Ming-Phos-Catalyzed Asymmetric Intermolecular [3 + 2

Nov 30, 2016 - Bing Xu , Zhan-Ming Zhang , Bing Liu , Shan Xu , Lu-Jia Zhou , Junliang ... Bing Liu , Zhan-Ming Zhang , Bing Xu , Shan Xu , Hai-Hong W...
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Copper(I)/Ming-Phos-Catalyzed Asymmetric Intermolecular [3 + 2] Cycloaddition of Azomethine Ylides with α‑Trifluoromethyl α, β‑Unsaturated Esters Bing Xu,‡ Zhan-Ming Zhang,‡ Shan Xu, Bing Liu, Yuanjing Xiao, and Junliang Zhang* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. China

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S Supporting Information *

ABSTRACT: The employment of α-trifluoromethyl α,βunsaturated esters as dipolarophiles pose considerable challenge due to expeditious defluorination and intrinsic steric hindrance. The present work provides an efficient access to valuable pyrrolidines bearing a trifluoromethylated all-carbon quaternary stereocenter through copper/M7-catalyzed asymmetric dipolar cycloaddition of α-trifluoromethyl α,β-unsaturated esters with azomethine ylides. The products were obtained in up to 98% yield with up to >20:1 d.r. and 99% ee. A broad substrate scope, good functional group tolerance, high stereoselectivity, as well as diverse synthetically valuable transformations of the products make this approach highly attractive. KEYWORDS: copper, azomethine ylides, Ming-Phos, dipolar cycloaddition, all-carbon quaternary stereocenter

T

During the course of studying the enantioselective coppercatalyzed [3 + 2] cycloaddition reaction of azomethine ylides with β-CF3 β,β-disubstituted enones (Scheme 1b),6m we wondered whether the more challenging α-CF3 α,β-unsaturated esters could be used as the dipolarophiles for the dipolar [3 + 2] cycloaddition of azomethine ylides. If successful, the valuable highly substituted pyrrolidines with one trifluoromethylated allcarbon quaternary stereocenter at C4 position would be easily accessed. However, this hypothesis may pose more considerable challenges than the enantioselective copper-catalyzed [3 + 2] cycloaddition reaction of azomethine ylides with β-CF3 β,β-disubstituted enones: (1) the competing β-F elimination of the carbanion intermediate of α-CF3 esters to give β,β-difluoroα,β-unsaturated esters.8,9 For example, Fuchikam8d and Kitazume8e have independently demonstrated that α-CF3 α,βunsaturated acids or esters easily undergo SN2′-type substitution with nucleophiles such as lithium aluminum hydride, Grignard reagent, butyllithium, and so on via a tandem nucleophilic addition and β-F elimination (Scheme 1c). Recently, our group also observed the defluorination products through tandem SN2′-and SNV-type substitution reactions of αCF3 α,β-unsaturated esters with bisnucleophiles such as 1,3dicarbonyl compounds and N-tosylated 2-aminomalonates;8g (2) The intrinsic steric hindrance would slow down the cyclization of a-CF3 carbanion intermediate3a,6b,10 and increase the risk of β-F elimination (Scheme 1d); (3) Theoretically, up to 16 stereoisomers and 16 regioisomers might be formed, and thus, the control of the product distribution will be beset with

he transition-metal-catalyzed asymmetric 1,3-dipolar [3 + 2]-cycloaddition of azomethine ylides with electrondeficient alkenes is one of the most powerful and straightforward synthetic tools for the construction of optically active highly substituted pyrrolidines,1−3 which are widely observed in an array of biologically active natural products, pharmaceuticals, and catalysts.4 However, the synthesis of highly substituted pyrrolidines with one all-carbon quaternary stereocenter5 at 3- or 4-position poses considerable challenge because of the requisite use of α,β- or β,β-disubstituted unsaturated compounds with intrinsic low reactivity. As a result, only a handful of examples have been developed to date. In 2010, Waldmann and co-workers applied 2-oxoindolin-3-yidene in asymmetric 1,3-dipolar [3 + 2]-cycloaddition of azomethine ylides to achieve chiral pyrrolidines with one all-carbon quaternary spiro-stereocenter at C4 position.6a Other α,βdisubstituted unsaturated compounds such as nitroalkenes, (E)3-benzylidene chroman-4-ones, N-tosyl-3-nitroindoles, and so on as dipolarophiles have been subsequently explored by Waldmann, Wang, Arai (Scheme 1a).6 However, readily available α-trifluoromethyl α,β-unsaturated compounds have not been developed as dipolarophiles so far, despite that the desired pyrrolidine products bearing one trifluoromethylated all-carbon quaternary stereocenter, a subunit frequently found in pharmaceuticals and agrochemicals (Figure 1).7 Herein, we reported our efforts to the first copper-catalyzed asymmetric [3 + 2] cycloaddition of azomethine ylides with α-trifluo-romethyl α,β-unsaturated esters, which provides an efficient, reliable, and atom-economic strategy for the efficient construction of valuable highly substituted pyrrole-dines featuring one trifluoromethylated all-carbon quaternary stereocenter with other three tertiary stereocenters in a highly stereoselective manner. © 2016 American Chemical Society

