Cobalt-catalyzed oxidative isocyanide insertion to amine-based bisnucleophiles: Diverse synthesis of substituted 2-aminobenzimidazoles, 2-aminobenzothiazoles, and 2-aminobenzoxazoles

Article abstract of DOI:10.1002/chem.201300239

Cobalt catalysis: Synthesis of substituted 2-aminobenzimidazoles, 2-aminobenzothiazoles, and 2-aminobenzoxazoles was achieved by using cobalt(II) acetate catalyzed isocyanide insertion to o-diaminobenzene, 2-aminobenzenethiol, and 2-aminophenol derivatives in 1,4-dioxane (see scheme). It was found that the reaction proceeded efficiently to give the desired products in up to 95 % isolated yields by C-N and C-S (O, N) formation in a single step. Copyright

Full text of DOI:10.1002/chem.201300239

COMMUNICATION
DOI: 10.1002/chem.201300239
Cobalt-Catalyzed Oxidative Isocyanide Insertion to Amine-Based
Bisnucleophiles: Diverse Synthesis of Substituted 2-Aminobenzimidazoles,
2-Aminobenzothiazoles, and 2-Aminobenzoxazoles
Tong-Hao Zhu, Shun-Yi Wang,* Gao-Nan Wang, and Shun-Jun Ji*[a]
2-Aminobenzoxazoles, 2-aminobenzothiazoles, and 2-ami-
nobenzimidazoles are privileged heteroaromatic scaffolds in
biologically active compounds and natural products
(Figure 1). For example, the core scaffold of the selective in-
pounds VI and VI’[6] all contain the 2-aminobenzimidazole
skeleton.
There are several reported approaches to these scaffolds:
1) The addition of amines to isothiocyanates or CS2 fol-
lowed by intramolecular condensation,[7,8] 2) the cross-cou-
pling reaction of bisnucleophiles with cyanogen bromide,[9]
3) intramolecular cyclization by C H activation,[10] and
À
4) passing through the intermediate formation of isocyanide
dibromides.[11] All these procedures suffer from the use of
unenviromental, toxic, and/or dangerous reagents and poor
step efficiency. Thus, the development of a new, general,
and atom-economic method to obtain these compounds re-
mains great challenge to chemists.
Recently, reactions using isocyanide insertion (similar to
carbon monoxide) have been developed for the synthesis of
N-heterocycles.[12] As a continuation of our work on the in-
[13]
À
sertion of isocyanide into an active N H bond, we hope
to apply this insertion strategy to the reaction of isocyanide
[14]
[15]
[16]
À
À
À
with SH,
OH, and NH2 based bisnucleophilic re-
agents, such as o-aminophenols, o-aminobenzenethiol, and
o-aminoanilines, for the synthesis of 2-aminobenzimidazoles,
2-aminobenzothiazoles, and 2-aminobenzoxazoles, respec-
tively (Scheme 1). Moreover, since the pioneering work of
Figure 1. Selected biologically active compounds.
Scheme 1. Direct strategy for the synthesis of 2-aminobenzimidazoles, 2-
aminobenzothiazoles, and 2-aminobenzoxazoles by isocyanide insertion.
