Cobalt-catalyzed direct C-C bond formation between tetrahydrofuran and alkynes

Article abstract of DOI:10.1016/j.tetlet.2014.09.042

We aimed to describe an efficient CoCl₂-catalyzed direct C-C bond formation of tetrahydrofuran (THF) with various alkynes in the presence of tert-butyl hydroperoxide and catalytic amount of acid to obtain vinyl-substituted THFs. Mono- and di-su

Full text of DOI:10.1016/j.tetlet.2014.09.042

Tetrahedron Letters 55 (2014) 6096–6100
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Cobalt-catalyzed direct C–C bond formation between
tetrahydrofuran and alkynes
Li Chen a, Jiajia Yang a, Lin Li b, Zhiqiang Weng a, Qiang Kang b,
⇑
a College of Chemistry and Chemical Engineering, Fuzhou University, 350108, PR China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2014
Revised 31 August 2014
Accepted 9 September 2014
Available online 16 September 2014
We aimed to describe an efficient CoCl2-catalyzed direct C–C bond formation of tetrahydrofuran (THF)
with various alkynes in the presence of tert-butyl hydroperoxide and catalytic amount of acid to obtain
vinyl-substituted THFs. Mono- and di-substituted alkynes were suitable for this transformation.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
C–C bond formation
Hydroalkylation
Tetrahydrofuran derivatives
Alkyl-substituted tetrahydrofuran (THF) moieties are key struc-
tures that exist extensively in biological and natural products.1
Selected examples of such structures are shown in Figure 1. Syn-
thesizing alkyl-substituted THFs is popular in the organic synthetic
community. The strategies of intramolecular cyclization and tan-
dem addition–cyclization are usually applied to construct THF
derivatives,2 and such a construction requires tedious processes.
The direct functionalization of THF via sp3 C–H bond activation3
improves accessibility of THF derivatives. However, information
on this transformation is limited because of the lack of reactivity
of THF.4 Recently, Li and Zhang found that THF can be coupled
directly with terminal alkynes using microwave assistance.5a Liu
et al. reported that tert-butyl hydroperoxide (TBHP) mediates the
coupling reaction of THF with phenyl acetylene.5b Zhang and co-
workers reported that copper catalyzes the hydroalkylation of
alkynes with THF.5c Lu and Tusun demonstrated that dirhodium
caprolactamate catalyzes the alkoxyalkylation of terminal alkynes
with THF in the presence of TBHP.5d However, these systems suffer
from high temperature, expensive cost of transition metals, and
substrate limitation (only terminal alkynes are employed in these
reactions). To determine greener synthetic methods, we focused
on the direct functionalization of THF under mild conditions. In
the present Letter, we reported the hydroalkylation of various
alkynes such as mono- and di-substituted alkynes with THF in
the presence of CoCl2 as catalyst.6
Initially, methyl propiolate (1a) was selected as the model sub-
strate to investigate the reaction conditions. In the presence of
10 mol % CoCl2 and 50 mol % TBHP, the reaction of 1a (from old
bottle without distillation) in THF at 50 °C efficiently provided a
separable mixture of the E/Z isomers of allyl ether 3a, and the total
yield was 30%. However, we failed to obtain the desired products
using freshly distilled 1a (Table 1, entry 1). A careful examination
of substrate 1a suggested that the old bottle of methyl propiolate
contained a small quantity of propiolic acid. This unexpected find-
ing revealed that acid might serve as an important function in the
reaction. Consequently, a catalytic amount of acid was added to
optimize the reaction conditions. After the supplementation of
20 mol % of 4-methylbenzoic acid as additive (Table 1, entry 2),
the reaction occurred, and we obtained a total of 55% 3a yield with
an E/Z ratio of 1:0.9. The yield of the desired product slightly
decreased when the amounts of acid (50 and 100 mol %, Table 1;
entries 3 and 4, respectively), TBHP (75 mol %, entry 5, Table 1),
and catalyst loading (20 mol % CoCl2, entry 6, Table 1) increased.
The reaction produced a trace amount of the desired product in
the absence of CoCl2 and TBHP (Table 1, entries 6 and 7). A total
yield of 58% was obtained for the desired product with an E/Z ratio
of 1:0.7 when the reaction time was extended to 5 h (entry 9,
Table 1). Several commercially available cobalt catalysts, such as
Co(OAc)2, Co(OAc)2.4H2O, CoBr2, Co(acac)2, and Co(acac)3, were
tested, and the reaction provided comparable yields (Table 1,
entries 10–14). CoCl2 produced the best results. Thus, these condi-
tions were selected for further analyses (Table 1, entry 9).
