Palladium-catalyzed intermolecular C-2 alkenylation of indoles using oxygen as the oxidant

Article abstract of DOI:10.1002/adsc.201300811

A general and efficient palladium-catalyzed intermolecular direct C-2 alkenylation of indoles using oxygen as the oxidant has been developed. The reaction is of complete regio- and stereoselectivity. All products are E-isomers at the C-2-position with no Z-isomers and 3-substituted products were detected.

Full text of DOI:10.1002/adsc.201300811

UPDATES
DOI: 10.1002/adsc.201300811
Palladium-Catalyzed Intermolecular C-2 Alkenylation of Indoles
Using Oxygen as the Oxidant
Zhao-Lei Yan,a Wen-Liang Chen,a Ya-Ru Gao,a Shuai Mao,a Yan-Lei Zhang,a
and Yong-Qiang Wanga,*
a
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of
Chemistry & Materials Science, Northwest University, Xi’an 710069, Peopleꢀs Republic of China
E-mail: wangyq@nwu.edu.cn
Received: September 6, 2013; Revised: December 1, 2013; Published online: March 17, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300811.
Abstract: A general and efficient palladium-cata-
lyzed intermolecular direct C-2 alkenylation of in-
doles using oxygen as the oxidant has been devel-
oped. The reaction is of complete regio- and stereo-
selectivity. All products are E-isomers at the C-2-
position with no Z-isomers and 3-substituted prod-
ucts were detected.
chiometric amount of CuACTHUNGETRNNUG
(OAc)2 as the oxidant.[7] In
2008, Miura, Satoh et al. reported the palladium(II)-
catalyzed C-2 vinylation of indole-3-carboxylic acids
using CuACTHUNGTRNEUNG(OAc)2·H2O (2 equiv.) as the oxidant, in
which the carboxyl group blocks the C-3 position and
acts as a removable directing group.[8] In 2009,
Arrayꢁs, Carretero et al. disclosed an elegant palla-
dium(II)-catalyzed regioselective direct C-2 alkenyla-
tion of indoles assisted by the removable N-(2-pyri-
À
Keywords: C-2 alkenylation; C H activation; in-
dyl)sulfonyl directing group using CuACHTUNRGTNEUNG(OAc)2·H2O (1–
doles; oxygen oxidation; palladium-catalyzed reac-
tion
2 equiv.) as the oxidant; nevertheless the excess of al-
kenes (2–5 equiv.) was employed.[9] More recently,
Prabhu et al. developed a ruthenium(II)-catalyzed
C-2
alkenylation
of
N-benzoylindoles
using
Cu(OAc)2·H2O (1 equiv.) as oxidant and AgSbF6
AHCTUNGTRENNUNG
The indole ring is a widespread structural unit in (20 mol%) as an activator.[10] In spite of these impor-
pharmaceuticals, agrochemicals, and functional mate- tant advances, some challenging issues still remain:
rials.[1] The development of efficient strategies for the for example, (i) a large excess of oxidants was used to
functionalization of the indole nucleus has been regenerate the catalyst;[8,9] (ii) stoichiometric amounts
ACTHNUTRGNEUNG
a long-standing topic in organic synthesis.[2] Among of the reduced external oxidant [such as Cu(OAc)2,[7,9]
them the metal-catalyzed alkenylation reaction is t-BuOOBz[6]] were produced as waste; and (iii) sub-
a very appealing approach for the direct functionali- strate scope was limited and some cases poor yields
zation of indoles. Because of the higher nucleophilic were obtained.[6] Herein, we describe a general, effi-
character of the C-3 position compared with the C-2 cient and structurally versatile palladium(II)-cata-
