Full Paper
Yang and Xu et al. reported the coupling reaction with mag-
netic copper–ferrite nanoparticles as catalysts.[12] Bolm and co-
workers reported the FeCl3-catalyzed CÀO coupling of phenols
with aryl iodides;[13] and later Buchwald and Bolm et al. studied
the iron-catalyzed CÀO coupling reaction in the presence of
trace amounts of copper.[14] Similar results have been described
by Likhar et al. using Fe3O4 mesoporouspolyaniline as a cata-
lyst.[15] Concurrently, several other groups (Singh, Olofsson, Zol-
figol, Obora, and so forth) have published their results in this
area.[16] Despite the aforementioned excellent examples, these
reactions usually require high temperatures, harsh conditions,
or expensive starting materials (Scheme 2). Herein, we report
Scheme 3. The strategy of CÀF bond activation by formation of the BÀF
bond.
Result and Discussion
On the basis of our analysis, the reaction of para-cyanfluoro-
benzene was used to test this idea by using B2pin2 as an addi-
tive. Interestingly, the self-coupling product was obtained, al-
though at a low yield. Next, the screening of reaction condi-
tions was performed to obtain a high yield. The experiments
showed that the reaction could be conducted smoothly. The
results are summarized in Table 1. In most cases, moderate to
excellent yields were obtained when [{Ir(cod)Cl}2]/dppe (cod=
cyclooctadiene; dppe=1,2-bis(diphenylphosphino)ethane) was
used as a catalyst in dimethylformamide (DMF) to give 95% of
the product (Table 1, entry 9). A control reaction showed that
the reaction could not occur without the iridium catalyst
(Table 1, entry 20).
Thus, having established the optimal conditions of [{Ir-
(cod)Cl}2] (2 mol%), dppe (4.1 mol%), and Cs2CO3 as the base,
and B2pin2, we explored the scope of the Ir-catalyzed etherifi-
cation reaction. The results were summarized in Table 2. The
reactions were carried out in DMF at 1408C in air. Generally,
Scheme 2. Several methods of synthesis of diaryl ethers.
that iridium-catalyzed CÀF activation has been realiz-
Table 1. Screening of reaction conditions.[a]
ed through the use of bis(pinacolato)diboron (B2pin2)
as a reagent with the formation of the BÀF bond and
homocoupling of aryl fluorides with the incorpora-
tion of an oxygen atom.
Entry
Catalyst[b]
Solvent
Yield [%][c]
We have recently shown that the stoichiometric
CÀF bond activation of a chloro-bridged iridium(III)
dimer, 2-(4,6-difluorophenyl)pyridyl (dfppy), leads to
the formation of a heteroleptic cyclometalated iridiu-
m(III) complex.[17] The point of concern, however, is
whether the iridium complex could be applied effec-
tively as a chemoselective catalyst for CÀF bond acti-
vation. Based on careful consideration, we believe CÀ
F bond activation can be achieved with the help of
reagents that involve Si, P, or B elements.[18] This is
because the formation of the FÀSi bond, FÀP bond,
or BÀF bond is much easier than that of MF (M=
metal).[19] Therefore, we have proposed a possible CÀ
F bond activation through the release of the BÀF
bond (Scheme 3). The oxidative addition of fluoro-
benzene to the iridium catalyst formed the complex
B species,[20] subsequently exchanging with B2pin2 to
form C by releasing the BÀF bond[21] to complete the
CÀF activation.
1
2
3
4
5
6
7
8
[(PPh3)3Ir(CO)H]/PPh3
[(PPh3)3Ir(CO)H]/P(C6H5OCH3)3
[(PPh3)3Ir(CO)H]/P(C4H9)3
[(PPh3)3Ir(CO)H]/dppe
[(PPh3)3Ir(CO)H]/X-Phos
[{Ir(cod)Cl}2]/PPh3
[{Ir(cod)Cl}2]/P(C6H5OCH3)3
[{Ir(cod)Cl}2]/P(C4H9)3
[{Ir(cod)Cl}2]/dppe
[{Ir(cod)Cl}2]/X-Phos
[{Ir(coe)Cl}2]/PPh3
[{Ir(coe)Cl}2]/P(C6H5OCH3)3
[{Ir(coe)Cl}2]/P(C4H9)3
[{Ir(coe)Cl}2]/dppe
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
43
32
25
67
<5
48
34
<5
95
<5
25
20
<5
58
<5
47
<5
<5
62
9
10
11
12
13
14
15
16
17
18
19
20[e]
[{Ir(coe)Cl}2]/X-Phos
[{Ir(cod)Cl}2]/dppe
[{Ir(cod)Cl}2]/dppe
[{Ir(cod)Cl}2]/dppe
[{Ir(cod)Cl}2]/dppe
DMSO
dioxane
toluene
DMA[d]
DMF
none
<5
[a] Conditions: 1a (0.5 mmol), [Ir] (0.02 mmol, 4 mol%), ligand (0.0205 mmol,
4.1 mol%), B2pin2 (0.55 mmol), Cs2CO3 (0.55 mmol, 1.1 equiv), solvent (3 mL), 24 h,
1408C, in air. [b] dppe=1, 2-bis(diphenylphosphino)ethane. X-Phos=2-dicyclohexyl-
phosphino-2’,4’,6’-triisopropylbiphenyl. coe=cyclooctene. [c] Isolated yields, separated
by column chromatography. [d] DMA=N,N-dimethylacetamide. [e] Catalyst was not
added.
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Chem. Asian J. 2014, 00, 0 – 0
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