Gd(OTf)3 Complex-Promoted Asymmetric Aldol

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Letter Cite This: Org. Lett. 2018, 20, 5314−5318

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N,N′‑Dioxide/Gd(OTf)3 Complex-Promoted Asymmetric Aldol Reaction of Silyl Ketene Imines with Isatins: Water Plays an Important Role Li Dai, Lili Lin, Jianfeng Zheng, Dong Zhang, Xiaohua Liu,* and Xiaoming Feng*

Org. Lett. 2018.20:5314-5318. Downloaded from pubs.acs.org by IMPERIAL COLLEGE LONDON on 09/08/18. For personal use only.

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: A highly diastereo- and enantioselective aldol reaction of isatins with silyl ketene imines was realized by using a chiral N,N′-dioxide/GdIII complex (1 mol %), and with the addition of water, all the reactions completed within 10 min and various β-hydroxy nitriles with adjacent tetrasubstituted stereocenters were obtained in excellent outcomes. The essential role of water was probed, and a possible transition state model was proposed based on an eight-coordinated N,N′-dioxide/GdIII complex.

S

ilyl ketene imines (SKIs),1 as a class of nitrile equivalent, have a pair of orthogonal substituent planes. Therefore, two dissimilar substituents on terminal carbon could impart an axis of chirality in SKIs. The unique structure enables them to be a highly versatile nucleophiles in organic synthesis, particularly in the construction of molecules with quaternary stereocenters (Scheme 1). In 2005,2 Fu et al. reported the first asymmetric acylation reactions of SKIs with anhydrides catalyzed by the chiral PPY [4-(pyrrolidino)pyridine] derivative. Later, Denmark and co-workers reported the aldol reactions of aldehydes with SKIs catalyzed by a chiral SiCl4/ bisphosphoramide catalyst system, which was also efficient to promote the Michael reaction of α,β-unsaturated aldehydes or ketones with SKIs.3 Subsequently, the protonation of SKIs was reported by List’s group, where chiral phosphoric acid was used as the catalyst.4 Moreover, Leighton’s group realized the Mannich reactions of acylhydrazones with SKIs in the presence of chiral silane Lewis acid.5 Our group applied chiral N,N′dioxide/metal complexes to realize the Mannich reactions of aldimines or ketimines with SKIs.6a,b Very recently, we also realized the conjugate addition of 3,3-disubstituted oxindoles with SKIs.6c Nevertheless, there is still no report on the aldol reaction of ketones with SKIs. The reasons are probably as follows: (1) the retro-aldol reaction may exist as a result of the large steric repulsion of the newly formed adjacent tetrasubstituted stereocenters in the aldolate intermediate;7 and (2) there is no additional assistant group on the oxygen of ketones compared with imines, which could form a bidentate coordination to improve chiral inducement. Isatins are a type of active ketones, and their aldol reaction provides direct access to 3-hydroxyindole alkaloids,8 which are © 2018 American Chemical Society

an important family of bioactive compounds.9 When we applied the chiral N,N′-dioxide/metal complex6a−c to the aldol reaction of SKIs with isatins, the stereoselectivity was found to be quite difficult to control and only moderate diastereo- and enantioselectivities were afforded. In addition, the strong background reaction was proven to be another issue for developing the catalytic version of this transformation. Though great challenges exist, the novel access to 3-hydroxyindole alkaloids bearing two adjacent tetrasubstituted stereocenters is attractive. Herein, we report our efforts in developing a chiral N,N′-dioxide/GdIII complex catalytic system10 with water as the essential additive to realize the highly diastereo- and enantioselective aldol reaction of SKIs with isatins. The addition of SKI 2a to N-benzyl protected isatin 1a was selected as the model reaction to optimize the reaction conditions (Table 1). Initially, various metal salts complexing with L-PiPr2 were identified in THF at 30 °C for 20 h (for more details, see the Supporting Information). Metal salts, including Sc(OTf)3, Ni(OTf)2, and Zn(OTf)2, which showed excellent efficiency in our previous work,6a−c gave poor diastereo- and enantioselectivities (Table 1, entries 1−3). To our delight, the complex of Gd(OTf)3 with L-PiPr2 could promote the reaction smoothly, providing the corresponding product in high yield (90%) and moderate diastereoselectivity (74:26 dr) with a promising ee value (66% ee for major diastereoisomer, Table 1, entry 4). It should be mentioned that the outcomes by the GdIII-L-PiPr2 system were not stable, Received: July 18, 2018 Published: August 23, 2018 5314

