Palladium-Catalyzed Stereoselective Defluorination Arylation

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

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Palladium-Catalyzed Stereoselective Defluorination Arylation/ Alkenylation/Alkylation of gem-Difluorinated Cyclopropanes Ebrahim-Alkhalil M. A. Ahmed,† Ayman M. Y. Suliman,† Tian-Jun Gong,* and Yao Fu* Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, iChEM, University of Science and Technology of China, Hefei 230026, China

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S Supporting Information *

ABSTRACT: A palladium-catalyzed cross-coupling of gem-difluorinated cyclopropanes with boronic acids, providing the corresponding arylated/alkenylated/alkylated 2-fluoroallylic scaffolds, is generated. This new approach has good functional group compatibility for both gem-difluorinated cyclopropanes and boronic acids; thus, an array of synthetic building blocks of monofluoroalkene scaffolds including conjugated fluorodiene and skipped fluorodiene gave good yields with high Z-selectivity. Moreover, proficient application was described for monofluoroalkene, whereas the corresponding alkyl fluoride was constructed through hydrogenation. he incorporation of fluorine-containing functional groups into agrochemicals, pharmaceuticals, and materials has become common practice due to the superior physicochemical and bioactivity characteristics that the fluorine atom can provide.1 Among them, monofluoroalkenes are one of the significant scaffolds as a peptide bond mimic in drug discovery, medicinal chemistry, and high-performance materials.2 Numerous studies have been carried out to construct monofluoroalkenes.3 The pioneering studies were usually focused on classical olefination (Peterson reaction, aldol condensation reaction, and the Horner−Wadsworth−Emmons reaction), electrophilic fluorination of metal alkenes, and fluorination of alkynes or allenes.4 More recently, catalytic reactions involving fluoroalkenylating building blocks, especially gem-difluoroalkenes5 and gem-difluorinated cyclopropane,6−8 have been demonstrated as the most efficient strategies to access monofluoroalkenes with high levels of stereoselectivity. For example, Loh, Li, and co-workers independently realized Rh(III)-catalyzed C(sp2)C−H activation and C−F cleavage of gem-difluoroalkene synthesis of monofluoroalkenes.5a,b Toste’s group documented a Pd-catalyzed defluorinative coupling of 1-aryl-2,2-difluoroalkenes with boronic acids toward synthesis of monofluorostilbenes.5c Our group reported a Ni-catalyzed reductive cross-coupling of gem-difluoroalkenes with secondary and tertiary alkyl halides.5d Moreover, some examples were reported for the synthesis of monofluoroalkenes using gem-difluorinated cyclopropane.6−8,9a In 2015, a practical method to synthesize monofluorinated allylic scaffolds via Pdcatalyzed C−C activation/C−F cleavage of gem-difluorinated cyclopropane with nucleophiles (e.g., N, O) was reported6 (Scheme 1a). Later, Gade’s group realized a Ni-catalyzed

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© XXXX American Chemical Society

Scheme 1. Palladium-Catalyzed C−C Activation/C−F Cleavage of gem-Difluorinated Cyclopropanes

hydrodefluorination of gem-difluorinated cyclopropanes.7 Wang’s group realized an example of Ag-catalyzed ringopening diarylation of siloxy-2,2-difluoro cyclopropanes.8 Despite the great successes achieved, a general method to obtain monofluoroalkenes, especially 2-fluoroallylic scaffolds, with readily available reagents (gem-difluorinated cyclopropanes and boronic acids) under mild conditions is in high demand to expand the scope and utility of monofluoroalkenes. Herein, we report the first example of Pd-catalyzed C−C activation/C−F cleavage Suzuki-type cross-coupling that combines readily available gem-difluorinated cyclopropanes9 Received: June 9, 2019

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DOI: 10.1021/acs.orglett.9b01979 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

11−13). Reducing the reaction temperature to 60 °C reduced the transformation and resulted in 45% of 3a. Optimal reaction conditions were demonstrated using Pd(OTFA)2, PtBu3·HBF4, and Cs2CO3 as base, giving 3a in excellent yield with perfect Zselectivity (38:1 Z/E, entry 11). Under these optimized conditions, Suzuki cross-coupling with phenylboronic acid neopentylglycol ester afforded the product 3a in moderate yield. No reaction occurred when this transformation was conducted without addition of ligand and Pd catalyst (entry 17). With these optimized conditions in hand, we preliminarily conducted the cross-coupling reaction using a range of aryland heteroarylboronic acids (as depicted in Scheme 2). This

with aryl-/alkenyl-/alkylboronic acids to afford the corresponding 2-fluorinated allylic scaffolds (Scheme 1b), which allows C(sp2)−C(sp3) or C(sp3)−C(sp3) bond formation under mild conditions with wide functional group toleration and excellent Z-selectivity. In addition, the monofluoroalkenes could be conducted on a gram scale and underwent hydrogenation to provide the alkylation product. We carried out the reaction with the model substrate of 2(2,2-difluorocyclopropyl)naphthalene (1a) and phenylboronic acid (2a) (Table 1). According to our previous study,6 Table 1. Optimization of Reaction Conditionsa

