Perfluorocyclopentadienyl Radical Derivative as an Organocatalyst for Oxidative Coupling of Aryl- and Thienylmagnesium Compounds under Atmospheric Oxygen

Article abstract of DOI:10.1021/acs.joc.8b00299

The oxidative homocoupling reaction of Grignard reagents in the presence of atmospheric oxygen molecules proceeded in the presence of a heptafluorotolyl-substituted perfluorocyclopentadienyl radical. The turnover number (TON) was over 30 for the coupling reactions of PhMgBr to give biphenyl. The organocatalyst could couple thienylmagnesium compounds to give bithiophene derivatives in up to 94% yield. Furthermore, a gram-scale synthesis of 6,6'-dimethoxybiphenyl-2,2'-diyl-bis(phosphonic acid diethyl ester) was demonstrated. Stabilization of the phenyl radical for the inhibition of the side reaction was also considered using DFT calculations.

Full text of DOI:10.1021/acs.joc.8b00299

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Perfluorocyclopentadienyl Radical Derivative as an
Organocatalyst for Oxidative Coupling of Aryl- and
Thienylmagnesium Compounds under Atmospheric Oxygen
Toshinobu Korenaga, Kaoru Nitatori, Hiroki Muraoka, Satoshi Ogawa, and Kazuaki Shimada
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00299 • Publication Date (Web): 20 Mar 2018
Downloaded from http://pubs.acs.org on March 20, 2018
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Perfluorocyclopentadienyl Radical Derivative as an Organocatalyst for Oxidative Coupling of
Aryl- and Thienylmagnesium Compounds under Atmospheric Oxygen
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Toshinobu Korenaga,* Kaoru Nitatori, Hiroki Muraoka, Satoshi Ogawa, Kazuaki Shimada
Department of Chemistry and Biological Sciences, Faculty of Science and Engineering, Iwate
University, 4ꢀ3ꢀ5 Ueda, Morioka, Iwate 020ꢀ8551, Japan
Eꢀmail:korenaga@iwateꢀu.ac.jp
Abstract: The oxidative homoꢀcoupling reaction of Grignard reagents in the presence of
atmospheric oxygen molecules proceeded in the presence of heptafluorotolylꢀsubstituted
perfluorocyclopentadienyl radical. The turnover number (TON) was over 30 for the coupling
reactions of PhMgBr to give biphenyl. The organocatalyst could couple thienylmagnesium
compounds to give bithiophene derivatives in up to 94% yield. Furthermore, gramꢀscale synthesis of
6,6'ꢀdimethoxybiphenylꢀ2,2'ꢀdiylꢀbis(phosphonic acid diethyl ester) was demonstrated. Stabilization
of the phenyl radical for the inhibition of the sideꢀreaction was also considered using DFT
calculations.
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The CꢀC coupling reactions were largely developed using transitionꢀmetal catalysts such as the
Pdꢀcatalyzed SuzukiꢀMiyaura coupling. On the other hand, transition metalꢀfree oxidative coupling
1
reactions were reported using stoichiometric nonꢀmetal oxidants. Recently, organocatalystꢀcatalyzed
oxidative homoꢀcoupling reactions of Grignard reagents in the presence of molecular oxygen were
2
reported. Although the catalytic reaction is one of the most efficient methods for the synthesis of
3
,4,5
symmetrical biaryls,
only three examples using organocatalysts have been reported to date. The
first and challenging example, used a TEMPOꢀcatalyzed reaction, was reported by Studer et al. in
2
a
2
008. Although the reaction of PhMgBr gave an 81% yield of biphenyl in the presence of 14 mol%
TEMPO, several sequential reaction cycles (coupling reaction of PhMgBr by TEMPO → purging
with O for recycling TEMPO → addition of PhMgBr → •••) were required to inhibit the formation
2
of phenol as a byꢀproduct. In 2015, we reported a four electronꢀaccepting organic oxidant 1 that
mediated the oxidative homoꢀcoupling of Grignard reagents bearing aryl, alkenyl, or benzyl group
2b
stoichiometry. Furthermore, a catalytic reaction using 10 mol% of 1 was also demonstrated under
2
b
dryꢀair conditions to give homoꢀcoupling products with over 90% yields. The catalytic coupling
reactions using 1 proceeded only by exposure to dry air under stirring except for the extra operation
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a
in contrast to the report by Studer et al. In 2016, Amaya and Hirao et al. reported the same reactions
using 15 mol% of
N,N'ꢀdiphenylꢀpꢀbenzoquinonediimine catalyst to give 61%–85% coupling
2c
products. More recently, Matano et al. also reported the same reaction using 1 mol% of porphyrin
1
1
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2
d
derivative to give 31% coupling product.