DOI: 10.1021/acs.orglett.8b02239 Org. Lett. 2018, 20, 5314−5318

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even under the same conditions, and two individual reactions may give different results. After careful investigation and analysis, it was found that water has an apparent impact on the current system and the addition of water11 could significantly improve diastereo- and enantioselectivities (91:9 dr, 88% ee, as shown in Table 1, entry 5). Moreover, the reaction could complete within 10 min (99% yield). Encouraged by these findings, other parameters were then probed. Diminishing the steric hindrance of the amide group in the ligand from isopropyl groups (L-PiPr2) to less bulky ethyl groups (LPiEt2) led to an increase in diastereo- and enantioselectivity (95:5 dr, 97% ee, Table 1, entry 6). A further decrease of the steric hindrance with methyl groups (L-PiMe2) or hydrogen atoms (L-PiPh), especially the latter, caused the drop of diastereo- and enantioselectivities (Table 1, entries 7 and 8). N,N′-Dioxides with different chiral backbones were also evaluated, but no better results were obtained (Table 1, entries 9 and 10). It is worth noting that the catalyst loading could be lowered to 1 mol % without any loss of reactivity, diastereoselectivity, and enantioselectivity (Table 1,entry 11). When the ratio of 1a to 2a was changed from 1:2 to 1:1, the yield of the product decreased sharply though the dr and ee values were still excellent (entry 12). With the optimized conditions in hand, the substrate scope of SKIs was then screened in the reaction of isatin 1a (Table 2). First, phenyl substituted SKIs (2a−2c) with aliphatic R2

Scheme 1. Asymmetric Construction of Tetrasubstituted Stereocenters by Reactions of SKIs

Table 2. Substrate Scope for SKIsa

Table 1. Optimization of Reaction Conditionsa

entry

metal salt

ligand

yield (%)b

drc

ee (%)c

d

Sc(OTf)3 Ni(OTf)2 Zn(OTf)2 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3 Gd(OTf)3

L-PiPr2 L-PiPr2 L-PiPr2 L-PiPr2 L-PiPr2 L-PiEt2 L-PiMe2 L-PiPh L-PrPr2 L-RaPr2 L-PiEt2 L-PiEt2

45 66 88 90 99 99 99 83 88 98 99 60

52:48 79:21 67:33 74:26 91:9 95:5 93:7 82:18 88:12 90:10 97:3 92:8

4/−16 4/36 27/−15 60/−15 88 97 95 −6/42 78 92 96 94

1 2d 3d 4d 5 6 7 8 9 10 11e 12e,f

entry

R1, R2

1 2 3 4 5 6 7 8 9 10 11 12 13d 14 15 16

Ph, Me Ph, Et Ph, Allyl 4-FC6H4, Me 4-ClC6H4, Me 4-BrC6H4, Me 4-MeC6H4, Me 4-MeOC6H4, Me 3-ClC6H4, Me 3-BrC6H4, Me 3-MeC6H4, Me 3-MeOC6H4, Me 2-FC6H4, Me 2-Naphthyl, Me 3-Thiophenyl, Me 2-Thiophenyl, Me

yield (%)b 99 98 98 89 99 77 99 93 93 95 99 99 55 98 79 72

(3aa) (3ab) (3ac) (3ad) (3ae) (3af) (3ag) (3ah) (3ai) (3aj) (3ak) (3al) (3am) (3an) (3ao) (3ap)

drc 97:3 99:1 99:1 96:4 96:4 98:2 95:5 93:7 98:2 98:2 96:4 97:3 79:21 96:4 95:5 97:3

ee (%)c 96 91 97 91 95 96 96 95 97 98 97 99 92 97 96 99

(1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R) (1S,2R)

a

Unless otherwise noted, the reactions were performed with Gd(OTf)3/L-PiEt2 (1:1, 1 mol %), 1a (0.10 mmol), 2 (0.20 mmol) in THF (0.5 mL) mixed with H2O (3 μL, 1.7 equiv) at 30 °C for 10 min. bIsolated yield. cDetermined by HPLC analysis or 1H NMR analysis. d5 mol % catalyst was used.

