Oxidative and Reductive Cross-Coupling Reactions Catalyzed by an

Article Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX

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Oxidative and Reductive Cross-Coupling Reactions Catalyzed by an Anionic “Ligandless” Palladium Complex Felix Schroeter, Swantje Lerch, and Thomas Strassner* Physikalische Organische Chemie, TU Dresden, Bergstr. 66, 01062 Dresden, Germany

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ABSTRACT: The anionic complex [NBu4][Pd(DMSO)Cl3], which can be synthesized on a gram scale in a single step starting from commercially available starting materials, has been shown to be an active catalyst in the Mizoroki−Heck reaction of aryl halides. We present two new catalytic applications of this complex: the base-free oxidative Heck reaction and the reductive homodimerization of aryl halides. This complex outperformed other palladium salts. In the latter reaction, the catalyst loading could be reduced to 0.01 mol %. The scope of the reactions has been explored, demonstrating the potential of the anionic palladium complex in these catalytic transformations. KEYWORDS: palladium, anionic complex, oxidative Heck reaction, reductive dimerization

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INTRODUCTION The formation of C−C bonds is the basis for every organic synthesis.1 Among the most straightforward and wellestablished methods are palladium-catalyzed cross-coupling reactions.2−5 While the Mizoroki−Heck reaction allows access to stilbenes (Scheme 1a),6−14 the Suzuki−Miyaura reaction,

a surrogate for the aryl halide) and an alkene for the synthesis of stilbenes (Scheme 1a).33−57 A viable alternative for the synthesis of symmetric biphenyls is the reductive homodimerization58 of aryl halides (Scheme 1b).59−80 These reactions are of interest for applications on an industrial scale.81−84 We recently presented the anionic palladium complex 1a (Scheme 1c) as an active catalyst in the Mizoroki−Heck reaction of activated aryl chlorides.85 We performed an indepth analysis of the reasons for the high catalytic activity,86 which explained the higher activity of 1a compared with 1b and 1c and suggested that complex 1a might also be suitable for other cross-coupling reactions. Earlier results by Dyson87,88 and Trzeciak,89−92 among others,93−98 on related anionic palladium complexes support this suggestion (Scheme 1d). Therefore, we investigated the performance of complex 1a in the oxidative Heck reaction and the reductive homodimerization in comparison with various commercially available palladium precatalysts. We explored the scope of these reactions and found that 1a is indeed a suitable precatalyst for these transformations.

Scheme 1. Investigated Reactions and Representative Catalyst Systems

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RESULTS AND DISCUSSION Oxidative Heck Reaction. As a model system for the oxidative Heck reaction, we chose the reaction of phenylboronic acid and styrene. To avoid excess amounts of the more expensive boronic acid, we used an excess of styrene in the reaction. We began the optimization using oxygen as the oxidizing agent (Table 1, entry 1). However, the yield was only 13%. With copper acetate monohydrate as a co-oxidant, the yield increased to 35%, but to some extent, undesired biphenyl formed by homocoupling of the boronic acid (Table 1, entry

among others, yields biphenyls (Scheme 1b).15−23 However, most of these cross-coupling reactions suffer from low atom economy and the necessity to use toxic, air-sensitive organometallic reagents.24−26 Among the required aryl halides, aryl chlorides are the reaction partners of choice since they are less expensive, but on the other side, they are also less reactive. One approach to attenuate these drawbacks is the development of the oxidative Heck reaction27−32 of a boronic acid (as © XXXX American Chemical Society

Special Issue: Work from the Organic Reactions Catalysis Society Meeting 2018 Received: August 21, 2018

A

DOI: 10.1021/acs.oprd.8b00274 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Article

acetonitrile complex also led to good yields of 88% and 86%, respectively (Table 2, entries 7 and 8). Next, we explored the scope of the reaction (Scheme 2). The reaction tolerates different methyl-substituted boronic

Table 1. Optimization of the Reaction Conditions for the Oxidative Heck Reactiona

Scheme 2. Substrate Scope of the Oxidative Heck Reactiona entry

catalyst (mol %)

1 2

1a (1) 1a (1)

