Biphasic Suzuki coupling reactions of aryl or benzyl bromides employing cobalt-containing phosphine ligand coordinated palladium complex

Article abstract of DOI:10.1016/j.tet.2005.01.134

Biphasic Suzuki-coupling reactions of aryl and benzyl bromides employing a cobalt-containing phosphine ligand chelated palladium complex 2 were carried out in various reaction conditions. Comparisons of the catalytic efficiencies in the presence/absence of a phase-transfer agent, TBAB, were presented. In addition, the effects of altering solvents, temperatures, catalysts, and substrates on the reactions were monitored and reported. Better yields were commonly observed while a phase-transfer agent TBAB was participated in the reactions. The factor of reaction time is more crucial than that of temperature in short reaction hour experiments. Obviously, an induction period for the reduction of Pd(II) to Pd(0) active species is needed for this type of reaction.

Full text of DOI:10.1016/j.tet.2005.01.134

Tetrahedron 61 (2005) 3835–3839
Biphasic Suzuki coupling reactions of aryl or benzyl bromides
employing cobalt-containing phosphine ligand coordinated
palladium complex
Chin-Pei Chang, Yi-Luen Huang and Fung-E Hong
*
Department of Chemistry, National Chung Hsing University, Taichung 40227, Taiwan
Received 17 November 2004; revised 26 January 2005; accepted 28 January 2005
Abstract—Biphasic Suzuki-coupling reactions of aryl and benzyl bromides employing a cobalt-containing phosphine ligand chelated
palladium complex 2 were carried out in various reaction conditions. Comparisons of the catalytic efficiencies in the presence/absence of a
phase-transfer agent, TBAB, were presented. In addition, the effects of altering solvents, temperatures, catalysts, and substrates on the
reactions were monitored and reported. Better yields were commonly observed while a phase-transfer agent TBAB was participated in the
reactions. The factor of reaction time is more crucial than that of temperature in short reaction hour experiments. Obviously, an induction
period for the reduction of Pd(II) to Pd(0) active species is needed for this type of reaction.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Palladium-catalyzed Suzuki cross-coupling of haloarenes
with arylboronic acid is among the most powerful Csp –Csp
bond-formation available to synthetic organic chemists.
2
2
1
Although N-heterocyclic carbenes (NHC) have been
emerged as potentially effective ligands for Suzuki reactions
Scheme 1.
2
recently, yet, phosphines remain the most employed
The reactants of the catalytic processes under investigation
were composed of 1.00 mmol of aryl halide, 1.5 equiv of
phenylboronic acid, 1 mol% of 2 (based on aryl halide),
3
ligands in accelerating or modifying the reactions. Even
though various synthetic methods have been extensively
explored in searching of more versatile and efficient organic
phosphine ligands in the palladium-catalyzed Suzuki–
1
mL toluene (or mixed solvent: THF/H OZ5 mL/1 mL),
2
4
and 2.0 equiv of K PO (based on aryl halide). The reaction
3 4
Miyaura cross-coupling reactions, nevertheless, to our
best knowledge, a systematical investigation of transition-
metal-containing phosphines (TM-phosphines) has yet
mixture was stirred at 65 8C for 16 h then workup followed.
In most of the cases, the reactions carried out in biphasic
media are more efficient than that in pure organic phase. The
fact that high efficiency was achieved for an alkyl bromide
5
,6
remained a relatively uncultivated territory.
8
,9
is noteworthy.
