Enantioselective CâH Annulation of Indoles with Diazo Compounds

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Enantioselective C-H Annulation of Indoles with Diazo Compounds through a Chiral Rh(III) Catalyst Xiaohong Chen, Shujing Yang, Helong Li, Bo Wang, and Guoyong Song ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00104 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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Enantioselective C−H Annulation of Indoles with Diazo Compounds through a Chiral Rh(III) Catalyst Xiaohong Chen, Shujing Yang, Helong Li, Bo Wang and Guoyong Song* Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China. KEYWORDS: enantioselective, C−H annulation, rhodium, indole, diazo ester

ABSTRACT: The asymmetric C–H annulation of O-pivaloyl 1-indolehydroxamic acid with donor/acceptor diazo compounds has been achieved for the first time by using a rhodium catalyst embedded in a chiral binaphthyl backbone. This protocol constitutes a straightforward route for the synthesis of a new family of 1,2-dihydro-3H-imidazo[1,5-a]indol-3-one derivatives having a quaternary carbon stereocenter in high yields and excellent enantioselectivity (up to 98:2 er).

Introduction Chiral indole moieties are among the most important heterocyclic structural motifs, existing widely in a large number of natural products, pharmaceuticals, and functional materials.1 Therefore, the development of efficient, atom-economical processes for the synthesis of enantioenriched indole derivatives through C–H functionalization of indoles has received intensive attention.2-4 Many asymmetric C–H functionalization of indoles with an electrophile through a classic EAS mechanism were reported, which often gave C3-selective indole products

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preferably because of its inherent nucleophilic property.2 However, direct asymmetric C–H functionalization of indoles at C2 position has met with limited success to date, especially in the cases of indoles that do not possess substituents at C3 position.3,4 Shibata reported Ir(I)-SDPcatalysed asymmetric C2−H bond addition of N-benzylindole to styrene, giving an alkylation product in 42% ee.4a Hartwig reported the intermolecular enantioselective C2–H addition of pyridines to bicycloalkenes in high yield with high ee values by a chiral Ir(I) catalyst.4b You and co-workers used Pd(OAc)2/Boc-Ile-OH as a chiral catalyst for C–H oxidative coupling of 5chloro-1-methyl-indole with a ferrocene compound, which yielded a C2-indole product in a 51% yield and 98% ee.4c It has been established that the use of a directing group could lead to regioselective C–H functionalization of indoles at C2 position in the presence of transition metal catalysts through a cyclometalation intermediate.5 This provides a possible access to indole derivatives in a regio- and enantio-selective version, when a chiral metal catalyst is used. Over past years, half-sandwich rhodium complexes (Rh(III)) have been recognized as competent catalysts for chelation-assisted C–H functionalization reactions.6 Enantioselective variants of these reactions became possible with the development of chiral rhodium complexes bearing C2-symmetric cyclopentadienyl ligands by Cramer7-11 or a metalloenzyme by Ward and Rovis.12 Recently, it was reported that diazo compounds could serve as one-carbon components to react with N-carboxamide indole derivatives by a half-sandwich rhodium catalyst, giving 1Himidazo[1,5-a]indol-3(2H)-one derivatives bearing a tetrasubstituted carbon.5f, 13 Given that their appearing in biologically active natural molecules and synthetic compounds,14 the ability to prepare chiral 1H-imidazo[1,5-a]indol-3(2H)-one derivatives through C2–H functionalization of indoles would be valuable. We reason that chiral half-sandwich rhodium complexes are proper catalysts for this transformation owning to a successful sample for asymmetric C–H activation of


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benzamides with diazo compounds has been established by Cramer.8d We herein report the efficient synthesis of a new family of chiral 1H-imidazo[1,5-a]indol-3(2H)-one derivatives by an intermolecular enantioselective C–H annulation of indoles with various diazo compounds catalysed by a chiral half-sandwich rhodium(III) complex. This method features high yields, excellent enantioselectivities, short reaction time and a broad substrate scope. Results and discussion First, the reaction of O-pivaloyl 1-indolehydroxamic acid (3a) with a donor/acceptor diazo ester (4a) was examined by a series of chiral half-sandwich rhodium complexes, and some representative results are summarized in Table 1. In the presence of AgSbF6 (20 mol%) and CsOAc (1 equiv) in CH3CN at 25 ºC, the rhodium(I) complex 1a (5 mol%) afforded the C–H annulation product 5a in 84% yield with an enantiomeric ratio (er) of 87:13 in 8 h (Table 1, entry 1). The C–H activation reaction occurred selectively at C2-position of indole unit rather than at C3 or C7 position.15 The activity and stereoselectivity of this reaction are little influenced by the steric hindrance of the ancillary ligands in the catalysts (see also Table 1, entries 2 and 3). When rhodium(III) complex 2, prepared from the reaction of 1a with I2 in benzene,16 was used as a catalyst, the yield of 5a increased to 94% in 3 h and the er was improved to 89:11 (Table 1, entry 4). The neutral complex 2 alone showed poor catalytic activity (Table 1, entry 5). The high ratio of AgSbF6/2 (8:1) is important to both reactivity and enantioselectivity of this reaction (Table 1, entry 6).17 These results suggested that AgSbF6 plays an important role in the present transformation.18 Further screening using different solvents gave acetone as the best choice, in which a very clean conversion of the starting materials was observed within 10 min, affording 5a in 97 % yield with an improved enantiomeric ratio value (90:10) (Table 1, entries 7–8).


