Letter Cite This: ACS Catal. 2018, 8, 11324−11329
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Nickel-Catalyzed Reductive Electrophilic Ring Opening of Cycloketone Oxime Esters with Aroyl Chlorides Decai Ding and Chuan Wang* Department of Chemistry, University of Science and Technology of China, Center for Excellence in Molecular Synthesis, Hefei National Laboratory for Physical Science at the Microscale, 96 Jinzhai Road, Hefei, Anhui 20237, P. R. China
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S Supporting Information *
ABSTRACT: By merging cross-electrophile coupling and C−C bond cleavage, we developed a Ni-catalyzed electrophilic ring opening of cycloketone oxime esters with aromatic acid chlorides in assistance of Mn as reductant. Notably, complete regioselectivity can be achieved in this C−C bond cleavage reaction, providing an efficient access to a variety of cyanoketones under cyanide-free conditions. A radical reaction pathway was proposed on the basis of the results of the mechanistic probing experiments. KEYWORDS: ring opening, cross-electrophile, C−C bond cleavage, Ni-catalysis, cyanoketones
R
Scheme 1. Redox-Neutral (A,B) and Reductive Strategy (C) To Utilize Cycloketone Oxime Esters in Organic Synthesis
ing-opening reactions of small strained molecules is one of the most important transformations in organic synthesis, and in the last decades, tremendous progresses have been achieved in the field of nucleophilic ring-opening reactions.1 In contrast, ring opening of the electrophilic small ring substrates with another electrophile are much less developed, although this type of reaction could provide diverse compounds inaccessible through the classic nucleophilic pathway. The challenge of developing new electrophilic ringopening reactions lies in finding an appropriate way to overturn the intrinsic nature of the precursors. Therefore, only a limited number of successful examples, especially in the catalytic version, have been reported up to date, and most of them focus on epoxides and aziridines.2,3 On the other side, great advancements have been achieved in the field of Nicatalyzed cross-electrophile coupling in the recent years.4,5 However, most of these reactions involve cleavage of carbon− halide or carbon−oxygen bonds, while the reductive crosscoupling through a C−C bond activation is very rare.6 Recently, the C−C bond cleavage of cylcoketone oxime esters proved to be a useful method to approach alkyl nitriles under cyanide-free conditions.7−9 To the best of our knowledge, all the reported reactions involving cycloketone oxime esters proceed in a redox-neutral pathway including direct couplings9a−f,h−j (Scheme 1A) and difunctionalizations of alkenes9k,l (Scheme 1B). Herein, we report a Ni-catalyzed electrophilic ring-opening reaction of cycloketone oxime esters with aroyl chlorides under reductive conditions providing an efficient cyanide-free synthesis of diverse cyanoketones (Scheme 1C). For optimization of the reaction conditions of this electrophilic ring-opening reaction, we used cyclobutanone O-acetyl oxime (1a) and benzoyl chloride (2a) as standard substrates (Table 1). Initially, we tested a series of ligands using NiCl2··glyme as catalyst and Mn as reductant in DMF at © XXXX American Chemical Society
room temperature. In the cases of bis (pyridine) and tris(pyridine) ligands, only a trace amount of the desired product 3a was formed (entries 1−4). In contrast, the reactions using phenanthroline-based ligands provided the nitrile 3a in moderate yields (entries 5 and 6). The main sidereaction was transacylation of benzoyl chloride (2a) with the solvent DMF providing N,N-dimethylbenzamide as the byproduct. Replacing Mn by Zn as reducing agent resulted in a lower yield (entry 7). Next, a brief solvent-screening was undertaken and no improved result was obtained (entries 8− 10). Furthermore, the influence of the Ni-salts was also explored (entries 11−14), and the best result was achieved in the case of Ni(acac)2 (entry 13). Increasing the amount of benzoyl chloride (2a) from 2 equiv to 3 equiv gave rise to a better yield (entry 15). Performing the reaction in a lower concentration also benefited the efficiency of the studied reaction (entry 16). Moreover, the temperature turned out to Received: September 29, 2018 Revised: October 28, 2018 Published: October 31, 2018 11324
