Pd-catalyzed coupling of aryl iodides with triarylbismuths as atom-economic multi-coupling organometallic nucleophiles under mild conditions

Article abstract of DOI:10.1016/j.tetlet.2010.09.053

The facile cross-coupling reactivity of triarylbismuth compounds with aryl iodides was achieved under mild heating conditions. The established catalytic protocol using Pd(OAc)₂(Cy₂NH)₂ system exhibited high coupling reactivity with a variety of triarylbismuth and aryl iodide compounds under mild conditions. These coupling reactions were completed in short reaction time affording good to high yields of functionalized biaryl products. The studies of multi-coupling reactions with tris(4-iodophenyl)amine, 8 also furnished moderate to good yields of coupled products, 8a-8f.

Full text of DOI:10.1016/j.tetlet.2010.09.053

Tetrahedron Letters 51 (2010) 6101–6104
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Pd-catalyzed coupling of aryl iodides with triarylbismuths
as atom-economic multi-coupling organometallic nucleophiles
under mild conditions
⇑
Maddali L. N. Rao , Debasis Banerjee, Ritesh J. Dhanorkar
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2010
Revised 7 September 2010
Accepted 10 September 2010
Available online 17 September 2010
The facile cross-coupling reactivity of triarylbismuth compounds with aryl iodides was achieved under
mild heating conditions. The established catalytic protocol using Pd(OAc)2(Cy2NH)2 system exhibited
high coupling reactivity with a variety of triarylbismuth and aryl iodide compounds under mild condi-
tions. These coupling reactions were completed in short reaction time affording good to high yields of
functionalized biaryl products. The studies of multi-coupling reactions with tris(4-iodophenyl)amine, 8
also furnished moderate to good yields of coupled products, 8a–8f.
Keywords:
Triarylbismuths
Aryl iodides
Ó 2010 Elsevier Ltd. All rights reserved.
Palladium
Atom-efficient
Mild conditions
Multi-coupling
There is a growing importance to coupling methodology using
organometallic reagents for C–C bond formations in organic synthe-
sis.1 Organobismuth compounds being non-toxic2a are emerging as
prospective organometallic green reagents for coupling reactions in
organic synthesis.2,3 With the program of developing atom-efficient
or atom-economic multi-coupling organometallic reagents useful
for organic synthesis, we have demonstrated the potential use of tri-
arylbismuth compounds for coupling reactions with various organic
electrophiles.3 The Suzuki–Miyaura, Stille, and Hiyama coupling
reactions now can be easily carried out at room temperature condi-
tions.4–6 However, the studies with new ligands are still continuing
to enrich these methods.7,8 Thus, several protocols with a variety of
ligands based on amines,7 carbenes,8a oximes,8b and imidazolium8c
salts in addition to phosphines8d are reported with promising
reactivity in coupling reactions.
In continuation of our studies with triarylbismuths as reagents
for organic synthesis, it was of interest to achieve BiAr3 couplings
under mild heating conditions. So far the known palladium cata-
lyzed cross-coupling reactions of BiAr3 compounds were per-
formed under 90–100 °C conditions with aryl halides and
triflates.2b,g,3d–f However, the homo-couplings9 of triarylbismuth
compounds readily occur either at room temperature9a or under
heating9b conditions. In this context, we report here a facile palla-
dium catalyzed coupling reaction of triarylbismuth compounds
with aryl iodides under mild heating condition.
To develop an efficient catalytic protocol we have chosen a sim-
ple palladium catalyst, Pd(OAc)2 with an amine as ligand for this
study, as this combination showed novel reactivity in coupling
reactions of aryl boronic acids with aryl bromides under room tem-
perature condition.7c,d As part of these efforts, a systematic screen-
ing was carried out using 4-iodoacetophenone and BiPh3 as a
model reaction under different base, solvent, and temperature con-
ditions (Table 1).
For example, this reaction was studied with Pd(OAc)2/2Cy2NH
system and potassium acetate base in different solvents such as
N,N-dimethylformamide (DMF), tetrahydrofuran (THF), 1,2-dime-
thoxyethane (DME), and acetonitrile at 90 °C (entries 1–4). This
revealed DMF as a suitable solvent to give high conversion to prod-
uct 1a (entry 1). However, the reaction with cyclohexylamine as
ligand provided only moderate conversion to the desired product
(entry 5). Incidentally, the base K3PO4 was proven to be effective
in DMF under different heating conditions with dicylohexyl amine
ligand (entries 6–9). At this stage we prepared Pd(OAc)2(Cy2NH)2
complex (DAPCy) using the literature procedure7c in order to use it
in our study. Encouragingly, the reaction using DAPCy complex also
showed good reactivity at 60 °C in DMF as solvent (entry 10). From
this, it was clear that the catalytic system is equally effective with
either Pd(OAc)2/2Cy2NH or preformed DAPCy system. Further check
with different bases revealed that KOAc is a better choice under
similar conditions (entries 11–13). In most of these reactions the
⇑
Corresponding author. Tel./fax: +91 512 2597532.
E-mail address: maddali@iitk.ac.in (M.L.N. Rao).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.053

