C-N coupling of indoles and carbazoles with aromatic chlorides catalyzed by a single-component NHC-nickel(0) precursor

Article abstract of DOI:10.1002/adsc.201500030

A new and efficient nickel-based protocol for the N-arylation of indoles and carbazoles with aromatic chlorides, the least expensive of the aryl halides, is described. The procedure provides selectively N-(hetero)arylation products in good to high yields, in short reaction times and without adding an excess of ligands.

Full text of DOI:10.1002/adsc.201500030

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DOI: 10.1002/adsc.201500030
VeÀry Important Publication
C N Coupling of Indoles and Carbazoles with Aromatic Chlor-
ides Catalyzed by a Single-Component NHC-Nickel(0) Precursor
Silvia G. Rull,+a Juan F. Blandez,+b Manuel R. Fructos,a Tomꢀs R. Belderrain,a,*
and M. Carmen Nicasiob,*
a
Laboratorio de Catꢀlisis Homogꢁnea, Unidad Asociada al CSIC, CIQSO – Centro de Investigaciꢂn en Quꢃmica
Sostenible and Departamento de Quꢃmica y Ciencias de los Materiales, Campus de El Carmen s/n, Universidad de
Huelva, 21007 Huelva, Spain
Fax : (+34)-959-219-942; phone: (+34)-959-219-955; e-mail: trodri@dqcm.uhu.es
Departamento de Quꢃmica Inorgꢀnica, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain
b
Fax : (+34)-954-455-7153; phone: (+34)-954-557-162; e-mail: mnicasio@us.es
+
These authors contributed equally to this work.
Received: January 14, 2015; Revised: February 2, 2015; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500030.
Abstract: A new and efficient nickel-based protocol
for the N-arylation of indoles and carbazoles with
aromatic chlorides, the least expensive of the aryl
halides, is described. The procedure provides selec-
tively N-(hetero)arylation products in good to high
yields, in short reaction times and without adding
an excess of ligands.
ACHTUNGTRENhNUGN aloarenes or ortho-alkenylhaloarenes and N-arylani-
lines as reactants, which are transformed into the N-
arylindole framework by palladium- or copper-medi-
ated
tandem
amination/cyclization
processes
(Scheme 1B).[12] The main drawback of this method
lies in the need to preform the ortho-disubstituted
À
Keywords: aryl chlorides; C N bond formation; N-
heterocyclic carbenes; nickel catalysts
The indole core is probably the most abundant, nitro-
gen-containing heterocycle occurring in bioactive nat-
ural products.[1] In this context, N-arylindoles are
pharmaceutically valuable compounds due to their in-
teresting biological activities including antifungal,[2]
antiviral,[3] antipsychotic,[4] antiallergic,[5] enzymes in-
hibitory,[6] and antigiotensin II-1 antagonistic.[7] In ad-
dition, structurally related N-arylcarbazoles exhibit
remarkable electroluminescense properties and have
been applied in the preparation of LEDs[8] and
charge-transporting/host materials.[9]
N-Arylindoles can be prepared by an annulation re-
action of enolizable N,N-diarylhydrazones under
acidic conditions (classical Fischer indole synthesis[10]
)
(Scheme 1A).[11] The poor commercial availability and
toxicity of N,N-diarylhydrazine precursors limited the
scope of this transformation. As a result, methods for
the synthesis of arylhydrazones involving the use of
transition metal catalysts have been devised.[11b,c] An-
other synthetic strategy makes use of ortho-alkynyl- Scheme 1. General methods for N-arylindole synthesis.
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Silvia G. Rull et al.
