Photoredox-Catalyzed Decarboxylative Alkylation of Silyl Enol Ethers

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Photoredox-Catalyzed Decarboxylative Alkylation of Silyl Enol Ethers To Synthesize Functionalized Aryl Alkyl Ketones Weiguang Kong, Changjiang Yu, Hejun An, and Qiuling Song* Institute of Next Generation Matter Transformation, College of Chemical Engineering & College of Material Sciences Engineering at Huaqiao University, 668 Jimei Boulevard, Xiamen, Fujian 361021, P. R. China S Supporting Information *

ABSTRACT: Photoredox-catalyzed decarboxylative alkylation of silyl enol ethers has been developed. Diverse functionalized aryl alkyl ketones were afforded in modest to good yields using N(acyloxy)phthalimide as an easy access alkyl radical source under mild and operationally simple conditions. The excellent performance of drug molecules such as fenbufen and indomethacin and naturally occurring carboxylic acids such as stearic acid and dehydrocholic acid further demonstrated the practicability of the reaction.

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decarboxylative halogenation of aliphatic carboxylic acids in the presence of stoichiometric silver salts.8 The Barton decarboxylation is another important application of this strategy that has been widely used in chemical synthesis.9 Moreover, decarboxylative functionalization catalyzed by transition metals has also been developed as an efficient option for the synthetic community.10 In addition to the reactions above, photoredox-catalyzed radical decarboxylative functionalization has been raised recently as a powerful tool to access diverse compounds.11 In consideration of the mechanism, the C-centered radical involved in the reactions can be generated via two paths from carboxylic acids and their derivatives under photoredox conditions: (1) carboxylic acids deliver one electron to an oxidative photocatalyst to generate a C-centered radical after CO2 extrusion, which is termed an reductive quenching process;12 (2) carboxylic acid derivatives such as N-(acyloxy)phthalimide receive one electron from a reductive photocatalyst to form a C-centered radical after CO2 and phthalimide anion extrusion, which is termed an oxidative quenching process (Scheme 1a).13 These two processes complement each other to make photoredox-catalyzed radical decarboxylative functionalization a novel and efficient method to create C−C and C−X bonds. On the other hand, silyl enol ethers, which are frequently used platforms to construct functionalized ketones and their derivatives, are electron-deficient alkenes, and they can react rapidly with the electrophilic carbon-centered radicals.14 On the basis of this property, MacMillan and co-workers disclosed the photoredoxcatalyzed trifluoromethylation of silyl enol ethers and highly valuable α-CF3 carbonyl compounds were synthesized in a gentle manner (Scheme 1b).15 Herein, we investigated if N-(acyloxy)phthalimide could act as a source of alkyl radical to react with silyl

etones are among the most important and valuable functional groups in organic molecules; they not only are widespread in natural and man-made biologically active molecules but also serve as basic building blocks for the synthesis of other complex structures.1 Because of their versatility, the development of diverse synthetic methods to prepare these compounds has been the subject of intense research. Generally, the oxidation of secondary alcohols could be employed to synthesize the lower members of them, but this method was not applicable to complicated structures because of the difficulty in procuring the necessary alcohols and the sensitivity of some substituent groups to oxidative conditions.2 Nucleophilic addition of organometallic reagents onto carboxylic acids or derivatives constitutes another important ketone synthesis protocol; the major challenges associated with these protocols are the limited scope of organometallics and sometimes overaddition of the reactive nucleophilic reagents to ketone products, which results in the formation of tertiary alcohols and other side reactions.3 Fortunately, the development of ketone synthesis based on Weinreb amide overcomes these drawbacks to some extent.4 Recent advances in cross-coupling reactions of activated carboxylic acid derivatives with various transmetalating reagents to forge new carbon−acyl bonds broaden the way of ketone construction; however, the requirement of noble metal catalysts and harsh reaction conditions makes this method far from acceptable.5 Thus, the development of a mild, operationally simple approach to the synthesis of functionalized ketones is highly desired. Carboxylic acids are basic synthetic materials because they are stable, inexpensive, and easily available. Their conversion into valuable fine chemicals such as medicinally relevant compounds represents an important goal in organic chemistry.6 Among the abundant transformations of carboxylic acids, decarboxylative functionalization has been established as a powerful strategy to construct all kinds of chemical bonds.7 In particular, the Hunsdiecker reaction dates back to the 1930s and involves the © XXXX American Chemical Society