Our previous work had demonstrated the catalytic capacity
of an unusual palladium complex 2, which is coordinated by
a cobalt-containing bidentate phosphine ligand [(m-Ph2-
PCH PPh )Co (CO) (m-PPh C^CPPh )] 1, on Suzuki
As known, Suzuki–Miyaura coupling reactions are more
efficient in organic phase while employing Pd(OAc) as
catalyst; on the contrary, it is more efficient in biphasic
media for using PdCl2. The idea of using biphasic media
in Suzuki–Miyaura coupling reaction is attractive because
of the ecological and safety reason for using water as a
2
2
2
2
4
2
2
7
cross-coupling reactions (Scheme 1). The reactions, using
bromobenzene (or bromothiophene) with phenylboronic
acid as reaction substrates, were carried out by employing 2
as catalyst in organic as well as biphasic media and results
1
0
1
1
8
major component of the reaction solvent. In addition,
water-soluble reagent such as NaOH, K PO (bases using in
were satisfactory.
3
4
Suzuki reaction) and NaCl (side product of Suzuki reaction)
can be dissolved extensively in water. Besides, the
disadvantage of low solubility of the metal catalysts in
Keywords: Biphasic Suzuki reaction; Palladium complex; Cobalt-contain-
ing phosphine; Phase-transfer agent.
*
Corresponding author. E-mail: fehong@dragon.nchu.edu.tw
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.01.134

3836
C.-P. Chang et al. / Tetrahedron 61 (2005) 3835–3839
a
Table 1. Suzuki-coupling reactions of benzyl bromides employing catalyst 2
Entry
Halide
Product
Solvent
THF–H
Toluene
Yield (%)
2
O(5:1)
O(5:1)
O(5:1)
99.0
90.0
1
THF–H
Toluene
2
79.2
64.9
2
3
THF–H
Toluene
2
90.2
72.4
2
THF–H O(5:1)
Toluene
NR
NR
4
a
Reactions were conducted in either THF with NaOH(aq) (3 M, 1.00 mL) or in toluene (1.00 mL) with K
with bromides (1.00 mmol) and phenylboronic acid (1.50 mmol).
3
PO
4
(3 equiv) at 65 8C for 16 h employing 1 mol% of
1
biphasic media might be overcome either by employing
often encountered while inactivated alkyl halides and alkyl
electrophiles are employed in Suzuki coupling reactions.
First, alkyl halides (even CH I) react slowly with Pd
3
1
2
phase-transfer agent or water-soluble phosphine ligands.
Phase transfer agent such as tetrabutylammonium bromide
TBAB) was reported frequently used in biphasic reac-
0
(
complexes, in contrast with the behavior observed for allyl,
17
benzyl, alkenyl, and aryl halides. Second, once the alkyl-
1
3
tions. Role played by the ammonium salt is thought to be
two-folds. First, it facilitates the solvability of the organic
substrates in the solvent medium. Second, it was thought to
enhance the rate of the coupling reaction by activating the
boronic acid through the formation of a boronate complex
II
Pd complex is formed, it should face the possibility of
decomposition by a fast b-hydride elimination, which
competes with the usually slower trans-metalation process.
b-Hydride elimination process requires several conditions
such as the existence of a vacant coordination site and the
feasibility of arranging the M–C(a)–C(b)–H atoms in the
same plane. This undesirable process may not be a problem
K
C
[
ArB(OH) ] [R N] . TBAB has been used recently in
3 4
conjunction with a palladium oxime catalyst for the Suzuki
coupling of aryl chlorides with phenylboronic acid in
1
4
3
water and as a promoter in the Pd(PPh3)4 catalyzed
Suzuki coupling reaction of 4-bromobenzonitrile and
in carbonylative couplings of C(sp ) centers since the fast
CO insertion prevents decomposition of the alkyl-Pd
1
5
18
intermediate. Third, the reductive elimination process is
phenylboronic acid in organic solvents.
II
normally slow for s-alkyl-s-aryl-Pd or di-s-alkyl-Pd
complexes.
