Cobalt-Catalyzed Cyclization of N-Methoxy Benzamides with Alkynes using an Internal Oxidant through C-H/N-O Bond Activation

Article abstract of DOI:10.1002/chem.201600471

The cyclization of substituted N-methoxy benzamides with alkynes in the presence of an easily affordable cobalt complex and NaOAc provides isoquinolone derivatives in good to excellent yields. The cyclization reaction is compatible with a range of functio

Full text of DOI:10.1002/chem.201600471

DOI: 10.1002/chem.201600471
Communication
&
Synthetic Methods |Hot Paper|
Cobalt-Catalyzed Cyclization of N-Methoxy Benzamides with
Alkynes using an Internal Oxidant through CÀH/NÀO Bond
Activation
Ganesan Sivakumar, Arjun Vijeta, and Masilamani Jeganmohan*[a]
sition metals, high abundance, and low cost.[3,4] It is also be-
Abstract: The cyclization of substituted N-methoxy benza-
mides with alkynes in the presence of an easily affordable
cobalt complex and NaOAc provides isoquinolone deriva-
tives in good to excellent yields. The cyclization reaction is
compatible with a range of functional group-substituted
benzamides, as well as ester- and alcohol-substituted al-
kynes. The cobalt complex [CoIIICp*(OR)2] (R=Me or Ac)
serves as an efficient catalyst for the cyclization reaction.
Later, isoquinolone derivatives were converted into 1-
chloro and 1-bromo substituted isoquinoline derivatives in
excellent yields in the presence of POCl3 or PBr3.
lieved that the catalytic activity of some cobalt complexes
would be similar to rhodium and ruthenium complexes. Very
recently, isoquinoline derivatives were synthesized efficiently
through a cobalt-catalyzed cyclization of aromatic ketoximes
with alkynes, using an internal oxidant.[5]
Isoquinolone is a key structural unit that is present in various
natural products, biologically active molecules, and pharma-
ceuticals.[6] Moreover, isoquinolone derivatives are widely used
as key intermediates in various organic transformations.[6] Sev-
eral methods have been reported for the synthesis of isoquino-
lone derivatives,[7] among which, metal-catalyzed cyclization of
aromatic amides or nitriles with alkynes through chelation-as-
sisted CÀH bond activation is an efficient method to synthesize
isoquinolone derivatives from easily available starting materials
with minimal waste.[8] Rhodium, palladium, and ruthenium
complexes are widely used as catalysts for this type of transfor-
mation. N-Alkyl or aryl benzamides reacted with alkynes in the
presence of a metal catalyst and stoichiometric amount of ex-
ternal oxidant to provide N-alkyl- or aryl-substituted isoquino-
lone derivatives.[8] Subsequently, a similar transformation was
achieved in the reaction of N-alkoxy benzamides with alkynes
in the presence of rhodium or ruthenium catalysts, with the N-
OR group as an internal oxidant.[2]
Transition metal-catalyzed chelation-assisted cyclization of het-
eroatom-substituted aromatics with carbon–carbon p-bonded
components through CÀH bond activation is a powerful
method to synthesize carbocyclic and heterocyclic molecules
in a highly atom- and step-economical manner.[1] Generally,
this type of cyclization is carried out in the presence of an ex-
ternal oxidant, which is used for the regeneration of the active
catalyst. However, this method leads to a stoichiometric
amount of reduced external oxidant as waste. Alternatively,
this type of reaction can also be carried out by using an inter-
nal oxidant, such as an oxidizing directing group.[2] This type of
group plays a dual role in the reaction, in that it acts as a di-
recting group, as well as an oxidant to regenerate the active
catalyst. For this type of cyclization, mainly second and third
row transition metals such as iridium, palladium, rhodium and
ruthenium complexes have been widely used as catalysts. The
use of cheaper, more abundant and environmentally benign
catalysts is still desirable in CÀH bond functionalization reac-
tions. In this context, complexes of cobalt have recently gained
much attention for CÀH bond functionalization reactions,
owing to their low toxicity in comparison with other early tran-
Our continuous interest in the development of versatile CÀH
bond transformations prompted us to explore the possibility
of cyclization of N-alkoxy benzamides with alkynes in the pres-
ence of an inexpensive cobalt(III) catalyst with an internal oxi-
dant.[9] Herein, we report a cobalt-catalyzed cyclization of sub-
stituted N-alkoxy benzamides with alkynes in the presence of
a catalytic amount of NaOAc, giving isoquinolone derivatives
in excellent yields. The catalytic reaction was compatible with
substrates incorporating various sensitive functional groups, in-
cluding I-, Br-, NO2-, and CN-substituted benzamides, as well as
ester- and alcohol-functionalized alkynes.
