Palladium-Catalyzed Oxidative Allylation of Sulfoxonium Ylides

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

Palladium-Catalyzed Oxidative Allylation of Sulfoxonium Ylides: Regioselective Synthesis of Conjugated Dienones Chunsheng Li,† Meng Li,† Wentao Zhong,† Yangbin Jin,† Jianxiao Li,† Wanqing Wu,† and Huanfeng Jiang*,†,‡ †

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Key Laboratory of Functional Molecular Engineering of Guangdong Province, Guangdong Engineering Research Center for Green Fine Chemicals, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China ‡ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China S Supporting Information *

ABSTRACT: The first examples of palladium-catalyzed allylic C−H oxidative allylation of sulfoxonium ylides to afford the corresponding conjugated dienones with moderate to good yields have been established. The features of this novel conversion include mild reaction conditions, wide substrate scope, and excellent regioselectivity.

S

Based on the previous endeavors, we envisaged that the allylPd intermediate could undergo a carbene insertion reaction with sulfoxonium ylides to afford conjugated dienones. We herein disclose a unique route toward conjugated dienones via palladium-catalyzed oxidative allylic C−H olefination of allylarenes (Scheme 1).

ulfoxonium ylides have served as privileged synthetic skeletons to form metal-carbene intermediates for diverse chemical transformations.1 Compared with diazo compounds,2 sulfoxonium ylides are easier to synthesize and safer in reaction, which can be employed as practical alternatives in metal-carbene reactions. Recently, the coupling reactions of sulfoxonium ylides with organometallic intermediates, generated via sp2 C−H bond activation, have been reported.3 In 2017, the groups of Aissa and Li independently reported the Rh-catalyzed C−H acylmethlyation of arenes assisted by the directing groups.4,5 Thereafter, Li and co-workers developed a distinct synthesis of 1-naphthols using sulfoxonium ylides as a dual functional directing group.6 Furthermore, Kim’s group developed a new method to synthesize 3-acyl (2H)-indazoles through the Rh(III)-catalyzed sp2 C−H functionalization and subsequent annulation.7 Despite the remarkable success that has been realized in the field of sp2 C−H activation, the application of sulfoxonium ylides in the metal-catalyzed allylic sp3 C−H functionalization has not yet been studied. Over the past decades, transition-metal catalyzed oxidative allylic C−H functionalization has been a useful and straightforward strategy to construct C−C and C−heteroatom (C−X) bonds in organic synthesis.8 In particular, palladium complexes have become efficient catalysts for allylic sp3 C−H functionalizations of a wide range of olefins.9 Owing to the high reactivity of the allylpalladium species, several categories of methods have been disclosed for the alkylation,10 oxygenation,11 amination,12 and silylation.13 Recently, our group has also realized a Pd-catalyzed allylic C−H alkylation and oxygenation through the coupling of terminal alkenes with various nucleophiles.14 © XXXX American Chemical Society

Scheme 1. Strategies for Sulfoxonium Ylides Utilization in C−H Functionalization

To examine our hypothesis, allylbenzene (1a) and sulfoxonium ylide (2a) were employed as the model substrates for the studies on reaction conditions. Fortunately, with Pd(OAc)2 (10 mol %) as catalyst, NQ (2 equiv) as oxidant, and PPh3 (20 mol %) as ligand, the desired product 3a was obtained in 15% yield (Table 1, entry 1). To improve the conversion efficiency, alternative readily available oxidants Received: November 11, 2018

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DOI: 10.1021/acs.orglett.8b03606 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Optimization of Reaction Conditionsa

Scheme 2. Scope of Allylarenesa

entry

catalyst

oxidant

ligand

solvent

yield (%)b

1 2 3 4 5 6 7 8 9 10c 11d 12e

Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2

NQ BQ DDQ DMBQ o-NQ DMBQ DMBQ DMBQ DMBQ DMBQ DMBQ DMBQ

PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3

DMSO DMSO DMSO DMSO DMSO DMA dioxane MeCN DMF DMSO DMSO DMSO

15 20 trace 62 ND trace trace 58 trace 70 89 77

a

Reaction conditions: 1 (0.2 mmol), 2a (0.1 mmol), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), DMBQ (0.2 mmol) in anhydrous DMSO (2 mL) at 65 °C for 24 h, R1= H for 3a−3r. Note: Isolated yields based on 2a.

a

Conditions: all reactions were performed with 1a (0.2 mmol), 2a (0.1 mmol), Pd catalyst (10 mol %), oxidant (2 equiv), and ligand (20 mol %) in anhydrous solvent (2.0 mL) at 80 °C for 24 h. NQ = 1,4naphthoquinone; o-NQ = 1,2-naphthoquinone, DMBQ = 2,6dimethylcyclohexa-2,5-diene-1,4-dione. bGC yields with n-dodecane as the internal standard. cThe reaction temperature was 60 °C. dThe reaction temperature was 65 °C. eThe reaction temperature was 70 °C.