II
1
9
In principle, the carbon–halide bond, either Csp3-, Csp2-, or
C –X, are all subjected to the attack of deliberately
chosen electrophiles while appropriate metal-containing
sp
Reported herein are some notable results from biphasic
Suzuki–Miyaura coupling reactions of aryl or benzyl
1
catalysts were employed. Nevertheless, difficulties are
6
a
Table 2. Biphasic Suzuki-coupling reactions of benzyl bromides with phenylboronic acid employing catalyst 2 and adding TBAB
Entry
Bromides
Product
Temperature (8C)
Time (h)
Yield (%)
40
65
0.5
16
98.2
99.0
1
4
65
0
0.5
16
72.0
86.8
2
3
40
65
0.5
16
74.2
90.2
4
5
40
0.5
47.8
40
40
0.5
0.5
37.4
6.8
b
6
a
Reactions were conducted in THF with 1 mol% of 2, NaOH(aq) (3 M, 1.00 mL), bromides (1.00 mmol), TBAB (0.20 mmol) and phenylboronic acid
1.50 mmol).
For comparison.
(
b

C.-P. Chang et al. / Tetrahedron 61 (2005) 3835–3839
3837
a
Table 3. Biphasic Suzuki coupling reactions employing 2 at various conditions
Entry
TBAB (equiv)
Temperature (8C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
—
0.2
—
0.2
—
0.2
—
65
65
65
65
40
40
40
40
16
16
0.5
0.5
16
16
0.5
0.5
O99
O99
10.8
16.2
75.4
87.5
10.5
6.8
0.2
a
Reactions were conducted in THF with 1 mol% of 2, NaOH(aq) (3 M, 1.00 mL), bromide (1.00 mmol) and phenylboronic acid (1.50 mmol).
bromides employing a cobalt-containing phosphine ligand
coordinated palladium complex 2 in the presence/absence of
a phase-transfer agent, TBAB.
pentaflurobenzylbromide, as demonstrated in entry 4, the
yield drops significantly, 47.8%. Obviously, the exceed-
ingly strong electron-withdrawing effect from the five
substituents as well as the steric effect caused by ortho-
position substituent play major role in preventing the aryls
from coupling. It is worthy of noting that the yield is
improved from almost nothing (Table 1, entry 4) to 37.4%
2
. Results and discussion
(
Table 2, entry 5) while adding TBAB to the reaction
system. On the contrary, the yield drops from 99.0% (Table
, entry 1) to 6.8% (Table 2, entry 6) while the reaction time
2.1. Suzuki reaction using cobalt-containing phosphine
ligand chelated palladium complex 2
1
8
In our previous work, an unusual high efficiency was
is shortened from 16 h to 30 min. It indicates that an
induction period is needed before the reaction can be
speeded up to a reasonable rate. In brief, the biphasic Suzuki
coupling reactions are more efficient in the presence of
phase-transfer agent TBAB than those without it.
achieved for a substituted alkyl bromide, benzylbromide, in
a 2-catalyzed Suzuki reaction. For comparison, the same
reaction conditions were employed for several structural
related substituted bromomethanes in either uni- or
bi-phasic media (Table 1). The reactants of the catalytic
reactions under investigation are consisted of 1.0 mmol of
aryl halide, 1.5 equiv of phenylboronic acid, 1 mol% of 2
Table 3 lists the results from the biphasic Suzuki coupling
reactions, employing 2 as catalyst and with 4-methylbenzyl
bromide and phenylboronic acid as substrates, in various
reaction conditions. This study has shown the importance of
two features. First, the yields are relatively lower for
reactions without adding TBAB (entry 3 vs 4; 5 vs 6).
Second, the factor of reaction time is more crucial than that
of temperature. Low yield was observed commonly for short
reaction time. Obviously, an induction period for the
reduction of Pd(II) to Pd(0) active species is needed for
this type of reaction.
(
based on aryl halide), 1.00 mL toluene (or mixed solvent:
THF/H OZ5 mL/1 mL), and 3.0 equiv of K PO . The
2
3
4
reaction mixture was stirred at 65 8C for 16 h then workup
followed. Encouraging results were obtained as shown
(
Table 1, entries 1–3). In these cases, reactions carried out in
biphasic media are more efficient than that in pure organic
solvent. Almost quantitative yield (99.0%) was obtained
when bromobenzene is used as the halide source (entry 1).
