Diastereoselective and Enantioselective Nickel-Catalyzed Reductive Coupling of Imines and Unactivated Alkenes

Open AccessCCS ChemistryCOMMUNICATIONS15 Nov 2022Diastereoselective and Enantioselective Nickel-Catalyzed Reductive Coupling of Imines and Unactivated Alkenes Biao Wang, Xian-Ming Liu, Kai-Xiang Zhang, Wei-Min Feng, Li-Jun Xiao and Qi-Lin Zhou Biao Wang State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Xian-Ming Liu State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Kai-Xiang Zhang State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Wei-Min Feng State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Li-Jun Xiao *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 and Qi-Lin Zhou *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 https://doi.org/10.31635/ccschem.022.202202506 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Herein we report a method for highly diastereoselective and enantioselective nickel-catalyzed reductive coupling of imines and unactivated alkenes. A monodentate chiral spiro phosphoramidite ligand is the key to controlling the enantioselectivity. This efficient, straightforward method allows for the construction of chiral benzocyclic and indole-fused cyclic amines with two contiguous stereogenic centers in good yields from readily accessible starting materials. Download figure Download PowerPoint Introduction Transition-metal-catalyzed reductive coupling of carbonyl compounds and unsaturated hydrocarbons has emerged as a promising alternative to conventional carbonyl addition reactions.1–5 Generally, in these transformations, the active C–M (M = transition metal) species are generated in situ via hydrometallation and oxidative cyclization, which minimizes the number of synthetic manipulations and reduces chemical waste, but stoichiometric metallic reductants, such as Mn, Zn, ZnEt2, BEt3, and R3SiH, are still needed (Scheme 1a).1–5 To further reduce metal salt waste, significant progress has been made by using nonmetallic reductants, including H2 and iPrOH,6,7 but such reactions are limited to activated unsaturated hydrocarbon substrates such as alkynes, dienes, and allenes.1–5,8–13 In contrast, unactivated alkenes (nonconjugated alkenes), which are more readily available, have been less studied, and asymmetric reductive coupling reactions remain a challenge.14–19 For exploring an atom-economic and environmentally benign method for the enantioselective synthesis of cyclic amines,20–22 we embarked on the development of nickel-catalyzed reductive cyclization of imines and unactivated alkenes. Scheme 1 | Transition-metal-catalyzed couplings of carbonyls with unsaturated hydrocarbons. PG = N-protecting group. Download figure Download PowerPoint Nickel(0) catalysts show high reactivity for reductive coupling reactions between carbonyls and activated alkenes, which proceed via nickelacycle intermediates generated by oxidative cyclization of the two unsaturated compounds with nickel(0).23–29 Recently, Ogoshi et al.30,31 achieved the first nickel-catalyzed intramolecular reductive coupling reactions involving unactivated alkenes by using Et3SiH or BEt3 as a reducing agent. However, attempts to develop an asymmetric version of the reaction were unsuccessful: they reported only one example, which used a chiral N-heterocyclic carbene (NHC) ligand and showed low enantioselectivity (36% ee) (Scheme 1b).30 Our group has developed a catalytic system which consists of a nickel complex and a Brønsted acid and shows high activity for inter- and intramolecular coupling of imines and alkenes.32–36 In these reactions, the Brønsted acid (e.g., PhCO2H) acts as a cocatalyst to protonate the Ni–N bond in the nickelacycle intermediate, thus facilitating the coupling reactions.33,34 Inspired by our findings and the alcohol-mediated carbonyl reductive coupling reactions of activated unsaturated hydrocarbons,37–39 we hypothesized that protonating the nickelacycle with an alcohol instead of a Brønsted acid would allow us to achieve reductive coupling of imines and unactivated alkenes and that we could use a chiral