One-pot gold-catalyzed aminofluorination of unprotected 2-alkynylanilines

Article abstract of DOI:10.1021/ol401098b

A tandem gold(I)-catalyzed aminocylization/fluorination and a two-step, one-pot gold(III)-catalyzed cyclization/electrophilic fluorination provide a convenient and general method for the synthesis of 3,3-difluoro-2-substituted- 3H-indoles in good yield un

Full text of DOI:10.1021/ol401098b

ORGANIC
LETTERS
2013
Vol. 15, No. 11
2766–2769
One-Pot Gold-Catalyzed
Aminofluorination of Unprotected
2‑Alkynylanilines
Antonio Arcadi,*,† Emanuela Pietropaolo,† Antonello Alvino,‡ and
,§
ꢀ
Veronique Michelet*
ꢁ
Dipartimento di Scienze Fisiche e Chimiche, Universita di L’Aquila, Via Vetoio-67010
Capitol(AQ), Italy, Dipartimento di Chimica, Sapienza, Universita di Roma,
ꢁ
P. le A. Moro 5, Roma, Italy, and Chimie ParisTech, Laboratoire Charles Friedel,
UMR 7223, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France
arcadi@univaq.it; veronique-michelet@chimie-paristech.fr
Received April 20, 2013
ABSTRACT
A tandem gold(I)-catalyzed aminocylization/fluorination and a two-step, one-pot gold(III)-catalyzed cyclization/electrophilic fluorination provide a convenient
and general method for the synthesis of 3,3-difluoro-2-substituted-3H-indoles in good yield under mild conditions. Extension of the procedure to the synthesis
of 2-aryl-3-fluoro-1H-indoles is described. The reaction proceeds smoothly in green ethanol and does not require any base, acid, or N-protective group.
Introducing fluorine into molecules represents a major
challenge in organic synthesis.1 The substitution of hydro-
gen with fluorine can lead to a dramatic impact in the
physicochemical and biological properties of organic
compounds, and utilization of fluorine derivatives spans
areas as diverse as pharmaceuticals, agrochemicals, and
polymers.2 In particular, strategic fluorination of hetero-
cyclic compounds has become an ever more important
weapon in the armamentarium of medicinal chemistry.3
Modification of the indole structure continues to attract
great interest from organic chemists reflecting its impor-
tance as an ubiquitous skeleton of pharmaceuticals and
bioactive natural products.4 Several studies have been
dedicated to the electrophilic substitution in the indole
core with fluorinating agents, and the methodologies
available allow the introduction of ꢀF to positions 2 or 3
(starting from corresponding trimethylstannyl indoles)5 to
positions 4,6 7,7 and to various positions of the benzenic
€
(4) (a) Knepper, K.; Vanderheiden, S.; Brase, S. Beilstein J. Org. Chem.
†
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Universita di L'Aquila.
2012, 8, 1191–1199. (b) Shiri, M. Chem. Rev. 2012, 112, 3508–3549. (c)
Karchava, A. V.; Melkonyan, F. S.; Yurovskaya, M. A. Chem. Heterocycl.
Compd. 2012, 48, 391–407. (d) Platon, M.; Amardeil, R.; Djakovitch, L.;
Hierso, J. C. Chem. Soc. Rev. 2012, 41, 3929–3968. (e) Vicente, R. Org.
Biomol. Chem. 2011, 9, 6469–6480. (f) Cacchi, S.; Fabrizi, G. Chem. Rev.
2011, 111, 215–283. (g) Palmisano, G.; Penoni, A.; Sisti, M.; Tibiletti, F.;
Tollari, S.; Nicholas, K. M. Curr. Org. Chem. 2010, 14, 2409–2441. (h)
Bartoli, G.; Bencivenni, G.; Dalpozzo, R. Chem. Soc. Rev. 2010, 39, 4449–
4465. (i) Zeng, M.; You, S.-L. Synlett 2010, 9, 1289–1301. (j) Bandini, M.;
Eichholzer, A. Angew. Chem., Int. Ed. 2009, 48, 9608–9644. (k) Driver, T. G.
