Palladium-catalyzed direct C-2 arylation of indoles with potassium aryltrifluoroborate salts

Article abstract of DOI:10.1021/jo801371w

(Chemical Equation Presented) Mild conditions have been developed to achieve Pd(OAc)₂-catalyzed regioselective cross-coupling between indoles and potassium aryltrifluoroarylborates in the presence of Cu(OAc) ₂ in acetic acid at room temperature. A variety of potassium aryltrifluoroborates selectively undergo the direct arylation across indoles and afford 2-aryl indoles in moderate to good yields.

Full text of DOI:10.1021/jo801371w

5
,6
Palladium-Catalyzed Direct C-2 Arylation of
Indoles with Potassium Aryltrifluoroborate Salts
rhodium and palladium catalysts, and the subsequent studies
showed that [Ph-I-Ph]BF4 underwent oxidative arylation ef-
7
ficiently with indoles under mild reaction conditions. More
recently, Shi and co-works reported that aryl boronic acids were
good coupling partners with the direct C-arylation of indoles
in the presence of palladium catalysts in an atmosphere of
Jinlong Zhao, Yuhong Zhang,* and Kai Cheng
Department of Chemistry, Zhejiang UniVersity,
Hangzhou 310027, People’s Republic of China
8
oxygen. Aryl boronic acids have been studied extensively in
9
Suzuki-Miyaura reactions, and the use of organotrifluoroborate
yhzhang@ zju.edu.cn
saltsasanalternativetoorganoboronicacidsintheSuzuki-Miyaura
reaction and other coupling reactions has been well developed
ReceiVed June 26, 2008
10,11
by Molander and others.
These salts can be readily prepared
through a variety of one-pot synthetic routes from readily
available starting materials or by the addition of KHF2 to a wide
1
2
variety of organoboron intermediates. By taking advantage
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2
Mild conditions have been developed to achieve Pd(OAc) -
catalyzed regioselective cross-coupling between indoles and
potassium aryltrifluoroarylborates in the presence of
Cu(OAc) in acetic acid at room temperature. A variety of
2
potassium aryltrifluoroborates selectively undergo the direct
arylation across indoles and afford 2-aryl indoles in moderate
to good yields.
(
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10.1021/jo801371w CCC: $40.75  2008 American Chemical Society
Published on Web 08/15/2008

