Palladium-catalyzed stereoselective synthesis of (E)-stilbenes via organozinc reagents and carbonyl compounds

Article abstract of DOI:10.1002/adsc.200606016

In the presence of a catalytic amount of PdCl₂(PPh ₃)₂ and a silylating agent, organozinc halides reacted with carbonyl compounds to give the corresponding (E)-stilbenes in good to excellent yields under mild conditions. The reaction mechanism is briefly discussed.

Full text of DOI:10.1002/adsc.200606016

FULL PAPERS
DOI: 10.1002/adsc.200606016
Palladium-Catalyzed Stereoselective Synthesis of (E)-Stilbenes
via Organozinc Reagents and Carbonyl Compounds
Jin-Xian Wang,a, b,* Kehu Wang,a Lianbiao Zhao,a Hongxia Li,a Ying Fu,a
and Yulai Hua
a
Institute of Chemistryand Department of Chemistry, Northwest Normal University,
805, An Ning Road (E.), Lanzhou 730070, Peopleꢀs Republic of China
Fax : (+86)-931-776-8159; e-mail: wangjx@nwnu.edu.cn
State KeyLaboratoryof Applied Organic Chemistry, Lanzhou University
b
Lanzhou 730000, Peopleꢀs Republic of China
Received: January13, 2006; Accepted: May7, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: In the presence of a catalytic amount of Keywords: carbonyl compounds; catalyst; organozinc
PdCl2(PPh3)2 and a silylating agent, organozinc hal- halides; palladium; (E)-stilbenes
G
ides reacted with carbonyl compounds to give the
corresponding (E)-stilbenes in good to excellent
yields under mild conditions. The reaction mecha-
nism is brieflydiscussed.
Introduction
be substituted byorganozinc halides under the cataly-
sis of Ni(acac)2 and a tertiaryamine to give 1-aryl-1-
AHCTREUNG
Stilbenes are important synthetic precursors to phen- alkenes in excellent yields. In addition, we have re-
anthrene alkaloids[1] as well as enantiomericallypure
ported[13] that stereoselectivelyalkenlayted com-
1,2-diphenylethane-1,2-diamines,[2] and diols.[3] They pounds can be obtained from substituted aromatic al-
are also used as optical brighteners[4] and therapeutic dehydes by reaction with organozinc halides in the
agents for liver disorders. Manymethods have been
presence of catalytic amount of NiCl
A
described for the synthesis of stilbenes.[5] The reduc- CoCl
AHCTREUNG
tion, dehydrogenation, and elimination reactions lead- have found that this reaction is sensitive to the molec-
ing to stilbenes without formation of new carbon- ular structure of the carbonyl compounds. When
carbon bonds are known to be veryconvenient meth-
NiCl
G
N
ods. Synthetically more important are the dimeriza- alyst, aromatic ketones, p-benzenedialdehyde etc.
tion reactions: oxidative or eliminative dimerization would undergo several side reactions.
of a suitable methylarene often constitutes the
We have now found that the (E)-stilbenes can be
method of the choice for the preparation of a sym- obtained from carbonyl compounds and arylmethyl-
metrical stilbene. Meerwein arylation[6] and Heck re- zinc halides in the presence of a catalytic amount of
action[7] are prominent examples for the synthesis of PdCl
2(PPh3)2 using a silylating agent. In this paper,
G
stilbenes from arenes and styrenes. Moreover, con- we report the synthesis of (E)-stilbenes from organo-
densation reactions of a nucleophilic with an electro- zinc reagents and carbonyl compounds. The reactions
philic arylmethyl compound including the Knoevena- are shown in Scheme 1.
gel-type reaction and the very general Wittig and
Wittig–Horner reactions[8] are also well known meth-
ods. Recently, a new procedure for the synthesis of Results and Discussion
stilbenes via reaction of aldehyde tosylhydrazones
with stabilized carbonions,[9] or benzotriazole-stabi- Various kinds of transition metal complex were em-
lized carbanions,[10] or trimethyl borate/lithium tert- ployed as catalyst for the synthesis of (E)-2-chlorostil-
butoxide[11] has been reported. We also have report- benes from 2-chlorobenzaldehyde with benzylzinc
ed[12] that the nitro group of 1-aryl-2-nitroethenes can chloride, and the results are summarized in Table 1.
