Arylations of allylic acetates with triarylbismuths as atom-efficient multi-coupling reagents under palladium catalysis

Article abstract of DOI:10.1016/j.tetlet.2009.07.148

Arylation of allylic acetates employing triarylbismuths as multi-coupling reagents under palladium-catalyzed conditions was reported. Triarylbismuths as nucleophilic coupling partners were used in sub-stoichiometric amounts with respect to allylic acetate

Full text of DOI:10.1016/j.tetlet.2009.07.148

Tetrahedron Letters 50 (2009) 5757–5761
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Arylations of allylic acetates with triarylbismuths as atom-efficient
multi-coupling reagents under palladium catalysis
*
Maddali L. N. Rao , Debasis Banerjee, Somnath Giri
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:
Arylation of allylic acetates employing triarylbismuths as multi-coupling reagents under palladium-cat-
alyzed conditions was reported. Triarylbismuths as nucleophilic coupling partners were used in sub-stoi-
chiometric amounts with respect to allylic acetates and thus served as atom-efficient multi-coupling
reagents. A variety of allylic acetates were cross-coupled with triarylbismuths to furnish the correspond-
ing functionalized 1,3-disubstituted propenes in good to excellent yields in short reaction times. The
reported palladium protocol also yielded chemo-selective allylic arylations in high yields.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 1 June 2009
Revised 27 July 2009
Accepted 30 July 2009
Available online 3 August 2009
Keywords:
Arylation
Triarylbismuths
Palladium-catalysis
Allylic acetates
Multi-coupling
Allylic substrates are useful for a variety of transformations in
organic synthesis. In the presence of metal catalyst these
substrates undergo selective transformations such as allylic alkyla-
tions, aminations, allylation of carbonyl compounds and Heck-type
couplings.1 Cross-coupling of allylic substrates with aryl halides or
organometallic reagents provides an easy access to functionalized
allylic scaffolds under metal catalysis. In specific, arylation of
allylic substrates using organometallic reagents is a versatile route
for the synthesis of 1,3-diarylpropene2,3 skeletons which are part
of some natural products4a–d and are useful for synthetic transfor-
mations.4e The high reactive nature of allylic substrates often leads
to regio- and stereo-selective reactions under metal-catalyzed con-
ditions.1,2 In the presence of metal catalyst such as palladium(0),
perform multi-couplings with organic electrophiles. This approach
would reduce loadings of organometallic reagents to a consider-
able extent in large-scale preparations.8,9 Triarylbismuths10 have
attracted our attention as multi-coupling organometallic reagents
for C–C bond formations in organic synthesis.11,12 Thus, triarylbis-
muths with their three aryl groups were employed in sub-stoichi-
ometric amounts with respect to organic electrophiles. Further,
triarylbismuths were considered as atom-efficient as all the three
aryl groups were utilized for coupling with three equivalents of or-
ganic electrophiles for C–C bond formations.12
Previously, coupling reactions of allylic bromides with triaryl-
bismuths were reported under palladium catalysis. These cou-
plings have been carried out with Pd(PPh3)4 catalyst in refluxing
tetrahydrofuran for longer reaction times.13,14 In this Letter, our fo-
cused efforts were directed to establish the reactivity of allylic ace-
tates with triarylbismuths under palladium catalysis. This led to
the development of an efficient palladium-catalyzed arylation of
allylic acetates with triarylbismuths as multi-coupling reagents.
Here, we wish to report a rapid and facile palladium-catalyzed che-
mo-selective arylation of allylic acetates with triarylbismuths in
sub-stoichiometric amounts to furnish the corresponding 1,3-
disubstituted propenes in high yields.
allylic substrates generally form p-allylpalladium(II) intermediate
through oxidative addition.3a This intermediate further reacts with
nucleophilic organometallic reagent through transmetalation fol-
lowed by reductive elimination and affords coupling product.3
However, these reactions are often sensitive to metal and ligand
combinations along with the stability and reactivity of allylic and
nucleophilic coupling partners.3
Organometallic reagents as nucleophilic coupling partners offer
a plethora of opportunities under metal catalysis for applications in
organic synthesis. Majority of these reagents react with stoichiom-
etric amounts of organic electrophiles in coupling reactions.5,6
Ever growing environmental concerns7 demand the develop-
ment of reactions with new organometallic reagents which can
To accomplish this, at first we have carried out the arylation
studies using triphenylbismuth and cinnamyl acetate under differ-
ent palladium catalytic conditions as illustrated in Table 1.
In this screening, coupling reactions were carried out using
Pd(PPh3)4 and in combination with bases such as sodium acetate,
potassium acetate, sodium carbonate and potassium phosphate.
These reactions did not furnish appreciable amount of allylic
* Corresponding author. Tel./fax: +91 512 2597532.
E-mail address: maddali@iitk.ac.in (M.L.N. Rao).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.148