a
TABLE 1. Effect of Catalysts and Oxidants
TABLE 2. Palladium-Catalyzed Direct Arylation of Indole with
a
Potassium Aryltrifluoroborate Salts
b
entry
catalyst (mol %)
Pd(OAc)2 (5%)
Pd(OAc)2 (5%)
Pd(OAc)2(5%)
Pd(OAc)2(5%)
Pd(OAc)2(5%)
Pd(OAc)2(5%)
Pd(OAc)2(5%)
oxidant
yield (%)
1
2
3
4
5
6
7
8
9
(PhCO2)2
mCPBA
H2O2
t-BuOOH
t-BuOOBu-t
K2S2O8
oxone
O2
air
air
N2
air
n.r.
trace
n.r.
c
15
25
c
72
63
71
62
81
Pd(OAc)2(5%)
Pd(OAc)2(5%)
10
11
12
13
14
15
Pd(OAc)2(5%)/Cu(OAc) 2 (10%)
Pd(OAc)2 (5%)/Cu(OAc) 2 (10%)
PdCl2(5%)/Cu(OAc) 2 (10%)
Pd(dba)2 (5%)/Cu(OAc) 2 (10%)
Pd(OTFA)2(5%)/Cu(OAc)2 (10%)
c
10
c
20
air
air
65
53
10
c
Pd(OAc)2 (5%)/Cu(OAc) 2 (100%) N2
a
Reaction conditions: indole (0.5 mmol), potassium phenyltrifluoroborate
0.75 mmol), Cu(OAc)2 (10 mol %), Pd(OAc)2 (5 mol %), room
(
b
c
temperature, 12 h. Isolated yields based on indole. GC yields based on
indole.
of the attractive features of organotrifluoroborate salts, we report
here an efficient and regioselective method for the C-arylation
of indoles with potassium trifluoroborate salts in the presence
of palladium catalyst. The oxidants were proved to be crucial
for the success of the reaction, and the reactions can be
performed in the absence of ligands without any precautions
with regard to the exclusion of air and moisture.
Our initial efforts toward direct coupling of the C-H bond
of indole with potassium phenyltrifluoroborate focused on the
use of oxidants (Table 1, entries 1-10). We first used the
previous oxidants described for arylation of 2-phenylpyridines
1
3
with aryl iodides by us, such as benzoyl peroxide, m-
chloroperbenzoic acid, and hydrogen peroxide, but in this case
they were inactive (Table 1, entries 1-3). Other peroxides
including 2-hydroperoxy-2-methylpropane and 2-(tert-butylp-
eroxy)-2-methylpropane afforded the desired coupling product,
but in very low yields (Table 1, entries 4 and 5). Potassium
peroxydisulfate and oxone showed an obvious effect on the
coupling reaction and moderate yields were achieved (Table 1,
entries 6 and 7). It should be noted that a 62% yield was
obtained for the coupling reaction in the presence of air, while
oxygen provided the higher yield (Table 1, entries 8 and 9).
Considering that oxygen is relatively hard to handle, 10 mol %
Cu(OAc)2 was added as cocatalyst for the reaction in air. It
was found that the rate of the coupling reaction was significantly
improved and the best yields were obtained (Table 1, entry 10).
The reaction was sluggish in nitrogen (Table 1, entry 11). When
PdCl2, Pd(dba)2,and Pd(OTFA)2 were used, the yields decreased
(
Table 1, entries 12-14). The homocoupling product from
potassium phenyltrifluoroborate was observed on a small scale,
and could be easily isolated by column chromatography.
Further investigations of efficiency and scope of the method
were performed and the Pd(OAc)2/Cu(OAc)2 was proved to be
an active catalyst for the direct C-H bond coupling of indoles
with potassium aryltrifluoroborates in air (Table 2).As shown
in Table 2, in most cases, good results were obtained for the
a
Reaction conditions: indole (0.5 mmol), potassium aryltrifluoroborate
salts (0.75 mmol), Cu(OAc) (10 mol %), Pd(OAc)2 (5 mol %), room
temperature, 12 h. Isolated yields based on indole. The yield with
arylboronate acids as substrates is in parentheses. The homocoupling
2
b
c
product from potassium 4-methoxylphenyl trifluoroborate was 54%.
Cu(OAc)2 (100 mol %) was used.
(
13) Cheng, K.; Zhang, Y. H.; Zhao, J. L.; Xie, C. S. Synlett 2008, 1325–
d
1
330.
J. Org. Chem. Vol. 73, No. 18, 2008 7429

