Water works: An efficient palladium-catalyzed cross-coupling reaction between boronic acids and bromoacetate with aminophosphine ligand

Article abstract of DOI:10.1016/j.tet.2010.08.047

Water greatly restrained the formation of self-coupling of boronic acids in a palladium-catalyzed cross-coupling reaction between boronic acids and ethyl bromoacetate with an aminophosphine ligand; good to excellent yields of cross-coupling product were obtained.

Full text of DOI:10.1016/j.tet.2010.08.047

Tetrahedron 66 (2010) 8238e8241
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Water works: an efficient palladium-catalyzed cross-coupling reaction between
boronic acids and bromoacetate with aminophosphine ligand
,c,
Zhi-Yong Peng a, Jian-Ping Wang a, Jiang Cheng b, Xiao-min Xie a, Zhaoguo Zhang a
*
a School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
b College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325027, China
c State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2010
Received in revised form 19 August 2010
Accepted 19 August 2010
Available online 9 September 2010
Water greatly restrained the formation of self-coupling of boronic acids in a palladium-catalyzed cross-
coupling reaction between boronic acids and ethyl bromoacetate with an aminophosphine ligand; good
to excellent yields of cross-coupling product were obtained.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Aminophosphine
Boronic acids
Ethyl bromoacetate
Palladium-catalyzed
Water
1. Introduction
functional groups,4 they were usually used as the aryl metallic
reagents.
a-Aryl carboxylic acids are of significant importance since many
OR
of them are commercially used as non-steroidal anti-inflammatory
and analgesic drugs.1 Continuing demands exist for general and
efficient procedures to supersede the current syntheses that need
multiple steps, special or toxic reagents, and usually are conducted
under harsh reaction conditions. Transition metal-catalyzed CeC
bond-forming reactions, characterized by mild reaction conditions,
that are high yielding and tolerate a wide range of functional
groups, have been widely applied in organic syntheses.
There are two general reactionpathways to realize the arylation of
(i)
ArX
ArM
+
M
X
O
O
OR
Ar
OR
O
+
(ii)
M = metal, X = halogen
Scheme 1.
corresponding esters to generate a-aryl carboxylic acid derivatives
Gooßen published the first paper that efficiently fulfilled the
cross-coupling reaction between arylboronic acids and bromo-
acetic acid derivatives by employing trinaphthyl phosphine as li-
gand5 without the traditional utilization of highly toxic thallium
salt as base.6 Deng discovered that Cu2O served as co-catalyst to
suppress the homo-coupling reaction of arylboronic acids and thus
enhanced the efficiency for the cross-coupling reaction between
arylboronic acids and ethyl bromoacetate or bromoacetamide with
commercially available Pd(PPh3)4 as catalyst.7 Luo found that hy-
droquinone accelerated the reaction remarkably,8 and this offers
reported by transition metal-catalyzed reactions (Scheme 1). Re-
actions via pathway (i), which involves a palladium-catalyzed cou-
pling reaction between aryl halide electrophiles and enolates, were
extensively studied by Buchwald2 and Hartwig.3 In these reactions,
bulky, electron-rich phosphine ligands and/or carbene ligands were
employed. Pathway (ii) is another promising alternative to achieve
the coupling reaction by employing a-halocarbonyl compounds and
aryl metal as the substrates. Because arylboronic acids are readily
available, fairly stable, and broadly compatible with an array of
another entry to achieve cross-coupling of a-halogenated carbonyl
compounds and boronic acids rather than self-coupling of boronic
acids.9 Employing a nickel catalyst, Lei reported an efficient cross-
coupling between arylboronic acids and ethyl bromoacetate.10
* Corresponding author. Tel./fax: þ86 21 54748925; e-mail address: zhaoguo@
sjtu.edu.cn (Z. Zhang).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.08.047

Z.-Y. Peng et al. / Tetrahedron 66 (2010) 8238e8241
8239
Aminophosphine ligands (Scheme 2) that we had applied to the
SuzukieMiyaura and Sonogashira coupling reactions11 are easily-
accessible, finely-tunable, efficient, and cheap ligands; however,
they were only recently employed as ligands in transition metal-
catalyzed cross-coupling reactions.12 A few reports on the co-
ordinative chemistry of aminophosphine compounds revealed that
the function of amino groups was more diversified from alkoxy
groups in phosphates.13 In mono- and di-aminophosphines, alkyl
and/or aryl amino groups served as strong electron-donating
To optimize the reaction conditions, we first tested the solvent
effect in reactions between phenylboronic acid and bromoacetate.
