Investigation of the catalytic activity of a Pd/biphenyl-based phosphine system in the Ullmann homocoupling of aryl bromides

Article abstract of DOI:10.1016/j.jorganchem.2011.04.032

We described Ullmann homocoupling promoted by a Pd/biphenyl-based phosphine system using DMF as solvent. Using Hammett equation it is found that the rate determining step of the reaction depends on the electronic nature of substituents of aryl bromides. Increase the rate of reaction with decreasing the electron donating of the substituent from NH₂ to H suggesting an oxidative addition step as the rate determining step. Decrease the rate of reaction with increasing the electron-withdrawing ability of the substituent from H to NO₂ indicating a reductive elimination step as the rate determining step.

Full text of DOI:10.1016/j.jorganchem.2011.04.032

Journal of Organometallic Chemistry 696 (2011) 2966e2970
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Investigation of the catalytic activity of a Pd/biphenyl-based phosphine system in
the Ullmann homocoupling of aryl bromides
Shirin Nadri , Ehsan Azadi , Ali Ataei , Mohammad Joshaghani ,*, Ezzat Rafiee a
a
a
a
a
,
b
,b
a
Chemistry Faculty, Razi University, Kermanshah 67149, Iran
Kermanshah Oil Refining Company, Kermanshah, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 February 2011
Received in revised form
We described Ullmann homocoupling promoted by a Pd/biphenyl-based phosphine system using DMF as
solvent. Using Hammett equation it is found that the rate determining step of the reaction depends on
the electronic nature of substituents of aryl bromides. Increase the rate of reaction with decreasing the
27 April 2011
2
electron donating of the substituent from NH to H suggesting an oxidative addition step as the rate
Accepted 28 April 2011
determining step. Decrease the rate of reaction with increasing the electron-withdrawing ability of the
substituent from H to NO
2
indicating a reductive elimination step as the rate determining step.
Keywords:
Ó 2011 Published by Elsevier B.V.
CeC coupling reaction
Ullmann reaction
Palladium
Biphenyl-based phosphine
Hammett correlation
1. Introduction
In addition to above mechanism, another reaction mechanism
(
abbreviated as 0/2) has already been proposed [12] in which, an
oxidative addition of aryl halide on Pd(0) takes place, followed by
disproportionation to afford intermediate ArePdeAr and PdX
. The
resulting bisarylpalladium(II) will be subjected to reductive elimi-
nation to yield the coupling product together with Pd(0) precursor.
The Ullmann reaction has evolved considerably from its original
guise as the homocoupling of aryl halides in the presence of an
excess of copper powder at elevated temperatures [1]. Improve-
ments to this method have emerged in recent years with
palladium-catalyzed systems under milder conditions which allow
easy access to biaryls from relatively simple substrates [2e5].
There are several reaction mechanisms for homocoupling. One
of the well-recognized mechanisms for Ullmann reaction [6,7]
2
PdX
amines.
Despite of these outstanding characteristics, the reports on the
2
can be reduced using a terminal reducing agent such as
mechanistic details about this homocoupling reaction are relatively
rare [13,14]. For example it is not clear which step is the rate
determining step of the overall reaction. Since such details are key
information in catalyst designing, and emanating from our interest
in palladium-catalyzed CeC coupling reactions [15e20] we decided
to investigate the Ullmann homocoupling reactions of aryl
bromides since this reaction is a oneecomponent reaction and does
not need any pseudo-conditioning pre-treatment. Among a series
of mechanistic tools, Hammett’ linear free energy relationship
(LFER) is more useful for one-component reactions and its
(
Scheme 1, abbreviated as 0/2/4) involves consecutive oxida-
tive addition steps of aryl halide on Pd(0) and Pd(II) species,
respectively. The resulting Pd(IV) species is very unstable and
readily undergoes reductive elimination to form Pd(II) species.
A terminal reducing agent is required to regenerate Pd(0) from
Pd(II) in the terminal reduction step and to complete the catalytic
cycle. Several types of reductants have been reported such as zinc
powder [8], tertiary amines [6], hydrazine [9], molecular hydrogen
[
10], hydroquinone [11]. The main characteristic of this mechanism
is the change of palladium oxidation state between zero, two and
four.
R
constants s and r are measure of the effects of chemical structure
on reactivity of compounds.
With this in mind, we used palladium acetate as a catalyst and
a synthesized biphenyl-based phosphine [15,16] in the Ullmann
homocoupling reaction (Scheme 2). Mild reaction conditions, low
catalyst loadings and short reaction times are only some reported
advantages of this class of phosphines in the Pd-catalyzed cross-
coupling reactions [21e28]. In fact, a suitable balance of structural,
*
Corresponding author. Chemistry Faculty, Razi University, Kermanshah 67149,
Iran. Tel./fax: þ98 831 4274559.
E-mail address: mjoshaghani@razi.ac.ir (M. Joshaghani).
0
022-328X/$ e see front matter Ó 2011 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2011.04.032