DOI: 10.1021/acscatal.8b03930 ACS Catal. 2018, 8, 11324−11329
Letter
ACS Catalysis Table 1. Optimization of the Reaction Conditions for the Ni-Catalzyed Ring-Opening Reactiona
entry
metal-salt
ligand
solvent
T (°C)
yield [%]b
1 2 3 4 5 6 7c 8 9 10 11 12 13 14d 15e 16e,f 17e,f,g 18e,f,h
NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2·glyme NiCl2 Ni(COD) Ni(acac)2 NiCl2(PCy3)2 Ni(acac)2 Ni(acac)2 Ni(acac)2 Ni(acac)2
L1 L2 L3 L4 L5 L6 L6 L6 L6 L6 L6 L6 L6 L6 L6 L6 L6 L6
DMF DMF DMF DMF DMF DMF DMF MeCN THF DMA DMF DMF DMF DMF DMF DMF DMF DMF
r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. r.t. −30 −55
traces traces traces traces 22 41 23 12 25 traces 39 43 51 28 57 67 73 84 (82i)
Table 2. Evaluation of the Substrate Scope the Ni-Catalyzed Reaction through Variation of the Structure of Acid Chloridesa
entry
Ar
yield [%]b
1 2 3 4 5 6 7 8 9 10 11 12 13
Ph (3a) 4-MeC6H4 (3b) 4-tBuC6H4 (3c) 3,5-(Me)2C6H3 (3d) 3-MeOC6H4 (3e) 4-MeOC6H4 (3f) 3,5-(MeO)2C6H3 (3g) 4-F3COC6H4 (3h) 4-FC6H4 (3i) 3-ClC6H4 (3j) 4-ClC6H4 (3k) 4-BrC6H4 (3l) 2-Naphthyl (3m)
83 93 75 93 (88c) 84 76 77 79 46 57 64 52 87
a Unless otherwise specified, reactions were performed on a 0.2 mmol scale of cyclobutanone O-acetyl oxime (1a) with 3 equiv of aroyl chloride 2, 10 mol % Ni(acac)2, 10 mol % ligand L6 and 3 equiv of Mn as reductant in 2 mL of DMF at −55 °C for 24 h. bYields of the isolated products. cReaction was performed on a 5 mmol scale.
oxime esters (Table 3). A panel of representative oxime esters were reacted with different aroyl chlorides under the optimum reaction conditions. When symmetrical cycloketone O-acetyl oximes were employed as substrates, the products 3n−s were afforded in moderate to high yields. In the case of unsymmetrical cyclic oxime esters, the challenge lies in the selective C−C bond cleavage. To our delight, our Ni-catalyzed ring-opening reactions all occurred selectively at the moresubstituted carbon-center furnishing the products 3t−af in complete regioselectivities and good to high yields. Moreover, the favored C−C bond cleavage at the benzylic position was observed in the cases of 3ag−ai. For the bi- and tricyclic oxime esters precursors with cis-configuration, the reactions proceeded with a conversion of the stereochemistry of the new formed stereocenter (3z−af). When dihydroindenone or dihydronaphthalenone O-acetyl oximes were used as the substrates, C−C cleavage at the benzylic position could be achieved at higher temperature in comparison to their fourmembered ring analogues. In this case, the products 3ah and 3ai were obtained in the enol-form instead of the keto-form. To demonstrate the utility of this method, some derivatizations based on the conversion of the cyano moiety were conducted (Scheme 2). The hydrolysis of 3z under basic conditions furnished a cyclic carboxylic acid 4 in a good yield, while a ZnCl2-mediated alcoholysis of 3z provided a butyl ester 5 with high efficiency (Scheme 2A). Furthermore, a ringopening product 3f was successfully converted into an α-amino cyclopentanone 6 via a two-step synthesis consisting of the imine formation and the followed Ti-catalyzed reductive cyclization (Scheme 2B).10 A series of control experiments were carried out to explore the mechanism of this Ni-catalyzed ring-opening reaction (Scheme 3). First, we performed a stoichiometric reaction between Ni(COD)2 and the aroyl chloride 2d (Scheme 3A).11 Upon completion the α-phenyl cyclobutanone O-acyl oxime
a Unless otherwise specified, reactions were performed on a 0.2 mmol scale of cyclobutanone O-acetyl oxime (1a) with 2 equiv benzoyl chloride (2a), 10 mol % nickel-salt, 10 mol % ligand, and 3 equiv Mn as reductant in 1 mL of solvent at room temperature. bYields were determined by 1H NMR spectroscopy using mesitylene as an internal standard. cZn was used as reductant instead of Mn. dNo additional ligand was used. e3 equiv of benzoyl chloride (2a) was used. fReaction was performed in 2 mL DMF. gReaction time: 8 h. hReaction time: 24 h. iYield of the isolated product.