Different acids were used to investigate the reaction of methyl
propiolate (1a) and THF. The results are summarized in Table 2.
⇑
Corresponding author. Tel.: +86 591 83707237.
E-mail address: kangq@fjirsm.ac.cn (Q. Kang).
http://dx.doi.org/10.1016/j.tetlet.2014.09.042
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

L. Chen et al. / Tetrahedron Letters 55 (2014) 6096–6100
6097
O
O
O
O
H
N
O
O
O
O
O
O
O
O
NMe2
Nemorensine
Pamamycin 607
O
AcO
O
O
O
O
O
O
H
H
Br
trans-Kumausyne
Amphidinolide
Figure 1. Representative natural products containing THF moieties.
Table 1
Table 2
Investigation of reaction conditionsa
Investigation of acidsa
CoCl2 (10 mol%)
TBHP (50 mol%)
CoCl2 (10 mol%)
TBHP (0.5 eq)
O
MeO2C
MeO2C
O
O
CO2Me
CO2Me
+
+
O
Acid (20 mol%), 50oC
P-Tolulic acid (x eq)
50oC
3a
1a
2a
3a
1a
2a
Entry
Acid (20 mol %)
Yieldb
Ratio of Z/Ec
Entry Catalyst
TBHP (x
mol %)
P-toluic acid Time
Yieldb Ratio
(10 mol %)
(x mol %)
(h)
of Z/Ec
1
2
3
4
5
6
7
8
9
P-Aminobenzoic acid
Ferrocenecarboxylic acid
4-Nitrophenylacetic acid
Chloropicolinic acid
Trifluoroacetic acid
2-Methoxybenzoic acid
2-Iodoxybenzoic acid
P-Toluic acid
Trace
Trace
Trace
8%
38%
58%
55%
58%
34%
n.d.
n.d.
n.d.
0.8:1
0.7:1
0.7:1
0.7:1
0.7:1
0.7:1
1
2
3
4
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
—
50 mol %
—
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
5
—
—
50 mol % 0.2
50 mol % 0.5
50 mol % 1.0
75 mol % 0.2
50 mol % 0.2
50 mol % 1.0
55%
40%
38%
48%
51%
Trace
Trace
58%
50%
52%
45%
39%
49%
0.9:1
0.8:1
0.9:1
0.7:1
0.9:1
—
5
6d
7
L
-Proline
8
9
CoCl2
CoCl2
Co(OAc)2
—
1.0
—
10
11d
58%
67%
0.7:1
0.8:1
50 mol % 0.2
50 mol % 0.2
0.7:1
0.8:1
0.7:1
0.6:1
0.7:1
0.7:1
L
-Glutamic acid
10
11
12
13
14
5
5
5
5
1-Adamantanecarboxylic acid
Co(OAc)2ꢀ4H2O 50 mol % 0.2
a
Reaction conditions: 1a (1 mmol), 10 mol % of CoCl2, and 50 mol % of TBHP in
THF (3 mL).
CoBr2
50 mol % 0.2
50 mol % 0.2
50 mol % 0.2
Co(acac)2
Co(acac)3
The yields were determined by 1H NMR using ethyl-4-bromobenzoate as an
b
5
internal standard.
a
E/Z ratios were determined by the 1H NMR analysis of crude reaction mixtures.
c
Reaction conditions: 1a (1 mmol) in THF (3 mL).
The yields were determined with 1H NMR using ethyl-4-bromobenzoate as the
b
d
The reaction was performed at 70 °C.
internal standard.
E/Z ratios were determined with the 1H NMR analysis of crude reaction
c
mixtures.
such as ester and ketone (Table 3, entries 1–3), performed well
in the reaction and provided an access to the functionalized 2-vinyl
THF 3a–3c. The di-substituted alkynes with ester groups were
more reactive and provided moderate yields of the preferred prod-
ucts: 3d and 3f (Table 3, entries 4 and 5, respectively). A significant
decrease in the yield was observed when phenyl acetylene (1g)
was used as the substrate under the optimal reaction conditions.
