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N-substituted pivalamides but also pivalamide and N,N-disubstituted pivalamides were tested in the β-arylations with a verity of iodobenzenes (Sc...
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Palladium-Catalyzed β‑Arylation of Amide via Primary sp3C−H Activation Ren Zhao and Wenjun Lu* Department of Chemistry, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China

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S Supporting Information *

ABSTRACT: A β-arylation of primary sp3C−H bonds on simple amides such as pivalamides with aryl iodides/ CF3CO2Ag has been established successfully at 120 °C in a Pd(OAc)2 (catalyst)/CF3CH2OH (solvent) system. Pivalamides including tBuCONH2, tBuCONHR, and tBuCONR2 undergo the arylations smoothly to afford β-aryl pivalamides in moderate to good yields. Various aryl iodides are available bearing either electron-donating or electron-withdrawing substituted groups in the coupling reactions.

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Scheme 1. Palladium-Catalyzed β-Arylation of sp3C−H Bonds from Amides

INTRODUCTION The formation of new C−C bonds from normal alkyl sp3C−H bonds through a coupling reaction is always a challenging objective to researchers in chemical synthesis.1 In the past decade, an efficient method was developed dramatically, which is a transition metal-catalyzed arylation of alkyl sp3C−H bonds with arylating reagents under assistance of directing groups. For example, Daugulis and co-workers disclosed an early example on catalytic β-arylation of alkyl sp3C−H bonds on 8aminoquoline amides with aryl iodides/AgOAc in 2005,2 in which CONH-8-quinoline as a special directing group could coordinate strongly with palladium complex not only to lead a β-sp3−H bond cleavage but also to stabilize a five-membered palladacycle, a key intermediate to form the target sp3C−sp2C bond. After that, other groups also successfully explored many palladium-catalyzed β-arylations of amides directed by different CONH−FGs (FG = functional group) with various arylating reagents via C−H activation (Scheme 1).3−11 For the regioselective β-arylations of alkyl sp3C−H bonds without special directing groups, the reports are found rarely, including use of transient directing groups generated from common functional groups such as carbonyl groups in situ under reaction conditions.12 In 2007, Yu and co-workers reported that a Pd(II)-catalyzed β-mono- and diarylation of pivalic acid with iodobenzene/Ag2CO3 could proceed smoothly in the presence of K2HPO4/NaOAc in tBuOH at 130 °C, which is a direct β-arylation of carboxylic acid.13 However, catalytic βarylations of simple amides have not been found until now. In our studies related to primary sp3C−H activation, we have established Pd(II)-catalyzed oxidative β-acyloxylation and β-mesylation of simple amides bearing CONHR groups with carboxylic acids and methanesulfonic anhydrides, respectively.14 Herein we report a Pd(II)-catalyzed arylation of primary β-sp 3C−H bonds on simple amides such as pivalamides containing CONH2, CONHR, or CONR2 groups with aryl iodides/CF3CO2Ag (AgTFA) in CF3CH2OH (TFE) © XXXX American Chemical Society

at 120 °C. It is reasonably assumed that a critical Pd(IV) species weakly coordinated with O atom of simple amide group, a five-membered palladacycle should be stabilized by the Thorpe−Ingold effect under weak acidic conditions during these couplings (Scheme 1).15 In fact, β-arylated pivalamides could be found in some of the biological active products (Scheme 2).16

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RESULTS AND DISCUSSION According to the previous work, the catalytic system consists of Pd(OAc)2(catalyst)/acid or TFE, which is very effective in the functionalization of inert alkyl and aryl C−H bonds via C−H activation.14,17 Thus, in the investigation of β-arylation of Nbutylpivalamide (1a) with p-iodonitrobenzene (2a), the Received: May 17, 2018

A

DOI: 10.1021/acs.organomet.8b00325 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Biological Active Products Containing the βArylation Amides Structure

time (see the Supporting Information), we found that 3a could be generated in 65% isolated yield from 1a and 2a (1 equiv/3 equiv) when Pd(OAc)2 (10 mol %) and AgTFA (3 equiv) was added in three batches in TFE at 120 °C (entry 12). The catalytic arylations of various N-substituted amides with p-iodonitrobenzene (2a) were studied further, and the results are shown in Scheme 3. For N-alkyl pivalamides, not only Scheme 3. Palladium-Catalyzed β-Arylation of NSubstituted Pivalamides with p-Iodonitrobenzenea