3 4 5 6 7 8 9

1a (1) 1a (1) 1a (1) 1a (1) 1a (0.5) 1a (1) 1a (1)

oxidant (equiv)

solvent

T (°C)

yield (%)

air Cu(OAc)2·H2O (0.2)/ air BQ (0.2)/air BQ (1.1) K2S2O8 CuSO4b BQ (1.1) BQ (1.1) BQ (1.1)

DMF DMF

100 100

13 35

DMF DMF DMF DMF DMF DMF DMF

100 100 100 100 100 80 120

48 100 4 2 25 24 53

a

Reaction conditions: 1 mmol of PhB(OH)2, 1.5 mmol of styrene, 2 mL solvent. Abbreviations: DMF = dimethylformamide, BQ = benzoquinone. b2.2 equiv instead of 1.1 equiv.

2). Benzoquinone (BQ) was more efficient as a co-oxidant (Table 1, entry 3). A quantitative yield of stilbene was achieved when BQ was used as a stoichiometric oxidant (Table 1, entry 4). Other oxidants were less efficient (Table 1, entries 5 and 6). Lowering the catalyst loading or altering the reaction temperature had a detrimental effect on the catalytic activity (Table 1, entries 7−9). While dioxane, acetonitrile, and dimethylacetamide also performed well (Table S1, entries 1− 3), the other tested solvents were inefficient (Table S1, entries 4−7). We chose dimethylformamide (DMF) for further optimization because of its lower volatility. Using equimolar amounts of styrene and phenylboronic acid is less efficient in the reaction (Table S1, entry 8); however, an excess of phenylboronic acid is as efficient as an excess of styrene (Table S1, entry 9). No base was required for this reaction, which further improved the atom economy of the process. Under the optimized reaction conditions, we compared different palladium precursors (Table 2). Analogously to

a

Reaction conditions: (A) 1 mmol of arylboronic acid, 1.5 mmol of styrene; (B) 1.5 mmol of arylboronic acid, 1 mmol of styrene. Isolated yields are shown.

acids. For example, 4-methylstilbene (2b) was isolated in 73% yield. The yield of 2-methylstilbene (2c) was lower, indicating that the catalyst is sensitive to steric demand, but to a lesser extent than in the Mizoroki−Heck reaction.85 Different alkenes can be employed in the reaction. For example, butyl acrylate and phenylboronic acid yield butyl cinnamate (2e) in 77% yield. We noticed that certain substrates only gave low yields using procedure A. Under the reaction conditions, decomposition of the boronic acid is possible. In these cases, using excess boronic acid instead of excess styrene led to improved results (Scheme 2, conditions B). With these adapted conditions, both electron-withdrawing and electron-donating substituents are tolerated well. The reaction is also applicable on gram scale (Scheme 3) without the need for flame-dried glassware. To obtain excellent

Table 2. Comparison of Different Precatalysts in the Oxidative Heck Reactiona

entry

catalyst

yield (%)

entry

catalyst

yield (%)

1 2 3 4

1a 1b 1c Na2PdCl4

100 79 66 50

5 6 7 8

Pd(OAc)2 Pd(DMSO)2Cl2 PdCl2 Pd(MeCN)2Cl2

52 58 88 86

Scheme 3. Gram-Scale Oxidative Heck Reactiona

a

Reaction conditions: 1 mmol of PhB(OH)2, 1.5 mmol of styrene.

a

earlier results,85 complex 1a performed best (Table 2, entry 1). The presence of bromide ions in 1b lowers the catalytic activity (Table 2, entry 2) because bromide binds more strongly to the palladium center and thus slows substrate binding.9,86,96,99−101 Using the sodium salt of the palladate anion (1c) is also less efficient, presumably because of the observed lower solubility (Table 2, entry 3). Other palladium salts were less efficient (Table 2, entries 4−6), although palladium chloride and its

The isolated yield is shown.

yields of the stilbene coupling product, styrene must be added dropwise to a stirred mixture of the remaining reagents in DMF at 50 °C. Then the reaction mixture is heated to 100 °C and stirred for 3 days. If styrene is added in one portion, the yield is significantly lower (14%). Under the optimized reaction conditions, 1.4 g (78% yield) of stilbene 2a was isolated. B



