Ni-Catalyzed Reductive Arylacylation of Alkenes toward

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Ni-Catalyzed Reductive Arylacylation of Alkenes toward CarbonylContaining Oxindoles Sheng Xu,‡ Kuai Wang,‡ and Wangqing Kong* The Center for Precision Synthesis (CPS), Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, People’s Republic of China

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S Supporting Information *

ABSTRACT: An easy-to-handle Ni-catalyzed three-component reductive arylacylation of alkenes using isobutyl chloroformate as a CO source was described. This reaction operates under mild reaction conditions without the need to use toxic CO gas or metal carbonyl reagents. In addition, this method allows for rapid synthesis of 3,3-disubstituted oxindoles with an all-carbon quaternary stereocenter containing a ketone group in good yields with broad substrate scope.

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intricate handling of CO gas or metal carbonyl complexes, a specially designed high-pressure reactor is usually required, which makes it less convenient to handle in most academic laboratories. Alternatively, radical arylacylation reaction have been developed by using α-oxocarboxylic acids,3 carboxylic acids,4 benzoins,5 aromatic carboxylic anhydrides,6 aromatic carboxylic chloride,7 α-diketones,8 or aldehydes9 as the acyl radical precursors (Scheme 1B). However, harsh conditions (ultraviolet (UV) irradiation, high temperature, or excess peroxides) are generally required. Therefore, the exploration of novel and mild carbonylation processes involving easy-tohandle procedures, and with less toxic CO surrogates, is highly demanded. Recently, Ni-catalyzed cross-coupling of alkyl halides with carboxylic acid derivatives under reducing conditions to synthesize dialkyl ketones was established.10 Because of our ongoing interest in the development of Nicatalyzed reductive dicarbofunctionalization of alkenes11 for efficient synthesis of N-heterocycles,12 herein, we report our studies toward Ni-catalyzed reductive arylacylation of alkenes using isobutyl chloroformate as a CO source13 to efficiently synthesize various 3,3-disubstituted oxindoles with an allcarbon quaternary stereocenter containing a ketone group (Scheme 1C). Initially, we selected N-(2-bromophenyl)-N-methylmethacrylamide (1a), iodocyclohexane (2a), and isobutyl chloroformate (3a) as benchmark substrates for our preliminary study (Table 1). As we expected, the desired product 4aa was

xindoles are a privileged scaffold and occur frequently in various biologically active alkaloid natural products and pharmaceutical compounds. Among them, 3,3-disubstituted oxindoles having a carbonyl group at the 3-position are particularly attractive synthons, because they are very valuable for the synthesis of indole alkaloids.1 Therefore, in the past few decades, significant efforts have been devoted to developing straightforward and highly efficient methods for constructing oxindoles containing carbonyl groups. In this context, transition-metal-catalyzed alkene arylacylation is an appealing strategy for the synthesis of diverse set of ketone-functionalized oxindoles (Scheme 1A).2 Because of the high toxicity and the Scheme 1. Arylacylation of Alkenes

Received: August 7, 2019

© XXXX American Chemical Society

A

DOI: 10.1021/acs.orglett.9b02788 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

Table 2. Effects of the Chloroformate Reagents 3

b

entry

ligand

solvent

additive

yield of 4aa (%)

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

L1 L2 L3 L4 L5 L6 L7 L8 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4

DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) THF DMA MeCN DMF DMSO DMA/THF(1/3) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1) DMA/THF(1/1)

TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl TMSCl MgCl2 NaI MnBr2 TMSCl TMSCl TMSCl TMSCl

37 59 40 63 trace 38 trace trace 24 51 24 trace trace 50 trace trace 60 trace 52 0 0

entry

R

yield of 4aa (%)

1 2 3 4 5

isobutyl (3a) propyl (3b) butyl (3c) isopropyl (3d) phenyl (3e)

63 62 62 33 trace

Scheme 2. Substrate Scope of Acrylamides 1a

1a (0.2 mmol), 2a (3 equiv), 3a (3 equiv), NiBr2·DME (10 mol %), ligand (20 mol %), additive (0.5 equiv), Mn (4 equiv) in solvent (2 mL) at 40 °C for 12 h, unless noted otherwise. bYields of isolated products. cZn0 used instead of Mn0. dNo TMSCl. eNo Mn0. fNo NiBr2·DME. a

