Cobalt-catalyzed formation of symmetrical biaryls and its mechanism

Article abstract of DOI:10.1002/chem.200900542

A method for cobalt-catalyzed formation of symmetrical biaryls and mechanism used for the formation, were reported. The study involved a new cobalt-catalyzed homo-coupling reaction in which cobalt was used with manganese dust as reducing agent. It was observed that the presence of allylchloride significantly reduced the amount of reduction compounds without modifying kinetics of the reactions and the reaction remained un-affected by the presence of air as oxidant. It was also found that no-reactions occurred in DMF, THF, and toluene in the presence of pyridine as co-solvent at room temperature, which indicated that MeCN acted as a ligand of the cobalt active species. The coupling process can be extended to less reactive aryl chlorides by changing some experimental conditions. DFT calculations revealed that the involvement of a Co^I/Co^III couple was realistic in the case of 1,3-diazadienes as ligands.

Full text of DOI:10.1002/chem.200900542

DOI: 10.1002/chem.200900542
Cobalt-Catalyzed Formation of Symmetrical Biaryls and Its Mechanism
[
a]
Aurꢀlien Moncomble, Pascal Le Floch,* and Corinne Gosmini*
[
12]
The biaryl motif plays a considerable role in organic
chemistry since it is commonly encountered in various areas
ranging from supramolecular chemistry to natural product
cobalt.
Recently, the “transition-metal-free” homocou-
pling reaction of organomagnesium compounds has also
been developed in the presence of a stoichiometric amount
of an organic oxidant. However, the direct reductive ho-
mocoupling of aryl halides turns out to be a more conven-
ient and straightforward approach than the self-coupling of
organometallic derivatives in the presence of an oxidant
since all synthetic difficulties, related to the preparation of
the organometallic reagent when functional groups are pres-
ent, are thus avoided.
[1]
[13]
synthesis. Therefore, a lot of attention has been paid over
the last century to the development of viable synthetic
methods allowing CÀC coupling between aryl groups. Two
classes of reactions allow the elaboration of the biaryl motif:
homo- and cross-coupling. Historically, the copper-catalyzed
coupling, known as the Ullmann reaction, was the first effi-
[2]
cient process enabling the synthesis of symmetrical biaryls.
However, development of this simple method was dramati-
cally hampered by the use of harsh conditions and stoichio-
metric amounts of copper. These drawbacks spawned the
development of various milder methods relying on the use
of nickel or palladium catalysts. Thus, symmetrical biaryls
can be obtained by the palladium-catalyzed homocoupling
of electrophilic aryl derivatives, ArX (X=I, Br, Cl, OTf) in
Recently, we have developed a very simple catalytic
system, relying on the use of CoBr /PPh as catalyst (in
2
3
DMF with pyridine as co-solvent), which allows the forma-
tion of unsymmetrical biaryls under mild conditions in good
[14]
yields with good selectivities. Motivated by our early suc-
[15]
cess, we recently launched a program aimed at expanding
the use of this new catalytic system to the direct homocou-
pling of functionalized aryl halides (X=I, Br and Cl).
Herein, we wish to report on the scope of a new cobalt-
catalyzed homocoupling reaction in which cobalt is used in
combination with manganese dust as reducing agent
(Scheme 1). In addition, results of DFT calculations that
shed some light on the mechanism of this coupling are also
presented.
[3]
the presence of a reducing agent. This Pd-reductive cou-
pling has been widely developed and its mechanism eluci-
[4]
dated. Among various available reducing agents, zinc has
probably been the most widely employed (electron source),
in combination with various transition metals (electron-
transfer catalyst), for the reductive homocoupling of aryl
[5]
halides. Note that some homogeneous reductants were
[1g]
also employed.
Preliminary studies, conducted with an activated aryl bro-
mide (ethyl 4-bromobenzoate), were highly encouraging and
allowed us to determine that the combination of 0.1 equiv of
Conversely, symmetrical biaryls can also be synthesized
by oxidative homocoupling of nucleophilic organometallic
aryl derivatives ArM in the presence of an oxidant. This re-
action which, in most cases, is catalyzed by palladium com-
CoBr with 3 equiv of manganese dust, activated by traces
2
of acid, was necessary to achieve the coupling within 3 h in
MeCN at room temperature (72% GC yield). GC monitor-
ing clearly showed the formation of ArH at the beginning of
[
3g–6]
plexes
was also developed with other metals such as
[
7]
[8]
[9]
[10]
[11]
nickel,
copper,
iron,
manganese,
gold
and
[16]
the reaction. As previously reported,
we found that the
presence of allylchloride proved to be beneficial since it
considerably reduces the amount of reduction compounds
without modifying kinetics of the reactions. The role played
by this additive has not been clarified thus far but the fact
that reaction rates are not modified indicates that it does
not interact with the catalytic active species. In previous
studies we showed that, when Zn powder was used as reduc-
ing agent, organozinc compounds were formed. Therefore,
as the reaction proceeds in the presence of Mn powder, we
[
a] A. Moncomble, Dr. P. Le Floch, Dr. C. Gosmini
Laboratoire “Hꢀtꢀroꢀlements et Coordination”
Ecole Polytechnique, CNRS
9
1128 Palaiseau Cedex (France)
Fax : (+33)1-6933-4440
E-mail: corinne.gosmini@polytechnique.edu
pascal.lefloch@polytechnique.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900542.
4770
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 4770 – 4774

