Chiral-at-Metal Rh(III) Complex-Catalyzed Michael Addition of

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Chiral-at-Metal Rh(III) Complex-Catalyzed Michael Addition of Pyrazolones with α,β-Unsaturated 2‑Acyl Imidazoles Shi-Wu Li,†,‡ Qian Wan,† and Qiang Kang*,† †

Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, 350002, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China S Supporting Information *

ABSTRACT: An efficient enantioselective conjugate addition of pyrazolones with α,β-unsaturated 2-acyl imidazoles catalyzed by chiral-at-metal rhodium complex is reported. The corresponding adducts were obtained in good yields (85%−96%) with excellent enantioselectivities (up to >99%). This protocol exhibits extraordinary reactivity, because of the fact that a complex with as little as 0.05 mol % Rh(III) can catalyze the title reaction on a gram-scale with excellent enantioselectivity.

T

reported a highly enantioselective Michael addition of 4substituted pyrazolones with 4-oxo-4-arylbutenoates catalyzed by chiral rare-earth metal complexes, delivering both enantiomers with the same chiral ligand by simply changing the metal center (see eq 1 in Scheme 1a).8 Later, the same group developed the highly Z-selective and enantioselective 1,4-

he pyrazole and pyrazolone derivatives, which is an important class of five-membered-ring heterocycles with two adjacent nitrogen atoms, have exhibited a variety of pharmacological and biological activities applied to pharmaceutical candidates (see Figure 1).1 For example, Remogliflozin

Scheme 1. Enantioselective Michael Addition of Pyrazolone with α,β-Unsaturated Ketones Catalyzed by Chiral Lewis Acid Complexes

Figure 1. Representative biologically active compounds with a pyrazolone motif.

etabonate (I)2 is an inhibitor of SGLT (II) for the treatment of type II diabetes. Both tetrahydropyrano[2,3-c]pyrazole (II) and its analogue (III) serve as fungicides.3 In this regard, several catalytic asymmetric reactions of pyrazolones with different Michael acceptors, such as nitro-alkenes,4 carbonyl compounds,5 maleimides,6 and isatylidene malononitriles,7 were developed in the past few years, affording various optically active pyrazolone derivatives. Among them, organocatalysts proved to be very powerful and versatile catalysts for achieving these transformations, albeit with its relatively large catalyst loading. However, successful examples of chiral metal complexcatalyzed addition reaction of pyrazolones with electrondeficient alkenes are quite rare. In 2011, the Feng group © XXXX American Chemical Society

Received: January 4, 2018

A

DOI: 10.1021/acs.orglett.8b00040 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

With the optimized conditions in hand (Table 1, entry 11), all sorts of α,β-unsaturated 2-acyl imidazoles and pyridine were employed to test the generality of this asymmetric Michael addition/esterification process (Scheme 2). First, α,β-unsatu-

addition reaction of pyrazolones to alkynones catalyzed by a N,N-dioxide scandium(III) complex (see eq 2 in Scheme 1a).9 Despite these elegant achievements, it is still highly desirable to develop new efficient catalytic asymmetric methods to access chiral multifunctionalized pyrazolone derivatives. As a continuation of our interest in the development of chiral-at-metal Rh(III) complexes10−13 as chiral Lewis acids in catalytic asymmetric reactions,14 herein we report an enantioselective Michael addition of pyrazolone with α,β-unsaturated 2-acyl imidazoles catalyzed by chiral-at-metal Rh(III) complexes. At the outset, the reaction between commercially available pyrazolone 1a and α,β-unsaturated 2-acyl imidazole 2a was conducted in the presence of 2 mol % chiral-at-metal Rh(III) complex Λ-Rh1 in CHCl3 at 30 °C and then treated with Ac2O under basic conditions, resulting in the esterified Michael addition product 3a in 63% yield with 59% ee (Table 1, entry

Scheme 2. Substrate Scope: α,β-Unsaturated 2-Acyl Imidazoles

Table 1. Optimization of the Reaction Conditionsa

entry 1 2 3 4 5 6 7 8 9 10 11

Λ-M (mol %) Λ-Rh1 (2) Λ-Rh2 (2) Λ-Rh3 (2) Λ-Ir (2) Λ-Rh2 Λ-Rh2 Λ-Rh2 Λ-Rh2 Λ-Rh2 Λ-Rh2