Angew. Chem., Int. Ed. 2009, 48, 7974–7976. (l) Joucla, L.; Djakovitch, L.
Adv. Synth. Catal. 2009, 351, 673–714. (m) Patil, S. A.; Patil, R.; Miller,
D. D. Curr. Med. Chem. 2009, 16, 2531–2565.
‡
ꢁ
Universita di Roma.
§ Chimie ParisTech.
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Verlag: Stuttgart, 1999.
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Cai, Ch. Chem. Commun. 2008, 5685–5694. (c) O’Hagan, D. Chem. Soc.
Rev. 2008, 37, 308–319. (d) Purser, S.; Moore, P. R.; Swallow, S.;
Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320–330. (e) Muller, K.;
Faeh, Ch.; Diederich, F. Science 2007, 317, 1881–1886.
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Chem. Soc. Rev. 2010, 39, 558–568. (b) Liu, P.; Sharon, A.; Chu, C. K.
J. Fluorine Chem. 2008, 129, 743–766. (c) Park, K. B.; Kitteringham,
N. R.; O’Neill, P. M. Annu. Rev. Pharmacol. Toxicol. 2001, 41, 443–470.
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r
10.1021/ol401098b
Published on Web 05/28/2013
2013 American Chemical Society

ring.8 Few other studies revealed the access to C3 mono- or
difluorinated indole derivatives. The syntheses of
3-substituted 3-fluorooxindoles,9 3,3-difluoroindoles
or indolines,10 or trans-2,3-difluoro-2,3-dihydroindoles,
monofluoroindole derivatives, or monofluoroindolines11
have been described in the presence of NFSI, DAST, and
Selectfluor. To the best of our knowledge, the synthesis of
2-substituted 3-fluoroindoles has not been previously de-
scribed. Our continuing interest in the synthesis of indole
derivatives by one-pot processes12 and in the formation of
carbonꢀheteroatom bonds via Au-catalyzed π-activation
of alkynes13 prompted us to investigate an alternative ap-
proach to 3,3-difluoro-2-arylindolines 3 with better flexibility
starting from unprotected o-alkynylanilines 1. Moreover, we
explored the extension of the procedure toward the unprece-
dented synthesis of 3-fluoro-2-aryl-indoles 2 (Scheme 1). We
wish to report herein our preliminary results.
First, the readily available 2-[[4-(methoxy)phenyl]ethynyl]-
aniline 1a was chosen as a model substrate and Selectfluor as
a source of electrophilic fluorine to carry out the desired
cyclizationꢀfluorination sequence. Indeed, Selectfluor has
shown very interesting activity in the Au-catalyzed amino-
fluorination of alkynes to prepare fluorinated pyrrolidines14
and pyrazoles.15,16
The results of Table 1 show that the reaction of 1a with
an excess of Selectfluor and water in CH3CN at room
temperature in the absence of any catalyst failed to give the
Scheme 1
desired C3 fluorinated indole derivatives ruling out their
formation by a direct electrophilic fluorocyclization pro-
cess14,17 (Table 1, entry 1). Under these conditions, a fast
degradationof thestarting N-unprotected2-alkynylaniline
1a was observed. When the same reaction was carried out
in the presence of Ph3PAuNTf218 (10 mol % of the com-
mercially available dimeric form), the formation of the
difluorinated indoline 2a was observed although in a
modest 35% yield (Table 1, entry 2). Subsequently, various
parameters such as solvents19 and the amount of water10b,20
were screened to improve the reaction efficiency. The
presence of a larger amount of water, e.g., 100 equiv
(Table 1, entry 3), led to a decrease of the yield as it may
speed up the protodeauration of indolylgold intermediate
species(42% of thecorresponding2-substitutedindolewas
detected). Ethanol has proven to be the best choice for the
reaction among solvents screened so far (MeCN, dioxane,
and acetone, Table 1, entries 3ꢀ6). Satisfyingly, the yield
of 3a was increased to 75% by using EtOH as the reaction
medium (Table 1, entry 4). By contrast with the results
observed in the gold(I)-catalyzed tandem aminofluorina-
tion of 1-phenyl-2-(4-phenylbut-3-yn-2-ylidene)hydrazine
to give the corresponding fluorinated pyrazole, the addi-
tion of base to the reaction mixture resulted detrimental
(7) Lee, J. H.; Kim, Y. G.; Cho, M. H.; Kim, J. A.; Lee, J. FEMS
Microbiol. Lett. 2012, 329, 36–44.
