Metal-free N-arylation of indolines with diaryliodonium salts

Article abstract of DOI:10.1055/s-0034-1379885

The N-arylation of indolines using diaryliodonium salts as electrophilic arylating reagents is described. Without the use of any additional additives, the desired N-aryl indolines could be obtained in up to 85% yield.

Full text of DOI:10.1055/s-0034-1379885

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Georg Thieme Verlag Stuttgart · New York
2015, 26, 651–655
651
letter
Synlett
S. Riedmüller, B. J. Nachtsheim
Letter
Metal-Free N-Arylation of Indolines with Diaryliodonium Salts
Stefan Riedmüllera,b
Boris J. Nachtsheim*a
R2
TFE, 70 °C
R2
I+
R1
R1
+
Ar
Ar
N
N
metal-free
a
H
Institut für Organische Chemie, Eberhard Karls Universität,
Ar
Auf der Morgenstelle 18, 72076 Tübingen, Germany
boris.nachtsheim@uni-tuebingen.de
Performance Materials Division, Merck KGaA, Frankfurter
b
Straße 250, 64293 Darmstadt, Germany
anilines,12 oxygen nucleophiles,
13,14
N-protected indoles,
15
Received: 05.11.2014
Accepted after revision: 27.11.2014
Published online: 14.01.2015
16
17
18
pyrroles, carbazoles, benzoazoles, or even ammonia to
produce primary aromatic amines.19
DOI: 10.1055/s-0034-1379885; Art ID: st-2014-d0922-l
The research interests of our group are focused on the
development of new C–X coupling strategies involving hy-
pervalent iodine reagents or (hypo)iodites.20 Very recently,
we could develop a novel palladium-catalyzed synthesis of
N-aryl carbazoles using anilines and stable cyclic dia-
Abstract The N-arylation of indolines using diaryliodonium salts as
electrophilic arylating reagents is described. Without the use of any ad-
ditional additives, the desired N-aryl indolines could be obtained in up
to 85% yield.
Key words amination, hypervalent iodine, metal-free, arylation, indo-
line
21
ryliodonium salts. As part of our ongoing research proj-
ects we were interested in direct metal-free arylation reac-
tions of nitrogen heterocycles utilizing diaryliodonium
salts. In this communication we want to present our initial
results describing the first metal free N-arylation of indo-
lines.
Indoline (2,3-dihydroindole) and related congeners are
widely found as a core structural element in alkaloid natu-
1
ral products or other pharmaceutically interesting mole-
2
cules. In particular N-aryl indolines were found to be inter-
In an initial attempt, indoline 1a was treated with 1.1
equivalents of diphenyliodonium triflate in DMF at 130 °C
(Table 1, entry 1). We were gratified to observe moderate
conversion and isolated 25% of N-phenylindoline 3a. In the
same experiment we also detected N-formylindoline as a
side product in significant amounts which results from an
undesired reaction of indoline with the solvent DMF.
Running the reaction at lower temperatures (60 °C) to
prevent side-product formation resulted only in trace
amounts of 3a (Table 1, entry 2). Next, N,N-dimethylacet-
amide (DMAc) was tested as a solvent, but again, only trace
amounts of 3a were observed (Table 1, entry 3). Acetonitrile
showed comparable results to DMF (Table 1, entry 4). Inter-
estingly, we observed a higher product formation with pro-
pionitrile giving 3a in 76% yield (Table 1, entry 5). Using
DMSO as another polar aprotic solvent did not improve the
outcome of the reaction (Table 1, entry 6). Finally, we inves-
tigated fluorinated solvents, in particular 1,1,1,3,3,3-hexa-
fluoroisopropanol (HFIP) and 2,2,2-trifluoroethanol (TFE).
Here, we observed a further increase in product formation
when we used TFE (Table 1, entry 7), while HFIP was com-
pletely inefficient in this transformation (Table 1, entry 8).
esting structural elements due to their recently explored
applications in different areas of organic electronics.3
In general, the N-aryl indoline framework is accessible
via partial hydrogenation of the corresponding fully unsat-
4
urated N-aryl indole precursor. Another recent methodolo-
gy is utilizing styrenes as precursors for the introduction of
5
the annelated nitrogen ring. Completely different is the
construction of the N-aryl indoline via arylation of amines
with nonaromatic ketones through a palladium-catalyzed
6
aerobic dehydrogenative aromatization. For late-stage
functionalization, the indoline-NH moiety is often used.
