Synthesis of tri- and tetrasubstituted olefins by palladium cross-coupling reaction

Article abstract of DOI:10.1055/s-2006-956467

Tri- and tetrasubstituted olefins were obtained in high yields and regioselectivities using stilbene as starting material. First, stilbene was converted into (E)-bromostilbene by a bromination-dehydrobromination sequence. Then, (E)-bromostilbene was coupled with arylboronic acids at room temperature and low loading of Pd catalyst precursor (0.5-0.05 mol%) to afford selectively (E)-1-aryl-1,2-phenylethylenes in high yields (87-98%). Bromination of triphenylethylene afforded directly the bromotriphenylethylene that also underwent coupling reactions with arylboronic acids under mild conditions to afford tetrarylethylene (88-90% yield). Under the same conditions attempted Suzuki cross-coupling reactions of (E)-bromostilbene or 1,1,2-triphenylethene with alkylboronic acids were unsuccessful. However, the alkyl group could be introduced under mild conditions and high yields by using a Pd-catalyzed Negishi coupling protocol. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-956467

LETTER
103
Synthesis of Tri- and Tetrasubstituted Olefins by Palladium Cross-Coupling
Reaction
Synthesis of
Tri-
a
and Tetrasu
r
bstitute
d
Olefins lina M. Nunes, Daniela Steffens, Adriano L. Monteiro*
Group of Molecular Catalysis, Instituto de Química, UFRGS. Av. Bento Gonçalves, 9500 Porto Alegre 91501-970, CP 15003, RS, Brazil
Fax +55(51)33167304; E-mail: almonte@iq.ufrgs.br
Received 5 September 2006
ethane to the synthesis of styrene derivatives and 1,1-di-
Abstract: Tri- and tetrasubstituted olefins were obtained in high
arylolefins, respectively.4 We have also shown that 1,2-
(dibromoethyl)arenes, easily obtained from the bromina-
tion of the corresponding styrene derivatives, can be
yields and regioselectivities using stilbene as starting material.
First, stilbene was converted into (E)-bromostilbene by a bromina-
tion–dehydrobromination sequence. Then, (E)-bromostilbene was
coupled with arylboronic acids at room temperature and low load- transformed into 2-arylacrylic esters by a one-pot de-
hydrobromination–carbonylation sequence.5 The use of
ing of Pd catalyst precursor (0.5–0.05 mol%) to afford selectively
(E)-1-aryl-1,2-phenylethylenes in high yields (87–98%). Bromina-
tion of triphenylethylene afforded directly the bromotriphenylethyl-
carbonylation of expensive and less available alkynes.
ene that also underwent coupling reactions with arylboronic acids
styrene in this reaction is an advantage over the traditional
The latter are also the starting material for most of the pal-
under mild conditions to afford tetrarylethylene (88–90% yield).
ladium-catalyzed syntheses of tri- and tetrasubstituted
olefins. Stilbene and substituted stilbenes are easily ob-
tained from a Pd-catalyzed Heck reaction. Herein we re-
port a method for the synthesis of tri- and tetrasubstituted
olefins using stilbene as a platform (Scheme 1) using a
bromination–dehydrobromination–Suzuki and Negishi
cross-coupling reaction sequence.
Under the same conditions attempted Suzuki cross-coupling reac-
tions of (E)-bromostilbene or 1,1,2-triphenylethene with alkylbo-
ronic acids were unsuccessful. However, the alkyl group could be
introduced under mild conditions and high yields by using a Pd-cat-
alyzed Negishi coupling protocol.