position in indole, the C-3-position of indole is the in- lyzed intermolecular C-2 alkenylation of indoles em-
herently more reactive site.[3,4] Therefore, the C-2 al- ploying the easily installed and removed N-(2-pyri-
kenylation of C-2/C-3-unsubstituted indoles is a chal- dyl)sulfonyl directing group, characterized by oxygen
lenge. To the best of our knowledge, only a few proto- as oxidizing agent, with complete regio- and stereose-
À
cols have been reported so far for the direct C H al- lectivity. Compared with CuACTHNUTRGEN(UNG OAc)2 and t-BuOOBz,
kenylation with alkenes at the C-2 position of indoles, oxygen is an ideal oxidant and offers attractive indus-
despite the potential utility of such products.[5] In trial prospects in terms of green and sustainable
2005, Gaunt et al. realized a palladium(II)-catalyzed chemistry while no reduced waste is produced.[11]
direct and solvent-controlled regioselective C-2 alke-
An effective strategy for regioselective C-2 alkeny-
nylation of indoles using tert-butyl benzoyl peroxide lation of indoles is to utilize the coordination of a di-
(t-BuOOBz, 0.9 equiv.) as the oxidant.[6] However, the recting group in the indole substrate to the metal
yields were low to moderate (ꢀ57%). In the same center of a catalyst, and the approach probably in-
year, Ricci et al. described a palladium(II)-catalyzed volves a five- or six-membered metal-cyclic intermedi-
regiocontrolled C-2 alkenylation of indole directed by ate to activate the C-2 position of indole.[4a,7,9b,10]
a non-removable N-2-pyridylmethyl group using a stoi- Therefore, our study commenced by examining the
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Table 1. Optimization of reaction conditions.[a]
Entry
R
Catalyst
Solvent
Time [h]
Conversion [%][b]
1
2
3
4
5
6
7
8
Boc- (1a)
Ts- (1b)
(2-pyridyl)SO2- (1c)
Ac- (1d)
Bz- (1e)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
(2-pyridyl)SO2- (1c)
Pd
Pd
Pd
Pd
Pd
PdCl2
Pd(TFA)2
Pd(OH)2
N
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMF
Anisole
AcOH
solvents[c]
AcOH
AcOH
24
24
24
24
24
24
24
24
24
24
24
24
24
24
4
–
<10
35 (30)
<10
<10
<10
80 (55)
–
–
–
62 (50)
92 (65)
100 (75)
<20
100 (73)
100 (93)
9
Pd
–
10
11
12
13
14
15[d]
16[d,e]
Pd
Pd
Pd
Pd
Pd
Pd
4
[a]
Reaction conditions: 1 (1 mmol), 2a (1.5 mmol), catalyst (0.1 mmol), O2 (1 atm) and TFA (8 mmol) in solvent (5 mL) at
608C.
[b]
[c]
[d]
[e]
The results in parentheses are isolated yields.
Solvents: 1,4-dioxane, toluene, DMSO, MeOH, CH3CN, THF, pyridine, CH3NO2.
At 808C.
TFA (2 mmol) was used.
indole N-protecting groups which have potential di- material and product (entry 15). The amount of TFA
recting feature for the C-2 functionalization of indole dramatically affected the yield, 2 equiv. of TFA were
instead of the more nucleophilic C-3 position. A set the optimal amount to afford the desired product in
of protecting groups was tested for the reaction of 93% yield (entry16, for details see the Supporting In-
indole derivatives (1) and ethyl acrylate (2a) with formation). Accordingly, the reaction conditions were
10 mol% PdACHTUNGTRENNUNG(OAc)2 as the catalyst, oxygen as the oxi- optimized as follows: PdAHCUTNGTREN(NUNG TFA)2 (10% mol), TFA
dant and trifluoroacetic acid (TFA) as additive in (2 equiv.) under oxygen atmosphere in AcOH at
N,N-dimethylacetamide (DMA) (Table 1). The reac- 808C.