Scheme 2. Scope of Aryl- and Heteroarylboronic Acidsa

entry

[Pd]

1 2 3 4 5 6 7 8 9 10 11 12 13 14d 15e 16f 17

Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2 Pd(OAc)2 Pd2(dba)3 Pd(OTFA)2 Pd(OTFA)2 Pd(OTFA)2

L

base

yielda (%)

Z/Ec

Bu-XPhos Bu-XPhos t Bu-XPhos t Bu-XPhos t Bu-XPhos SPhos XPhos DavePhos XantPhos QPhos PtBu3·HBF4 PtBu3·HBF4 PtBu3·HBF4 PtBu3·HBF4 PtBu3·HBF4 PtBu3·HBF4

K2CO3 K3PO4 K2HPO4 KF Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3

40 50 45 NR 47 68 53 78 NR 76 90 (85b) 54 NR 80 45 45 NR

29:1 30:1 27:1

t t

28:1 23:1 27:1 28:1 35:1 38:1 37:1 30:1 38:1 30:1

a

Standard reaction conditions: gem-difluorinated cyclopropane 1a (0.2 mmol), aryl/heteroarylboronic acids (1.5 equiv), Pd(OTFA)2 (0.02 mmol), PtBu3.HBF4 (0.03 mmol), and Cs2CO3 (0.2 mmol) in 1 mL of CH3CN at 80 °C for 16 h under Ar atmosphere. Results are an average of two experiments, and yield represents isolated yield after purification by silica gel chromatography, Z/E selectivity ⩾ 30:1 unless noted. Nap = 2-naphthyl.

a

Standard reaction conditions: 1a (0.2 mmol), phenylboronic acid 2a (1.5 equiv), [Pd] (10 mol %), ligand (15 mol %), and base (1 equiv) in 1 mL of CH3CN at 80 °C for 24 h under Ar atmosphere. The yield was determined by 19F NMR using trifluoromethylbenzene as internal standard. bYield represents isolated yield after purification by silica gel chromatography. cThe ratio of Z/E was determined by using 19F NMR. dReaction was conducted in THF. eThe reaction was performed at 60 °C. fPhenylboronic acid neopentylglycol ester (0.3 mmol) was used instead of phenylboronic acid.

reaction exhibited good tolerance with both electron-donating and electron-withdrawing groups at the para-position of arylboronic acids. For instance, the arylboronic acids with t Bu, OMe, and Me groups reacted well, providing products (3b−d) in good yields with high Z-selectivity. Functional groups at different positions of aryls (ortho-, meta-, and para-, 3d−f) showed good compatibility. The aryls with withdrawing groups (such as CF3 and CN) were well tolerated in this process and afforded the products in moderate to good yield. The toleration of aryl chloride (3h) provided opportunities for further transformation at the carbon−halogen bond through a transition-metal-catalyzed cross-coupling reaction. Meanwhile, other functional groups such as ether, ester, and ketone also

sequences of bases were screened in this reaction, and we noticed that the use of K2CO3, K3PO4, K2HPO4, and Cs2CO3 was suitable in this reaction (entries 1−3 and 5), while KF cannot promote the reaction (entry 4). A range of ligands were employed (entries 6−11), yielding product 3a in moderate to good yields with high Z-selectivity (see the Supporting Information for more details). The effects of other Pd catalysts were also studied under these reaction conditions, and it was found that PdII has better reaction efficiency than Pd0 (entries B

DOI: 10.1021/acs.orglett.9b01979 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters survived during the cross-coupling process to give the monofluoroalkenes in good to excellent yields (3i−l). Moreover, amine was suitable under the standard conditions and provided C−C-coupled product (3o) with no amination product observed.6 Finally, heteroarylboronic acids containing ketal and thianthrenyl groups were compatible and resulted 3r and 3s in yields of 65% and 68%, respectively. Next, the scope of a variety of gem-difluorinated cyclopropanes was also examined under the optimized conditions, as shown in Scheme 3. A series of aryls with different

Scheme 4. Pd-Catalyzed Cross-Coupling of Alkenyl- and Methylboronic Acidsa

Scheme 3. Scope of gem-Difluorinated Cyclopropanesa

a

Isolated yield for 0.2 mmol scale reaction.

To demonstrate the utility of this defluorination crosscoupling, the reaction of 2-(2,2-difluorocyclopropyl)naphthalene (1a) with boronic acid was carried out on a gram scale under mild conditions, and the desired product 3i was isolated in excellent yield (2.87 g, 94% yield) (Scheme 5). Scheme 5. Gram-Scale Synthesis and Transformation of 3ia

a

Standard reaction conditions: gem-difluorinated cyclopropane (0.2 mmol), phenylboronic acid 2a (1.5 equiv), Pd(OTFA)2 (0.02 mmol), PtBu3.HBF4 (0.03 mmol), and Cs2CO3 (0.2 mmol) in 1 mL of CH3CN at 80 °C for 24 h under Ar atmosphere. Results are an average of two experiments, and yield represents isolated yield after purification by silica gel chromatography, Z/E selectivity ⩾ 30:1 unless noted.