We found that THF and ethanol were suitable solvents, and water
plays a dramatic role in controlling the selectivity between homo-
coupling and cross-coupling reactions. When 10% (v/v) of H2O was
added, the yield of cross-coupling was greatly increased and the
quantity of biphenyl was obviously decreased. This is quite different
from previous reports, in which water was detrimental to the cross-
coupling.5,8,9
groups, making the phosphines stronger
s
-donor ligands. Hartwig
To explain why water can improve the selectivity, the equilib-
rium between the CePd and OePd species should be considered
(Scheme 3). After the oxidative addition of Pd(0) with ethyl bro-
moacetate, two possible palladium species may be formed. If
a CePd bond-containing complex A is the only species, trans-
metallation in the presence of phenylboronic acid will proceed via
route c to generate complex D, which is transformed to ethyl
phenylacetate after reductive elimination. However, if complex B (a
palladium enolate) is the predominant species (equilibrium a), or
its presence is not negligible, the transmetallation step might be
a different story. Because boron is oxophilic, the B(OH)2 moiety
could replace PdBr in complex B to generate a new boron-con-
taining enolate and a palladium complex PhPdBr (E). E is formally
the direct oxidative addition product of Pd(0) and bromobenzene,
which undergoes further transmetallation with phenylboronic acid
to yield biphenyl as the product. When water was added, there
would exist OHꢀ in the basic system, which displaces the halide in
species A to give an hydroxopalladium species C in solution
(equilibrium b),4a,15 transmetallation in the presence of phenyl-
boronic acid will proceed via route e to generate complex D, which
also undergoes reductive elimination to give ethyl phenylacetate.
The addition of water favors equilibrium b, and thus leads to the
decrease of the formation of biphenyl product and the increase of
cross-coupling product.
reported that both the enolate itself and ligand structure determine
the connection pattern of the enolate to palladium during his study
toward the reductive elimination of a palladium intermediate in
palladium-catalyzed a
-arylation of carbonyl compounds.14 Gooßen
found that the sterically bulky and moderately electron-donating
phosphines were suitable ligands for the cross-coupling reaction
between arylboronic acids and bromoacetate. This enlightening
discovery prompted us to test our aminophosphine ligands in this
reaction.
NR1R2
Ar
Ar
P
NR1R2
L1
Ar P
NR1R2
L2
L1a, L2a: Ar = Ph, R1 = R2 = Pr
i
L1b, L2b: Ar = Ph, R1-R2 = (CH2)5
L1c, L2c: Ar = Ph, R1-R2 = (CH2)2O(CH2)2
L1d, L2d: Ar = Ph, R1-R2 = (CH2)4
Scheme 2.
2. Results and discussion
We chose phenylboronic acid (1a) and ethyl bromoacetate (2) as
substrates, L1a as ligand and Pd2(dba)3 as palladium species to in-
vestigate the cross-coupling reaction. A degassed Schlenk reaction
tube was charged with ethyl bromoacetate (2 mmol), phenyl-
boronic acid (3 mmol), potassium carbonate (6 mmol), Pd2(dba)3
(0.05 mmol), ligand L1a (0.15 mmol), and THF (5 mL). The resulting
suspension was refluxed for 8 h under nitrogen atmosphere. Be-
sides the desired cross-coupling product, the homo-coupling
product of phenylboronic acid, was also formed (Table 1, entry 1).