(2) (2) (2) (2) (2) (1)

solvent

time (h)

yieldb (%)

enantiomeric excess, eec (%)

CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 DCM THF toluene CH3OH DCE DCE

26 26 26 26 26 26 26 26 26 26 26

63 76 54 45

59 90 45 56

85 80 90 93 89 91

90 89 73 92 95 96

rated 2-acyl imidazoles with an electron-donating substituted phenyl ring were evaluated, and, in all cases (3b−3e), high yields (89%−94%) with excellent enantioselectivities (93%− 96%) were achieved. The introduction of electron-withdrawing groups (3f−3i), such as Cl and Br substituents on the phenyl ring, also afforded the corresponding products in 88%−93% yields with 93%−97% ee values. The substrates with 1-naphthyl or heteroaromatic substituents worked well, affording 3j−3l in 90%−96% yields with up to >99% ee. In addition to aromatic substituents, alkyl substituents such as methyl, trifluoromethyl, benzyloxy-methyl, styryl-substituted α,β-unsaturated 2-acyl imidazoles were also tolerated, delivering the desired products (3m−3p) in 90%−95% yields with 92%−98% ee. In addition, the replacement of N-Me with iPr or Ph of α,βunsaturated 2-acyl imidazoles has no influence on the outcome of the reaction, affording the corresponding products in good yields with excellent enantioselectivities (Scheme 3, 3q−3r). Scheme 3. Substrate Scope: α,β-Unsaturated 2-Acyl Imidazoles, Pyrazole, and Pyridine

Reaction conditions: 1a (0.15 mmol), 2a (0.1 mmol), Λ-Rh (2 mol %), solvent (1 mL) at 30 °C under an argon atmosphere. bIsolated yields. cDetermined by chiral HPLC analysis. a

1). Inspired by this promising result, a variety of chiral Rh(III) and Ir(III) complexes with different achiral ligands were investigated in the title reaction (entries 2−4). To our delight, Λ-Rh2 was the best one, in terms of reactivity and enantioselectivity, giving the desired product in 76% yield with 90% ee (entry 2), probably due to the steric hindrance effect of bulky substituent. Whereas, in the absence of catalyst, the reaction could not afford any product (entry 5). Further screening of solvents revealed that DCE was the superior solvent, which afforded 3a in 89% yield with 95% ee (entry 10). Further decreasing the catalyst loading to 1 mol %, the reaction could still maintain the reactivity and generate desired product 3a in 91% yield with 96% ee (entry 11).

When 2-pyridyl was used in place of N-methylimidazole, the reaction still generated the expected product 3t in 91% yield, albeit with 80% ee. However, α,β-unsaturated N-acyl pyrazoles could not realize this transformation, although it also has the bidentate coordination motif. B

DOI: 10.1021/acs.orglett.8b00040 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Further investigation of the substrate scope of pyrazolones were conducted (Scheme 4). Replacement of methyl Scheme 4. Substrate Scope: Pyrazolones

Figure 2. X-ray derived ORTEP of 5a with thermal ellipsoids shown at the 35% probability level.

In summary, we have developed a highly enantioselective Michael addition of pyrazolones with α,β-unsaturated 2-acyl imidazoles. In the presence of 1 mol % of chiral-at-metal Rh(III) complex Λ-Rh2, the corresponding adducts were obtained in good yields (85%−96%) with excellent enantioselectivities (up to >99%). Remarkably, this protocol exhibits extraordinary advantages, in terms of reactivity and enantioselectivity, given the fact that as low as 0.05 mol % of Λ-Rh2 can promote the title reaction on gram scale to afford the desired product with excellent enantioselectivity. Moreover, this protocol could be extended to synthesize highly functionalized chiral pyrazol-5(4H)-ones, with adjacent quaternary and tertiary stereocenters with excellent diastereoselectivity and enantioselectivity by a one-pot sequential Michael addition/ dearomative chlorination strategy.