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Palladium-Catalyzed Stereoselective Synthesis of (E)-Stilbenes
FULL PAPERS
Scheme 1.
Table 1. Optimized conditions for the synthesis of (E)-stilbenes via the addition-elimination of benzylzinc chloride to 2-chloro-
benzaldehydes.[a]
EntsrtyC[matoall%y ]
Lewis acid
Solvent
T [8C]
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
PdCl2
A
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiCl
none
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiCl
AlCl3
BF3·OEt2
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiCl
THF
THF
THF
THF
THF
THF
THF
THF
DMF
CH3CN
C6H6
THF
THF
THF
THF
THF
THF
ꢀ18!r.t.
ꢀ18!r.t.
r.t.
8
8
8
8
8
8
3
6
8
8
8
8
8
8
8
8
8
89
91
70
84
90[c]
0
70
86
85
78
61
69
78
86
78
83
81
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2(PPh3)2 (2.0)
PdCl2 (2.0)
Ni(acac)2 (3.0)/Et3N
CoCl(PPh3)3 (3.0)
NiCl2(PPh3)2 (3.0)
ACHTREUNG
ACHTREUNG
G
0
R
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
ꢀ18!r.t.
U
U
ACHTREUNG
9
ACHTREUNG
10
11
12
13
14
15
16
17
ACHTREUNG
AHCTREUNG
AHCTREUNG
U
ACHTREUNG
ACHTREUNG
ACHTREUNG
[a]
All reactions were conducted on a scale of 11 mmol benzylzinc chloride with 10 mmol 2-chlorobenzaldehyde, and
20 mmol Lewis acid in 20 mL solvent.
[b] Yields of isolated products.
[c]
Product is the corresponding diphenylethanol.
PdCl2
CoCl(PPh3)3, NiCl2
(Table 1, entries 2, 14, 15, 16, and 17). In the absence 78, and 61%, respectively(Table 1, entries 2, 9, 10,
of PdCl2(PPh3)2, the reaction was not successful and 11). THF was found to be the best solvent.
(Table 1, entry6). The amount of PdCl 2(PPh3)2 affect- We have also investigated the effects of time and
A
,
the presence of 2% PdCl2(PPh3)2 and Me3SiCl, the
A
2
A
N
ACHTREUNG
ACHTREUNG
AHCTREUNG
ed the yield, and the use of 2 mol% of the catalyst temperature on the reaction. It was found that a high
based on 2-chlorobenzaldehyde gave the best result. yield of (E)-stilbene could be obtained at ꢀ188C to
Furthermore, the reaction was influenced markedly room temperature for 8 h (Table 1, entries 2, 3, 4, 7,
bythe solvent. When THF, DMF, CH CN, and ben- and 8). In the absence of Me3SiCl, the olefination re-
3
zene were employed as the solvent in the reaction of actions was not successful. For example, no olefinic
benzylic zinc chloride with 2-chlorobenzadehyde in product was isolated after the reaction of 2-chloro-
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Jin-Xian Wang et al.