hibitor of the human P2Y1 receptor I[1] and the drug for
treating insomnia II[2] is 2-aminobenzoxazole. The core
structure of the anti-HIV agent III[3] and the phosphodiester-
ase 10 inhibitor IV[4] is 2-aminobenzothiazole. The histamine
H1-receptor V, named astemizloe,[5] and bioactive com-
Kharasch[17] on the metal-catalyzed homocoupling reaction
of aromatic Grignard reagents in the middle of the 20th cen-
tury, cobalt-catalyzed cross-coupling reactions have received
particular attention.[18] Herein, we report a safe and efficient
À
À
cobalt-catalyzed C N, C OACTHNUTRGNE(NUG S,N) bond formation by isocya-
[a] T.-H. Zhu, Prof. Dr. S.-Y. Wang, G.-N. Wang, Prof. Dr. S.-J. Ji
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering and
Materials Science
nide insertion to o-diaminobenzene, 2-aminobenzenethiol,
and 2-aminophenol derivatives. Whilst we were preparing
this paper, Orru and Maesꢀs group reported a palladium-cat-
alyzed isocyanide insertion to bisnucleophiles.[19]
Soochow University, Suzhou 215123 (P.R. China)
Fax: (+86)0512-65880307
E-mail: shunyi@suda.edu.cn
We initiated our study by investigating isocyanide inser-
tion reactions involving 2-aminophenol 1a and tert-butyl iso-
cyanide 2a. When the model reaction was performed in the
presence of nano-Co3O4, the desired product N-tert-butyl-
shunjun@suda.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300239.
Chem. Eur. J. 2013, 00, 0 – 0
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Table 2. Cobalt-catalyzed isocyanide insertion reactions with 2-amino-
benzo[d]oxazol-2-amine (3a) was obtained in 33% LC yield
(Table 1, entry 2). After optimization of the reaction condi-
tions (see the Supporting Information and Table 1 for de-
tails), it was found that the optimal reaction conditions in-
phenols 1a–h.[a]
volved CoACHTUNGTRENNUNG(OAc)2·4H2OACHUTNGTREN(NGUN 10 mol%), NaOAc (2 equiv), and
K2S2O8 (1 equiv) in 1,4-dioxane at 1008C (Table 1, entry 15).
Entry
1
2
Product Yield [%][b]
Table 1. Cobalt-catalyzed insertion reactions with 2-amino phenol 1a of
tert-butyl isocyanide 2a.[a]
1
2
3
4
5
6
7
R1 =H (1a)
R2 =tBu (2a)
R2 =tBu (2a)
R2 =tBu (2a)
R2 =tBu (2a)
R2 =tBu (2a)
R2 =tBu (2a)
3a
3b
3c
3d
3e
3 f
3g
3h
3i
87
92
85
95
83
89
81
67
81
68
71
R1 =5-NO2 (1b)
R1 =4-NO2 (1c)
R1 =3-NO2 (1d)
R1 =4-Cl (1e)
R1 =5-CH3 (1 f)
R1 =4-OCH3 (1g) R2 =tBu (2a)
8[c]
9[c]
10[c]
11[c]
R1 =H (1a)
R1 =H (1a)
R1 =H (1a)
R1 =H (1a)
R2 =Cy (2b)
R2 =nBu (2c)
Entry
[Co]
Base
Solvent
Yield [%][b]
R2 =2,6-Me2C6H3 (2d)
R2 =Bn (2e)
3j
3k
1
2
3
4
5
6
7
8
–
–
THF
THF
THF
THF
THF
THF
THF
THF
1,4-dioxane
DME[c]
toluene
DMF[d]
DMSO[e]
1,4-dioxane
1,4-dioxane
0
33
55
23
trace
14
trace
80
93
83
50
nano-Co3O4
Co
[Co
[Co
CoCl2·6H2O
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
N
12
R2 =tBu (2a)
3l
trace
[a] Reaction Conditions: 2-aminophenol 1a–h (0.5 mmol), isocyanide 2
(0.6 mmol), Co(OAc)2·4H2O (10 mol%), NaOAc (2 equiv), K2S2O8
(1 equiv), 1,4-dioxane (3 mL), 1008C, 18 h. [b] Isolated yield. [c] DMF in-
CoSO4
AHCTUNGTRENNUNG
Co
Co
Co
Co
Co
Co
Co
Co
E
9
N
stead of 1,4-dioxane was used as the reaction solvent.
10
11
12
13
14[f]
15[g]
R
N
G
90
85
82
96
G
T
aminobenzenethiol (1i) and o-aminoanilines (1j–m), under
the optimal conditions. The results are listed in Table 3. The
reactions of 2-aminobenzenethiol (1i) with different isocya-
nides furnished the corresponding products 4a–f in up to
90% yields (Table 3). Introducing a Cl group at the para-po-
sition of the aromatic ring maintained a high yield (4g, 87%
yield).
