Pd-catalyzed addition of boronic acids to vinylogous imines: A convenient approach to 3-sec-alkyl substituted indoles

Article abstract of DOI:10.1016/j.tetlet.2012.05.056

A convenient approach to 3-sec-alkyl substituted indoles was developed via palladium-catalyzed addition of arylboronic acids to vinylogous imines generated in situ from sulfonylindoles under mild conditions.

Full text of DOI:10.1016/j.tetlet.2012.05.056

Tetrahedron Letters 53 (2012) 3873–3875
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Pd-catalyzed addition of boronic acids to vinylogous imines: a convenient
approach to 3-sec-alkyl substituted indoles
⇑
Liang-Liang Cao, Xu-Ni Li, Fan-Yan Meng, Guo-Fang Jiang
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 March 2012
Revised 3 May 2012
Accepted 11 May 2012
Available online 17 May 2012
A convenient approach to 3-sec-alkyl substituted indoles was developed via palladium-catalyzed addition
of arylboronic acids to vinylogous imines generated in situ from sulfonylindoles under mild conditions.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Palladium
Conjugate addition
Sulfonylindoles
Vinylogous imines
Boronic acids
The conjugate addition of carbon nucleophiles to activated ole-
fins is one of the most important reactions for carbon–carbon bond
formation.1 Transition metal-catalyzed addition reactions of aryl-
boronic acids to carbon–carbon double bonds-containing com-
pounds and derivatives have emerged as useful transformations
for organic synthesis in part due to the nature of nontoxic and
air/moisture stability of arylboronic acids.2 Recently, Rh(I)-cata-
lyzed addition reactions of arylboronic acids to various electro-
philes have been remarkably developed and widely used.3
Compared with numerous reports of Rh(I)-catalyzed addition
reactions to unsaturated bonds, Pd(II)-catalyzed nucleophilic addi-
tion reactions of arylboronic acids are relatively rare. This is mostly
due to that arylrhodium species are more nucleophilic and capable
of addition to electrophiles, while arylpalladium species are rela-
tively more electrophilic and the addition reactions will be trou-
bled by the formation of the Heck-type coupling products.4 But,
as an important complement to Rh(I)-catalyzed addition reactions,
these years, Pd(II)-catalyzed nucleophilic addition reactions of
arylboronic acids have also attracted much attention and many
successful examples have been reported.5
motif using 3-(1-arylsulfonylalkyl) indole precursors.9 This kind of
sulfonylindole is able to eliminate arenesulfinic acid under the ba-
sic condition, leading to reactive vinylogous imine intermediate.
Several addition reactions involving this intermediate have been
reported, and the desired substituted indoles were obtained
conveniently.10
Given the fact that sec-alkyl-3-substituted indoles are common
structural motifs in
a number of biologically active natural
products and pharmaceutical compounds,11 the significance of
developing a new synthetic method for this kind of indole deriva-
tives is obvious.12,13 Very recently, we have developed a catalyzed
system to access sec-alkyl-3-substituted indoles by Rh(I)-catalyzed
addition reaction of arylboronic acids to vinylogous imines gener-
ated in situ from sulfonylindoles.10j Focus on the synthesis of
Pd(ΙΙ)Ln
Ar-Pd(ΙΙ)Ln
Ar-B(OH)2
Arylpalladium
R
Ar
Addition
Vinylogous imine intermediate based on indole skeleton has
been proved to be a good electrophilic species and useful interme-
diate in indole transformations.6 It can be generated using both
acidic7 and basic8 reaction conditions and upon nucleophilic addi-
tion affords the corresponding substituted indoles. In 2006, Petrini
and co-workers described an innovative approach to this structural
R
R
N
N
Ts
H
Base
-TolSO2H
N
H
Vinylogous Imine
Intermediates
⇑
Corresponding author. Tel.: +86 731 88821861; fax: +86 731 88713642.
E-mail address: guofangjiang@yahoo.com.cn (G.-F. Jiang).
Scheme 1. Nucleophilic addition to vinylogous imines generated in situ from
sulfonylindoles.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.056

