Palladium-catalyzed oxidative cyclization of N-aryl enamines: From anilines to indoles

Article abstract of DOI:10.1002/anie.200802482

(Chemical Equation Presented) The special advantage of the title reaction to form substituted indoles 2 lies within the broad scope of the transformation: A multitude of N-aryl enaminones 1 can be prepared readily in one step from commercially available anilines. Furthermore, anilines can be converted directly in a one-pot process into the indole products. R¹ = H, Me, OMe, Cl, F, carbonyl functionality, CN, fused aryl; R² = alkyl, aryl; R ³ = alkyl, O-alkyl.

Full text of DOI:10.1002/anie.200802482

Communications
DOI: 10.1002/anie.200802482
Heterocycles
Palladium-Catalyzed Oxidative Cyclization of N-Aryl Enamines: From
Anilines to Indoles**
Sebastian Würtz, Souvik Rakshit, Julia J. Neumann, Thomas Dröge, and Frank Glorius*
Table 1: Optimization of the reaction conditions.[a]
The indole unit is one of the most abundant and relevant
heterocycles in natural products and pharmaceuticals.[1]
Despite the existence of numerous methods for the synthesis
and derivatization of indoles,[2] the development of new, more
efficient methods is of great importance. In this context, direct
À
oxidative C C coupling by the selective activation of two
À
C H bonds[3] is a promising synthetic strategy.[4,5] In contrast
Entry
Variation from the “standard conditions”
Yield[b] [%]
to established cross-coupling methods,[6] such as the Suzuki–
Miyaura coupling, prefunctionalization of the reaction cen-
ters is not required. For example, electron-rich aniline
substrates can be activated and functionalized by electrophilic
aromatic palladation under acidic conditions to give indole-
quinones[7] or carbazoles.[8,9] However, the limited scope of
these reactions, the frequent requirement of a stoichiometric
amount of a palladium complex, and the low yields often
observed limit the usefulness of these methods. Furthermore,
simple non-annulated indoles could not be prepared under
these acidic conditions.
Herein, we report an efficient synthesis of functionalized
indoles from commercially available anilines by palladium-
catalyzed, intramolecular oxidative coupling. As this cycliza-
tion does not proceed through electrophilic aromatic palla-
dation, a large variety of anilines can be used in this reaction.
Our investigation commenced with the cyclization of methyl
(Z)-3-(phenylamino)but-2-enoate (1a) to give the corre-
sponding indole 2a. In experiments to optimize the reaction,
the best results were obtained with a catalytic amount of
Pd(OAc)2, Cu(OAc)2 as the oxidant, and K2CO3 as the base in
DMF (Table 1, entry 1). Under these conditions, conversion
was complete within 3 h at 808C (72% yield of the isolated
product), or within less than 15 min at 1408C, even when only
5 mol% of Pd(OAc)2 was used (not shown). Variation of the
oxidant (Table 1, entries 3–6), the base (Table 1, entries 7 and
8), or the solvent (Table 1, entries 9–11) led to a decrease in
1
2
3
4
5
6
7
8
9
10
11
12
13
14
–
80 (72)[c]
<5
23
<5
60
47
64
62
<5
32
no Pd(OAc)2
Ag2CO3 instead of Cu(OAc)2
benzoquinone instead of Cu(OAc)2
benzoquinone[d]
Cu(OAc)2 (2.1 equiv)
Cs2CO3 instead of K2CO3
no K2CO3
toluene instead of DMF
toluene instead of DMF, 1108C
HOAc instead of DMF
Pd(TFA)2 instead of Pd(OAc)2
<5
75
41
[d]
PPh3
NaCl[e]
80
[a] Standard reaction conditions: 1a (0.25 mmol), Cu(OAc)2
(0.75 mmol), K2CO3 (0.75 mmol), Pd(OAc)2 (10 mol%), DMF (3 mL),
808C, 14 h. [b] The yield was determined by GC. [c] The yield of the
isolated product is given in brackets. [d] Included as an additive
(20 mol%). [e] Included as an additive (50 mol%). DMF=N,N-dime-
thylformamide.