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Lowering the reaction temperature to -20 oC in acetone slightly slowed the reaction (95% yield in 30 min), but increased the stereoselectivity obviously (95:5 er) (Table 1, entry 9). Table 1. Optimization of the enantioselective C−H Annulation of 3a with 4aa

time (h) temp (oC) yieldb (%)

erc

entry

[Rh] (mol %)

[Ag] (mol%)

sol

1

1a (5)

20

CH3CN

8

25

84

87:13

2

1b (5)

20

CH3CN

6

25

95

88:12

3

1c (5)

20

CH3CN

6

25

96

83:17

4

2 (2.5)

20

CH3CN

3

25

94

89:11

5

2 (2.5)

0

CH3CN

24

25

5

6

2 (2.5)

10

CH3CN

24

25

88

85:15

7

2 (2.5)

20

acetone

0.1

25

97

90:10

8

2 (2.5)

20

CH2Cl2

1

25

94

85:15

9

2 (2.5)

20

acetone

0.5

-20

95

95:5

10

2 (2.5)

20

acetone

24

-30

65

95:5

a

All reaction were carried out with 0.2 mmol of 3a and 0.3 mmol of 4a in 2 mL of solvent. b Isolated yield. c Determined by HPLC analysis on a chiral stationary phase.


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On the basis of the above screening results, the 2/AgSbF6 combination was then chosen as a catalyst to examine the reaction of various N-(pivaloyloxy)-1H-indole-1carboxamide derivatives with 4a in acetone in the presence of CsOAc. Some representative results are summarized in Scheme 1. The introduction of an electrowithdrawing or electro-donating or phenyl functional group at C5-position of the indole was tolerated, affording the corresponding annulated product 5b-5g in high yields (8095%) and high enantioselectivity (92:8 – 95:5 er). Indoles bearing a chloro or CO2Me substituent at C6-position also reacted with high levels of stereoselectivity (5h and 5i). In the case of 4-benzyloxy-substituted indole substrate, the C–H activation reaction occurred selectively at C2-position of indole skeleton, giving 5k as a major isomer (13:1) in excellent enantioselectivity (98:2 er). As a contrast, this reaction catalysed by a non-chiral catalyst [Cp*RhCl2]2 yielded a C2−H/C7−H activation product mixture with a ratio of 1.2:1 (see SI).15 The asymmetric annulation of 3-methyl- and 3-allyl-substituted indoles could also be achieved analogously in high yields (5l: 92%, 5m: 94%) and high enantioselectivity (5l: 95:5 er, 5m: 96:4 er), indicative of compatibility of the steric effect. N-(pivaloyloxy)-1H-pyrrole-1-carboxamide (3n) was also suitable substrate for the asymmetric C–H annulation with 4a, giving 5n in 75% isolated yield with good stereoselectivity (93:7 er). The absolute configuration of 5i was unequivocally determined to be 3R by X-ray crystallography with a copper radiation source (see SI).


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Scheme 1. Rhodium-catalyzed Enantioselective Annulation of Indoles with 4aa

a

Reaction conditions: 3 (0.2 mmol), 4a (0.3 mmol), catalyst 2 (2.5 mol %), AgSbF6 (20 mol %), CsOAc (0.2 mmol), acetone (2.0 mL), isolated yields. The er values were determined by HPLC analysis on a chiral stationary phase.