Article

Reductive Homodimerization of Aryl Bromides. The dimerization of 4-bromoacetophenone takes place under reductive reaction conditions. We began the optimization with a search for a suitable reductant (Table 3). Earlier

at that temperature but dropped to 94% when the temperature was lowered to 100 °C (Table 3, entries 15 and 16). Under the optimized reaction conditions, we compared the different precatalysts (Table 4). Again, complex 1a out-

Table 3. Optimization of the Reaction Conditions for the Reductive Homodimerizationa

Table 4. Precatalyst Comparison in the Reductive Homodimerizationa

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

loading of 1a (mol %) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.03 0.01 0.01 0.001 0.1 0.1

reductant TBACl ascorbic acid HQ HCO2Na Zn Mg HQ HQ HQ HQ HQ HQ HQ HQ HQ HQ

base

T (°C)

conv. (%)

yield (%)

NaOAc NaOAc

140 140

41 6

0 3

NaOAc NaOAc NaOAc NaOAc Cs2CO3 K2CO3 NaOH K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4

140 140 140 140 140 140 140 140 140 140 140 140 120 100

58 75 96 100 100 100 100 100 100 86 100b 49c 100 94

40 4 1 3 93 98 48 100 100 86 100b 45c 100 94

entry

catalyst

yield (%)

entry

catalyst

yield (%)

1 2 3 4

1a 1b 1c Pd(OAc)2

100 70 30 21

5 6 7 8

Pd(DMSO)2Cl2 PdCl2 Pd(MeCN)2Cl2 Na2PdCl4

23 31 36 38

a

Reaction conditions: 1 mmol of 4-bromoacetophenone.

performed all of the other palladium sources (Table 4, entry 1). In this reaction, the reactivities of the other precatalysts were considerably lower (Table 4, entries 4−8). For example, palladium chloride led to a yield of only 31% (Table 4, entry 6). At these low catalyst loadings, decomposition of the catalyst has a more pronounced detrimental effect on the catalytic activity, which is reflected by the low yield obtained with palladium acetate (Table 4, entry 4). Next, we explored the scope of this reaction (Table 5). The model substrate was efficiently coupled in 95% isolated yield at Table 5. Best Results upon Screening the Substrate Scope of the Reductive Homodimerization with Electron-Deficient Substratesa

a

Reaction conditions: 1 mmol of 4-bromoacetophenone, 3 h. Abbreviations: TBACl = tetrabutylammonium chloride; HQ = hydroquinone. bThe reaction time was 6 h instead of 3 h. cThe reaction time was 24 h instead of 3 h.

observations suggested that tetrabutylammonium chloride (TBACl) would be suitable for this transformation (Table 3, entry 1).86 However, the desired product was not formed. Instead, debromination to give acetophenone occurred, which accounted for the conversion of 41%. Ascorbic acid decomposed under the reaction conditions and led to only trace amounts of product (Table 3, entry 2), while a 40% yield was obtained with hydroquinone (HQ) (Table 3, entry 3). Stronger reductants, such as sodium formate, zinc, and magnesium, led to nearly quantitative debromination (Table 3, entries 4−6). The use of a stronger base was required for a high yield of the desired product (Table 3, entries 7−10). Among the tested bases, potassium phosphate gave the dimerization product in quantitative yield (Table 3, entry 10). Under these reaction conditions, the catalyst loading could be reduced to 0.03 mol % and even to 0.01 mol % with a slight decrease in yield (Table 3, entries 11 and 12). At this stage, a quantitative yield of the product was obtained when the reaction time was slightly prolonged (Table 3, entry 13). However, when the catalyst loading was further reduced, the reaction remained incomplete even after 24 h of reaction time (Table 3, entry 14). Alternatively, the reaction can be performed at 120 °C with a catalyst loading of 0.1 mol %. The yield of 100% was retained

yields (%)b entry

Ar

conditions

3

4

5

1 2 3 4 5 6

4-Ac-Ph (a) 4-Ac-Ph (a) 4-NC-Ph (b) 4-OHC-Ph (c) 4-NO2-Ph (d) 3,5-(CF3)2-Ph (e)

A C, 6 h B C, 72 h C, 6 h A

95 89 73 72 (3) 17

0 0 (3) (8) 65 0

0 0 (2) (3) 0 29

Reaction conditions: (A) 0.03 mol % 1a, 140 °C, 3 h; (B) 0.1 mol % 1a, 140 °C, 3 h; (C) 0.1 mol % 1a, 100 °C, until conversion was complete (see the indicated times). bIsolated yields, except for values in parentheses, which were determined by GC.