ligand to control the enantioselectivity. Indeed, we herein report that we have developed a method for highly enantioselective nickel-catalyzed intramolecular reductive coupling of imines and unactivated alkenes using isopropanol as the reductant (Scheme 1c). A chiral spiro phosphoramidite ligand is the key to controlling the enantioselectivity of the reaction. Experimental Methods General procedure for Ni-catalyzed enantioselective reductive coupling of imines and unactivated alkenes: In an argon-filled glove box, an oven-dried sealed tube was charged with a stir bar, catalyst precursor Ni(COD)2 (COD = 1,5-cyclooctadiene) (2.8 mg, 0.01 mmol), ligand (R,R,R)- L6 (6.1 mg, 0.01 mmol), imines 1 (0.1 mmol), and iPrONa (0.01 mmol). Then toluene (0.9 mL) and iPrOH (0.1 mL) were injected into the tube. Then the tube was sealed and removed from the glove box. The reaction mixture was stirred at 20–60 °C. After cooling to room temperature, the solvent was removed under vacuum, and the residue was purified by preparative thin layer chromatography (TLC; petroleum ether/ethyl acetate: 4∶1, v/v) to give pure product 2. Results and Discussion Optimization of reaction conditions Initially, we selected allylbenzaldehyde imine 1a as a model substrate to determine whether we could accomplish an imine-alkene reductive cyclization by using isopropanol as a reducing agent. We were delighted to observe that 1a smoothly underwent intramolecular reductive cyclization in the presence of 10 mol % Ni(COD)2, 20 mol % of PCy3 (Cy = cyclohexyl), and 10 mol % iPrONa in 9:1 (v/v) toluene/isopropanol, giving desired product 2a in 89% yield (see Supporting Information Table S1). Encouraged by this result, we explored various chiral ligands with the goal of achieving enantioselective reductive cyclization of 1a (Table 1). We started by evaluating various monodentate chiral spiro phosphorus ligands developed in our laboratory40,41 (entries 1–4) and found that spiro phosphoramidite ligand (R,R,R)- L5 gave the desired product in 64% yield with 57% ee (entry 4). By modifying the phenyl group on the amine moiety of L5, we prepared two novel ligands: one with a 1-naphthyl group ( L6) and one with a 2-naphthyl group ( L7). Use of these ligands resulted in higher enantioselectivities (84% and 78% ee, respectively; entries 5 and 6). Notably, (S,R,R)- L5, which is a diastereomer of the (R,R,R)- L5, afforded 2a with only 6% ee (entry 7), indicating that the matching combination of the chiral ligand configurations was R,R,R. A phosphoramidite ligand with a binaphthyl scaffold ( L8) showed a lower enantioselectivity (29% ee, entry 8), and bidentate phosphine ligands, such as BINAP and DuPhos, showed no enantioselectivity at all (entries 9 and 10). Notably, the diastereoselectivity of the reaction was excellent with all the tested chiral ligands: no trans product was detected. Table 1 | Optimization of the Reaction Conditionsa Entry Ligand Additive Temp. (°C) Yield (%)b ee (%)c 1 (R)- L2 — 30 28 4 2 (R)- L3 — 30 19 4 3 (R)- L4 — 30 40 30 4 (R,R,R)- L5 — 30 64 57 5 (R,R,R)- L6 — 30 90 84 6 (R,R,R)- L7 — 30 82 78 7 (S,R,R)- L5 — 30 75 6 8 (R,R,R)- L8 — 30 51 29 9 (R)-BINAP — 30 20 0 10 (S,S)-DuPhos — 30 12 0 11e (R,R,R)- L6 — 20 64 90 12e (R,R,R)- L6 — 10 32 90 13e (R,R,R)- L6 LiCl 20 93 89 14e (R,R,R)- L6 MS (4Å) 20 86 87 15e (R,R,R)- L6 LiCl+MS (4Å) 20 95 (92)d 92 16e,f (R,R,R)- L6 LiCl+MS (4Å) 20 54 92 17 — LiCl+MS (4Å) 20 0 ND aReaction conditions: LiCl (1.0 equiv), 4Å MS (50 mg). bThe yields were determined by 1H NMR analysis using 1-isobenzofuranone as an internal standard. cDiastereo- and enantioselectivity were determined by chiral HPLC. Only cis product was obtained for all reactions. dIsolated yield is given in parenthesis. COD = 1,5-cyclooctadiene, Ts = 4-toluenesulfonyl. ND = not determined. eReaction time: 8 h. fWithout iPrONa. In an attempt to improve the enantioselectivity, we lowered the reaction temperature and were pleased to find that the enantioselectivity improved to 90% ee at both 20 and 10 °C (entries 11 and 12). However, the yields of 2a decreased to 64% and 