Received: November 18, 2017

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DOI: 10.1021/acs.orglett.7b03587 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

we next studied the influence of water on our reactions and found that better results can be achieved with 25 equiv of water added in dry CH3CN (entries 12−14, Table 1). However, other acids including Brønsted acids and Lewis acids did not improve the yield (entries 15−19, Table 1). An 88% yield was obtained when trimethyl((1-phenylvinyl)oxy)silane (1a) was used on 0.2 mmol (entry 20, Table 1). Control experiments found that light and photoredox catalyst were essential to the reactions. The reactions did not proceed without irradiation or in the absence of the photocatalyst (entries 21 and 22, Table 1). With the optimized conditions in hand, we set out to explore the substrate scope of the reaction with a range of different qN(acyloxy)phthalimides (Scheme 2). It was demonstrated that this

Scheme 1. (a) Mechanism of C-Centered Radical Generation via Photoredox-Catalyzed Radical Decarboxylation; (b) and (c) Photoredox-Catalyzed Functionalization of Silyl Enol Ethers

Scheme 2. Scope of the N-(Acyloxy)phthalimide Compoundsa

enol ethers so that highly functionalized alkyl ketones would be formed under mild photoredox-neutral conditions (Scheme 1c). To evaluate the feasibility of our hypothesis on photoredoxcatalyzed decarboxylative alkylation of silyl enol ethers as a mild, operationally simple approach to synthesize functionalized ketones, trimethyl((1-phenylvinyl)oxy)silane (1a) and 1,3dioxoisoindolin-2-yl cyclohexanecarboxylate (2a) were selected as model substrates to conduct the optimization of reaction conditions (Table 1). Initially, screening of common photoTable 1. Optimization of Photoredox-catalyzed Decarboxylative Alkylation of Silyl Enol Ethers

a Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), [Ir(ppy)2(dtbbpy)](PF6) (2.5 mol %), H2O (25 equiv), CH3CN (2 mL), 5 W blue LED, N2, rt, 12 h. bThe reaction was scaled up to 7.5-fold.

reaction showed good functional group compatibility. Simple alkyl-substituted N-(acyloxy)phthalimides worked smoothly in this transformation, and all of the corresponding products were obtained in moderate yields (3ab−ae). Substituents such as Br, alkynyl, ester, and carbonyl groups were compatible in the reaction. The desired ketones that are useful compounds for further transformation were obtained in moderate to good yields (3af−ai). It is worth mentioning that when the reaction was scaled up to 1.5 mmol scale, 44% (165 mg) of 3ai was obtained; the drop in yield might be due to the formation of a decarboxylative protonation side product of N-(acyloxy)phthalimide. N-(Acyloxy)phthalimide derived from 2-(4-methoxyphenyl)acetic acid could also participate in the reaction but with a relatively low yield (3aj). This was probably due to the relatively strong stability of the benzyl radical that would lead to the 1-methoxy-4-methylbenzene side product. In addition to primary alkyl groups, secondary alkyl groups could also be successfully introduced into the silyl enol ethers through this photoredox-catalyzed decarboxylative alkylation reaction and

a GC yield. bUsed directly from commercial source. cDry solvent. d1a (0.2 mmol) and 2a (0.3 mmol) were used. eIsolated yield. fNo light.

catalysts found that fac-Ir(ppy)3 and [Ir(ppy)2(dtbbpy)](PF6) were efficient candidates to promote the reaction with the desired 2-cyclohexyl-1-phenylethan-1-one (3aa) obtained in 60% and 67% yields, respectively (entries 1−5, Table 1). Further investigation of solvents revealed that CH3CN was essential to the reaction. Other solvents such as DMF, DMSO, THF, toluene, DCE, and acetone led to terrible results (entries 3, 6−11, Table 1). Inspired by Glorius’ work that water can activate N-(acyloxy)phthalimide through hydrogen-bond interactions,16 B