The role of oxygen molecules in all
organocatalystꢀcatalyzed oxidative homoꢀcoupling is the reꢀgeneration of the catalyst from its
reduced form to turn over the catalytic cycle. However, an oxygenꢀmolecule induced side reaction
with the Grignard reagent that produced an oxide product, such as phenol from PhMgBr, inhibited
2
a
the high yield of homoꢀcoupling products. In contrast, the catalytic system using 1 forms only a
trace amount of the byꢀproduct under atmospheric conditions, and high yields of the coupling
2b
products can be achieved as mentioned above. The mechanism of homoꢀcoupling of Grignard
reagents using 1 involves a) 1 mediated stoichiometric homoꢀcoupling of four molar RMgX to give
two moles of RꢀR and an anion species 2 derived from 1, b) 2 converted to the
perfluorocyclopentadienyl radical 3 in the presence of atmospheric oxygen, and 3 catalyzed the
2b
homoꢀcoupling reaction of the rest of the RMgX to give RꢀR (Figure 1).
1
/2 R-R
R-MgX
stoichiometric reaction
F
F
F
F
4
R-MgX 2R-R
F
F
F
F
F
F
catalytic
reaction
F
F
ArF
ArF
3 MgFX
ArF
[MgX]
ArF
ArF
ArF
1
3
ArF = 4-heptafluorotolyl
2
atmospheric O2
Figure 1 Mechanism of oxidative homoꢀcoupling of RMgX using 1.
Unfortunately, the active catalyst species 3 could not be isolated from the coupling reaction mixture.
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If 3 can be used for the reaction directly, higher catalytic activity could be expected. In this study, an
alternative synthesis of the active species 3 from 1 was performed, and its utilization for the
oxidative coupling reaction was demonstrated.
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Firstly, the generation of active species 3 was studied. Purified 1 was converted to active species 3
by treatment with Mg; the oxidant 1 was treated with Mg turnings in the presence of trace I
2
in THF
at 40°C for 4 h to give anion species 2. It was readily converted to active species 3 under dry air.
19
2b,7
These species were confirmed by F NMR or ESR analysis (Figure 2).
Isolation of 3 was failed
because 3 was decomposed by concentration of the THF solution.
19
Figure 2 Confirmation of 2 and 3 by F NMR and ESR.
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The active species 3 was used for the oxidative homoꢀcoupling of PhMgBr (5a). When the solvent
effect was studied, the THF solvent gave the best result as with the cases using 1 (Table 1, entries 1,
2b
3
–5). However, the addition of triethylamine in the THF remarkably decreased the yield of
1
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biphenyl (6a) (entry 2). Because the catalyst 3 was not destroyed with triethylamine in THF, we
assumed that the triethylamine altered the reactivity of 5. The reaction using 10 mol% of 3 at 50°C
for 6 h afforded 6a in 98% yield (entry 6). When the catalyst loading was 5 mol% and 2 mol%, 6a
was obtained in 81% yield for 24 h and 62% yield for 3 days, respectively (entries 8, 10). Although
the low catalyst loading of 3 gave a somewhat low yield compared to the reaction using 1 (entries 8,
10 vs. 9, 11), the turnover numbers (TON) were roughly similar. The TON in the cases of 1 could not
be directly calculated from the catalyst loading. As described above, 1 initially acted as a
2b
stoichiometric oxidant and then as a catalyst in the form of 3. For instance, in the case of the
reaction using 10 mol% of 1 (Table 1, entry 7), 20% of 6a was obtained through the stoichiometric
8
reaction and 73% of 6a was obtained through the catalytic reaction; TON = 7.3. In contrast, the
TON of the reaction using 10 mol% of catalyst 3 as a oneꢀelectron oxidant was calculated to be 9.8
(entry 6). When the catalyst loading was 10 mol%, the TON of 3 was slightly larger than that of 1. In
contrast, when the catalyst loading was 2 mol%, the TON of 3 was slightly lower than that of 1.
Comparison of the results of 1 and 3 showed that the catalytic activity of 3 did not exceed the
reaction activity of 1 for initial stoichiometric coupling.
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a
Table 1 Homoꢀcoupling Reaction of PhMgBr (5a)
Catalyst
Ph-MgBr
Ph Ph
THF
5a
6a
50 °C
Entry
Catalyst
(mol%)
Solvent
Time
(h)
Yield
(%)
TON
b
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
c
1
3 (5)
3 (5)
3 (5)
3 (5)
3 (5)
3 (10)
1 (10)
3 (5)
1 (5)
3 (2)
1 (2)
THF
6 h
62
d
c
2
THF
6 h
18
c
3
CPME
6 h
51
c
4
(CH
2
Cl)
2
6 h
49
c
5
toluene
THF
THF
THF
THF
THF
THF
6 h
34
6
6 h
98
93
81
87
9.8
7.3
16
15
31
36
7
6 h
8
24 h
24 h
9
10
3 days 62
3 days 76
1
1
a
The use of 1.0 mmol of 5a (1.0 M THF solution,
which was titrated with 1,10ꢀphenanthroline and
ꢀPrOH) purchased from Aldrich in the presence of
i
b
c
catalyst 1 or 3. Isolated yield. Conversion by
d
GC analysis. Triethylamine was added.