a

For the preparation of 3i, isolated yield for 10 mmol scale reaction; for the final product of alkylation, isolated yield for 0.2 mmol scale reaction.

substituents, such as neutral, donating, and withdrawing groups, were employed in this reaction to produce 4a−f in good to high yields, respectively. For example, products containing a variety of substituents (such as tBu, Ph, MeO, and TsO) exhibited good tolerance for this cross-coupling. Additionally, pyridine 4f was obtained in good yield. Further attempts were also made; e.g., allylic gem-difluorinated cyclopropane was appropriate for this coupling to occur, providing the corresponding conjugated fluorodiene 4g in moderate yield. This presents the first example of Pd-catalyzed cross-coupling with alkenyl gem-difluorinated cyclopropane, which provides an effective method for synthesis of conjugated fluorodiene. Finally, drug molecules such as estrone-derived gem-difluorinated cyclopropane reacted smoothly to form 4h with 73% isolated yield. To further demonstrate the compatibility of this defluorination cross-coupling, alkenylboronic acid was tested under optimal reaction conditions; unfortunately, only a trace of product was obtained. To our delight, when the reaction was modified by using precatalyst Pd-G3 with Q-Phos as ligand,10 alkenylboronic acid underwent this coupling successfully, generating skipped fluorodiene 5a (Scheme 4, eq 1). Hence, this strategy presents a practical and beneficial approach for the synthesis of skipped fluorodiene. Methylboronic acid was also tolerated in this transformation by using the modified reaction conditions and yielded the corresponding product 5b in 72% isolated yield with high Z/E selectivity (Scheme 4, eq 2).

The 2-fluoroallylic moiety is regarded as one of the more valuable building blocks and has more derivation possibilities. Indeed, ((Z)-2-(2-fluoro-3-(3-methoxy-4-methylphenyl)prop1-en-1-yl)naphthalene (3i) could undergo hydrogenation to produce the alkylation product. To investigate the chemoselectivity of the reaction, competition experiments were conducted in the following studies (Scheme 6). When equivalent 2-(2,2Scheme 6. Competition Experiments of Different Cyclopropanea

a

C

Isolated yield for 0.2 mmol scale reaction. DOI: 10.1021/acs.orglett.9b01979 Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

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dichlorocyclopropyl)benzene (7a) and 1a were treated with phenylboronic acid (2a) under optimal conditions, we found that only carbon−carbon bond formation at gem-difluorinated cyclopropane occurred, whereas 2-(2,2-dichlorocyclopropyl)benzene did not take place. Similar selectivity was also found when 1a and cyclopropylbenzene (7b) were treated with phenylboronic acid (2a) under similar conditions, and such substrates (7b) are regarded as unreactive electrophiles to trap in this reaction. Based on our previous work,6 we propose a plausible pathway of this cross-coupling as illustrated in Scheme 7. First,

Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ebrahim-Alkhalil M. A. Ahmed: 0000-0002-8658-8110 Ayman M. Y. Suliman: 0000-0002-4149-0646 Tian-Jun Gong: 0000-0001-5484-8531 Yao Fu: 0000-0003-2282-4839 Author Contributions †

E.-A.M.A.A. and A.M.Y.S. contributed equally.

Scheme 7. Proposed Reaction Mechanism

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from National Key R&D Program of China (2017YFA0303502, 2018YFB1501600), National Natural Science Foundation of China (21572212, 21732006, 21702200, and 51821006), Strategic Priority Research Program of CAS (XDA21060101, XDB20000000), and the Major Program of Development Foundation of Hefei Centre for Physical Science and Technology (2017FXZY001). CAS President’s International Fellowship Initiative and Fundamental Research Funds for the Central Universities.

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the Pd(0) catalyst reacted with the gem-difluorinated cyclopropane through a C−C bond activation step to generate an intermediate (I). Second, β-F elimination of I generated active complex II. This formed complex II could be attacked by a suitable nucleophile (aryl-/alkenyl-/alkylboronic acids) via transmetalation to afford intermediate III. Finally, C−C bond elimination occurred to generate the target product with high stereoselectiviy (Z-isomer as the major product) with released Pd(0) catalyst contributing to the next catalytic cycle. In conclusion, we report a novel palladium-catalyzed carbon−carbon coupling reaction of gem-difluorinated cyclopropanes with boronic acid derivatives. A range of aryl- and heteroarylboronic acids were well-coupled with gem-difluorinated cyclopropanes with good functional group compatibility. Excitingly, C(sp3)−C(sp3) cross-coupling was also well demonstrated. A diverse number of monofluoroalkenes were synthesized, especially conjugated and skipped fluorodienes, and isolated in moderate to excellent yield with high Zselectivity. This strategy expanded our previous method to cross-coupling of gem-difluorinated cyclopropane via C−C activation and C−F cleavage. Other related work and further transformations of monofluoroalkenes are currently underway in our laboratory.

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REFERENCES

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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.9b01979. Detailed experimental procedures and spectra data for all compounds (PDF) D

DOI: 10.1021/acs.orglett.9b01979 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b01979 Org. Lett. XXXX, XXX, XXX−XXX