DOI: 10.1021/acs.orglett.7b03587 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters achieved good results (3aa, 3ak−an). As a special case, the product of N-(acyloxy)phthalimide derived from trans-2-phenyl1-cyclopropanecarboxylic acid was afforded in 57% yield with excellent diastereoselectivity (3an). Moreover, N-(acyloxy)phthalimides derived from α-amino acids were efficiently decarboxylated under the reaction conditions, and valuable β-amino ketones were generated in moderate yields (3ao, 3ap). We know that tertiary alkyl radicals are more stable and sterically demanding than primary and secondary alkyl radicals. The reflection of this characteristic was 3aq, which was obtained in a relatively lower yield. To our surprise, drug molecules such as fenbufen and indomethacin and naturally occurring carboxylic acids such as stearic acid and dehydrocholic acid could be readily applied to this photoredox-catalyzed ketone synthesis reaction. All of the desired products were obtained in excellent yields (3ar−au). The success of these remarkable cases further demonstrated the practicability of the reaction. Next, we examined the generality of this transformation with respect to different silyl enol ethers. N-(Acyloxy)phthalimide derived from 4-oxo-4-phenylbutanoic acid (2i) was selected as another reactant to participate in the reaction as the obtained 1,5dicarbonyl compounds are highly valuable building blocks for the synthesis of other complex structures. As shown in Scheme 3,

Scheme 4. Experimental Probes on the Reaction Mechanism

Scheme 5. Proposed Mechanism

Scheme 3. Scope of the Silyl Enol Ethersa

interactions, single-electron transfer from visible light excited photocatalyst to the N-(acyloxy)phthalimide 2 generates a radical anion A, which then undergoes homolytic cleavage of the N−O bond, leading to carboxyl radical B and phthalimide. Next decarboxylation of B generates alkyl radical C. Radical addition of alkyl radical C to silyl enol ether 1 generates the α-O radical D, which is subsequently oxidized by the oxidative photocatalyst to form the intermediate E. Finally, hydrolysis of the intermediate E affords the desired ketone product. In summary, photoredox-catalyzed decarboxylative alkylation of silyl enol ethers as a mild and operational simple approach to synthesize functionalized aryl alkyl ketones has been developed. N(Acyloxy)phthalimides were utilized as an easy access alkyl radical source to afford the desired products in moderate to good yields. The excellent performance of drug molecules such as fenbufen and indomethacin and naturally occurring carboxylic acids such as stearic acid and dehydrocholic acid makes this transformation valuable and practical in organic synthesis.

a Reaction conditions: 1 (0.2 mmol), 2i (0.3 mmol), [Ir(ppy)2(dtbbpy)](PF6) (2.5 mol %), H2O (25 equiv), CH3CN (2 mL), 5 W blue LED, N2, rt, 12 h.

α-aryl silyl enol ethers with functional groups such as −Me, −OMe, −F, −Cl, and −Br proved to be well adapted to our conditions without the influence of the position of functional groups (3bi−mi). Furthermore, trimethyl((1-(naphthalen-2yl)vinyl)oxy)silane also exhibited robust reactivity in reaction with the desired product obtained in good yield (3ni). However, heteroaromatics such as pyridine and thiofuran inhibited the reaction strongly (3oi−pi). In addition, an alkyl-derived silyl enol ether was also proven to be inappropriate in this protocol (3qi). To gain insight into the mechanism of the reaction, a radical cascade experiment and radical-trapping experiments were conducted (Scheme 4). When N-(acyloxy)phthalimide derived from 6-heptenoic acid (2v) reacted with 1a under standard conditions, radical cyclization product 3av was obtained in 50% yield (eq 1). The reaction was totally suppressed with the radicaltrapping reagent (BHT or TEMPO or 1,1-diphenylethylene) added, and radical-trapping products were all detected by GC−MS (eqs 2−4). These results support the involvement of an alkyl radical and its reaction with silyl enol ether. According to the results above and other previous investigations, a plausible mechanism for the photoredox-catalyzed decarboxylative alkylation of silyl enol ethers is proposed in Scheme 5. First, with the help of water through hydrogen-bond

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03587. Experimental details and spectral data for all products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. C

DOI: 10.1021/acs.orglett.7b03587 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters ORCID

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Qiuling Song: 0000-0002-9836-8860 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of Fujian Province (2016J01064), the Recruitment Program of Global Experts (1000 Talents Plan), Program of Innovative Research Team of Huaqiao University (Z14 × 0047), and the Graduate Innovation Fund of Huaqiao University (to W. K.) is acknowledged. We also thank the Instrumental Analysis Center of Huaqiao University for analysis support.

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DOI: 10.1021/acs.orglett.7b03587 Org. Lett. XXXX, XXX, XXX−XXX