Reductive couplings of 2-halopyridines without external ligand: Phosphine-free nickel-catalyzed synthesis of symmetrical and unsymmetrical 2,2'-bipyridines

Article abstract of DOI:10.1021/jo402084m

An unexpectedly facile synthetic approach for symmetrical and unsymmetrical 2,2'-bipyridines through the Ni-catalyzed reductive couplings of 2-halopyridines was developed. The couplings were efficiently catalyzed by 5 mol % of NiCl2.6H2O without the use of external ligands. A variety of 2,2'-bipyridines including caerulomycin F have been efficiently synthesized.

Full text of DOI:10.1021/jo402084m

Note
pubs.acs.org/joc
Reductive Couplings of 2‑Halopyridines without External Ligand:
Phosphine-Free Nickel-Catalyzed Synthesis of Symmetrical and
Unsymmetrical 2,2′-Bipyridines
Lian-Yan Liao, Xing-Rui Kong, and Xin-Fang Duan*
College of Chemistry, Beijing Normal University, Beijing, 100875 China
S
* Supporting Information
ABSTRACT: An unexpectedly facile synthetic approach for
symmetrical and unsymmetrical 2,2′-bipyridines through the
Ni-catalyzed reductive couplings of 2-halopyridines was
developed. The couplings were efficiently catalyzed by 5 mol
% of NiCl2·6H2O without the use of external ligands. A variety
of 2,2′-bipyridines including caerulomycin F have been
efficiently synthesized.
2,2′-Bipyridines represent a family of most widely used
bidentate ligands due to their exceptional coordination
chemistry. They have been extensively used as versatile building
blocks in the fields of analytical, photo-, supra-, nano-, and
macromolecular chemistry.1 In organic chemistry, 2,2′-bipyr-
idines can function as effective ligands for transition-metal-
catalyzed coupling reactions,2 and meanwhile, their chiral
homologues are still a central class of chiral ligands for
asymmetric transformations.1b,3 In addition, these structural
units can also be found in natural products such as
caerulomycins and collismycins.4 Hence, there is a great
demand for efficient synthetic methods to access various 2,2′-
bipyridine derivatives.
chromatography, which extremely limits their industrial
application.
We report herein a simple and efficient protocol for the
synthesis of symmetrical and unsymmetrical 2,2′-bipyridines
through Ni-catalyzed reductive couplings of 2-halopyridines.
The most prominent advantage of this protocol is that the
couplings are facilely catalyzed by 5 mol % of NiCl2·6H2O
without the use of external ligands. This finding is in sharp
contrast to the general impression that the syntheses of 2,2′-
bipyridines by Ni-catalyzed coupling reactions are usually
difficult due to their adverse effects on the catalytic cycle.
Importantly, in these cases the in situ formed 2,2′-bipyridines
did not inhibit the catalytic cycle; instead, their nickel
complexes actually catalyzed the couplings.
Transition-metal-catalyzed coupling reactions have found
application in the synthesis of various biaryl compounds
including arylpyridines and bipyridines; however, the prepara-
tion of 2,2′-bipyridines by these coupling reactions is usually
considered to be most difficult relative to 2,3′- or 2,4′-
bipyridines.1d,5 This is due to two main problems associated
with 2,2′-bipyridine-forming cross-coupling reactions: (a) the
coordination ability of 2,2′-bipyridines can have an inhibition
effect on the catalytic cycle1d,5 and (b) the preparation of the 2-
pyridyl organometallics can be problematic. To circumvent the
difficulty with the 2-pyridyl organometallic preparation, Ni-
catalyzed direct couplings of 2-halopyridines in the presence of
reducing agents provide an alternative.6 To date, the synthesis
of 2,2′-bipyridines by this coupling has been mostly limited to
those by homocoupling.1c,6 Moreover, these homocouplings
require a large amount of catalyst metal and ligands to offset the
adverse effect of 2,2′-bipyridines on the catalytic cycle. For
example, the previously reported Ni-mediated homocouplings
of 2-halopyridines required stoichiometric NiCl2 or Ni(OAc)2
and 4 equiv of triphenylphosphine.7 An improved and most
commonly used procedure for this coupling still required 30
mol % of NiBr2(PPh3)2, and 100 mol % of Et4NI.8 The large
amounts of triphenylphosphine necessarily used in the above-
mentioned methods usually require subsequent separation by
Our research is inspired by the fact that Ni(bpy)X2 can
efficiently catalyze the couplings of 2-halopyridines.9 We thus
surmise that the Ni-catalyzed reductive couplings of 2-
halopyridines can also be catalyzed by the in situ formed Ni−
bpy complex and can proceed autocatalytically if the excessive
bipyridine continuously generated in the reaction can be
removed timely. During our previous synthesis of 2,2′-
bipyridine derivatives through a Negishi cross-coupling, we
found that the product bipyridines could precipitate from the
reaction mixture in the form of zinc complexes.4a,10 This
observation leads us to realize the possibility of achieving the
above-mentioned coupling by using zinc as a reducing agent, in
which the adverse binding of the excessive 2,2′-bipyridine with
nickel can be prevented by forming a complex with
simultaneously formed ZnX2 [Scheme 1 (I)].11 To test this
premise, we chose the reductive homocoupling of 2-
bromopyridine (1a) as the model reaction. After a series of
optimization experiments (see the table in the Supporting
Information), it was found that without any external ligands 2-
bromopyridine could homocouple in a 83% yield under the
Received: September 19, 2013
© XXXX American Chemical Society
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Scheme 1. Effect of 2,2′-Bipyridine in Ni-Catalyzed Reductive Coupling of Halopyridine
catalysis of 5 mol % of NiCl2·6H2O in DMF using Zn−LiCl12
Table 1. Ni-Catalyzed Reductive Homocouplings of 2-
Halopyridines
a
as a reducing agent. 2-Chloropyridine could also be coupled
well under these optimized conditions. It is worth mentioning
that the dilution of the reaction mixture with toluene would
result in the precipitation of zinc complex of 2,2′-bipyridine,
indicating the formation of the zinc complex during the
reaction as proposed in Scheme 1 (I). Meanwhile, the product
could also be isolated through zinc complex precipitate in a
74% yield.
b
entry
1(X, R)
product
yield(%)
The above results confirmed our initial hypothesis: this
reductive coupling could be catalyzed by the in situ formed Ni−
bpy complex and proceeded smoothly in the absence of
external ligands.13 To further confirm that this coupling was
catalyzed by the in situ formed Ni−bpy complex, a control
experiment using 3-bromopyridine was performed [Scheme 1
(II)]. It clearly indicated that NiCl2 could hardly catalyze the
coupling in the absence of 2,2′-bipyridine; instead, the addition
of external 2,2′-bipyridine promoted the reaction in 83% yield.
