A practical palladium catalyzed dehalogenation of aryl halides and α-haloketones

Article abstract of DOI:10.1016/j.tet.2007.03.061

A practical and high-yielding protocol for the dehalogenation of aromatic halides is presented. In the presence of palladium acetate, triphenylphosphine, and potassium carbonate, a number of highly functionalized aromatic halides as well as α-haloketones were dehalogenated with alcohols as hydrogen donors.

Full text of DOI:10.1016/j.tet.2007.03.061

Tetrahedron 63 (2007) 4266–4270
A practical palladium catalyzed dehalogenation
of aryl halides and a-haloketones
Jingbo Chen, Yushun Zhang, Liquan Yang, Xiang Zhang, Jianping Liu,
*
Liang Li and Hongbin Zhang
Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University), Ministry of Education,
School of Chemical Science and Technology, Yunnan University, Kunming, Yunnan 650091, PR China
Received 6 January 2007; revised 8 March 2007; accepted 9 March 2007
Available online 12 March 2007
Abstract—A practical and high-yielding protocol for the dehalogenation of aromatic halides is presented. In the presence of palladium
acetate, triphenylphosphine, and potassium carbonate, a number of highly functionalized aromatic halides as well as a-haloketones were
dehalogenated with alcohols as hydrogen donors.
Ó 2007 Elsevier Ltd. All rights reserved.
O
O
1. Introduction
N
H
N
H
Debromination
A halogen atom is introduced to a given position of arene
ring system in organic synthesis either as an activating group
or a blocking group.1 Occasionally, it is introduced to arene
ring simply from side reactions of other desired synthetic
operation.2 Dehalogenation is an important chemical trans-
formation in organic synthesis and in industrial applica-
tions.3 Because aryl halides are deleterious environmental
pollutants, this transformation is now assumed to be more
important for decontamination of aromatic halides.4 A great
number of dehalogenation methods have been developed
over years.5 Recent advances in this field have led to several
new methods by employing palladium,6,4b rhodium,7 iron,8
and nickel9 catalysts. Although numerous reagents or pro-
tocols are available for dehalogenation, many reported
methods suffered from some limitations. Functional group
compatibility as well as selectivity is rarely addressed. In
our work on the synthesis of galanthamine derivatives, we
needed a debromination method compatible with various
functional groups (Scheme 1).
MeO
Br
MeO
BnO
OBn
1
2
OMe
MeO
Scheme 1. Desired debromination of highly functional substrate.
(100 ꢀC). The advantage of Helquist’s method is the use of
a very simple, readily available, inexpensive, and compara-
tively nonhazardous source of hydrogen donor. Unfortu-
nately, utilization of Helquist’s condition toward substrate
1 resulted in a complicated mixture of products. Having
failed to promote the desired debromination reaction, we
thus initiated a research program aiming to develop a dehalo-
genation process compatible with various functional groups.
In this paper, we reported a practical, functional group com-
patible, and high-yielding protocol for the dehalogenation of
aromatic halides and a-haloketones.
It was unlikely that substrate 1 could sustain conventional
hydrogenation (Pd/C, H2) or metal hydride reduction with-
out affecting the benzyloxyl group and the amide group. A
method published in 1978 by Helquist drew our attention.2a
A number of aryl bromides were reduced to arenes by uti-
lization of sodium methoxide as hydrogen donor in the
presence of tetrakis(triphenylphosphine)palladium in DMF
2. Results and discussion
Because triphenylphosphine is readily available and cheap,
it was chosen as a ligand to optimize the palladium catalyzed
dehalogenation. The reaction was initially carried out in
2-butanol in the presence of potassium carbonate. To our
delight, desired debromination product was obtained in
98% isolated yield at 100 ꢀC (oil bath) (Scheme 2).
Keywords: Dehalogenation; Catalytic; Hydrogen donor; Aromatic halides;
a-Haloketones.