Pd(OAc)2(10 mol %)/TFE/silver salt system was still tested first. As shown in Table 1, no products were detected if Table 1. Palladium-Catalyzed β-Arylation of NButylpivalamidea

entry

Ag salts

solvent

1 2 3 4 5 6d 7 8 9e 10f 11g 12h 13 14 15 16

Ag2CO3 Ag2O AgOAc AgTFA AgOAc AgTFA AgTFA AgTFA AgTFA AgTFA AgTFA AgTFA AgTFA AgTFA AgTFA AgTFA

TFE TFE TFE TFE TFE AcOH DCE DCE TFE TFE TFE TFE TFE TFE TFE TFE

additiveb

TFA

TFA

Na2CO3 NaHCO3 KH2PO4 K2HPO4

yield (%)c 0 0 0 60 40 36 6 6 30 30 0 71(65) 0 12 30 5

a

Conditions: 1a (0.1 mmol), 2a (0.3 mmol), Pd(OAc)2 (0.01 mmol, 10 mol %), Ag salts (0.3 mmol), solvent (0.3 mL), 120 °C, 48 h. b TFA (0.2 mmol, entries 5 and 8); others (0.1 mmol, entries 13−16). c The yield is based on 1a and detected by 1H NMR analysis using CH2Br2 as an internal standard. The isolated yield is in parentheses. d N-Butyl-3-hydroxy-2,2-dimethylpropanamide is produced in 20% yield. ePdCl2 instead of Pd(OAc)2. fPd(TFA)2 instead of Pd(OAc)2. g Without palladium catalysts. hCatalysts and Ag salts are added in three batches.

Ag2CO3, Ag2O, or AgOAc was used as an iodine scavenger just in TFE (entries 1−3). Fortunately, when AgTFA was employed, a target product, β-aryl N-butylpivalamide (3a) was produced in 60% yield under identical conditions (entry 4), indicating that AgTFA was more useful than any other silver salts in the arylation.17b In the cases of AgOAc/TFE/ CF3CO2H(TFA) or AgTFA/AcOH instead of AgTFA/TFE, the yield of 3a was decreased, and a β-oxygenated byproduct was formed in AcOH (entries 5 and 6). In other solvents such as dichloroethane (CH2ClCH2Cl, DCE), the yield was very poor (entries 7 and 8). When some weak bases were added in the solution, the arylation was also hampered (entries 13−16). Without palladium catalysts, no reaction happened (entry 11). Compared with PdCl2, (CF3CO2)2Pd (Pd(TFA)2), and others, Pd(OAc)2 was an effective catalyst in this reaction. After screening the amounts of substrate, catalyst, and silver salt and other reaction conditions including temperature and reaction

a Conditions: 1 (0.1 mmol), 2a (0.3 mmol), Pd(OAc)2 (10 mol %), AgTFA (0.3 mmol), TFE (0.3 mL), 120 °C, 48 h. Catalysts and Ag salts are added in three batches. The yield is based on the substrate 1 and detected by 1H NMR analysis using CH2Br2 as an internal standard. The isolated yield is in parentheses. b2a (0.12 mmol), AgTFA (0.12 mmol). c2a (0.5 mmol), AgTFA (0.5 mmol).

linear but also branched aliphatic substituted amides underwent the arylations smoothly to generate the β-aryl amides (3a−3e) in moderate to good isolated yields. Cyclic aliphatic substituted amides especially containing four-, five-, or sixmembered ring could give their corresponding compounds (3f−3h) in 48−65% yields, and N-(adamantan-1-yl)pivalamide afforded its β-arylated product (3i) in 75% yield under same conditions. Heterocyclic aliphatic substituted pivalamide (1j) was also available in the arylation. For Nbenzyl amides containing strong electron-withdrawing group B