O
OPdBr
BrPd
OEt
-OR
a
OEt
B
A
b
O
ROPd
OEt
PhB(OH)2
PhB(OH)2
BrB(OH)2
C
c
e
d
PhB(OH)2
ROB(OH)2
OEt
OB(OH)2
PhPdBr
O
PhPd
OEt
E
D
PhB(OH)2
BrB(OH)2
Biphenyl
Table 1
Effect of solvents using L1a as liganda
O
Pd2(dba)3 / L1a
Ph
+
PhCH2CO2Et
OEt
Scheme 3.
PhB(OH)2
BrCH2CO2Et
Ph-Ph
+
solvent, base
1a
2
3a
4a
We then turned our attention to ligands, palladium species, and
the effect of bases. The results are summarized in Table 2. Only L1a
is a highly effective ligand in this reaction (Table 2, entry 1), while
other ligands are inferior. In our previous work,11 ligands L1b and
L1c were workable ligands in the SuzukieMiyaura cross-coupling
reaction; however, they gave poor yields in this reaction (Table 2,
entries 3e8).
When different palladium sources were tested, tris(dibenzyli-
deneacetone) dipalladium(0) turned out to be the best palladium
(0) precursors. Palladium(II) acetate gave products with moderate
yields, PdCl2(MeCN)2 and Pd(PPh3)4 gave products with poor yields
along with much more biaryl product (Table 2, entries 9e11).
The choice of the base also affects the product selectivity (Table 2,
entries 12e15). K2CO3 is the most suitable base, K3PO4 and KFare also
workable bases, NaOAc and NEt3 are significantly less active.
Entry
Solvent
3a (%)b
4a (mg)c
1
2
3
4
5
6
7
8
9
THF
Dioxane
Toluene
Ethanol
DMF
Acetone
75
34
33
66
16
38
91
65
70
35
70
88
33
49
51
65
41
67
78
34
THF/H2Od
Dioxane/H2Od
Toluene/H2Od
Ethanol/H2Od
10
a
All reactions were run with phenylboronic acid (3 mmol), ethyl bromoacetate
(2 mmol), K2CO3 (6 mmol), Pd (0.05 mmol), and ligand L1a (0.15 mmol) in 5 mL of
solvent at 65 ꢁC for 8 h.
b
Isolated yields based on ethyl bromoacetate.
Isolated mass.
Organic solvent 4.5 mL, water 0.5 mL.
c
d

8240
Z.-Y. Peng et al. / Tetrahedron 66 (2010) 8238e8241
Table 2
boronic acids and ethyl bromoacetate successfully. We optimized
The effects of ligands, palladium sources, and basesa
the reaction condition and found that water was crucial in the
cross-coupling reaction. A number of arylboronic acids and alke-
nylboronic acids were tested and good to excellent yields were
obtained.
Pd / ligand
+
3a
+
1a
2
4a
THF/H2O, base
Entry
Palladium source
Ligand
Base
3a (%)b
4a (mg)c
4. Experimental section
1
2
3
4
5
6
7
8
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(OAc)2
PdCl2(MeCN)2
Pd(PPh3)4
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
L1a
L1b
L1c
L1d
L2a
L2b
L2c
L2d
L1a
L1a
L1a
L1a
L1a
L1a
L1a
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
KF
91
27
15
18
42
25
11
16
79
58
42
69
62
32
21
41
139
232
182
81
143
231
147
87
72
124
52
33
11
4.1. Ethyl 2-phenylacetate (3a)
A degassed Schlenk reaction tube was charged in sequence with
PhB(OH)2 (3 mmol), potassium carbonate (6 mmol), Pd2(dba)3
(0.05 mmol), ligand L1a (0.15 mmol), THF (4.5 mL), water (0.5 mL),
and ethyl bromoacetate (2 mmol). The resulting mixture was
degassed by freezeethaw techniques and was kept stirring for 8 h
at 65 ꢁC. The reaction was quenched with water (5 mL), and the
solution was extracted with ethyl acetate (3ꢂ10 mL). The organic
layers were combined, and dried over sodium sulfate. The solvent
evaporated in vacuo to give the crude product, which was purified
by flash chromatography (PE then 2% EA/PE), biphenyl (41.2 mg,
a white solid) and 3a (295.2 mg, 90%, a colorless oil) were obtained
in sequence. Characterization of 3a: 1H NMR (400 MHz, CDCl3)
9
10
11
12
13
14
15
NaOAc
NEt3
27
a
All reactions were run with phenylboronic acid (3 mmol), ethyl bromoacetate
(2 mmol), base (6 mmol), Pd (0.05 mmol) and ligand (0.15 mmol) in 5 mL of solvent
(THF/H2O¼9:1, v/v) at 65 ꢁC for 8 h.