Key words: cross-coupling reactions, Suzuki reaction, palladium
We used N- and S-based palladacycles to produce stilbene
on a 20-gram scale.6 Bromination of the stilbene at 0 °C
using CH2Cl2 as solvent gave the 1,2-dibromo-1,2-di-
phenylethane in 71%, that underwent dehydrobromina-
tion (K2CO3 as base in a 1:1 mixture of THF–MeOH at
r.t.) furnishing the (E)-bromostilbene in 70–88% yield
and 97–99% regioselectivity.5
The construction of tri- and tetraarylolefins with a high
degree of stereocontrol remains a significant challenge in
organic synthesis. Amongst the several approaches de-
scribed in the literature, Pd-catalyzed reactions have been
widely applied in the synthesis of tri- and tetrasubstituted
olefins.1 The palladium-catalyzed cross-coupling of aryl
halides with arylboronic acids (Suzuki reaction) is a well
established and efficient method for the construction of
An initial screening was performed in order to determine
the best conditions for the Pd-catalyzed Suzuki cross-cou-
pling of (E)-bromostilbene with 4-methoxyphenylboronic
acid. We have found similar results to those obtained for
the coupling of arylboronic acids with vinyl bromide, gen-
erated in situ from 1,2-dibromoethane: Pd(OAc)2/PPh3 as
catalyst precursor, KOH as base, and a solvent mixture
(MeOH–THF). However, the reaction can be carried out
under milder conditions, such as at room temperature,
with lower loadings of palladium (0.05–0.5 mol%). It is
also important to note that under these conditions
Pd(OAc)2 without triphenylphosphine ligand gave also
Caryl–Caryl bonds and has found widespread use in organic
and polymer synthesis. On the other hand, the Suzuki
reaction with vinyl halide derivatives affording tri- and
tetraarylolefins has attracted less attention.2 For instance,
the use of [PdCl(C3H5)]2/Tecicyp {cis,cis,cis-1,2,3,4-
tetrakis[(diphenylphosphanyl)methyl]cyclopentane} at
low loadings enables the coupling of vinyl halides at
130 °C.2a,3 We have recently applied the Suzuki reaction
of arylboronic acids with vinyl bromides generated in situ
from 1,2-dibromoethane and 1,2-dibromo-1-phenyl-
R1
R1
R2
1. bromination
2. dehydrobromination
1. bromination
2. Suzuki or Negishi
reaction
3. Suzuki or Negishi
reaction
Scheme 1 Synthesis of tri- and tetrasubstituted olefins from trans-stilbene
SYNLETT 2007, No. 1, pp 0103–0106
0
4.
0
1.
2
0
0
7
Advanced online publication: 20.12.2006
DOI: 10.1055/s-2006-956467; Art ID: S16906ST
© Georg Thieme Verlag Stuttgart · New York

104
C. M. Nunes et al.
LETTER
the expected coupling product. However, since higher re- THF and methanol, and the mixture was stirred overnight
action times and palladium loadings are necessary for the at room temperature. The reaction mixture was filtered off
ligandless reaction we chose to follow our studies using a and the reagents for the coupling reaction were added [p-
mixture of Pd(OAc)2/PPh3 as catalyst precursor. The best methoxyphenylboronic acid, Pd(OAc)2, PPh3, and KOH].
conditions so far developed were applied to a variety of The mixture was stirred at room temperature for one hour.
arylboronic acids (Table 1). Under these conditions, the After work-up, (E)-1,2-diphenyl-1-(p-methoxyphen-
reaction proceeds smoothly and the (E)-1-aryl-1,2-di- yl)ethene was obtained in 91% yield and 98%, regioselec-
phenylethylenes were isolated in high yields. Long tivity.
reaction times were necessary for the coupling with aryl-
boronic acids containing electron-withdrawing groups
(Table 1, entries 5–8). The regioselectivity was main-
tained in the cross-coupling reaction since it was the same
MeO
1. K2CO3 (2 equiv),
MeOH–THF,
25 °C, over-night
H
Br
as that observed for the starting stilbene (95–98%).