Table 1. Optimization studies for the N-arylation of indole
arene precursor for the assembly of the indole nu-
cleus. In recent years, procedures based on the use of
monosubstituted or ortho-disubstituted benzenoid
precursors other than those described above have ap-
peared in the literature.[13]
with 4-chlorotoluene catalyzed by [(IPr)Ni(styrene)2].[a]
A different approach to the synthesis of N-arylin-
À
doles consists of the direct C N functionalization of
the preformed indole rings. Classically, this strategy
has been achieved through SNAr (nucleophilic aro-
matic substitution) of p-electron rich NH-indoles with
electron-deficient fluorobenzenes (Scheme 1C),[14] or
through stoichiometric Ullmann condensations.[15]
With the development of metal-mediated catalytic
Entry Base
Temperature [8C] Time [h] Yield [%][b]
1
2
3
4
5
6
LiO-t-Bu 80
LiO-t-Bu 110
NaO-t-Bu 110
Cs2CO3
LiO-t-Bu 110
LiO-t-Bu 110
1
1
1
1
1
2
25
81
<5
<5
78[c]
87
À
110
procedures for C N bond formation, limitations asso-
ciated with those traditional methods such as narrow
substrate scope, high reaction temperatures and the
need for stoichiometric amounts of copper salts have
been partially overcome. Copper and palladium are
the metals of choice to catalyze the N-arylation of in-
doles (Scheme 1D).[16] Copper-mediated protocols
make use of simple, cost-effective nitrogen-based li-
gands like diamines,[17a–c] a-amino acids,[17d] Schiff
bases[17e] oximes,[17f,g] etc., and require temperatures
[a]
Reaction conditions: indole (1.2 mmol); 4-chlorotoluene
(1.0 mmol); base (1.2 mmol); catalyst (5 mol%); dioxane
(1 mL).
Yields of isolated products.
The reaction was performed in toluene.
[b]
[c]
between 80–1208C[18] and catalyst loadings between oxane at 808C in the presence of LiO-t-Bu
5–20 mol%. However, they are limited almost exclu- (1.2 equiv.) giving the expected coupling product in
sively to the use of aryl iodides and bromides as cou- low yield (25%) within 1 hour (Table 1, entry 1). In-
pling partners.[19] Compared to copper, the number of creasing the temperature up to 1108C produced a no-
palladium-based catalytic systems able to perform the table enhancement of the reaction yield (entry 2).
N-arylation of indoles in an effective manner remains Different bases were tested but LiO-t-Bu proved to
scarce.[12f–20] This could be attributed to the poor nu- be the most effective (entries 3 and 4). Moreover, the
cleophilicity, high acidity and strong coordination N-arylation reaction was successfully conducted in
ability of the NH group of indole to the Pd center.[21] toluene although the yield did not change substantial-
Moreover, selectivity problems associated with com- ly (entry 5). Finally, the highest yield of the coupling
peting N- and C-arylation have also been observ- product was obtained when the reaction was carried
ed.[20b,c] To avoid these problems, reactions have to be out for 2 h (entry 6).
carried out using loadings of 1–8 mol% Pd and excess
of ligands, generally phosphines (Pd/L ratio up to the preparation of N-arylindoles, a selection of aryl
1:4). chlorides was reacted with several indoles under the
To examine the scope of this nickel precatalyst for
À
C N coupling reactions catalyzed by nickel com- optimized conditions. The results of the couplings are
plexes have received increased attention in the past presented in Table 2. Reactions were accomplished
few years.[22] Recently, we became interested in the within a fairly short time, 2 h in most cases. Electron-
development of single-component nickel catalysts for neutral and electron-rich aryl chlorides reacted with
[23a]
[23b,c]
À
À
C C
that the Ni(0) complex [(IPr)Ni(styrene)2] [IPr=1,3- high yields (3a–3f), whereas for substituted indoles
bis(2,6-diisopropylphenyl)imidazol-2-ylidene] effi- yields were slightly lower (3g–3l). In such cases, no
and C N
bond couplings. We reported the parent indole affording the coupling products in
ciently catalyzed the coupling of aryl tosylates with further improvement of yields was observed by ex-
cyclic secondary amines and anilines.[23c] Since no tending the reaction time, as previously observed for
nickel catalyst showing a wide scope for the N-aryla- other Ni-catalyzed amination reactions.[22c,d] It is
tion of indole has been reported so far, we decided to worth noting that competing C-arylation products
explore the use of our IPr-Ni(0) complex as catalyst were not observed for reactions of 2-methylindole.[20c]
À
in this challenging C N coupling. As nickel(0) com- Finally, no desired product was obtained with aryl
plexes are highly reactive toward the oxidative addi- chlorides containing electron-withdrawing groups
tion of aryl chlorides, the most affordable and avail- (CF3 and COPh) or substituents in ortho positions of
able among the aryl halides, we focused on the N-ary- the ring[20c] even under prolonged reaction times.
lation of indoles with chloroarenes.
