Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
N‑Sulfonyl Bisimidazoline Ligands and Their Applications in Pd(II)Catalyzed Asymmetric Addition toward α‑Tertiary Amines Jia-Yin Wang,† Meng-Wei Li,† Meng-Fan Li,† Wen-Juan Hao,† Guigen Li,*,‡,§ Shu-Jiang Tu,*,† and Bo Jiang*,† Org. Lett. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 09/14/18. For personal use only.
†
School of Chemistry & Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou 221116, P. R. China ‡ Institute of Chemistry & BioMedical Sciences, Collaborative Innovation Center of Chemistry for Life Sciences, Nanjing University, Nanjing 210093, P. R. China § Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States S Supporting Information *
ABSTRACT: A new class of chiral N-sulfonyl bisimidazoline (Bim) ligands have been designed, prepared, and applied in Pd(II)-catalyzed asymmetric addition of arylboronic acids to isatin-derived N-Boc ketimines. The combination of Pd(OCOCF3)2 and N-tosyl Bim ligand shows high catalytic activity and excellent asymmetric induction, enabling asymmetric addition to offer α-tertiary amines with generally good to high yields and excellent enantioselectivity (up to 96% yield, 96% ee). This asymmetric Pd(II) catalysis can tolerate air conditions, providing a practical and operationally simple protocol toward the construction of an enantioenriched α-tertiary stereocenter.
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have remained unexploited for asymmetric catalysis so far. Inspired by the recent success of chiral bis(imidazolidine)pyridine (PyBidine) ligands in asymmetric catalysis,5 we believed that the Bim ligands with easily tunable functional modifications would generate a new catalyst class for unique activity and asymmetric controls (Figure 1). Meanwhile, it is well-known that chiral quaternary carbon stereocenters exist in a wide assortment of natural products, bioactive substances, pharmaceuticals, and agrochemical ingredients.6 Therefore, the enantioselective construction of these stereocenters has attracted extensive interest in the organic community but poses a particular challenge in asymmetric catalysis due to the inherent steric congestion.7 In fact, the control of chiral α-tertiary amines is also very important owing to the versatile chemical and pharmaceutical applications of chiral amines.8,9 In this regard, asymmetric addition to ketimines has been a straightforward and practical strategy. Numerous organocatalytic and metal-catalyzed asymmetric reactions have been reported for the formation of chiral α-tertiary amines.9,10 Specifically, asymmetric nucleophilic additions to isatin-derived ketimines provide an efficient and direct entry to chiral α-tertiary amines by using various C-,11 O-,12 P-,13 and S-nucleophiles (Scheme 1b).14 Pd(II)-catalyzed asymmetric addition of arylboron to ketimines has attracted considerable attention.15 Recently,
he development of more powerful chiral ligands has been one of the most attractive topics in chemical synthesis, especially for those ligands which form extremely efficient catalytic complexes and result in excellent enantioselectivity and chemical yields.1 A variety of structurally diverse chiral ligands containing phosphines and/or amines have been designed and synthesized, and some of them have exhibited outstanding catalytic performances in numerous enantioselective transformations.2 Among these ligands, chiral bisoxazolines (Box, type I, Figure 1) have been widely utilized for
Figure 1. Design of chiral ligands.
forming transition-metal complexes for many asymmetric reactions.3 However, structural modifications of these ligands are limited by the availability of their backbone starting materials.4 As shown in Figure 1, chiral C2-symmetric bisimidazolines (Bim, type II) showed great structural flexibility and availability in regard to N-protection groups via N-sulfonylation, N-alkylation, and N-acylation to tune the catalytic reactivity and asymmetric induction by catalysts. Surprisingly, the above chiral C2-symmetric bisimidazolines © XXXX American Chemical Society
Received: July 26, 2018
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DOI: 10.1021/acs.orglett.8b02374 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Optimization of the Reaction Conditionsa
Scheme 1. Profiles of Asymmetric Addition of IsatinDerived Ketimines