Re-examination of the reaction conditions showed that the reac-
tion could yield a total of 59% 3g with an E/Z ratio of 1.4:1 using
two equivalents of TBHP (Table 3, entry 6). Based on this finding,
we examined several aryl acetylenes. Generally, the alkynes with
an electron-withdrawing group (Cl, Br, or F) or electron-donating
group (Me or MeO) on the para site of phenyl rings produced the
corresponding products in moderate yields (Table 3, entries 7–
11). The ortho-substituted aryl acetylenes performed well in the
reaction and provided the desired products (Table 3, entries 12–
14). The terminal alkyne with heterocyclic substitution, such as
2-ethynylthiophene, was less reactive than aryl acetylenes and
produced a total of 38% yield (Table 3, entry 15). The internal
alkynes such as 1-phenyl-1-propyne (1p) and ethyl 3-phenylpropi-
olate (1q) were also tolerated in the reaction conditions to afford
the desired products in 31% and 43% yields, respectively (Table 3,
d
20 mol % of CoCl2 was used.
p-Aminobenzoic acid, ferrocenecarboxylic acid, and 4-nitrophenyl-
acetic acid failed to produce the desired product (Table 2, entries
1–3), whereas chloropicolinic acid produced only 8% yield (Table 2,
entry 4). Substituting benzoic acids, including 2-methoxybenzoic
acid, 2-iodoxybenzoic acid, and p-toluic acid, produced comparable
results (Table 2, entries 6–8). Two natural amino acids were also
used in the reaction and produced moderate yields (Table 2, entries
9 and 10). The best result was obtained using 20 mol % of 1-ada-
mantanecarboxylic acid at an elevated reaction temperature of
70 °C (Table 2, entry 11). Ligands and peroxides failed to improve
the yield of 3a (detailed information are listed in Supporting infor-
mation). The optimization studies indicated that the best condi-
tions were as follows: reaction of 1a and THF with 10 mol % of
CoCl2, 20 mol % of 1-adamantanecarboxylic acid, and 50 mol % of
TBHP at 70 °C.
Various alkynes were reacted with THF using the obtained opti-
mization conditions to examine the generality of the current
method. The terminal alkynes with electron-withdrawing groups,

6098
L. Chen et al. / Tetrahedron Letters 55 (2014) 6096–6100
Table 3
Substrate scope of the cobalt-catalyzed hydroalkylation of THF with alkynesa
CoCl2 (10 mol%)
TBHP (x eq.)
R2
O
R2
R1
O
+
R1
1-Adamantanecarboxylic acid
(20 mol%), 70oC
1
2a
3
Entry
1
Alkyne
CO2Me
TBHP (x equiv)
Time (h)
5
Product
Yieldb (%)
67
Ratio of Z/Ec
MeO2C
EtO2C
O
O
0.5
0.5
0.8:1
1a
3a
3b
CO2Et
O
2
3
5
5
61
50
1:1
1b
O
O
0.5
0.5
0.5
0.6:1
1c
3c
CO2Me
CO2Et
CO2Me
O
MeO2C
EtO2C
MeO2C
EtO2C
4
5
2
2
53
57
0.8:1
0.8:1
1d
3d
CO2Et
O
1e
3e
O
6
7
2
2
5
5
59
60
1.4:1
1.4:1
1f
3f
O
Me
Me
1g
3g
3h
O
8
2
2
2
5
5
5
38
55
50
1.4:1
1.9:1
1.2:1
MeO
1h
MeO
Cl
O
9
Cl
Br
1i
1j
3i
O
10
Br
F
3j
O
11
12
2
2
5
5
68
56
1.5:1
1.1:1
F
1k
3k
O
Me
Cl
Me
Cl
1l
3l
O
13
2
5
49
2.0:1
1m
3m

L. Chen et al. / Tetrahedron Letters 55 (2014) 6096–6100
6099
Table 3 (continued)
Entry
Alkyne
TBHP (x equiv)
Time (h)
Product
Yieldb (%)
Ratio of Z/Ec
O
14
2
5
52
1.6:1
F
3n
3o
F
1n
O
15
16
2
2
5
5
38
31
0.8:1
0.8:1
S
1o
S
O
1p
3p
EtO2C
CO2Et
O
17
2
5
43
0.5:1
1q
3q
a
Reaction conditions: 1a (1 mmol), 10 mol % of CoCl2, and 20 mol % of 1-adamantanecarboxylic acid in THF (3 mL).