Encouraged by this result, we employed 3,4,5-trimethoxy-
phenyl bromide as substrate to study the influence of bases
* Corresponding author. Fax: +86 871 5035538; e-mail: zhanghb@ynu.
edu.cn
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.061

J. Chen et al. / Tetrahedron 63 (2007) 4266–4270
4267
Table 2. Pd catalyzed dehalogenation of aromatic halides with alcohol as
hydrogen donor
MeO
BnO
MeO
BnO
1%Pd(OAc)2, 4%Ph3P
2-Butanol, K2CO3
100 °C oil bath, 12 hours
Pd(OAc)2, Ph3P,K2CO3
Alcohol, 100 °C (oil bath)
X
H
Br
O
O
R
R
X = Cl, Br, I
HN
98%
HN
O
O
Entry Method
Substrates
Yield (%)
99*
1
2
Br
O
Scheme 2. Pd catalyzed debromination in 2-butanol.
BnO
MeO
1
2
3
4
[A]
[A]
[A]
[A]
OMe
Table 1. Studies on palladium catalyzed debromination of aromatic
bromidesa
Br
BnO
OMe
OMe
98
O
Pd(OAc)2, Ph3P
MeO
MeO
MeO
MeO
MeO
Br
Alcohol, Base
100 °C (oil bath), 12 hours
Br
H
BnO
MeO
N
OH 99
Entry
Base
Hydrogen donors
Times (h)
Yield (%)
1
2
3
4
5
6
NaHCO3
K2CO3
Cs2CO3
KOH
K2CO3
K2CO3
2-Butanol
2-Butanol
2-Butanol
2-Butanol
iso-Propanol
n-Butanol
1
1
1
1
1
1
5
35
44
99
16
47
Br
BnO
MeO
OH
90
O
O
O
a
5
6
[A]
[A]
98
93
Yields represent isolated yields. Aryl bromide (1 mmol), Pd(OAc)2
(1 mmol %), Ph3P (4 mmol %), and K2CO3 (2 mmol) in commercially
available alcohol (5 mL) were degassed and purged with nitrogen, then
stirred at 100 ꢀC (oil bath).
Br
I
OMe
Br
and solvents. The experimental results are shown in Table 1.
The debromination reaction tended to be carried out under
stronger basic condition. Utilization of n-butanol as a solvent
enhanced the reaction rate.
7
8
[A]
[A]
99
99
N
O
Br
In consideration of functional group compatibility, potas-
sium carbonate was chosen as a base and a number of aryl
halides were subjected to dehalogenation in the presence
of palladium acetate and triphenylphosphine. The results
are summarized in Table 2. It is noteworthy that a variety
of functional groups such as double bond, benzyloxyl group,
carbonyl group, ester, phenol, amino group, and amide are
compatible under our reaction conditions. The bromo group
in 1-bromo-4-chlorobenzene or in 1-bromo-3,5-difluorobenz-
ene could be selectively removed (see entries 11 and 24 in
Table 2) in the presence of a chloro or a fluoro moiety. De-
iodination and dechlorination were also achieved. It is inter-
esting that iso-propanol was a better hydrogen source when
halophenols were used as substrates. Dehalogenation of
halophenol in n-butanol system led to decomposition of
palladium catalysts and formation of a black precipitate.
Reduction of 4-fluoroanisole was not observed under our
reaction conditions.
9
[A]
99
Br
10
11
[A]
[A]
97
95
I
I
Br
Cl
H2N
Br
12
[B]
93
H2N
13
14
[B]
[B]
95
93
Br
O
HN
Br
HN
This dehalogenation procedure could also be applied to
a-haloketones. Excellent results were obtained for a number
of a-haloketones. The results are shown in Table 3.