Scheme 3. Scope of Sulfoxonium Ylidesa

were investigated (Table 1, entries 2−5). DMBQ (2,6dimethylbenzoquinone) proved to be an inferior oxidant (Table 1, entry 4). Further exploration of the solvents showed that, except for MeCN, other solvents could not promote this oxidative allylation process (Table 1, entries 6−9). Subsequently, the screening of catalysts and ligands revealed that Pd(OAc)2 and PPh3 were the optimal (see additional details in the Supporting Information). Particularly, the reaction temperature could also influence the yield of the desired products, and the reaction proceeded in excellent yield at 65 °C (Table 1, entry 11). Therefore, the optimal reaction conditions were defined as Pd(OAc)2 (10 mol %), PPh3 (20 mol %), and DMBQ (2 equiv) in anhydrous DMSO (2.0 mL) at 65 °C for 24 h. The generality of this Pd-catalyzed oxidative olefination reaction was examined, and the results are shown in Scheme 2. Gratifyingly, allylarenes with different type of substituents were compatible in this transformation, generating the corresponding products in moderate to good yields. Substrates with electron-donating groups (−CH3, −OCH3, and −N(CH3)2), or electron-withdrawing substituents (−F, −Cl, −Br, and −CF3) on the phenyl ring were successfully transformed into the desired olefination products 3a−3l. Moreover, polysubstituted substrates were also suitable for this cascade procedure, affording the respective products 3m-3p in excellent yields. Additionally, 2-allylthiophene (2q) and 2naphthylpropene (2r) smoothly converted to the desired 3q and 3r in 73% and 85% yields, respectively. As for substrate 2methyl-3-phenylpropene (3s), only a trace amount of desired products was obtained, probably owing to the steric effect of methyl group. Moreover, when we examined internal alkenes such as (E)-prop-1-en-1-ylbenzene or 4-phenyl-1-butene, neither of them could not be converted into the corresponding products (see Supporting Information for details). We next explored the generality and limitation of sulfoxonium ylides (Scheme 3). Aroyl sulfoxonium ylides

a

Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), Pd(OAc)2 (10 mol %,), PPh3 (20 mol %), DMBQ (0.2 mmol) in anhydrous DMSO (2 mL) at 65 °C for 24 h. Note: Isolated yields based on 2.

with electron-donating substituents such as ethyl and tert-butyl at the para-position of the phenyl ring, afford 4a and 4b in 84% and 80% yields. Substrates bearing electron-withdrawing groups also reacted to deliver the desired products in moderate yields (4e−4h). Besides, disubstituted substrate was also tolerated in the reaction, generating the desired product 4k in 85% yield. Notably, (1E,4E,6E)-1,7-diphenylhepta-1,4,6-trien3-one (4l) could be formed in 69% yield with excellent regioselectivity. Substrates bearing a thiophene (4m), naphthalene ring (4n) and alkyl substituent (4o) were also compatible for this transformation. Conjugated dienones motifs and their derivatives have potent biological and pharmaceutical activities. For example, leinamycin, a naturally occurring antibiotic containing extended conjugation, exhibits effective anticancer activity.15 Moreover, chalcones (1,3-diaryl-2-propen-1-ones) are also B

DOI: 10.1021/acs.orglett.8b03606 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

bond. The following coordination of sulfoxonium ylide 2a delivers the Pd species II. Then, the reactive π-allylpalladium carbenoid species III is obtained after the α-elimination of DMSO. Then, the alkylpalladium intermediate IV is generated after the migratory insertion process. The conjugated dienone product would be obtained after the β-H elimination process. Finally, the Pd(II) catalyst is regenerated in the presence of DMBQ and HOAc. In summary, we have achieved the first example of Pdcatalyzed oxidative allylation of sulfoxonium ylides, which delivered various functionalized conjugated dienones. This strategy featured broad substrate scope, good functional group tolerance, and excellent regioselectivity. This method also complements Wittig and aldol strategies to access extended chalcone products, and broadens the applications of sulfoxonium ylides. Moreover, further research on the applications of this method in the synthesis of bioactive molecules are ongoing in our laboratory.

bioactive compounds that contain a broad range of pharmacological activities including antioxidant and antiinflammatory.16 Besides, conjugated dienones are also valuable synthons in contemporary organic synthesis.17 Therefore, in view of the potential application of conjugated dienone products, the reaction between 1a and 2a was performed at 5 mmol scale, which afforded 3a in 80% isolated yield (0.992 g, Scheme 4). Scheme 4. Gram-Scale Synthesis of Conjugated Dienonesa

a

Conditions: see the Supporting Information for details

In order to get insight into the mechanism of this transformation, we conducted several competition experiments. First, we investigated the competitive conversion rate of 1-allyl-4-methoxybenzene (1c) and 1-allyl-4(trifluoromethyl)benzene (1i). The yields of the corresponding products were 62% and 25%, which indicated that the phenyl rings bearing electron-donating groups proceed more efficiently. Moreover, the reaction exhibited an intramolecular kinetic isotope effect (kH/kD = 4.0) implying that the allylic C−H cleavage was involved in the reaction (Scheme 5).

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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.8b03606. Experimental procedures, condition screening table, characterization data, and copies of NMR spectra for all products (PDF)

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Scheme 5. Controlled Experiments

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wanqing Wu: 0000-0001-5151-7788 Huanfeng Jiang: 0000-0002-4355-0294 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Key Research and Development Program of China (2016YFA0602900), the National Natural Science Foundation of China (21420102003), and Guangdong Province Science Foundation (2017B090903003) for financial support.

In light of the previous literature4,18 and control experiments, the mechanism of this coupling is proposed in Scheme 6. First, the palladium catalyst coordinates with olefins to form the intermediate I. Afterward, the key π-allylic palladium intermediate is generated from the cleavage of allylic C−H

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Scheme 6. Proposed Catalytic Cycle

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DOI: 10.1021/acs.orglett.8b03606 Org. Lett. XXXX, XXX, XXX−XXX