Interestingly, it is generally believed that the preparation of
2,2′-bipyridines through the coupling reaction is usually more
difficult relative to those of 3,3′-bipyridines,1a,5 our findings
lead to the opposite conclusion: the present reductive coupling
of 2-halopyridines can proceed facilely without the external
ligands, leading to 2,2′-bipyridine, conversely, the coupling of 3-
bromopyridines to 3,3′-bipyrdine cannot take place under the
same conditions.
At this point, the Ni-catalyzed reductive coupling of 2-
halopyridines that used to require a large amount of catalyst
metal and ligands to offset the product-inhibition effect turned
out to be a remarkably facile reaction under the present
conditions. We suppose that Zn−LiCl played an important role
in this coupling by means of (a) the appropriate reducing
activity; and (b) the synergic complexing of Zn2+ with the
continuously producing 2,2′-bipyridine that keeps the concen-
tration of 2,2′-bipyridine around nickel suitable for the efficient
catalysis [Scheme 1, (I)]. To the best of our knowledge, the
above coupling represents the first example of external-ligand-
free Ni-catalyzed reductive coupling of 2- halopyridines with
low catalyst loading. It also represents a very simple, practical,
and highly efficient protocol to prepare 2,2′-bipyridines. This
reaction was thus further explored with the results being
summarized in Table 1. It was found that a variety of 2-bromo-
and 2-chloropyridines could be readily homocoupled to furnish
the corresponding 2,2′-bipyridines in moderate to high yields.
Overall, the reaction showed a remarkable tolerance toward
different functional groups such as ketone (entries 13 and 14),
ester (entries 6 and 7), cyano (entries 8, 9, 19), amide (entry
12), benzyl ether, or alcohol (entries 11, 15−17). In addition,
2-chloroquinoline or quinoxaline could also be coupled (entries
20 and 21). Even more interesting is the fact that 2-bromo-5-
chloropyridine was selectively coupled to give 5,5′-dichloro-
1
1b (Br, 3-Me)
2b
2c
2d
2d
2e
2f
80
c
c
2
1c (Br, 4-Me)
81 (82)
81 (80)
79
3
1d (Br, 5-Me)
4
1d (Cl, 5-Me)
5
1e (Br, 6-Me)
78
6
1f (Br, 5-MeOOC)
1f (Cl, 5-MeOOC)
1g (Br, 5-NC)
77
7
2f
74
8
2g
2h
2i
90
9
1h (Br, 6-NC)
83
10
11
12
13
14
15
16
17
18
19
20
21
1i (Br, 5-MeO)
82
1j (Br, 5-BnO)
2j
78
1k (Br, 5-PhNHCO)
1l (Br, 5-PhCO)
1m (Br, 5-MeCO)
1n (Br, 5-PhCHOH)
1n (Cl, 5-PhCHOH)
1o (Br, 6-PhCHOH)
1p (Br, 5-Cl)
2k
2l
81
67
2m
2n
2n
2o
2p
2q
2r
62
77
75
65
68
1q (Cl, 5-NCCH2)
1r (2-chloroquinoline)
1s (2-chloroquinoxaline)
78
72
2s
65
a
NiCl2·6H2O (0.5 mmol), Zn (12 mmol), LiCl (10 mmol), and 2-
halopyridine (10 mmol), 50−60 °C, 3 h. Isolated yields. The yield in
parentheses was obtained on a 100 mmol scale
b
c
2,2′-bipyridine (entry 18). This tolerance to chlorine in such
couplings has been observed for the first time and allows the
products containing a chlorine handle that can readily be
derivatized to yield other functional bipyridines. Furthermore,
the present procedure proved to be very easy to scale-up. We
carried out the couplings of 1c and 1d on a 100 mmol scale; the
yields were almost identical to those obtained on a small scale
(entries 2 and 3).
To further extend the scope of this facile synthetic approach
for 2,2′-bipyridines, we went on to explore the reductive cross-
couplings of two different 2-halopyridines. Our preliminary
experiments indicated that the homocouplings of 2-halopyr-
idine or 2-halopicoline proceeded faster than those of the 2-
halopyridines with a functional group such as OMe, NEt2,
NHAc, CH2OH, CH2NH2, etc., and the couplings of the
equivalent of the two different 2-halopyridines usually resulted
in predominantly homocoupling products. In addition,
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attempts to take advantage of the different reactivity of C−Cl/
C−Br (such as the coupling between 2-chloropyridine with 2-
bromo-6-methoxypyridine) to increase the selectivity of the
cross-couplings gave rise to disappointing results. After a series
of trials, we found that 2-halopyridine or 2-halopicoline could
be selectively cross-coupled with another functionalized 2-
halopyridine in a 2.5:1 ratio. Moreover, the dropwise addition
of the solution of two 2-halopyridines in DMF over a period of
3 h helped to increase the yields of the cross-coupling products.