Received: October 22, 2016 Revised: November 24, 2016 Published: November 30, 2016 210

DOI: 10.1021/acscatal.6b03015 ACS Catal. 2017, 7, 210−214

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ACS Catalysis

challenging.11 We hypothesized that a bifunctional chiral ligand bearing a hydrogen-bond donor, upon the binding of the enolization of the carbanion intermediate of α-CF3 esters to the ligand via hydrogen bond interaction (Scheme 1d, path b), may inhibit β-F elimination and promote the cyclization of a-CF3 carbanion intermediate (Scheme 1d, path a). More importantly, the spatial relationship between the carbanion of α-CF3 esters and the chiral ligand via hydrogen bond interaction should be confined by the chiral backbone of the ligand, thereby providing a more favorable setting for achieving excellent diastereo- and enantioselectivity. To examine the above hypothesis, α-CF3 unsaturated ester 1a and azomethine ylide 2a were selected as the model substrates. A series of commercially available chiral ligands such as (R)-BINAP, (S)-TF-BiphamPhos, (R)-MOP, (R,S)-OPINAP, (S,S)-iPr-FOXAP, (S,R)-PPFA, Binol-derived phosphoramidite, and chiral oxazoline ligands and our recently developed chiral Ming-Phos M1−M7 (Figure 2, Table S1)

Scheme 1. Asymmetric [3 + 2]-Cycloadditions of Azomethine Ylides with α,β- or β,β-Disubstituted Unsaturated Compounds

Figure 2. Screening chiral ligands.

were examined. Among them, both (S,S)-iPr-FOXAP and (R,Rs)-M7 could deliver 3aa in high yields with excellent ees and >20:1 diastereoselectivity. With more than 10 g of (R,Rs)M7 in our hand, which could be easily prepared in good yields and in large scale from inexpensive commercially materials in two steps via simple operation,6m,12 we chose (R,Rs)-M7 as the chiral ligand for further screening. Subsequently, a number of copper(I) salts such as Cu(CH3CN)4PF6, Cu(CH3CN)4NTf2 and Cu(CH3CN)4ClO4 were tested with the use of (R,Rs)-M7 as the chiral ligand, delivering better diastereoselectivity but lower enantioselectivity (Table S2, entries 2−4). At the same time, other metal salts were examined, such as AgOTf, Cu(OTf)2, Cu(OAc)2, and CuCl2, but only trace amount of product could be detected (Table S2, entries 5−8). Other solvents, such as TBME, Et2O, acetone, toluene, and iPr2O could not give a better result (Table S2, entries 9−13). With the optimal reaction conditions in hand, we next examined the scope by variation of the azomethine yield component 2 (Table 1). Not only electron-donating but also electron-withdrawing groups on the othro-, meta-, and parapositions of aryl moiety of azomethine ylides were compatible

Figure 1. Pharmaceutical and agrochemical featuring pyrrolidine bearing one trifluoromethylated all-carbon quaternary stereocenter.

difficulties; (4) The construction of contiguous four chiral stereocenters, especially with one trifluotromethylated allcarbon quaternary stereocenter in one step remains extremely 211

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ACS Catalysis Table 1. Exploration of Azomethine Ylide Scopea

Table 2. Exploration of Dipolarophile Scopea

entry

R

(−)-3

d.r.b

yield [%]c

ee [%]

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

4-CH3OC6H4 (2a) 4-FC6H4 (2b) 4-ClC6H4 (2c) 4-BrC6H4 (2d) 4-CNC6H4 (2e) 4-CF3C6H4 (2f) 4-CH3C6H4 (2g) 4-PhC6H4 (2h) Ph (2i) 2-CH3C6H4 (2j) 3-CH3C6H4 (2k) 3-BrC6H4 (2l) 2-naphthyl (2m) 2-thienyl (2n) styryl (2o)