tion was found to proceed in low conversion (<10%)
After identifying the optimized conditions, we
when the N-protecting group was Boc, Ts, Ac or Bz moved on to explore the scope of the C-2 alkenyla-
(entries 1, 2, 4, 5). The N-(2-pyridyl)sulfonyl group, tion reaction. First, we studied the effect of electronic
which can be easily removed by mild reduction condi- and structural variations on the alkene (Table 2). The
tions,[9a] turned out to be the best protecting group to present reaction tolerated a variety of alkenes. Mono-
give 35% conversion (entry 3). To our delight, N-(2- substituted alkenes, not only electrophilic alkenes but
pyridyl)sulfonylindole (1c) provided complete C-2 re- also the more challenging non-activated styrene, re-
gioselectivity and E stereoselectivity. Then the palla- acted with 1c to give the corresponding C-2 alkenyla-
dium source was investigated (entries 6–9), and the tion products with complete regio- and stereoselectiv-
conversion improved remarkably to 80% when ity in good to excellent yields (65–98%) (entries 1–7).
PdACHTUNGTRENNUNG(TFA)2 was used. In the absence of a palladium Indole phosphonate 3ch could also be generated from
catalyst, the reaction would not occur (entry 10). vinyl phosphonate 2h in excellent yield (entry 8).
After careful solvent screening, acetic acid (AcOH) Pleasingly, 1,1-disubstituted alkenes successfully cou-
proved to be the best solvent to give complete conver- pled with indole at the C-2 position to give the corre-
sion and good yield (entries 11–14). Increasing the sponding double-bond isomerized products in high
temperature could greatly improve the reaction rate; yields (entries 9, 10). Particularly noticeable is the
the reaction was done within 4 h at 808C with a similar performance of 1,2-disubstituted alkenes in the reac-
yield, and a higher temperature led to a significant tion in view of the small number of precedents and
drop in the yield due to decomposition of the starting lower reactivity of this kind of olefin in oxidative al-
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Palladium-Catalyzed Intermolecular C-2 Alkenylation of Indoles
Table 2. Alkenylation of 1c with various alkenes.[a]
Entry
1
Alkene
Product
Time [h]
4
Yield [%][b]
93
2
3
4
98
83
24
4
5
6
6
95
65
97
24
8
7
18
12
2.5
90
94
92
8
9[c]
10[c]
11
4
97
24
63[d]
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UPDATES
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Table 2. (Continued)
Entry
Alkene
Product
Time [h]
6
Yield [%][b]
12
97
13[c]
24
75
[a]
Reaction conditions: 1c (1 mmol), 2 (1.5 mmol), PdACHTNUTRGNE(UNG TFA)2 (0.1 mmol), O2 (1 atm) and TFA (2 mmol) in AcOH (5 mL) at
808C.
Isolated yields.
Double bond isomer of the alkenylation product.
After hydrogenation.
[b]
[c]
[d]
Figure 1. X-ray crystal structures of compounds 3cf, 3cl and 3cm.
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Palladium-Catalyzed Intermolecular C-2 Alkenylation of Indoles
Table 3. The C-2 H-alkenylation for structural variation of the indole with ethyl acrylate and styrol.[a]
Entry
Alkene
Product
Time [h]
4
Yield [%][b]
96
1
15
12
15
8
73
95
2
3
4
5
91
87
8
93
24
24
18
24
40 (96%)[c]
56 (93%)[c]
93
78
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Zhao-Lei Yan et al.
Table 3. (Continued)
Entry
Alkene
Product
Time [h]
24
Yield [%][b]
87
6
18
89
[a]
Reaction conditions: 1c (1 mmol), 2 (1.5 mmol), PdACHTNUTRGNE(UNG TFA)2 (0.1 mmol), O2 (1 atm) and TFA (2 mmol) in AcOH (5 mL) at
808C.
Isolated yields.
Yield in parentheses are based on recovered indole 1i.