Under the above-optimized conditions, the cross-couplings
between two different 2-halopyridines were conducted, and the
results are illustrated in Table 2. It can be seen that although
reaction. Since 2,2′-bipyridines have been extensively used as
versatile building blocks in various fields, the efficient
preparation of the 2,2′-bipyridines bearing an easily derivatized
functional group is highly desired, for they can be easily linked
to a target molecule. Utilizing the present cross-coupling, this
type of 2,2′-bipyridines such as 3f, 3g, 3k, 3l, or 3n could be
conveniently prepared in a one-pot manner. The application of
this cross-coupling was finally highlighted in the one-step
synthesis of natural product, caerulomycin F (Scheme 2).14
Notably, caerulomycin F can be facilely transformed into
caerulomycins E and A in a one-pot manner using the reported
procedure.4a
In summary, we have developed an unexpectedly facile
synthetic approach for symmetrical and unsymmetrical 2,2′-
bipyridines through the Ni-catalyzed reductive couplings of 2-
halopyridines, which have previously been considered to be
highly difficult due to the product-inhibition effect. Contrary to
the common methods that need to use a large amount of
catalyst metal and ligands to offset the adverse effect of 2,2′-
bipyridine on the catalytic cycle, we took advantage of the in
situ formed Ni−bpy complex to catalyze the coupling and
performed the reaction with low catalyst loading in the absence
of external ligands. Furthermore, the reaction pattern presented
herein makes us realize that many of the couplings of 2-
halopyridine and N-heterocyclic analogues can probably be
performed in an external-ligand-free way. The further
mechanism study and other external-ligand-free couplings of
2-halopyridines are being investigated in our laboratories.
Table 2. Ni-Catalyzed Reductive Cross-Couplings of Two
a
,b
Different 2-Halopyridines
c
entry
R
R′
product
yield (%)
1
2
H
6-CN
3a
3b
3c
3d
3e
3f
81
85
75
73
84
75
80
68
72
82
67
68
78
73
H
6-OMe
3
6-Me
5-Me
5-Me
H
6-OMe
4
6-OMe
5
6-NEt2
6
6-CH2OH
6-CH(OCH2)2
5-OMe
d
7
H
3g
8
6-Me
H
3h
3i
9
5-NHAc
EXPERIMENTAL SECTION
■
10
H
5-OBn
3j
General Procedure for the Preparation of Symmetrical 2,2′-
Bipyridines via Homocouplings of 2-Halopyridines. A 100 mL
round-bottom flask was charged with NiCl2·6H2O (0.12 g, 0.5 mmol)
and DMF (20 mL). The resulting solution was stirred and heated to
40 °C, and then 2-halopyridine (10 mmol), anhydrous LiCl (0.43 g, 10
mmol), and zinc dust (0.78 g, 12 mmol) were added. When the
temperature rose to 50 °C, a grain of iodine crystal and two drops of
acetic acid were added to the mixture. An immediate rise in
temperature and color change to black was caused, indicating the
reaction was triggered (note 1). The mixture was stirred at 55−60 °C
for 2−3 h until complete conversion of 2-halopyridine (monitored by
TLC). To the cooled reaction mixture was added 1 N HCl aqueous
solution (15 mL) to consume the remaining zinc dust. The resulting
mixture was made alkaline with aqueous ammonia (25%) and taken up
with CH2Cl2 (note 2). The organic layers were collected, dried over
anhydrous Na2CO3, and concentrated. The crude material was purified
by flash chromatography to give the desired product.
e
11
H
5-CH(OH)Ph
5-CH2NHPh
5-CH2CN
5-CH2NH2
3k
3l
e
12
H
f
13
H
3m
3n
f
14
H
a
NiCl2·6H2O (0.15 mmol), Zn (12.6 mmol), LiCl (10.5 mmol), 2-
halopyridine (picoline) (7.5 mmol), functionalized 2-halopyridine (3
b
c
mmol), 60−70 °C, 6 h. X = Br unless otherwise noted. Isolated
d
e
yields. 3g was isolated as 2,2′-bipyridine-6-carbaldehyde. The mole
ratio of 2-bromopyridine to the other pyridine was 4.0:1. X = Cl.
f
these cross-couplings resulted in the homocoupling products of
2-halopyridine or 2-halopicoline as the side products, which
could be separated by chromatography, the yields of cross-
coupling products were moderate to high. A variety of common
functional groups such as OMe, OBn, NEt2, NHAc, CN,
CH(OR)2, CH2OH, CH2NH2, and CH2CN could facilitate the
selective cross-couplings, demonstrating the generality of the
Note 1: The reaction is highly exothermic in the initiation stage and
may cause a runaway when carried out on a large scale! In a 100 mmol
scale experiment, only 10 mmol of 2-halopyridine was added to the
Scheme 2. One-Step Synthesis of Caerulomycin F
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flask to initiate the reaction. The remaining 2-halopyridine was dripped
slowly into the reaction mixture at 50−60 °C. The reaction was then
performed and post-treated as described above.
(100 MHz, CDCl3) δ 157.0, 152.1, 140.4, 121.6, 116.4, 110.8. Data
was consistent with that reported in the literature.19
2,2′-Bipyridine-6,6′-dicarbonitrile (2h). The product was
isolated as a white solid in 83% yield (427 mg): mp = 264.5−265.9
°C; Rf = 0.22 (petroleum ether/ethyl acetate = 5:1); IR (cm−1, KBr):
2236; 1H NMR (400 MHz, CDCl3) δ 8.73 (d, J = 8.1 Hz, 2H), 8.02 (t,
J = 7.8 Hz, 2H), 7.78 (d, J = 7.6 Hz, 2H); 13C NMR (100 MHz,
CDCl3) δ 155.6, 138.4, 133.4, 129.0, 124.6, 117.0. Data was consistent
with that reported in the literature.20
5,5′-Dimethoxyl-2,2′-bipyridine (2i). The product was isolated
as a white solid in 82% yield (443 mg): mp = 135−137.3 °C; Rf = 0.35
(petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ
8.26 (d, J = 2.7 Hz, 2H), 8.21 (d, J = 8.8 Hz, 2H), 7.25 (dd, J = 2.8 Hz,
J = 8.8 Hz, 2H), 3.84 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 155.6,
148.9, 136.5, 129.6, 121.0, 55.7. Data was consistent with that reported
in the literature.21
Note 2: The product could also be isolated through the zinc
complex precipitate as follows: after the reaction was complete, the
reaction mixture was diluted with 70−80 mL toluene, and stirred at
room temperature for 2 h. The precipitate was collected by filtration,
then dissolved with aqueous ammonia (25%), and extracted with
CH2Cl2. The desired product was then isolated as described above.
