-crown-6 (8.0 equiv), K2CO3 (8.0 equiv), MeCN (0.7 M), 80 °C. Download figure Download PowerPoint Considering that 1,4-diketones are known precursors for the synthesis of furans in the Paal–Knorr reaction,10,11,52–58,62 we wondered whether the reaction would first generate 1,4-diketones and then undergo intramolecular cyclization and dehydration to form the final products. However, when we added 1,4-diketone 1ah into the reaction of 1a and 2a, there was no formation of 3as, and 1ah was recovered in 89% yield, indicating that 1,4-diketones were not likely to be the intermediates (Scheme 4a). Moreover, when substrate 1ai was used, the nonaromatized product 16 was obtained in 45% yield, supporting the observation that the [4+1] cyclization product was the intermediate in the reaction (Scheme 4b). The strong electron-withdrawing CF3 might stabilize compound 16. In the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf), compound 16 was successfully converted to furan 3at in 87% yield. As shown in Scheme 4c, compound 2a had strong absorption in the green light range, which was consistent with our experimental results. However, α,β-unsaturated ketone 1a had no absorption in the visible wavelength range, and there was no obvious formation of electron donor–acceptor complex between 1a and 2a. Then, we monitored the reaction profile of the reaction, which showed that the transformation was completely suppressed without light, suggesting that the reaction did not proceed through the chain-reaction mechanism (Scheme 4d, for details, see Supporting Information Figure S1). Scheme 4 | Mechanism study. (a) Reaction of 1a and 2a in the presence of 1ah. (b) Isolation of the [4+1] cyclization product 16 between the reaction of α,β-unsaturated ketone 1ai and acylsilane 2b. (c) UV/vis spectra of 1a, 2a, and their 1∶1 mixture. (d) Light on/off experiments of the reaction between 1a and 2a. Download figure Download PowerPoint A plausible reaction mechanism is proposed in Scheme 5, based on our experimental results and related literature.7 Initially, direct excitation of acylsilane 2 by visible light generates excited state I, which then produces siloxycarbenes II through radical 1,2-silyl transfer. Subsequent nucleophilic addition of II to the C=C bond of unsaturated ketone generates intermediate III, which may be in equilibrium with intermediate IV. Afterward, the intramolecular cyclization of IV afforded intermediate V. The existence of V is verified by the analysis of the reaction solution by high-resolution mass spectrometry (HRMS; for details, see Supporting Information) and supported by the isolation of compound 3at. Finally, aromatization of V generates furan product 3, with the elimination of PhSiMe2OH. It is worth noting that PhSiMe2OH was also detected by HRMS but might be unstable under the reaction conditions and was converted to PhSiMe2OSiMe2Ph as the side product of the reaction (44% yield; for details, see Supporting Information). Scheme 5 | Proposed mechanism. Download figure Download PowerPoint Conclusion We have developed an unprecedented direct synthesis of furans with α,β-unsaturated ketones and acylsilanes, which proceeds under visible-light-induced, catalyst-free, and additive-free conditions. This reaction represents a novel reaction mode of siloxycarbenes. The methodology displays broad substrate scope and excellent functional-group compatibility and affords the corresponding furans in moderate to good yields. The downstream transformation of the multiple-functionalized furan demonstrates the synthetic potential of our method. The mechanism study supports the [4+1] cyclization-aromatization mechanism rather than the generation of 1,4-diketones as the intermediates. Footnote a Deposition number 2116090 (for 3q) contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures Supporting Information Supporting Information is available and includes experimental procedures, characterization data of all new compounds, and X-ray crystallography of compound 3q. Conflict of Interest A patent application based on this work has been filed, and X.S., S.L., and Z.Z. may benefit from royalty payments. Funding Information We are grateful to the National Natural Science Foundation of China (grant no. 21901191), the Fundamental Research Funds for the Central Universities, and Wuhan University for financial support. References 1. 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