Palladium catalyzed cross-dimerization of terminal acetylenes and acrylates

Tetrahedron Letters 60 (2019) 1234–1237

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Palladium catalyzed cross-dimerization of terminal acetylenes and acrylates Shanshan Chen ⇑, Shi-Hang Xie, Cheng-Ying Ai, Xiu-Li Zhang Division of Applied Chemistry, School of Natural Sciences, Anhui Agricultural University, Hefei 230036, China

a r t i c l e

i n f o

Article history: Received 7 December 2018 Revised 21 March 2019 Accepted 23 March 2019 Available online 26 March 2019

a b s t r a c t A palladium catalyzed oxidative cross-coupling reaction between terminal alkynes and acrylates was developed. In the presence of palladium catalyst and copper(II) acetate as oxidant, the cross dimerization products with were formed. This protocol provides a novel method to construct the tetrasubstituted functionalized alkenes. Ó 2019 Elsevier Ltd. All rights reserved.

Keywords: Palladium catalyst Alkynes Acrylates Cross-dimerization Conjugated skeleton

The demand of green and efficient methodologies is increasingly intense in modern organic chemistry. Therefore, oxidative dehydrogenative coupling reactions have attracted much attention from organic chemists due to their powerful and versatile performances in forging new CAC bonds during the past several decades [1]. So far, the transition-metal catalyzed coupling reactions of sp[2](aryl)-sp[2](aryl) [2], sp[2](alkenyl)-sp[2](alkenyl) [3], sp[2](aryl)-sp[2](alkenyl) [4], sp[3](alkyl)-sp[2](aryl) [5], sp[3](aryl)sp(alkynl) [6], sp[2](alkyl)-sp[3](alkyl) [7], and sp[2](alkynl)-sp (alkynl) [8] etc have been well documented to synthesize diverse functionalized compounds. Among them, the transition-metal catalyzed direct CAC bond formation between alkynes and alkenes is an attractive reaction to access b-alkynyl carbonyl compounds via an addition process (Scheme 1, eq 1) [9], or to form the conjugated 1,3-dienes via the possible sequences of alkenyl CAH bond activation and hydrometallation of the triple bond (Scheme 1, eq 2) [10]. These methods are highly atom-economic to construct new carbon-carbon bonds with use of the simple alkynes and alkenes. It is worth noting that the triple or double p-bond functionality will be completely or partially reduced in the final products. Recently, a direct palladium catalyzed oxidative cross-coupling reaction between the terminal alkynes and acrylates to form the conjugated enynes was established by Jun and co-workers (Scheme 1, eq 3) [11]. It is the most straightforward and ideal pathway to synthesize the conjugated enyne derivatives. ⇑ Corresponding author. E-mail address: [email protected] (S. Chen). https://doi.org/10.1016/j.tetlet.2019.03.062 0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.

Despite aforementioned investigations, there has not been a report regarding the cross-dimerization of terminal alkynes and acrylates. It is well known that the polyconjugated enyne fragments are widely observed in natural products and applied into the synthesis of various organic functional materials [12]. Previous works on the preparation of these compounds involved the palladium catalyzed coupling reactions with use of organohalides and organometallic reagents [13]. However, these methods are less environmentally friendly and atom-economic due to their need of tedious steps to prepare the starting materials and the formation of waste byproducts in reactions. Therefore, the development of direct and efficient methodologies to synthesize these compounds are highly desirable. Herein, we would like to communicate the results on the palladium catalyzed cross-dimerization of terminal alkynes and acrylates to construct the polyconjugated fragments (Scheme 1, eq 4). The model reaction was carried out with phenylacetylene (1a) and methyl acrylate (2a) to optimize the reaction conditions for this cross-dimerization reaction (Table 1). First, different palladium catalysts were tested for this coupling reaction. Pd(OAc)2 proved to be more efficient, although only a moderate yield was obtained (Table 1, entry 1). Other palladium sources such as PdCl2, Pd(CH3CN)2Cl2, Pd(PPh3)2Cl2, Pd(MeCN)4(BF4)2, Pd(TFA)2, and White catalyst were all found to be unreactive or inefficient to catalyze this reaction (Table 1, entris 2–7). Subsequent screening of different copper slats showed that other neutral and ionic copper (II) slats could not improve the yield (Table 1, entries 7–13). Additionally, the copper(I) salts were also not useful to form the desired

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entries 3b and 3d). And the tert-butyl acrylate and phenyl acrylate were less efficient to react with phenylacetylene, affording a slightly lower yield, respectively (Table 2, entries 3b and 3e). Only trace amount of desired product was detected when the tert-butyl methacrylate was used in this coupling reaction (Tables 2, 3f). Next, the substrate scope of terminal alkynes was examined under the optimized reaction conditions (Table 3). It was observed that the substrate with an electron-withdrawing group on the phenyl ring more favored the product formation (Table 3, entries 3 h, 3j and 3 l). In contrast, the expected cross-coupling products were obtained only in low yields when the strong electron-donating groups were installed on the phenyl ring of the substrates (Table 3, entries 3g, 3i, and 3k). The halides functionalities on the phenyl ring permit the products to be further modified in next step. Unfortunately, the aliphatic-substituted terminal alkyne such as 1octyne failed to give the desired product (Table 3, 3m). Scheme 1. Transition-metal catalyzed oxidative coupling reactions between terminal alkynes and acrylates.