General Procedure for the Preparation of Unsymmetrical
2,2′-Bipyridines via Cross-Couplings between Two Different 2-
Halopyridines. A 100 mL round-bottom flask was charged with
NiCl2·6H2O (0.04g, 0.15 mmol) and DMF (20 mL). The resulting
solution was stirred and heated to 40 °C, and then 2-halopyridine or 2-
halopicoline (2.5 mmol), functionalized 2-halopyridine (1 mmol),
anhydrous LiCl (0.44g, 10.5 mmol), and zinc dust (0.82 g, 12.6 mmol)
were added. When the temperature rose to 50 °C, a grain of iodine
crystal and two drops of acetic acid were added to initiate the reaction.
The solution of the remaining 2-halopyridine (2-halopicoline) (5
mmol) and functionalized 2-halopyridine (2 mmol) in 20 mL of DMF
was added dropwise into the mixture at 60−70 °C over a period of 3 h.
The resulting mixture was then stirred at 60−70 °C for an additional 3
h until complete conversion of 2-halopyridine (monitored by TLC).
The product was isolated as described in the procedure for the
homocouplings.
5,5′-Bis(benzyloxy)-2,2′-bipyridine (2j). The product was
isolated as a white solid in 78% yield (718 mg): mp = 195.2−197.4
1
°C; Rf = 0.15 (petroleum ether/ethyl acetate = 5:1); H NMR (400
MHz, CDCl3) δ 8.32 (s, 2H), 8.17 (d, J = 8.4 Hz, 2H), 7.19−7.40 (m,
12H), 5.09 (s, 4H), 13C NMR (100 MHz, CDCl3) δ (ppm) 154.8,
148.9, 137.2, 136.1, 128.7, 128.3, 127.6, 122.4, 121.1, 70.5; MS (ESI)
[M + H]+ (m/z, 369). Anal. Calcd for C24H20N2O2: C, 78.24; H, 5.47;
N, 7.60. Found: C, 77.98; H, 5.74; N. 7.53.
N5,N5′-Diphenyl-2,2′-bipyridine-5,5′-dicarboxamide (2k).
The product was isolated as a white solid in 81% yield (798 mg):
mp > 300 °C; Rf = 0.22 (petroleum ether/ethyl acetate =3:1); IR
2,2′-Bipyridine (2a). The product was isolated as a white solid in
83% yield (647 mg): mp = 67−68.5 °C; Rf = 0.46 (petroleum ether/
1
1
(cm−1, KBr) 1669; H NMR (400 MHz, DMSO-d6) δ 10.47 (s, 2H),
ethyl acetate = 5:1); H NMR (400 MHz, CDCl3) δ 8.62 (d, J = 4.3
Hz, 2H), 8.33 (d, J = 8.0 Hz, 2H), 7.74−7.77 (m, 2H), 7.32−7.26 (m,
2H); 13C NMR (100 MHz, CDCl3) δ 156.1, 149.1, 135.8, 123.6,
121.0. Data was consistent with that reported in the literature.8a
3,3′-Dimethyl-2,2′-bipyridine (2b). The product was isolated as
a colorless oil in 80% yield (736 mg): Rf = 0.32 (petroleum ether/ethyl
8.92 (d, J = 2.2 Hz, 2H), 8.33 (dd, J = 2.4 Hz, J = 8.2 Hz, 2H), 7.74−
7.69 (m, 6H), 7.36 (t, J = 7.7 Hz, 4H), 7.12 (t, J = 7.6 Hz, 2H); 13C
NMR (125 MHz, DMSO-d6) δ 163.4, 153.2, 149.8, 139.5 139.1, 130.5,
129.2, 124.6, 120.9 (one carbon missing due to overlap). Data was
consistent with that reported in the literature.22
2,2′-Bipyridine-5,5′-diylbis(phenylmethanone) (2l). The
product was isolated as a white solid in 67% yield (610 mg): mp =
211.2−212.9 °C; Rf1= 0.28 (petroleum ether/ethyl acetate = 5:1); IR
(cm−1, KBr) 1648; H NMR (400 MHz, CDCl3) δ 8.67 (s, 2H), 7.79
(d, J = 7.2 Hz, 4H), 7.68 (d, J = 7.3 Hz, 4H), 7.65 (m, 2H), 7.53 (t, J =
7.6 Hz, 4H); 13C NMR (100 MHz, CDCl3) δ 191.0, 145.2, 141.5,
135.5, 134.4, 133.9, 129.8, 129.0, 126.9, 125.9. Data was consistent
with that reported in the literature.16
1
acetate = 5:1); H NMR (400 MHz, CDCl3) δ 8.44 (d, J = 4.6 Hz,
2H), 7.55 (d, J = 7.7 Hz, 2H), 7.20−7.15 (m, 2H), 2.09 (s, 6H); 13C
NMR (100 MHz, CDCl3) δ 157.6, 146.5, 138.3, 131.5, 122.9, 18.4.