COMMUNICATION
logically wondered about the possible formation of ArÀMn
This new coupling process was also successfully extended
to less reactive aryl chlorides by changing several experi-
mental conditions. Thus, reactions were conducted at 508C
since no reaction occurred at 258C and addition of pyridine
was required. Presumably, the use of pyridine as co-solvent
results in stabilization of the aryl cobalt species as previous-
derivatives. Two experiments clearly demonstrated that,
under these conditions, ArÀMn compounds are not formed.
First, we showed that the reaction is not affected by the
presence of air as oxidant since it is well known oxidants
[10]
provoke the homocoupling of RÀMn derivatives. Whatev-
er the experimental conditions used (inert atmosphere or in
air), reactions led to the same result in terms of kinetics and
conversion yields. Second evidence was given when the reac-
[18]
ly reported.
Under these conditions we found that aryl
chlorides bearing an electron-withdrawing group could be
homocoupled in good yields (Scheme 2). These results are
reported in Table 2.
[17]
tion was run in the presence of acetic anhydride since no
ketone was produced. Several other parameters such as the
nature of the solvent or the presence of a co-solvent were
shown to affect the reaction. Thus, no reactions occurred in
DMF, THF and toluene even in presence of pyridine as co-
solvent at room temperature. This led us to conclude that
MeCN probably acts as a ligand of the cobalt active species.
Scheme 2. Homocoupling of various arylchlorides catalyzed by CoBr
Table 2. Homocoupling of various aryl chlorides.
2
/Py.
[
a]
Entry
FG
t
Yield [%]
20
21
22
23
24
p-MeOCO
p-CN
p-MeCO
1.5 h
3.5 h
2 h
3 h
7 h
84
80
44
74
2
Scheme 1. Homocoupling of aryl halides catalyzed by CoBr .
p-CF
3
H
(32)
As can be seen in the following table, a large array of
ortho-, para- or meta-substituted bromoarenes underwent
the desired homocoupling reaction under these experimen-
tal conditions, in good to excellent isolated yields. As ex-
pected, reactions rates are highly dependent on the electron-
ic richness of the arene and electron-deficient arenes (see
entries 1 to 8, Table 1) proved to be much more reactive
than electron rich derivatives (see entries 14 to 19). Note
that with deactivated arenes the use of iodides derivatives
led to higher conversion yields.
[a] Yield of the isolated product (yield determined by GC).
Encouraged by these results, we extended our study to
the effects of ligands. Various nitrogen- and phosphorus-
based ligands such as 2,2’-bipyridine, 1,3-diazadienes, PPh3,
dppe and dppp were evaluated. The most significant results
were obtained with the 1,4-bis-(m-xylyl)-1,3-diazadiene
ligand (Scheme 3). As can be seen in Table 3, addition of
Table 3. Homocoupling of aryl bromides using the diazadiene based cat-
alyst.
[
a]
Entry
FG
t
Yield [%]
Table 1. Homocoupling of various aryl bromides and iodides.
2
2
5
6
p-EtOCO
o-EtOCO
p-CN
p-CF3
H
3 h
15 min
1 h
4.5 h
6 h
1 d
66
81
75
60
66
48
32 (55)
[
a]
Entry
FG
X
t
Yield [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
p-EtOCO
o-EtOCO
p-MeCO
p-CN
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
Br
I
Br
I
Br
Br
4 h
4 h
4 h
3 h
3 h
6 h
6 h
8 h
6 h
4 h
6 h
8 h
3 h
5 h
3 h
5 d
3 h
6 d
1 d
90
86
72
92
78
81
64 (84)
43 (74)
86
66 (94)
44
81
67
75
75
27
28
29
30
31
p-MeO
o-MeO
o-CN
2 d
p-CF
m-CF
3
[
a] Yield of the isolated product (yield determined by GC).
3
o-CF
p-F
p-Cl
p-Br
H
3
the diazadiene ligand to the reaction mixture, after the acti-
vation of Mn powder, proved to be beneficial in terms of re-
actions rates and, in all cases, shorter reaction times were
obtained whatever the nature of the substituent (compare
for example entries 16 and 30 and entries 2 and 26).
In order to shed some light on the mechanism of this ho-
mocoupling process, DFT calculations were carried using
the B3LYP functional and the PCM method (see Support-
ing Information for further details regarding these calcula-
tions). A first series of calculations, only considering MeCN
0
1
2
3
4
5
6
7
8
9
H
p-Me
p-Me
p-MeO
p-MeO
o-MeO
48
75
21 (45)
45 (75)
[19]
p-NMe
2
[
a] Yield of the isolated product (yield determined by GC).
Chem. Eur. J. 2009, 15, 4770 – 4774
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www.chemeurj.org
4771