The catalytic activity of 3 in the reactions of 4ꢀsubstitutedꢀaryl Grignard reagents was also similar to
3 6 4
that of 1. The reactions of 4ꢀtolylꢀMgBr (5b) or 4ꢀCF ꢀC H ꢀMgBr (5c) in the presence of 10 mol%
of catalyst at 50 °C for 6 h gave homoꢀcoupling products 6b in 95% yield using 3 vs. 91% using 1 or
6c in 92% using 3 vs. 91% using 1, respectively. Although an advantageous use of 3 could not be
found in the reaction of 5, the superiority of 3 was shown in the homoꢀcoupling of Grignard reagents
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bearing thienyl groups (7) to give bithiophene derivatives 8, which were utilized for the functional
9
molecules. The reaction of 2ꢀthienyl magnesium bromide (7a) using 20 mol% of the oxidant 1 at
6
0 °C for 12 h gave only a low yield of 2,2’ꢀbithiophene (8a) (Table 2, entry 2) unlike the reactions
of 5. In contrast, the reaction of 7a using 20 mol% of 3 afforded 94% of 8a (entry 1). In this case,
2(5 )ꢀthiophenone as a byꢀproduct was observed in only ca. 2%. The organic catalyst 3 was
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
effective for other Grignard reagents bearing thienyl group (7bꢀ7e), and bithiophene products (8bꢀ8e)
were obtained in over 80% yields. On the contrary, the reactions using 1 produced the coupling
products 8 in low yield. The results showed that the rate of electronꢀtransfer from the Grignard
reagent 7 with 1 was slower than that with 3, and the active species 3 hardly formed from 1 during
the reaction. When the reaction of 2ꢀpyridylꢀMgBr (7f) was performed using 3, 2,2’ꢀbipyridyl (8f)
was obtained in 44% yield. However, the reaction of 2ꢀfurylꢀMgBr did not proceed.
a
Table 2 Homoꢀcoupling Reaction of Grignard Reagents bearing Thienyl Groups (7)
Ar = thienyl group
Catalyst
(20 mol%)
Ar-MgBr
Ar Ar
THF
7
8
60 °C
b
entry Grignard reagent
catalyst time (h)
yield (%)
1
2
S
3
1
12
12
94 (8a)
36 (8a)
MgBr
7
a
3
4
S
3
1
24
24
88 (8b)
45 (8b)
MgBr
7
b
5
6
S
3
1
24
24
82 (8c)
43 (8c)
Me
Me
MgBr
7c
7
8
S
3
1
24
24
85 (8d)
39 (8d)
MgBr
7d
9
S
3
1
24
24
80 (8e)
35 (8e)
BrMg
10
Me
7e
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1
1
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1
1
1
1
1
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2
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2
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3
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3
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3
3
3
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5
5
5
5
5
5
5
6
a
The use of 1.0 mmol of 7 (1.0 M THF solution,
which was titrated with 1,10ꢀphenanthroline and
b
iꢀPrOH) in the presence of 20 mol% of 3 or 1.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
1
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Isolated yield.
Finally, synthesis of (6,6'ꢀdimethoxy[1,1'ꢀbiphenyl]ꢀ2,2'ꢀdiyl)bisꢀphosphonic acid tetraethyl ester (9)
using catalyst 3 was demonstrated. The compound 9 has been used as a synthetic intermediate for the
1
0
MeOꢀBIPHEP ligand and its derivatives. Thus far, 9 had been synthesized using Ullman coupling
11
of diethyl (2ꢀiodoꢀ3ꢀmethoxyphenyl)phosphonate or oxidative coupling of lithium diethyl
12
11
3
ꢀmethoxyphenylphosphonate. Both the reactions mediated a stoichiometric transition metal (Cu
12
or FeCl
3
) to give 9 in medium yields (ca. 40%–60%). One of the great drawbacks of both reactions
was the postꢀtreatment of the reactions. After the coupling reactions, a tarꢀlike paste of
transitionꢀmetal origin obstructed the purification of 9. The paste adhered to the surface of a filter,
®
even if the Celite was used, and filtration was made very difficult to be carried out, especially
remarkable in large scale synthesis. Therefore, the synthesis of 9 using organocatalyst 3 was tried
(Figure 3). The Grignard reagent was prepared from diethyl 3ꢀmethoxyphenylphosphonate (10) with
2
LDA followed by the addition of MgBr in THF. This was added to the THF solution containing 20
mol% of 3, and the solution was then stirred at 60°C for 24 h to give the coupling product of 9 in
4
4% yield; at 48 h reaction time, the yield improved to 59%. Gramꢀscale synthesis was then tried
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under the latter condition. In the scale of 2.9 g of 10, a somewhat viscous and dark solution was
obtained after the coupling reaction. The viscosity could be easily removed by filtration through a
®
short pad of Celite , and easy purification of the concentrated filtrate by silicaꢀgel chromatography
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
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4
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4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
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0
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0
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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
gave the 1.6 g of 9 in 55% yield.