FULL PAPERS
Table 2. Synthesis of (E)-stilbenes via addition-elimination of organozinc reagents to aromatic aldehydes.[a]
1
EntryR (Ar)
R
X
Product[b]
Yield [%][c]
90
1
H
H
H
H
H
Cl
2a[14]
2a[14]
2b[15]
2c[16]
2
3
4
H
Br
Br
Br
85
81
86
3-Br
4-Cl
5
6
4-Br
4-Br
H
Br
Br
2d[15]
88
89
4-OMe
2e
7
H
4-OH
H
Br
Br
Br
Br
Br
2f[17]
2g[18]
2h[19]
2i[16]
2j[20]
85
84
78
85
91
8
3-Cl
H
9
2-OH
2-OMe
4-OMe
10
11
H
H
12
H
3,5-(OMe)2-4-OH
Cl
2k
2l
73
86
13
14
15
H
H
H
4-OCH2Ph
2,4-(NO2)2
2-OCH2Ph
Cl
Cl
Cl
no reaction
2m
2n
2o
75
68
82
16
17
H
2,6-Br2-4-OH
4-OMe
Cl
3-Br
Br
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Palladium-Catalyzed Stereoselective Synthesis of (E)-Stilbenes
FULL PAPERS
Table 2. (Continued)
1
EntryR (Ar)
R
X
Product[b]
Yield [%][c]
18
3-Br
3-OMe-4-OH
H
Br
2p
78
74
19
(b-naphthyl)
Br
2q[21]
[a]
All reactions were conducted on a scale of 10 mmol aldehydes with 11 mmol organozinc reagent, 0.2 mmol [PdCl2(PPh3)2],
A
and 20 mmol Me3SiCl in 20 mL THF at ꢀ188C to room temperature for 8 h.
[b] All products were characterized bymelting point, IR, 1H NMR 13C NMR, MS and elemental analyses.
[c]
Yields of isolated products.
benzaldehyde and PhCH2ZnCl without Me3SiCl. The
products were the corresponding 1,2-diphenethyl al-
cohol (Table 1, entry5).
In order to determine the scope and limitation of
this new method for the synthesis of (E)-stilbenes,
various aldehydes were allowed to react with organo-
zinc reagents in the presence of THF catalyzed by
PdCl2(PPh3)2 using a silylation agent were examined
as shown in Table 2.
Scheme 3. Reagents and conditions: 1) [PdCl2(PPh3)2],
A
ACHTREUNG
Me3SiCl, THF, ꢀ288C to room temperature; 2) saturated
NH4Cl solution.
We have also found that the reaction of substituted
aromatic aldehydes with benzylzinc chlorides or bro-
mides in the presence of catalytic amount of
PdCl2(PPh3)2 also gave the corresponding (E)-stil-
U
benes in good to excellent yields. The reactions are
shown in Scheme 2 and the results are summarized in
Table 2 (68–90%, entries 1–13 and 15–19).
Various substituents on the phenyl ring, either elec-
tron-withdrawing or electron-donating, such as chloro,
bromo, methoxy, benzyloxy, and hydroxy, could be
tolerated and had little effect on the yields except for
the case of 2,4-dinitrobenzaldehyde which gave no
olefination product (Table 2, entries 3–13, 15–18, and
14). Aromatic ketones also reacted with benzylzinc
chlorides or bromides similarlygiving the correspond-
ing substituted (E)-stilbenes in excellent yields
Scheme 4. Reagents and conditions: 1) [PdCl2(PPh3)2],
A
Me3SiCl, THF, ꢀ188C to room temperature; 2) saturated
NH4Cl solution.
(Scheme 3, and Table 3). The p-benzenedialdehyde ing substituted (1E,3E)-1,4-diphenyl-1,3-butadiene in
reacted similarlyand provided the corresponding sub-
stituted
(Scheme 4, and Table 4).
excellent isolated yields (Scheme 5, and Table 5). One
(E,E)-distylbenzene in excellent yields feature of this reaction that makes it a particularlyat-
A
tractive synthetic procedure is its regiospecificity. The
Unsaturated aldehydes, such as phenylacrylic alde- double bond is formed between the carbonyl carbon
hyde, reacted similarly and provided the correspond- atom of the carbonyl compounds and the carbon
atom of organozinc reagents. Another feature of this
reaction is its stereoselectivity. In all cases only the
(E)-stilbenes were formed. The (E)-stilbene can be
1
detected by H NMR ,13C- NMR, MS and elemental
analysis.