Treatment of N-methoxy 4-methoxybenzamide (1a) with di-
phenylacetylene (2a) in the presence of [CoCp*(CO)I2]
(10 mol%) and NaOAc (30 mol%) in 2,2,2-trifluoroethanol (TFE)
at 1208C for 24 h gave isoquinolone derivative 3aa in 92%
yield (isolated product; Scheme 1). In the reaction, the N-OMe
group of amide 1a acts as an internal oxidant. Next, the reac-
tion was examined with various N-substituted amides 1b–d
(Scheme 1) under similar reaction conditions. In the reactions
of 1b and 1c with 2a, the expected cyclization product 3aa
was formed in trace and 41% yield, respectively, whereas sub-
strate 1d underwent no reaction. This result clearly reveals
[a] G. Sivakumar, A. Vijeta, Dr. M. Jeganmohan
Department of Chemistry
Indian Institute of Science Education and Research
Pune 411021 (India)
Fax: (+91)20-25865315
E-mail: mjeganmohan@iiserpune.ac.in
Supporting information for this article, including spectroscopic data and
1
copies of H and 13C NMR spectra of all compounds, is available on the
WWW under http://dx.doi.org/10.1002/chem.201600471.
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Table 1. The reaction of benzamides 1 with diphenylacetylene (2a).[a]
Entry
Amide
Product
Yield [%][b]
1
2
3
4
5
6
7
8
9
1e: R2 =Me
3ea: R2 =Me
90
87
90
89
88
87
84
85
82
1 f: R2 =H
1g: R2 =I
3 fa: R2 =H
3ga: R2 =I
Scheme 1. Cyclization of N-substituted benzamides 1a–d with alkyne 2a.
1h: R2 =Br
1i: R2 =Cl
1j: R2 =F
1k: R2 =CF3
1l: R2 =CN
1m: R2 =NO2
3ha: R2 =Br
3ia: R2 =Cl
3ja: R2 =F
3ka: R2 =CF3
3la: R2 =CN
3ma: R2 =NO2
that the N-OMe acts as an efficient internal oxidant in
comparison to other N-OR sources.
The choice of solvent is crucial for the success of the reac-
tion. Various solvents, including TFE, dichloroethane (DCE),
methanol, tert-amyl alcohol, and 1,4-dioxane were examined.
In comparison to TFE (92% yield of 3aa; see above), DCE was
less effective for the reaction, giving a 15% yield of isolated
product 3aa. Other solvents were totally ineffective (see the
Supporting Information, Table 1). Although the amide’s N-OMe
group acts as an internal oxidant, a catalytic amount of acetate
base is necessary to start the catalytic reaction. Thus, the reac-
tion was examined with various bases, including NaOAc, KOAc,
CsOAc, AgOAc, and Cu(OAc)2·H2O. NaOAc and KOAc were
equally effective for the reaction, giving isolated 3aa in 92%
and 90% yield, respectively. CsOAc, AgOAc, and Cu(OAc)2,
were partially effective, providing 3aa in 45%, 50% and 15%
yields, respectively. The reaction temperature of 1208C was
also crucial to optimizing the yield of 3aa. When reactions
were carried out at 60 and 808C, 3aa was obtained in only
40% and 65% yields, respectively.
10
11
86
82
[a] Reaction conditions: 1e–o (50 mg), diphenylacetylene (2a; 1.2 equiv),
[CoCp*(CO)I2] (10 mol%), and NaOAc (30 mol%) in TFE (3.0 mL) at 1208C
for 24 h; [b] isolated product.