H
2. Pd(OAc)2 (0.5 mol%),
PPh3 (1 mol%); KOH (2 equiv),
p-MeOC6H4B(OH)2;
Br
It is worthwhile to mention that dehydrobromination and
coupling reaction occur in a basic media in a common sol-
vent media (THF–MeOH) making a one-pot protocol pos-
sible (Scheme 2). Therefore, in a resealable Schlenk flask
were placed 1,2-dibromo-1,2-diphenylethane, K2CO3,
MeOH–THF, 25 °C, 1 h
Scheme 2 Synthesis of trisubstituted olefins from 1,2-dibromo-1,2-
diphenylethane
Table 1 Pd-Catalyzed Suzuki Cross-Coupling Reaction of (E)-Bro-
Since important arylated ethylenes such as Tamoxifen
contain an ethyl group attached to the double bond,2a we
have also investigated the cross-coupling reaction of (E)-
bromostilbene with alkylboronic acids. Under the same
conditions no conversion of starting material was ob-
served when we used methyl- and butylboronic acid
(Table 1, entries 9 and 10). Some coupling product (up to
10%) was obtained by raising the temperature to 100 °C
and increasing the palladium loading [4 mol% Pd(OAc)2]
but the reduced product stilbene was the main product (up
to 90%). It is know that alkylboronic acids are less prone
to react with aryl halides than arylboronic acids.7 On the
other hand, it has been shown that (E)-bromostilbene
could be converted into 1-aryl-1,2-diphenylethene by a
cross-coupling reaction with arylzinc chloride in the pres-
ence of catalytic amounts of Pd(PPh3)4.1 Therefore, we
decided to insert an alkyl group by a Pd-catalyzed cross-
coupling reaction of (E)-bromostilbene with alkyl zinc ha-
lides. We investigated the coupling of (E)-bromostilbene
with ethylzinc chloride in THF at room temperature using
different catalyst precursors and phosphine ligands
(Table 2). Low yields were obtained in the absence of
phosphine ligands (Table 2, entries 1 and 2). Dppf was the
best ligand (Table 2, entries 3–6) when Pd(OAc)2 was
used as the catalyst precursor. Triphenylphosphine is a
very cheap phosphine and the difference was not so pro-
nounced when a phosphine–palladium complex was used
(Table 2, entries 7 and 8). Therefore, the reaction was car-
ried out using PdCl2(PPh3)2 as catalyst precursor on a 1.5-
mmol scale and the (Z)-1,2-diphenyl-1-butene was ob-
tained in 90% isolated yield. Once again the regioselectiv-
ity was maintained in the cross-coupling reaction
(Table 2, entry 9).
mostilbene with Arylboronic Acidsa
Br
Ar
Pd(OAc)2/PPh3
+ ArB(OH)2
KOH, 25 °C
MeOH–THF
Entry
1
ArB(OH)2
Time (h)b Yield (%)c
1
1
1
1
94
98
95
96
B(OH)2
2
3
4
MeO
Me
B(OH)2
B(OH)2
CH3
B(OH)2
5
6
7
15
15
15
97
87
90
Cl
B(OH)2
B(OH)2
CF3
CF3
B(OH)2
8
15
93
CF3
B(OH)2
9
MeCH2CH2CH2B(OH)2
MeB(OH)2
24
24
NR
NR
10
The synthesis of tetraarylethylenes was also possible by
using the same Suzuki reaction protocol established for
the synthesis of triarylethylenes. Bromotriphenylethyl-
ene, obtained from the bromination of triphenylethylene,
underwent a coupling reaction with arylboronic acids us-
ing Pd(OAc)2/PPh3 as catalyst precursor at room temper-
a Reaction conditions: (E)-bromostilbene (1 mmol), arylboronic acid
(1.2 mmol), KOH (2 mmol), Pd(OAc)2 (0.005 mmol), PPh3 (0.01
mmol), MeOH (2.5 mL), THF (2.5 mmol), 25 °C.
b Reaction times were not optimized.
c Isolated yields. NR = no reaction.
Synlett 2007, No. 1, 103–106 © Thieme Stuttgart · New York

LETTER
Synthesis of Tri- and Tetrasubstituted Olefins
105
ature to afford tetraarylethylenes in high yields (Table 3,
entries 1 and 2). On the other hand, the protocol obtained
for the reaction of (E)-bromostilbene with ethyl zinc chlo-
ride was not directly transposable for the coupling of bro-
motriphenylethylene. Using PdCl2(PPh3)2 as catalyst
precursor, lower selectivities were observed for the reac-
tion of bromotriphenylethylene with ethylzinc chloride,
the reduced product (triphenylethylene) was obtained in
the same proportion as the coupling product (Table 3, en-
try 3). The selectivity in coupling product was improved
by using diphosphines instead of monophosphine ligands
(Table 3, entries 3–10), but, since lower activities were
observed, a higher temperature (100 °C) was necessary to
achieve high conversion. Therefore, using BINAP as
ligand and 2 mol% of palladium catalyst precursor, bro-
motriphenylethylene reacted cleanly with ethyl zinc chlo-
ride at 100 °C affording (Z)-1,1,2-triphenyl-1-butene in
97% selectivity and 91% yield after work-up.