a

a catalyst loading of 0.03 mol % (Table 5, entry 1, conditions A). However, for other electron-deficient aryl bromides, noncatalyzed nucleophilic aromatic substitution of the bromide by HQ becomes a major side reaction. Another important side reaction is protodebromination, which accounts for the high conversion of the substrates (Table S2). On the basis of the results of the optimization, two alternatives were tested: (1) the catalyst loading was increased, since it might accelerate the C



Article

Scheme 4. Gram-Scale Reductive Aryl Halide Dimerizationa

catalyzed reaction relative to the noncatalyzed side reaction (Table 5, conditions B); (2) the reaction temperature was lowered to 100 °C to minimize side reactions, and subsequently, the reaction time was increased until conversion of the substrates was complete (Table 5, conditions C). These approaches are summarized in Table S2. We found that the optimum reaction conditions vary from substrate to substrate. For example, 4,4-dicyanobiphenyl (3b) was accessible in a good yield of 73% with conditions B (Table 5, entry 3). In contrast, 4,4-diformylbiphenyl (3c) can be obtained in 72% using conditions C (Table 5, entry 4). Nevertheless, neither of these approaches was successful in the syntheses of 3d and 3e, where only small amounts of the desired product were formed (Table 5, entries 5 and 6). Since the reaction conditions have to be separately optimized for each compound to reach maximum efficiency, we preoptimized the reaction conditions in the reaction of electron-rich aryl bromides (Table S3). While these substrates do not allow nucleophilic aromatic substitution, we found that hydrodebromination was still an issue. A slightly increased catalyst loading offered an optimum between the desired reaction and the side reaction. Then we compared the yields under two different sets of reaction conditions (Table 6).

a

available on a gram scale, in the oxidative Heck reaction and the reductive dimerization of aryl halides. In both reactions, the complex outperformed other conventional and commercially available palladium salts. In the oxidative Heck reaction, the reaction conditions can be adapted to yield various substituted stilbenes. In the reductive dimerization, the reactivity of the complex depends on balancing the reaction conditions to minimize side reactions, such as nucleophilic aromatic substitution and hydrodebromination. Excellent isolated yields were also obtained upon 10-fold scale-up of the reactions with only slight adaptions to the reaction protocol. These results show that complex 1a is also suitable for cross-coupling reactions on a larger scale.

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Table 6. Reaction Scope of the Reductive Homodimerization with Electron-Rich Substratesa

entry 1 2 3 4 5 6

Ar Ph 4-Me-Ph 4-MeO-Ph

conditions A B, 48 h A B, 48 h A B, 72 h

EXPERIMENTAL SECTION General Considerations. All of the reactions were performed under an argon atmosphere unless stated otherwise. Solvents of at least 99.0% purity were used in all of the reactions in this study. Toluene and DMF were distilled from calcium hydride and stored over molecular sieves (4 Å). The following compounds were synthesized according to literature procedures: 1a−c, 8 5 Pd(DMSO) 2 Cl 2 , 1 0 8 and Pd(MeCN)2Cl2.109 All of the other chemicals were obtained from common suppliers and used without further purification. 1 H and 13C spectra were acquired on Bruker NMR Avance 300 and Bruker DRX 500 spectrometers. 1H and 13C spectra were referenced internally using the solvent resonances (1H at 7.26 ppm and 13C at 77.0 ppm for CDCl3). Chemical shifts (δ) are given in parts per million and coupling constants (J) in Hertz. Elemental analyses were performed by the microanalytical laboratory of our institute on a Hekatech EA 3000 Euro Vector elemental analyzer. GC−MS measurements were carried out on an Agilent 7890A gas chromatograph with an Agilent 5975 series mass-selective detector. Small-Scale General Catalytic Procedure. All of the solid reagents were placed in a 10 mL crimp vial, which was capped with a butyl rubber septum. The vial was evacuated and subsequently filled with argon three times, and then the solvent and afterward all of the liquid reagents were added via syringe. The vials were placed in a preheated aluminum block and stirred for the indicated time. For determination of the GC yield, the mixture was diluted with DCM (2 mL). Dodecane (175 mg) was added as an internal standard, and 50 μL aliquots of the reaction mixture were dissolved in DCM (2 mL). Then 1 mL of this solution was placed onto a plug of silica and rinsed with DCM (2 mL). The obtained solution was analyzed by GC−MS. The yield was determined using a calibration curve. All yields given are averages of two separate runs. For determination of the isolated yields, the reaction mixture was filtered over Celite and poured into water (10 mL), and the aqueous phase was extracted with DCM (2 × 10 mL). The

yield (%) 35 43 35 57 35 46

The isolated yield is shown.