the group of Zhang reported an elegant and efficient palladium-catalyzed enantioselective addition of arylboronic acids to isatin-derived N-sulfonyl ketimines with the 2,6dicholobenzyl protecting group, but the Boc-protected counterparts were not applicable for this transformation (Scheme 1b).15a Therefore, the catalytic asymmetric addition of aryl nucleophiles to isatin-derived N-Boc ketimines is still challenging (Scheme 1c).16 We elaborate herein the facile synthesis of modular N-sulfonyl Bim ligands and their successful application in the Pd(II)-catalyzed asymmetric addition of arylboronic acids 2 to isatin-derived N-Boc ketimines 1 for enantioselective construction of α-tertiary amines 3 (Scheme 1d). Initially, the benzyl-protected isatin-derived N-Boc ketimine 1a and phenylboronic acid (2a) were selected as representative substrates to optimize the reaction conditions (Table 1). With Pd(OCOCF3)2 and Ph-Pyox (L1) as a catalytic system, the reaction in the presence of LiOH in toluene at room temperature did not give the desired product 3a (entry 1). A number of other bidentate oxazoline ligands often used in asymmetric catalysis, such as tBu-Pyox (L2), tBu-Pyox (L3), and Box (L3−L5) were next assayed with the aim of promoting the desired addition reaction. However, these chiral ligands did not show any improvement on both the reactivity and enantioselectivity (entries 2−5). Next, we considered using imidazolidine ligands as bidentate nitrogen ligands to investigate this asymmetric addition reaction. The easily available imidazolidine ligands, such as Pyim (L6), Quinim (L7), and PyBim (L8), were then evaluated. Unfortunately, none of the ligands L6−L8 were able to allow a successful reaction (entries 6−8). Afterward, our first synthesized Nsulfonyl-protected Bim ligands L9−L13 were applied in this reaction system. Delightfully, the use of ligands L9−L13 all could drive the conversion of 1a into 3a (entries 9−13), in which the combination of Pd(II) catalyst and N-tosyl Bim ligand L10 exhibited the best catalytic performance in this transformation regarding the reactivity and enantioselectivity, affording the adduct product (R)-3a in 86% yield and 90% ee value (entry 10). The absolute configuration of 3a was confirmed by X-ray diffraction analysis, and its derivatives were
entry
L (mol %)
solvent
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22d 23e
L1 (5) L2 (5) L3 (5) L4 (5) L5 (5) L6 (5) L7 (5) L8 (5) L9 (5) L10 (5) L11 (5) L12 (5) L13 (5) L14 (5) L10 (5) L10 (5) L10 (5) L10 (5) L10 (5) L10 (5) L10 (5) L10 (5) L10 (10)
toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene MTBE EA THF DCE benzene PhCl PhCF3 toluene toluene
ND ND ND ND ND ND NR NR 65 86 71 42 58 NR 70 85 79 63 80 55 58 88 70
eec (%)
90 90 90 90 87 85 81 84 68 90 86 79 91 90
a
Reaction conditions: 1a (0.1 mmol), 2a (0.3 mmol), Pd(O2CCF3)2 (5 mol %), ligand (5 mol %), LiOH (0.5 equiv), solvent (1.0 mL), under air conditions, 8 h, at room temperature. bIsolated yield based on substrate 1a. cThe ee value was determined by HPLC. d0 °C. eUse of Pd(O2CCF3)2 (10 mol %).
assigned by analogy (see the Supporting Information). C2linked Bim ligand L14 proved to be ineffective in the asymmetric catalysis (entry 14). The following screening of the solvents, such as methyl tert-butyl ether (MTBE), CH3CO2Et (EA), tetrahydrofuran (THF), 1,2-dichloroethane (DCE), benzene, chlorobenzene (PhCl), and trifluoromethylbenzene (PhCF3), revealed that all these reaction media gave relatively inferior outcomes with respect to reaction yields and/or ee value as compared with toluene (entries 15−21 vs entry 10). Moreover, decreasing the reaction temperature to 0 °C is beneficial to both the yield and the enantioselectivity, and the product 3a was obtained in 88% yield and 91% ee (entry B
DOI: 10.1021/acs.orglett.8b02374 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 3. Substrate Scope of Arylboronic Acidsa−c
22). However, the increase in the loading of the catalyst and the ligand did not facilitate the reaction process (entry 23). With the effective catalytic system identified by the screening of bidentate nitrogen ligands in hand, we next evaluated the scope of asymmetric catalysis by varying isatinderived N-Boc ketimines and arylboronic acid components. The representative results are listed in Scheme 2. N-Boc Scheme 2. Substrate Scope of Isatin-Derived N-Boc Ketiminesa−c
a Reaction conditions: 1 (0.1 mmol), arylboronic acids (0.3 mmol), Pd(TFA)2 (5 mol %), ligand (5 mol %), LiOH (0.5 equiv), toluene (1.0 mL), under air conditions, at 0 °C. bIsolated yield based on substrate 1. cThe ee value was determined by chiral HPLC.