5758
M. L. N. Rao et al. / Tetrahedron Letters 50 (2009) 5757–5761
Table 1
Catalyst screening with cinnamyl acetatea-c
[Pd]
+
BiPh3
Ph
OAc
Ph
Ph
2.1
1.1
3.1
(3 equiv)
(1 equiv)
(3 equiv)
Entry
Catalyst
Base/additive (equiv)
Solvent
Temp (°C)
Time (h)
Convb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
None
NaOAc
KOAc
Na2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4/KBr (1)
K3PO4/KBr (2)
K3PO4
K3PO4/KBr (2)
K3PO4/KI (2)
None
DMF
DMF
DMF
DMF
DMA
MeCN
THF
DME
Toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
90
90
90
90
90
90
90
90
90
80
60
90
90
90
90
90
90
90
90
1
1
1
1
1
1
1
1
1
2
3
1
1
1
1
1
1
1
1
15
21
22
38
0
0
7
0
0
46
40
50
55
39
68
78 (76)
39
39
0
None/KI (2)
K3PO4/KI (2)
a
Conditions: BiPh3 (1 equiv, 0.5 mmol), cinnamyl acetate (3.5 equiv, 1.75 mmol), base (1 equiv, 0.5 mmol), palladium catalyst (0.09 equiv, 0.045 mmol), additive (0, 1 or 2
equiv) and solvent (6 mL).
b
Conversion based on GC analysis of the crude reaction mixture. Isolated yield is given in parenthesis.
Homo-coupling product biphenyl from triphenylbismuth formed as a minor side product.
c
arylation product 1,3-diphenylpropene, 3.1 (Table 1, entries 1–4).
The reaction with potassium phosphate base yielded 38% of the
product in N,N-dimethylformamide (DMF) solvent (Table 1, entry
4). Further screening was carried out in different solvents using
Pd(PPh3)4 and potassium phosphate as base. In this, polar aprotic
N,N-dimethylacetamide (DMA), acetonitrile and tetrahydrofuran
(THF) solvents failed to deliver good conversion (Table 1, entries
5–7). The solvents such as 1,2-dimethoxyethane (DME) and tolu-
ene (Table 1, entries 8 and 9) were also found to be equally ineffi-
cient. Then, the reaction was studied by lowering the temperature
condition. This provided a slight increase in the cross-coupling
product 3.1 with improved conversion of up to 46% (Table 1, en-
tries 10 and 11). To further drive the reaction towards cross-cou-
pling, additives have been employed to check their efficacy.
Although we have not observed any dramatic change in the overall
cross-coupling conversion (Table 1, entries 12 and 13), this attempt
yielded a positive outcome at a later stage with the change of cat-
alyst combination. Thus, the initial reaction with PdCl2(PPh3)2 in
DMF solvent did not show any improvement in combination with
K3PO4 as base (Table 1, entry 14). Satisfyingly, under the same con-
dition but with potassium bromide as additive the reaction deliv-
ered conversion up to 68% (Table 1, entry 15). The reaction was
further driven towards high cross-coupling with potassium iodide
as additive which afforded the conversion up to 78% with 76% iso-
lated yield (Table 1, entry 16). The increase in conversion with salt
Further, we have also noted the hydrolysis of cinnamyl acetate
to cinnamyl alcohol in minor amounts during the reaction. Nota-
bly, this hydrolysis was prominent in DMA, acetonitrile, THF,
DME and toluene solvents and that led to no or poor conversion
to cross-coupling product (Table 1, entries 5–9). Although three
equiv of cinnamyl acetate is enough to react with one equivalent
of triphenylbismuth theoretically, we have employed 0.5 equiv of
cinnamyl acetate in excess for all the reactions to compensate
the loss due to partial hydrolysis.
Our systematic study revealed the facile coupling reactivity of
triphenylbismuth for arylation of cinnamyl acetate under the
established palladium-catalyzed conditions (Table 1, entry 16).
This prompted us to further elaborate this study with different tri-
arylbismuths to establish the scope of allylic arylations using var-
ious allylic acetates (Table 2).
The arylation studies carried out with different triarylbismuths
and a variety of functionalized allylic acetates furnished a variety
of 1,3-disubstituted propenes in regio- and chemo-selective man-
ner (Table 2).16 In all these reactions 3.5 equiv of allylic acetate
was employed with 1 equiv of triarylbismuth under the standard
palladium protocol. Firstly, the arylating ability of different triaryl-
bismuths was found to be facile with cinnamyl acetate to afford
1,3-diarylpropenes in high yield (Table 2, entries 1–3). Further ary-
lations were carried out using divergent triarylbismuths in combi-
nation with p-methyl cinnamyl acetate. These reactions proved to
be facile and afforded the functionalized 1,3-diarylpropenes in
high yields (Table 2, entries 4–12). The coupling reaction with tri-
thiophen-2-ylbismuth also afforded the corresponding coupling
product in moderate yield (Table 2, entry 13). Reaction with
3-(naphthalene-2-yl)allylic acetate also yielded the cross-coupling
product in high yield. (Table 2, entry 14). Notably, reactions of
3-bromocinnamyl acetates and 4-bromocinnamyl acetates were
chemo-selective, yielding allylic arylation products in high isolated
yields (Table 2, entries 15–20). Reactivity of 3-methoxycinnamyl
acetate was efficient furnishing high yields of coupled products
additives may be ascribed to the expected stabilization of the g3
-
palladium intermediate with the exchange of acetate for halide an-
ion.3 Additional control experiments were carried out without
base, additive and catalyst combinations. These controls revealed
a definitive role for the catalyst, base and additive to yield higher
cross-coupling conversion (Table 1, entries 17–19). As known be-
fore, the formation of biphenyl as homo-coupling side product
from triphenylbismuth was observed in minor amounts in these
screening reactions.15 However, the amount of biphenyl varied
with respect to cross-coupling conversions.

