Ni-Catalyzed Enantioconvergent Coupling of Epoxides with Alkenylboronic Acids: Construction of Oxindoles Bearing Quaternary Carbons

Open AccessCCS ChemistryCOMMUNICATION1 Apr 2020Ni-Catalyzed Enantioconvergent Coupling of Epoxides with Alkenylboronic Acids: Construction of Oxindoles Bearing Quaternary Carbons Liang Wu†, Guoqiang Yang† and Wanbin Zhang Liang Wu† Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China) , Guoqiang Yang† Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China) and Wanbin Zhang *Corresponding author: E-mail Address: [email protected] Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China) https://doi.org/10.31635/ccschem.019.201900064 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail We have developed a nickel- nickel/bisphosphine-catalyzed stereoconvergent cross-coupling reaction of epoxides with alkenylboronic acids. Racemic spiroepoxyoxindoles were converted to chiral homoallylic alcohols bearing quaternary carbon stereogenic centers via a stereoablative enantioconvergent transformation. The subsequently fabricated oxindoles-carrying quaternary carbon products were obtained in good yields and enantioselectivity. A wide range of substrates and alkenylboronic acids was tolerated under the catalytic system. This reaction provided a rare example of a nickel-catalyzed enantioselective cross-coupling reaction of tertiary alkyl electrophiles and an enantioconvergent transformation of racemic epoxides, beneficial as a low-cost, sustainable, and efficient catalyst in the preparation of chiral oxindole-containing natural and pharmaceutical compounds. Download figure Download PowerPoint Introduction Enantioconvergent coupling is an efficient process for converting a racemate directly into a product high in chemical yield and with high enantioselectivity during the construction of molecular skeleton via coupling reactions, which circumvent the drawback of stereospecific and kinetic resolution couplings. nickel-Catalyzed enantioconvergent couplings of racemic alkyl electrophiles have become important in the area of coupling reactions due to their ability to construct a wide variety of C–C(sp3) bonds while controlling stereochemistry.1–13 The enantioconvergent coupling of racemic alkyl electrophiles with organometallic reagents has been reported extensively.14–37 Also, the enantioconvergent reductive coupling of two distinguished organohalides has attracted much attention.38–41 However, in general, the alkyl electrophiles for Ni-catalyzed enantioconvergent coupling reactions are limited to secondary alkyl halides or their analogs, which leads to the generation of chiral tertiary carbon stereocenters (Scheme 1a).12–31,38,39 Quaternary carbon stereocenters exist in a wide assortment of natural and pharmaceutical compounds; therefore, the development of efficient methods for the construction of quaternary carbon stereocenters is desired considerably.42–44 The sole example of a highly enantioselective construction of quaternary carbon stereocenters via such reactions was reported very recently by the Fu group (Scheme 1b).45 α-Halo-α-alkyl-β-lactams were coupled with olefins in the presence of a hydrosilane to generate β-lactams bearing a quaternary carbon stereocenter. The Doyle group also reported an example of a Ni-catalyzed stereoconvergent cross-coupling of tertiary electrophiles (aziridine) with a single substrate.46 However, the desired product obtained possessed only 27% enantiomeric excess (ee). Thus, due to the low ee achieved, the consideration of a kinetic resolution process could not be precluded. Scheme 1 | Ni-Catalyzed enantioconvergent cross-coupling of racemic alkyl electrophiles. Download figure Download PowerPoint Taking the above issues into account, the Ni-catalyzed asymmetric couplings of tertiary alkyl electrophiles still impose a significant challenge, possibly due to the difficulty in differentiating between the two faces of the tertiary carbon radical and/or concomitantly controlling the formation of the alkyl–alkyl bonds and the stereochemistry.45–54 Herein, we present the first, favorable Ni-catalyzed enantioconvergent coupling of tertiary alkyl electrophiles with organoboron reagents (Scheme 1c).14–22 We took advantage of the following preexisting knowledge in our strategic experimental design for the Ni-catalyzed enantioconvergent coupling reaction: (1) Ring-opening reactions of epoxides have been recognized as useful transformations owing to their simplicity, high reactivity, and the broad applicability of the alcohol products in organic synthesis and bioactive molecules.55,56 The enantioselective ring-opening of epoxides is one of the most