15
16
[B]
[B]
92
98
O
Br
Br
In summary, we have developed a practical and efficient
method compatible with various functional groups for the
dehalogenation of aryl halides and a-haloketones. The
advantage of this procedure is that it is easy to handle and
highly compatible with a lot of functionalities. Furthermore,
triphenylphosphine ligand is readily available and inexpen-
sive. In combination with this method, utilization of bromine
OMe
Br
17
[B]
99
(continued)

4268
J. Chen et al. / Tetrahedron 63 (2007) 4266–4270
Table 2. (continued)
Table 3. Pd catalyzed debromination of a-haloketones with n-butanol as
hydrogen sourcea
Entry Method
Substrates
HO
Yield (%)
1% Pd(OAc)2
4 % Ph3P
O
O
Br
H
[B]
[C]
55
92
R1
R1
Alcohol, K2CO3
100 °C (oil bath), 90-98%
18
Cl
Br
Entry
1
Substrates
Yield (%)
90
HO
[B]
[C]
30
82
19
O
Br
MeO
HO
O
[B]
[C]
50
98
20
O
I
2
3
O
92
Br
21
22
23
[C]
[C]
[C]
95
97
15
HO
HO
Br
O
92b
Br
Br
O
HO
Cl
4
5
6
94
99
F
F
Br
24
[A]
Br
O
97
40
O
Br
25
26
[B]
Cl
O
Br
95
Starting
material
[A], [B], [C]
F
OMe
O
61b
[A] Aryl halides (1 mmol), Pd(OAc)2 (1 mmol %), Ph3P (4 mmol %), and
K2CO3 (2 mmol) in commercially available n-butanol (5 mL) were stirred
at 100 ꢀC (oil bath) for 1 h. [B] Aryl halides (1 mmol), Pd(OAc)2 (2–
5 mmol %), Ph3P (8–20 mmol %), and K2CO3 (2 mmol) in commercially
available n-butanol (5 mL) were stirred at 100 ꢀC (oil bath) for 14 h. [C]
Aryl halides (1 mmol), Pd(OAc)2 (3–5 mmol %), Ph3P (12–20 mmol %),
and K2CO3 (2 mmol) in commercially available iso-propanol (5 mL) were
stirred at 90 ꢀC (oil bath) for 14 h. [*] The methyl ester was also converted
to n-butyl ester by a transesterification process. Yields represented isolated
yields or determined by GC–MS. The isolated products were characterized
by 1H NMR, 13C NMR, and GC–MS, and all known compounds are identi-
cal to authentic samples.
Cl
7
Ph
N
H
a
a-Bromoketones (1 mmol), Pd(OAc)2 (1 mmol %), Ph3P (4 mmol %),
and K2CO3 (2 mmol) in commercially available n-butanol (5 mL) were
stirred at 100 ꢀC (oil bath).
Substrate (1 mmol), Pd(OAc)2 (2–5 mmol %), Ph3P (8–20% mmol), and
K2CO3 (2 mmol) in commercially available n-butanol (5 mL) were stirred
at 100 ꢀC (oil bath) for 14 h. Yields represented isolated yields. The iso-
lated products were characterized by 1H NMR and 13C NMR, and are
identical to authentic samples.
b
atom as a blocking group in aromatic ring system should
practically be of usefulness.
3.2. General method for palladium catalyzed
dehalogenation
3. Experimental
A mixture of palladium acetate (2.2–11 mg, 0.01–
0.05 mmol), triphenylphosphine (10.5–52.5 mg, 0.04–
0.20 mmol), potassium carbonate (276 mg, 2 mmol,
2.0 equiv), and aromatic halides (1 mmol) in n-butanol
(5 mL) (iso-propanol was used for halophenols as substrate)
was degassed and purged with nitrogen (two times). The
resulting mixture was then stirred at 100 ꢀC (oil bath) under
nitrogen. The reaction was monitored by thin layer chroma-
tography (TLC). After removal of the solvent, the residuewas
diluted with water (50 mL) and extracted with ethyl acetate
(3ꢁ15 mL). The organic phases were combined, washed
with brine (10 mL), and dried over anhydrous Na2SO4. The
organic solvent was removed and the residue was chromato-
graphed on silica gel to afford the products. All compounds
were characterized by 1H NMR and 13C NMR.
3.1. General experimental
Infrared (IR) spectra (nmax) were recorded on a Perkin–
Elmer 1800 Fourier transform infrared spectrophotometer
in KBr plates. Proton nuclear magnetic resonance (1H
NMR) spectra were recorded on a Bruker Avance 300 spec-
trometer at 300 MHz. Carbon-13 nuclear magnetic reso-
nance (13C NMR) was recorded on Bruker Avance 300
spectrometer at 75 MHz. Chemical shifts are reported as
d values in parts per million (ppm) relative to tetramethyl-
silane (TMS) for all recorded NMR spectra. High-resolution
mass spectra were taken on AB QSTAR Pulsar mass spec-
trometer. Starting materials and reagents used in reactions
were obtained commercially from Acros, Aldrich, and
Fluka, and were used without purification, unless otherwise
indicated.