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L.-L. Cao et al. / Tetrahedron Letters 53 (2012) 3873–3875
indoles and indole derivatives, we herein disclosed a Pd(II)-cata-
lyzed addition reaction of vinylogous imines with organoboronic
acids, and by this strategy a potential entry to biologically active
indole derivatives would be supplied (Scheme 1).
ucts with moderate to excellent yields. When the substituents R2
are aromatic ones, this palladium-catalyzed reaction exhibited a
superior activity than the rhodium-catalyzed reaction (2 mol %
Rh[(COD)Cl]2, KOH as base, dioxane/water = 10/1, reflux, 12 h)
which we have reported recently10j (Table 2, entries 1–4). For
example, phenyl or para-position of the phenyl ring with Me or
MeO reacted with phenyl boronic acid and all gave good yields of
products. It is worth noting that sulfonylindoles with aliphatic sub-
stituents also exhibited good reactivity in this reaction, aliphatic
substituents such as isopropyl, pentyl, cyclohexyl, and tert-butyl
all afforded good yields of conjugate addition products (Table 2,
entries 5–8). At last the influence of different arylboronic acids
was also examined. The steric hindrance on the phenyl ring of aryl-
boronic acid inhibited the transformation (Table 2, entries 10 and
11), and electron-donating substituents at the phenyl ring of boro-
nic acids were beneficial for this transformation (Table 2, entries
12 and 13), whereas the electron-withdrawing group decreased
the efficiency (Table 2, entry 14).
Our investigation started with the reaction of sulfonylindole 1a
with phenylboronic acid 2a (3.0 equiv) using PdCl2 (5.0 mol %) as
the catalyst, 2,20-bipyridine (6 mol %) as the ligand in THF/H2O
(10:1) under basic condition (K2CO3 2.0 equiv). To our disappoint-
ment, although the mixture was stirred for 12 h and only little
amounts of the desired product 3a were isolated (Table 1, entry
1), some undetermined byproducts were also detected. After
screening the solvent, the best result could be obtained in Tolu-
ene/H2O (10:1), moderate yields of 3a were obtained (53%)
(Table 1, entries 2–4). A number of bases were evaluated, KOH,
Cs2CO3, and NaOH were little effective, K3PO4 gave moderate yields
and K2CO3 was found to be the most suitable base for this reaction
(Table 1 entries 5–8). Next, the effect of different ligands was
examined. Bidentate nitrogen ligand such as 1,10-phenanthroline
afforded the product in poor yield (Table 1, entry 9). Monophos-
phine ligand is more suitable in this reaction: PPh3 gave moderate
yield of 3a, and when the bulky and electron-rich P(1-nap)3 was
used, the product 3a was obtained in 82% yield (Table 1, entries
10 and 11). However, other bidentate phosphines such as dppf,
dppe, and dppb exhibited lower performance (Table 1, entries
12–14). Subsequently, the effect of palladium sources was studied
and we found that Pd(OAc)2 showed better catalytic activity than
PdCl2, the yield of product 3a was increased to 88% (Table 1, entries
15–17). Finally, without addition of water the yield of 3a decreased
sharply to 56% (Table 1, entry 18). Thus, phenylboronic acid 2a
(3.0 equiv), Pd(OAc)2 (5 mol %), P(1-nap)3 (6 mol %), K2CO3
(2.0 equiv), and Toluenen/H2O (10:1) were chosen as the optimized
conditions.
Previously 1,4-addition reactions using arylpalladium species
were mostly based on
a,b-unsaturated carbonyl compounds, and
the mechanism has been well studied: insertion of an unsaturated
ketone into the Ar–Pd bond gives a C- or O-bound enolate, which
yields 1,4-adduct via hydrolysis with water.2c Based on this mech-
anism and excellent reports,5 a possible mechanism for the Pd(II)-
catalyzed addition reaction of arylboronic acids to the vinylogous
imines is proposed as shown in Scheme 2. First, transmetalation
generates the arylpalladium species A from arylboronic acid and
palladium catalyst, followed by insertion of the olefin into the car-
bon–palladium bond giving intermediates B. Rearomatization of
indole leading the formation of intermediates C. Protonolysis of C
gives the corresponding conjugate addition products 3 with the
regeneration of the divalent palladium species making the catalytic
cycle possible.
With the optimized conditions in hand, we turned our attention
to investigate the scope of this transformation. The results are
summarized in Table 2. As expected, various sulfonylindoles
underwent the reaction smoothly, giving the corresponding prod-
In summary, palladium-catalyzed addition of arylboronic acids
to vinylogous imines generated in situ from sulfonylindoles was
successfully developed. This method is an important complement
Table 1
Optimizing the conditions for the reaction of arylsulfonyl indole 1a with phenyl boronic acid 2a
Cl
TolO2S
Pd source (5 mol%)
Cl
L (6 mol%)
+
PhB(OH)2
3.0 equiv
2a
Base (2 equiv)
Sovlent/H2O = 10/1
reflux, 12 h
N
H
1a
N
H
3a
Entrya
Pd source
Ligand
Base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Pd(OAc)2
Pd(OCOCF3)2
Pd(CH3CN)2Cl2
Pd(OAc)2
2,20-Dipridyl
2,20-Dipridyl
2,20-Dipridyl
2,20-Dipridyl
2,20-Dipridyl
2,20-Dipridyl
2,20-Dipridyl
2,20-Dipridyl
1,10-Phenanthroline
PPh3
P(1-nap)3
dppf
dppe
dppb
P(1-nap)3
P(1-nap)3
P(1-nap)3
P(1-nap)3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
THF
12
53
27
<5
21
45
14
18
18
45
82
<5
<5
18
88
74
77
56
Toluene
Dioxane
CH2Cl2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
K3PO4
Cs2CO3
NaOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
9
10
11
12
13
14
15
16
17
18c
a
b
c
Reaction conditions: Pd source (5 mol %), L (6 mol %), 1a (0.2 mmol), 2a (0.6 mmol), solvent (3 mL), H2O (0.3 mL), reflux, 12 h.
Isolated yields based on 1a.
No water was added.