TABLE 3. Direct Arylation of Various Substituted Indoles with
potassium trifluoroarylborates with an electron-donating group
in the aromatic ring (Table 2, entries 1-6). However, the
homocoupling of potassium 4-methoxylphenyltrifluoroborate
was very fast and a 54% isolated yield of this byproduct was
obtained, which led to the decrease of yield of desired cross-
coupling product (Table 2, entry 6). Potassium aryltrifluroborate
with ortho substitutent delivered the corresponding arylated
indole in poor yield (Table 2, entry 2), illustrating that the steric
hindrance played a role in the reaction. The presence of strong
electron-withdrawing groups such as -F and -CF3 in the
aromatic ring of aryltrifluoroborate salts led to lower efficiency
even with 100 mol % Cu(OAc)2 (Table 2, entries 8 and 9), while
a moderate yield was obtained for potassium 4-chlorophenyl-
trifluoroborate (Table 2, entry 6). Potassium 2-naphthalenyltri-
fluoroborate carried out the reaction smoothly and a 70% yield
was afforded (Table 2, entry 10). In the case of 1-methyl-1H-
indoles, the efficiency was generally enhanced in respect to that
of free indoles (Table 2, entries 11-19). In particular, the
coupling of sterically hindered potassium trifluoroborates was
feasible, as exemplified by the reaction of potassium 2-meth-
ylphenyltrifluoroborate with 1-methyl-1H-indole to give arylated
product in 82% yield (Table 2, entry 12), which is consistent
a
PhBF3K
8
with the results reported by Shi using aryl boronic acids. The
potassium aryltrifluoroborates need not be in excess for electron-
rich indoles and potassium aryltrifluoroborates, but an excess
is required for electron-deficient indoles and potassium aryltri-
fluoroborates. We also examined the reactivity of aryl boronic
acids and found that the aryl boronic acids were less efficient
than the corresponding potassium aryltrifluoroborates under the
1
4
reaction conditions (Table 2, yield in parentheses). However,
only a trace of the desired product was observed by the analysis
of GC-MS for 3-methylindole, and 2-methylindole was inactive.
The substituent groups on the indoles influenced the direct
arylation reactions as summarized in Table 3. Indoles containing
strongly electron-withdrawing substituents exhibited lower yields
to the arylation than their electron-rich counterparts. For
example, the yields of -CN- and -CO2Et-substituted indoles
were considerably depressed in contrast to Me- and MeO-
substituted indoles (Table 3, entries 1-8). Again, the N-
substituted indoles were more reactive than the free (N-H)
indoles (Table 3, entries 9-16), and N-benzylindole was well
tolerated (Table 3, entry 17). In contrast, N-acetylindole was
inactive to the reaction (Table 3, entry 18).
The reaction mechanism is not clear currently. In the study
of palladium-catalyzed alkenylation of indoles, Gaunt and co-
workers found that the selectivity of C2- and C3-alkenylated
indoles was strongly influenced by solvents, and acidic reaction
1
5
conditions favor C2-alkenylation production. They proposed
that the deprotonation of palladation intermediate 1 at C3 would
be slowed at acidic conditions, which could allow the migration
of C3fC2 to form the intermediate 2 (Scheme 1). The
mechanism studies by Sames et al. also revealed that the first
electrophilic substitution of the indole ring at C3-position
followed by a C3 f C2 migration (Scheme 1, intermediate 1
to 2) was the most probable pathway for the selective C2-
6f
arylation in the direct palladium-catalyzed arylation of indoles.
In our experiment, the electron-rich indoles are more reactive
(
14) (a) Batey, R. A.; Thadani, A. N.; Smil, D. V. Org. Lett. 1999, 1, 1683–
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9202.
a
Reaction conditions: indoles (0.5 mmol), potassium phenyltrifluoro-
2
borate (0.75 mmol), Cu(OAc)2 (10 mol %), Pd(OAc)2 (5 mol %), room
temperature, 12 h. Isolated yields based on indole. Cu(OAc)2 (100 mol
) was used.
b
c
(
15) Grimser, N. P.; Gauntlett, C.; Godfrey, C. R. A.; Gaunt, M. J. Angew.
%
Chem., Int. Ed. 2005, 44, 3125–3129.
7
430 J. Org. Chem. Vol. 73, No. 18, 2008