[b]
[c]
kenylation (Fujiwara–Moritani) reactions.[12] Under dyl)sulfonyl group was not only a readily removable
these reaction conditions, cyclohexene underwent directing group but also an activating group in the C-
a smooth reaction with 1c to afford the corresponding 2 alkenylation reaction of indole.
trisubstituted alkene products 3ck (63% yield, after
Removal of the 2-pyridysulfonyl group from the al-
hydrogenation) (entry 11). It is interesting that di- kenylation products was easily achieved by reduction
methyl maleate which has the Z-configration reacted with zinc in NH4Cl (aq)/THF (1:1) at room tempera-
with 1c to afford a single E- configration diastereomer ture, with the stereochemistry of the olefin moiety un-
3cl (93% yield) (entry 12). Butadiene sulfone, as touched. For instance, the products 3ca and 3hf were
a solid source of butadiene, was also compatible with converted to their free indole derivatives 4ca and 4hf
the reaction to provide the double-bond isomerized under the conditions in 92% and 85% yields, respec-
product 3cm in 75% yield (entry 13). The molecular tively (Scheme 2).
structures of compound 3cf, 3cl and 3cm were con-
In summary, we have developed a general, simple
firmed
by
X-ray
crystallographic
analysis and efficient method for the intermolecular direct C-2
alkenylation of indoles using palladium(II) as catalyst
(Figure 1).[13]
Next, we explored the reaction of various indole and oxygen as the oxidant. The reaction not only can
derivatives with ethyl acrylate and styrol (Table 3). proceed well without Cu(II), but shows complete
Indoles possessing either electron-withdrawing regio- and stereoselectivity. All products are E-iso-
groups or electron-donating groups smoothly reacted mers at the C-2 position, and no Z-isomers and 3-sub-
and provided C-2 alkenylation products 3 in good to stituted products can be detected on analyzing the re-
excellent yields (73–96%), albeit 5-bromoindole (1i) action mixtures. The method should have many appli-
proved to be less reactive than other substituted in- cations in organic and medical chemistry. Detailed
doles (entry 4). Many functional substituents such as
F, Cl, Br and MeO were compatible under these con-
ditions, which could be further transformed into other
functionalities.
As shown in Scheme 1, blocking the C-2 position of
indole resulted in the formation of the C-3 alkenyla-
tion product. Compared with C-2 alkenylation of N-
(2-pyridyl)sulfonyl-3-methylindole (1f), C-3 alkenyla-
tion of N-(2-pyridyl)sulfonyl-2-methylindole (1l) was
much slower. This case indicated that the N-(2-pyri- indoles.
Scheme 2. Deprotection of 2-alkenyl-N-(2-pyridyl)sulfonyl-
Scheme 1. The C-3 alkenylation of 2-substituted indoles.
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UPDATES
Palladium-Catalyzed Intermolecular C-2 Alkenylation of Indoles
[2] a) M. Bandini, A. Eichholzer, Angew. Chem. 2009, 121,
9786–9824; Angew. Chem. Int. Ed. 2009, 48, 9608–9644;
b) R. Rossi, F. Bellina, M. Lessi, Synthesis 2010, 4131–
4153.
mechanistic investigations and the development of
a more efficient catalytic system are currently under-
way.
[3] a) Y. Fujiwara, O. Maruyama, M. Yoshidomi, H. Tani-
guchi, J. Org. Chem. 1981, 46, 851–855; b) C.-G. Jia, W.
Lu, T. Kitamura, Y. Fujiwara, Org. Lett. 1999, 1, 2097–
2100; c) C. Liu, X.-Q. Han, X. Wang, R. A. Widenhoe-
fer, J. Am. Chem. Soc. 2004, 126, 3700–3701; d) E.
Capito, J. M. Brown, A. Ricci, Chem. Commun. 2005,
1854–1856; e) H.-F. Yu, Z.-K. Yu, Angew. Chem. 2009,
121, 2973–2977; Angew. Chem. Int. Ed. 2009, 48, 2929–
2933; f) S. Mochida, K. Hirano, T. Satoh, M. Miura, J.
Org. Chem. 2011, 76, 3024–3033; g) W.-L. Chen, Y.-R.