3.2.1. 3-(3-Benzyloxy-2-bromo-4-methoxy-phenyl)-
acrylic acid methyl ester. White solid; mp 105.4–

J. Chen et al. / Tetrahedron 63 (2007) 4266–4270
4269
106.7 ꢀC. IR (KBr) (nmax, cmꢂ1): 3127 (s), 1709 (s), 1585
(m), 1487 (s), 1402 (s), 1265 (s), 1221 (s), 1027 (s), 802
(m), 733 (m). H NMR (300 MHz, CDCl3) d (ppm): 8.06
(1H, d, J¼15.9 Hz), 7.56 (2H, d, J¼6.6 Hz), 7.54–7.33
(4H, m), 6.90 (1H, d, J¼8.7 Hz), 6.30 (1H, d, J¼15.9 Hz),
5.03 (2H, s), 3.90 (3H, s), 3.81 (3H, s). 13C NMR (75 MHz,
CDCl3) d (ppm): 167.19, 155.21, 145.75, 143.42, 137.08,
128.57, 128.47, 128.28, 128.05, 123.48, 121.99, 118.77,
111.56, 74.73, 56.31, 51.82. HRMS (ESI+) calcd for
C18H17O4NaBr [M+Na]+: 399.0207, found: 399.0214.
158.48, 155.11, 145.47, 137.03, 131.16, 130.89, 129.92,
128.57, 128.49, 128.31, 125.44, 116.17, 114.24, 111.33,
74.81, 56.29, 55.41, 41.53, 34.69. HRMS (ESI+) calcd for
C24H25NO4Br [M+H]+: 470.0966, found: 470.0969.
1
3.2.6. 3-Benzyloxy-4-methoxy-N-[2-(4-methoxy-phenyl)-
ethyl]benzamide. White solid; mp 169.3–170.1 ꢀC. IR
(KBr) (nmax, cmꢂ1): 3295 (m), 3128 (s), 3017 (m), 1632
(m), 1507 (s), 1400 (s), 1314 (w), 1257 (m), 1185 (w), 1137
(w), 1019 (w). 1H NMR (300 MHz, CDCl3) d (ppm): 7.50–
7.30 (6H, m), 7.22 (1H, dd, J¼8.4, 2.0 Hz), 7.13 (2H, d, J¼
8.6 Hz), 6.86 (2H, d, J¼8.6 Hz), 6.09 (1H, br s), 5.14 (2H, s),
3.89 (3H, s), 3.79 (3H, s), 3.63 (2H, q, J¼6.8 Hz), 2.84 (2H,
t, J¼6.8 Hz). 13C NMR (75 MHz, CDCl3) d (ppm): 167.03,
158.48, 152.52, 148.24, 136.80, 131.10, 129.89, 128.70,
128.13, 127.66, 127.42, 119.98, 114.26, 113.19, 111.04,
71.19, 56.17, 55.40, 41.41, 34.95. HRMS (ESI+) calcd for
C24H26NO4 [M+H]+: 392.1861, found: 392.1853.
3.2.2. 3-(3-Benzyloxy-4-methoxy-phenyl)acrylic acid
butyl ester. White solid; mp 129.8–130.2 ꢀC. IR (KBr)
(nmax, cmꢂ1): 3433 (s), 3129 (s), 1715 (m), 1635 (m), 1590
(w), 1512 (m), 1399 (s), 1265 (m), 1168 (m), 1011 (m). 1H
NMR (300 MHz, CDCl3) d (ppm): 7.58 (1H, d, J¼
15.9 Hz), 7.46–7.30 (5H, m), 7.10 (1H, d, J¼7.8 Hz), 7.11
(1H, s), 6.86 (1H, d, J¼7.8 Hz), 6.24 (1H, d, J¼15.9 Hz),
5.15 (2H, s), 4.19 (2H, t, J¼6.6 Hz), 3.88 (3H, s), 1.76–
1.56 (2H, m), 1.46–1.39 (2H, m), 0.96 (3H, t, J¼7.2 Hz).
13C NMR (75 MHz, CDCl3) d (ppm): 167.43, 151.89,
148.47, 144.53, 136.84, 128.74, 128.14, 127.50, 127.49,
123.00, 116.08, 112.77, 111.68, 71.19, 64.40, 56.10,
30.95, 19.34, 13.89. HRMS (ESI+) calcd for C21H24O4Na
[M+Na]+: 363.1572, found: 363.1573.