obtained in 37% yield utilizing the combination of catalytic amounts of NiBr2·DME (10 mol %), bipyridine L1 (20 mol %), TMSCl (0.5 equiv) as an additive, and Mn as a reductant (Table 1, entry 1). Subsequently, different nitrogen ligands were investigated (Table 1, entries 2−8), and L4 was found to be the most effective, producing 4aa in 63% isolated yield (Table 1, entry 4). Different solvents were also examined (Table 1, entries 9−14). Using tetrahydrofuran (THF) or N,N-dimethylacetamide (DMA) as the sole solvent, the yield was reduced (Table 1, entries 9 and 10). Various additives were also tested (Table 1, entries 15−17), and TMSCl was still the most effective. Replacing Mn0 with Zn0 led to only a trace amount of product. Finally, control experiments confirmed that Mn0 or NiII precatalysts are indispensable for this transformation (Table 1, entries 20 and 21). We further studied the effect of various chloroformate reagents on the reaction outcome. Bulkier isobutyl chloroformate (3a) or linear propyl and butyl chloroformates (3b and 3c) afforded the corresponding ketone in good yields (Table 2, entries 1−3). Chloroformates with sterically hindered isopropyl (3d) were less efficient (Table 2, entry 4). No reaction occurred with phenyl chloroformate (3e). With isobutyl chloroformate as the optimal CO source, we set out to evaluate the reaction scope of acrylamides 1 (see Scheme 2). First, different aryl electrophiles were tested and the utilization of aryl iodide afforded oxindole 4aa in higher

a 1 (0.2 mmol), 2a (3 equiv), 3a (3 equiv), NiBr2·DME (10 mol %), L4 (20 mol %), TMSCl (0.5 equiv), Mn (4 equiv) in DMA/THF(1/ 1) (2 mL) at 40 °C for 12 h. bThe reaction was conducted on a 1 mmol scale.

yield, compared to aryl bromide and aryl triflate. Nevertheless, aryl bromides are more readily available. The aromatic ring with different substitution patterns, as well as with electrondonating or electron-withdrawing groups, were all tolerated to furnish the corresponding oxindoles 4ba−4ka in good yields. Attractively, various heterocycles such as azaoxindole 4la and quinoline-2-one 4ma could also be efficiently constructed under our standard conditions. The acrylamide double bond bearing an isopropyl, n-hexyl, methoxymethyl, or benzyl group, were all proceeded smoothly to give the corresponding B

DOI: 10.1021/acs.orglett.9b02788 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

formation of 4ap in 30% and 4ap′ containing a cyclopentyl ring in 30% yield, respectively (Scheme 4A). This result

oxindoles 4na−4qa in good yields. Finally, benzyl protected substrate 1r was also tolerated, as 4ra was obtained in 60% yield. Next, we continue to explore the substrate scope of alkyl halides 2 (Scheme 3). Various primary and secondary alkyl

Scheme 4. Mechanistic Study

Scheme 3. Substrate Scope for Alkyl Halides 2a

confirm the involvement of alkyl radical in this transformation. A control experiment of 1a and 2a was conducted under our standard conditions in the absence of ClCO2iBu, the noncarbonylative product 6aa was isolated in 46% yield (Scheme 4B). In addition, diisobutyl carbonate was observed in 17% yield as a byproduct in the reaction mixture (Scheme 4C). This result indicate that ClCO2iBu is the CO source and consistent with Ni-catalyzed decarbonylation from ClCO2iBu, which afforded an alkyl alkoxide that was reacted with ClCO2iBu. It is worth noting that only a trace amount of 6aa was detected under standard conditions. To gather direct evidence on the reaction intermediates involved in this transformation, we decided to synthesize the σalkyl-NiII complex 5 according to our previous report12a and examine the reactivity of complex 5 with 2a and 3a. Interestingly, the target product 4aa was isolated in 36% yield (Scheme 4D). The control experiment without nickel catalyst did not consume the aryl bromide 1a (Table 1, entry 21), suggesting that the formation of aryl manganese species is not likely. On the basis of the above results, we believe that σalkyl-NiII species 5 is a key intermediate in this transformation. Based on these experiments and previous studies on reductive cross-couplings,10 a possible mechanism can be proposed (Scheme 5). Under the reductive reaction conditions, oxidative addition of aryl bromide 1 to in-situgenerated Ni0 species followed by migratory insertion of double bond gives σ-alkyl-NiII−X species A.14 Reduction of the intermediate A with Mn0 will afford σ-alkyl-NiI intermediate B, which undergoes further oxidative addition with ClCO2iBu to form σ-alkyl-NiIII−CO2iBu species C. Subsequently, NiII acyl complex D is generated upon decarbonylation, CO insertion, and reduction by stoichiometric Mn0. In parallel, NiI species produced in the reaction mixture mediates the formation of an alkyl radical from alkyl halide. The addition of alkyl radical to intermediate D furnishes the acyl-NiIII-alkyl species E, which undergoes reductive elimination to produce the corresponding ketone product 4 and NiI (Path A). In the absence of ClCO2iBu, the alkyl radical can add to intermediate A to give