the yield. The use of acetic acid as the solvent resulted in the
rapid decomposition of the substrate and therefore no
product formation (Table 1, entry 11). The results with
Pd(TFA)2 (TFA = trifluoroacetate) were similar to those
observed under the optimal conditions (Table 1, entry 12),
whereas the addition of PPh3 resulted in the formation of a
less active catalyst (Table 1, entry 13). Interestingly, chloride
anions do not influence the reaction at all (Table 1,
entry 14).[10]
A great variety of substituted anilines can be transformed
into the corresponding indoles under the optimized reaction
conditions (Table 2). In some cases, an increased reaction
temperature (and, consequently, a shorter reaction time) led
to higher yields. Substrates with a variety of electron-donating
(Table 2, entries 2–8) and electron-withdrawing substituents
(Table 2, entries 9–19) were converted directly into the indole
products, which are versatile building blocks for subsequent
synthetic modification, for example, through modern cross-
coupling reactions (Table 2, entries 12–14).[11,12] The ability to
vary the aniline moiety so broadly is a distinct advantage of
this new indole synthesis.
[*] S. Würtz, S. Rakshit, J. J. Neumann, T. Dröge, Prof. Dr. F. Glorius
Westfälische Wilhelms-Universität Münster
Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Münster (Germany)
Fax: (+49)251-833-3202
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/research/
glorius/welcome.html
[**] We thank the Fonds der Chemischen Industrie, the Alfried Krupp
von Bohlen und Halbach Foundation, the Deutsche Forschungs-
gemeinschaft, and the International Graduate School of Chemistry
for generous financial support, Prof. Dr. M. Christl for mechanistic
discussions, and Dr. H. Luftmann and Dr. K. Bergander for
analytical support.
In the case of meta-substituted substrates 1, two regioiso-
meric indole products 2 can be formed. Intriguingly, exclusive
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802482.
7230
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Table 2: Scope of the reaction.[a]
(Table 2, entries 7 and 15). The substrate derived
from naphthylamine also underwent the desired
reaction to give just one regioisomer of the product
in good yield (85%; Table 2, entry 20). Whereas good
selectivities were observed with methyl and chloro
substituents (Table 2, entries 3 and 13), the presence
of the small fluoro group led to the nonselective
formation of the 4- and 6-substituted indole
regioisomers (Table 2, entry 10). Evidently, the
steric demand of the substituents is more important
for selectivity in this transformation than their
electronic character.
The enamine moiety was also varied successfully
with the efficient formation of different carbonyl
derivatives (Table 3, entries 1, 4, and 5), including a
2-aryl-substituted indole (Table 3, entry 6). We dem-
onstrated on a 10 mmol scale that it is sufficient to
Entry
1[c]
Product
Yield[b]
[%]
Entry
11[d]
Product
Yield[b]
[%]
72
74
53
2[d]
82
68
12[c]
13[c]
3[d]
2c
2m
76
(6-Me/4-Me)
(92:8[e]) (6-Cl/4-Cl)
(88:12[e])
Table 3: Variation of the enamine unit.[a]
4[c]
72
62
64
14[c]
15[f]
16[f]
17[f]
18[f]
19[f]
64
54
5[d]
6[f]
7[f]
8[f]
9[d]
Entry
Product
Yield[b] [%]
(>99:1[e])
1[c]
71
70
62
2[c,d]
3[e]
52
64
70
65
4[c]
79
68
(>99:1[e])
5[f]
85
68
64
78
6[g]
[a] Reaction conditions:
1 (1 mmol), Cu(OAc)2 (3 mmol), K2CO3
(3 mmol), Pd(OAc)2 (5–10 mol%), DMF (12 mL). [b] Yield of the
isolated product. [c] Pd(OAc)2 (5 mol%), 1408C, 30 min. [d] The reaction
was carried out with Cu(OAc)2·H2O instead of Cu(OAc)2. [e] Aniline 1
(10 mmol), Cu(OAc)2·H2O (30 mmol), K2CO3 (30 mmol), Pd(OAc)2
(2 mol%), 1408C, 17 h. [f] Pd(OAc)2 (10 mol%), 1408C, 24 h. [g] Pd-
(OAc)2 (10 mol%), 1808C (microwave), 1 h.