efficient methods for the construction of chiral alcohol compounds and so has attracted much attention from the chemical community. (2) Despite the considerable progress made in this field, the enantioconvergent transformation of racemic epoxides, which could be used to prepare chiral alcohol products with high conversion and high enantioselectivity in an atom economical manner, has not been widely studied.57–63 (3) With respect to the construction of quaternary stereocenters, Pd- and Cu-catalyzed enantioconvergent couplings of 1,1-disubstituted epoxides with nucleophiles were reported by Trost and Nishibayashi, respectively.57,58 A elegant example of redox-triggered C–C coupling of alcohols and 1,1-disubstituted epoxides via iridium catalysis, has been developed by Krische and co-workers.59 However, an installed alkynyl or alkenyl group was required for these examples because of their inherent SN2′ oxidative addition mechanism. (4) On the other hand, Weix and Zhao64 realized the first example of an enantioselective cross-coupling of epoxides with aryl halides catalyzed by a combination of nickel and a chiral titanocene. However, such a catalytic system is limited to meso-epoxides because the enantiodetermining step is the titanium-promoted ring-opening step. (5) With regards to the importance of chiral oxindole skeletons for the discovery of bioactive molecules65–67 and our current work concerning Ni-catalyzed enantioselective reactions,68–71 especially alkenylation, we sought to develop an enantioconvergent alkenylation of spiroepoxyoxindoles.72–75 Hereby, we report a Ni-catalyzed enantioconvergent ring-opening alkenylation of spiroepoxyoxindoles with alkenylboronic acids for the construction of chiral oxindoles bearing a homoallylic alcohol moiety and quaternary carbon stereocenter, in good yields and ees, and with excellent regioselectivity (Scheme 1c). Results and Discussion Spiroepoxyoxindole is an easily accessible and strained compound that has been used in organocatalytic kinetic resolutions previously.74,75 Thus, this compound represents an ideal substrate for testing our proposal. We chose spiroepoxyoxindole 1 and styrenyl boronic acid 10a as the coupling partners to screen reaction conditions. After initial optimization, we screened different chiral ligands systematically (Scheme 2). Unfortunately, Box and Pybox ligands, which showed excellent catalytic activity and enantio-inducing ability in previous reports,13,45 were not suitable for this reaction ( L1 and L2). Gratifyingly, we found that phosphine ligands promoted this reaction to afford the ring-opening alkenylation product 11 regioselectively ( L3– L12). Axially-chiral biphenyl bisphosphine ligands ( L4– L7) with ortho-oxygen substituents gave the desired product in good to excellent yields (62–95%), with good ees (74–78%), while very low ee value was obtained when Binap ( L3) was used as the ligand. Screening of the aryl groups on the phosphine showed that a phenyl group was the best choice ( L7– L9). Other types of chiral ligands, such as monophosphoramide ( L10), an electron-rich bisphosphine ligand bearing center-chirality ( L11), and phosphine–oxazoline ligands ( L12– L13), also enabled the coupling reaction (except L13), but with poor enantio-inducing ability. Thus, the 6,6′-dimethoxy ligand, L7 (MeO-Biphep), was used for the screening of different protecting groups on the nitrogen of the oxindole. A protecting group free substrate was also coupled to give the alkenylation product 12 (Scheme 2) in good yield (89%), but with moderate ee (65%). Several substrates, bearing simple alkyl groups, denoted as R, reacted with 10a to construct the stereocenter with good ee ( 13; 86%, and also see ), but these functional groups could not be removable. Accordingly, we decided to find a removable R group that was also capable of inducing a high level of ee. Thus, we tested allyl-, benzyl-, Boc-, and alkoxylmethyl-type as protecting groups ( 14–19a). To our delight, the reaction of a substrate bearing a benzyloxymethyl acetal (BOM) protecting group (—CH2OBn) gave the corresponding product 19a in good yield (74%) with high ee (84%). With increased ligand loading and employing calcium hydride (CaH2) as an additive (see ), the yield and ee were improved to 86% and 87%, respectively, signifying that CaH2 might function as a desiccant and a base to promote the transmetalation step. Scheme 2 | Condition optimization for the Ni-catalyzed enantioconvergent cross-coupling reactions. Notes: Reactions were carried out on a 0.10 mmol scale using Nickel(II) bromide ethylene glycol dimethyl ether ether complex (NiBr2•glyme; 10 mol %) and ligand (12 mol %) in analytical reagent (AR) grade acetonitrile (MeCN; 1.0 mL) under N2 at 30 °C for 24 h. Isolated yields are shown, as well as ees, determined by high-performance liquid chromatography using a chiral column.aL7 (20 mol %), and calcium hydride (CaH2; 100 mol %) was added as an additive. Download figure Download PowerPoint With the optimized conditions in hand, the substrate scope of this Ni-catalyzed enantioconvergent coupling was examined (Scheme 3). The reaction proceeded efficiently with substrates bearing electron-donating or electron-withdrawing substituents ( 19a– 19n). In general, substrates bearing 4-substituents gave the corresponding chiral oxindole products in good to high yields and ees, especially for substrates bearing halide, vinyl, and phenyl substituents ( 19b– 19i). We also obtained good ees for substrates bearing substituents at 5-positions ( 19j, 19k). Substituents at the 6- or 7-positions gave the corresponding alkenylation products with lower, yet synthetically useful enantioselectivities ( 19l– 19n). Substrates bearing two substituents at different positions of the benzene ring were also converted successfully to their corresponding products ( 19o– 19t). Substituents such as cyclopropanyl, vinyl, bromide, and iodide were tolerated well under the reaction conditions, with the aryl bromide and iodide being highly active for subsequent cross-coupling reactions. Scheme 3 | Scope of spiroepoxyoxindoles. Note: Unless stated otherwise, reactions were carried out on a 0.10 mmol scale using NiBr2•glyme (10 mol %) and (S)-(6,6′-dimethoxybiphenyl-2,2′-diyl)bis(diphenylphosphine) [(S)-MeO-Biphep; L7, 20 mol %)] in AR grade MeCN (1.0 mL) under N2 at 30 °C for 24 h. Isolated yields are shown and ees were determined by high-performance liquid chromatography using a chiral column.aL6 (20 mol %) instead of L7. bL4 (20 mol %) instead of L7. Download figure Download PowerPoint In consideration that halides can be converted into other functional group, we chose 9c as the standard substrate for the examination of the scope of the alkenylboronic acids (Scheme 4). First, substituted styrenylboronic acids were assessed, and the results showed that substituents on the benzene ring of the styrenylboronic acids, irrespective of their position, had no noticeable effect on the reaction ( 19u– 19aj). The enantioselectivity reduced slightly for styrenylboronic acids bearing electron-withdrawing substituents. Again, halide substituents were compatible with the catalytic system. A styrenylboronic acid bearing two substituents also gave the corresponding product in similar yield and ee ( 19ak). The compatibility of an aromatic heterocycle was also examined ( 19al), which showed high enantioselectivity, albeit with a lower yield, compared with the results of styrenylboronic acid. It was also possible to couple 2-alkyl and 2,2-disubstituted vinylboronic with the epoxide substrate with good reactivity and enantioselectivity ( 19am and 19an). However, the reaction of 1,2-disubstituted vinylboronic acid was unsuccessful ( 19ao), possibly due to steric interactions with the bulky tertiary alkyl-nickel species. Currently, we have not yet been able to demonstrate reactivity with alkyl- and phenylboronic acids under this catalytic system. Scheme 4 | Scope of alkenylboronic acids. Note: Reactions were carried out on a 0.10 mmol scale using NiBr2•glyme (10 mol %) and (S)-MeO-Biphep (L7, 20 mol %) in AR grade MeCN (1.0 mL) under N2 at 30 °C for 24 h. Isolated yields are shown under the structures; ees were determined by high-performance liquid chromatography using a chiral column. Download figure Download PowerPoint Further, several transformations of chiral product 19c were conducted to demonstrate the potential utility of this enantioselective coupling reaction (Scheme 5). The homoallylic alcohol moiety of 19c could be converted to chiral tetrahydrofuran skeletons. Using N-bromosuccinimide (NBS) or meta-chloroperoxybenzoic acid (mCPBA) as the electrophilic oxidants, spirofuran-oxindoles 20 and 21, bearing three chiral centers, were prepared in good (85%) and moderate (56%) yields, respectively, with good ees and excellent diastereoselectivities (92% ee, >20∶1 dr and 93% ee, >20∶1 dr, respectively). The N-BOM group could be altered or removed selectively. When 19c was treated with boron trichloride (BCl3), the benzyl (Bn) group of the BOM was removed to generate 22 bearing two hydroxyl groups, while the treatment of 19c with concentrated HCl in EtOH at high temperature yielded the BOM-removed product 23. Notably, both of these reaction conditions did not influence the chiral quaternary stereocenter. By employing a mild