Pd(II)-catalyzed direct olefination of arenes with allylic esters and ethers

Article abstract of DOI:10.1055/s-0031-1290078

A convenient Pd(II)-catalyzed direct olefination of unactivated arenes with allylic esters and ethers via C-H activation was demonstrated. Under the typical conditions, various aryl-substituted allylic esters and ethers can be prepared. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0031-1290078

LETTER
259
Pd(II)-Catalyzed Direct Olefination of Arenes with Allylic Esters and Ethers
D
irec
t
O
lefination
i
of
A
re
a
State Key Laboratory of Applied Organic Chemistry and Department of Chemistry, Lanzhou University, Lanzhou, Gansu 730000,
P. R. of China
Fax +86(931)8915557; E-mail: liuzhq@lzu.edu.cn
Received 25 September 2011
Initially, we chose toluene and allylic acetate as the model
Abstract: A convenient Pd(II)-catalyzed direct olefination of unac-
substrates to optimize suitable conditions for this reaction
tivated arenes with allylic esters and ethers via C–H activation was
(Table 1). It was found that the oxidants, catalysts, and
demonstrated. Under the typical conditions, various aryl-substituted
solvent affected the reaction efficiency critically. The ox-
idant AgOAc was better than others such as Ag2CO3,
Ag2O, Ag2SO4, benzoquinone (BQ), and O2 (Table 1, en-
tries 1–10). The desired product was obtained in good
yield by using a mixed solvent of 5% DMSO in toluene
(Table 1, entry 11). Other solvents such as AcOH, DMF,
1,4-dioxane, t-BuOH, and DCE were not effective
(Table 1, entries 12–16). As for the catalyst, Pd(OAc)2
was more efficient than Pd(O2CCF3)2, PdCl2, Pd(acac)2,
PdCl2(Cy3P)2, and Pd(Ph3P)4 (Table 1, entries 17–21). Al-
though PdCl2(Ph3P)2 gave a slightly higher yield than
Pd(OAc)2, the formation of cinnamyl acetate as byprod-
uct, which was believed to be generated directly from the
catalyst and allylic acetate, caused problems in product
isolation (Table 1, entry 22).11 The product was obtained
in low yields when the reaction was carried out at lower
temperature such as 80 °C and 50 °C (Table 1, entries 23
and 24).
Under the typical reaction conditions,12 various electron-
rich and electron-deficient arenes were effective sub-
strates in the direct olefination reaction (Table 2). Elec-
tron-rich arenes gave moderate to good yields of the E-
aryl-substituted allylic acetates (Table 2, entries 1–8). The
moderate electron-deficient arene such as chlorobenzene
also gave moderate yield of the corresponding product
(Table 2, entry 9). The highly electron-deficient arenes
such as nitrobenzene gave very low yield of the desired
products (Table 2, entry 10). The scope of the allylic es-
ters and ether has been investigated (Table 2, entries 11–
15). Allylic benzoate gave 35% yield of the desired prod-
uct (Table 2, entry 11). It is interesting that the aryl-sub-
stituted allylic cinnamate and methacrylate are obtained in
34% and 30% yields, respectively (Table 2, entries 12 and
13). It indicates that another C=C bond could be tolerated
allylic esters and ethers can be prepared.
Key words: palladium catalysis, olefination, C–C bond formation,
C–H bond activation, allylic ester
The direct carbon–carbon bond formation via C–H bond
activation and functionalization remains a critical chal-
lenge for modern synthetic organic chemists.1 Recently,
direct olefination of unactivated arenes draws much atten-
tion.2 Although several efficient protocols to construct a
C–C bond via the direct alkenylation of arenes have been
explored since the pioneering work developed by Fuji-
wara,3 most of them suffer from limited substrate scope,
such as electron-rich arenes, electron-deficient olefins,
and acidic conditions.4 Recently, Yu5 and Zhang6 reported
two efficient methods to olefinate electron-deficient are-
nes. In addition, some useful ortho olefination of arenes
using a directing group has also been developed.7 Howev-
er, most of them use electron-deficient alkenes as the ole-
finating reagents except for few examples using aliphatic
olefins and styrenes.8 The allylic carboxylic ester moiety
has rarely been employed in the direct alkenylation of ar-
enes via C–H activation since it readily undergoes b-ace-
tate elimination to provide the terminal alkene.9 Very
recently, Jiao developed some efficient olefination reac-
tions using allyl esters with aromatic halides and arenes to
prepare aryl-substituted allylic esters which are even dif-
ficult to synthesize by the traditional Heck reaction.10
However, there are still some drawbacks in the direct ole-
fination reactions such as acidic conditions and relatively
long reaction time.10c Herein, we wish to report an effi-
cient Pd(II)-catalyzed olefination of unactivated arenes
using allylic esters and ethers (Scheme 1).
R2
O
Pd(OAc)2 (10 mol%)
AgOAc (2 equiv)
R2
O
H
O
R3
FG
+
FG
O
R3
R1
DMSO–solvent (5:95)
110 °C, 12 h
R1
allylic esters
and ethers
FG = H, Me, OMe
Cl, NO2
Scheme 1 Pd(II)-catalyzed direct olefination of arenes with allylic esters and ethers
SYNLETT 2012, 23, 259–262
x
x.
x
x.
2
0
1
2
Advanced online publication: 03.01.2012
DOI: 10.1055/s-0031-1290078; Art ID: W20611ST
© Georg Thieme Verlag Stuttgart · New York