2j
74
85
10[d]
20[c]
(6-F/4-F)
(53:47[e])
(>99:1[e])
[a] Reaction conditions: 1 (1 mmol), Cu(OAc)2 (3 mmol), K2CO3 (3 mmol),
Pd(OAc)2 (10 mol%), DMF (12 mL), reaction time: 0.5 h at 1408C, 12–16 h at
80 or 1108C. [b] Yield of the isolated product. [c] The reaction was carried out at
808C. [d] The reaction was carried out at 1108C. [e] The ratio of the regioisomers
was determined by GC analysis. [f] The reaction was carried out at 1408C.
use 2 mol% of the palladium catalyst and to use
Cu(OAc)2 hydrate as the oxidant (Table 3, entries 2
and 3). Furthermore, a one-pot reaction sequence
enables rapid access to indoles from commercially
available anilines.[13] Within minutes, aniline was
formation of the 6-substituted indole regioisomer was
observed with both mesomerically electron-donating
methoxy and an electron-withdrawing acetyl substituent
condensed at room temperature with methyl acetoacetate in
the presence of InBr3 (1 mol%) without a solvent to give the
corresponding enamine carboxylate,[14] which underwent
a
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Communications
Scheme 1. One-pot synthesis of indole 2a from aniline and methyl
acetoacetate.
subsequent cyclization under the standard conditions to give
the desired indole product in 72% yield (Scheme 1).
Scheme 3. Competition experiments for the determination of the
kinetic isotope effect.
N-Aryl enamine carboxylates tend to react with electro-
philes at the a C atom;[15] similar reactivity was also reported
recently for a palladium-catalyzed coupling of enaminones
with aryl borates.[16] Therefore, a plausible mechanism begins
with an electrophilic palladation of the nucleophilic enam-
ine,[17] followed by deprotonation (Scheme 2). The resulting
palladium complex A is suitable for electrophilic aromatic
s-bond metathesis and base-assisted deprotonation, but not
an electrophilic aromatic palladation.
In summary, we have developed an efficient and broadly
applicable direct oxidative synthesis of indoles from com-
mercially available anilines. This transformation of anilines
into indoles can also be carried out in a one-pot sequence.
None of the substrates reported in this study were previously
converted into indoles by cyclization. Thus, the method is an
attractive alternative to Heck coupling reactions, which
require the use of ortho-halogen-substituted anilines.[22] Initial
mechanistic results speak unambiguously against an electro-
philic aromatic palladation of the aniline ring and favor a s-
bond-metathesis or deprotonation pathway. The exploration
of the use of air as an oxidant is under way, as well as further
investigations into the mechanism and synthetic applications
of this method.
palladation[18] or intramolecular C H activation, such as
À
Experimental Section
Representative procedure: Methyl (Z)-3-(phenylamino)but-2-enoate
(1a; 191 mg, 1 mmol) was stirred in DMF (12 mL) together with
Pd(OAc)2 (22.5 mg, 10 mol%), Cu(OAc)2 (545 mg, 3 mmol), and
K2CO3 (415 mg, 3 mmol) at 808C under an atmosphere of argon.
After completion of the reaction (as indicated by TLC, GC, or GC–
MS), the reaction mixture was cooled to room temperature, diluted
with EtOAc (15 mL), and filtered through a short pad of silica, which
was then washed with EtOAc (60 mL). Removal of the solvent in
vacuo and purification of the residue by flash chromatography (SiO2,
pentane/EtOAc 5:1) provided indole 2a (136 mg, 0.72 mmol, 72%) as
an orange solid. The analytical data obtained were in agreement with
the data reported in the literature.[23]
Scheme 2. Possible mechanistic pathways for the activation of the
À
aromatic C H bond of the aniline moiety.
Received: May 27, 2008
Published online: August 13, 2008
s-bond metathesis[19a,b] or base-assisted deprotonation.[19,20]
Subsequent reductive elimination generates the indole prod-
uct and a Pd0 complex that can be reoxidized to the PdII
complex by the oxidant Cu(OAc)2.
The basic reaction conditions would lower the electro-
philicity of a cationic [PdX]+ species and thus render an
electrophilic aromatic palladation mechanism unlikely.[5a]
Furthermore, competition experiments with para-substituted
substrates 1 showed that electron-donating groups in the para
position slow down the transformation.[21] This observation is
in agreement with a mechanism involving s-bond metathesis
or base-assisted deprotonation, but not with an electrophilic
palladation (Scheme 2). The observation of an intramolecular
kinetic isotope effect of kH/kD = 4.6 (Scheme 3) also supports
À
Keywords: C H activation · heterocycles · indoles · oxidation ·
palladium
.
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À
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