6102
M. L. N. Rao et al. / Tetrahedron Letters 51 (2010) 6101–6104
Table 1
Screening conditionsa,b,c
[Pd]
BiPh3
Ac
(3 equiv)
I
+
Ac
Ac Ac
+
Ph
Ph-Ph
+
(1 equiv)
1a
B
A
1
Entry
Catalyst/ligand
Base (equiv)
Solvent
Time (h)
Temp (°C)
1 (%)
A (%)
B (%)
1a (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2CyNH2
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2Cy2NH
Pd(OAc)2/2Cy2NH
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
Pd(OAc)2(Cy2NH)2
None
KOAc (4)
KOAc (4)
KOAc (4)
KOAc (4)
K3PO4 (4)
K3PO4 (4)
K3PO4 (4)
K3PO4 (4)
K3PO4 (4)
K3PO4 (4)
KOAc (4)
K2CO3 (4)
AgOAc (4)
KOAc (4)
KOAc (4)
KOAc (4)
KOAc (3)
KOAc (2)
KOAc (1)
None
DMF
THF
4
4
4
4
4
4
1
1
1
1
2
2
2
2
2
2
2
2
2
2
1
2
2
2
90
90
90
90
90
90
90
80
60
60
60
60
60
40
35
25
35
35
35
35
35
35
35
35
2
50
56
47
2
0
0
4
11
10
7
15
42
9
7
35
7
24
46
63
10
40
23
93
10
11
18
11
15
16
12
15
13
14
9
11
34
9
4
10
4
12
8
10
4
15
6
10
6
0
3
6
4
0
0
0
0
0
0
0
0
76
9
16
34
66
78
77
75
75
72
77
69
DME
MeCN
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
24
81 [73]
80 [81]
55 [45]
89 [89]
64
12
13
20
9
6
8
41
17
80
KOAc (3)
KOAc (3)
KOAc (3)
KOAc (3)
0
0
0
0
52 [44]d
65 [65]e
0
0
a
Reaction conditions: BiPh3 (0.25 mmol, 1 equiv), 4-iodoacetophenone (0.825 mmol, 3.3 equiv), Pd(OAc)2 (0.0225 mmol, 0.09 equiv), amine ligand (0.045 mmol,
0.18 equiv), base, solvent (3 mL), temperature, and time.
b
Pd(OAc)2(Cy2NH)2 (0.0225 mmol, 0.09 equiv) was employed.
GC conversions are given and isolated yields are given in parenthesis. Isolated yields are calculated considering three aryls from BiPh3 for couplings. Thus, 0.75 mmol of
c
product as 100% yield.
d
With 3 mol % of Pd(OAc)2(Cy2NH)2.
With 6 mol % of Pd(OAc)2(Cy2NH)2.
e
formation of biphenyl (A) from BiPh3 and homo-coupled product (B)
from aryl iodide were formed as the side products.
in DMF as solvent at 35 °C in 2 h emerged as an optimal condition
for this reaction (entry 17). This is a remarkable reactivity from the
view point that the cross-coupling of BiPh3 as multi-coupling orga-
nometallic nucleophile is demonstrated under mild heating
conditions.
To suppress these homo-couplings and to further enhance
cross-coupling reactivity, the reactions were further carried out
by lowering the temperature (entries 14–16). Overall, this reaction
was found to be effective even at mild heating conditions either at
35 °C or 40 °C with a high cross-coupling yield and minimized side
products (entries 14 and 15). However, the reaction was found to
be moderate at room temperature (entry 16). Thus, it was clear
that the cross-coupling is facile between 35 and 40 °C
temperatures with high cross-coupling yield. Further study using
different amounts of KOAc base between 1 and 3 equiv revealed
that 3 equiv of base is good enough to get high coupling yield of
1a (entries 17–19). In this case, the product 1a was isolated in
89% yield (entry 17). It is to be noted that a control reaction with-
out a base afforded very poor conversion to the product (entry 20).
At this stage, we have checked the reaction with 1 h reaction time
and this furnished lower yield of the product (entry 21). Impor-
tantly, with 3 and 6 mol % of the catalyst loading the reaction deliv-
ered lower yields of 1a (entries 22 and 23). Further, in the absence
of a catalyst, formation of the cross-coupled product was not
observed (entry 24). Hence, the presence of a catalyst and its
amount is important to obtain higher yield of the coupling product
(entry 17).
This prompted us to extend the scope of this study with differ-
ent BiAr3 compounds as given in Table 2.10 The coupling reactions
using 4-iodoacetophenone as an organic electrophile with triaryl-
bismuths as atom-efficient multi-coupling organometallic nucleo-
philes afforded good to excellent yields of functionalized
biphenyls. Effective participation of both electron-rich and -defi-
cient BiAr3 compounds as multi-coupling nucleophiles was dem-
onstrated in all these reactions. The coupling reactions of
triarylbismuths with phenyl group containing protected formyl
and acetyl groups also fared well under the optimized conditions.
Further, we have also carried out couplings with a diverse range
of aryl iodides under the optimized condition. These results are
summarized in Table 3. This study also revealed efficient couplings
with electron-deficient aryl iodides furnishing excellent yields of
functionalized biaryls. The reactivity of electron-rich iodides was
found to be impressive with moderate yields as these substrates
are otherwise poor substrates in such couplings. The reactions of
ortho-substituted aryl iodide with different BiAr3 compounds also
furnished good yield of products.
With these studies, it was clear that the coupling reaction is
effective with 3 equiv of KOAc base to provide high yield of the
product in 2 h reaction time (entry 17). Essentially, the condition
with KOAc (3 equiv) using Pd(OAc)2(Cy2NH)2 (0.09 equiv) catalyst
Then, we turned our attention to establish the multi-coupling
reactivity of aryl iodides. For this, we have chosen tris(4-iodo-
phenyl)amine (8) for the reaction with different BiAr3 compounds
in a one-pot operation (Scheme 1). These multi-couplings with

