Palladium-Catalyzed Decarboxylative γ-Olefination of 2, 5

Letter Cite This: Org. Lett. 2018, 20, 1949−1952

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Palladium-Catalyzed Decarboxylative γ‑Olefination of 2,5Cyclohexadiene-1-carboxylic Acid Derivatives with Vinyl Halides Chi-Hao Chang and Chih-Ming Chou* Department of Applied Chemistry, National University of Kaohsiung, 700 Kaohsiung University Road, Nanzih District, Kaohsiung 81148, Taiwan S Supporting Information *

ABSTRACT: This study explores a Pd-catalyzed decarboxylative Heck-type Csp3−Csp2 coupling reaction of 2,5cyclohexadiene-1-carboxylic acid derivatives with vinyl halides to provide γ-olefination products. The olefinated 1,3cyclohexadienes can be further oxidized to produce meta-alkylated stilbene derivatives. Additionally, the conjugated diene products can also undergo a Diels−Alder reaction to produce a bicyclo[2.2.2]octadiene framework.

T

Moreover, MacMillan reported a photocatalytic decarboxylative olefination of alkyl, α-amino, and α-oxy carboxylic acids with vinyl sulfones or vinyl halides using a photoredox nickel dual catalysis activation, realizing Csp3−Csp2 bond formation.13 In 2011, Studer and co-workers reported a Pd-catalyzed stereospecific decarboxylative γ-arylation using 2,5-cyclohexadiene-1-carboxylic acids as substrates coupled with aryl iodides to afford arylated 1,3-cyclohexadienes (Scheme 1b),14a,b which was applied to the total synthesis of resveratrols, macheriols, and THC.14c,d Recently, we and Studer’s group have independently

he alkene moiety is an important building block in organic synthesis and widely exists in pharmaceuticals, natural products, and functional organic π-materials.1 Thus, the development of synthetic protocols toward these scaffolds has attracted considerable attention. In general, conventional methods for the construction of alkene moieties with good stereoselectivity include Wittig−type reactions,2 Mizoroki− Heck reactions,3 and transition-metal catalyzed cross-coupling reactions.4 However, the shortcomings of these methods include being step-noneconomic and waste-exhaustive and requiring expensive or air-sensitive organometallic reagents (organozinc, -boron, and -tin compounds). During the past decade, transition-metal catalyzed decarboxylative cross-couplings have become powerful tools for the formation of carbon−carbon bonds.5 Decarboxylative crosscouplings take advantage of the fact that the nucleophilic coupling partners are generated in situ from readily available carboxylic acids by extrusion of carbon dioxide instead of toxic metal halides. Decarboxylative couplings can provide a greener, more practical, and step-economic synthetic alternative to the use of common organometallic reagents. Since Myers and co-workers reported the first decarboxylative Heck-type olefination of aromatic carboxylic acids with alkenes in 2002,6 extensive research on decarboxylative olefinations using a variety of carboxylic acids coupled with a broad range of alkene sources has been developed. For example, Su,7 Gooβen,8 and others9 showed that successful decarboxylative couplings of aromatic carboxylic acids with alkenes or vinyl halides can be achieved using various metal catalysts or oxidants. In addition, decarboxylative olefinations using alkynyl,10 cinnamyl,11 or αketo carboxylic acids12 as substrates have also been disclosed. © 2018 American Chemical Society

Scheme 1. Decarboxylative Olefinations or Arylation of Various Carboxylic Acid Derivatives

Received: February 9, 2018 Published: March 14, 2018 1949

DOI: 10.1021/acs.orglett.8b00486 Org. Lett. 2018, 20, 1949−1952

Letter

Organic Letters showed that 2,5-cyclohexadiene-1-carboxylic acids undergo tandem Pd-catalyzed decarboxylative C−H olefination/rearomatization reactions yielding ortho-alkylated vinylarenes.15 We envisioned that this versatile substrate class, 2,5-cyclohexadiene1-carboxylic acids, can also be utilized in the decarboxylative olefination with vinyl halides to enable a new decarboxylative Csp3−Csp2 bond-forming reaction. Herein, we disclose a Pdcatalyzed decarboxylative γ-olefination of 2,5-cyclohexadiene-1carboxylic acids with vinyl bromides or vinyl iodides to provide olefinated 1,3-cyclohexadienes that can be further used in the preparation of meta-alkylated stilbenes or bicyclo[2.2.2]octadiene frameworks. We began with the optimization of the decarboxylative γolefination reaction by using 1.0 equiv of 1-isopropyl-2,5cyclohexadiene-1-carboxylic acid (1b) and 1.1 equiv of (Z)-βbromostyrnene (2a) as model substrates, 10 mol % of Pd(OAc)2 catalyst, and Cs2CO3 as a base in toluene at 110 °C for 3 h, affording γ-olefination product 3b in 45% isolated yield (Table 1,

loading, but only a 66% yield was obtained (entry 13). Therefore, for further study we used 1.0 equiv of vinyl halides, 1.25 equiv of acids, 5 mol % of Pd(OAc)2, and 1.4 equiv of Cs2CO3 in toluene at 110 °C for 3 h as the standard conditions in the following experiments. With the optimized reaction conditions in hand, we turned our attention toward the substrate scope of this Pd-catalyzed decarboxylative γ-olefination reaction. As shown in Scheme 2, a Scheme 2. Substrate Scope of the Pd-Catalyzed Decarboxylative γ-Olefinationa,b