3.2.7. (3-Benzyloxy-2-bromo-4-methoxy-benzyl)-[2-(4-
hydroxy-phenyl)ethyl]methyl-amine. Colorless oil. IR
(KBr) (nmax, cmꢂ1): 3129 (s), 2940 (s), 1600 (s), 1481 (s),
1
1451 (s), 1403 (s), 1273 (s), 1031 (s), 819 (s), 740 (s). H
NMR (300 MHz, CDCl3) d (ppm): 7.86 (1H, br s), 7.60
(2H, d, J¼7.3 Hz), 7.45–7.30 (3H, m), 7.24 (1H, d, J¼
8.5 Hz), 7.04 (2H, d, J¼8.2 Hz), 6.87 (1H, d, J¼8.5 Hz),
6.77 (2H, d, J¼8.2 Hz), 5.04 (2H, s), 3.85 (3H, s), 3.78 (2H,
s), 2.92–2.73 (4H, m), 2.41 (3H, s). 13C NMR (75 MHz,
CDCl3) d (ppm): 154.87, 152.82, 144.95, 137.04, 130.79,
129.58, 128.38, 128.18, 127.95, 126.42, 121.05, 115.50,
111.10, 74.41, 60.48, 59.44, 55.96, 41.53, 32.20. HRMS
(ESI+) calcd for C24H27NO3Br [M+H]+: 456.1174, found:
456.1169.
3.2.3. 3-(3-Benzyloxy-2-bromo-4-methoxy-phenyl)prop-
2-en-1-ol. White solid; mp 69.2–70.0 ꢀC. IR (KBr) (nmax
,
cmꢂ1): 3421 (s), 3135 (s), 3021 (s), 1634 (m), 1589 (m),
1482 (m), 1401 (s), 1290 (s), 1215 (w), 1029 (s), 963 (m),
806 (w), 744 (m). H NMR (300 MHz, CDCl3) d (ppm):
1
7.57 (2H, d, J¼6.5 Hz), 7.40–7.31 (3H, m), 7.25 (1H, d, J¼
8.7 Hz), 6.91 (1H, d, J¼15.8 Hz), 6.85 (1H, d, J¼8.7 Hz),
6.18 (1H, dt, J¼15.8, 5.8 Hz), 5.00 (2H, s), 4.31 (2H, dd,
J¼5.8, 1.5 Hz), 3.85 (3H, s). 13C NMR (75 MHz, CDCl3)
d (ppm): 153.29, 145.39, 137.30, 130.48, 130.04, 130.00,
128.56, 128.42, 128.17, 122.43, 120.09, 111.72, 74.67,
63.77, 56.31. HRMS (ESI+) calcd for C17H17O3NaBr
[M+Na]+: 371.0258, found: 371.0263.
3.2.8. (3-Benzyloxy-4-methoxy-benzyl)-[2-(4-hydroxy-
phenyl)ethyl]methyl-amine. White solid; mp 139.2–
139.8 ꢀC. IR (KBr) (nmax, cmꢂ1): 3437 (s), 3124 (s), 3009
(s), 1599 (m), 1512 (s), 1455 (m), 1398 (s), 1254 (s), 1162
1
(m), 1125 (m), 1010 (m), 811 (m). H NMR (300 MHz,
CDCl3) d (ppm): 7.40 (2H, d, J¼6.8 Hz), 7.35–7.20 (3H,
m), 6.97 (2H, d, J¼8.4 Hz), 6.96 (1H, s), 6.92 (2H, s),
6.75 (2H, d, J¼8.4 Hz), 5.03 (2H, s), 3.84 (3H, s), 3.47
(2H, s), 2.80–2.65 (2H, m), 2.65–2.50 (2H, m), 2.22 (3H, s).
13C NMR (75 MHz, CDCl3) d (ppm): 155.57, 148.82,
148.06, 137.20, 131.02, 130.78, 129.76, 128.49, 127.79,
127.53, 122.08, 115.65, 114.97, 111.47, 70.78, 61.81, 59.23,
56.09, 41.87, 32.61. HRMS (ESI+) calcd for C24H28NO3
[M+H]+: 378.2069, found: 378.2074.