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

20:1 >20:1 19:1 20:1 >20:1 >20:1 17:1 18:1 18:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1

94 87 93 84 75 94 82 80 82 92 82 84 98 77 76

96 92 91 95 90 94 92 94 90 96 96 90 94 94 80

entry

R1/R2

(−)-3

d.r.b

yield [%]c

ee [%]

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

Ph/H (1a) 4-FC6H4/H (1b) 4-ClC6H4/H (1c) 4-BrC6H4/H (1d) 4-CNC6H4/H (1e) 4-NO2C6H4/H (1f) 4-CF3C6H4/H (1g) 4-CH3OC6H4/H (1h) 4-PhC6H4/H (1i) 2-BrC6H4/H (1j) 3-BrC6H4/H (1k) 2-Naphthyl/H (1l) 2-thienyl/H (1m) styryl/H (1n) H/Ph (1o)

(−)-3ad (−)-3bd (−)-3 cd (−)-3dd (−)-3ed (−)-3fd (−)-3gd (−)-3hd (−)-3id (−)-3jd (−)-3kd (−)-3ld (−)-3md (−)-3nd (−)-3oa

20:1 >20:1 >20:1 >20:1 14:1 12:1 >20:1 18:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1

84 94 96 88 71 70 81 77 94 93 87 85 93 76 44

95 91 91 90 91 94 92 92 92 99 92 92 94 94 0

a

Unless otherwise noted, all reactions were carried out with 0.2 mmol of 1a, 0.4 mmol of 2, 5 mol % of catalyst ([Cu]/M7) in 4.0 mL THF at −60 °C for 4−12 h. bThe diastereomeric ratio was determined by 1 H, 19F NMR analysis of the crude products. cIsolated yield. d(S,R)PPFA as the chiral ligand. eUse (S,S)-iPr-FOXAP as the chiral ligand.

a

Unless otherwise noted, all reactions were carried out with 0.2 mmol of 1, 0.4 mmol of 2d, 5 mol % of catalyst ([Cu] /M7) in 4.0 mL THF at −60 °C for 4−12 h. bThe diastereomeric ratio was determined by 1 H, 19F NMR analysis of the crude products. cIsolated yield. dUse (S,S)- iPr-FOXAP as the chiral ligand. eUse (S,R)-PPFA as the chiral ligand. fThe substrate 2a was used.

(Table 1, entries 1−12) and the desired products 3aa−3al were produced in 75−94% yields with 90−96% ees and up to >20:1 d.r.. Moreover, the 2-naphthyl and 2-thienyl derived azomethine ylides 2m and 2n also worked well, delivering the corresponding pyrrolidines 3am and 3an in 98% yield, 94% ee and 77% yield, 94% ee (Table 1, entries 13 and 14). Gratifyingly, the styryl-substituted 2o was also applicable for this asymmetric cycloaddition and pyrrolidine 3ao in 76% yield with >20:1 d.r. and 80% ee. Next, we examined the scope with respect to the α-CF3 α,βunsaturated ester component 1 by reaction with 2d under the optimal reaction conditions, and the representative results are shown in Table 2. Esters 1b−1k bearing either electron-rich or deficient aryl group could afford the desired products 3bd−3kd in 70−96% yields, up to >20:1 d.r., and 90−99% ees (Table 2, entries 1−11). The β-naphthyl-substituted 1l could also deliver the single diastereomer (>20:1 d.r.) 3ld in 85% yield with 92% ee (Table 2, entry 12). Moreover, the 2-thienyl-derived 1m was also applicable to this transformation, producing the corresponding product 3md with 94% ee and >20:1 d.r. value (Table 2, entry 13). Notably, the diene 1n was also comptible, and the reaction proceeded in high regio-, diastereo-, and enantioselectivity (Table 2, entry 14). Then, ethyl (E)-3-phenyl-2(trifluoromethyl)acrylate 1o was also examined, but only the racemic product 3oa was obtained in moderate yield (Table 2, entry 15). The structure and absolute configuration of the cycloadduct 3fd was determined by X-ray crystallographic analysis. This reaction is amenable to a gram-scale synthesis of the highly substituted pyrrolidines without loss of the efficiency and selectivity, as exemplified with 3ad (Scheme 2). The multifunctionality present in the product provides many opportunities for derivatization. First, the ester group at the 2-postion