S. Nadri et al. / Journal of Organometallic Chemistry 696 (2011) 2966e2970
2967
Pd(II)
steric and electronic properties, made them as one of the most
effective classes of phosphine ligands identified so far for such
reactions.
Initial reduction
Terminal reduction
Pd(0)
First oxidative addition
Ar
2. Material and methods
4
1
X
2.1. General information
Pd(II)X
2
Ar Pd(II)X
All reactions were performed under an atmosphere of dry
nitrogen. All chemicals were purchased commercially from Fluka
and/or Merck companies that were used without further purifica-
tion. The biphenyl-based phosphine was prepared according to our
previous work [15,16].
3
2
Reductive elimination
Second oxidative addition
Ar
Pd(IV)X2
Ar
1H (200 MHz), 13C (100 MHz) and 31P (81 MHz) NMR spectra
were recorded on a Bruker Avance Spectrometer. Elemental anal-
ysis was performed using CHN Herause rapid model. Gas chroma-
tography was performed on a Varian CP-3800 (column: CP-Sil 8 CB
fused silica capillary column). Thin layer chromatography on pre-
coated silica gel Fluorescent 254 nm (0.2 mm) on aluminum plates
were used for monitoring the reactions. The cross coupling prod-
Ar Ar
Ar
X
Scheme 1. General catalytic cycle for the Pd(0)-catalyzed homocoupling of aryl
halides.
Pd(OAc)2
1
2
Br
ucts were characterized by their H NMR spectra or GC analysis.
R
Biphenyl-based phosphine
R
R
2.2. General procedure for palladium-catalyzed homocoupling of
PPh2
aryl bromides
Phosphine:
A mixture of palladium acetate (0.2 mol%), biphenyl-based
3
phosphine (0.4 mol%), aryl bromide (5 mmol) and NEt (7.5 mmol)
ꢀ
Me
in DMF (10 ml) was stirred under nitrogen atmosphere at 100 C for
appropriate time. The reaction mixture was then cooled to room
temperature. After extraction with water and ether, the combined
Scheme 2. Homocoupling of aryl bromides catalyzed by palladium.
organic layer was dried over MgSO
the crude product was characterized by H NMR spectroscopy.
4
. The solvent was evaporated and
1
Table 1
Initial optimizations for homocoupling reaction of bromobenzene.
a
[
Cat.]
2
Br
Base, Solvent,100 oC
Entry
Pd(OAc)
2
(mol%)
Phosphine:Pd
Base
Solvent
Yieldb (%)
TONb
1
2
3
4
5
6
7
8
9
0.001
0.01
0.1
0.2
0.3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
1:1
1:1
1:1
1:1
1:1
0:1
2:1
3:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
3
3
3
3
3
3
3
3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
0
0
21
58
63
0 (22)
100
0
0
210
290
210
0 (110)
500
41
205
No base
NaOH
NaOAc
0 (<5)
44 (65)
65 (73)
67 (82)
91 (100)
0 (<5)
0 (<5)
0 (13)
39 (85)
12 (41)
33 (76)
(12)
0 (<25)
220 (325)
325 (365)
335 (410)
455 (500)
0 (<25)
0 (<25)
0 (65)
195 (425)
60 (205)
165 (380)
(60)
10
11
12
13
14
15
16
17
18
19
20
21
K
K
2
CO
CO
3
c
2
3
DMF
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
3
3
3
3
3
3
3
3
Toluene
1,4-Dioxane
THF
d
d
CH
CH
3
NO
CN
2
3
DMSO
DMF
DMF
e
f
(29)
(145)
a
b
c
d
e
f
ꢀ
Reaction conditions: bromobenzene (5 mmol), base (7.5 mmol), solvent (10 ml), 100 C, 5 h, under N
GC yield. The numbers in parentheses are yields and TON after 24 h.
2
.
In the presence of isopropanol.
ꢀ
60
C.
Room temperature.
Bath temperature (75 C).
ꢀ