(3f) (3f) (3g) (3g) (3h) (3h)

Reaction conditions: (A) 0.25 mol % 1a, 140 °C, 6 h; (B) 0.25 mol % 1a, 100 °C, until conversion was complete (see the indicated times). Isolated yields are shown.

a

Again, at 140 °C, the yields were low, while the substrate was completely consumed (Table 6, conditions A). Performing the reaction at a lower temperature until the substrate was completely consumed led to improved yields (Table 6, conditions B). The best yield of 57% was obtained for 3g (Table 6, entry 4). Aryl chlorides were unreactive under the employed conditions. The occurrence of debromination of the substrates indicates that the catalyst might also be used in hydrodehalogenation reactions,102−104 which are frequently discussed for the treatment of organic-halide-containing wastewater.105−107 Excellent yields on a larger scale could also be achieved upon increasing the catalyst load and prolonging the reaction time (Scheme 4). Again, it was observed that the order of addition of the reagents was important for obtaining a high yield and that no flame-dried glassware was necessary to perform the reaction.

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CONCLUSION We investigated the catalytic activity of the anionic “ligandless” palladium complex [NBu4][Pd(DMSO)Cl3] (1a), which is D



Article

combined organic phases were washed with water (3 × 20 mL) and dried over sodium sulfate. The solvent was evaporated under reduced pressure, and the raw product was adsorbed onto silica and purified by column chromatography on silica gel. Product characterization data are given in the Supporting Information. Gram-Scale Procedure for the Oxidative Heck Reaction. In an argon-purged Schlenk tube containing a magnetic stirring bar, phenylboronic acid (1.22 g, 10 mmol), benzoquinone (1.31 g, 1.1 equiv), and catalyst 1a (53 mg, 1 mol %) were dissolved in 20 mL of dry DMF. The mixture was heated to 50 °C under stirring, and styrene (1.56 g, 1.5 equiv) was added dropwise during a period of 30 min. The mixture was heated to 100 °C for 72 h under stirring, cooled to room temperature, and filtered over Celite. Then water (50 mL) was added to the reaction mixture, and the aqueous phase was extracted with DCM (3 × 50 mL). The combined organic phases were washed with water (3 × 30 mL) and dried over MgSO4. The crude product was adsorbed onto silica and purified by column chromatography using silica gel (column diameter, 60 mm; eluent, hexanes). The product was dried under reduced pressure at 50 °C and obtained as a colorless solid (1.407 g, 78% yield). Gram-Scale Procedure for the Reductive Aryl Halide Dimerization. In an argon-purged Schlenk tube containing a magnetic stirring bar, 4-bromoacetophenone (1.99 g, 10 mmol) and hydroquinone (1.21 g, 1.1 equiv) were dissolved in 20 mL of dry DMF. The mixture was heated to 50 °C under stirring, and catalyst 1a (2.6 mg, 0.05 mol %, via stock solution) and potassium phosphate (2.56 g, 1.1 equiv) were added successively. The mixture was heated to 140 °C for 24 h, cooled to room temperature, and filtered over Celite. Then water (50 mL) was added to the reaction mixture, and the aqueous phase was extracted with DCM (3 × 50 mL). The combined organic phases were washed with water (3 × 30 mL) and dried over MgSO4. The crude product was adsorbed onto silica and purified by column chromatography using silica gel (column diameter, 60 mm; eluent, 2:1 hexanes/ethyl acetate). The product was dried under reduced pressure at 50 °C and obtained as gray solid (1.038 g, 87% yield).

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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.oprd.8b00274.

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Supplemental tables and characterization data for the products (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas Strassner: 0000-0002-7648-457X Notes

The authors declare no competing financial interest.

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REFERENCES

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