260
X. Shang et al.
LETTER
Table 1 Optimization of the Typical Reaction Conditionsa
(continued)
in the allylic substrates. But-3-en-2-yl acetate was effec-
tive substrate (Table 2, entry 14). It is noteworthy that al-
lylic ether is also used successfully as the olefination
partner (Table 2, entry 15).
O
cat. Pd
oxidant
H
O
O
+
Table 1 Optimization of the Typical Reaction Conditionsa
solvent, 12 h
O
Entry Catalyst (mol%)
Solvent
Additive
(equiv)
Yield
(%)b
O
cat. Pd
oxidant
H
O
O
23c
24d
Pd(OAc)2 (10)
Pd(OAc)2 (10)
toluene
(5% DMSO)
AgOAc (2)
49
+
solvent, 12 h
O
toluene
(5% DMSO)
AgOAc (2)
trace
Entry Catalyst (mol%)
Solvent
Additive
(equiv)
Yield
(%)b
a Reaction conditions: toluene (1.2 equiv; 1–2 mL when used as sol-
vent), DMSO–solvent (5:95, v/v), allylic acetate (0.6 mmol), 110 °C,
12 h, unless otherwise noted.
1
2
Pd(OAc)2 (5)
Pd(OAc)2 (10)
Pd(OAc)2 (20)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
Ag2CO3 (2)
Ag2CO3 (2)
Ag2CO3 (2)
Ag2CO3 (1)
Ag2CO3 (3)
AgOAc (2)
Ag2O (2)
16
43
b Yields determined by 1H NMR spectroscopy of the crude products.
c The reaction was carried out at 80 °C.
3
49
d The reaction was carried out at 50 °C.
4
31
Table 2 Olefination of Arenes with Allyl Esters and Ethersa
5
41
6
53
Entry Product
Yield of E-isomer (%)b
7
trace
8
O
8
Ag2SO4 (2)
BQ (1)
1
O
O
O
43
9
trace
trace
70
10
11
O2 (1 atm)
AgOAc (2)
2a
O
toluene
(5% DMSO)
O
O
12
13
Pd(OAc)2 (10)
Pd(OAc)2 (10)
AcOH
AgOAc (2)
AgOAc (2)
trace
2
2b
65
DMF
(5% DMSO)
2
(2b/2b¢ = 2:1)
14
15
16
17
18
19
20
21
22
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(TFA)2 (10)
PdCl2 (10)
1,4-dioxane
(5% DMSO)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
10
3
t-BuOH
(5% DMSO)
2b¢
O
DCE
(5% DMSO)
11
18
10
60
23
trace
72
O
MeO
toluene
(5% DMSO)
2c
64
3
(2c/2c¢ = 5:4)
toluene
(5% DMSO)
O
Pd(PPh3)4 (10)
Pd(acac)2 (10)
toluene
(5% DMSO)
OMe
2c¢
toluene
(5% DMSO)
PdCl2(PCy3)2 (10) toluene
(5% DMSO)
PdCl2(PPh3)2 (10) toluene
(5% DMSO)
Synlett 2012, 23, 259–262
© Thieme Stuttgart · New York