2968
S. Nadri et al. / Journal of Organometallic Chemistry 696 (2011) 2966e2970
3
. Results and discussion
significant influence of the phosphine is due to its steric-electronic
effects on palladium center.
3
.1. Initial optimization
A 2:1 phosphine:Pd mole ratio appears to provide the active
catalyst (Table 1, entry 7). This mole ratio is in consistent with our
results regarding Suzuki [16] and Heck [18] cross-couplings using
the same phosphine. Using a higher mole ratio (Table 1, entry 8)
gave low yield which may be due to decrease of rate of oxidative
addition step by formation of a bis-phosphine Pd complex and
blocking of free coordination sites on the Pd species.
We paid attention to the effects of base in homocoupling reac-
tions. Indeed, the base is necessary for terminal reduction and
without base, the catalytic cycle is not completed. A among a series
Initial optimizations for homocoupling reaction of bromo-
benzene were carried out to find the optimum catalyst loading,
phosphine:Pd mole ratio, base and solvent (Table 1).
As the results show, the yield of reaction increases significantly
with increasing the catalyst loading till 0.2 mol% Pd(OAc)
increases smoothly with higher catalyst loading (Table 1, entries
e5). The 0.2 mol% catalyst was selected according economically
2
and then
1
point of view.
The influence of the phosphine:Pd mole ratio on the outcome of
the reaction (yield and turnover number [TON]) was investigated
using different mole ratios 0:1, 1:1, 2:1 and 3:1 (Table 1, entries 4,
3
of bases, NEt was selected as a base of choice (Table 1, entry 7). Its
more efficiency is probably due to its solubility in most organic
solvents as well as its high reducing ability. Low conversion was
6
e8). Phosphine-free reaction led to the biphenyl product with low
2 3
achieved using K CO (Table 1, entry 12). This feature may be
yield even after 24 h (Table 1, entry 6). An effortless interpretation is
exist which suggests the phosphine facilitates the homocoupling
reactions by promoting the initial reduction of the Pd(OAc) to
2
Pd(0). However, other factors should also be considered. For
examples, it has been reported that biphenyl-based phosphines
contribute in the stabilization of the intermediates by avoiding the
interpreted by the fact that in the presence of an inorganic base,
DMF decomposes into the carbon monoxide and dimethylamine;
the latter could reduce aryl halides in the presence of Pd (0) cata-
lysts [34].
Panvela et al. [35] reported that a reductive reagent such as
isopropanol could promote the homocoupling of aryl halides using
catalyst degradation through the formation of
p-interactions with
inorganic bases such as K
2 3
CO . In our system addition of iso-
the aryl ring [29e33]. As a result, it may be concluded that
propanol to the reaction mixture using K
2
CO
3
as base could really
Table 2
a
The Ullmann homocoupling of various aryl bromides.
Pd(OAc) / Phosphine
2
2
R
Br
o
Et N, DMF, 100 C
3
R
R
Entry
1
Aryl bromides
Yieldb
100
s
p
k
rel
c
TONb
500
Br
Br
Br
Br
Br
Br
0
1
2
3
4
5
6
59 (74)
27 (53)
18 (39)
5 (28)
ꢁ0.17
ꢁ0.27
0.5
0.59
0.27
0.18
0.05
0.1
295 (370)
135 (265)
90 (195)
25 (140)
50 (120)
CH3
OCH3
COCH3
NO2
0.78
ꢁ0.66
10 (24)
NH2
Br
7d
<5 (73)
e
e
e
<25 (365)
8
<5
e
<25
Br
a
ꢀ
2 3 2
(0.2 mol%), phosphine (0.4 mol%), NEt (7.5 mmol), DMF (10 ml), 100 C, 5 h, N .
Reaction condition: aryl bromide (5 mmol), Pd(OAc)
The results in parentheses is conversion after 24 h.
b
c
krel is equal with (k
R
/k
H
), that k
H
is related to homocoupling reaction of bromobenzene and para-substituent is H.
(5 mol%), phosphine (10 mol%).
d
The number in parentheses is from the Pd(OAc)
2