Scheme 2. Synthetic Transformations of 3ada

a

Condition: (a) LiOh (2.0 equiv), THF, 1.5 h. (b) LiBH4 (2.0 equiv), PrOH, RT, 3 h. (c) DDQ (10.0 equiv), toluene, 70 °C, 5 h. (d) MCPBA (1.1 equiv), DCM, RT, 5 h. (e) MCPBA (2.5 equiv), DCM, RT, 12 h. DCM = dichloromethane, DDQ = 2,3-dichloro-5,6-dicyano1,4-benzoquinone, MCPBA = m-chloroperbenzoic acid, THF = tetrahydrofuran.

i

of 3ad was easily selectively hydrolyzed in the presence of LiOH, leading to the highly substituted proline 4 in 93% yield. The ester group at the 2-position could be selectively reduced to the corresponding alcohol 5 in 89% yield. Moreover, the oxidation of 3ad with DDQ could produce the highly 212

DOI: 10.1021/acscatal.6b03015 ACS Catal. 2017, 7, 210−214

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substituted 2-pyrroline 6 in 94% yield. Treatment of 3ad with different equivalents of m-CPBA delivered the N-hydroxyl pyrrolidine 7 and nitrone 8 in 75% and 85% yields, respectively. Of note, for all of these transformations, the chirality information is not affected at all and all enantioenriched products are obtained. In order to gain insight of the function of the N−H bond in (R, Rs)-M7, further control experiments were carried out by using the N-methylated M7 as the chiral ligand (Scheme 3).

Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junliang Zhang: 0000-0002-4636-2846 Author Contributions ‡

These authors contributed equally (B.X. and Z.-M.Z.).

Notes

The authors declare no competing financial interest.

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Scheme 3. Performance of N-Methylated (R, Rs)-N-Me-M7

ACKNOWLEDGMENTS We gratefully acknowledge the funding support of NSFC (21425205, 21672067), 973 Program (2015CB856600), and the Program of Eastern Scholar at Shanghai Institutions of Higher Learning.