1) LDA
2) MgBr2
O
3) 3 (20 mol%)
O
MeO
MeO
P(OEt)
2
MeO
P(OEt)2
P(OEt)2
10
O
9
MeO-BIPHEP and its derivatives
Figure 3 Synthesis of the synthetic intermediate of MeOꢀBIPHEP using 3.
As a whole, in this and the authors’ previous studies, oxidative coupling of RMgX using 3 or 1 gave
homoꢀcoupling products in high yield and high TON as compared to the reactions using the other
2a,2c, 2d
two organocatalysts.
One of the reasons for this good result could be the inhibition of the
formation of the byꢀproduct. For instance, the coupling reaction of PhMgBr using 20 mol% of
2
TEMPO with continual bubbling of pure O yields 61% and 24% biphenyl and a phenol,
2
a
respectively. In contrast, the current study’s catalytic reaction yielded only ca. 1% of phenol in
entry 1 of Table 2. The explanation of the results is very difficult because the detail mechanism of the
oxidative coupling of Grignard reagents is complicated and still unclear. One of the most feasible
13
mechanism of the biaryl coupling is oxidantꢀmediated elimination of biaryl from Ar
2
Mg, and the
aryl radical (Ar•) which was generated from ArMgBr with oxidant does not participate in the
coupling. In contrast, the Ph• was observed in the reaction of PhMgBr using pꢀbenzoquinone as an
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d
oxidant in THF. Therefore, it can't be denied the possibility that Ar• is associated with the side
2
reaction with O to give oxidizing byꢀproduct such as phenol. As one possibility, the stabilization of
Ph• with 2 was demonstrated by DFT calculations. The interacted structure of Ph• with 2 was
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searched by DFT calculations at the uB97D/6ꢀ31+G(d) using Gaussian 09. As a result, the structure
+
of {Ph• + 2 + MgBr [2 THF]} (11) was found (Figure 4). The two molecules of THF binding to Mg
were very important for the stabilization of this structure (See Supporting Information). In the
structure 11, it was certain that Ph• and the anion species 2 existed (in other words, the phenyl anion
and radical species 3 did not exist) because the sum of the Mulliken charges or the NPA charges of
the phenyl ring were +0.07 or −0.06, and for the moiety of 2 (C19
Figure 5). Furthermore, the NPA charge (+0.10) and the natural electron configuration (2
at the ipsoꢀcarbon of the phenyl ring in structure 11 were similar to those of the phenyl radical
F
17), they were −1.04 or −0.88
1
.02
2.85
(
s
2
p
)
1
.02 2.82
1.23
3.13
(
+0.15; 2
s
2
p
) but not those of the phenyl anion (−0.41; 2
s
2
p
) (Figure S1 in Supporting
Information). In the orbital analysis of the SOMO of 11, the lobes were located at the ipsoꢀcarbon of
the phenyl ring and the cyclopentadienyl ring (Figure 4). In contrast, the lobes at the ipsoꢀcarbon
were not observed in SOMOꢀ1 and SOMOꢀ2 (See Supporting Information). The orbital distribution
in the SOMO predicted the existence of an interaction between the ipsoꢀcarbon of phenyl ring and
the cyclopentadienyl ring. NBO analysis clarified the relatively strong interaction of the natural bond
ipso
orbital of the ipsoꢀcarbon [C (Lp)] with a localized antiꢀbonding orbital, CꢀC(π*) or CꢀC(σ*), in
the cyclopentadienyl ring (Figure S2 in Supporting Information). The corresponding interacting
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energies, computed using secondꢀorder perturbation theory [E(2)], were 11.96 or 12.12 kcal/mol,
respectively. These relatively large interacting energies would play a role in the stabilization of Ph•.
As a result, Ph• in 11 was stabilized by 11.6 kcal/mol as compared to free Ph• (Figure S3 in
Supporting Information). Therefore, if Ph• was generated, the side reaction of Ph• to give phenol
would be retarded.
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Figure 4 Optimized structure of 11.
In summary, the radical species 3 could be alternative synthesized, and it acted as an organocatalyst
for the oxidative homoꢀcoupling of Grignard reagents. The catalyst showed a relatively high TON for
the coupling of PhMgBr, and it enabled catalytic oxidative homoꢀcoupling of Grignard reagents
bearing thienyl groups to give bithiophene compounds. The catalytic system seemed to stabilize the
radical intermediate derived from a Grignard reagent to inhibit the side reaction. In future work, it is
expected that the radical species 3 will be diverted to the other oxidative reactions under aerobic
14
condition.
Experimental Section
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General experimental methods.
All reactions were carried out under an argon atmosphere with dry solvents under anhydrous
conditions, unless otherwise noted. All solvents were purchased from Kanto Chemical Co. and then
were stored in Schlenk tubes under an argon atmosphere. Reagents were purchased at the highest
commercial quality and used without further purification, unless otherwise noted. Preparative
column chromatography was carried out by using silica gel (Kanto Chemical Co. 60N, 63ꢀ210 ꢁm).