Based on our experimental results and other relat-
ed studies,[26,27] we propose the general reaction
mechanism shown in Scheme 6. The first step of this
mechanism is the reaction of PdCl2L2 with organozinc
reagent or zinc powder to form [Pd(0)L2] and
then [Pd(0)L2] reacts with ArCH2ZnX 1 to give
Scheme 2. Reagents and conditions: 1) [PdCl2(PPh3)2],
A
Me3SiCl, THF, ꢀ188C to room temperature; 2) saturated
NH4Cl solution.
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Jin-Xian Wang et al.
FULL PAPERS
Table 3. Palladium-catalyzed stereoselective olefination of benzylzinc bromides with aromatic ketones.[a]
1
EntryR
R
Product[b]
Yield [%][c]
78
1
4-CH3
H
H
H
3a
3b
2
3
72
75
H
4-Br
4-Br
4-Cl
3c
3d
3e
4
4-Cl
H
76
81
5
[a]
All reactions were conducted on the following scale: 10 mmol substituted acetophenones with 11 mmol benzylzinc bro-
mides, 2 mmol% [PdCl2 (PPh3)2] and 20 mmol Me3SiCl in 20 mL THF at ꢀ288C to room temperature.
[b] All products were characterized bymelting point, IR, 1H NMR 13C NMR, MS and elemental analyses.
[c]
Yields of isolated products.
[ArCH2PdXL2] 6. The second step is the 1,2-addition stituents, simple operation and excellent yields of the
of 6 to an aromatic aldehyde, which is activated by products. This method offers the possibilityfor the
chlorotrimethylsilane,[28] to give palladium complex 7. formation of carbon-carbon double bonds and the
The loss of the [PdXClL2] 8 from 7 in the presence of synthesis of the natural active products.
the chlorotrimethylsilane yields a secondary silyl
ether 9, which has been isolated. Compound 9 can
also be detected in the reaction byGC-MS. Loss of
the Me3SiOH from 9 gives the final product (E)-stil-
benes 2. The [PdXClL2] 8 in the presence of
ArCH2ZnX 1 gives [Pd0L2] and the catalyst is thus re-
generated.
Experimental Section
All reactions were carried out in over-dried glassware under
argon or nitrogen. All substrates and reagents were obtained
commerciallyexcept for the organozinc reagents and cata-
lysts, which were prepared by standard procedures.[29,30]
THF was distilled from sodium/benzophenone. 1H NMR
spectra were recorded on a Bruker AM 400 MHz or a
Bruker AC-E 200 MHz spectrometers in CDCl3 with TMS
as an internal standard. 13C NMR spectra were obtained on
a Bruker AM-400 operating at 100 MHz or a Bruker AC-E
200 operating at 50 MHz in CDCl3 with TMS as an internal
standard. IR spectra were recorded on an Alpha Centauri
FI-IR spectrometer. Mass spectra were recorded on an HP
5988 A and GC/MS/DS instrument. Elemental analyses
were carried out on a Carlo–Erba 1106 instruments. The
stereochemistries of the product stilbenes were determined
bycomparison of the 1H NMR spectra with those reported
in the literature and GC/MS analyses on an HP-6890/HP-
5973 gas chromatograph equipped with a 30 m ” 0.25 mm
HP-5MS capillarycolumn and a flame ionization detector.
Conclusions
In conclusion, we found that the reaction between or-
ganozinc halide and carbonyl compound is actually a
competitive reaction. When catalysts, reagents, reac-
tion conditions and material ratio are changed, the
types of reactions change accordingly. They range
from dialkylation reaction, stereoselective olefination
reaction, to an addition reaction. We have developed
a new and convenient method for an efficient and
highlystereoselective synthesis of ( E)-stilbenes from
the corresponding carbonyl compounds with benzylic
zinc halides in the presence of catalytic amount of
PdCl2(PPh3)2 using a silylating agent in THF under
G
mild conditions. The main advantages of this new
method are mild reaction conditions, high stereoselec-
tivity, tolerance of unsaturated double bonds and sub-
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Palladium-Catalyzed Stereoselective Synthesis of (E)-Stilbenes
FULL PAPERS
Table 4. Palladium-catalyzed stereoselective olefination of benzylzinc bromides with p-benzenedialdehyde.[a]
[b]
EntryR (Ar)
Product
Yield [%][c]
78
1
H
4a[22]
2
3
4
5
6
4-Cl
4b[23]
73
89
82
70
3-Cl
4-Br
3-Br
4c
4d[22]
4e
4-CH3
4f[23]
71
80
7
b-C10H7
4g
[a]
All reactions were conducted on the following scale: 10 mmol substituted p-benzenedialdehyde with 11 mmol benzylzinc
bromides, 2 mmol% [PdCl2
C
[c]
Yields of isolated products.