Data was consistent with that reported in the literature.15
4,4′-Dimethyl-2,2′-bipyridine (2c). The product was isolated as
a white solid in 81% yield (745 mg): mp = 169.5−170.5 °C; Rf = 0.40
(petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ
8.48 (d, J = 5.0 Hz, 2H), 8.21 (s, 2H), 7.10 (d, J = 4.8 Hz, 2H), 2.39
(s, 6H); 13C NMR (100 MHz, CDCl3) δ 156.0, 148.9, 148.1, 124.6,
122.0, 21.1. Data was consistent with that reported in the
literature.15,16
5,5′-Dimethyl-2,2′-bipyridine (2d). The product was isolated as
a white solid in 81% yield (745 mg): mp = 119.8−121.2 °C; Rf = 0.52
(petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ
8.42 (s, 2H), 8.19 (d, J = 8.1 Hz, 2H), 7.55 (d, J = 7.8 Hz, 2H), 2.32
(s, 6H); 13C NMR (100 MHz, CDCl3) δ 153.8, 149.5, 137.3, 132.9,
120.3, 18.3. Data was consistent with that reported in the literature.17
6,6′-Dimethyl-2,2′-bipyridine (2e). The product was isolated as
a white solid in 78% yield (718 mg): mp = 88.6−90.4 °C; Rf = 0.55
(petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ
8.11 (d, J = 7.8 Hz, 2H), 7.61 (t, J = 7.8 Hz, 2H), 7.08 (d, J = 7.6 Hz,
2H), 2.56 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 157.8, 155.9,
137.0, 123.0, 118.1, 24.7. Data was consistent with that reported in the
literature.15,16
Dimethyl 2,2′-Bipyridine-5,5′-dicarboxylate (2f). The product
was isolated as a white solid in 77% yield (524 mg), mp = 260.6−261.9
°C; Rf = 0.28 (petroleum ether/ethyl acetate = 3:1); IR (cm−1, KBr)
1727; 1H NMR (400 MHz, CDCl3) δ 9.23 (s, 2H), 8.52 (s, 2H), 8.39
(s, 2H), 3.93 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 165.6, 158.3,
150.6, 138.1, 122.5, 121.3, 52.5. Data was consistent with that reported
in the literature.15,18
2,2′-Bipyridine-5,5′-dicarbonitrile (2g). The product was
isolated as a white solid in 90% yield (464 mg): mp = 267.6−268.8
°C; Rf1= 0.46 (petroleum ether/ethyl acetate = 5:1); IR (cm−1, KBr):
2239; H NMR (400 MHz, CDCl3) δ 8.96 (d, J = 1.3 Hz, 2H), 8.64
(d, J = 8.3 Hz, 2H), 8.14 (dd, J = 2.0 Hz, J = 8.3 Hz, 2H); 13C NMR
1,1′-(2,2′-Bipyridine-5,5′-diyl)diethanone (2m). The product
was isolated as a pale yellow solid in 62% yield (372 mg): mp = 210−
211 °C; Rf = 0.60 (petroleum ether/ethyl acetate =3:1); IR (cm−1,
1
KBr) 1684; H NMR (400 MHz, CDCl3) δ 9.25 (d, J = 1.6 Hz, 2H),
8.61 (d, J = 8.3 Hz, 2H), 8.38 (dd, J = 8.3 Hz, J = 2.2 Hz, 2H), 2.70 (s,
6H); 13C NMR (100 MHz, CDCl3) δ 196.5, 158.2, 149.7, 136.7,
132.4, 121.7, 26.9. Data was consistent with that reported in the
literature.23
2,2′-Bipyridine-5,5′-diylbis(phenylmethanol) (2n). The prod-
uct was isolated as a white solid in 77% yield (708 mg): mp = 175.2−
177 °C, Rf = 0.35 (petroleum ether/ethyl acetate = 3:1); IR (cm−1,
1
KBr): 3297, 1599; H NMR (400 MHz, CD3COCD3) δ 8.65 (d, J =
1.5 Hz, 2H), 8.27 (d, J = 8.2 Hz, 2H), 7.83 (dd, J = 1.9 Hz, J = 8.2 Hz,
2H), 7.41 (d, J = 7.4 Hz, 4H), 7.33 (d, J = 7.4 Hz, 4H), 7.25 (m, 2H),
6.10 (d, J = 3.4 Hz, 2H), 5.83 (br s 2H); 13C NMR (125 MHz,
DMSO-d6) δ 154.4, 147.9, 145.3, 141.6, 135.4, 128.8, 127.5, 126.7,
120.5, 72.6; MS (ESI) [M + H]+ (m/z, 369). Anal. Calcd for
C24H20N2O2: C, 78.24; H, 5.47; N, 7.60. Found: C, 78.47; H, 5.72; N,
7.53.
2,2′-Bipyridine-6,6′-diylbis(phenylmethanol) (2o). The prod-
uct was isolated as a white solid in 65% yield (598 mg): mp = 66−68
°C; Rf = 0.25 (petroleum ether/ethyl acetate = 3:1); IR (cm−1, KBr)
3114, 1594; 1H NMR (500 MHz, CDCl3) δ 8.59 (d, J = 2.8 Hz, 2H,),
7.65 (t, J = 6.0 Hz, 2H), 7.40 (d, J = 5.8 Hz, 4H), 7.36 (t, J = 6.0 Hz,
4H), 7.32−7.29 (m, 2H), 7.23 (t, J = 4.6 Hz, 2H), 5.79 (s, 2H), 5.19
(br s, 2H); 13C NMR (100 MHz, CDCl3) δ 161.0, 147.8, 143.2, 136.8,
128.6, 127.8, 127.1,122.4, 121.3, 75.0. Data was consistent with that
reported in the literature.24
D
dx.doi.org/10.1021/jo402084m | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
5,5′-Dichloro-2,2′-bipyridine (2p). The product was isolated as a
white solid in 68% yield (383 mg): mp = 207−208 °C; Rf = 0.56
(petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ
8.55 (d, J = 2.2 Hz, 2H), 8.34 (d, J = 8.5 Hz, 2H), 7.75 (dd, J = 2.5 Hz,
J = 8.5 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 152.3, 147.1, 135.7,
131.5, 120.8. Data was consistent with that reported in the literature.25
2,2′-Bipyridine-5,5′-diacetonitrile (2q). The product was