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(1) For recent reviews about 1,3-dipolar cycloadditions of azomethine ylides, see: (a) Nair, V.; Suja, T. D. Tetrahedron 2007, 63, 12247−12275. (b) Stanley, L. M.; Sibi, M. P. Chem. Rev. 2008, 108, 2887−2902. (c) Alvarez-Corral, M.; Munoz-Dorado, M.; Rodrıguez-Garcıa, I. Chem. Rev. 2008, 108, 3174−3198. (d) Naodovic, M.; Yamamoto, H. Chem. Rev. 2008, 108, 3132−3148. (e) Engels, B.; Christl, M. Angew. Chem., Int. Ed. 2009, 48, 7968−7970. (f) Adrio, J.; Carretero, J. C. Chem. Commun. 2011, 47, 6784−6794. (g) Moyano, A.; Rios, R. Chem. Rev. 2011, 111, 4703−4832. (h) Albrecht, Ł.; Jiang, H.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2011, 50, 8492−8509. (i) Maroto, E. E.; Izquierdo, M.; Reboredo, S.; Marco-Martínez, J.; Filippone, S.; Martín, N. Acc. Chem. Res. 2014, 47, 2660−2670. (j) Hashimoto, T.; Maruoka, K. Chem. Rev. 2015, 115, 5366−5412. (k) Nájera, C.; Sansano, J. M. J. Organomet. Chem. 2014, 771, 78−92. (l) Kissane, M.; Maguire, A. R. Chem. Soc. Rev. 2010, 39, 845−883. (m) Adrio, J.; Carretero, J. C. Chem. Commun. 2014, 50, 12434− 12446. (2) For selected leading examples of asymmetric [3 + 2] cycloaddition reactions of azomethines, for metal-catalytic ones without formation of all-carbon quaternary stereocenter, see: (a) Longmire, J. M.; Wang, B.; Zhang, X. J. Am. Chem. Soc. 2002, 124, 13400− 13401. (b) Chen, C.; Li, X.; Schreiber, S. L. J. Am. Chem. Soc. 2003, 125, 10174−10175. (c) Knöpfel, T. F.; Aschwanden, P.; Ichikawa, T.; Watanabe, T.; Carreira, M. E. Angew. Chem., Int. Ed. 2004, 43, 5971− 5973. (d) Cabrera, S.; Arrayás, R. G.; Carretero, J. C. J. Am. Chem. Soc. 2005, 127, 16394−16395. (e) Yan, X.-X.; Peng, Q.; Zhang, Y.; Zhang, K.; Hong, W.; Hou, X.-L.; Wu, Y.-D. Angew. Chem., Int. Ed. 2006, 45, 1979−1983. (f) Zeng, W.; Chen, G.-W.; Zhou, Y.-G.; Li, Y.-X. J. Am. Chem. Soc. 2007, 129, 750−751. (g) Saito, S.; Tsubogo, T.; Kobayashi, S. J. Am. Chem. Soc. 2007, 129, 5364−5365. (h) Nájera, C.; Retamosa, M. D. G.; Sansano, J. M. Angew. Chem., Int. Ed. 2008, 47, 6055−6058. (i) Yamashita, Y.; Guo, X.-X.; Takashita, R.; Kobayashi, S. J. Am. Chem. Soc. 2010, 132, 3262−3263. (j) Kim, H. Y.; Li, J.-Y.; Kim, S.; Oh, K. J. Am. Chem. Soc. 2011, 133, 20750−20753. (k) Yamashita, Y.; Imaizumi, T.; Kobayashi, S. Angew. Chem., Int. Ed. 2011, 50, 4893−4896. (l) Potowski, M.; Schürmann, M.; Preut, H.; Antonchick, A. P.; Waldmann, H. Nat. Chem. Biol. 2012, 8, 428−430. (m) Bai, X.-F.; Song, T.; Xu, Z.; Xia, C.-G.; Huang, W.-S.; Xu, L.-W. Angew. Chem., Int. Ed. 2015, 54, 5255−5259. (n) Xu, H.; Golz, C.; Strohmann, C.; Antonchick, A. P.; Waldmann, H. Angew. Chem., Int. Ed. 2016, 55, 7761−7765. (3) For selected leading examples of asymmetric non-[3 + 2] cycloaddition of azomethine ylides, see: (a) Xue, Z.-Y.; Li, Q.-H.; Tao, H.-Y.; Wang, C.-J. J. Am. Chem. Soc. 2011, 133, 11757−11765. (b) Potowski, M.; Bauer, J. O.; Strohmann, C.; Antonchick, A. P.; Waldmann, H. Angew. Chem., Int. Ed. 2012, 51, 9512−9516. (c) He, Z.-L.; Teng, H.-L.; Wang, C.-J. Angew. Chem., Int. Ed. 2013, 52, 2934− 2938. (d) Tong, M.-C.; Chen, X.; Tao, H.-Y.; Wang, C.-J. Angew. Chem., Int. Ed. 2013, 52, 12377−12380. (e) Guo, H.; Liu, H.; Zhu, F.-

With the use of the (R, Rs)-N-Me-M7 as the chiral ligand, cycloadduct 3ad could be still furnished but with lower yield and much low enantioselectivity (Condition B) than the result from the corresponding precursor M7 with free N−H bond (Condition A). On the basis of the above result, it is not hard to conclude that the free N−H bond contribute significantly to the enhancement of the enantioselectivity and reactivity. Moreover, M7 with free N−H bond may stabilize the enol ion via hydrogen bond interaction, thereby effectively inhibiting β-F elimination. In summary, we have demonstrated for the first time that αCF3, α,β-unsaturated esters can serve as dipolarophiles in asymmetric [3 + 2]-cycloaddition reaction with azomethine ylide under the catalysis of M7/copper complexes. This method provides an efficient, reliable, and atom-economic strategy for the excellent diastereo- and enantioselectivity construction of valuable highly substituted pyrrolidines featuring one trifluoromethylated all-carbon quaternary stereocenter with contiguous other three tertiary stereocenters. Moreover, the broad substrate scope, easy scale-up, easily made ligand in large scale, and the versatile tansformations of the [3 + 2] cycloadducts make this reaction practical and highly attractive. Further studies including mechanism, synthetic application of this effcient transformation, and the employment of the chiral catalyst to other reactions are currently in progress.

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REFERENCES

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03015. Experimental procedures, spectroscopic data for the substrates and products (PDF) Accession Codes

The X-ray crystal structure information is available at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers CCDC 1500207 (−)-3fd. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif 213

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DOI: 10.1021/acscatal.6b03015 ACS Catal. 2017, 7, 210−214