Nevertheless, the reaction was not very efficient (79.2% in
THF/H O and 64.9% in toluene) when a less reactive
2
4
-methyl-benzyl bromide, having a s electron-donating
Table 4. Biphasic Suzuki coupling reaction of aryl bromides employing
catalyst 2 and adding TBAB
group, was used (entry 2). The reaction with 4-fluoro-benzyl
bromide, having a s electron-withdrawing group at the
para-position, gave a better yield, that is, 90.2% in THF/
a
Entry
Halide
Product
Yield
%)
(
H O and 72.4% in toluene (entry 3). However, no reactivity
2
was observed for using an aliphatic bromide as substrate
entry 4).
1
2
6.8
(
Biphasic Suzuki-coupling reactions, using benzyl bromides
and phenylboronic acid as reaction substrates, were carried
out employing catalyst 2 (Table 2, entries 1–5). A phase-
transfer agent, TBAB, was intentionally added to enhance
the reactivity of the reaction. Satisfactory results were
obtained for using benzyl bromide or mono-substituted
benzyl bromides as reaction substrates (entries 1–3). The
substituent, either electron-withdrawing/donating, in the
para-position does not affect the outcome notably
Trace
3
12.3
27.2
4
a
Reactions were conducted in THF at 40 8C for 0.5 h employing 1 mol% of
2, NaOH(aq) (3 M, 1.00 mL), bromides (1.00 mmol), TBAB (0.20 mmol)
and phenylboronic acid (1.50 mmol).
(entries 2–3). Although having five strong s electron-
withdrawing groups at the ortho, meta, para-position for

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C.-P. Chang et al. / Tetrahedron 61 (2005) 3835–3839
The necessity of long reaction hours for this type of reaction
is also demonstrated in the following experiments (Table 4).
The following reactions were carried out for only 0.5 h
rather than the commonly employed 16 h. A rather low
yield, 6.8%, was obtained while bromobenzene is used as
the halide source (Table 4, entry 1). By contrast, almost
quantitative yield (99.0%) was obtained at higher tempera-
ture, 65 8C, and long reaction hours, 16 h (Table 1, entry 1).
The account is also valid for entries 2–4 no matter which
type of halides, with electron-withdrawing/donating sub-
stituents, are used. It is concluded here that longer reaction
in either uni- or bi-phasic Suzuki’s reactions. It has shown
that Suzuki-coupling reactions of benzyl bromides (sub-
strates of Csp
performed better than using aryl bromides (substrates of
Csp substituent). In most cases, the reactions carried out in
3
substituent) employing catalyst 2 was
2
biphasic media were much efficient than that in pure organic
phase. Better yields were commonly observed while a
phase-transfer agent TBAB was added in the reactions. The
factor of reaction time is more crucial than that of reaction
temperature. Not many differences were observed either
using Pd(OAc) or (COD)PdCl as the palladium source.
2
2
time is more crucial for aryl bromides (Csp
3
2
substituent)
than benzyl bromides (Csp substituent) to produce better
yield in Suzuki coupling reaction employing the kind of
catalyst like 2.
4. Experimental
4
.1. General
2.2. Suzuki reactions using 3, 4, 5-chelated palladium
complexes and 2
All operations were performed in a nitrogen flushed glove
box or in a vacuum system. Freshly distilled solvents were
used. H NMR spectra were recorded over Varian-400
spectrometer at 400.00 MHz. The chemical shifts are
reported in ppm relative to internal standard CDCl3.