Here, transition-metal-mediated N-arylation is a common
7
procedure. However, metal-free procedures involving
8
arynes are also known. In the last decade new chemical
transformations based on hypervalent iodine reagents have
9
become very popular in organic chemistry. In particular,
10
diaryliodonium salts could demonstrate their great po-
tential as efficient nontoxic, electrophilic arylating reagents
which are particularly useful in metal-free arylations.11 Ex-
amples from the literature describe arylation reactions of
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Synlett
S. Riedmüller, B. J. Nachtsheim
Letter
Table 1 Optimization Studiesa
X–
H
N
N
additive
+
I+
solvent, temperature
1a
2a
3a
Entry
X–
OTf
Solvent
Additive (equiv)
Time (h)
Temp (°C)
Yield (%)b
1
2
3
4
5
6
7
8
9d
DMF
DMF
DMAc
MeCN
EtCN
DMSO
TFE
–
15
14
66
14
16
18
14
23
18
15
96
13
24
30
42
130
60
60
75
90
110
70
55
70
70
70
75
70
70
70
25
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTs
–
tracec
tracec
27
–
–
–
76
–
tracec
83
–
HFIP
TFE
–
tracec
85
–
10
11
12
13
14
15
TFE
NaOH (1.5)
tracec
tracec
tracec
44
TFE
NaH (1.1)
TFE
KOt-Bu (1.5)
TFE
–
–
–
CF
BF
3
COO
TFE
81
4
TFE
73
a
b
c
Typical reaction conditions: 1a (0.21 mmol), 2a (1.1 equiv), solvent (3–4 mL).
Isolated yield after column chromatography.
Product not isolated.
d
1
a (0.21 mmol), 2a (1.3 equiv).
Base additives, such as sodium hydroxide, sodium hydride,
and potassium tert-butoxide did not improve the product
yield (Table 1, entries 10–12). To complete the optimization,
we investigated diphenyliodonium salts with other counte-
rions (Table 1, entries 13–15). The tosylate salt of 2a was
not efficient at all (Table 1, entry 13), whereas 2a associated
with trifluoroacetate or tetrafluoroborate performed simil-
iarly to the triflate salt. Diphenyliodonium tetrafluorobo-
rate showed a slightly reduced yield of 3a of 73%.
aryliodonium salts bearing 4-bromo or 4-trifluoromethyl
substituents furnished N-aryl indolines 3d and 3e in yields
of 65 and 62%, respectively (Table 2, entries 3 and 4).
Unsymmetrically substituted diaryliodonium salts are
known to react chemoselectively with different types of nu-
cleophiles. However, the chemoselectivity strongly depends
22
1
0a,24
on the nature of the nucleophile
and the presence of a
transition metal.1
1f,12,14a,c,25
To verify the chemoselectivity for the metal-free aryla-
tion of indolines, we reacted 1a under our standard proto-
col together with (2,4,6-trimethylphenyl)(phenyl)iodonium
triflate.2 Here, we isolated a product mixture containing
both possible N-arylated indolines, albeit in low yields.
Compounds 3a and 3f were isolated in 16% and 5% yield.
This result matches well with the general observed chemo-
selectivity trend in nonmetal-catalyzed reactions with hy-
pervalent diaryliodonium salts. Furthermore, a recent re-
port2 from Olofsson and coworkers stimulated us to inves-
tigate N-heteroaryl-containing diaryliodonium triflates.
However, a variety of heteroaryl-containing iodonium tri-
flates 4a–d did not react with indoline 1a under our opti-
Next, we used a variety of iodonium salts with different
substituents in 4-position of the arene moiety.23 Dia-
ryliodonium salts bearing electron-donating (4-Me or 4-
3c
MeO) and electron-withdrawing groups (4-Br or 4-F C)
3
were selected as arylating reagents. From the results sum-
marized in Table 2, a clear trend in aryl-group-transfer ca-
pability is visible. Compared to unsubstituted di-
phenyliodonium triflate, the more electron-rich 4-methyl-
substituted congener reacted in lower yields giving 3b in
5a
4
1% (Table 2, entry 1). By using the 4-methoxy-substituted
1
2
3
iodonium salt (R = H, R = R = OMe), no N-aryl indoline 3c
could be isolated at all. In contrast, electron-deficient di-
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S. Riedmüller, B. J. Nachtsheim
Letter
Table 2 Using Different Substituted Symmetrical and Unsymmetrical Iodonium Saltsa
R2
TfO–
R1
TFE
5 °C
R1
R2
I+
R3
1a
+
R1
7
N
R1
2
3
Entry
R1
R2
R3
Me
Time (h)
N-Aryl indoline 3
Yield (%)b
1
H
Me
OMe
Br
48
88
22
4
3b
3c
3d
3e
41
0
2
H
OMe
Br
3
H
65c
62
16
4
H
CF
3
CF
H
3
5d
Me
Me
24
3a R , R = H
3
1
2
1
2
e
f R , R = Me
5
a
Typical reaction conditions: 1a (0.21 mmol), 2 (1.1 equiv), solvent (3–4 mL).
Isolated yield after column chromatography.
Average of two runs.