Gao, S. Mao, Y.-L. Zhang, Y.-F. Wang, Y.-Q. Wang,
Org. Lett. 2012, 14, 5920–5923.
Experimental Section
General Information
All commercial reagents and solvents were used as received
without further purification. Flash chromatographies were
carried out on silica gel 200–300 mesh. All NMR spectra
were obtained at ambient temperature using a Varian
INOVA-400 MHz spectrometer. The following abbreviations
are used to show the multiplicities: s=singlet, d=doublet,
t=triplet, q=quartet, m=multiplet. All new products were
further characterized on Bruker EQUINOX55 instrument
and AXIMA-CFRꢃplusMALDI-TOF mass spectrometers.
In addition, X-ray crystal structure analyses were measured
on a Bruker Smart APEXIICCD instrument using Mo-Ka
radiation.
[4] For C-3 alkenylation of indoles with the directing
group: a) A. Maehara, H. Tsurugi, T. Satoh, M. Miura,
Org. Lett. 2008, 10, 1159–1162; b) F. Wang, G.-Y. Song,
X.-W. Li, Org. Lett. 2010, 12, 5430–5433; c) T. Ueyama,
S. Mochida, T. Fukutani, K. Hirano, T. Satoh, M.
Miura, Org. Lett. 2011, 13, 706–708; d) B. Li, J.-F. Ma,
N.-C. Wang, H.-L. Feng, S.-S. Xu, B. Wang, Org. Lett.
2012, 14, 736–739; e) L. Ackermann, L.-H. Wang, R.
Wolfram, A. V. Lygin, Org. Lett. 2012, 14, 728–731.
[5] a) M. Tao, C. H. Park, R. Bihovsky, G. J. Wells, J.
Husten, M. A. Ator, R. L. Hudkins, Bioorg. Med.
Chem. Lett. 2006, 16, 938–942; b) B. D. Palmer, A. M.
Thompson, R. J. Booth, E. M. Dobrusin, A. J. Kraker,
H. H. Lee, E. A. Lunney, L. H. Mitchell, D. F. Ortwine,
J. B. Smaill, L. M. Swan, W. A. Denny, J. Med. Chem.
2006, 49, 4896–4911; c) T. J. Opperman, J. D. Williams,
C. Houseweart, R. G. Panchal, S. Bavari, N. P. Peet,
D. T. Moir, T. L. Bowlin, Bioorg. Med. Chem. 2010, 18,
2123–2130; d) T. Mendgen, C. Steuer, C. D. Klein, J.
Med. Chem. 2012, 55, 743–753; e) C. G. Fortuna, V.
Barresi, C. Bonaccorso, G. Consiglio, S. Failla, A. Tro-
vato-Salinaro, G. Musumarra, Eur. J. Med. Chem. 2012,
47, 221–227; f) T. T. B. Nguyen, T. Lomberget, N. C.
Tran, E. Colomb, L. Nachtergaele, S. Thoret, J. Dubois,
J. Guillaume, R. Abdayem, M. Haftek, R. Barret,
Bioorg. Med. Chem. Lett. 2012, 22, 7227–7231; g) S.
Barzen, C. B. Rçdl, A. Lill, D. Steinhilber, H. Stark, B.
Hofmann, Bioorg. Med. Chem. 2012, 20, 3575–3583;
h) R. G. Panchal, D. Lane, H. I. Boshoff, M. M. Butler,
D. T. Moir, T. L. Bowlin, S. Bavari, J. Antibiot. 2013,
66, 47–49. i) For a selected example of directed C-2 al-
kenylation of indoles with alkynes, see: Z. Ding, N.
Yoshikai, Angew. Chem. 2012, 124, 4776–4779; Angew.
Chem. Int. Ed. 2012, 51, 4698–4701.