For comparison, biphasic Suzuki-coupling reaction of
benzyl bromides employing a number of palladium catalysts
were carried out (Table 5). Rather low yields were observed
for using 3 as an acting diphosphine ligand in the cases or
either Pd(OAc) or (COD)PdCl as the palladium source
1
2
2
4.2. General procedures for the Suzuki cross-coupling
reactions
(
entries 1–2). Slightly improved yields were obtained while
a mono-dentate phosphine ligand 4 was used (entries 3–4).
Better yields were seen while 2-(di-tert-butylphosphino)bi-
phenyl 5, the powerful phosphine ligand which was used by
Suzuki cross-coupling reactions were performed according
to the following procedures.
2
0
Buchwald, were employed (entries 5 and 6). Not many
differences were observed either using Pd(OAc)2 or
4
.2.1. Method I (conducted in a THF–H O biphasic
2
(
COD)PdCl as the palladium source. In summary, the
2
medium). Complex 2 (0.012 mg, 0.01 mmol) and boronic
acid (0.183 g, 1.50 mmol) were charged into a 20 mL
Schlenk flask. The flask was evacuated and backfilled with
nitrogen before adding THF (5 mL), 3 M NaOH solution
(1 mL), aryl halide (1.00 mmol) and (in the presence/
absence of) tetrabutylammonium bromide (TBAB, 0.65 g,
catalytic efficiencies of these types of ligands are in the
sequence of 5O4O3. The best yield was found while 2 was
used as the catalyst (entry 7).
Table 5. Biphasic Suzuki-coupling reaction of benzyl bromides employing
various palladium catalysts
a
0
1
2
.20 mmol). The solution was stirred at 65 8C for 30 min–
6 h. The mixture was washed with aqueous NaOH (1 M,
0 mL), and then the aqueous layer was extracted with ether
(
30 mL). The combined organic layers were washed with
b
Entry
Pd
Ligand
Isolated yield
%)
brine (20 mL), and dried over with anhydrous magnesium
sulfate, filtered, and concentrated in vacuo. The crude
material was purified by flash chromatography on silica gel.
(
1
2
3
4
5
6
7
Pd(OAc)
2
3
3
4
4
5
5
—
6.4
4.6
(COD)PdCl
Pd(OAc)
(COD)PdCl
Pd(OAc)
(COD)PdCl
2
2
2
2
2
12.9
12.6
43.5
38.7
72.0
4
.2.2. Method II (conducted in a toluene medium).
Complex 2 (0.012 mg, 0.01 mmol), the boronic acid
0.183 g, 1.50 mmol), and K PO (0.425 g, 2.00 mmol),
2
(
3
4
were charged into a Schlenk flask. The flask was evacuated
and backfilled with nitrogen before adding toluene (1 mL)
and the aryl halide (1.00 mmol). The solution was stirred
at 65 8C, 16 h. The same procedures as Method I were
followed from here accordingly.
a
Reactions were conducted in THF with 1 mol% of Pd catalysts, NaOH(aq)
3 M, 1 mL), bromides (1.00 mmol), TBAB (0.20 mmol) and phenyl-
boronic acid (1.50 mmol).
(
b
4
.2.3. Method III (conducted in a THF–H O biphasic
2
medium). Ligand 1 (1.00 mmol), boronic acid (0.183 g,
.50 mmol) and 1 mol% of Pd complex (Pd(OAc)2 or
PdCl ), were charged into a Schlenk flask. The flask was
1
2
evacuated and backfilled with nitrogen before adding THF
(5 mL), 3 M NaOH solution (1 mL), tetrabutylammonium
bromide (TBAB, 0.065 g, 0.20 mmol) and aryl halide
(1.00 mmol). The flask was sealed with Teflon screw cap,
and the solution was stirred at 40 8C for 0.5 h. The same
3
. Concluding remarks
We have demonstrated the catalytic capacity of a novel
cobalt-containing phosphine chelated palladium complex 2

C.-P. Chang et al. / Tetrahedron 61 (2005) 3835–3839
3839
procedures as Method I were followed from here
accordingly.
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