Conditions: 1a (0.36 mmol), 2 (1.1 equiv).
Isolated by column chromatography.
b
c
d
e
mized reaction conditions (Scheme 1). We generally isolat-
ed significant amounts of the corresponding 4-substituted
iodoarenes as the sole reaction products.
Nitrogen heterocycles, such as 1H-indole, 1H-ben-
zo[d]imidazole gave trace amounts of product. 1,2,3,4-Tet-
rahydroquinoline did not react at all. With 1H-indazole at
least 12% of the N-arylated product 7 was observed. Next,
we submitted 1H-benzotriazole together with a variety of
diaryliodonium salts. Under optimized reaction conditions
(see Supporting Information) we observed a high N2-selec-
TfO–
R1
N
TFE
R2
I+
1a
+
N
N
26
70 °C
tivity and isolated the N2-arylated products 8a–d in mod-
erate yields (25–46%).
R1
14–70 h
Indolines with a phenyl or ester group in the 2-position
of the indoline underwent reoxidation in the presence of
the hypervalent iodine arylating reagent. In both cases the
corresponding indoles 9a and 9b were isolated (Scheme 2).
a R1 = R = i-Pr
2
4
5
1
2
4
4
4
b R = R = Me
not isolated
c R1 = H, R2 = OMe
d R = H, R2 = F
1
Scheme 1 Attempts using N-heteroaryl diaryliodonium triflates as ary-
lating reagents for indoline 1a
H
H
N
N
R
+
2a OTf
R
TFE, 75 °C
–4 d
2
We next concentrated our work on the exploration of
the substrate scope using other indolines and other nitro-
gen heterocycles (Figure 1). Methyl-substituted indolines
gave the corresponding products 6a and 6b in moderate
yields (50% and 41%, respectively). When we used indolines
with a hydroxy functionality, we were interested if we
could observe a difference in chemoselectivity. However,
from the reaction mixture we only isolated 6c and 6e as
sole N-arylated products in low yields, which indicates that
the free hydroxyl group remained untouched. 3-Nitrile-
substituted indoline gave 6d in 18% yield, while the 2-
methyl derivative yielded 6f in 29% yield.
(1.1 equiv)
9
a 15%
1
1
b R = Ph
c R = COOMe
9b quant.
Scheme 2 Arylation of 2-substituted indolines 1b and 1c yield the cor-
responding indoles
To conclude, we have demonstrated for the first time
the transition-metal-free N-arylation of indolines by utiliz-
ing diaryliodonium salts as mild and nontoxic arylating re-
agents. Beside other indolines, 1H-benzotriazole could be
used as well in our N-arylation procedure. The correspond-
ing N-arylated products could be isolated in moderate to ac-
ceptable yields. In future research we want to look deeper
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S. Riedmüller, B. J. Nachtsheim
Letter
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N
N
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OH
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6b 41%
6c 15%
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N
N
N
N
N
N
N
(
2
7
12%
N
8a 46%
8b 25%
(
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N
N
Cl
N
CF3
N
N
8c 40%
8d 37%
Figure 1 Scope of isolated N-arylated indolines and 2-arylated benzo-
triazoles in our investigation
(
(
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Acknowledgment
2
(
(
(
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20) (a) Kloeckner, U.; Weckenmann, N. M.; Nachtsheim, B. J. Synlett
Financial support by the Deutsche Forschungsgemeinschaft (DFG)
and the Fonds der Chemischen Industrie (Sachkostenzuschuss) is ac-
knowledged. S.R. gratefully acknowledges Merck KGaA for financial
support.
2
012, 23, 97. (b) Hempel, C.; Weckenmann, N. M.; Maichle-
Moessmer, C.; Nachtsheim, B. J. Org. Biomol. Chem. 2012, 10,
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Supporting Information
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1202.
(
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1379885.
S
u
p
p
ortioIgnfrm oaitn
S
u
p
p
ortioIgnfrm oaitn
(22) When diaryliodonium salts with more nucleophilic counterions
e.g., halogens) were used, in some cases they have a negative
(
influence on the specific reaction due to the nucleophilic nature
of the counterion; cf. ref. 12 and 14b.
References and Notes
(1) Gan, Z.; Reddy, P. T.; Quevillon, S.; Couve-Bonnaire, S.; Arya, P.
Angew. Chem. 2005, 117, 1390.
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Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 651–655

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55
Synlett
S. Riedmüller, B. J. Nachtsheim
Letter
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23) (a) Zhu, M.; Jalalian, N.; Olofsson, B. Synlett 2008, 592.
slowly to the solution, which was then heated to 70 °C for 14 h.
After the solution was cooled to r.t., the mixture was diluted
with H O and sat. NaHCO . The aqueous phase was extracted
(b) Bielawski, M.; Olofsson, B. Chem. Commun. 2007, 2521.