substitution of 1a by phenyl or propyl group has great influence on the enantio-control of reactions, which gave the corresponding products in 77% and 79% ee, respectively (see structures 4a and 4b inScheme 4). Moreover, we found that replacing or removing the N-phenyl group of pyrazolones (1d and 1e) failed to deliver the expected products and starting material were recovered. Unfortunately, 4-benzoyl-5-pyrazolone 1f was not a suitable substrate for the current protocol to generate a chiral quaternary center. To evaluate the synthetic potential of this catalytic system, a gram-scale reaction of α,β-unsaturated 2-acyl imidazole 2a (1.60 g/7.55 mmol) with pyrazolone 1a (1.97 g/11.32 mmol) was conducted in the presence of 0.05 mol Λ-Rh2. Gratifyingly, the reaction proceeded smoothly to afford 3a in 91% yield with 92% ee (see eq 1 in Scheme 5). Notably, in the presence of 1

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 5. Gram-Scale Experiment and Synthetic Transformation

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00040. X-ray data for compound 5a, experimental procedures, characterization data, and copies of 1H and 13C NMR spectra and HPLC chromatograms for obtained compounds (PDF) Accession Codes

CCDC 1579028 contains 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

mol % Λ-Rh2, 20 mol % Et3N, and 1.2 equiv NCS in DCE, the reaction of 1a with 2h could afford highly functionalized chiral pyrazol-5(4H)-one (5a) with adjacent quaternary and tertiary stereocenters in 80% yield with excellent diastereoselectivity (>20:1 dr) and enantioselectivity (96% ee; see eq 2 in Scheme 5). Moreover, the conversion of the imidazole moiety to useful synthetic building blocks was also investigated. It was found that the imidazole auxiliary could be easily converted to an ester without erosion of enantioselectivity (see eq 3 in Scheme 5).15 The relative and absolute configuration of product 5a was unequivocally established using single-crystal X-ray analysis (Figure 2; for details, see the Supporting Information).

Qiang Kang: 0000-0002-9939-0875 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Daqiang Yuan (Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences) for his kind help in an X-ray analysis. This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000) and 100 Talents Programme of the Chinese Academy of Sciences. C

DOI: 10.1021/acs.orglett.8b00040 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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M.; Meggers, E.; Gong, L. Chem.Asian J. 2017, 12, 963. (l) Yuan, W.; Zhou, Z. J.; Gong, L.; Meggers, E. Chem. Commun. 2017, 53, 8964. (12) For selected examples on chiral-at-metal iridium complexes, see: (a) Huo, H.; Fu, C.; Harms, K.; Meggers, E. J. Am. Chem. Soc. 2014, 136, 2990. (b) Huo, H.; Shen, X.; Wang, C.; Zhang, L.; Rose, P.; Chen, L. A.; Harms, K.; Marsch, M.; Hilt, G.; Meggers, E. Nature 2014, 515, 100. (c) Huo, H.; Wang, C.; Harms, K.; Meggers, E. J. Am. Chem. Soc. 2015, 137, 9551. (d) Shen, X.-D.; Huo, H.-H.; Wang, C.-Y.; Zhang, B.; Harms, K.; Meggers, E. Chem.Eur. J. 2015, 21, 9720. (13) Zheng, Y.; Tan, Y. Q.; Harms, K.; Marsch, M.; Riedel, R.; Zhang, L. L.; Meggers, E. J. Am. Chem. Soc. 2017, 139, 4322. (14) (a) Gong, J.; Li, K.; Qurban, S.; Kang, Q. Chin. J. Chem. 2016, 34, 1225. (b) Sun, G.-J.; Gong, J.; Kang, Q. J. Org. Chem. 2017, 82, 796. (c) Li, S.-W.; Gong, J.; Kang, Q. Org. Lett. 2017, 19, 1350. (d) Li, K.; Wan, Q.; Kang, Q. Org. Lett. 2017, 19, 3299. (e) Lin, S.-X.; Sun, G.-J.; Kang, Q. Chem. Commun. 2017, 53, 7665. (15) For the protocols of transformation of 2-acyl imidazoles to a wide variety of carbonyl compounds, see: (a) Ohta, S.; Hayakawa, S.; Nishimura, K.; Okamoto, M. Chem. Pharm. Bull. 1987, 35, 1058. (b) Miyashita, A.; Suzuki, Y.; Nagasaki, I.; Ishiguro, C.; Iwamoto, K.-I.; Higashino, T. Chem. Pharm. Bull. 1997, 45, 1254.

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DOI: 10.1021/acs.orglett.8b00040 Org. Lett. XXXX, XXX, XXX−XXX