1a (0.2 mmol), 2 (Y = Cl, 3 equiv), 3a (3 equiv), NiBr2·DME (10 mol %), L4 (20 mol %), TMSCl (0.5 equiv), Zn (4 equiv) in 2 mL of DMA/THF (1/3) at 60 °C for 12 h.

a

iodides could be coupled. Alkyl iodides containing phenyl (4ab), cyano (4ac), ester (4ad), ether (4ae), and amide (4af) groups were all compatible. Remarkably, alkyl chloride could be survived to afford 4ag in 57% yield. Substrates containing a heterocycle group, such as indole, was perfectly accommodated to furnish the desired product 4ai in 69% yield. The alkyl iodide derived from dihydrocholesterol was subjected to our standard conditions. To our delight, 4aj was obtained in 59% yield. This result indicates that the application of this method for late-stage functionalization of complex molecules is feasible. Moreover, benzyl chlorides were found reactive to this system (4ak−4ao) with altered reaction conditions (Zn as the reducing agent, 60 °C). The aryl ring of the benzyl chlorides bearing electron-withdrawing groups such as CF3, F, Cl (4al− 4an) or electron-donating group such as OMe (4ao) were well-tolerated. To probe the mechanism of this transformation, control experiments were designed. A radical-clock substrate 6iodohex-1-ene (2p) was subjected to the three-component arylacylation reaction with 1a and 3a and resulted in the C

DOI: 10.1021/acs.orglett.9b02788 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Notes

Scheme 5. Mechanistic Proposal

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The “1000-Youth Talents Plan”, National Natural Science Foundation of China (No. 21702149), Wuhan University, and Fundamental Research Funds for the Central Universities (No. 2042018kf0012) are greatly appreciated for financial support.

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alkyl-NiIII-alkyl species F, which affords noncarbonylative product 6, following reductive elimination (Path B). Since the σ-alkyl-NiII species was confirmed to be the key intermediate in this transformation, the oxidative addition of Ni0 with aryl bromide 1 should be faster than with ClCO2iBu. A possible explanation is that the amide carbonyl in substrate 1 may act as a directing group, and promote the previous process. The alkyl radical reacts faster with the acyl species D than with the σ-alkyl-NiI intermediate A, which leads to the selective formation of dialkyl ketone 4, rather than the noncarbonylative product 6. Since Ni-catalyzed enantioselective reductive dicarbofunctionalization of alkenes have been realized very recently,11k−m,12 several chiral ligands were tested in order to render the reaction asymmetric. 4aa was obtained in an enantiomeric ratio (er) of 75:25 when a phosphoramidite ligand was used, albeit with a low yield (see Table S3-3 in the Supporting Information). In conclusion, we have discovered a Ni-catalyzed threecomponent reductive alkene arylacylation reaction for the synthesis of a variety of 3,3-disubstituted oxindoles having an all-carbon quaternary stereocenter containing a ketone group from alkyl halides and isobutyl chloroformate, which is a safe and easy handle CO surrogate. This strategy has the advantages of good yields, simple operation, mild conditions, and high compatibility of functional groups.

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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.9b02788. Experimental procedures, analytical data, and spectra data for all new compounds (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wangqing Kong: 0000-0003-3260-0097 Author Contributions ‡

These authors contributed equally. D

DOI: 10.1021/acs.orglett.9b02788 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b02788 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters (13) (a) Rérat, A.; Michon, C.; Agbossou-Niedercorn, F.; Gosmini, C. Synthesis of Symmetrical Diaryl Ketones by Cobalt-Catalyzed Reaction of Arylzinc Reagents with Ethyl Chloroformate. Eur. J. Org. Chem. 2016, 2016, 4554. (b) Shi, R.; Hu, X. From Alkyl Halides to Ketones: Nickel-Catalyzed Reductive Carbonylation Utilizing Ethyl Chloroformate as the Carbonyl Source. Angew. Chem., Int. Ed. 2019, 58, 7454. (14) (a) Li, Y.; Wang, K.; Ping, Y.; Wang, Y.; Kong, W. Nickelcatalyzed domino Heck cyclization/Suzuki coupling for the synthesis of 3,3-disubstituted oxindoles. Org. Lett. 2018, 20, 921. (b) Yen, A.; Lautens, M. Nickel-catalyzed intramolecular arylcyanation for the synthesis of 3,3-disubstituted oxindoles. Org. Lett. 2018, 20, 4323. (c) Yoon, H.; Marchese, A. D.; Lautens, M. Carboiodination catalyzed by nickel. J. Am. Chem. Soc. 2018, 140, 10950.

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DOI: 10.1021/acs.orglett.9b02788 Org. Lett. XXXX, XXX, XXX−XXX