Next, the cyclization reaction was examined with various un-
symmetrical benzamides 1p–u (Scheme 2). In all of these ben-
zamides, there are two CÀH bonds for activation. Interestingly,
the present cyclization is highly regioselective and only
The cyclization reaction was examined with various substi-
tuted N-methoxy benzamides under the optimized reaction
conditions (Table 1). The catalytic reaction was compatible with
benzamide substrates incorporating sensitive functional
groups such as I, Br, Cl, F, CN, and NO2. Treatment of N-me-
thoxy 4-methyl benzamide (1e) and N-methoxy benzamide
(1 f) with diphenylacetylene (2a) gave the expected isoquino-
lone derivatives 3ea and 3 fa in excellent yields (Table 1, en-
tries 1 and 2). N-methoxy benzamides 1g-j with highly sensi-
tive halide substituents such as I, Br, Cl and F also efficiently
participated in the reaction, providing isoquinolone derivatives
3ga–ja in 87–90% yield (Table 1, entries 3–6). Interestingly, N-
methoxy benzamides 1k–m, incorporating electron-withdraw-
ing substituents CF3, CN, and NO2, reacted with 2a, yielding
isoquinolone derivatives 3ka–ma in 82–85% yield (Table 1, en-
tries 7–9). N-Methoxy ortho-methyl benzamide (1n) reacted
with 2a to afford isoquinolone derivative 3na in 86% yield
(Table 1, entry 10). The heteroaromatic amide N-methoxythio-
phene-2-carboxamide (1o) was also a suitable substrate for
the reaction, affording the expected cyclization product 3oa in
82% yield (Table 1, entry 11). In all of these reactions, the cycli-
zation products were obtained in excellent yields (82–90%)
and electronic effect on the aromatic system did not influence
the reactivity of the reaction or the yield of product.
Scheme 2. Regioselectivity studies.
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a single regioselective cyclization product was observed. Thus,
meta-methoxy-, bromo-, and chloro-substituted N-methoxy
benzamides (1p–r) reacted with diphenylacetylene (2a) afford-
ing the expected isoquinolone derivatives 3pa–ra in 76–82%
yield, in which the CÀH bond activation selectively takes place
at a less hindered C6 position. In a similar fashion, N-methoxy-
2-naphthamide (1s) and N-methoxy 3,4-dimethoxy benzamide
(1t) provided the expected isoquinolone derivatives 3sa and
3ta in excellent 87 and 89% yields, respectively, in which the
CÀH bond activation also takes place selectively at a less hin-
dered side. In contrast, benzamide 1u reacted with 2a under
similar reaction conditions affording the expected cyclization
product 3ua in 81% yield, in which the CÀH bond activation
selectively takes place at the hindered C2 position. It is possi-
ble that, owing to the dual coordination of an ether O atom
and the nitrogen atom of amide 1u, the cyclization takes place
at C2 (see the Supporting Information, page 4), whereas in the
substrate 1t, the CÀH bond activation takes place at the less
hindered side (C6). This is probably due to the steric hindrance
of the methyl group of OMe.[9e]
(Scheme 3). Less reactive symmetrical alkynes, such as 2-
butyne (2b), 3-hexyne (2c) and 1,4-dimethoxybut-2-yne (2d),
reacted efficiently with 1t, providing cyclization products 3tb–
td in 72–76% yield (Scheme 3). Unsymmetrical alkynes, such as
1-phenyl-1-propyne (2e) and 1-phenyl-1-hexyne (2 f) reacted
with 1t, giving mixtures of cyclization products 3te and 3te’
in 79% total yield in a 3:1 ratio and 3tf and 3tf’in a separable
56% and 20% yields, respectively. Methyl 3-phenylpropiolate
(2g) reacted with 1t, providing mixtures of cyclization prod-
ucts 3tg and 3tg’ in 88% yield in a 3:1 ratio. Interestingly, in
the cyclization of methyl hex-2-ynoate (2h) with 1i, only
a single regioisomeric cyclization product 3ih was obtained in
60% yield. In the reaction, the CÀH bond of 1i was connected
at the alkyl-substituted carbon of alkyne 2h (see the Support-
ing Information for NOE studies). Furthermore, in the cycliza-
tion of 3-phenylprop-2-yn-1-ol (2i) with 1 f or 1t, the expected
cyclization products 3 fi and 3ti were obtained in 76% and
75% yields, respectively, in a highly regioselective manner. In
the reaction, the CÀH bond of 1t was connected at the Ph-
substituted carbon of alkyne 2i (see the Supporting Informa-
tion for NOE studies). It is important to note that in the previ-
ously reported rhodium- and ruthenium-catalyzed reactions,
The scope of the cyclization reaction was further
examined with various symmetrical and unsymmetrical alkynes
Scheme 3. Scope of alkynes.