(8) Schlosser, M.; Ginanneschi, A.; Leroux, F. Eur. J. Org. Chem.
2006, 2956–2969 and references cited therein.
(9) For examples of 3-substituted 3-fluorooxindoles, see: (a) Seki, T.;
Fujiwara, T.; Takeuchi, Y. J. Fluorine Chem. 2011, 132, 181–185. (b)
Fujiwara, T.; Seki, T.; Miura, M.; Takeuchi, Y. Heterocycles 2009, 79,
427–432. (c) Fujiwara, T.; Yin, B.; Jin, M.; Kirk, K. L.; Takeuchi, Y.
J. Fluorine Chem. 2008, 129, 829–835. (d) Ishimaru, T.; Shibata, N.;
Horikawa, T.; Yasuda, N.; Nakamura, S.; Toru, T.; Shiro, M. Angew.
Chem., Int. Ed. 2008, 47, 4157–4161.
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Z.; Jiao, N. Org. Lett. 2011, 13, 4498–4501.
16
and we failed to obtain 3a under the presence of NaHCO3
Other gold catalysts such as AuCl, AuCl3, and NaAuCl4
(Table 1, entries 7ꢀ9) were tested and allowed the formation
of 2a in 38ꢀ56%. In addition, it was found that other
transition-metal catalysts such as PdCl2, PtCl2,CuCl2 2H2O,
3
RuCl3 2H2O, or AgNTf2 were ineffective. Although less
effective in the model study, we selected the cheaper
NaAuCl4 2H2O catalyst that previously, in our hands,
3
(11) Yin, B.; Wang, L.; Inagi, S.; Fuchigami, T. Tetrahedron 2010, 66,
6820–6825.
3
(12) (a) Arcadi, A.; Cacchi, S.; Fabrizi, G.; Goggiamani, A.; Iazzetti, A.;
Marinelli, F. Org. Biomol. Chem. 2013, 11, 545–548. (b) Abbiati, G.;
Arcadi, A.; Chiarini, M.; Marinelli, F.; Pietropaolo, E.; Rossi, E. Org.
Biomol. Chem. 2012, 10, 7801–7808. (c) Arcadi, A.; Chiarini, M.; Marinelli,
F.; Picchini, S. Synthesis 2011, 4084–4090 and references cited therein.
allowed a mild and green procedure for the synthesis of
indoles from 2-alkynylanilines17 and explored the Au(III)-
catalyzed cyclization of 2-alkynylanilines combined in a
one-pot procedure with the C3 fluorination of the in situ
formed 2-substituted indole by Selectfluor.
A one-pot procedure was used with the aim of accomplish-
ing efficiently the process and avoiding the use of time- and
resource-consuming workup and purification procedures.21
Alkynes 1were converted into the corresponding 2-substituted
indoles in EtOH at room temperature by using 5 mol %
^
(13) (a) Antoniotti, S.; Genin, E.; Michelet, V.; Genet, J.-P. J. Am. Chem.
Soc. 2005, 127, 9976–9977. (b) Genin, E.; Toullec, P. Y.; Antoniotti, S.;
^
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3113. (c) Genin, E.; Toullec, P.-Y.; Marie, P.; Antoniotti, S.; Brancour, C.;
^
Genet, J.-P.; Michelet, V. ARKIVOC 2007, 5, 67–78. (d) Neatu, F.; Li, Z.;
^
Richards, R.; Toullec, P. Y.; Genet, J.-P.; Dumbuya, K.; Gottfried, J. M.;
^
€
Steinruck, H.-P.; Parvulescu, V. I.; Michelet, V. Chem.;Eur. J. 2008, 14,
^
9412–9418. (e) Toullec, P. Y.; Genin, E.; Antoniotti, S.; Genet, J.-P.;
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Michelet, V. Synlett 2008, 707–711. (f) Nea-tu, F.; Parvulescu, V. I.; Michelet,
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V.; Genet, J.-P.; Goguet, A.; Hardacre, C. New J. Chem. 2009, 33, 102–106.