S. Nadri et al. / Journal of Organometallic Chemistry 696 (2011) 2966e2970
2969
bromides containing electron-withdrawing or donating substitu-
ents in the Ullmann homocoupling reaction (Table 2).
Regardless 4-bromotoluene which has acceptable conversion
and coupling yield (Table 2, entry 2), both electron-rich and
electron-deficient studied aryl bromides had low to negligible
conversions and yields. 1-bromo-4-nitrobenzene gave significant
conversion but very low yield (Table 2, entry 5) after 24 h which
may be due its activity toward dehalogenation [36]. Similarly,
4-brommoaniline had excellent conversion but negligible yield
(Table 2, entry 6) because of substrates like amines strongly inhibit
or prevent formation of biaryls by providing alternative reaction
pathways [37e39]. Thus amines are arylated at N-atoms affording
triarylamines [37].
Table 2 shows that 1-bromonaphthalene and 4-bromobiphenyl
were inactive substrates for this reaction (entries 7, 8). It seems
that steric factor is mostly responsible for the decreased yield in
these cases. In the case of 1-bromonaphthalene increasing catalyst
loading to 5 mol% gave 73% yield indicating the efficiency of the
high catalyst loading (Table 2, entry 7).
Fig. 1. LFER for the Ullmann homocoupling reactions of various aryl bromides.
increase the yield of the desired biphenyl (Table 1, entry 13), but it
was still less efficient than NEt .
3
Examination of entry 9 of Table 1 shows that no reductive
homocoupling product (biphenyl) was formed in the absence of
base, indicating that the base is essential for achieving the
palladium-catalyzed reductive homocoupling of aromatic halides
in DMF solution.
3.3. Mechanistic study by Hammett correlation
In the next stage, the effect of solvent was investigated. As shown
in Table 1, the reaction rate is greatly affected by the kind of solvent
used. Among the commonly used organic solvents, the best results
were obtained with DMF (Table 1, entry 7) and in some extent with
CH NO and DMSO (Table 1, entries 17, 19). DMF has an important
3 2
advantage which made them as a unique solvent. Its water solubility
makes the product extraction and purification very easy.
As briefly discussed in the introduction, there are several
mechanisms for the Ullmann coupling, among them, the 0/2/4
and 0/2 mechanisms are most common. In order to investigate
which of them is responsible in our reaction condition; competitive
reaction using equimolar amounts of 4-iodobenzene and
4
-bromoanisole was performed. If a 0/2/4 mechanism accounts
the Ullmann reactions, the coupling product should be mainly
biphenyl formed from the homocoupling of iodobenzene. While, in
a 0/2 mechanism, both homo and cross coupling products should
be observed [6]. In our preliminary tests, no cross coupling product
The effect of temperature was finally examined and it is found
that the reaction temperature has a great effect; the reaction being
ꢀ
very slow below 100 C (Table 1, entries 20e21). Thus, temperature
ꢀ
of 100 C was the optimal one in this study.
was observed indicating
responsible.
a 0/2/4 mechanism should be
3
.2. The Ullmann homocoupling reactions
After determination of the reaction mechanism, we should
concentrated on a question arouse, which step in the overall
reaction is likely to be the slow and hence rate-limiting one?
Among two consecutive oxidative addition steps, the first one is
Using the optimized reaction conditions, we explored the
general applicability of Pdephosphine system with different aryl
Fig. 2. LFER for the Ullmann homocoupling reactions in other catalytic systems, (a) Cat: Pd(dppf)Cl
PdCl /EDTA/ascorbic acid in watereethanol, see Ref. [43].
2 2
, in DMSO see Ref. [41] (b) Cat: Pd(dppf)Cl , in 3-Pentanol; see Ref. [42] (c) Cat:
2

2970
S. Nadri et al. / Journal of Organometallic Chemistry 696 (2011) 2966e2970
unlikely to be since oxidative addition for more electron-rich Pd(0)
is faster than Pd(II). In addition, more steric crowding about Pd(II)
decrease more the rate of second oxidative addition respect to
Pd(0). The terminal reduction step is unlikely to be rds as well since
it is independent to aryl halide. This leaves steps (2) and (3) as
possible candidates for the rate determining step. Using Hammett
correlation we are capable to undertake mechanistic studies. If only
one of these steps (2 or 3) is the rate determining step for all aryl
bromides, independent of electron-withdrawing or electron-
donating properties of substituents, a straight line should be
expected in the Hammett plot.
increasing the electron-withdrawing ability of the substituent
from H to NO indicating a reductive elimination step as the rate
determining step. In the case of bromobenzene, the rate of
oxidative addition and reductive elimination steps are compa-
rable and it is not clear to decide what step is the rate-limiting
step.
2
Acknowledgments
The authors thank the Razi University Research Council and
Kermanshah Oil Refining Company for support of this work.
As shown in Table 2, both electron-donating and electron-
withdrawing substituents reduce the reaction yield. This behavior
may be interpreted by considering different rate determining step
in the catalytic cycle. In this context, to evaluate the above
substituent effects we have attempted to correlate the data repor-
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r
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4
the electron-withdrawing ability of the substituent has opposite
effect; decreases the rate of coupling reaction. These results are
more consisting by choosing the reductive elimination step (step 3)
as probable rate-limiting step. On the other word, increasing the
electron-withdrawing ability of the substituent increases the rate
of oxidative addition to the extent where the oxidative addition
become fast than the reductive elimination and hence the reductive
elimination become the rate determining step.
(
[
[
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oxidative addition (step 2). In such point, it is not clear to decide
what step is the rate-limiting step. In our case study, this point was
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1
[
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In summary, the results presented in this paper show that
Pd/biphenyl-based phosphine system is capable of catalyzing
homocoupling of aryl bromides. More prominently, Hammett
study on our results as well as literature results shows that
electronic properties of aryl bromide affect on the rate of the
Ullmann reaction. Depending on the nature of substituent, the
rate determining step may be either oxidative addition step or
reductive elimination step. Electron-withdrawing groups
prompt the oxidative addition. Therefore, increase the rate of
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6
[
uent from NH
2
to H suggesting an oxidative addition step as the
2
rate determining step. Decrease the rate of reaction with