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H NMR spectra were measured at 400 MHz or 500 MHz using tetramethylsilane (TMS) as an
13
internal standard (δ 0 ppm). C NMR spectra were measured at 101 MHz or 126 MHz, and chemical
19
shifts are given relative to chloroformꢀ
d
(δ 77.16 ppm). F NMR spectra were measured at 376 MHz,
3
1
3 6 6
and chemical shifts are given relative to CCl F using C F as secondary reference (ꢀ162.9 ppm). P
NMR spectrum was measured at 162 MHz, and chemical shifts are given relative to 85% H
externally.
3 4
PO
Formation of catalyst 3 from 1
A flameꢀdried 20 mL Schlenk flask was flushed with argon and charged with powder Magnesium
2
(12.2 mg, 0.5 mmol), catalytic amount of I ,
1,2ꢀbisperfluorotolylꢀ3,3,4,4,5,5ꢀhexafluorocyclopentene (1, 121.6 mg, 0.2 mmol) and THF (1.0 mL).
The reaction mixture was stirred at 40 °C for 4 h under argon atmosphere to give dark brown
solution. Then, the solution was expose to the dry air into the reactor. The reaction mixture was
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turned to dark purple solution to form the radical active species 3. The solution of 3 was used for the
homoꢀcoupling reactions without purification because the radical species 3 could not be isolated.
1
1
1
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General Procedure for the Catalytic Homo-coupling Reaction of Grignard Reagent.
A 20 mL Schlenk flask was flushed with freshly prepared radical active species 3 (0.2 mmol, 20
mol%). And then, freshly prepared Grignard reagent in THF (1.0 mmol, titrated by using
1,10ꢀphenanthroline and isopropanol) was added. The reaction mixture was stirred at appropriate
temperature for 6 to 72 h under dry air. The reaction mixture was quenched with saturated aqueous
4 2 2 2 4
NH Cl solution, and extracted with EtOAc or CH Cl . The organic layer was dried over Na SO and
was concentrated under reduced pressure. The resulting mixture was purified by silica gel column
chromatography to give the corresponding product.
General Procedure for the Oxidative Homo-coupling Reaction of Grignard Reagent.
A flameꢀdried 20 mL Schlenk flask was flushed with argon and charged with
1,2ꢀbisperfluorotolylꢀ3,3,4,4,5,5ꢀhexafluorocyclopentene (1, 121.63 mg, 0.2 mmol) and THF (1.0
mL) at room temperature. And then, purchased or freshly prepared Grignard reagent in THF (1.0
mmol, titrated by using 1,10ꢀphenanthroline and isopropanol) was added. The reaction mixture was
stirred at appropriate temperature for 12 to 24 h under dry air. The reaction mixture was quenched
with saturated aqueous NH
4
2 2
Cl solution, and extracted with EtOAc or CH Cl . The organic layer was
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dried over Na
2
SO
4
and was concentrated under reduced pressure. The resulting mixture was purified
by silica gel column chromatography to give the corresponding product.
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b
Biphenyl (6a)
Catalytic homoꢀcoupling of PhꢀMgBr (5a, 1.0 mmol) was performed in the presence of 10 mol% 3 at
1
50 °C for 6 h to give 6a (75.3 mg, 0.488 mmol, 98%) as a white solid. H NMR (400 MHz, CDCl3):
13
δ
7.30 ꢀ 7.36 (m, 2H), 7.40 ꢀ 7.45 (m, 4H), 7.57 ꢀ 7.60 (m, 4H). C NMR (101 MHz, CDCl3):
δ
127.3, 127.4, 128.9, 141.4.
2
b
4,4’-Dimethylbiphenyl (6b)
Catalytic homoꢀcoupling of 4ꢀtolylꢀMgBr (5b, 1.0 mmol) was performed in the presence of 10 mol%
1
3
at 50 °C for 6 h to give 6b (86.7 mg, 0.476 mmol, 95%) as a white solid. H NMR (500 MHz,
13
CDCl
CDCl
3
3
):
):
δ
2.39 (s, 6H), 7.24 (d,
J = 8.0 Hz, 4H), 7.48 (d, J = 8.0 Hz, 4H). C NMR (126 MHz,
δ
21.2, 126.9, 129.5, 136.8, 138.4.
2b
4,4’-Bis(trifluoromethyl)biphenyl (6c)
Catalytic homoꢀcoupling of 4ꢀCF
3
ꢀC
6
H
4
ꢀMgBr (5c, 1.0 mmol) was performed in the presence of 10
1
mol% 3 at 50 °C for 6 h to give 6c (133.2 mg, 0.459 mmol, 92%) as a white solid. H NMR (500
13
MHz, CDCl
3
):
δ
7.72 (dd,
J
3
= 9.5, 8.5 Hz, 8H). C NMR (126 MHz, CDCl ): δ 124.2 (q, J = 271.7
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Hz), 126.0 – 126.1 (m), 127.7, 130.3 (q, J = 32.4 Hz), 143.3.