Table 2 Pd-Catalyzed Cross-Coupling Reaction of (E)-Bromo-
stilbene with Ethylzinc Chloridea
Br
[Pd]
+
EtZnCl
THF, 25 °C
Entry
Catalyst
Ligand
Conv. (%) Yield (%)b
1
2
3
4
5
6
7
8
9c
Pd(OAc)2
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
–
31
9
3
–
5
PPh3
45
46
35
77
85
65
100
41
PCy3
34
P(o-Tol)3
21
dppf
73
PdCl2(dppf)
PdCl2(PPh3)2
PdCl2(PPh3)2
–
–
–
68
In summary, we have found that a simple catalyst precur-
sor prepared in situ from palladium acetate and triphen-
ylphosphine can be used in the synthesis of tri- and
tetraarylethylene, which were obtained in high yields and
regioselectivities, using stilbene as starting material. The
Suzuki cross-coupling reactions can be carried out at low
catalyst loadings at room temperature. Alkyl groups could
be introduced under mild conditions and high yields by
using a Pd-catalyzed Negishi coupling protocol. The
64
98 (90)
a Reaction conditions: (E)-bromostilbene (0.25 mmol), ethylzinc
chloride (1.25 equiv), Pd (1 mol%), Pd(OAc)2/PR3 ratio = 1:2,
Pd(OAc)2/dppf ratio = 1:1, THF (4 mL), 25 °C, 1 h.
b GC yields. Isolated yields are giving in parentheses.
c Reactions conditions: (E)-bromostilbene (1.5 mmol), 15 h.
Table 3 Synthesis of Tetrasubstituted Ethylenes
Br
R
RB(OH)2
or RZnCl
+
Entry
1b
RM
Ph-B(OH)2
Catalyst Precursor
Pd(OAc)2/PPh3
Conv. (%)
Coupling product (%)a Reduced product (R = H) (%)
100
100
94
100 (88)
100 (90)
51
–
2b
p-MeOC6H4B(OH)2 Pd(OAc)2/PPh3
–
3c,d
4c,d
5c,e
6c,e
7c,e
8c,e
9c,e
10c,e
EtZnCl
EtZnCl
EtZnCl
EtZnCl
EtZnCl
EtZnCl
EtZnCl
EtZnCl
PdCl2(PPh3)2
49
54
31
45
5
Pd(OAc)2/PCy3
Pd(OAc)2/dppf
Pd(OAc)2/dppe
Pd(OAc)2/dppp
Pd(OAc)2/dppb
76
46
100
53
69
55
99
95
100
92
8
Pd(OAc)2/p-tolyl BINAP 100
Pd(OAc)2/BINAP 100
90 (88)
97 (91)
10
3
a Isolated yields are given in parentheses.
b Reaction conditions: bromotriphenylethylene (1 mmol), arylboronic acid (1.2 mmol), KOH (2 mmol), Pd(OAc)2 (0.5 mol%, 0.005 mmol),
PPh3 (0.01 mmol), MeOH (2.5 mL), THF (2.5 mmol), 25 °C, overnight.
c Reactions conditions: bromotriphenylethylene (0.25 mmol), Pd (2 mol%, 0.005 mmol), EtZnCl (0.3 mmol), overnight.
d At 25 °C.
e At 100 °C.
Synlett 2007, No. 1, 103–106 © Thieme Stuttgart · New York

106
C. M. Nunes et al.
LETTER
142.95, 143.47 ppm. IR (mull): 2952, 2925, 2854, 1461, 1376, 760,
700 cm–1. GC-MS (IE, 70 eV): m/z (%) = 284 (43) [M+], 78 (100),
77 (57), 51 (52), 91 (47), 191 (47), 39 (41), 165 (39).
extension of these protocols for the selective synthesis of
biologically active tri- and tetraarylated olefins is now
under investigation in our group.
Acknowledgment
General Experimental Procedures
We thank FAPERGS, PRONEX and CNPq for partial financial sup-
port. We also thank CNPq (C.N. M) and FAPERGS (D.S.) for scho-
larships.
All reactions were carried out under argon atmosphere in oven dried
resealable Schlenk tube. Iodobenzene was purchased from Acros,
and styrene was purchased from Aldrich and dried before use.
MeOH and THF were degassed and dried, respectively. Arylboron-
ic acids were prepared according to the previously published proce-
dure.8 Chemicals were used without purification. NMR spectra
were recorded on a Varian XL300 spectrometer, infrared spectra
performed in a SHIMADZU FTIR-8300 spectrometer, and mass
spectra obtained on a GC/MS Shimadzu QP-5050 (EI, 70eV). Gas
chromatography analyses were performed on a HP column DB-17
GC with a FID and 30 m capillary column with a dimethylsiloxane
stationary phase.