C. Gosmini, P. Le Floch and A. Moncomble
model for describing Mn aggregates but these steps are sup-
posed to be fast.
The overall energetic pathway of this mechanism is de-
picted in Figure 1. Calculations were realized considering
I
III
that Co and Co complexes can adopt either a singlet or a
triplet state. As can be seen, kinetic of the reaction is relat-
ed to the triplet state.
Scheme 3. Homocoupling of various arylbromides catalyzed by the
CoBr /diazadiene system.
2
as a potential ligand of cobalt, proved to be difficult to ach-
ieve due to the number of potential active species involving
one, two or three molecules of MeCN in the coordination
sphere of the Co. Therefore, we deliberately focused our
study on the diazadiene based catalyst which proved to be
easier to model since two coordination sites are occupied.
The parent diazadiene ligand [HN=CHÀCH=NH] was first
chosen as model. Our mechanism, which is presented in
Scheme 4, involves as starting precursor the dibromide com-
plex 1a which is presumably formed when diazadiene is
added to the reaction mixture.
Figure 1. Energetic pathway (singlet and triplet state) computed for the
proposed mechanism with R=H (B3LYP functional and PCM method
considering MeCN as solvent).
Complex 3a is the only species where the singlet state
was found to be slightly more stable than the triplet configu-
ration but this has no influence on the kinetic. The B3LYP
functional being known to overestimate the stability of high-
[20]
spin compounds compared to low-spin systems,
single-
point calculations were also conducted on the B3LYP-opti-
[19a,21]
mized structures using two others functionals (B3PW91
[22]
and PBE0 ). Results were not found to be significantly dif-
ferent and with B3PW91, the relative positions of singlet
and triplet states are the same than with B3LYP. On the
contrary with PBE0, the triplet state was found to be slightly
more stable than the singlet state. To check the validity of
this approach, complex 3a was reoptimized with the two
latter functionals. The singlet–triplet gap energies for com-
plex 3a are given in Table 4.
Scheme 4. Proposed mechanism (a: R=H; b: R=1,4-bis-(m-xylyl)).
I
This mechanism which relies on the formation of Co and
III
Co species involves four distinct steps. The first step is the
monoelectronic reduction by Mn powder to afford the
I
active Co species 2a. We then computed that this complex
is reactive enough to undergo an oxidative addition with the
The absolute difference between the different methods
being very weak led us to assume that the chosen approach
constitute a good compromise. Note that the two activation
barriers of the oxidative additions are comparable (about
III
first equivalent of ArÀBr to afford the Co species 3a.
Then, after a second reduction step by Mn powder, the re-
sulting arylcobalt(I) complex 4a undergoes an oxidative ad-
dition with the second equivalent of ArÀBr to form the dia-
À1
20 kcalmol ) and in good agreement with experimental ob-
rylcobalt complex 5a. Finally the last step is a reductive
elimination step that yields the biaryl derivative and the
active species 2a. Note that reduction processes by Mn
powder were not computed due to the lack of a suitable
servations since coupling takes place at room temperature.
This observation also suggests that the two reduction pro-
III
I
cesses of Co into Co by Mn powder are probably not the
rate-determining steps. As can be expected, the driving
4772
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Chem. Eur. J. 2009, 15, 4770 – 4774