b
Isolated yields based on ethyl bromoacetate.
Isolated mass.
d
7.35e7.30 (m, 5H), 4.17 (q, J¼7.0 Hz, 2H), 3.63 (s, 2H), 1.27 (t,
c
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3): 171.6, 134.1, 129.2, 128.5,
127.0, 60.8, 41.4, 14.1.
The generality and selectivity of the reaction were investigated
using a number of arylboronic acids and alkenylboronic acids in
combination with ethyl bromoacetate; the results are summarized
in Table 3.
4.2. Ethyl 2-o-tolylacetate (3b)
1H NMR (400 MHz, CDCl3)
d
7.19e7.17 (m, 4H), 4.16 (q, J¼7.2 Hz,
2H), 3.63 (s, 2H), 2.32 (s, 3H), 1.25 (t, J¼7.0 Hz, 3H). 13C NMR
(100 MHz, CDCl3): 171.7, 137.1, 133.2, 130.6, 130.4, 127.6, 126.3, 61.0,
39.5, 19.8, 14.4.
Table 3
Pd-catalyzed synthesis of arylacetic acid derivativesa
Pd2(dba)3 / L1a
4.3. Ethyl 2-m-tolylacetate (3c)
+
1
2
3
THF/H2O, K2CO3
1H NMR (400 MHz, CDCl3)
3H), 4.16 (q, J¼7.2 Hz, 2H), 3.57 (s, 2H), 2.34 (s, 3H), 1.25 (t, J¼7.0 Hz,
3H). 13C NMR (100 MHz, CDCl3): 171.9, 138.4, 134.3, 130.2, 128.7,
128.0, 126.5, 61.0, 41.6, 21.6, 14.4.
d 7.23e7.20 (m, 1H), 7.11e7.07 (m,
Entry
ReB(OH)2
3
Yieldb (%)
1
2
3
4
5
6
7
8
Phenyl
o-Tolyl
m-Tolyl
p-Tolyl
p-Chlorophenyl
p-Formylphenyl
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
90
91
90
98
98
92
85
84
97
90
97
60
88
86
94
84
4.4. Ethyl 2-p-tolylacetate (3d)
p-Trifluoromethylphenyl
p-Trifluoromethoxyphenyl
p-Methoxyphenyl
p-(tert-Butyl)phenyl
1,3-Benzodioxol-5-yl
2,6-Dimethylphenyl
1-Naphthyl
2-Methoxy-1-Naphthyl
(E)-Styryl
(E)-3-(Benzyloxy)prop-1-enyl
1H NMR (400 MHz, CDCl3)
d
7.18e7.11 (m, 4H), 4.14 (q, J¼7.2 Hz,
2H), 3.56 (s, 2H), 2.32(s, 3H),1.24 (t, J¼7.0Hz, 3H).13C NMR(100 MHz,
9
CDCl3): 171.6, 136.4, 130.9, 129.0, 128.9, 60.6, 40.8, 20.9, 14.0.
10
11
12
13
14
15
16
4.5. Ethyl 2-(4-chlorophenyl)acetate (3e)
1H NMR (400 MHz, CDCl3)
d 7.30e7.26 (m, 2H), 7.23e7.20 (m,
2H), 4.16 (q, J¼7.1 Hz, 2H), 3.58 (s, 2H), 1.25 (t, J¼7.0 Hz, 3H). 13C
NMR (100 MHz, CDCl3): 171.4, 133.2, 132.8, 130.9, 128.9, 61.2,
40.9, 14.4.
a
All reactions were run with phenylboronic acid (3 mmol), ethyl bromoacetate
(2 mmol), K2CO3 (6 mmol), Pd (0.05 mmol), and 1a (0.15 mmol) in 5 mL of solvent
(THF/H2O¼9:1, v/v,) at 65 ꢁC for 8 h.
b
4.6. Ethyl 2-(4-formylphenyl)acetate (3f)
Isolated yields based on ethyl bromoacetate.