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Typical for the Suzuki Coupling of 1-Bromo-1,2-diphenyl-
ethene and Arylboronic Acids
An oven-dried resealable Schlenk flask was charged with 1-bromo-
1,2-diphenylethene (259 mg, 1 mmol), evacuated and black-filled
with argon. Then, Pd(OAc)2 (1.1 mg, 0.005 mmol), PPh3 (2.6 mg,
0.01 mmol), phenylboronic acid (146 mg, 1.2 mmol), KOH (112
mg, 2 mmol), MeOH (2.5 mL), THF (2.5 mL) were added. The re-
action mixture was stirred at r.t. for 1 h. The solution was then taken
up in Et2O (30 mL) and washed with aq NaOH (1 M, 10 mL) and
brine (2 × 5 mL). The organic layer was dried over MgSO4, filtered,
and concentrated under reduced pressure. The crude material was
purified by flash chromatography on silica gel with cyclohexane,
affording triphenylethene (241 mg, 94% yield).
Triphenylethylene: white solid, mp 68.6 °C (lit. mp 67–69 °C).9 1
H
NMR (300 MHz, CDCl3): d = 6.96 (s, 1 H), 7.01–7.33 (m, 15 H)
ppm. 13C NMR (75.4 MHz, CDCl3): d = 126.71, 127.38, 127.47,
127.57, 127.92, 128.17, 128.60, 129.51, 130.35, 137.33, 140.32,
142.54, 143.38 ppm. IR (mull): 2924, 2854, 1463, 1377, 760, 695
cm–1. GC-MS (IE, 70 eV): m/z (%) = 256 (100) [M+], 178 (89), 120
(71), 126 (60), 179 (50), 113 (46), 51 (42), 165 (38), 255 (26).
Typical for the Negishi Coupling of 1-Bromo-1,2-diphenyl-
ethene and Ethyl Zinc Chloride
An oven-dried resealable Schlenk flask was charged with ZnCl2
(273 mg, 2 mmol), THF (3 mL), diethyl zinc (2 mL of a 1 M solu-
tion in hexane, 2 mmol). The mixture was stirred at r.t. for 1 h before
use. The mixture was transferred to an oven-dried resealable
Schlenk flask containing 1-bromo-1,2-diphenylethene (388 mg, 1.5
mmol) in THF (5 mL). Finally, Pd(PPh3)2Cl2 (10.5 mg, 0.015
mmol) was added and the reaction was stirred at r.t. for 2 h. The sol-
vent was evaporated under reduced pressure, and the crude material
was purified by flash chromatography on silica gel with hexane fur-
nishing (Z)-1,2-diphenyl-1-butene (281 mg, 90% yield).
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1
(Z)-1,2-Diphenyl-1-butene: white solid, mp 170.8 °C. H NMR
(300 MHz, CDCl3): d = 1.06 (t, J = 7.5 Hz, 3 H), 2.50 (q, J = 7.4 Hz,
2 H), 6.42 (s, 1 H), 6.90–7.40 (m 10 H) ppm. 13C NMR (75.4 MHz,
CDCl3): d = 12.88, 33.52, 125.07, 126.00, 126.78, 127.76, 128.44,
128.51, 128.96, 137.53, 141.47, 144.93 ppm. IR (mull): 3079, 3056,
3023, 2967, 2931, 2873, 1599, 1494, 1445, 756, 697 cm–1. GC-MS
(IE, 70 eV): m/z (%) = 208 (52) [M+], 115 (100), 91 (41), 178 (40),
179 (38), 129 (28), 193 (26), 89 (25).
1,1,2-Triphenyl-1-butene: white solid, mp 77.8 °C (lit. mp 78–79
°C).1 1H NMR (300 MHz,CDCl3): d = 0.94 (t, J = 7.5 Hz, 3 H), 2.48
(q, J = 7.5 Hz, 2 H), 6.86–7.34 (m, 15 H) ppm. 13C NMR (75.4
MHz, CDCl3): d = 13.55, 28.96, 125.68, 126.11, 126.56, 127.31,
127.76, 128.10, 129.42, 129.65, 130.72, 138.78, 142.13, 142.16,
(8) Bean, F. R.; Johnson, K. R. J. Am. Chem. Soc. 1932, 54,
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Synlett 2007, No. 1, 103–106 © Thieme Stuttgart · New York

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