^
(g) Nea-tu, F.; Toullec, P. Y.; Michelet, V.; Parvulescu, V. I. Pure Appl. Chem.
of NaAuCl4 2H2O. Selectfluor (3 equiv) was then added
3
2009, 81, 2387–2396. (h) Tomas-Mendivil, E.; Toullec, P. Y.; Diez, J.;
Conejero, S.; Michelet, V.; Cadierno, V. Org. Lett. 2012, 14, 2520–2523.
(17) Arcadi, A.; Bianchi, G.; Marinelli, F. Synthesis 2004, 610–619.
(18) Mezailles, N.; Ricard, L.; Gagosz, F. Org. Lett. 2005, 7, 4133–
4137.
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(14) Simonneau, A.; Garcia, P.; Goddard, J. P.; Mouries-Mansuy,
V.; Malacria, M.; Fensterbank, L. Beilstein J. Org. Chem. 2011, 7, 1379–
1386.
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(19) Nyfeller, P. T.; Duron, S. G.; Burkart, M. D.; Vincent, S. P.;
(15) Quian, J.; Liu, Y.; Zhu, J.; Jiang, B.; Xu, Z. Org. Lett. 2011, 13,
4220–4223.
(16) For a recent review on transition metal-catalyzed general fluor-
ination reactions, see: Liu, G. Org. Biomol. Chem. 2012, 10, 6243–6248
and references cited therein.
Wong, C.-H. Angew. Chem., Int. Ed. 2005, 44, 192–212.
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2011, 7, 565–569.
Org. Lett., Vol. 15, No. 11, 2013
2767

Table 1. Optimization of Reaction Conditions for the Tandem
Aminodifluorination of 2((4-Methoxyphenyl)ethynyl)aniline 1aa
Table 2. Substrate Scope*
entry [M] 10 mol % solvent (H2O x equiv) time (h) yieldb (%)
1
CH3CN (10)
CH3CN (10)
CH3CN (100)
EtOH (100)
1,4-dioxane (100)
acetone (100)
EtOH (100)
EtOH (100)
EtOH (100)
3
2c
3c
4c
5c
6c
7
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
AuCl
3
35
17
75
0
3
1.5
2
2
0
2
56
38
45
8
AuCl3
2
9
NaAuCl4
0.25
a Conditions: 0.45 mmol of alkyne 1a and 2.5ꢀ3 equiv of Selectfluor,
10 mol % of [M], 0ꢀ100 equiv of water in 4.5 mL of solvent at room
temperature. b Isolated yields. c The formation of the aminocylization
derivative 2-(4-methoxyphenyl)-1H-indole 4a was observed (42% yield).
when full conversion of the gold(III)-catalyzed cyclization
of 1 was observed. The results are summarized in Table 2.
A variety of alkynyl derivatives 1 underwent cyclization/
fluorination smoothly, displaying a broad substrate com-
patibility. It was found that alkynyl derivatives 1 bearing
both electron-rich and electron-poor aryl-substituted moi-
eties furnished the corresponding products 2 in moderate
to good yields (57ꢀ86%). o-Substituent tolerance was also
proved as exemplified by the cyclization/functionalization
of o-methoxy- or naphthyl-substituted derivatives (Table 2,
entries 2 and 4). The in situ cyclization of 2-alkynylanilines
has the advantage that the resulting products, 2-substituted
indoles, are easily functionalized by electrophilic aromatic
substitution at position 3 without isolation and purification
of the 3-unsubstituted indole intermediate.10 The difluoro
or dichloro substitutions in the ortho- and para-positions of
the anilines were tolerated (Table 2, entries 8 and 9).
Interestingly, the polyfluorinated indoline 2h was isolated
in high yield (Table 2, entry 8). It is noteworthy that, in the
presence of the electron-withdrawing groups in the aniline
ring, the cyclization step was to slow at room temperature
and the reactions were therefore carried out in EtOH at
reflux. The increase of the reaction temperature exhibited
benefical effects also in the presence of an ortho substituent
group in the aryl moiety of 1.