General Procedure
Benzylzinc halides (11 mmol) was prepared according to
Knochelꢀs procedure.[29] PdCl2
(PPh3)2 (0.2 mmol) and THF
AHCTREUNG
(2 mL) were mixed and heated at 608C for 10 min in over-
dried glassware under argon or nitrogen. The solution of
benzylzinc halide in THF was added at room temperature,
and the resulting mixture was cooled to ꢀ188C (for aromat-
ic aldehydes and unsaturated aldehydes or ꢀ288C for aro-
matic ketones). A solution of the carbonyl compound
(10 mmol) and Me3SiCl (2.18 g, 20 mmol) in THF (10 mL)
was added dropwise. Then the temperature was allowed to
rise to room temperature. After stirring for 8 h, saturated
NH4Cl solution (10 mL) and Et2O (10 mL) were added and
Scheme 5. Reagents and conditions: 1) [PdCl2(PPh3)2],
A
Me3SiCl, THF, ꢀ188C to room temperature; 2) saturated
NH4Cl solution.
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Jin-Xian Wang et al.
FULL PAPERS
Table 5. Palladium-catalyzed stereoselective olefination of benzylzinc bromides with a,b-unsaturated aldehydes.[a]
1
EntryR
R
Product[b]
Yield [%][c]
76
1
H
H
H
H
5a[24]
5b
2
3
CH3
Br
91
61
5c
4
5
6
4-Cl
4-Cl
4-Cl
H
5d
5e
5f
85
94
63
CH3
Br
7
2-Cl
2-Cl
3-Cl
3-Cl
3-Cl
3-Br
2-Br
2-Br
CH3
H
5g
55
51
50
69
65
72
67
75
8
5h
5i[25]
5j
9
H
10
11
12
13
14
CH3
Br
5k
5l
CH3
CH3
H
5m
5n
15
4-Br
H
5o
90
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Palladium-Catalyzed Stereoselective Synthesis of (E)-Stilbenes
FULL PAPERS
Table 5. (Continued)
1
EntryR
R
Product[b]
Yield [%][c]
85
16
4-Br
3-F
CH3
H
5p
5q
17
63
[a]
All reactions were conducted on the following scale: 10 mmol substituted cinnamaldehydes with 11 mmol benzylzinc bro-
mide, 2 mmol% [PdCl2
C
[c]
Yields of isolated products.
Scheme 6. The general reaction mechanism of palladium-catalyzed stereoselective olefination of organozinc reagents with
carbonyl compounds.
the mixture was stirred for 10 min. The organic layer was
separated, dried over anhydrous MgSO4 and concentrated.
The product was isolated from the crude reaction mixture
bychromatographyon a silica gel column using petroleum/
ethyl acetate as an eluent.
Most of the products are know, except for 2e, 2k, 2l, 2m,
2n, 2o, 2p in Table 2, 3a–e in Table 3, 4c, 4e, 4g in Table 4,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 20272047, 20572086), the Gansu
Natural Science Foundation of China (3ZS051 A25–001) and
Key Laboratory of Eco-Environment-Related Polymer Mate-
rials (Northwest Normal University), Ministry of Education
of China.
and 5b–h, 5j–q in Table 5. All new compounds were charac-
terized byIR, 1H NMR, 13C NMR, MS, and elemental anal-
ysis.
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Adv. Synth. Catal. 2006, 348, 1262 – 1270
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FULL PAPERS
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