L.-L. Cao et al. / Tetrahedron Letters 53 (2012) 3873–3875
3875
Table 2
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Lett. 2009, 11, 4744; (j) Chen, Q.-A.; Dong, X.; Chen, M.-W.; Wang, D.-S.; Zhou,
Y.-G.; Li, Y.-X. Org. Lett. 2010, 12, 1928.
Substrate scope for Pd-catalyzed reaction of sulfonyl indoles 1 with arylboronic acids
214
R2
R2
SO2Tol
Ar
R1
Pd(OAc)2 (5 mol%)
P(1-nap)3 (6 mol%)
R1
+
ArB(OH)2
3.0 equiv
2
K2CO3 (2 equiv)
Toluene/H2O = 10/1
reflux, 12 h
N
N
H
H
1
3
4. (a) Ueura, K.; Satoh, T.; Miura, M. Org. Lett. 2005, 7, 2229; (b) Lin, S.; Lu, X. J. Org.
Chem. 2007, 72, 9757.
a
Entry
R1
R2
Ar
Yieldb (%)
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1
2
3
4
5
6
7
8
Me
Me
Me
Me
Me
Me
Me
Me
H
Me
Me
Me
Me
Me
4-ClC6H4
Ph
4-MeC6H4
4-MeOC6H4
i-Pr
n-Pentyl
Cy
t-Bu
n-Pentyl
n-Pentyl
n-Pentyl
n-Pentyl
n-Pentyl
n-Pentyl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
88 (3a)
86 (3b)
82 (3c)
81 (3d)
90 (3e)
74 (3f)
87 (3g)
76 (3h)
64 (3i)
32 (3j)
67 (3k)
80 (3l)
82 (3m)
50 (3n)
9
10
11
12
13
14
2-MeOC6H4
3-MeOC6H4
4-MeOC6H4
4-MeC6H4
4-CF3C6H4
a
Reaction conditions: Pd(OAc)2 (5 mol %), P(1-nap)3 (6 mol %), 1 (0.2 mmol), 2
(0.6 mmol), Toluene (3 mL), H2O (0.3 mL), reflux, 12 h.
b
Isolated yields based on 1.
8. (a) Semenov, B. B.; Granik, V. G. Pharm. Chem. J. 2004, 38, 287; (b) Semenov, B.;
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2008, 64, 5262.
ArB(OH)2
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2
Ar-Pd(II)
A
Pd(II)
1
3
Base
Ph
Ar
Ar
(II)Pd
Ph
Ph
H2O
N
N
N
Vinylogous Imine
Intermediates
11. (a) Bosch, J.; Bennasar, M.-L. Synlett 1995, 587; (b) Faulkner, D. J. Nat. Prod. Rep.
2002, 19, 1; (c) Agarwal, S.; Caemmerer, S.; Filali, S.; Froehner, W.; Knoell, J.;
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O’Connor, S. E.; Maresh, J. J. Nat. Prod. Rep. 2006, 23, 532.
C
Pd(II)
B
Scheme 2. Proposed mechanism for the Pd(II) catalyzed conjugate addition reaction.
12. For recent some reviews on synthesis of indoles and substituted indoles, see:
(a) Agarwal, S.; Cammerer, S.; Filali, S.; Frohner, W.; Knoll, J.; Krahl, M. P.;
Reddy, K. R.; Knolker, H. J. Curr. Org. Chem. 2005, 9, 160; (b) Cacchi, S.; Fabrizi,
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to Rh(I)-catalyzed addition reactions which we have reported re-
cently and provides a potential and facile access to 3-sec-alkyl
substituted indole derivatives. Further study will be directed to-
ward the development of asymmetric version of this reaction.
Acknowledgments
13. Selected examples of synthesis of indoles and substituted indoles, see: (a) He,
Q.-L.; Sun, F.-L.; Zeng, X.-J.; You, S.-L. Synlett 2009, 1111; (b) Sun, F.-L.; Zeng,
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We are grateful to financial support from the National Natural
Science Foundation of China (20502005, J0830415), Department
of Science and Technology Foundation of Hunan Province
(2010GK3157), and Hunan Nature Science Foundation (09JJ6024).
Supplementary data
Supplementary data associated with this article can be found, in
theonline version, at http://dx.doi.org/10.1016/j.tetlet.2012.05.056.
14. General procedure for Pd-catalyzed reactions of boronic acids to sulfonylindoles: A
10 mL Schlenk tube was charged with Pd(OAc)2 (2.25 mg, 5 mol %), P(1-nap)3
(4.95 mg, 6 mol %), sulfonylindoles 1 (0.2 mmol), boronic acids 2 (0.6 mmol),
K2CO3 (54.40 mg, 0.4 mmol) and then evacuated under vacuum and placed
under a nitrogen atmosphere. Toluene (3 mL) and water (0.30 mL) were added
subsequently. The mixture was stirred at 100 °C for 12 h. Then water (10 mL)
was added and the mixture was extracted with CH2Cl2 (15 mL) for three times.
The combined organic phase was dried over Na2SO4 and concentrated under
reduced pressure, the residue was subjected to flash chromatography on silica
gel (eluent: petroleum ether/ethyl acetate, 10:1) to yield the corresponding
products 3.
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