M. L. N. Rao et al. / Tetrahedron Letters 50 (2009) 5757–5761
5759
Table 2
Arylations of allylic acetates with triarylbismuthsa-c
OAc
+
[Pd]
Bi
R2
2
R2
3
R1
R1
3
1
(3 equiv)
(3 equiv)
(1 equiv)
Entry
1
Triarylbismuth
Allylic acetate
Product
Yieldb
OAc
OAc
Bi
76
82
3.1
3.2
3
Bi
Bi
Me
2
3
3
Me
OAc
OMe
3.3
86
77
80
85
85
77
80
75
75
70
65
70
80
3
OMe
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
OAc
3.4
3.5
3.6
3.7
Bi
Bi
4
3
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
OMe
5
3
OMe
Me
Me
Me
Me
Me
Bi
Bi
Bi
Me
6
3
Me
OMe
F
OMe
7
3
3.8
3.9
F
3
8
Bi
Bi
Cl
3
9
Cl
Me
3.10
3.11
10
11
12
13
14
15
16
3
Me
Me
Me
O
O
Bi
Bi
O
O
3
3
O
O
3.12
3.13
O
O
Me
Me
S
S
Bi
Bi
Bi
Bi
3
OAc
OAc
OAc
Me
3.14
3.15
3
Me
Br
Br
Br
Br
3
3.16
Me
83
3
Me
(continued on next page)

5760
M. L. N. Rao et al. / Tetrahedron Letters 50 (2009) 5757–5761
Table 2 (continued)
Entry
Triarylbismuth
Allylic acetate
Product
Yieldb
80
Br
Br
3.17
OAc
OAc
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
OMe
17
3
3
OMe
OMe
OMe
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
3.18
90
73
80
82
86
82
82
80
82
70
84
79
83
90
85
70
Br
Br
Br
Br
Br
OAc
OAc
3.19
3.20
3
Me
3
Me
Br
OAc
OAc
OAc
MeO
3.21
3
OMe
OMe
MeO
MeO
Me
3.22
3.23
3
Me
OMe
Me
3
OMe
OMe
OAc
OAc
OAc
3.24
3
Me
Cl
Cl
Cl
OMe
3.25
3.26
3
OMe
Cl
Cl
3
Cl
Cl
OAc
OAc
OAc
3.27
3.28
3.29
3
Cl
Cl
Cl
Cl
OMe
3
OMe
Me
Me
3
Cl
OAc
OAc
OAc
3.30
3
Cl
Cl
3.31
3.32
Me
3
Me
Cl
Cl
Cl
Cl
OMe
3
OMe
OAc
OAc
3.33
3.34
3
Bi
Me
34
70
3
Me