The results depicted in Table 2 compare well with
Using 5 mol% of IPr-Ni(0) precatalyst, 4-chloroto- those described recently by the group of Stradiotto
luene (1 equiv.) and indole (1.2 equiv.) reacted in di- using the BippyPhos/[Pd(cinnamyl)Cl]2 catalytic sys-
2
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C N Coupling of Indoles and Carbazoles with Aromatic Chlorides
Table 2. Scope of N-arylation of indoles with aryl chlorides
Table 3. Scope of the N-arylation of indoles and carbazoles
catalyzed by [(IPr)Ni(styrene)2].[a]
with heteroaryl chlorides catalyzed by [(IPr)Ni(styrene)2].[a]
[a]
Reagents and conditions: indole (1.2 mmol), aryl chloride
(1.0 mmol); LiO-t-Bu (1.2 mmol), catalyst (5 mol%), di-
oxane (1 mL), 1108C, 2 h. Yields of isolated products.
Reaction performed for 3 h.
[b]
tem[12f] which shows the widest scope for the N-aryla-
tion of indoles with chloroarenes ever reported. Reac-
tions were conducted with lower loadings of Pd-based
catalyst (1 mol%) as compared to those used here,
however a large excess of BippyPhos ligand (4 mol%)
was required in order to make effective the couplings
in longer reaction times than those displayed in
Table 2.
À
The C N coupling between two heteroaromatic re-
agents is a far more challenging process, since the co-
ordination abilities of both heteroaryl groups to the
metal center might inhibit the catalysis. Thus, we de-
cided to examine the application of the IPr-Ni(0) pre-
[a]
Reagents and conditions: indole (1.2 mmol), heteroaryl
À
catalyst to the C N coupling of indoles with hetero-
chloride (1.0 mmol); LiO-t-Bu (1.2 mmol), catalyst
(10 mol%), dioxane (1 mL), 1108C, 2 h. Yields of isolated
products.
A
were applied to the N-arylation of indole with 2-
chloroquinoline the expected product was obtained in
51% isolated yield. No significant improvement in
ACHTUNGTRENNUNG
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Silvia G. Rull et al.
thanks the Ministerio de Economꢀa y Competitividad for a re-
search grant.
yield was observed by increasing the reaction time;
however, the coupling product was formed quantita-
tively in 2 h when the reaction was conducted with
10 mol% of IPr-Ni(0) complex (Table 3, 5a).
Under these conditions, diverse indoles were cou-
pled with unsubstituted N-containing heteroaryl chlor-
ides such as 2-chloroquinoline, 2-chloroquinoxaline
and 2-chloropyridine, in good to excellent isolated
yields (5a–k, Table 3). In addition, 2-chlorobenzoxa-
zole and 2-chlorobenzothiazole could be successfully
used as coupling partners, affording the corresponding
products in high yields (5l–n). However, non-benzo-
fused five-membered heteroaryl chlorides did not un-
dergo the reaction. Finally, using the same conditions
carbazole was effectively coupled with different heter-
oaryl chlorides in yields ranging from 71–94% (5o–r).
Again, it should be stressed that all these couplings
proceed with high yields in only 2 h and without
adding an excess of the ligand. To the best of our
knowledge, this is the shortest reaction time reported
to date for the N-arylation of indoles with (hetero)ar-
yl chlorides.
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À
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À
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This research was financially supported by the MINECO
(Proyectos CTQ2011-24502 and CTQ2ß14-52769-C3-3-R),
the Junta de Andalucꢀa (Proyecto P07-FQM-02745) and
Consolider Ingenio 2010, (Grant No. CSD2006-003). S.G.R
4
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COMMUNICATIONS
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6 C N Coupling of Indoles and Carbazoles with Aromatic
Chlorides Catalyzed by a Single-Component NHC-Nickel(0)
Precursor
Adv. Synth. Catal. 2015, 357, 1 – 6
Silvia G. Rull, Juan F. Blandez, Manuel R. Fructos,
Tomꢀs R. Belderrain,* M. Carmen Nicasio*
6
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