Isolated yields.
b
c
E/Z ratios were determined with the 1H NMR analysis of crude reaction mixtures.
entries 16 and 17). However, the reactions of alkyl substituted
alkynes such as 1-pentyne and 4-octyne failed to deliver the corre-
sponding products.
tuted alkynes were tolerated under these reaction conditions,
and the desired products were produced. Preliminary mechanistic
studies suggested that this transformation possibly occurs through
a radical pathway. Further investigations on the reaction mecha-
nism and other applications of this catalytic system are currently
ongoing in our laboratory.
Subsequently, we examined the reciprocal transformation
between Z and E isomers by conducting control experiments. (E)-
3a and (Z)-3a were introduced separately to the optimal reaction
conditions (Scheme 1). No mutual transformations were observed
between the Z/E isomers for both reactions. However, the recovery
yields were low because starting materials were further trans-
formed to other unidentified products.
Currently, the exact mechanism of this reaction remains ambig-
uous. Previous reports showed that peroxides can be used as oxi-
dant reagents to generate radical species7. Therefore, a free
radical process is possibly involved in the reaction. Radical-trap-
ping experiment was conducted to support the radical pathway
of this transformation (Eq. (1)). When one equivalent of (2,2,6,6-
tetramethylpiperidin-1-yl)oxidanyl was used as the radical-trap-
ping reagent, the reaction failed to produce the desired product
Acknowledgments
We thank the National Natural Science Foundation of China
(21302183) and the ‘Strategic Priority Research Program’ of the
Chinese Academy of Sciences (Grant No. XDA09030102) for gener-
ous financial support.
Supplementary data
Supplementary data (detailed experimental procedures and
spectroscopic data for all new compounds) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.09.042.
3a. Thus, this reaction is probably preceded by a radical pathway
8
catalyzed by CoCl2
.
CO2Me
MeO2C
O
CoCl2 (10 mol%)
TBHP (50 mol%)
References and notes
TEMPO
+
ð1Þ
1. (a) McCann, P. A.; Pogell, B. M. J. Antibiot. 1979, 32, 673; (b) Kondo, S.; Yasui, K.;
Katayama, M.; Marumo, S.; Kondo, T.; Hattori, H. Tetrahedron Lett. 1987, 28,
5861; (c) Dillon, M. P.; Lee, N. C.; Stappenbeck, F.; White, J. D. J. Chem. Soc., Chem.
Commun. 1995, 1645; (d) Chakraborty, T. K.; Das, S. Curr. Med. Chem. Anti-Cancer
Agents 2001, 1, 131; (e) Suzuki, T.; Koizumi, K.; Suzuki, M.; Kurosawa, E. Chem.
Lett. 1983, 1643.
2. selected examples: (a) Wolfe, J. P.; Rossi, M. A. J. Am. Chem. Soc. 2004, 126, 1620;
(b) Qian, H.; Han, X.; Widenhoefer, R. A. J. Am. Chem. Soc. 2004, 126, 9536; (c)
Yao, T.; Zhang, X.; Larock, R. C. J. Am. Chem. Soc. 2004, 126, 11164; (d) Sromek, A.
W.; Kel’in, A. V.; Gevorgyan, V. Angew. Chem., Int. Ed. 2004, 43, 2280; (e)
Antoniotti, S.; Genin, E.; Michelet, V.; Genêt, J. P. J. Am. Chem. Soc. 2005, 127,
9976; (f) Jung, H. H.; Floreancig, P. E. Org. Lett. 1949, 2006, 8; (g) Xu, T.; Ko, H. M.;
Savage, N. A.; Dong, G. B. J. Am. Chem. Soc. 2012, 134, 20005; (h) Xu, T.; Dong, G.
B. Angew. Chem., Int. Ed. 2012, 51, 7567; (i) Xu, T.; Savage, N. A.; Dong, G. B.
Angew. Chem., Int. Ed. 2014, 53, 1891.