M. L. N. Rao et al. / Tetrahedron Letters 50 (2009) 5757–5761
5761
Table 2 (continued)
Entry
Triarylbismuth
Allylic acetate
Product
Yieldb
OAc
Bi
OMe
3.35
35
71
3
OMe
a
Reaction conditions: BiAr3 (1 equiv, 0.5 mmol), allylic acetate (3.5 equiv, 1.75 mmol), PdCl2(PPh3)2 (0.09 equiv, 0.045 mmol), K3PO4 (1 equiv, 0.5 mmol), KI (2 equiv,
1 mmol), DMF (6 mL), 90 °C and 1 h.
b Isolated yields were calculated considering all the three aryl groups for coupling from triarylbismuths. Thus, cross-coupling product 3 equiv (1.5 mmol) corresponds to 100%
yield. All products were characterized by 1H NMR, 13C NMR, IR and ESI-HRMS data and in comparison with the literature data.
c In general, homo-coupling bi-aryls from triarylbismuths were formed in all the reactions and the amount varied with respect to the degree of the cross-coupling product.
325; (f) Kinoshita, H.; Shinokubo, H.; Oshima, K. Synlett 2002, 1916–1918; (g)
Mariampillai, B.; Herse, C.; Lautens, M. Org. Lett. 2005, 7, 4745–4747; (h)
Gomes, P.; Gosmini, C.; Perichon, J. Org. Lett. 2003, 5, 1043–1045; (i) Matsuzaka,
(Table 2, entries 21–23). Further, chloro-substituted allylic acetates
in para, meta and ortho positions afforded chemo-selective aryla-
tion in allylic position (Table 2, entries 24–32). It is worth mention-
ing that arylation of 3-cyclohexylallylic acetate was effective under
the present protocol and furnished 1,3-disubstituted propenes in
good yields (Table 2, entries 33–35). Thus, the general reactivity
of various electronically divergent triarylbismuths was facile for
the arylation of various functionalized allylic acetates under the
established coupling conditions. The present protocol also proved
to be efficient for chemo-selective arylation of allylic acetates.
The effectiveness of triarylbismuths to function as multi-coupling
reagents was demonstrated with allylic acetates in all these cou-
pling reactions. This established the atom-efficient transfer of
three aryl groups from triarylbismuths for the purpose of allylic
arylation.
The noteworthy points regarding the cross-couplings of triaryl-
bismuths with allylic acetates (Tables 1 and 2) in comparison with
the corresponding reactivity known with allylic bromides13 are (i)
the catalyst and solvent combination suitable for allylic bromides
was found to be ineffective for allylic acetate couplings. Reactions
of allylic bromide with triarylbismuth were reported to be facile
with Pd(PPh3)4 in tetrahydrofuran, whereas this combination did
not furnish effective coupling with allylic acetate in our screening
conditions, (ii) the coupling reactivity was not improved either in
different solvents with Pd(PPh3)4 catalyst (Table 1), (iii) allylic ace-
tate was hydrolyzed in some solvents and led to no or poor cross-
coupling conversions during our screening, (iv) allylic acetates
were efficiently coupled under our coupling conditions in very
short reaction time of 1 h compared to longer reaction times re-
ported with allylic bromides and (v) in our protocol allylic acetates
were employed in 3.5 equiv ratio with 1 equiv of triphenylbismuth,
whereas it was higher in amounts with allylic bromides.
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In summary, we have demonstrated an efficient arylation of
allylic acetates with electronically different triarylbismuths under
palladium-catalyzed conditions. This reaction also demonstrated
triarylbismuths as sub-stoichiometric multi-coupling and atom-
efficient reagents for chemo-selective allylic arylation of allylic
acetates under the established palladium protocol conditions.
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Acknowledgements
16. Representative procedure: To an oven-dried Schlenk tube, cinnamyl acetate
(3.5 equiv, 1.75 mmol, 309 mg) was added followed by triphenylbismuth
(1 equiv, 0.5 mmol, 220 mg), K3PO4 (1 equiv, 0.5 mmol, 106 mg), KI (2 equiv,
1.0 mmol, 166 mg), PdCl2(PPh3)2 (0.09 equiv, 0.045 mmol, 31.5 mg) and DMF
(6 mL) under nitrogen atmosphere. The reaction mixture was stirred in an oil
bath maintained at 90 °C for 1 h. After the reaction was complete, the contents
in the Schlenk tube were cooled to room temperature, quenched with 10 mL
water and extracted with ethyl acetate (3 Â 20 mL). The combined organic
extract was washed with water (2 Â 10 mL) followed by brine (20 mL) and
dried over anhydrous MgSO4. The organic extract was concentrated under
reduced pressure. The crude product was purified by column chromatography
using petroleum ether as eluent to afford 1,3-diphenylpropene, 3.1 (221 mg,
76%).
We thank the Department of Science and Technology (DST),
New Delhi for financial support. D.B. also thanks IIT Kanpur and
S.G. thanks CSIR, New Delhi for research fellowships.
References and notes
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Am. Chem. Soc. 2001, 123, 9525–9534; (b) Takeuchi, R.; Kashio, M. J. Am. Chem.
Soc. 1998, 120, 8647–8655; (c) Giambastiani, G.; Poli, G. J. Org. Chem. 1998, 63,
9608–9609; (d) Jang, T.-S.; Keum, G.; Kang, S. B.; Chung, B. Y.; Kim, Y. Synthesis
2003, 775–779; (e) Pretot, R.; Pfaltz, A. Angew. Chem., Int. Ed. 1998, 37, 323–