have a crucial effect on the outcome of this reaction (entries 17 and 18). Through lowering the reaction temperature and extending the reaction time, the reaction yields could be significantly improved, providing the best result at −55 °C (entry 18). After establishing the best reaction conditions, we started to evaluate the substrate spectrum of this Ni-catalyzed reductive ring-opening reaction (Table 2). First, we reacted O-acetyl oxime (1a) with diverse aromatic acid chlorides 2a−m bearing electron-donating or -withdrawing groups on different positions. To our delight, all the reactions proceeded smoothly, furnishing the corresponding nitriles 3a−m in moderate to good yields. Notably, the 5 mmol scale synthesis of 3d provided a similar yield (entry 4). One limitation of this method was observed in the reactions employing aliphatic acid chlorides, which failed to deliver the desired products. In this case, only decomposition of the oxime esters was observed. Subsequently, we continued to explore the scope of this Nicatalyzed reaction by varying the structure of the cycloketone 11325
DOI: 10.1021/acscatal.8b03930 ACS Catal. 2018, 8, 11324−11329
Letter
ACS Catalysis
Table 3. Evaluation of the Substrate Scope the Ni-Catalyzed Reductive Ring-Opening Reaction through Variation of the Structure of the Cycloketone O-Acetyl Oximes13,a,b
a Unless otherwise specified, reactions were performed on a 0.2 mmol scale of cycloketones O-acetyl oximes 1 with 3 equiv aroyl chloride 2, 10 mol % Ni(acac)2, 10 mol % ligand L6, and 3 equiv of Mn as reductant in 2 mL DMF at −55 °C for 24 h. bYields of the isolated products. cReaction was performed at room temperature for 3 h. dDetermined by 1H NMR-spectroscopy. eReaction was performed at −30 °C.
(1i) was added to the reaction mixture both with and without the reductant Mn. In either case, the ring-opening product 3u was not formed. This result implies that this Ni-catalyzed reaction is likely not initiated through the interaction between a Ni(0)-species and the aroyl chlorides. Next, the oxime ester 1i was subjected to a stoichiometric reaction to Ni(COD)2 (Scheme 3B). In this case, a complete consumption of 1i was observed after 30 min at room temperature. The subsequent addition of aroyl chloride 2d to the reaction mixture in the absence of Mn did not lead to the formation of the γcyanoketone 3u. In contrast, the desired product 3u was obtained in a 55% yield in the presence of Mn, indicating that Mn is required in a step of an-intermediate-reduction instead of serving as a terminal reductant. Furthermore, we conducted the reaction under the standard reactions with TEMPO as a radical scavenger (Scheme 3C). In this case, the reaction was completely inhibited, suggesting the existence of a key radical intermediate in this studied reaction. However, we did not obtain any radical trapping product. In addition, a radical clock experiment reaction employing the cyclobutanone oxime ester
Scheme 2. Derivatizations of the Ni-Catalyzed RingOpening Products
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DOI: 10.1021/acscatal.8b03930 ACS Catal. 2018, 8, 11324−11329
Letter
ACS Catalysis
ring opening of the resultant iminyl radical gives a C-centered radical, which recombines rapidly with the Ni(II) complex. In the next step, the generated Ni(III) intermediate III is reduced by Mn providing a Ni(I) species IV, which undergoes subsequently an oxidative addition with aroyl chlorides 2. Finally, reductive elimination on the Ni(III) complex V furnished the cyanoketones 3 as products and regenerates the Ni(I) species I for the next catalytic cycle. In summary, a Ni-catalyzed electrophilic reductive ring opening of cycloketone oxime esters with aromatic acid chlorides has been accomplished through merging C−C bond cleavage and cross-electrophile coupling. This new method demonstrates a broad substrate scope with respect to both coupling partners. Remarkably, this reaction proceeds with complete regioselectivity for the key C−C cleavage step allowing the synthesis of diverse cyanoketones under cyanidefree conditions. The preliminary mechanistic studies indicate that this Ni-catalyzed reaction might proceed in a radical pathway initiated through the interaction between oxime esters and a Ni(I) species with intermediate-reduction in the following steps.