DOI: 10.1021/acs.organomet.8b00325 Organometallics XXXX, XXX, XXX−XXX

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Organometallics such as −NO2 or −CF3 groups on aromatic rings, the yields of their products (3k−3p) were 47−73%. Methyl 4-(pivalamidomethyl)-benzoate (1t), N-(4-(methylsulfonyl)benzyl)pivalamide (1u), and N-(3,4-dichlorobenzyl)pivalamide (1v) could also give their β-aryl amides (3t−3v) in moderate yields. Meanwhile, halogen atoms such as fluorine, chlorine, or bromine at the para positions of aromatic rings were tolerant in the couplings (3q−3s). However, neither N-benzyl amides without electron-withdrawing groups nor N-aryl amides could be arylated effectively since many byproducts were generated under the reaction conditions, like in the β-mesylation of amides.14b When there were both primary and secondary βsp3C−H bonds on the amide substrates such as N-(4nitrobenzyl)-2,2-dimethylhexanamide (1w) and N-(4-nitrobenzyl)-2,2-dimethylbutanamide (1x), only primary sp3C−H bonds were arylated successfully. Certainly, for the amides with only secondary β-sp3C−H bonds, no arylation occurred (3aa), showing a perfect selectivity in the functionalization of primary sp3C−H bonds. Since the Thorpe−Ingold effect might be very important in the couplings, as shown in Scheme 1, the simple amides containing α-H could not give their β-arylated products (3y and 3z). Moreover, not only N-substituted pivalamides but also pivalamide and N,N-disubstituted pivalamides were tested in the β-arylations with a verity of iodobenzenes (Scheme 4). In the coupling reactions of N-alkyl pivalamide 1a with iodobenzene and its derivatives bearing electron-donating groups including −OMe, −NPhth, and −Me, 1a was used in excess to avoid biaryls generated from aryl iodides under the reaction conditions (4a−4d). The aryl iodides substituted by various electron-withdrawing groups such as −Br, −Cl, −F, −Ms, −CHO, −COMe, −CO2Me, −NO2, −CF3, and so on. were available to react with 1 equiv of 1a, and most of the target products (4e−4s) were obtained in moderate to good yields. Interestingly, pivalamide with −CONH2 group could also be used as the substrate in the couplings (4t−4w), and the yield of β-aryl pivalamide (4w) was up to 72%. In the comparison with the β-arylation of pivalic acid under weak basic conditions,13 only β-monoaryl pivalic acid (4x) was produced in 66% yield in this coupling reaction. In the case of N,N-dialkyl pivalamides, it was found that they were more reactive than N-alkyl pivalamides and pivalamides, and the yields of their β-aryl pivalamide products were 55−81% (4y− 4ae). It is remarkable that β-monoaryl pivalamides (4ac−4ae) from 2,2-dimethyl-1-(pyrrolidin-1-yl)propan-1-one, 2,2-dimethyl-1-(piperidin-1-yl)propan-1-one and N,N-dibutylpivalamide could be prepared in 81, 72, and 70% yields respectively, just using 1.2 equiv of 2a/AgTFA under the reaction conditions. In contrast, in the couplings of the same three N,N-disubstituted pivalamides, when the loading of 2a/AgTFA was 3 equiv, the corresponding β-diaryl pivalamides (4af−4ah) were generated in 65−77%. It means that the selectivity of mono- and diaryl products could be under control, depending on the amount of inert aryl iodide/AgTFA added in the arylations of some active pivalamides. On the basis of the current and previous studies, a proposed reaction mechanism on the β-arylation of pivalamide is shown in Scheme 5. The first step is a sp3C−H activation, in which a Pd(II) species coordinates with an amide group and cleaves its β-sp3C−H bond to form an alkyl−Pd(II) five-membered ring intermediate. Other two methyl groups can stabilize this fivemembered ring intermediate through the Thorpe−Ingold effect, and Pd(II) seems prefer to coordinate with O atom of

Scheme 4. Palladium-Catalyzed β-Arylation of Pivalamides with Aryl Iodidesa

a

Conditions: 1 (0.1 mmol), 2 (0.3 mmol), Pd(OAc)2 (10 mol %), AgTFA (0.3 mmol), TFE (0.3 mL), 120 °C, 48 h. Catalysts and Ag salts are added in three batches. The yield is based on the substrates 1 and detected by 1H NMR analysis using CH2Br2 as an internal standard. The isolated yield is in parentheses. b1 (1.0 mmol), 2 (0.1 mmol), AgTFA (0.1 mmol). The yield is based on 2. c2 (0.12 mmol), AgTFA (0.12 mmol). C

DOI: 10.1021/acs.organomet.8b00325 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

detected by 1H NMR analysis using CH2Br2 as an internal standard. The residue was purified by preparative TLC or silica gel column chromatogrphy to afford the desired product. Method C. A reaction tube (15 mL) with magnetic stir bar was charged with the amide (1 mmol), aryl iodide (0.1 mmol), Pd(OAc)2 (0.01 mmol, 2.2 mg), AgTFA (0.1 mmol, 22.3 mg), and TFE (0.30 mL). The tube was sealed and heated to 120 °C for at least 48 h until the amide was consumed. The crude reaction mixture was cooled to room temperature, diluted with EtOAc, and filtered with Celite, and the cake was washed with EtOAc. The combined solvents were removed under reduced pressure. The crude product was detected by 1 H NMR analysis using CH2Br2 as an internal standard. The residue was purified by preparative TLC or silica gel column chromatography to afford the desired product. Method D. A reaction tube (15 mL) with magnetic stir bar was charged with the amide (0.1 mmol), aryl iodide (0.5 mmol), Pd(OAc)2 (0.01 mmol, 2.2 mg), AgTFA (0.5 mmol, 22.3 mg), and TFE (0.30 mL). The tube was sealed and heated to 120 °C for at least 48 h until the amide was consumed. The crude reaction mixture was cooled to room temperature, diluted with EtOAc, and filtered with Celite, and the cake was washed with EtOAc. The combined solvents were removed under reduced pressure. The crude product was detected by 1H NMR analysis using CH2Br2 as an internal standard. The residue was purified by preparative TLC or silica gel column chromatography to afford the desired product.