15
2,2’-Bithiophene (8a)
1
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1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
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Catalytic homoꢀcoupling of 2ꢀthienylꢀMgBr (7a, 1.0 mmol) was performed in the presence of 20
1
mol% 3 at 60 °C for 12 h to give 8a (78.2 mg, 0.470 mmol, 94%) as a yellow solid. H NMR (400
1
3
MHz, CDCl
3
):
δ
7.00 ꢀ 7.03 (m, 2H), 7.17 (d,
J = 3.5 Hz, 2H), 7.20 (d, J = 5.1 Hz, 2H). C NMR
(101 MHz, CDCl
3
): δ 123.9, 124.5, 127.9, 137.5
16
3,3’-Bithiophene (8b)
Catalytic homoꢀcoupling of 3ꢀthienylꢀMgBr (7b, 1.0 mmol) was performed in the presence of 20
1
mol% 3 at 60 °C for 24 h to give 8b (73.0 mg, 0.439 mmol, 88%) as a yellow solid. H NMR (400
13
MHz, CDCl
3
):
3
δ 7.32 – 7.36 (m, 4H), 7.36 ꢀ 7.38 (m, 2H). C NMR (101 MHz, CDCl ): δ 119.9,
126.2, 126.5, 137.3.
17
5,5’-Dimethyl-2,2’-bithiophene (8c)
Catalytic homoꢀcoupling of 5ꢀMeꢀ2ꢀthienylꢀMgBr (7c, 1.0 mmol) was performed in the presence of
1
20 mol% 3 at 60 °C for 24 h to give 8c (79.8 mg, 0.411 mmol, 82%) as a white solid. H NMR (400
13
3
MHz, CDCl ): δ 2.46 (s, 6H), 6.63 (d, J = 3.1 Hz, 2H), 6.87 (d, J = 3.5 Hz, 2H). C NMR (101 MHz,
CDCl ): 15.5, 123.0, 125.9, 135.6, 138.6.
3
δ
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2
2
2
2
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8
5
,5’-Dimethyl-3,3’-bithiophene (8d)
Catalytic homoꢀcoupling of 5ꢀMeꢀ3ꢀthienylꢀMgBr (7d, 1.0 mmol) was performed in the presence of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
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1
2
3
4
5
6
7
8
9
0
1
2
0 mol% 3 at 60 °C for 24 h to give 8d (82.8 mg, 0.426 mmol, 85%) as a white solid. H NMR (400
1
3
3 3
MHz, CDCl ): δ 2.49 (s, 6H), 6.95 (s, 2H), 7.05 (s, 2H). C NMR (101 MHz, CDCl ): δ 15.5, 117.4,
124.7, 137.4, 140.3.
19
3,3’-Dimethyl-2,2’-bithiophene (8e)
Catalytic homocoupling of 3ꢀMeꢀ2ꢀthienylꢀMgBr (7e, 1.0 mmol) was performed in the presence of
1
2
0 mol% 3 at 60 °C for 24 h to give 8e (77.9 mg, 0.401 mmol, 80%) as a yellow oil. H NMR (400
13
3
MHz, CDCl ): δ 2.17 (s, 6H), 6.91 (d, J = 5.2 Hz, 2H), 7.25 (d, J = 5.2 Hz, 2H). C NMR (101 MHz,
CDCl
3
):
δ
14.8, 125.1, 129.5, 130.2, 136.6.
20
2,2'-Bipyridyl (8f)
Catalytic homoꢀcoupling of 2ꢀpyridylꢀMgBr (7f, 1.0 mmol) was performed in the presence of 20
1
mol% 3 at 60 °C for 24 h to give 8f (34.5 mg, 0.221 mmol, 44%) as a white solid. H NMR (500
MHz, CDCl
3
):
δ
7.31 (t,
J
= 1.2 Hz, 2H), 7.82 (t,
J = 1.2 Hz, 2H), 8.39 (d, J = 8.0 Hz, 2H), 8.68 (t, J
13
=
2.0 Hz, 2H). C NMR (126 MHz, CDCl
3
): 121.2, 123.8, 137.0, 149.3, 156.2.
δ
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1
(
6,6'-dimethoxy[1,1'-biphenyl]-2,2'-diyl)bis-phosphonic acid tetraethyl ester (9)
Diisopropylamine (1.62 g, 16.0 mmol), 1.60 M
added to the flameꢀdried 50 mL Schlenk flask A under argon. The reaction mixture was stirred at
78 °C for 30 min. After addition of a solution of diethylꢀ3ꢀmethoxyphenylphosphonate (10) (3.42 g,
nꢀBuLi (9.38 mL, 15.0 mmol), THF (10 mL) were
1
1
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1
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1
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2
2
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2
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ꢀ
14.0 mmol) in 5 mL of THF to the solution dropwise, the reaction mixture was stirred at ꢀ78 °C for 1
h.