LETTER
Direct Olefination of Arenes
261
Table 2 Olefination of Arenes with Allyl Esters and Ethersa
(continued)
Table 2 Olefination of Arenes with Allyl Esters and Ethersa
(continued)
Entry Product
Yield of E-isomer (%)b
Entry Product
Yield of E-isomer (%)b
O
O
Cl
O
O
2d
2i¢
55
4
O
(2d/2d¢ = 3:1)
O2N
O
O
O
10
17
35
O
2j
O
2d¢
11
O
Ph
O
O
2k
O
2e
45
5
(2e/2e¢ = 5:1)
12
O
Ph
34
30
O
2l
O
2e¢
O
13
O
O
6
O
O
32
56
2m
O
2f
14
O
O
47
43
7
2n
Ph
2g
15
O
2o
O
O
O
a Reaction conditions: arene (1–2 mL), DMSO (5% v/v relative to ar-
ene), allylic ester or ether (0.6 mmol), Pd(OAc)2 (10 mol%, 0.06
mmol), AgOAc (2 equiv, 1.2 mmol), 110 °C, 12 h.
b Isolated yield of the E-isomer.
2h
71
8
(2h/2h¢ = 5:4)
O
O
O
A competing olefination would occur between the allylic
and the acrylic double bonds. However, the C–C bond for-
mation took place in the position of the terminal allylic
C=C bond although 52% yield of ethyl cinnamate was
produced in the olefination of benzene with ethyl acrylate
under the typical conditions (Scheme 2). The selectivity
might be attributed to the stability of the alkyl–Pd inter-
mediate via chelation of Pd atom by the carbonyl O atom
which was previously proposed.13 Additionally, this che-
lation effect could also result in the high regioselectivity
since the (Pd)C–C(O) bond cannot rotate freely.
O
2h¢
O
O
Cl
2i
46
9
(2i/2i¢/2i¢¢ = 5:3:2)
O
O
In conclusion, we demonstrated an efficient Pd(II)-cata-
lyzed direct olefination of unactivated arenes, fluorinated
arenes, and heteroarenes by using allylic esters and ether.
As the olefination partner, various allylic esters bearing
another C=C bond and allylic ether give moderate to good
yields of the desired products. The present protocol would
Cl
2i¢
© Thieme Stuttgart · New York
Synlett 2012, 23, 259–262

262
X. Shang et al.
LETTER
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O
typical
O
O
conditions
O
+
52%
O
O
typical
conditions
arene
+
Ar
O
O
major
Pd(OAc)2
Pd
O
Ar
O
Scheme 2 Competing coupling of arenes with allyl ester and acry-
late catalyzed by Pd
be useful to prepare substituted allylic alcohols. Further
investigation of this procedure is under way in our labora-
tory.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This project is supported by National Science Foundation of China
(No. 21002045) and the Fundamental Research Funds for the Cen-
tral Universities (No: lzujbky-2009-115). We also thank the State
Key Laboratory of Applied Organic Chemistry and Lanzhou Uni-
versity for financial support.
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(11) The cinnamyl acetate was also generated in the absence of
toluene when PdCl2(Ph3P)2 was used as catalyst.
(12) Typical Procedure
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Synlett 2012, 23, 259–262
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