The reactions of O-pivaloyl 1-indolehydroxamic acid (3a) with various diazo esters were then examined (Scheme 2). The diazo ester substrates having linear, branched or arylcontaining alkyl substituents all afforded the corresponding annulation products 5o-5t in


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high yields (87-95%) and in a highly stereoselective fashion (93:7–97:3 er). Neither electron-donating (Me, 5p) nor electron-withdrawing (Cl, 5q) substituents on the aryl ring disturbed the catalyst activity or enantioselectivity. The ether (5u), ketal (5v) or ester (5w) moieties were compatible with the reaction conditions, gave desired products stereoselectively. In the case of diazo ester substrates having a smaller ester group, a drop

Scheme 2: Enantioselective annulation of different diazo esters with 3aa

a

Reaction conditions: 3a (0.2 mmol), 4 (0.3 mmol), catalyst 2 (2.5 mol %), AgSbF6 (20 mol %), CsOAc (0.2 mmol), acetone (2.0 mL), isolated yields. The er values were determined by HPLC analysis on a chiral stationary phase.


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in product yields and stereoselectivities was observed as seen in the case of tert-butyl 2diazopropanoate (5x, 64%, 86:14 er) and isopropyl 2-diazopropanoate (5y, 61% 84:16 er), that is because of the reduced size difference of the two substituents on the diazo compound.8d A competition reaction of 3b and 3e with diazo ester 4a was allowed to react under standard conditions, which yielded a mixture of 5b (Me) and 5e (Br) with a ratio of 1.5:1, suggesting C−H annulation reaction is favored for indole having electron-donating groups (eq 1). When diazo esters 4p and 4q were used in a competition reaction, the electrondonating substrate was favored 1.3:1 (eq 2), suggesting that migratory insertion or reductive elimination favors for diazo esters with electron-donating group in benzyl group. The reaction of deuterated N-(pivaloyloxy)-1H-indole-1-carboxamide (3a-D) with 4a catalysed by 2/AgSbF6 afforded 5a in 94% yield in 1 h through the C–D bond cleavage with observation of slightly drop of enantioselectivity (91:9 er) (eq 3). Treatment of 3a under the standard conditions in the absence of a diazo compound but with the addition of D2O, no deuterium incorporation was observed at C2 or C3 position (eq 4). Furthermore, the catalytic reaction of 3a and 4a in the presence of D2O led to recovered 3a with no observable deuteration. These results suggested that cyclorhodation is largely irreversible under these conditions, which is consistent with previous reports.13b The reaction of a 1:1 mixture of 3a-D and 3j with 4a yielded a mixture of C–D and C–H annulation products 5a and 5j with a KIE (kinetic isotope effect) value of 1.1 (eq 5),19 suggesting that C–H bond cleavage is fast relative to the other steps of the catalytic cycle.


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A possible reaction mechanism for the present asymmetric C–H annulation is proposed in Scheme 3. A five-membered chiral rhodium species such as A is formed by the C−H cleavage of N-(pivaloyloxy)-1H-indole-1-carboxamide (3a) at the C2 position with a rhodium species CpchiralRh(OCOR)2 which is generated through ligand exchange reactions. The diastereoselective coordination of a diazo ester (4a) to the metal center in A affords metal-carbene specie B-1 preferably to avoid steric repulsion between the large ester substituent and the bulk pivalate group.20 The subsequent migratory insertion gives


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C-1, which undergoes a C−N bond formation reaction to give D1. Protonolysis by HOCOR furnishes the final product (R)-5a with the regeneration of the catalyst.

Scheme 3. Possible Reaction Pathway

Conclusions In summary, we have developed the enantioselective C−H annulation of O-pivaloyl 1indolehydroxamic acid with diazo ester compounds by using a chiral cyclopentadienyl rhodium catalyst, leading to formation of a variety of enantioenriched 1,2-dihydro-3Himidazo[1,5-a]indol-3-one derivatives bearing a tetrasubstituted stereogenic center. This


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protocol features high yield, excellent stereoselectivity, and good functional group compatibility. Further investigations into the mechanism and the development of more efficient catalytic systems are currently under investigation in our laboratory.

ASSOCIATED CONTENT Supporting Information. Experimental procedures, product characterization data (PDF) and X-ray crystallographic data of 5i (CIF). AUTHOR INFORMATION Corresponding Author *Email for G. S.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Program for Thousand Young Talents of China and Research Foundation for Talented Scholars from Beijing Forestry University. REFERENCES (1) For reviews, see: (a) Sundberg, R. J. Indoles, Academic Press: San Diego, 1996; (b) Humphrey, G. R.; Kuethe, J. T. Chem Rev. 2006, 106, 2875; (c) Somei, M.; Yamada, F. Nat. Prod. Rep. 2005, 22, 73.