* Reaction conditions: 1 equiv of diyne, NaAuCl4 2H2O (5 mol %)
in EtOH (0.1 M), then Selectfluor (3 equiv) was added to the reaction
mixture. a Isolated yield. b Reflux. c 13% of the monofluorinated indole
was also isolated.
3
For substituted anilines bearing a chloro or fluoro group,
the procedure allowed the exclusive formation of 3h and 3i
in modest to good yields.
On the basis of the previous literature,22 since the gold(I)
complex could be oxidizedintothe gold(III) complex in the
presence of Selectfluor23 and the gold(III) complex also
catalyzed the reaction, two main mechanistic manifolds
regarding the tandem gold(I)-catalyzed aminofluorination
of 1 can occur (Scheme 3). First, the reaction may proceed
via a gold(I) catalytic cycle via intermediates A and B.
Intermediate B could be either oxidized into the gold(III)
The isolation of the monofluorinated product 3f to-
gether with the difluoronitated 2f prompted us to modify
the standard reaction conditions trying to access selectively
to monoflurinated derivatives. Gratifyingly, by only using
1.1 equiv of Selectfluor, the desired 3-fluoro-2-arylindoles
2eꢀgwere isolated in satisfactory yield (Scheme 2). Whereas
a mixture of mono- and difluorinated adducts was obtained
in the case of 3-Br- and 4-CF3-substituted arylakynes 1f and
1g, the monofluorinated indole 3e wasisolatedin75% yield.
(22) (a) Engle, K. M.; Mei, T.-S.; Wang, X.; Yu, J.-Q. Angew. Chem.,
Int. Ed. 2011, 50, 1478–1491. (b) Wegner, H. A.; Auzias, M. Angew.
Chem., Int. Ed. 2011, 50, 8236–8247. (c) Hopkinson, M. N.; Gee, A. D.;
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2768
Org. Lett., Vol. 15, No. 11, 2013

Scheme 2
Scheme 3. Mechanism Rationale
complex C in the presence of Selectfluor or protodemeta-
lated to form indole 4. Upon reductive elimination of
intermediate C, the gold(I) catalyst is regenerated and
the 2-substituted-3-fluoro indole 3 is formed.24 A second
mechanistic pathway involving redox Au(I)/Au(III) cata-
lytic cycle may also be possible, leading after oxidation
of Au(I) catalyst to intermediate C.25 Moreover the 2-
substituted indole derivative 4 derived by the electrophilic
protodeauration of the indolyl gold(I)26 or indolyl
gold(III) species27 might undergo an uncatalyzed mono-
fluorination21 reaction which will be followed by difluorina-
tion28 in the presence of an excess of Selectfluor.
In conclusion, the unprecedented gold-catalyzed amino-
flurination of unprotected 2-alkynylanilines was investigated.
An alternative approach to 3,3-difluoroindole derivatives
with better flexibility over the previously described proce-
dures has been developed. A two-step, one-pot gold(III)-
catalyzed cyclization/electrophilic fluorination was achieved
in green ethanol and allowed the synthesis of 3,3-difluoro-2-
substituted-3H-indoles in good yield under mild condi-
tions. Extension of the procedure to the synthesis of
2-aryl-3-fluoro-1H-indoles was explored. The metho-
dology is simple to perform and requires use of neither
bases, acids, nor N-protective groups. Further investi-
gation is in progress to direct the tandem gold-catalyzed
oxidative cycloamination reaction toward different
indoles.29
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Acknowledgment. This work was supported by Uni-
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0
ꢁ
ꢁ
versita dell’Aquila, the Ministere de l Education et de la
Recherche, and the Centre National de la Recherche
Scientifique (CNRS). Johnson Matthey, Inc., is acknowl-
edged for a generous loan of AuCl3.
Supporting Information Available. Experimental pro-
cedures and characterization data of new products 1f,I,
2aꢀi, and 3eꢀi. This material is available free of charge
via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 11, 2013
2769