a

Unless otherwise noted, the reactions were performed with metal salt/L-PiPr2 (1:1, 10 mol %), 1a (0.10 mmol), 2a (0.20 mmol), and H2O (3 μL, 1.7 equiv) in THF (0.5 mL) at 30 °C for 10 min. b Isolated yield. cDetermined by HPLC analysis. dWithout H2O, the reaction time is 20 h e1 mol % catalyst loading. fWith 2a (1.0 equiv).

groups, such as methyl, ethyl, and allyl, were proved to be suitable, and the corresponding products (3aa−3ac) were obtained in 98−99% yields, 97:3−99:1 dr and 91−97% ee (entries 1−3). Regardless of the electronic nature and meta- or para- positions of substituents on the aryl R1 group in SKIs (2d−2l), excellent results were obtained (77−99% yields, 5315

DOI: 10.1021/acs.orglett.8b02239 Org. Lett. 2018, 20, 5314−5318

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with SKI 2a (2.0 equiv), delivering 3aa in 99% yield (1.10 g), 96:4 dr, and 96% ee (Scheme 2). Furthermore, the product

93:7−98:2 dr, and 91−99% ee; entries 4−12). Comparatively, the reaction of substrate 2m with an ortho-substituted aryl group provided the corresponding product 3am with 79:21 dr and 92% ee but in moderate yield (55%), even when the catalyst loading was increased to 5 mol % (entry 13). Fusedring and thiophene-substituted SKIs (2n−2p) were also well accommodated in this transformation, affording the desired products in good results (72−98% yields, 95:5−97:3 dr, and 96−99% ee; entries 14−16). Next, different substituted isatins were also investigated. As shown in Table 3, different substituents on the aromatic ring of

Scheme 2. Gram-Scale Synthesis and Further Transformations of 3aa

Table 3. Substrate Scope for Isatinsa

3aa could be transformed into γ-amino alcohol 4 by reduction with NaBH4 in the presence of CoCl2, which possesses the same core structure with donaxaridine. Additionally, a pyrrolidinoindoline-type alkaloid CPC-1 analogue 5 was synthesized in 43% yield, >99:1 dr, and 91% ee by protecting the hydroxy group and subsequent treatment with LiAlH4. For the catalytic mechanism, of key importance is to understand the essential role of water with respect to reactivity and selectivity. The water might provide a proton to help the leaving of the silyl group to generate the final product 3, and to promote the dissociation of the product from the catalyst, which could not only accelerate catalytic cycle but also decrease the lifetime of instable GdIII/N,N′-dioxide/alcoholate-3aa thus impeding a retro-aldol reaction. In order to understand the influence of water for the enantioselectivity, we tried to obtain the crystal of the catalyst. Though the single crystal of the optimal ligand L-PiEt2 with Gd(OTf)3 failed, we obtained the crystal of L-RaPr2/Gd(OTf)3 complex in a mixed solvent system of methanol/EtOAc/DCM/hexane instead. The crystal data showed that Gd3+ coordinated with the four oxygens of L-RaPr2, three methanol molecules, and one OTf− ion in an eight-coordinated manner (Figure 1; CCDC 1828765). This geometry is obviously different to octahedral coordination with Sc3+ and Ni2+ in our previous studies.12 The angle among two amides and Gd3+ is around 150° rather than an approximately linear fashion. The other four coordination sites locate in an open space, indicating the coodination of the substrates or reagents occurs in varied forms. Furthermore, the HRMS spectra of a N,N′-dioxide L-PiEt2, Gd(OTf)3, and 1a mixture in a 1:1:1 ratio exhibited the ion peak at m/z 1167.3712, in refernce to the intermediate [L-PiEt2 + Gd3+ + OTf− + MeOH + 1a-H+]+ (calcd for 1167.3718). It indicates the coordination of 1a to the catalyst in a ratio of 1:1 with the dissociation of two methanol molecules. Control experiments showed that the addition of methanol could also improve the results significantly (Scheme 3), evidencing water and methanol might play similar roles in the reaction. Hence, in the optimal reaction system, it was extremely possible that three H2O molecules coordinate to the Gd3+ center and two of them dissociate followed by the coordination of isatin 1a. The roles of water on chiral N,N′-dioxide metal catalysis could be