3.2.4. 3-(3-Benzyloxy-4-methoxy-phenyl)prop-2-en-1-ol.
White solid; mp 60.3–60.8 ꢀC. IR (KBr) (nmax, cmꢂ1):
3140 (s), 1590 (m), 1481 (m), 1400 (s), 1289 (s), 1265 (s),
1228 (m), 1147 (m), 1091 (w), 1008 (s), 962 (s), 802 (m),
1
745 (m), 697 (m). H NMR (300 MHz, CDCl3) d (ppm):
7.60–7.30 (5H, m), 7.05–6.80 (3H, m), 6.49 (1H, d, J¼
15.9 Hz), 6.14 (1H, dt, J¼15.9, 5.8 Hz), 5.16 (2H, s), 4.27
(2H, dd, J¼5.8, 1.0 Hz), 3.88 (3H, s). 13C NMR (75 MHz,
CDCl3) d (ppm): 149.68, 148.30, 137.51, 131.18, 129.74,
128.70, 128.01, 127.43, 126.59, 120.29, 111.84, 111.76,
71.15, 63.95, 56.14. HRMS (ESI+) calcd for C17H18O3Na
[M+Na]+: 293.1153, found: 293.1155.
3.2.9. 3-Benzyloxy-2-bromo-4-methoxy-benzaldehyde.
1H NMR (300 MHz, CDCl3) d (ppm): 10.27 (1H, s), 7.75
(1H, d, J¼8.7 Hz), 7.58–7.53 (2H, m), 7.45–7.32 (3H, m),
6.97 (1H, d, J¼8.7 Hz), 5.06 (2H, s), 3.95 (3H, s). 13C NMR
(75 MHz, CDCl3) d (ppm): 191.07, 158.91, 145.35, 136.82,
128.63, 128.54, 128.44, 127.67, 126.69, 123.61, 111.13,
74.97, 56.45.
3.2.5. 3-Benzyloxy-2-bromo-4-methoxy-N-[2-(4-meth-
oxy-phenyl)ethyl]benzamide. White solid; mp 129.5–
129.7 ꢀC. IR (KBr) (nmax, cmꢂ1): 3466 (m), 3129 (s), 3013
(m), 1640 (m), 1541 (w), 1401 (s), 1286 (w), 1256 (w),
1
1
1182 (w), 1030 (w). H NMR (300 MHz, CDCl3) d (ppm):
3.2.10. 3-Benzyloxy-4-methoxy-benzaldehyde. H NMR
7.52 (2H, d, J¼8.5 Hz), 7.41–7.30 (3H, m), 7.25 (1H, d,
J¼8.5 Hz), 7.15 (2H, d, J¼8.5 Hz), 6.85 (1H, d, J¼
8.5 Hz), 6.83 (2H, d, J¼8.5 Hz), 6.09 (1H, br s), 4.97 (2H, s),
3.85 (3H, s), 3.77 (3H, s), 3.66 (2H, q, J¼6.7 Hz), 2.87 (2H,
t, J¼6.7 Hz). 13C NMR (75 MHz, CDCl3) d (ppm): 167.36,
(300 MHz, CDCl3) d (ppm): 9.81 (1H, s), 7.50–7.42 (4H,
m), 7.42–7.23 (3H, m), 6.98 (1H, d, J¼8.5 Hz), 5.18 (2H,
s), 3.94 (3H, s). 13C NMR (75 MHz, CDCl3) d (ppm):
190.91, 155.06, 148.71, 136.32, 129.99, 128.70, 128.18,
127.54, 126.98, 111.29, 110.80, 70.84, 56.22.

4270
J. Chen et al. / Tetrahedron 63 (2007) 4266–4270
4. (a) Vincent, T.; Spinelli, S.; Guibal, E. Ind. Eng. Chem. Res.
Acknowledgements
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This work was supported by a grant (20562013) from Natu-
ral Science Foundation of China and a grant (2003B0001Q)
from Natural Science Foundation of Yunnan Provincial
Science & Technology Department. H.Z. thanks Eli Lilly
& Company for an Eli Lilly Scientific Excellence Award.
5. Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2002, 102,
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