results indicated not only the
compatibility of the CÀF bond
with the investigated conditions,
but offered also the potential
opportunity to further function-
[13]
alize the product.
However,
more electron deficient aryl
bromides such as 4-bromoben-
zotrifluoride resulted not suit-
AHCTUNGTRENNUNG
(
we found that this reaction is
very sensitive to the electronic
properties of the arene unit.
For example, only low or mod-
erate yields were achieved
when electron-rich arenes were
applied, except for the case of
3
3
ae with a Mo catalyst (3ab,
ad, and 3ae). On the other
Figure 2. Cross-coupling of 4-bromoanisole with benzene catalyzed by various transition metals. The yields hand, steric hindrance also
were determined by GC using dodecane as the internal standard. Catalysts: A=[V
C=[Mn(acac) ]*, D=[Mn(acac) ]*, E=FeCl , F=[FeCl ]*, G=[Fe(acac) ], H=[Fe
J=[Fe(OAc) ], K=[Fe(OAc) ]*, L=[Co(acac) ], M=[Co(acac) ], N=[Co(OAc) ], O=[Ni
Q=[Cu(OAc) ], R=[Cu(acac) ], S=CuI, T=[Mo(OAc) ]*, U=NbCl *, V=TaCl *,W=ReCl
3
A
H
U
G
R
N
U
G
3
]*, B=[Cr
]*, I=[Fe
(acac) ], P=NiI
*, X=AgOTs,
A
H
U
G
R
N
U
G
3
]*,
highly affected the efficiency.
Actually, such transformation
was completely inhibited in the
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
H
U
G
R
N
N
3
3
3
A
H
U
T
E
N
N
3
A
H
U
G
R
N
U
G
3
A
H
U
G
R
N
U
G
2
],
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
H
U
G
R
N
N
2
A
H
U
G
R
N
U
G
3
A
H
U
G
R
N
U
G
2
A
T
N
T
E
N
N
2
2
,
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
H
U
G
R
N
U
2
A
T
N
T
E
N
N
2
5
5
Y=AuCl, Z=none*. *In these reactions, 1,10-phenanthroline (30 mol%) was used rather than DMEDA as case of the sterically hindered
ligand.
mesitylene or 1,3,5-trimethoxy-
benzene—this result is not in
agreement with a recent re-
[9k]
Although Cu salts exhibited high reactivity in the coupling
port Notably, an enhancement of the acidity of the CÀH
bonds has dramatically increased the efficiency of the reac-
tion. For example, 1,4-difluorobenezene showed a much
better reactivity and the desired products were obtained in
[6a]
of electron-deficient arenes and heterocycles, the present
study implies that, using proper Cu catalysts, cross-coupling
between electron-neutral benzene and aryl bromides also
leads to moderate yields (Figure 2, Q–S). To our surprise,
other transition-metal salts located on the fifth row of the
[14]
moderate to good yields (3ac).
Most importantly, with the same substrate, different
metal-based catalytic systems showed various efficiencies.
For example, 1-bromo-4-fluorobenzene and anisole (3ka
periodic table, such as NbCl and [Mo CAHTUNGTERNN(UNG OAc) ], also showed
2
5
good catalytic abilities (Figure 2, T and U). It was notewor-
thy that also AgOTs and AuCl showed partial catalytic abili-
ty in this transformation (Figure 2, X and Y). The control
experiments indicated that the absence of corresponding
transition-metal salts did not lead to the desired coupling
products from aryl bromides at 808C (Figure 2, Z).
and 3ae), [Mo
ACHTUNGTNERUNNG( OAc) ] showed good reactivity. However,
2
[Co(acac) ] showed lower catalytic ability in both reactions
AHCTUNGTRENNUNG
3
while the catalytic ability of NbCl was completely inhibited
5
by anisole although it showed partial catalytic activity in the
coupling of 1-bromo-4-fluorobenzene with benzene.
To explore such an interesting transformation, currently
used Co complexes and rarely used Nb and Mo compounds
were selected as catalysts to examine their catalytic ability
towards various aromatic bromides and several arenes
In a recent report of Fe-catalyzed cross-coupling between
aryl iodides and arenes, 4-bromotoluene only showed a
[9k]
moderate reactivity. In both results, the SET process was
highly preferred based on primary reaction-mechanism stud-
ies. We agreed with this hypothesis and considered that the
here developed transformation went through a similar cata-
lytic pathway. To verify this hypothesis, several preliminary
experiments were further conducted. In the presence of
TEMPO as a radical scavenger, all the transformations pro-
moted by Nb/Mo/Co catalysts were terminated (reaction a
(
Scheme 1). To our satisfaction, electron-donating substitu-
ents such as methoxy and methyl groups on the phenyl rings
of ArBr promoted the cross-coupling with high efficiency
and the desired products were isolated in moderate to good
yields, regardless of the position of those groups (para, meta,
or ortho; 3aa, 3ba, 3ca, and 3da). Polysubstituted electron-
rich aryl bromides also worked very well (3ea and 3 fa).
Also 1-bromo-4-fluorobenzene was tested and the corre-
sponding coupling product was isolated in moderate to good
yields, thus leaving the CÀF bond untouched (3ka). These
in Scheme 2). SmI showed similar property to shut down
2
these transformations (reaction a in Scheme 2). Moreover,
with 1,1-diphenylethylene as an indicator of radical inter-
mediates, the addition products were also observed under
3594
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3593 – 3597