M. L. N. Rao et al. / Tetrahedron Letters 51 (2010) 6101–6104
6103
Table 2
Table 3
Cross-couplings with different BiAr3 compoundsa,b,c
Couplings of aryl iodides with BiAr3 compoundsa,b,c
Pd(OAc)2(Cy2NH)2
Me
Pd(OAc)2(Cy2NH)2
(0.09 equiv)
Me
O
I
Bi
(0.09 equiv)
+ Bi
+
I
O
3
R1
(3 equiv)
R2
KOAc (3 equiv)
R2
R1
(3 equiv)
3
KOAc (3 equiv)
1
1a-1k
R
R
º
DMF, 35 C, 2 h
º
DMF, 35 C, 2 h
(3 equiv)
(1 equiv)
(3 equiv)
(1 equiv)
Entry
1
Ar3Bi
Biaryl
Yield (%)
Entry Aryl iodide
BiAr3
Biaryl
Yield (%)
Me
Bi
2a
(2)
1a
NC
I
NC
89
90
Bi
1
2
80
87
3
O
Me
3
Bi
Bi
Me
Bi
Bi
Me NC
Me
2b
1b
Me
Me
2
3
4
5
2
3
3
O
NC
2c
Me
3
3
2
82
3
85
86
80
1c
Me
Me
OMe
2d
Me
O
Me
NC
Bi
Bi
Bi
OMe
4
5
6
7
8
2
2
84
78
75
75
88
Bi
Bi
OMe
OMe 1d
OMe
3
3
O
2e
F
NC
F
Me
3
3
1e
OMe
O
Me
2f
Cl
NC
Cl
2
3
Bi
Bi
Bi
OEt
1f
OEt
6
7
8
85
70
78
3
O
Me
3a
(3)
O2N
I
O2N
O2N
O2N
Bi
Bi
Bi
3
Cl
Cl 1g
3
O
Me
3b
Me
Me
3
3
3
3
F
1h
F
3
O
3c
9
86
3
Me
Cl
Me
Me
OMe
1i
OMe
9
60
Bi
Bi
3d
Bi
Bi
Bi
Bi
Bi
Cl
O2N
O2N
Cl
O
Me
3
10
11
12
13
75
84
75
75
3
O
O
1j
10
11
70
70
OMe
OMe
3e
4a
O
O
O
O
Me
O
3
O
O
3
3
3
1k
Bi
(4)
Cl
I
OMe
OMe
O
3
a
F3C
I
OMe F3C
OMe
Me
5a
6a
(5)
Reaction conditions: 4-iodoacetophenone (0.825 mmol, 3.3 equiv), BiAr3
(0.25 mmol, 1 equiv), Pd(OAc)2(Cy2NH)2 (0.0225 mmol, 0.09 equiv), KOAc
(0.75 mmol, 3 equiv), DMF (3 mL), 35 °C, 2 h.
3
MeO
I
MeO
COOEt
Me
Me
(6)
14
15
50
65
b
Isolated yields were calculated considering three aryl from BiAr3 and 0.75 mmol
3
3
of cross-coupling product as 100% yield.
COOEt
All the products were characterized by 1H NMR, 13C NMR, IR, and HRMS
c
Bi
Bi
7a
I
Me
(7)
analysis.
COOEt
3
16
17
7
7
61
60
7b
7c
Me
Me
COOEt
tris(4-iodophenyl)amine under the conditions studied provided
moderate to high yields of the coupling products, 8a–8f. However,
biaryls as side products were also formed in these reactions as we
have used 0.5 equiv excess of triarylbismuths to facilitate the
multi-coupling process with 8. The excess amount was used to
compensate the loss of BiAr3 due to homo-couplings. Although
not optimized for multi-couplings with 8, the coupling studies
provided moderate to high yield of the products. Thus, the novel
coupling reactivity of BiAr3 compounds was also demonstrated in
the synthesis of molecular motifs useful for photo-physical
applications.12
Our further attempts to establish the reactivity using aryl bro-
mide and aryl triflates with BiAr3 compounds provided mixed re-
sults under the established conditions. While the reaction with
aryl bromide furnished poor yield of the coupling product, aryl tri-