isolated as a yellow solid in 78% yield (456 mg): mp = 215.9−217.8
°C, Rf = 0.15 (petroleum ether/ethyl acetate = 3:1); IR (cm−1, KBr)
2255; 1H NMR (400 MHz, CDCl3) δ 8.71 (s, 2H), 8.59 (d, J = 7.2 Hz,
2H), 7.95 (d, J = 7.9 Hz, 2H) 3.88 (s, 4H); 13C NMR (100 MHz,
CDCl3) δ 155.4, 148.5, 136.6, 128.9, 121.4, 116.7, 29.7. Data was
consistent with that reported in the literature.26
2,2′-Bipyridin-6-ylmethanol (3f). The product was isolated as a
light yellow oil in 75% yield (419 mg): Rf = 0.26 (petroleum ether/
ethyl acetate = 3:1); H NMR (400 MHz, CDCl3) δ 8.68 (m, 1H),
8.40 (d, J = 8.0 Hz, 1H), 8.30 (d, J = 7.8 Hz, 1H), 7.82 (m, 2H), 7.32
(m, 1H), 7.27 (d, J = 7.4 Hz, 1H), 4.84 (s, 2H), 3.11 (s,1H); 13C
NMR (100 MHz, CDCl3) δ 158.6, 155.7, 154.9, 149.2, 137.0, 123.9,
122.3, 121.1, 120.5, 119.7, 64.2. Data was consistent with that reported
in the literature.31
1
2,2′-Bipyridine-6-carbaldehyde (3g). The product was isolated
as a pale yellow solid in 80% yield (442 mg): mp = 166−167 °C; Rf =
1
0.14 (petroleum ether/ethyl acetate = 5:1); IR (cm−1, KBr) 1698; H
NMR (400 MHz, CDCl3) δ 10.18 (s, 1H), 8.66 (m, 1H), 8.49 (d, J =
8.0 Hz, 1H), 8.41 (dd, J = 0.7 Hz, J = 7.9 Hz, 1H), 7.86 (t, J = 7.8 Hz,
1H), 7.81 (dd, J = 1.8 Hz, J = 8.2 Hz, 1H), 7.56 (m, 1H), 7.29 (m,
1H); 13C NMR (100 MHz, CDCl3) δ 190.8, 156.7, 155.9, 149.1,
137.7, 136.9, 123.8, 121.5, 121.3, 120.5, 104.0. Data was consistent
with that reported in the literature.10
2,2′-Biquinoline (2r). The product was isolated as a white solid in
72% yield (461 mg): mp =193−195 °C; Rf = 0.66 (petroleum ether/
1
ethyl acetate = 3:1); H NMR (400 MHz, CDCl3) δ 8.81 (d, J = 8.6
Hz, 2H), 8.28 (d, J = 8.6 Hz, 2H), 8.20 (d, J = 8.3 Hz, 2H), 7.83 (d, J
= 8.1 Hz, 2H), 7.70 (t, J = 7.2 Hz, 2H), 7.52 (t, J = 7.4 Hz, 2H); 13C
NMR (100 MHz, CDCl3) δ 150.0, 146.5, 136.8, 130.2, 127.7, 127.1,
125.8, 122.4, 121.6. Data was consistent with that reported in the
literature.17
4-Methoxy-6′-methyl-2,2′-bipyridine (3h). The product was
isolated as a pale yellow solid in 68% yield (408 mg): mp = 45−46 °C;
1
Rf = 0.3 (petroleum ether/ethyl acetate = 5:1); H NMR (400 MHz,
CDCl3) δ 8.47 (d, J = 5.6 Hz, 1H), 8.16 (d, J = 7.8 Hz, 1H), 7.97 (d, J
= 2.5 Hz, 1H), 7.67 (t, J = 7.7 Hz, 1H), 7.15 (d, J = 7.6 Hz; 1H), 6.81
(m, 1H), 3.92 (s, 3H), 2.62 (s, 3H); 13C NMR (100 MHz, CDCl3) δ
163.5, 153.6, 153.5, 149.5, 139.3, 137.3, 133.1, 120.5, 113.3, 110.6,
53.2, 18.3; MS (ESI) [M + H]+ (m/z, 201). Anal. Calcd for
C12H12N2O: C, 71.98; H, 6.04; N, 13.99. Found: C, 72.02; H, 6.08; N,
13.92.
2,2′-Biquinoxaline (2s). The product was isolated as a pale yellow
solid in 65% yield (445 mg): mp 224−225 °C; Rf = 0.55 (petroleum
1
ether/ethyl acetate = 3:1); H NMR (400 MHz, CDCl3) δ 10.14 (s,
2H), 8.28 (m, 2H), 8.23 (m, 2H), 7.87 (m, 4H); 13C NMR (100 MHz,
CDCl3) δ 148.6, 144.3, 142.8, 141.7, 130.9, 130.6, 130.0, 129.4. Data
was consistent with that reported in the literature.27
N-(2,2′-Bipyridin-4-yl)acetamide (3i). The product was isolated
2,2′-Bipyridine-6-carbonitrile (3a). The product was isolated as
a white solid in 81% yield (670 mg): mp = 125−126 °C; Rf = 0.66
(petroleum ether/ethyl acetate = 3:1); IR (cm−1, KBr) 2234; 1H NMR
(400 MHz, CDCl3) δ 8.68 (d, J = 8.1 Hz, 2H), 8.47 (d, J = 8.0 Hz,
1H), 7.96 (t, J = 7.9 Hz, 1H), 7.87 (m, 1H), 7.71 (d, J = 7.6 Hz, 1H),
7.39 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 157.7, 154.0, 149.3,
137.9, 137.2, 133.2, 128.1, 124.8, 124.2, 121.6, 117.4. Data was
consistent with that reported in the literature.28
6-Methoxy-2,2′-bipyridine (3b). The product was isolated as a
colorless oil in 85% yield (474 mg): Rf = 0.67 (petroleum ether/ethyl
acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ 8.64 (s, 1H), 8.39 (d, J
= 8.0 Hz, 1H), 8.02 (d, J = 7.4 Hz, 1H), 7.77 (m, 1H), 7.68 (t, J = 8.0
Hz, 1H), 7.25 (m, 1H), 6.77 (d, J = 8.1 Hz, 1H), 4.03 (s, 3H); 13C
NMR (100 MHz, CDCl3) δ 163.5, 156.0, 153.3, 149.0, 139.4, 136.9,
123.5, 121.1, 113.8, 111.1, 53.2. Data was consistent with that reported
in the literature.29
6-Methoxy-6′-methyl-2,2′-bipyridine (3c). The product was
isolated as a white solid in 75% yield (450 mg): mp = 56−57 °C; Rf =
0.89 (petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz,
CDCl3) δ 8.19 (d, J = 7.8 Hz, 1H), 8.04 (d, J = 7.4 Hz, 1H), 7.67 (m,
2H), 7.13 (d, J = 7.6 Hz, 1H), 6.75 (d, J = 8.2 Hz, 1H), 4.0 (s, 3H),