methyl functionality reacted efficiently with arylboronic acids 2 with both electronically poor (chloro 2b) and rich (methyl 2c) groups. However, the ee value is excellent for the former product 3m, while an appreciably reduced ee value was observed for the latter 3n. These results indicate that the electronic nature of arylboronic acids seems to impose a remarkable effect on the enantioselectivity. This similar observation also appears in the conversion of ketimine 1d possessing C6 chloro functionality into α-tertiary amine products 3o−u. Arylboronic acids bearing a functional group, such as halogen (e.g., fluoro, chloro, and bromo), ether (PMP = p-methoxyphenyl), and alkyl, proved to be good candidates for the reaction, in which the presence of electron-withdrawing groups is generally required to achieve excellent enantioselectivity (3o−3q vs 3t−3u). As F-containing compounds exhibit important pharmacological properties, we decided to investigate the generality of asymmetric Pd-catalysis by combining with fluoro-containing N-Boc ketimines 1m−o and 4-chlorophenylboronic acid 2b. As expected, the reaction could proceed smoothly, and high levels of stereochemical control were established regardless of their structural features (3v−x). Alternatively, the opposite enantiomer of (S)-3p was formed with 83% yield and 96% ee when the chiral (S,S,S,S)bisimidazole behaved as the bidentate nitrogen ligand. The prepared N-sulfonyl bisimidazoline ligand shows excellent chiral induction, perhaps because this ligand is an anion, rather than a strong base, and most of the negative charge is on the nitrogen. Thus, the ligand is a relatively strong electrondonating ligand, but not strong enough to totally neutralize one positive charge of Pd(II), which will increase the nucleophilic ability of the Pd−Ar species and keep the ability of Pd(II) for activation of ketimine. It is a good balance between electron-withdrawing and electron-donating ligands, leading to the high reactivity of this catalytic system.17
a
Reaction conditions: 1 (0.1 mmol), PhB(OH)2 (0.3 mmol), Pd(TFA)2 (5 mol %), ligand (5 mol %), LiOH (0.5 equiv), toluene (1.0 mL), under air conditions, at 0 °C. bIsolated yield based on substrate 1. cThe ee value was determined by chiral HPLC.
ketimines with diverse functional groups were first investigated in combination with benzoboric acid under the standard conditions. Substituents with different electronic nature residing in different positions of arene ring would be accommodated, confirming the efficiency of asymmetric catalysis, as products 3b−g were generated in 70%−89% yields and between 82% and 94% ee except for 3g. Generally, introduction of a functional groups into C6 or C7 position of isatin-derived ketimines 1 allowed for the addition of 2a to proceed with the better enantioselectivity control in the presence of Pd(II)-Bim system as compared with C4 or C5 substituents (Scheme 2, 3c and 3d vs 3f and 3g). The 4-chloro substituent at the C4 position of indolin-2-one ring 1g seems reluctant to go through this process in which both low yield and poor chiral induction were observed (3g, 43%, 60% ee), maybe due to steric effects. Next, we investigated the effect of the substituent group at the 1-position of the ketimines 1. Isatin-derived ketimines with different functional groups at this nitrogen atom, such as 4-methylbenzyl (1h), 4-bromobenzyl (1i), phenyl (1j), N-Boc (1k), and methyl (1k), were efficiently transformed into the corresponding products with high enantioselectivity (86−94% ee). To broaden the application of this Pd(II)-catalyzed asymmetric addition, N-Boc ketimines with C6 or C7 substituents were adopted to react with arylboronic acids 2 with different substitution patterns, due to substituents at both these positions favor to show higher reactivity and better enantioselectivity (Scheme 3). N-Boc ketimine 1b bearing C7 C
DOI: 10.1021/acs.orglett.8b02374 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters The significant potential of our approach based on the amplification reaction for the synthesis of (R)-3p was amply demonstrated with the excellent enantioselectivity being retained (Scheme 4a). Removal of the Boc protecting group
Bo Jiang: 0000-0003-3878-515X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the NSFC (No 21472071), PAPD of Jiangsu Higher Education Institutions, the Outstanding Youth Fund of JSNU (YQ2015003), and the Robert A. Welch Foundation (D-1361, USA).
Scheme 4. Amplification Reaction and Deprotection of 3p
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of 3p provided the free chiral amine product 4 in high yield without loss of stereochemical purity, which is a useful chiral amine precursor in organic chemistry (Scheme 4b). In conclusion, we have established a class of new N-sulfonyl bisimidazoline ligands for asymmetric catalysis by forming Pd(II) catalysts and demonstrated both modularity and versatility. In comparison with known chiral ligands, the ability of N-sulfonyl Bim ligand to easily vary the steric and electronic properties of the N-protecting groups over a wide range is extensively practical. By applying this Pd(II)-N-tosyl Bim catalytic system, asymmetric addition of arylboronic acids to isatin-derived N-Boc ketimines was first realized, enabling a direct and practical protocol toward the highly enantioselective construction of α-tertiary amines. Furthermore, the reaction worked readily under the air conditions, exhibiting a remarkable tolerance to oxygen and making the asymmetric transformation simple and mild. Further expanding the scope and application of ligands for distinct catalysis are currently in process in our laboratory.
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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/acs.orglett.8b02374. Experimental procedures, characterization data for all products, detailed optimized geometries, and free energies (PDF) Accession Codes
CCDC 1856780−1856781 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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DOI: 10.1021/acs.orglett.8b02374 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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