Symmetrical Biaryls
COMMUNICATION
Table 4. Singlet and triplet gap energies for complex 3a with different
functionals.
Functional
PBE0
B3PW91
E
E
PCM(s)ÀEPCM(t) (B3LYP geom.)
PCM(s)ÀEPCM(t) (optim. geom.)
3.1
2.9
0.2
À0.3
À0.1
À0.2
absolute difference
force of the reaction is the strongly exothermic formation of
Figure 3. A view of the computed transition state between 2b and 3b
(
2bTS3b). Selected bond lengths [ꢂ] and angles [8]: CÀBr1 2.302, CoÀC
the CÀC bond, which only requires a very weak activation
2.115, Co
ÀBr1 2.531, CoÀBr2 2.472, CoÀN1 2.153, CoÀN2 2.249, CoÀCÀ
À1
energy (DE
=0.8 kcalmol ).
PCM
Br1 69.77, C-Br1-Co 51.64, Br1-Co-C 58.59, N1-Co-N2 76.20. Selected
torsion angle [8] between the mean plane of the incoming aromatic ring
and the CÀBr1 bond 31.6.
These results encouraged us to improve our model by
computing the real system (1,4-bis-(m-xylyl)-1,3-diazadiene)
instead of the parent diazadiene. All calculations were car-
ried out at the same level of theory for the metal, reactants
(
aryl bromides) and the diazadiene backbone but, in order
servations and the thermodynamic of the reaction is still
driven by the reductive elimination. No transition state
AHCTUNGTRENNUNG( 5bTS2b) could be located for the last reductive elimination
that yields the biphenyl derivative. All attempts to optimize
this transition state resulted in the formation of the starting
complex 5b or products of the reductive elimination what-
ever the method employed. However, this is conceivable
when we consider the high exothermicity of the process and
one may logically propose that the structure of this transi-
tion state is very close to that of 5b.
to save computer time, substituents of the diazadiene were
computed with a smaller basis set (3-21G) (see Supporting
Information for further details). The new energetic pathway
was only computed for the triplet spin state since we previ-
ously showed with the model compound that the kinetics of
the reaction is related to this spin stat. The whole computed
energetic pathway is depicted in Figure 2. The computed
transition state for the first oxidative addition (2bTS3b,
from 2b to 3b) is presented in Figure 3.
In conclusion, we showed that the tolerance of this new
coupling procedure towards a wide variety of functional
groups enables the synthesis of a broad spectrum of valuable
compounds in satisfactory to high yield under simple and
mild conditions. This versatile process compares favorably
with other procedures that use other transition-metal-based
catalysts. Additionally, preliminary DFT calculations have
I
III
shown that the involvement of a Co /Co couple is realistic
at least in the case of 1,3-diazadienes as ligands. Further
studies aimed at extending this coupling procedure to other
derivatives and to precisely assess ligands effects are cur-
rently under progress in our laboratories and will be report-
ed in due course.
Experimental Section
Cobalt bromide (0.5 mmol, 110 mg) and manganese powder (30 mmol,
820 mg) are added in acetonitrile (5 mL). Allyl chloride (1.5 mmol,
130 mL) and dodecane (as an intern reference, 100 mL) were then intro-
duced in the solution. The medium was activated by traces of trifluoro-
acetic acid (0.7 mmol, 50 mL). The reaction was stirred for five minutes.
Then, the aryl halide was added (5 mmol). For aryl iodide or bromide,
the solution was stirred at room temperature. For aryl chloride, pyridine
(1 mL) was added in the solution which was heated at 508C. The reaction
was monitored by GC until the disappearance of all the reagent. The
mixture was diluted in dichloromethane, filtered and hydrolyzed by hy-
drochloric acid (2m). The organic fraction was dried over magnesium sul-
fate, filtered and concentrated under reduced pressure. The resulting
product was purified on silica gel or by recrystallization and character-
Figure 2. Energetic pathway (triplet state) computed for the proposed
mechanism with R=1,4-bis-(m-xylyl) (B3LYP functional and PCM
method considering MeCN as solvent).
As can be seen, differences between this pathway and
that computed for the parent diazadiene are quite low. The
increase of the steric hindrance around the metal logically
leads to an increase of the activation energy for the oxida-
tive addition processes. However activations barriers (about
À1
1
13
2
5 kcalmol ) are still in agreement with experimental ob-
ized by H and C NMR spectroscopy.
Chem. Eur. J. 2009, 15, 4770 – 4774
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4773

C. Gosmini, P. Le Floch and A. Moncomble
Acknowledgements
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ect 081616).
Keywords: biaryls
·
CÀC coupling
·
heterogeneous
catalysis · cobalt · homocoupling
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Received: February 27, 2009
Published online: March 26, 2009
4774
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