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the ortho CÀH bond of aromatic moiety was connected at the
CH2OH substituted carbon of alkyne 2i.[2b,c] In a similar fashion,
1-phenyl-1-trimethylsilane (2j) reacted with 1i under similar re-
action conditions, affording the expected cyclization product
3ij in 76% yield in a highly regioselective manner. However,
a silyl group was cleaved in the product under the reaction
conditions. To avoid the SiMe3 cleavage, the cyclization reac-
tion was examined at a lower temperature (808C) under similar
reaction conditions. However, in this reaction, product 3ij was
obtained in only 45% yield with SiMe3 cleavage. In the reac-
tion, the phenyl group of the alkyne was connected at the ni-
trogen atom of the amide group of 1i. Furthermore, the cycli-
zation reaction of 1a as well as 1c with terminal alkyne, phe-
nylacetylene, was examined under the optimized reaction con-
ditions at 1208C, as well as at 808C. However, no cyclization
product was obtained.
Isoquinolone derivatives can be used as a key synthetic pre-
cursor for various organic transformations.[6] In particular,
highly useful 1-chloro and 1-bromo isoquinoline derivatives
can be prepared easily from isoquinolone derivatives. Treat-
ment of 3aa with POCl3 at 1008C for 2 h gave 1-chloroisoqui-
noline derivative 4a in 92% yield (Scheme 4). Similarly, 3aa re-
acted with PBr3 at 1208C for 6 h to give 1-bromoisoquinoline
derivative 4b in 91% yield (Scheme 4).
Scheme 5. Proposed mechanism. R=Me or Ac.
Scheme 4. Synthesis of 1-halo isoquinoline derivatives.
To investigate the mode of reactivity of benzamides and
alkynes, intermolecular competitive experiments were per-
formed. Treatment of 1a (1.0 equiv) and 1m (1.0 equiv) with
2a (1.0 equiv) under the optimized reaction conditions gave
only product 3ma in 74% yield [Eq. (1)]. Product 3aa was not
formed. This result clearly reveals that the reactivity of elec-
tron-withdrawing NO2-substituted benzamide 1m is more re-
active than 1a. Furthermore, treatment of compound 1i
(1.0 equiv) with 2a (1.0 equiv) and 2c (1.0 equiv) under similar
reaction conditions gave product 3ia in 58% yield along with
compound 3ic in 30% yield [Eq. (2)]. This result clearly reveals
that the reactivity of 2a is greater than that of 2c.
tion of the nitrogen atom of 1 into a cobalt species 5 followed
by ortho-metalation provides a five-membered cobaltacycle in-
termediate 6 and ROH. Coordinative insertion of alkyne 2 into
the CoÀC bond of intermediate 6 provides intermediate 7. Re-
ductive elimination followed by cleavage of the NÀOMe bond
of intermediate 7 in the presence of ROH gives cyclization
product 3 and regenerates the active cobalt species 5 for the
next catalytic cycle. In the reaction, only 30 mol% of NaOAc
was used as an external acetate source to start the catalytic re-
action. For the remaining conversion, the N-OMe group of the
amide, as well as ROH, acts as a base to deprotonate the
amide NÀH bond and the ortho CÀH bond of the benzamide.
In conclusion, we have described an inexpensive cobalt-cata-
lyzed route to the cyclization of N-methoxy-substituted benza-
mides with alkynes providing isoquinolone derivatives in good
to excellent yields. Thereafter, isoquinolone derivatives were
A possible reaction mechanism is proposed in Scheme 5 to
account for the present cyclization reaction. NaOAc likely re-
moves the iodide ligand from [CoCp*(CO)I2] complex, followed
by ligand exchange, giving the active CoIII species 5. Coordina-
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2015, 7, 732; f) K. Gao, N. Yoshikai, Acc. Chem. Res. 2014, 47, 1208; g) M.
Moselage, J. Li, L. Ackermann, ACS Catal. 2016, 6, 498; h) T. Yamakawa, N.
Yoshikai, Org. Lett. 2013, 15, 196; i) P.-S. Lee, T. Fujita, N. Yoshikai, J. Am.
Chem. Soc. 2011, 133, 17283; j) B. J. Fallon, E. Derat, M. Amatore, C.
Aubert, F. Chemla, F. Ferreira, A. Perez-Luna, M. Petit, J. Am. Chem. Soc.
2015, 137, 2448; k) B. J. Fallon, J. B. Garsi, E. Derat, M. Amatore, C.
Aubert, M. Petit, ACS Catal. 2015, 5, 7493; l) D. Tilly, G. Dayaker, P. Bachu,
Catal. Sci. Technol. 2014, 4, 2756.
converted into highly useful 1-chloro- and 1-bromo-substituted
isoquinoline derivatives in excellent yields.