product. Of the typical solvents used in cross-coupling reactions that were screened, dimethyl sulfoxide (DMSO) gave the best yield. The careful screening mixed solvents was also failed to improve the yield (Table 1, entries 14–19). As shown in Table 1, the best yield was obtained 10 mol% loading of Pd(OAc)2 and Cu(OAc)2 (2 equiv.) as oxidant in DMSO at 60 °C, to give the product (3a) in 54% yield (entry 1). With the optimized conditions at hand, different acrylates were tested as coupling partners (Table 2). It was found that the yields of the corresponding cross-dimerization products would be decreased with prolonging the carbon chain of the esters (Table 2,

In order to probe the mechanism of this cross-dimerization reaction, we designed and synthesized the possible intermediates generated in situ. Unfortunately, no desired product was detected when the 1,4-diphenylbuta-1,3-diyne and the (Z)-but-1-en-3-yne1,4-diyldibenzene were subjected into the reaction under the standard reaction conditions, respectively.

Table 1 Optimization of oxidative cross-dimerization coupling for the phenylacetylene with methyl acrylate.a

entry c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20c 21d

catalyst

oxidant

solvent

yield (%)b

Pd(OAc)2 PdCl2 Pd(CH3CN)2Cl2 Pd(PPh3)2Cl2 Pd(MeCN)4(BF4)2 White catalyst Pd(TFA)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 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2

Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 CuCl2 CuSO4 Cu(OTf)2 CuBr2 CuCl CuI Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO/HOAc (1:1) DMSO/HOAc (1:10) DMSO/HOAc (10:1) DMSO/dioxane (1:1) DMSO/CH3CN(1:1) DMSO/toluene (1:1) DMSO DMSO

54 0 0 6 22 4 10 0 5 19 0 0 0 32 34 18 35 22 41 33 33

a The reactions were carried out with phenylacetylene (0.2 mmol), methyl acrylate (0.2 mmol), Pd(TFA)2 (0.02 mmol), Cu(OAc)2 (0.4 mmol) at 60 °C in DMSO with air (1 atm). b Isolated yield. c The temperature is 50 °C. d The temperature is 70 °C.

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Table 2 Cross-coupling of phenylacetylene with various acrylatesa.

Table 3 Cross-coupling of terminal alkynes with methyl acrylatea.

a

The reactions were carried out with terminal alkynes (0.2 mmol), methyl acrylate (0.2 mmol), Pd(TFA)2 (0.02 mmol), Cu(OAc)2 (0.4 mmol) at 60 °C in DMSO with air (1 atm). b Isolated yield.

a The reactions were carried out with phenylacetylene (0.2 mmol), acrylate (0.2 mmol), Pd(TFA)2 (0.02 mmol), Cu(OAc)2 (0.4 mmol) at 60 °C in DMSO with air (1 atm). b Isolated yield.

Albeit without direct evidence to disclose the detailed pathway of this coupling reaction currently, a plausible mechanistic pathway was proposed as shown in Scheme 2. Firstly, an alkynylpalladium species generated in situ would react with another molecular alkyne to form a cis-vinylpalladium intermediate (II0 ) which could isomerize into a more stable trans-vinylpalladium intermediate (II) in the presence of acid. Following, a migratory addition to acrylate and b-hydride elimination processes would result in an intermediate 4. Possibly, with the chelation assistance of p-bond [14], a vinylpalladium intermediate IV would be generated via CAH bond activation, which can undergo migratory addition to acrylate and b-hydride elimination with affording the desired product. The palladium(0) species generated in situ would be reoxidized by the copper(II) salt and oxygen of air to regenerate the palladium(II) species fro next catalytic cycle. In summary, we have established a novel protocol for the oxidative cross-dimerization of terminal alkynes and acrylates by using palladium(II) catalyst. The desired cross-coupling products were obtained in reasonable yields. Further investigation into the mechanism of this coupling reaction and its application to the synthesis of organic functional materials are currently in progress.

Young Scientists of China (21405002), Anhui Provincial Natural Science Foundation (1608085MB26), and Anhui Agricultural University for financial support.

Acknowledgments

Appendix A. Supplementary data

We thank the Anhui Provincial Foundation for Selected Young Scientists Studying Abroad, the National Science Foundation for

Supplementary data to this article can be found online at https://doi.org/10.1016/j.tetlet.2019.03.062.

Scheme 2. Proposed mechanism for the cross-dimerization of terminal alkynes and acrylates.

S. Chen et al. / Tetrahedron Letters 60 (2019) 1234–1237

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