To another flameꢀdried 50 mL Schlenk flask B flushed with argon, magnesium powder (390 mg, 16.0
mmol) and THF (10 mL) was added at 0 °C. After addition of 1,2ꢀdibromoethane (2.82 g, 15.0
mmol) to the solution dropwise, the turbid solution was stirred at 0 °C to room temperature for 2 h.
The resulting solution was slowly added to the Schlenk A, and the reaction mixture was stirred for 1
h at room temperature to give the corresponding Grignard reagent.
The prepared Grignard reagent (12.0 mmol, titrated by using 1,10ꢀphenanthroline and isopropanol)
was added to the THF solution of 3 (from 1.46 g of 1, 2.4 mmol, 20 mol%) in 50 mL Schlenk flask.
The reaction mixture was stirred at 60 °C for 48 h under dry air. The reaction mixture was quenched
4
with saturated aqueous NH Cl solution, and extracted with EtOAc. The organic layer was dried over
Na
2
SO
4
and was concentrated under reduced pressure. The resulting mixture was purified by silica
1
gel column chromatography to give 9 (1.60 g, 3.29 mmol, 55%) as a pale yellow solid. H NMR
(400 MHz, CDCl
3
): δ 1.12 (t, J = 7.1 Hz, 6H), 1.16 (t, J = 7.1 Hz, 6H), 3.71 (s, 6H), 3.76 – 3.94 (m,
31
8
H), 7.11 (d,
J
3
= 8.2 Hz, 2H), 7.42 (m, 2H), 7.56 (m, 2H). P NMR (162 MHz, CDCl ): δ 18.0 (s).
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1
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2
2
2
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2
2
2
2
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3
3
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4
4
4
4
4
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5
5
5
5
5
5
5
5
5
6
1
3
C NMR (101 MHz, CDCl
3
): δ 16.3 (t, J = 6.9 Hz), 55.9, 61.5 (d, J = 5.7 Hz), 61.6 (d, J = 6.1 Hz),
1
25.0 (d, = 8.6 Hz), 128.4, 128.6, 129.1, 129.5 – 129.8 (m), 131.0, 157.6 (d, 19.8 Hz).
J
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Acknowledgments
This work was supported in part by JSPS KAKENHI (Grantꢀin Aid for Scientific Research (C), no.
16K05764 (T.K.)).
Supporting Information
The copies of NMR spectra and computational data. This material is available free of charge via the
Internet at http://pubs.acs.org.
References
1
Recent reviews: (a) Guo, S.ꢀr.; Kumar, P. S.; Yang, M. Adv. Synth. Catal. 2017, 359, 2. (b)
Morimoto, K.; Dohi, T.; Kita, Y. Synlett 2017, 28, 1680. (c) Dohi, T.; Kita, Y. Current Org. Chem
016, 20, 580. (d) Sun, C.ꢀL.; Shi, Z.ꢀJ. Chem. Rev. 2014, 114, 9219.
.
2
2 (a) Maji, M. S.; Pfeifer, T.; Studer, A. Angew. Chem., Int. Ed. 2008, 47, 9547. (b) Korenaga, T.;
Nitatori, K.; Muraoka, H.; Ogawa, S.; Shimada, K. Org Lett 2015, 17, 5500. (c) Amaya, T.; Suzuki,
R.; Hirao, T. Chem. Commun. 2016, 52, 7790. (d) Sudoh, K.; Satoh, T.; Amaya, T.; Furukawa, K.;
Minoura, M.; Nakano, H.; Matano, Y. Chem. – Euro. J. 2017, 23, 16364.
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3
Transitionꢀmetal catalyzed homoꢀcoupling of Grignard reagents in combination with a
stoichimetric oxidant: (a) Nagano, T.; Hayashi, T. Org. Lett. 2005,
C.; MahuteauꢀBetzer, F.; Ahr, M. Org. Lett. 2005, , 1943. (c) Li, X.; Li, D.; Li, Y.; Chang, H.; Gao,
W.; Wei, W. RSC Adv. 2016, , 86998, and references cited therein.
7, 491; (b) Cahiez, G.; Chaboche,
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4 Transitionꢀmetal catalyzed homoꢀcoupling of Grignard reagents in combination with an oxygen
molecule as an oxidant: (a) Cahiez, G.; Moyeux, A.; Buendia, J.; Duplais, C. J. Am. Chem. Soc. 2007,
1
29, 13788. (b) Liu, W.; Lei, A. Tetrahedron Lett. 2008, 49, 610. (c) Mayer, M.; Czaplik, W. M.; von
Wangelin, A. J. Synlett 2009, 2919. (d) Aparna, P. I.; Bhat, B. R. J. Mol. Catal. A: Chem. 2012, 358,
3. (e) Bhat, A. P. I.; Inam, F.; Bhat, B. R. Eur. J. Org. Chem. 2013, 7139. (f) Bhat, A. P. I.; Inam, F.;
Bhat, B. R. RSC Adv. 2013, , 22191. (g) Bottoni, A.; Cahiez, G.; Calvaresi, M.; Moyeux, A.;
Giacinto, P.; Miscione, G. P. J. Organomet. Chem. 2016, 814, 25.