Experimental Section
General procedure for the cyclization of N-substituted benza-
mides: In
a 15 mL pressure tube, N-substituted benzamide
[4] Selected studies of high-valent cobalt complexes: a) T. Yoshino, H. Ikemo-
to, S. Matsunaga, M. Kanai, Angew. Chem. Int. Ed. 2013, 52, 2207; Angew.
Chem. 2013, 125, 2263; b) T. Yoshino, H. Ikemoto, S. Matsunaga, M. Kanai,
Chem. Eur. J. 2013, 19, 9142; c) B. Sun, T. Yoshino, S. Matsunaga, M.
Kanai, Adv. Synth. Catal. 2014, 356, 1491; d) J. Li, L. Ackermann, Angew.
Chem. Int. Ed. 2015, 54, 3635; Angew. Chem. 2015, 127, 3706; e) D. Zhao,
J. H. Kim, L. Stegemann, C. A. Strassert, F. Glorius, Angew. Chem. Int. Ed.
2015, 54, 4508; Angew. Chem. 2015, 127, 4591; f) J. R. Hummel, J. A.
Ellman, Org. Lett. 2015, 17, 2400; g) J. R. Hummel, J. A. Ellman, J. Am.
Chem. Soc. 2015, 137, 490; h) A. B. Pawar, S. Chang, Org. Lett. 2015, 17,
660; i) P. Patel, S. Chang, ACS Catal. 2015, 5, 853; j) H. Ikemoto, T. Yoshino,
K. Sakata, S. Matsunaga, M. Kanai, J. Am. Chem. Soc. 2014, 136, 5424.
[5] a) L. Grigorjeva, O. Daugulis, Angew. Chem. Int. Ed. 2014, 53, 10209;
Angew. Chem. 2014, 126, 10373; b) B. Sun, T. Yoshino, M. Kanai, S. Matsu-
naga, Angew. Chem. Int. Ed. 2015, 54, 12968; Angew. Chem. 2015, 127,
13160; c) H. Wang, J. Koeller, W. Liu, L. Ackermann, Chem. Eur. J. 2015,
21, 15525; d) M. Sen, D. Kalsi, B. Sundararaju, Chem. Eur. J. 2015, 21,
15529; e) S. Prakash, K. Muralirajan, C.-H. Cheng, Angew. Chem. Int. Ed.
2016, 55, 1844; f) O. Planas, C. J. Whiteoak, A. Company, X. Ribas, Adv.
Synth. Catal. 2015, 357, 4003; g) D. Kalsi, B. Sundararaju, Org. Lett. 2015,
17, 6118.
(50 mg), [Cp*Co(CO)I2] (10 mol%), NaOAc (30 mol%) and, if solid,
alkyne [for example, diphenylacetylene 2a (1.2 equiv)] were added.
The tube was sealed with a septum and then evacuated and
purged with nitrogen gas three times. To the tube were then
added trifluoroethanol (TFE; 3.0 mL) and, if liquid, alkyne (2b–j;
1.2 equiv) via syringe and the tube was evacuated and purged
with nitrogen gas three times. After that, the septum was removed
and immediately replaced with a screw cap. Then, the reaction
mixture was allowed to stir at 1208C for 24 h. After cooling to am-
bient temperature, the reaction mixture was diluted with CH2Cl2
(50 mL), filtered through Celite and silica gel, and the filtrate was
concentrated under reduced pressure. The crude residue was puri-
fied by columbn chromatography on silica gel with ethyl acetate in
dichloromethane (for ratios, see the Supporting Information) as
eluent to give pure product 3.
Acknowledgements
[6] a) J. R. Lewis, Nat. Prod. Rep. 1994, 11, 329; b) B. D. Krane, M. O. Fagbule,
M. Shamma, J. Nat. Prod. 1984, 47, 1; c) K. W. Bentley, Nat. Prod. Rep.
1992, 9, 365; d) B. D. Krane, M. Shamma, J. Nat. Prod. 1982, 45, 377.
[7] Selected examples: a) R. P. Korivi, Y.-C. Wu, C.-H. Cheng, Chem. Eur. J.
2009, 15, 10727; b) C.-C. Liu, K. Parthasarathy, C.-H. Cheng, Org. Lett.
2010, 12, 3518; references therein; c) S. Pimparkar, M. Jeganmohan,
Chem. Commun. 2014, 50, 12116; d) X. Peng, W. Wang, C. Jiang, D. Sun,
Z. Xu, C.-H. Tung, Org. Lett. 2014, 16, 5354; e) W. Wang, X. Peng, X. Qin,
X. Zhao, C. Ma, C.-H. Tung, J. Org. Chem. 2015, 80, 2835.
[8] Selected examples: a) S. Mochida, N. Umeda, K. Hirano, T. Satoh, M.