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(2) For selected reviews, see: (a) Lancianesi, S.; Palmieri. A.; Petrini, M. Chem. Rev. 2014, 114, 7108; (b) You, S.-L.; Cai, Q.; Zeng, M. Chem. Soc. Rev. 2009, 38, 2190; (c) Bandini, M.; Eichholzer, A. Angew. Chem., Int. Ed. 2009, 48, 9608; (d) Poulsen, T. B.; Jørgensen, K. A. Chem Rev. 2008, 108, 2903. (3) For selected examples of asymmetric C2−H functionalization of 3-substituted indoles, see: (a) Cheng, H.-G.; Lu, L.-Q.; Wang, T.; Yang, Q.-Q.; Liu, X.-P.; Li, Y.; Deng, Q.-H.; Chen, J.R.;. Xiao, W.-J. Angew. Chem., Int. Ed. 2013, 52, 3250; (b) Zhang, Y.; Liu, X.; Zhao, X.; Zhang, J.; Zhou, L.; Lin, L.; Feng, X. Chem. Commun. 2013, 11311; (c) Tan, W.; Li, X.; Gong, Y.-X.; Ge, M.-D.; Shi, F. Chem. Commun. 2014, 15901; (d) Shi, F.; Zhu, R.-Y.; Dai, W.; Wang, C.-S.; Tu, S.-J. Chem. Eur. J. 2014, 20, 2597; (e) Seayad, J.; Seayad, A. M.; List, B. J. Am. Chem. Soc. 2006, 128, 1086; For an intramolecular example, see: (f) Wilson, R. M.; Thalji, R. K.; Bergman, R. G.; Ellman, J. A. Org. Lett. 2006, 8, 1745. (4) (a) Pan, S.; Ryu, N.; Shibata, T. J. Am. Chem. Soc. 2012, 134, 17474; (b) Sevov, C. S.; Hartwig, J. F. J. Am. Chem. Soc. 2013, 135, 2116; (c) Gao, D.-W.; Gu, Q.; You, S.-L. J. Am. Chem. Soc. 2016, 138, 2544. (5) For selected examples of direct C2−H functionalization of indoles, see: (a) Potavathri, S.; Pereira, K. C.; Goresky, S. I.; Pike, A.; LeBris, A. P.; DeBoef, B. J. Am. Chem. Soc. 2010, 132, 14676; (b) Schipper, D. J.; Hutchinson, M.; Fagnou, K. J. Am. Chem. Soc. 2010, 132, 6910; (c) Truong, T.; Daugulis, O. J. Am. Chem. Soc. 2011, 133, 4243; (d) Ding, Z.; Yoshikai, N. Angew. Chem. Int. Ed. 2012, 51, 4698; (e) Ikemoto, H.; Yoshino, T.; Sakata, K.; Matsunaga, S.; Kanai, M. J. Am. Chem. Soc. 2014, 136, 5424; (f) Zhang, Y.; Zheng, J.; Cui, S. J. Org. Chem. 2014, 79, 6490; (g) Liang, L.; Fu, S.; Lin, D.; Zhang, X.-Q.; Deng, Y.; Jiang, H.; Zeng, W. J. Org. Chem.