32%, respectively, and we detected allylbenzaldehyde generated by hydrolysis of the imine substrate. To prevent this side reaction, we evaluated various additives in the reaction at 20 °C (entries 13 and 14, also see Supporting Information). Surprisingly, adding 1.0 equiv of LiCl and molecular sieves improved both the yield and the ee (95% yield, 92% ee; entry 15). Control experiments showed that a catalytic amount of iPrONa was curial to the high yield of the reaction but had no influence on the enantioselectivity (entry 16) while the chiral spiro phosphoramidite ligand ( L6) was indispensable for chiral induction (entry 17). Substrates scope Using the optimized conditions (Table 1, entry 15), we investigated the substrate scope by carrying out nickel-catalyzed intramolecular reductive coupling reactions of allylbenzaldehyde imines 1. All the imine substrates listed in Table 2 exclusively gave single diastereomers of the corresponding products ( 2a– 2r). The absolute configuration of (S,S)- 2a was determined by means of single-crystal X-ray diffraction analysis (see Supporting Information Table S7). Imine substrates with various substituents on the aryl ring smoothly underwent the cyclization to give the corresponding five-membered cyclic amines ( 2a– 2g, 2i– 2k) in 61%–92% yields with ee values of 88%–97%. An exception was imine 1h, which has an o-methoxyl substituent on the aryl ring and provided cyclized product 2h in high yield but relatively low enantioselectivity (76% ee), a result that we attributed to the steric effect of the methoxyl group near the imine group. Notably, the 4-Cl atom of substrate 1j was tolerated, and the Cl atom in the product can be expected to facilitate subsequent functionalization. Also notable is the result for benzodioxole substrate 1k, which gave 2k in 75% yield with 93% ee. We also examined the effects of various protecting groups on the nitrogen atom of o-allylbenzaldehyde imine substrates. In addition to 4-methylphenylsulfonyl, N-phenyl-sulfonyl ( 2l, 78% yield, 87% ee), N-4-methoxylphenylsulfonyl ( 2m, 89% yield, 91% ee), and N-4-trifluoromethylphenylsulfonyl ( 2n, 56% yield, 88% ee) were suitable protecting groups. However, the protecting groups N-2-nitrobenzenesulfonyl ( 2o) and N-diphenylphosphinoyl ( 2p) were not compatible with the reaction and could not give the desired product. These results indicated that the suitable N-protecting group of imines is crucial for the reaction. In addition, the last two examples are particularly noteworthy, as the challenging substrates bearing more hindered, disubstituted double bonds (R2 = Me or Et) underwent reductive coupling smoothly to deliver chiral cyclic amines with an all-carbon quaternary center ( 2q, 82% yield, 89% ee; 2r, 65% yield, 92% ee, respectively). Table 2 | Scope of Allyl Iminesa aThe diastereo- and enantioselectivity were determined by chiral HPLC; Isolated yield. bUsing 0.5 equiv iPrONa at 25 °C for 12 h. cUsing 0.5 equiv iPrONa at 30 °C for 12 h. NR = no reaction. Next, we studied intramolecular reductive coupling reactions of various substituted homoallylbenzaldehyde imines 3 with catalysis by Ni/(R,R,R)- L6, expecting to obtain six-membered cyclic amines with a tetralin core structure (Table 3). Homoallylbenzaldehyde imines 3a– 3i provided desired products 4a– 4i in good yields (65%–90%) with high enantioselectivities (87%–92% ee). Like the last two examples in Table 2, the present catalyst system was also applied to the synthesis of six-membered cyclic amines bearing an all-carbon quaternary center. An quaternary stereogenic carbon was constructed at the adjacent position of the chiral amine in good yields with high enantioselectivity ( 4j, 4k). The absolute configuration of (S,R)- 4j was determined by means of single-crystal X-ray diffraction analysis (See Supporting Information Table S8). Table 3 | Scope of Homoallyl Iminesa aThe diastereo- and enantioselectivity were determined by chiral HPLC. No trans product was detected in any reaction. Isolated yield. Encouraged by these results, we tried to extend our protocol to build up chiral polycyclic 