Miura, Chem. Lett. 2010, 39, 744; b) T. K. Hyster, T. Rovis, J. Am. Chem. Soc.
2010, 132, 10565; c) F. Wang, G. Song, X. Li, Org. Lett. 2010, 12, 5430;
d) L. Ackermann, A. V. Lygin, N. Hofmann, Angew. Chem. Int. Ed. 2011, 50,
6379; Angew. Chem. 2011, 123, 6503; e) L. Ackermann, A. V. Lygin, N. Hof-
mann, Org. Lett. 2011, 13, 3278; f) B. Li, H. Feng, S. Xu, B. Wang, Chem.
Eur. J. 2012, 18, 12873; g) K. Parthasarathy, N. Senthilkumar, J. Jayakumar,
C.-H. Cheng, Org. Lett. 2012, 14, 3478; h) M. C. Reddy, R. Manikandan, M.
Jeganmohan, Chem. Commun. 2013, 49, 6060.
We thank the DST-SERB (EMR/2014/000978), India for support
of this research. G.S. thanks the CSIR for a fellowship.
Keywords: annulation · CÀH activation · cobalt · heterocycles ·
homogeneous catalysis
[1] Selected reviews: a) P. Thansandote, M. Lautens, Chem. Eur. J. 2009, 15,
5874; b) T. Satoh, M. Miura, Chem. Eur. J. 2010, 16, 11212; c) G. Song, F.
Wang, X. Li, Chem. Soc. Rev. 2012, 41, 3651; d) D. A. Colby, R. G. Bergman,
J. A. Ellman, Chem. Rev. 2010, 110, 624; e) J. Wencel-Delord, T. Drçge, F.
Liu, F. Glorius, Chem. Soc. Rev. 2011, 40, 4740; f) P. B. Arockiam, C. Bru-
neau, P. H. Dixneuf, Chem. Rev. 2012, 112, 5879; g) B. Li, P. H. Dixneuf,
Chem. Soc. Rev. 2013, 42, 5744; h) L. Ackermann, Chem. Rev. 2011, 111,
1315.
[2] a) N. Guimond, C. Gouliaras, K. Fagnou, J. Am. Chem. Soc. 2010, 132,
6908; b) N. Guimond, S. I. Gorelsky, K. Fagnou, J. Am. Chem. Soc. 2011,
133, 6449; c) B. Li, H. Feng, S. Xu, B. Wang, Chem. Eur. J. 2011, 17, 12573;
d) L. Ackermann, S. Fenner, Org. Lett. 2011, 13, 6548; e) B. Li, J. Ma, N.
Wang, H. Feng, S. Xu, B. Wang, Org. Lett. 2012, 14, 736; f) S. Rakshit, C.
Grohmann, T. Besset, F. Glorius, J. Am. Chem. Soc. 2011, 133, 2350; g) G.
Song, X. Gong, X. Li, J. Org. Chem. 2011, 76, 7583; h) R. K. Chinnagolla, S.
Pimparkar, M. Jeganmohan, Org. Lett. 2012, 14, 3032; references therein.
[3] Selected studies of low-valent cobalt complexes: a) Q. Chen, L. Llies, E.
Nakamura, J. Am. Chem. Soc. 2011, 133, 428; b) W. Song, L. Ackermann,
Angew. Chem. Int. Ed. 2012, 51, 8251; Angew. Chem. 2012, 124, 8376; c) K.
Gao, P.-S. Lee, C. Long, N. Yoshikai, Org. Lett. 2012, 14, 4234; d) K. Gao, N.
Yoshikai, J. Am. Chem. Soc. 2013, 135, 9279; e) N. Yoshikai, ChemCatChem
[9] a) R. K. Chinnagolla, M. Jeganmohan, Eur. J. Org. Chem. 2012, 417; b) R. K.
Chinnagolla, M. Jeganmohan, Chem. Commun. 2012, 48, 2030; c) R. K.
Chinnagolla, S. Pimparkar, M. Jeganmohan, Chem. Commun. 2013, 49,
3703; d) R. Manoharan, M. Jeganmohan, Chem. Commun. 2015, 51,
29296; e) R. Manikandan, P. Madasamy, M. Jeganmohan, Chem. Eur. J.
2015, 21, 13934.
Received: February 1, 2016
Published online on March 16, 2016
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