Scheme 3. Control Experiments for Investigation of the Mechanism
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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/acscatal.8b03930. Representative experimental procedures and necessary characterization data for all new compounds (PDF)
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1q incorporating a bishomoallyl group was carried out under the optimum reaction conditions providing a cyclic product with a moderately good diastereomeric ratio (Scheme 3D). This result supports the possible generation of an iminyl radical, which results in the formation of a C-centered radical via the ring-opening reaction and the following cyclization reaction.9c,g,12 On the basis of the experimental results aforementioned, we proposed the following plausible mechanism for this Nicatalyzed reaction (Scheme 4). Initially, under the reductive reaction conditions a Ni(I)-species I is generated, which interacts with cycloketone oxime esters 1, to afford a cage II consisting of an iminyl racial and a Ni(II) complex. The instant
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chuan Wang: 0000-0002-9219-1785 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by “1000-Youth Talents Plan” starting up funding, National Natural Science Foundation of China (Grant No. 21772183), the Fundamental Research Funds for the Central Universities (WK2060190086), as well as by the University of Science and Technology of China.
Scheme 4. Proposed Mechanism of the Ni-Catalyzed Electrophilic Ring Opening of Cycloketone Oxime Esters
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DOI: 10.1021/acscatal.8b03930 ACS Catal. 2018, 8, 11324−11329
Letter
ACS Catalysis
Cleavage Cascades. Angew. Chem., Int. Ed. 2018, 57, 744−747. (f) Zhao, B.; Shi, Z. Copper-Catalyzed Intermolecular Heck-Like Coupling of Cyclobutanone Oximes Initiated by Selective C−C Bond Cleavage. Angew. Chem., Int. Ed. 2017, 56, 12727−12731. (g) Yang, H.-B.; Pathipati, S. R.; Selander, N. Nickel-Catalyzed 1,2-Aminoarylation of Oxime Ester-Tethered Alkenes with Boronic Acids. ACS Catal. 2017, 7, 8441−8445. (h) Ai, W.; Liu, Y.; Wang, Q.; Lu, Z.; Liu, Q. Cu-Catalyzed Redox-Neutral Ring Cleavage of Cycloketone OAcyl Oximes: Chemodivergent Access to Distal Oxygenated Nitriles. Org. Lett. 2018, 20, 409−412. (i) Yang, L.; Gao, P.; Duan, X.-H.; Gu, Y.-R.; Guo, L.-N. Direct C−H Cyanoalkylation of Quinoxalin-2(1H)ones via Radical C−C Bond Cleavage. Org. Lett. 2018, 20, 1034− 1037. (j) Yu, X.-Y.; Chen, J.-R.; Wang, P.-Z.; Yang, M.-N.; Liang, D.; Xiao, W.-J. A Visible-Light-Driven Iminyl Radical-Mediated C−C Single Bond Cleavage/Radical Addition Cascade of Oxime Esters. Angew. Chem., Int. Ed. 2018, 57, 738−743. (k) Li, L.; Chen, H.; Mei, M.; Zhou, L. Visible-Light Promoted γ-Cyanoalkyl Radical Generation: Three-Component Cyanopropylation/Etherification of Unactivated Alkenes. Chem. Commun. 2017, 53, 11544−11547. (l) Wu, J.; Zhang, J. Y.; Gao, P.; Xu, S.-L.; Guo, L.-N. Copper-Catalyzed Redox-Neutral Cyanoalkylarylation of Activated Alkenes with Cyclobutanone Oxime Esters. J. Org. Chem. 2018, 83, 1046−1055. (10) Frey, G.; Luu, H. T.; Bichovski, P.; Feurer, M.; Streuff, J. Convenient Titanium(III)-Catalyzed Synthesis of Cyclic Aminoketones and PyrrolidinonesDevelopment of a Formal [4 + 1] Cycloaddition. Angew. Chem., Int. Ed. 2013, 52, 7131−7134. (11) Wotal, A. C.; Ribson, R. D.; Weix, D. J. Stoichiometric Reactions of Acylnickel(II) Complexes with Electrophiles and the Catalytic Synthesis of Ketones. Organometallics 2014, 33, 5874−5881. (12) For a kinetic study of the generation of iminyl radical, see: Griller, D.; Schmid, P.; Ingold, K. U. Cyclization of 4-Cyanobutyl Radical. Can. J. Chem. 1979, 57, 831−833. (13) The trans configuration of compound 3aa and the Zconfiguration of 3ah were confirmed by 2D-NOESY-measurements. The relative configuration of 3z, 3ab−af, and 3ai was assigned by analogy assuming a common reaction pathway.
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DOI: 10.1021/acscatal.8b03930 ACS Catal. 2018, 8, 11324−11329