General Procedure for Palladium-Catalyzed Inter-
molecular C-2 Alkenylation of Indoles Using Oxygen
as the Oxidant
A sealed tube containing the N-(2-pyridyl)sulfonylindole de-
rivative 1c (0.39 mmol), PdACHTNUTRGENNG(U TFA)2 (10 mol%), was evacuat-
ed and filled with dioxygen gas using an oxygen containing
balloon. Then, AcOH (2.0 mL), alkene 2 (0.58 mmol) and
trifluoroacetic acid (TFA) (0.78 mmol) were sequentially
added to the system via syringe under an oxygen atmos-
phere. The mixture was heated to 808C for 8–24 h (as indi-
cated in each case). Then the reaction mixture was cooled
to room temperature, diluted with EtOAc (30 mL) and
washed with saturated aqueous NaHCO3 (3ꢄ5 mL). The
combined organic phase was dried (Na2SO4) and concentrat-
ed under reduced pressure. Purification by flash chromatog-
raphy afforded the C.2 alkenylated indole derivative 3.
Acknowledgements
This research was supported by National Natural Science
Foundation of China (NSFC-20972126, 21272185), the Pro-
gram for New Century Excellent Talents in University of the
Ministry of Education, China (NCET-10-0937), and Educa-
tion Department of Shaanxi Provincial Government
(09JK776).
[6] N. P. Grimster, C. Gauntlett, C. R. A. Godfrey, M. J.
Gaunt, Angew. Chem. 2005, 117, 3185–3189; Angew.
Chem. Int. Ed. 2005, 44, 3125–3129.
[7] E. Capito, J. M. Brown, A. Ricci, Chem. Commun.
References
2005, 1854–1856.
[8] A. Maehara, H. Tsurugi, T. Satoh, M. Miura, Org. Lett.
2008, 10, 1159–1162.
[1] a) R. J. Sundberg, Indoles, Academic, New York, 1996;
b) M. Somei, F. Yamada, Nat. Prod. Rep. 2005, 22, 73–
103; c) R. C. Yanagita, Y. Nakagawa, N. Yamanaka, K.
Kashiwagi, N. Saito, K. Irie, J. Med. Chem. 2008, 51,
[9] a) A. Garcꢅa-Rubia, R. G. Arrayꢁs, J. C. Carretero,
Angew. Chem. 2009, 121, 6633–6637; Angew. Chem. Int.
Ed. 2009, 48, 6511–6515; b) A. Garcꢅa-Rubia, B.
Urones, R. G. Arrayꢁs, J. C. Carretero, Chem. Eur. J.
2010, 16, 9676–9685.
46–56;
d) A. J.
Kochanowska-Karamyan,
M. T.
Hamann, Chem. Rev. 2010, 110, 4489–4497; e) M. Shiri,
Chem. Rev. 2012, 112, 3508–3549.
Adv. Synth. Catal. 2014, 356, 1085 – 1092
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1091

UPDATES
Zhao-Lei Yan et al.
[10] V. Lanke, K. R. Prabhu, Org. Lett. 2013, 15, 2818–2821.
[11] a) Z.-Z. Shi, C. Zhang, C.-H. Tang, N. Jiao, Chem. Soc.
Rev. 2012, 41, 3381–3430; b) A. N. Campbell, S. S.
Stahl, Acc. Chem. Res. 2012, 45, 851–863.
[12] a) E. M. Beck, N. P. Grimster, R. Hatley, M. J. Gaunt,
J. Am. Chem. Soc. 2006, 128, 2528–2529; b) E. M.
Beck, R. Hatley, M. J. Gaunt, Angew. Chem. 2008, 120,
3046–3049; Angew. Chem. Int. Ed. 2008, 47, 3004–3007;
c) Y.-H. Zhang, B.-F. Shi, J.-Q. Yu, J. Am. Chem. Soc.
2009, 131, 5072–5074; d) Y. Lu, D.-H. Wang, K. M.
Engle, J.-Q. Yu, J. Am. Chem. Soc. 2010, 132, 5916–
5921.
[13] CCDC 958552 (3cf), CCDC 958553 (3cl), and CCDC
958554 (3cm) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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