2.62 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 163.5, 157.8, 155.5,
153.8, 139.3, 136.9, 123.1, 118.0, 113.7, 110.8, 53.2, 24.6. Data was
consistent with that reported in the literature.29
6′-Methoxy-5-methyl-2,2′-bipyridine (3d). The product was
isolated as a white solid in 73% yield (438 mg): mp = 45−46 °C; Rf =
0.62 (petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz,
CDCl3) δ 8.47 (s, 1H), 8.27 (d, J = 8.1 Hz, 1H), 7.97 (d, J = 7.4 Hz,
1H), 7.66 (t, J = 7.9 Hz, 1H), 7.56 (dd, J = 8.1 Hz, J = 1.4 Hz, 1H),
6.73 (d, J = 8.2 Hz, 1H), 4.02 (s, 3H), 2.35 (s, 3H); 13C NMR (100
MHz, CDCl3) δ 157.7, 155.9, 149.3, 138.9, 123.0, 122.4, 121.7, 120.9,
118.1, 117.4, 55.6, 24.6. Data was consistent with that reported in the
literature.30
as a pale yellow solid in 72% yield (460 mg): mp = 178−179 °C; Rf =
1
0.60 (petroleum ether/ethyl acetate = 5:1); IR (cm−1, KBr) 1675; H
NMR (400 MHz, CDCl3) δ 11.52 (s, 1H), 8.95 (d, J = 1.3 Hz, 1H),
8.68 (t, J = 7.0 Hz, 1H), 8.33 (dd, J = 1.5 Hz, J = 6.7 Hz, 1H), 8.24 (d,
J = 10.8 Hz, 1H), 8.01 (m, 1H), 7.90 (m, 1H), 7.46 (m, 1H), 2.35 (s,
3H); 13C NMR (100 MHz, CDCl3) δ 171.9, 154.7, 149.9, 148.1,
146.1, 142.5, 130.8, 126.8, 114.3, 111.5, 104.8, 24.9. Data was
consistent with that reported in the literature.32
5-(Benzyloxy)-2,2′-bipyridine (3j). The product was isolated as a
pale yellow solid in 82% yield (644 mg): mp = 84−85 °C; Rf = 0.45
(petroleum ether/ethyl acetate = 5:1); 1H NMR (400 MHz, CDCl3) δ
8.67 (d, J = 4.8 Hz, 2H), 8.36 (d, J = 7.7 Hz, 2H), 7.81 (m, 2H), 7.26
(m, 1H), 7.17 (m, 2H), 6.74 (m, 1H), 6.65 (d, J = 7.7 Hz, 2H), 4.41
(s, 2H); 13C NMR (125 MHz, CDCl3) δ 155.8, 154.9, 149.1, 147.7,
143.2, 139.9, 137.1, 135.3, 128.7, 127.8, 126.6, 123.8, 121.4, 121.1,
73.7; MS (ESI) [M + H]+ (m/z, 263). Anal. Calcd for C17H14N2O: C,
77.84; H, 5.38; N, 10.68. Found: C, 77.88; H, 5.40; N, 10.65.
2,2′-Bipyridin-5-yl(phenyl)methanol (3k). The product was
isolated as a pale yellow solid in 67% yield (527 mg): mp = 97−98 °C;
Rf = 0.38 (petroleum ether/ethyl acetate = 3:1); 1H NMR (400 MHz,
CDCl3) δ 8.69 (m, 2H), 8.38 (m, 2H), 7.82 (m, 2H), 7.65 (dd, J = 1.5
Hz, J = 7.7 Hz, 1H); 7.31 (m, 5H), 6.32 (s, 1H), 2.90 (br s, 1H); 13C
NMR (125 MHz, CDCl3) δ 156.0, 155.3, 149.2, 137.6, 137.0, 128.8,
128.4, 127.6, 123.8, 123.0, 122.0, 121.7, 121.0, 120.4, 70.4; MS (ESI)
[M + H]+ (m/z, 263). Anal. Calcd for C17H 14N2O: C, 77.84; H, 5.38;
N, 10.68. Found: C, 77.81; H, 5.41; N, 10.66.
N-(2,2′-Bipyridin-5-ylmethyl)aniline (3l). The product was
isolated as a pale yellow solid in 68% yield (506 mg): mp = 77−78
1
°C; Rf = 0.15 (petroleum ether/ethyl acetate = 5:1); H NMR (400
MHz, CDCl3) δ 8.67 (s, 2H), 8.37 (t, J = 7.8 Hz, 2H), 7.80 (m, 2H),
7.20 (m, 1H), 7.18 (m, 2H), 6.74 (t, J = 7.3 Hz, 1H), 6.66 (d, J = 7.9
Hz, 2H), 4.42 (s, 2H), 4.09 (br s, 1H); 13C NMR (100 MHz, CDCl3)
δ 156.0, 155.4, 149.2, 148.5, 147.7, 136.9, 136.1, 135.1, 129.4, 123.7,
121.1, 121.0, 118.1, 113.1, 45.8; MS (ESI) [M + H]+ (m/z, 262). Anal.
Calcd for C17H15N3: C, 78.13; H, 5.79; N, 16.08 Found: C, 78.06; H,
5.72; N, 16.02.
N,N-Diethyl-5′-methyl-2,2′-bipyridin-6-amine (3e). The prod-
uct was isolated as a light yellow oil in 84% yield (607 mg): Rf = 0.24
(petroleum ether/ethyl acetate = 2:1); 1H NMR (400 MHz, CDCl3) δ
8.45 (m, 1H), 8.27 (J = 8.1 Hz, 1H), 7.60 (d, J = 7.3 Hz, 1H), 7.53 (m,
2H), 6.47 (d, J = 8.3 Hz, 1H), 3.58 (q, J = 7.0 Hz, 4H), 2.34 (s, 3H),
1.22 (t, J = 7.1 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 156.9, 154.9,
154.1, 149.1, 137.9, 137.1, 132.5, 120.4, 107.8, 105.5, 42.6, 18.2, 13.0;
MS (ESI) [M + H]+ (m/z, 242). Anal. Calcd for C15H19N3: C, 74.65;
H, 7.94; N, 17.41. Found: C, 74.62; H, 7.97; N, 17.38.
2-(2,2′-Bipyridin-5-yl)acetonitrile (3m). The product was
isolated as a pale yellow solid in 78% yield (456 mg): mp = 87−88
°C; Rf = 0.18 (petroleum ether/ethyl acetate = 5:1); IR (cm−1, KBr)
2249; 1H NMR (400 MHz, CDCl3) δ 8.67 (m, 2H), 8.42 (dd, J = 2.0
Hz, J = 8.2 Hz, 2H), 7.83 (m, 2H), 7.34 (m, 1H), 3.84 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ 163.5, 155.9, 153.6, 148.9, 147.8, 139.33,
E
dx.doi.org/10.1021/jo402084m | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
139.32, 124.5, 121.7, 113.9, 110.8, 21.2; MS (ESI) [M + H]+ (m/z,
196). Anal. Calcd for C12H9N3: C, 73.83; H, 4.65; N, 21.52. Found: C,
73.80; H, 4.68; N, 21.47.