hydrogenation system, the olefin bond reduced selectively with deprotection of the Bn group ( 24). When ammonium formate (HCO2NH4) was used as the reductant at a higher temperature, the C–Cl bond was cleaved, giving rise to the oxindole 25, bearing a free N–H group, in good yield (82%) with no loss of ee (94%). Thus, the 4-Cl group could be considered as a protecting group. It is noteworthy that the stepwise alkenylation and reduction of the olefin bond could represent an alternative method for the enantioselective coupling of tertiary electrophiles with alkyl nucleophiles. Scheme 5 | Transformations of 19c using different reagents. Note: (a) NBS, dichloromethane (CH2Cl2), 30 °C; (b) mCPBA, dichloromethane (CH2Cl2), 30 °C; (c) BCl3, CH2Cl2, −78 °C; (d) concentrated hydrochloric acid (conc. HCl), ethanol (EtOH), 60 °C; (e) palladium on carbon (Pd/C), hydrogen balloons (H2 balloon), methanol (MeOH), at room temperature (rt); (f) Pd/C, HCO2NH4, EtOH, reflux. Download figure Download PowerPoint To gain more insight into the stereochemical course of this reaction, we carried out the following two reactions. (1) The enantiopure, (R)– 9c, was subjected to reaction conditions in which a racemic ligand was used with shorter reaction time (Scheme 6a, reaction equation [eq 1]). The results revealed that the ee of the recovered (R)– 9c remained >99%, whereas the ee obtained for the coupling product, (S)– 19c, was only 17%, suggesting that a stereoablative process occurred at or after the irreversible oxidative addition step.76 Based on these results, the regioselectivity of the reaction, and the results of the radical trapping reactions with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (see ), a single-electron transfer mechanism via a stabilized tertiary radical intermediate ( 26) is more likely to have occurred during the oxidative addition (Scheme 6b),46,77,78 rather than an SN2-type of oxidative addition of Ni to substrate 9, which would have been expected to favor the less-substituted site of the epoxide.79–81 (2) To understand the reason as to why (S)- 19c was obtained in 17% ee, we carried out the reaction of rac– 9c under standard conditions, but for shorter reaction time (Scheme 6a, eq 2). A slight kinetic resolution effect was observed, indicating (S)-MeO-Biphep matched slightly better with (S)– 9c (Scheme 6b); thus, the ring-opening step of (R)– 9c should be expected to match slightly better with (R)-Segphos to promote the generation of (S)– 19c. Therefore, the fact that (S)– 19c was obtained in 17% ee is reasonable (see Scheme 6a, eq 1). Scheme 6 | Stereochemical course-related experiments and the proposed mechanism for the generation of oxindoles bearing quaternary carbons by the Ni-catalyzed enantioconvergent coupling of epoxides with alkenylboronic acids. Download figure Download PowerPoint Conclusions We have developed a mild chiral bisphosphine ligand-promoted Ni-catalyzed enantioconvergent ring-opening alkenylation of racemic epoxides, to fabricate chiral oxindoles bearing a homoallylic alcohol moiety and quaternary carbon stereocenters, with good to excellent enantiopurity. The catalytic coupling reaction tolerated 2-aryl- and 2-alkyl-substituted vinylboronic acids and a wide range of substrates bearing different substituents. Our preliminary mechanistic study showed that this reaction proceeded via a single-electron transfer oxidative addition mechanism. The development of other challenging enantioselective coupling reactions for the construction of quaternary stereocenters is currently underway in our laboratory and would serve a vital purpose in the streamline fabrication of complex, and enantioenriched carbon framework. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no competing interests. Acknowledgments This research was made possible as a result of a generous grant from the National Natural Science Foundation of China (nos. 21620102003, 21772119, and 21831005) and Shanghai Municipal Education Commission (no. 201701070002E00030). References 1. Rosen B. M.; Quasdorf K. W.; Wilson D. 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Google Scholar FiguresReferencesRelatedDetails Issue AssignmentVolume 2Issue 2Page: 623-631Supporting Information Copyright & Permissions© 2019 Chinese Chemical SocietyKeywordsepoxideenantioconvergent couplingnickel catalysisquaternary carbonalkenylboronic acidoxindoleAcknowledgmentsThis research was made possible as a result of a generous grant from the National Natural Science Foundation of China (nos. 21620102003, 21772119, and 21831005) and Shanghai Municipal Education Commission (no. 201701070002E00030). Downloaded 2,352 times Loading ...