Stoichiometric organic oxidantꢀmediated homoꢀcoupling of Grignard reagents (a) Cheng, J.ꢀW.;
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Luo, F.ꢀT. Tetrahedron Lett. 1988, 29, 1293. (b) Nishiyama, T.; Seshita, T.; Shodai, H.; Aoki, K.;
Kameyama, H.; Komura, K. Chem. Lett. 1996, 549. (c) Krasovskiy, A.; Tishkov, A.; del Amo, V.;
Mayr, H.; Knochel, P. Angew. Chem. Int. Ed. 2006, 45, 5010. (d) Ramnial, T.; Taylor, S. A.; Clyburne,
J. A. C.; Walsby, C. J. Chem. Commun. 2007, 2066. (e) Maji, M. S.; Pfeifer, T.; Studer, A. Angew.
Chem. Int. Ed. 2008, 47, 9547. (f) Blangetti, M.; Fleming, P.; O’Shea, D. F. J. Org. Chem. 2012, 77
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Studer, A. Chimia 2012, 66, 413.
,
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Purification of 1 was required before performing the conversion of 1 to the active species 3, if 1
was stored in the atmosphere. Although no appreciable change of 1 was observed after storing under
19
argon for a month by F NMR and GCꢀMS analyses, 1 was allowed to stand under argon with
0
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moisture for a month to give ca. 10% of decomposed impurity. The impurity could be removed by
silicaꢀgel column chromatography and recrystallization.
7 In the ESR spectrum, a broad singlet peak at the position of
g = 2.00268 was observed at room
temperature, which is the evidence of the presence of a radical in the THF solution. The radical can
be expected to be the cyclopentadienyl radical 3 generated by oneꢀelectron oxidation of the
cyclopentadienyl anion 2, considering the
gꢀvalue is in good agreement with that of 3 generated in
the homoꢀcoupling reaction ( = 2.00309). See ref. 2b.
g
8 Active species 3 acted as a oneꢀelectron oxidant. Twice turning over the catalytic cycle produced
one molecule of 6 from two molecules of 5.
9
Recent examples of symmetrical bithiophene derivatives as a unit for functional materials: (a) Aoki,
H.; Saito, H.; Shimoyama, Y.; Kuwabara, J.; Yasuda, T.; Kanbara, T. ACS Macro Lett. 2018, , 90.
b) Long, X.; Dou, C.; Liu, J.; Wang, L. Macromolecules 2017, 50, 8521. (c) Liu, M.; Yang, J.; Lang,
C.; Zhang, Y.; Zhou, E.; Liu, Z.; Guo, F.; Zhao, L. Macromolecules 2017, 50, 7559.
7
(
10 Recent examples of effective utilization of MeOꢀBIPHEP and derivatives: (a) Vickerman, K. L.;
Stanley, L. M. Org. Lett. 2017, 19, 5054. (b) Wu, Z.; Leboeuf, D.; Retailleau, P.; Gandon, V.;
Marinetti, A.; Voituriez, A. Chem. Commun. 2017, 53, 7026. (c) Zhu, L.; Qi, X.; Li, Y.; Duan, M.;
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Zou, L.; Bai, R.; Lan, Y. Organometallics 2017, 36, 2107.
1 Suto, Y.; Tsuji, R.; Kanai, M.; Shibasaki, M., Org. Lett. 2005,
2 Ma, M.ꢀL.; Peng, Z.ꢀH.; Chen, L.; Guo, Y.; Chen, H.; Li, X.ꢀJ. Chin. J. Chem. 2006, 24, 1391.
3 Murarka, S.; Moebus, J.; Erker, G.; MueckꢀLichtenfeld, C.; Studer, A. Org. Biomol. Chem. 2015,
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13, 2762.
14 Environmentally sustainable benefits of catalytic aerobicꢀoxidation systems: (a) Stahl, S. S.
Science 2005, 309, 1824. (b) McCann, S. D.; Stahl, S. S. Acc. Chem. Res. 2015, 48, 1756. and
references cited therein.
1
5 Nising, C. F.; Schmid, U. K.; Nieger, M.; Brase, S. J. Org. Chem. 2004, 69, 6833.
6 Billingsley, K.; Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 3366.
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17 Kar, A.; Mangu, N.; Kaiser, H. M.; Tse, M. K. J. Organomet. Chem. 2009, 694, 537.
18 Fukuzumi, K.; Nishii, Y.; Miura, M. Angew. Chem. Int. Ed. 2017, 56, 12750.
19 Dohi, T.; Ito, M.; Yamaoka, N.; Morimoto, K.; Fujioka, H.; Kita, Y. Tetrahedron, 2009, 65, 10815.
20 Ghodse, S. M.; Telvekar, V. N. Tetrahedron Lett. 2017, 58, 524.
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