1-Adamantanecarboxylic acid (20 mol%)
70 oC, 5 h
O
We demonstrated that CoCl2 catalyzed the hydroalkylation of
THF with various alkynes in the presence of a catalytic amount of
1-adamantanecarboxylic acid as additive. Mono- and di-substi-
CoCl2 (10 mol%)
MeO2C
O
3a
59% of
recovered
TBHP (50 mol%), THF
1-Adamantanecarboxylic acid
(20 mol%)
3a
(E)-
70 oC, 5 h
3. (a) Yoo, W. J.; Li, C. J. In C-H Activation; Yu, J. Q., Shi, Z., Eds.; Springer: Berlin,
2010; Vol. 292, p 281; (b) Li, C. J. Acc. Chem. Res. 2009, 42, 335.
as above
40% of 3a
recovered
4. (a) Xiang, J.; Fuchs, P. L. J. Am. Chem. Soc. 1996, 118, 11986; (b) Clark, A. J.; Rooke,
S.; Sparey, T. J.; Taylor, P. C. Tetrahedron Lett. 1996, 37, 909; (c) Xiang, J.; Jiang,
W.; Gong, J.; Fuchs, P. L. J. Am. Chem. Soc. 1997, 119, 4123; (d) Inoue, A.;
Shinokubo, H.; Oshima, K. Synlett 1999, 1582; (e) Davies, H. M. L.; Hansen, T.;
Churchill, M. R. J. Am. Chem. Soc. 2000, 122, 3063; (f) Díaz-Requejo, M. M.;
O
MeO2C
3a
(Z)-
Scheme 1. Investigation of the reciprocal transformation between Z and E isomers.

6100
L. Chen et al. / Tetrahedron Letters 55 (2014) 6096–6100
Belderraín, T. R.; Nicasio, M. C.; Trofimenko, S.; Pérez, P. J. J. Am. Chem. Soc. 2002,
H. Chem. Eur. J. 2008, 14, 10876; (h) Hess, W.; Treutwein, J.; Hilt, G. Synthesis
2008, 3537; (i) Cahiez, C.; Moyeux, A. Chem. Rev. 2010, 110, 1435.
124, 896; (g) Hirano, K.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 2002, 43, 3617;
(h) Jang, Y. J.; Shih, Y. K.; Liu, J. Y.; Kuo, W. Y.; Yao, C. F. Chem. Eur. J. 2003, 9,
2123; (i) Yamada, K.; Yamamoto, Y.; Tomioka, K. Org. Lett. 2003, 5, 1797; (j)
Albone, D. P.; Challenger, S.; Derrick, A. M.; Fillery, S. M.; Irwin, J. L.; Parsons, C.
M.; Takada, H.; Taylor, P. C.; Wilson, D. J. Org. Biomol. Chem. 2005, 3, 107; (k)
Fructos, M. R.; Trofimenko, S.; Díaz-Requejo, M. M.; Pérez, P. J. J. Am. Chem. Soc.
2006, 128, 11784; (l) He, L.; Yu, J.; Zhang, J.; Yu, X. Q. Org. Lett. 2007, 9, 2277; (m)
Cao, K.; Jiang, Y. J.; Zhang, S. Y.; Fan, C. A.; Tu, Y. Q.; Pan, Y. J. Tetrahedron Lett.
2008, 49, 4652; (n) Chen, Z.; Zhang, Y. X.; An, Y.; Song, X. L.; Wang, Y. H.; Zhu, L.
L.; Guo, L. Eur. J. Org. Chem. 2009, 5146; (o) Sun, H.; Zhang, Y.; Guo, F.; Zha, Z.;
Wang, Z. J. Org. Chem. 2012, 77, 3563; (p) Singh, P.; Gudup, S.; Aruri, H.; Singh,
U.; Ambala, S.; Yadav, M.; Sawant, S. D.; Vishwakarma, R. A. Org. Biomol. Chem.
2012, 10, 1587; (q) Tortoreto, C.; Achard, T.; Zeghida, W.; Austeri, M.; Gune, L.;
Lacour, J. Angew. Chem., Int. Ed. 2012, 51, 5847.
7. (a) Li, Z.; Yu, R.; Li, H. Angew. Chem., Int. Ed. 2008, 47, 7497; (b) Cheng, K.; Huang,
L.; Zhang, Y. Org. Lett. 2009, 11, 2908; (c) Pan, S.; Liu, J.; Li, H.; Wang, Z.; Guo, X.;
Li, Z. Org. Lett. 1932, 2010, 12; (d) Chen, L.; Shi, E.; Liu, Z.; Chen, S.; Wei, W.; Li, H.;
Xu, K.; Wan, X. Chem. Eur. J. 2011, 17, 4085; (e) Liu, D.; Liu, C.; Li, H.; Lei, A. W.