As illustrated in Table 3, most substrates gave good to excellent
yields. Electron-poor and electron-rich compounds are equally
suitable for the transformation (Table 3, entries 1e11), sterically
hindered compounds gave moderate yields (Table 3, entry 12), 1-
naphthylboronic acid, 2-methoxy-1-naphthylboronic acid, and
alkenylboronic acid also gave good yields (Table 3, entries 13e16).
1H NMR (400 MHz, CDCl3)
7.46 (d, J¼8.0 Hz, 2H), 4.16 (q, J¼7.2 Hz, 2H), 3.71 (s, 2H), 1.26 (t,
J¼7.0 Hz, 3H). 13C NMR (100 MHz, CDCl3): 192.0, 170.8, 141.2, 135.5,
130.2, 130.1, 61.3, 41.6, 14.3.
d
10.0 (s, 1H), 7.85 (d, J¼8.4 Hz, 2H),
4.7. Ethyl 2-(4-(trifluoromethyl)phenyl)acetate (3g)
3. Conclusions
1H NMR (400 MHz, CDCl3)
J¼8.0 Hz, 2H), 4.16 (q, J¼7.2 Hz, 2H), 3.68 (s, 2H), 1.26 (t, J¼7.2 Hz,
3H). 13C NMR (100 MHz, CDCl3): 171.0, 138.3, 129.9, 129.4, 125.7,
123.0, 61.4, 41.3, 14.3.
d
7.58 (d, J¼8.0 Hz, 2H), 7.40 (d,
In conclusion, we have applied aminophosphine as an efficient
ligand in a palladium-catalyzed cross-coupling reaction between

Z.-Y. Peng et al. / Tetrahedron 66 (2010) 8238e8241
8241
4.8. Ethyl 2-(4-(trifluoromethoxy)phenyl)acetate (3h)
1H NMR (400 MHz, CDCl3)
4.16. (E)-Ethyl 5-(benzyloxy)pent-3-enoate (3p)
1H NMR (400 MHz, CDCl3)
d 7.34e7.24 (m, 5H), 5.88e5.69 (m,
2H), 4.49 (s, 2H), 4.13 (q, J¼7.0 Hz, 2H), 4.00 (d, J¼7.2 Hz, 2H), 3.09
(d, J¼6.6 Hz, 2H), 1.25 (t, J¼7.0 Hz, 3H). 13C NMR (100 MHz, CDCl3):
171.3, 138.0, 130.2, 128.2, 127.6, 127.4, 125.5, 71.9, 70.0, 60.5, 37.6,
14.0.
d
7.31 (d, J¼8.4 Hz, 2H), 7.17 (d,
J¼8.4 Hz, 2H), 4.16 (q, J¼7.2 Hz, 2H), 3.62 (s, 2H), 1.26 (t, J¼7.2 Hz,
3H). 13C NMR (100 MHz, CDCl3): 171.3, 148.5, 133.0, 130.8, 121.2,
119.4, 61.2, 40.7, 14.2. IR (KBr): n 2358, 2339, 1735, 1509, 1253, 1217,
1153, 1030, 922 cmꢀ1. HRMS (MICROMASS GCT-MS EI) exact mass
calcd for (C11H11F3O3): m/z 248.0660, found m/z 248.0633.
Acknowledgements
4.9. Ethyl 2-(4-methoxyphenyl)acetate (3i)
We thank the National Natural Science Foundation of China, the
Science and Technology Commission of Shanghai Municipality, and
the Education Commission of Shanghai Municipality for financial
support.