Scheme 5. Proposed Reaction Mechanism

sp2CO to afford a rigid ring. The secondary step is an oxidation of the Pd(II) intermediate by iodobenzene to form a Pd(IV) species. The final step is a reductive elimination of the Pd(IV) species to give the target β-arylated amide and the Pd(II) catalyst to complete the catalytic cycle. The Pd(IV) species with strong reductive ability is beneficial to give the coupling product during the reductive elimination. In summary, we developed a Pd(II)-catalyzed coupling reaction of primary β-sp3C−H bonds on simple amides with aryl iodides to afford β-aryl amides in moderate to good yields in the presence of AgTFA as an iodine scavenger in TFE. In the β-arylations, the amide substrates without special functional groups including N-alkyl or benzyl pivalamides, N,Ndialkyl pivalamides, pivalamide, and similar amides are available, and either electron-rich or -deficient aryl iodides are compatible. Some N,N-dialkyl pivalamides can provide βdiaryl products using aryl iodide/TFA in excess.

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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.organomet.8b00325. Details for optimization of the reaction conditions, 1 mmol scale example, preparations and characterization of compounds and spectroscopic data (PDF)

EXPERIMENTAL SECTION

General information. NMR spectra were obtained on a Varian Mercury 400 plus instrument, Bruker AMX-400 instrument (400 MHz for 1H, 101 MHz for 13C and DEPT-135) and Bruker AMX-500 instrument (500 MHz for 1H, 126 MHz for 13C and DEPT-135). 19F NMR spectra were recorded on Bruker AMX-400 instrument (376 MHz) and Bruker AMX-500 instrument (470 MHz). HRMS data were recorded on ACQUITYTM UPLC and Q-TOF MS Premier. Melting points were obtained on an INESA SGW X-4 melting point apparatus. All commercial materials were used as received unless otherwise noted. Solvents were distilled before use. Flash chromatography was performed on silica gel (200−300 mesh). General Procedures. Method A. A reaction tube (15 mL) with magnetic stir bar was charged with the amide (0.1 mmol), aryl iodide (0.10 mmol), Pd(OAc)2 (0.01 mmol, 2.2 mg), AgTFA (0.10 mmol, 22.3 mg), and TFE (0.30 mL). The tube was sealed and heated to 120 °C for 16 h. Then after being cooled to room temperature, the second batch of aryl iodide (0.10 mmol) and AgTFA (0.10 mmol, 22.3 mg) was added to the tube. The mixture was heated for another 16 h. After cooling to room temperature, the third batch of aryl iodide (0.10 mmol) and AgTFA (0.10 mmol, 22.3 mg) was added to the tube. The resulting mixture was heated for at least 16 h until the amide was consumed. The crude reaction mixture was diluted with EtOAc and filtered with Celite, and the cake was washed with EtOAc. The combined solvents were removed under reduced pressure. The crude product was detected by 1H NMR analysis using CH2Br2 as an internal standard. The residue was purified by preparative TLC or silica gel column chromatogrphy to afford the desired product. Method B. A reaction tube (15 mL) with magnetic stir bar was charged with the amide (0.1 mmol), aryl iodide (0.12 mmol), Pd(OAc)2 (0.01 mmol, 2.2 mg), AgTFA (0.12 mmol, 22.3 mg), and TFE (0.30 mL). The tube was sealed and heated to 120 °C for at least 48 h until the amide was consumed. The crude reaction mixture was cooled to room temperature, diluted with EtOAc, and filtered with Celite, and the cake was washed with EtOAc. The combined solvents were removed under reduced pressure. The crude product was

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ren Zhao: 0000-0002-7602-6085 Wenjun Lu: 0000-0001-8077-7060 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grant No. 21372153) for financial support. REFERENCES

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DOI: 10.1021/acs.organomet.8b00325 Organometallics XXXX, XXX, XXX−XXX