flate did not furnish the desired coupling.
In conclusion, we have demonstrated an efficient cross-cou-
pling of aryl iodides with BiAr3 compounds as atom-efficient mul-
ti-coupling organometallic nucleophiles using mild heating
conditions under palladium catalysis. Overall, the catalytic system
with an easily available palladium precursor and dicyclohexyl-
amine as ligand proved to be very effective for cross-couplings
with triarylbismuth compounds. Further efforts are under way to
broaden the scope of these couplings in organic synthesis.
Bi
F
F
3
a
Reaction conditions: aryl iodide (0.825 mmol, 3.3 equiv), BiAr3 (0.25 mmol,
1 equiv), Pd(OAc)2(Cy2NH)2 (0.0225 mmol, 0.09 equiv), KOAc (0.75 mmol, 3 equiv),
DMF (3 mL), 35 °C, 2 h.
b
Isolated yields were calculated considering three aryl from BiAr3 and 0.75 mmol
of product as 100% yield.
All the products were characterized by 1H NMR, 13C NMR, IR, and HRMS
c
analysis.
Ar
Pd(OAc)2(Cy2NH)2
Ar
I
I
N
N
I
(0.135 equiv)
8
8a-8f
BiAr3 (1.5 equiv),
KOAc (4.5 equiv)
DMF, 35 C, 6 h
º
Ar
8a, Ar = -tolyl(73%)
m
8b, Ar = p-tolyl (75%)
8c,
8d,
8e,
Ar= -trifluoromethylphenyl (55%)
p
Ar = -trifluoromethylphenyl (60%)
m
Ar= -ethoxyphenyl (61%)
p
8f, Ar = -methoxyphenyl (61%)
m
Scheme 1. Multi-coupling reactions of tris(4-iodophenyl)amine.11

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M. L. N. Rao et al. / Tetrahedron Letters 51 (2010) 6101–6104
Chem. Soc. 2006, 128, 4101–4111; (c) Adjabeng, G.; Brenstrum, T.; Wilson, J.;
Acknowledgments
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The authors thank the Department of Science and Technology
(DST), New Delhi for supporting this work. D.B. thanks the Indian
Institute of Technology Kanpur and R.J.D thanks the Council of Sci-
entific and Industrial Research (CSIR), New Delhi for research
fellowships.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.09.053.
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References and notes
10. General procedure: A hot-oven dried Schlenk tube was charged with aryl iodide
(3.3 equiv, 0.825 mmol), followed by triarylbismuth (1 equiv, 0.25 mmol),
KOAc (3 equiv, 0.75 mmol, 0.075 g), Pd(OAc)2(Cy2NH)2 (0.09 equiv,
0.0225 mmol, 0.0132 g) and dry DMF (3 mL) solvent under nitrogen
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35 °C for 2 h. The contents of the mixture were quenched with water (10 mL)
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of 8a–8f correspond to 100% yield.
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