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2014, 79, 9472; (h) Chen, X.; Hu, X.; Bai, S.; Deng, Y.; Jiang, H.; Zeng, W. Org. Lett. 2016, 18, 192. For a review, see: (i) Cacchi, S.; Fabrizi, G. Chem. Rev. 2011, 111, PR215. (6) (a) For reviews on Rh(III)-catalyzed C−H functionalization, see: (a) Satoh, T.; Miura, M. Chem. Eur. J. 2010, 16, 11212; (b) Patureau, F. W.; Wencel-Delord, J.; Glorius, F. Aldrichimica Acta. 2012, 45, 31; (c) Song, G.; Wang, F.; Li, X. Chem. Soc. Rev. 2012, 41, 3651; (d) Song, G.; Li, X. Acc. Chem. Res. 2015, 48, 1007. (7) For reviews, see: (a) Newton, C. G.; Kossler, D.; Cramer, N. J. Am. Chem. Soc. 2016, 138, 3935; (b) Ye, B.; Cramer, N. Acc. Chem. Res. 2015, 48, 1308. (8) For selected examples on enantioselective Rh(III)-catalysed C−H functionalization, see: (a) Ye, B.; Cramer, N. Science, 2012, 338, 504; (b) Ye, B.; Cramer, N. J. Am. Chem. Soc. 2013, 135, 636; (c) Ye, B.; Donets, P. A.; Cramer, N. Angew. Chem., Int. Ed. 2014, 53, 507; (d) Ye; B.; Cramer, N. Angew. Chem., Int. Ed. 2014, 53, 7896; (e) Zheng, J.; You, S.-L. Angew. Chem., Int. Ed. 2014, 53, 13244; (f) Zheng, J.; Wang, S.-B.; Zheng, C.; You, S.-L. J. Am. Chem. Soc. 2015, 137, 4880; (g) Chidipudi, S. R.; Burns, D. J.; Khan, I.; Lam, H. W. Angew. Chem., Int. Ed. 2015, 54, 13975; (h) Pham, M. V.; Cramer, N. Chem. Eur. J. 2016, 22, 2270; (i) Zheng, J.; Cui, W.-J.; Zheng, C.; You, S.-L. J. Am. Chem. Soc. 2016, 138, 5242. (9) Kossler, D.; Cramer, N. J. Am. Chem. Soc. 2015, 137, 12478. (10) Dieckmann, M.; Jang, Y.-S.; Cramer, N. Angew. Chem., Int. Ed. 2015, 54, 12149. (11) (a) Song, G.; O, W. W. N.; Hou, Z. J. Am. Chem. Soc. 2014, 136, 12209; (b) Teng, H.-L.; Luo, Y.; Wang, B.; Zhang, L.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2016, 55, 15406. (12) Hyster, T. K.; Knörr, L.; Ward, T. R.; Rovis, T. Science 2012, 338, 500. (13) For examples of Rh(III)-catalyzed C−H functionalization with diazo compounds, see: (a) Chan, W.-W.; Lo, S.-F.; Zhou, Z.; Yu, W.-Y.; J. Am. Chem. Soc. 2012, 134, 13565; (b) Hyster,


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T. K.; Ruhl, K. E.; Rovis, T. J. Am. Chem. Soc. 2013, 135, 5364; (c) Hu, F.; Xia, Y.; Ye, F.; Liu, Z.; Ma, C.; Zhang, Y.; Wang, J. Angew. Chem. Int. Ed. 2014, 53, 1364; (d) Yu, S.; Liu, S.; Lan, Y.; Wan. B.; Li, X. J. Am. Chem. Soc. 2015, 137, 1623; (e) Cui, S.; Zhang, Y.; Wang, D.; Wu, Q. Chem. Sci. 2013, 4, 3912. See also ref 5h. For an asymmetric example, see ref 8d. (14) (a) Wright, Jr., W. B.; Brabander, H. J. J. Med. Chem. 1968, 11, 1164; (b) Varasi, M.; Heidempergher, F.; Caccia, C.; Salvati, P. US Pat. 5637593, 1997; (c) Lennox, W.; Qi, H.; Lee, D.-H.; Choi, S.; Moon, Y.-C. US Pat. 9271960 B2, 2016. (15) The C−H functionalization could occur at C7-position through a six-membered cyclorhodation intermediate, especially in the case of introduction of an electro-donating group such as BnO at C4 position. (16) During the preparation of this manuscript, the synthesis of complex 2 was reported, see: Potter, T. J.; Kamber, D. N.; Mercado, B. Q.; Ellman, J. A. ACS Catal. 2017, 7, 150. (17) For examples on AgSbF6-increased the rate of Rh(III)-catalyzed C−H functionalization reactions, see: (a) Li, X.; Yu, S.; Wang, F.; Wan, B.; Yu, X. Angew. Chem. Int. Ed. 2013, 52, 2577; (b) Tauchert, M. E.; Incarvito, C. D.; Rheingold, A. L.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2012, 134, 1482. (18) To understand the role of AgSbF6 in present reaction, several catalytic reactions between 3a and 4a were carried out. A poor conversion was observed in the presence NaI (20 mol%), suggesting that I- inhibits the reaction possible through the formation of Rh-I bond. The kinetic reactions catalyzed by [Cp*RhCl2]2 and [Cp*RhCl2]2/AgSbF6 gave similar reaction rates at room temperature in CH3CN, suggesting that SbF6- may be not involved in the catalytic cycle (see SI).


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(19) A competition reaction carried out by 3a and 3j (equimolar amounts) with 4a led to a mixture of 5a and 5j with a 1:1 ratio, suggesting that the rate of the reaction do not be influenced by the 4-methyl group on the indole unit (see SI). (20) Gladysz, J. A.; Boone, B. J. Angew. Chem., Int. Ed. 1997, 36, 550.


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Enantioselective CâH Annulation of Indoles with Diazo Compounds

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