Table 1. Optimization of the Pd-Catalyzed Decarboxylative γOlefination Reactiona

entry

1b:2a

ligand

based

3b (%)e

1 2 3 4 5 6 7 8 9 10 11 12 13

1:1.1 1:1.1 1:1.1 1:1.1 1:1.1 1:1.1 1:1.1 1:1.1 1:1.1 1:1.5 1.25:1 1.25:1 1.25:1

− PPh3b Dppec SPhosb XantPhosc − − − − − − − −

Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 t BuOK t BuONa K2CO3 Ag2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3

45 30f 31f 22g 0g 0g 0g 0g 0f 59 87 88h 66i

a

Reaction was performed with 1 (0.125 mmol), and 2 (0.1 mmol) in 2 mL of toluene. bIsolated yields. cReaction conducted at 1 mmol scale of 2a. dCombined yield of isomers.

series of 1-alkylated 2,5-cyclohexadiene-1-carboxylic acids 1a−l were prepared from readily available aromatic carboxylic acids derivatives using the well-established Birch reduction (see Supporting Information (SI) for details),16 and coupled with a variety of (Z)- or (E)-vinyl halides under optimized conditions to give 3a−3t. Methyl, isopropyl, sec-butyl, n-hexyl, and benzyl substituents at the 1-position of the 2,5-cyclohexadiene-1carboxylic acids coupled with (Z)-β-bromostyrene were compatible with this procedure, leading to the corresponding olefinated 1,3-cyclohexadienes 3a−e in good yields. As we expected, an additional substituent (methyl or fluoro) at the 2- or 3-position of the 2,5-cyclohexadiene-1-carboxylic acid was well tolerated, and reactions proceeded with excellent regioselectivity to deliver 3e−3h in 60−83% isolated yields. This finding is consistent with the decarboxylative γ-arylation of 2,5cyclohexadiene-1-carboxylic acids reported by Studer.14a As for the 4-methyl 2,5-cyclohexadiene-1-carboxylic acid substrate, compound 3i was obtained and isolated with its isomers in 53% combined yields. However, 4-isopropyl or 4-tert-butyl 2,5cyclohexadiene-1-carboxylic acids did not work with this process due to the large steric effect. It is worth noting that undesired double bond isomerization products, which are commonly found as side products in Heck reactions, were not observed using this procedure. Substituted (E)-β-bromostyrenes were also examined using these reaction conditions. Para-substituents with hydrogen, benzyloxy, chloro, or nitro groups were well tolerated, delivering 3j−m in high yields. Using para-, meta-, and ortho-methoxy (E)-

a

Reaction conditions: 0.1 mmol scale at 0.05 M with 10 mol % of Pd(OAc)2 under an argon atmosphere. bWith 20 mol % of additive. c With 10 mol % of additive. dBase/acid = 1.1:1. eIsolated yields. f Isopropylbenzene as major product. gRecovering of 1b. hWith 5 mol % of Pd(OAc)2. iWith 1 mol % of Pd(OAc)2.

entry 1). Next, a variety of mono- or bidentate phosphine ligands were examined. The decarboxylative rearomatization of 1b occurred in the presence of PPh3 or Dppe formed isopropylbenzene as the major product, instead of forming 3b (entries 2 and 3). Using Buchwald ligands, Sphos and XantPhos, poor reactivity was observed and resulted in the recovery of 1b (entries 4 and 5). Other bases, such as tBuOK, tBuONa, K2CO3, or Ag2CO3 were also tested and either were found to be ineffective or resulted in the formation of isopropylbenzene (entries 6−9). Upon increasing the amount of 2a to 1.5 equiv, the reaction yield increased to 59% (entry 10). Interestingly, the reaction provided the γ-olefination product in 87% yield when increasing the amount of 1b from 1.1 to 1.25 equiv (entry 11), and decreasing the amount of 2a from 1.1 to 1. Moreover, decreasing the catalyst loading to 5 mol % still gave a comparable result (entry 12). We further examined the reaction with a 1 mol % of Pd(OAc)2 1950