1,2-aindoles (Table 4), which are found in various naturally occurring and synthetic bioactive compounds.42,43 Indeed, the method was successfully applied for the construction of chiral polycyclic 1,2-aindoles, including 6-5-5 ( 6a, 6b) and 6-5-6 tricyclic systems ( 6c, 6d), from the corresponding N-allylindole-2-imines and N-homoallylindole-2-imines, in good yields with high enantioselectivities. Table 4 | Scope of N-Tethered Indole Alkenyl Iminesa aThe diastereo- and enantioselectivity were determined by chiral HPLC. No trans product was detected in any reaction. Isolated yield. Because chiral amine moieties are found in a wide variety of naturally occurring alkaloids and pharmaceuticals, we demonstrated the utility of our method by performing a gram-scale synthesis of versatile cyclic amine 2a (Scheme 2a), which was converted to an E1-activating enzyme inhibitor by sequential deprotection and arylation of the amine group (Scheme 2b).44 In addition, product 4a, which was prepared from a readily accessible o-homoallylbenzaldehyde imine, is a key intermediate for the synthesis of agents with antiobesity activity and activity against Huntington's disease (Scheme 2c).45,46 Scheme 2 | Gram-scale experiment and synthetic transformations of the reductive cyclization products. Download figure Download PowerPoint Mechanistic study To gain insight into the reaction mechanism, we performed deuterium-labeling experiments (Scheme 3). 1H NMR spectroscopic analysis of the product indicated that isopropanol was the reducing agent (Scheme 3a) and that the hydride of the Ni–H intermediate was derived not from the OH bond of the isopropanol but from the α-C–H bond (Scheme 3b,c). Scheme 3 | Control and deuterium-labeling experiments. Download figure Download PowerPoint On the basis of the aforementioned results and previous reports,32–36,47–49 we propose the mechanism outlined in Scheme 4. The coordination of imine and alkene with nickel(0) and subsequent oxidative cyclization yield nickelacycle intermediate C. A stereo induction model shows that the diastereo- and enantioselectivity were determined during the oxidative cyclization. Isopropanol or sodium isopropoxide promotes the ring-opening of C by cleavage of the Ni–N bond, forming intermediate D.50 Subsequently, β-hydride elimination from D forms intermediate E and acetone, which can be detected in the reaction mixture. Finally, a reductive elimination reaction of intermediate E releases the product and regenerates the nickel catalyst. In addition, the role of the additives of LiCl and molecular sieves is to promote the formation of imine and to prevent hydrolysis. Scheme 4 | Proposed mechanism. Download figure Download PowerPoint Conclusion We have developed a highly efficient method for nickel-catalyzed asymmetric reductive coupling reactions of imines with unactivated alkenes. This reaction is enabled by a nickel catalyst-bearing monodentate chiral spiro phosphoramidite ligand and features the use of readily available and environmentally benign isopropanol as the reductant. This diastereo- and enantioselective method for the synthesis of chiral cyclic amines has potential applications in medicinal chemistry and natural product synthesis. Supporting Information Supporting Information is available and includes general information, optimization details, synthetic procedures and characterization data of new compounds, as well as mechanistic studies. Characterization data include NMR spectra and high-performance liquid chromatography (HPLC) charts. X-ray crystallographic data of compounds 2a and 4j are available. Conflict of Interest There is no conflict of interest to report. Funding Information We thank the National Natural Science Foundation of China (Nos. 21790332, 91956000, 22188101), the Fundamental Research Funds for the Central Universities, and the Haihe Laboratory of Sustainable Chemical Transformations for their financial support. Dedication This article is dedicated to Professor Michael P. Doyle on the occasion of his 80th birthday. References 1. 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