a

Unless otherwise noted, the reactions were performed with Gd(OTf)3/L-PiEt2 (1:1, 1 mol %), 1 (0.10 mmol), 2a (0.20 mmol) in THF (0.5 mL) mixed with H2O (3 μL, 1.7 equiv) at 30 °C for 10 min. bIsolated yield. cDetermined by HPLC analysis or 1H NMR analysis. dDetermined by X-ray analysis.

isatin were tolerated. Generally, isatins with electron-donating groups showed higher reactivities and better diastereo- and enantioselectivities than those with electron-withdrawing groups (entries 4, 8, and 11−13 vs entries 1−3, 5−7, and 9−10). It is worth mentioning that substrates 1o and 1p with fused naphthyl or tetrahydronnaphthyl ring could undergo the reaction smoothly, delivering the corresponding products in good yields and excellent stereoselectivities (77−99% yields, 93:7−97:3 dr, and 97−99% ee; entries 14−15). The absolute configuration of the product 3ia was determined to be (1S,2R) by X-ray crystallography analysis (CCDC 1561440). The configuration of other products was also determined to be (1S,2R) by comparing their CD spectra with that of compound 3ia. To show the practicability of this methodology, a gram-scale synthesis of product 3aa was carried out. In the presence of 1 mol % of the catalyst, isatin 1a (3 mmol) reacted smoothly 5316

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silyl group which are assisted by water, the (1S,2R)-product 3aa is afforded with high efficiency. In summary, we have successfully realized the first asymmetric aldol reaction of SKIs with isatins by using as low as 1 mol % chiral N,N′-dioxide-Gd(OTf)3 complex as the catalyst and water as an essential additive. This strategy provided a new access to chiral β-hydroxy nitriles bearing two congested tetrasubstituted carbon centers in a single step within 10 min, as well as a new approach to the core structure of natural molecule, such as CPC-1. Besides, a possible transition-state model was proposed to understand the possible reaction mechanism and the essential role of water.

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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.8b02239. Experiment procedures, full spectroscopic data for all new compounds, and copies of 1H, 13C, 19F NMR and HPLC spectra (PDF) Accession Codes

CCDC 1561440 and 1828765 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.

Figure 1. Proposed transition-state model based on the X-ray crystal structure of L-RaPr2 with Gd(OTf)3.

Scheme 3. Control Experiments for Mechanism

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaohua Liu: 0000-0001-9555-0555 Xiaoming Feng: 0000-0003-4507-0478

concluded as follows: (1) water can serve as the ligands to participate into the coordination of Gd3+. (2) The coordinated water in the complex might form the hydrogen bond with isatin 1a to adjust it into a better orientation for nucleophilic attack by substrate 2a. And (3) the addition of water could protonate the instable species (Gd3+/N,N′-dioxide/3aa) to deliver the final product with the release of the catalyst, which could accelerate the rate of the reaction. Based on the studies above and the absolute configuration of products, a possible transition-state model was illustrated in Figure 1 (above). Preliminarily, the four oxygen atoms of ligand L-PiEt2 coordinate to Gd3+ together with at least three water molecules. With the addition of isatin, 1a coordinates to Gd3+ by taking the place of two waters with its two oxygens in a bidentate fashion. The steric hindrance caused by the amide and amino acid backbone of the ligand cause the isatin 1a to coordinate like that shown in the transition state (the Bn group of isatin orientated inside). Additionally, the vicinal water might interact with the carbonyl group at the active site via hydrogen bonding, and transfer proton after the nucleophilic addition. Since the Re face of isatin is obstructed by the aryl ring of the amide in the ligand, the SKI molecule would be more likely to approach from the Si face. On the other hand, the steric hindrance between the aryl group of SKI and the gadolinium complex makes model A1 favorable. Last, with the protonation of the alcoholate−Gd bond and the leaving of the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Nos. 21432006 and 21772127) for financial support. REFERENCES

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