DOI: 10.1021/acs.orglett.8b00486 Org. Lett. 2018, 20, 1949−1952

Letter

Organic Letters β-bromostyrenes, the reaction proceeded with good yields (3n− p). In the case of methylene methyl ester substituted 2,5cyclohexadiene-1-carboxylic acid or substituted 1,4-dihydro-1naohthoic acids, treatment of para-(E)-β-iodostyrene instead of para-(E)-β-bromostyrene gave satisfying results (3q−s). The cyclic substrate, 1-bromocyclohexene, also worked well with this procedure, affording 3t in 85% yield. Meta-substituted stilbenes are an important class of compounds and are widely found in many natural products, pharmaceuticals, and functional organic π-materials.1f,g,17 Recently, alternative methods in the preparation of metasubstituted vinylarenes by the strategies of template directing groups (Yu,18 Maiti,19 Li20) or traceless directing groups (Miura,21 Gooβen,22 Ackermann,23 Zhao24) have been disclosed in the area of arene C−H functionalization. However, a large template directing group is needed to be preformed, and the expensive catalyst systems are indispensable. Therefore, the development of an efficient protocol for the synthesis of metasubstituted stilbenes is highly desired. To show the synthetic utility of the γ-olefination products produced above in Scheme 2, we subjected the olefinated 1,3cyclohexadienes 3 to a dichloromethane solution of DDQ (2,3dichloro-5,6-dicyano-1,4-benzoquinone, 1.5 equiv) at room temperature. The corresponding oxidative rearomatization products 4 were obtained in good to excellent yields (Scheme 3). With this established protocol, meta-alkylated stilbenes with

in THF (Scheme 4; see the optimization studies of Lewis acids in the SI). As a result, the corresponding bicyclo[2.2.2]octadienes 6a and 6b were obtained in 67% and 55% yields, respectively. Scheme 4. Application of Pd-Catalyzed Decarboxylative γOlefination Products in the Preparation of Bicyclo[2.2.2]octadienesa,b

a

Reaction was performed with 3 (0.2 mmol), 5 (0.6 mmol) and Cu(OTf)2 (0.02 mmol) in 1 mL of tetrahydrofuran. bIsolated yields.

On the basis of our experiment results, a plausible mechanism for this Pd-catalyzed decarboxylative γ-olefination reaction was proposed and is illustrated in Scheme 5. First, the Pd(OAc)2 Scheme 5. Proposed Mechanism for the Pd-Catalyzed Decarboxylative γ-Olefination

Scheme 3. Application of Pd-Catalyzed Decarboxylative γOlefination Products in the Preparation of Meta-Alkylated Vinylarenesa,b

reduced to active Pd(0)-species in the presence of 2,5cyclohexadiene-1-carboxylic acids 1, followed by oxidative addition with vinyl halides 2 affording Pd(II)-species A, which then undergoes ligand exchange with the Cs-carboxylate of 1 to provide Pd(II)-carboxylate species B. Decarboxylation of B provides 2,5-cyclohexadienyl Pd(II)-species C. Next, 1,3-Pd migration to the γ-position affords the thermodynamically stable 2,4-cyclohexadienyl Pd(II)-species D. Interestingly, the 1,3-Pd migration process to the sterically hindrance position is prohibited due to the large steric effect of the methyl group at the 2-position. Finally, the intermediate D undergoes reductive elimination providing olefinated 1,3-cyclohexadienes 3 and regenerating the Pd(0)-species to complete the catalytic cycle. In summary, we have developed a Pd-catalyzed decarboxylative γ-olefination of 2,5-cyclohexadiene-1-carboxylic acids with vinyl bromides or vinyl iodides. This method most likely proceeds through a Pd(0)/Pd(II) catalytic cycle. We have also demonstrated that the synthesized olefinated 1,3-cyclohexadienes can be rearomatized to meta-alkylated stilbenes or can undergo the Diels−Alder reaction to construct bicyclo[2.2.2]octadiene frameworks. The corresponding γ-olefination products

a

Reaction was performed with 3 (0.1 mmol), and DDQ (0.15 mmol) in 2 mL of dichloromethane. bIsolated yields.

highly diverse structures and specific double bond stereochemistry can be rapidly accessed from readily available benzoic acids derivatives in three steps. Bicyclo[2.2.2]octadiene frameworks are interesting and versatile building blocks in organic synthesis.25 In order to incorporate these frameworks into our γ-olefination system, we performed Diels−Alder reactions of olefinated 1,3-cyclohexadiene 3b or 3j with dienophile 5 (dimethyl acetylenedicarboxylate) in the presence of catalytic amounts of Cu(OTf)2 1951

DOI: 10.1021/acs.orglett.8b00486 Org. Lett. 2018, 20, 1949−1952

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Organic Letters

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are highly useful building blocks in organic synthesis. Future studies to elucidate the reaction mechanism and apply this methodology to a natural product approach are being currently carried out in our laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00486. Experimental details and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chih-Ming Chou: 0000-0002-9894-5365 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the National University of Kaohsiung and the Ministry of Science and Technology of Taiwan (MOST 1042113-M-390-004-MY2 and MOST 106-2113-M-390-001-MY2) for their financial support.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b00486 Org. Lett. 2018, 20, 1949−1952