2,2′-Bipyridin-5-ylmethanamine (3n). The product was isolated
as a pale yellow solid in 75% yield (405 mg): mp = 68−69 °C; Rf =
0.10 (petroleum ether/ethyl acetate = 3:1); 1H NMR (400 MHz,
CDCl3) δ 8.63 (s, 1H), 8.54 (s, 1H), 8.27 (dd, J = 2.0 Hz, J = 8.0 Hz,
2H), 7.78 (m, 2H), 7.30 (m, 1H), 4.70 (s, 2H), 3.53 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ 155.9, 155.1, 149.1, 147.9, 137.1, 136.7,
135.8, 123.8, 121.3, 121.1, 62.3. Data was consistent with that reported
in the literature.31
S.; Kira, M. J. Organomet. Chem. 2000, 612, 117. (e) Lin, G.; Hong, R.
J. Org. Chem. 2001, 66, 2877.
́
(9) (a) De Franca
Nedel
E.; Polissaint, F.; Ned
̧
, K. W. R.; Navarro, M.; Leo
ec, J.-Y. J. Org. Chem. 2002, 67, 1838. (b) Sengmany, S.; Leo
́
nel, E.; Durandetti, M.;
́
́
́
nel,
́
́
elec, J.-Y.; Pipelier, M.; Thobie-Gautier, C.;
Dubreuil, D. J. Org. Chem. 2007, 72, 5631. (c) Gosmini, C.; Bassene-
Ernst, C.; Durandetti, M. Tetrahedron 2009, 65, 6141 and references
cited therein.
(10) Petzold, H.; Heider, S. Eur. J. Inorg. Chem. 2011, 1249.
(11) (a) Durandetti, M.; Devaud, M.; Perichon, J. New J. Chem. 1996,
20, 659. (b) Jutand, A.; Mosleh, A. J. Org. Chem. 1997, 62, 261.
Caerulomycin F. The product was isolated as a pale yellow solid in
(12) For the reactions using Zn−LiCl, see: Samann, C.; Schade, M.
̈
74% yield (480 mg): mp = 63−64 °C; Rf = 0.10 (petroleum ether/
A.; Yamada, S.; Knochel, P. Angew. Chem., Int. Ed. 2013, 52, 9495 and
references cited therein.
1
ethyl acetate = 3:1); H NMR (400 MHz, CDCl3) δ 8.67 (d, J = 4.6
Hz, 1H), 8.39 (d, J = 8.0 Hz, 1H), 7.89 (s, 1H), 7.88−7.79 (m, 1H),
7.33−7.30 (m, 1H), 6.77 (d, J = 2.0 Hz, 1H), 4.77 (s, 2H), 4.14 (br s,
1H), 3.94 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 167.3, 160.9,
156.5, 155.5, 148.9, 136.9, 123.9, 121.3, 106.6, 105.3, 64.3, 55.3. Data
was consistent with that reported in the literature.14
(13) The couplings of 2-halopyridine could also be promoted by
Ni(0) in the absence of ligands when a stoichiometric amount of
Ni(0) was applied, and thus, the couplings described in this note were
triggered by this coupling to initiate the formation of the bipyridine;
see: Rode, T.; Breitmaier, E. Synthesis 1987, 574.
(14) Fu, P.; Wang, S.; Hong, K.; Li, X.; Liu, P.; Wang, Y.; Zhu, W. J.
Nat. Prod. 2011, 74, 1751.
(15) Marker, A.; Canty, A. J.; Brownlee, R. T. C. Aust. J. Chem. 1978,
31, 1255.
(16) Sase, W. H. F.; Whittle, C. P. J. Chem. Soc. 1961, 1347.
(17) Verniest, G.; Wang, X.; De Kimpe, N.; Padwa, A. J. Org. Chem.
2010, 75, 424.
ASSOCIATED CONTENT
■
S
* Supporting Information
General information, optimization experiments, and copies of
1H and 13C NMR spectra for all products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(18) Muldoon, J.; Ashcroft, A. E.; Wilson, A. J. Chem.Eur. J. 2010,
AUTHOR INFORMATION
Corresponding Author
*E-mail: xinfangduan@vip.163.com.
Notes
16, 100.
■
(19) Veauthier, J. M.; Carlson, C. N.; Collis, G. E.; Kiplinger, J. L.;
John, K. D. Synthesis 2005, 2683.
(20) Baxter, P. N. W.; Connor, J. A.; Schweizer, W. B.; Wallis, J. D. J.
Chem. Soc., Dalton Trans. 1992, 3015.
(21) Fukuda, Y.; Seto, S.; Furuta, H.; Ebisu, H.; Oomori, Y.;
Terashima, S. J. Med. Chem. 2001, 44, 1396.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(22) Beer, P. D.; Fletcher, N. C. Polyhedron 1996, 15, 1339.
We gratefully acknowledge the National Science Foundation of
China (21072022, 21242006, 21372031) and the Specialized
Research Fund for the Doctoral Program of Higher Education
(20100003110010).
(23) Munavalli, S.; Gratzel, M. Chem. Ind. 1987, 20, 722.
̈
(24) Ishikawa, S.; Hamada, T.; Manabe, K.; Kobayashi, S. Synthesis
2005, 2176.
(25) Oae, S.; Kawai, T.; Furukawa, N. Phosphorus Sulfur 1987, 34,
123.
(26) Lindner, E.; Veigel, R.; Ortner, K.; Nachtigal, C.; Steimann, M.
Eur. J. Inorg. Chem. 2000, 959.
(27) Seggio, A.; Chevallier, F.; Vaultier, M.; Mongin, F. J. Org. Chem.
2007, 72, 6602.
(28) Ishizuka, T.; Sinks, L. E.; Song, K.; Hung, S.-T.; Nayak, A.;
Clays, K.; Therien, M. J. J. Am. Chem. Soc. 2011, 133, 2884.
(29) Kim, S.-H.; Rieke, R. D. Tetrahedron 2010, 66, 3135.
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