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(
2
3
349, 2610. (d) Kuriyama, M.; Hamaguchi, N.; Onomura, O. Chem.
several times with CH Cl . The organic phase is washed with
2 2
Eur. J. 2012, 18, 1591.
H O, dried over MgSO , filtered, and concentrated in vacuo. The
2 4
(24) Malmgren, J.; Santoro, S.; Jalalian, N.; Himo, F.; Olofsson, B.
Chem. Eur. J. 2013, 19, 10334.
crude residue was purified by column chromatography on silica
gel, eluting with cyclohexane–CH Cl (2:1, v/v) giving 34 mg
2
2
(
25) (a) Bielawski, M.; Malmgren, J.; Pardo, L. M.; Wikmark, Y.;
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(83%) of 3a as a colorless solid.
2,3-Dihydro-1-(4-methylphenyl)-1H-indole (3b)
Pale yellow oil, partially crystalline (18 mg, 41%); eluent: cyclo-
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1338. (e) Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S. J.
2
2
1
H NMR (400 MHz, CDCl ): δ = 7.17–7.13 (m, 5 H), 7.07–7.04
3
(m, 2 H), 6.75–6.69 (m, 1 H), 3.93 (t, 2 H, J = 8.5 Hz), 3.12 (t, 2 H,
13
1
Am. Chem. Soc. 2005, 127, 7330. (f) Deprez, N. R.; Sanford, M. S.
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A. J.; Xu, W.; Suero, M. G.; Gaunt, M. J. J. Am. Chem. Soc. 2013,
J = 8.5 Hz), 2.33 (s, 3 H). C{ H} NMR (100.6 MHz, CDCl ): δ =
3
147.7, 141.9, 131.2, 130.8, 129.8, 127.2, 125.1, 118.6, 118.3,
108.0, 52.5, 28.3, 20.9. IR (neat): 2360, 2341, 1770, 1742, 1597,
1515, 1482, 1456, 1384, 1312, 1296, 1246, 1166, 1127, 1063,
1049, 924, 869, 815, 742, 703 cm . HRMS (APCI): m/z calcd for
C15H15N: 209.12045; found [M + H] : 210.12714.
–
1
135, 12532. (i) Phipps, R. J.; Grimster, N. P.; Gaunt, M. J. J. Am.
+
Chem. Soc. 2008, 130, 8172. (j) Phipps, R. J.; McMurray, L.; Ritter,
S.; Duong, H. A.; Gaunt, M. J. J. Am. Chem. Soc. 2012, 134, 10773.
26) (a) Beletskaya, I. P.; Davydov, D. V.; Moreno-Mañas, M. Tetrahe-
dron Lett. 1998, 39, 5621. (b) Tomas, F.; Abboud, J. L. M.; Laynez,
J.; Notario, R.; Santos, L.; Nilsson, S. O.; Catalan, J.; Claramunt, R.
M.; Elguero, J. J. Am. Chem. Soc. 1989, 111, 7348. (c) Liu, Y.; Yan,
W.; Chen, Y.; Petersen, J. L.; Shi, X. Org. Lett. 2008, 10, 5389.
27) General Procedure for the N-Arylation of Indoline (1a) with
Diphenyliodonium Triflate (2a) to 1-Phenylindoline (3a)
Diphenyliodonium triflate (2a, 0.1 g, 0.23 mmol, 1.1 equiv) was
charged in a vial and sealed with a septum. After adding TFE (3
mL), indoline (1a, 0.024 mL, 0.21 mmol) was added dropwise
1-(4-Chlorophenyl)-2-methylindoline (6f)
(
Pale yellow oil (15 mg, 29%); eluent: cyclohexane–CH Cl (5:1,
2
2
1
v/v). H NMR (500 MHz, CDCl ): δ = 7.31 (d, 2 H, J = 8.8 Hz), 7.18
3
(d, 2 H, J = 8.8 Hz), 7.14 (d, 1 H, J = 7.2 Hz), 7.03 (t, 1 H, J = 7.5
Hz), 6.78 (d, 1 H, J = 7.9 Hz), 6.74 (t, 1 H, J = 7.4 Hz), 4.36–4.30
(m, 1 H), 3.33 (dd, 1 H, J = 8.8 Hz, J = 15.5 Hz), 2.76 (dd, 1 H,
1
2
13
1
(
J = 7.4 Hz, J = 15.4 Hz), 1.31 (d, 3 H, J = 6.1 Hz). C{ H} NMR
1
2
(125.8 MHz, CDCl ): δ = 148.4, 142.3, 129.7, 129.4, 127.7, 127.3,
3
125.1, 122.9, 119.3, 108.5, 60.1, 37.3, 20.1. HRMS (APCI): m/z
calcd for C15H14ClN: 243.08148 (100%), found [M
244.08851.
+
+
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Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 651–655

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