Angew. Chem., Int. Ed. 2013, 52, 4453; (f) Wei, W.; Zhang, C.; Xu, Y.; Wan, X.
Chem. Commun. 2011, 10827; (g) Chen, S.; Xu, Y.; Wan, X. Org. Lett. 2011, 13,
6152; (h) Liu, Z.; Zhang, J.; Chen, S.; Shi, E.; Xu, Y.; Wan, X. Angew. Chem., Int. Ed.
2012, 51, 3231; (i) Shi, E.; Shao, Y.; Chen, S.; Hu, H.; Liu, Z.; Zhang, J.; Wan, X. Org.
Lett. 2012, 14, 3384; (j) Zhang, J.; Jiang, J.; Li, Y.; Zhao, Y.; Wan, X. Org. Lett. 2013,
15, 3222; (k) Zhang, J.; Jiang, J.; Li, Y.; Wan, X. J. Org. Chem. 2013, 78, 11366; (l)
Zhang, C.; Liu, C.; Shao, Y.; Bao, X.; Wan, X. Chem. Eur. J. 2013, 19, 17917.
8. selected recent example: (a) Ikeda, Y.; Nakamura, T.; Yorimitsu, H.; Oshima, K. J.
Am. Chem. Soc. 2002, 124, 6514; (b) Affo, W.; Ohmiya, H.; Fujioka, T.; Ikeda, Y.;
Nakamura, T.; Yorimitsu, H.; Oshima, K.; Imamura, Y.; Mizuta, T.; Miyoshi, K. J.
Am. Chem. Soc. 2006, 128, 8068; (c) Fries, P.; Halter, D.; Kleinschek, A.; Hartung, J.
J. Am. Chem. Soc. 2011, 133, 3906; (d) Lyaskovskyy, V.; Suarez, A. I. O.; Lu, H.;
Jiang, H.; Zhang, X. P.; de Bruin, B. J. Am. Chem. Soc. 2011, 133, 12264; (e) Weiss,
M. E.; Kreis, L. M.; Lauber, A.; Carreira, E. M. Angew. Chem., Int. Ed. 2011, 50,
11125; (f) Kuzmina, O. M.; Steib, A. K.; Markiewicz, J. T.; Flubacher, D.; Knochel,
P. Angew. Chem., Int. Ed. 2013, 52, 4945; (g) Sawano, T.; Ashouri, A.; Nishimura,
T.; Hayashi, T. J. Am. Chem. Soc. 2012, 134, 18936; (h) Mulzer, M.; Whiting, B. T.;
Coates, G. W. J. Am. Chem. Soc. 2013, 135, 10930; (i) Chen, C.; Dugan, T. R.;
Brennessel, W. W.; Weix, D. J.; Holland, P. L. J. Am. Chem. Soc. 2014, 136, 945.
5. (a) Zhang, Y.; Li, C. J. Tetrahedron Lett. 2004, 45, 7581; (b) Liu, Z. Q.; Sun, L.; Wang,
J. G.; Han, J.; Zhao, Y. K.; Zhou, B. Org. Lett. 2009, 11, 1437; (c) Huang, L.; Cheng,
K.; Yao, B.; Zhao, J.; Zhang, Y. Synthesis 2009, 20, 3504; (d) Tusun, X.; Lu, C. D.
Synlett 2013, 1693.
6. Reviews on cobalt-catalyzed reactions: (a) Pattenden, G. Chem. Soc. Rev. 1988,
17, 361; (b) Iqbal, J.; Sanghi, R.; Nandy, J. P. In Radicals in Organic Synthesis;
Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim, 2001; Vol. 1, p 135; (c)
Shinokubo, H.; Oshima, K. Eur. J. Org. Chem. 2004, 2081. and references therein;
(d) Yorimitsu, H.; Oshima, K. Pure Appl. Chem. 2006, 78, 441; (e) Gosmini, C.;
Bégouin, J. M.; Moncomble, A. Chem. Commun. 2008, 3221; (f) Rudolph, A.;
Lautens, M. Angew. Chem., Int. Ed. 2009, 48, 2656; (g) Jeganmohan, M.; Cheng, C.