1H NMR (400 MHz, CDCl3)
d 7.20 (m, 2H), 6.86 (m, 2H), 4.15 (q,
J¼6.8 Hz, 2H), 3.79 (s, 3H), 3.55 (s, 2H), 1.25 (t, J¼7.2 Hz, 3H). 13C
NMR (100 MHz, CDCl3): 172.2, 158.9, 130.5, 126.5, 114.2, 61.0, 55.4,
40.7, 14.4.
References and notes
4.10. Ethyl 2-(4-tert-butylphenyl)acetate (3j)
1. (a) Lloyd-Jones, G. C. Angew. Chem., Int. Ed. 2002, 41, 953e956; (b) Millard, A. A.;
Rathke, M. W. J. Am. Chem. Soc. 1977, 99, 4833e4835.
1H NMR (400 MHz, CDCl3)
J¼8.4 Hz, 2H), 4.15 (q, J¼7.2 Hz, 2H), 3.58 (s, 2H), 1.31 (s, 9H), 1.26 (t,
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3): 172.0, 150.0, 131.4, 129.2,
125.7, 61.0, 41.1, 34.7, 31.6, 14.5.
d
7.34 (d, J¼8.4 Hz, 2H), 7.22 (d,
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4.11. Ethyl 2-(benzo[d][1,3]dioxol-5-yl)acetate (3k)
1H NMR (400 MHz, CDCl3)
d 6.79e6.72 (m, 3H), 5.94 (s, 2H), 4.15
(q, J¼7.2 Hz, 2H), 3.51 (s, 2H), 1.25 (t, J¼7.0 Hz, 3H). 13C NMR
(100 MHz, CDCl3): 171.9,147.9,146.9,127.9,122.6,109.9,108.4,101.2,
61.1, 41.2, 14.4.
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4.12. Ethyl 2-(2,6-dimethylphenyl)acetate (3l)
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d 7.09e7.02 (m, 3H), 4.15 (q,
J¼7.2 Hz, 2H), 3.68 (s, 2H), 2.33 (s, 6H), 1.24 (t, J¼6.8 Hz, 3H). 13C
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4.13. Ethyl 2-(naphthalen-1-yl)acetate (3m)
1H NMR (400 MHz, CDCl3)
d 8.01e7.98 (m, 1H), 7.86e7.77 (m,
2H), 7.54e7.41 (m, 4H), 4.15 (q, J¼7.2 Hz, 2H), 4.05 (s, 2H), 1.21 (t,
J¼6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): 171.9, 134.1, 132.4, 131.0,
129.0, 128.3, 128.2, 126.8, 126.1, 125.8, 124.1, 61.2, 39.6, 14.5.
4.14. Ethyl 2-(2-methoxynaphthalen-1-yl)acetate (3n)
1H NMR (400 MHz, CDCl3)
d 7.73e7.70 (m, 2H), 7.62 (s, 1H),
7.42e7.39 (m, 1H), 7.34e7.29 (m, 1H), 7.10 (s, 1H), 4.16 (q, J¼7.2 Hz,
2H), 3.90 (s, 3H), 3.76 (s, 2H), 1.24 (t, J¼6.8 Hz, 3H). 13C NMR
(100 MHz, CDCl3): 171.9, 156.3, 134.4, 130.2, 128.9, 127.6, 126.7,
126.4, 126.3, 125.3, 124.0, 105.4, 60.9, 55.7, 37.0, 14.5.
4.15. (E)-Ethyl 4-phenylbut-3-enoate (3o)
1H NMR (400 MHz, CDCl3)
d 7.38e7.20 (m, 5H), 6.49 (d,
J¼15.6 Hz, 1H), 6.30 (dt, J¼16.0, 7.2 Hz 1H), 4.17 (q, J¼7.2 Hz, 2H),
3.24 (dd, J¼6.8, 1.2 Hz, 2.0 H), 1.28 (t, J¼7.2 Hz, 3H). 13C NMR
(100 MHz, CDCl3): 171.8, 137.1, 133.6, 128.8, 127.7, 126.5, 122.1, 61.0,
38.7, 14.5.