SCHEME 1. Proposed Mechanism for the Direct Arylation
of Indoles with ArBF
combined with copper as a cocatalyst using air as the oxidant.
The addition of 10 mol % Cu(OAc)2 significantly improved the
reaction and the direct arylation could be performed under
surprisingly mild conditions.
3
K
Experimental Section
General Procedure. A mixture of indole (0.5 mmol), Pd(OAc)
6 mg, 5 mol %), Cu(OAc) (9 mg, 10 mol %; in Table 2, entries
2
(
2
8
1
, 9, 18, and 19, and in Table 3, entries 7, 8, 15, and 16, 91 mg,
00 mol %), potassium aryltrifluoroborate (0.75 mmol), and acetic
acid (5 mL) was stirred at room temperature under air for 12 h.
Afterward, the reaction mixture was filtered through a plug of Celite
and the filtrate was evaporated. The resulting oil was dissolved in
ether (25 mL) and washed with aqueous NaHCO
3
(2 × 15 mL).
The organic layer was dried with Na SO and concentrated. The
2
4
residue was purified by flash column chromatography to afford the
corresponding products.
1
2
-Phenyl-1H-indole [Table 2, entry 1]. H NMR (400 MHz,
than the electron-deficient indoles, which is consistent with the
electrophilic substitution mechanism. On the basis of these
results, we proposed a plausible mechanism for the arylation
of indoles as shown in Scheme 1. First, the aryl is transferred
CDCl
3
, TMS) δ 8.32 (br s, 1 H), 7.62-7.66 (m, 3 H), 7.38-7.45
(m, 3 H), 7.32 (t, 1 H, J ) 7.2 Hz), 7.19 (t, 1 H, J ) 7.2 Hz), 7.12
1
3
(t, 1 H, J ) 7.2 Hz), 6.82 (s, 1 H); C NMR (CDCl
3
, 100 MHz)
δ 137.9, 136.38, 132.3, 129.2, 129.0, 127.7, 125.2, 122.4, 120.7,
1
6
+
1
20.3, 110.9, 100.0; MS (EI) m/z (%) 194 (16) [M + 1], 193
from the metal boron to metal palladium, which undergoes
the electrophilic palladation at the preferential C3-position of
indole. The subsequent 1,2-migration gives intermediate 2, and
the deprotonation leads to the production of intermediate 3,
which liberates the biaryl product after the reductive elimination
with the generation of Pd(0). The pivotal step in this process
involves the oxidation of Pd(0) to Pd(II). We found that
Cu(OAc)2 has a poor effect on the reaction as an oxidant, but
it played some role for the activation of oxygen. For example,
the yields of biaryl increased from 62% to 81% in air after the
addition of 10% Cu(OAc)2, and the reaction was sluggish in
the absence of oxygen even in the presence of 100 mol %
Cu(OAc)2 (Table 1, entries 9-10 and 15).
+
+
7
(
100) [M ], 192 (13) [M - 1], 165 (20).
-Methoxy-2-phenyl-1H-indole [Table 3, entry 5]. H NMR
400 MHz, CDCl , TMS) δ 8.23 (br s, 1 H), 7.65 (d, 2 H, J ) 7.6
1
5
(
3
Hz), 7.43 (t, 2 H, J ) 7.6 Hz), 7.31 (q, 1 H, J ) 7.6 Hz), 7.26 (s,
1
6
1
H), 7.09 (d, 1 H, J ) 2.0 Hz), 6.86 (dd, 1 H, J ) 8.8, 2.4 Hz),
.75 (s, 1 H), 3.87 (s, 3 H); C NMR (CDCl
13
3
, 100 MHz) δ 154.5,
38.6, 132.4, 132.0, 129.7, 129.0, 127.6, 125.0, 112.6, 111.6, 102.2,
+
+
99.8, 55.8; MS (EI) m/z (%) 224 (18) [M + 1], 223 (100) [M ],
208 (72), 180 (21).
8
Acknowledgment. Funding from Zhejiang Provincial Natural
Science Foundation of China (R407106) and the Natural Science
Foundation of China (No. 205710631) is acknowledged
In conclusion, we have developed a palladium-catalyzed
direct arylation of indoles with potassium aryltrifluoroborates
Supporting Information Available: General experimental
procedures and spectroscopic data for all compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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JO801371W
J. Org. Chem. Vol. 73, No. 18, 2008 7431