A
[a] Reaction Conditions: 2-aminophenol 1a (0.5 mmol), tert-butyl isocya-
nide 2a (0.6 mmol), cobalt catalyst (20 mol%), base (2 equiv), oxidant
K2S2O8 (1 equiv), solvent (3 mL), 1008C, 18 h. [b] Yield was determined
by LC analysis with biphenyl as the internal standard. [c] DME: di
oxyethane. [d] DMF. [e] DMSO. [f] 5 mol% cobalt catalysts is used.
[g] The dosage of cobalt catalyst is 10 mol%.
ACHTUNGTNERmNUNG eth-
ACHTUNGTRENNUNG
Based on the above results, we believed that this insertion
reaction would enable concise syntheses of o-aminobenzimi-
dazoles by using o-aminoanilines instead of o-aminobenze-
nethiol as the substrate. When the reaction utilizing bis-
With the optimized conditions in hand, the scope of this
reaction was investigated and the results are summarized in
Table 2. The substituted 2-aminophenols (1b–g) reacted well
with tert-butyl isocyanide (2a) (Table 2, entries 2–7) to give
the desired products 3b–g in moderate to excellent yields. It
is noteworthy that 2-aminophenols bearing electron-donat-
ing groups (Me or OMe), or electron-withdrawing groups
(NO2) could afford the desired products in good yields. In
addition, relatively bulky substrate 1d also efficiently under-
went the transformation, generating the desired products 3d
in 95 (Table 2, entry 2). Then, several structurally varied iso-
cyanides were also investigated. When other aliphatic and
aromatic isocyanides (2b–e) were applied to the reaction,
the reactions also proceeded smoothly in moderate yields
(67–81%) under slightly modified conditions (DMF was
used as the solvent instead of 1,4-dioxane) (Table 2, en-
tries 8–11). However, when 2-aminoethanol (1h) was sub-
jected to this reaction, no desired product was observed
(Table 2, entry 12).
AHCTUNGTREGnNNNU ucleophiles o-aminoanilines was applied to the optimized
conditions, the cobalt-catalyzed isocyanide insertion reac-
tions also proceeded well to afford the desired products in
yields ranging from 45 to 91%. It was observed that the use
of Ts-substituted o-aminoaniline 1k gave the desired prod-
ucts in relatively lower yields than when Me-substituted o-
aminoaniline 1l or Bn-substituted o-aminoaniline 1k were
used (Table 3).
In conclusion, a novel and practical CoACHTUNTGRNEUNG(OAc)2·4H2O-cata-
À
lyzed direct insertion reaction of isocyanides into active N
À
H, O
U
not only expands the scope of the cobalt-catalyzed cross-
coupling reaction but also provides a general, reliable, and
diverse approach leading to the valuable substituted 2-ami-
nobenzimidazole, 2-aminobenzothiazole, and 2-aminoben-
zoxazole frameworks. Further studies to understand the
cobalt-catalyzed insertion reaction mechanism and extend-
ing this strategy to synthetic applications are ongoing in our
laboratory.
To evaluate the general performance of our method, we
further tested other amine-based bisnucleophiles, such as o-
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Cobalt-Catalyzed Oxidative Isocyanide Insertion
COMMUNICATION
Table 3. Cobalt-catalyzed isocyanide insertion reactions with other bisnu-
(10 mol%), NaOAc (2 equiv), and K2S2O8 (1 equiv). The mixture was
stirred by magnetic stirrer in an appropriate time at 1008C until the
amino compound was completely consumed (checked by TLC analysis).
Then, the solvent was evaporated under reduced pressure. The residue
was purified by flash column chromatography with ethyl acetate and pe-
troleum ether as the eluents to afford pure product.
cleophiles 1i–m.[a]
Acknowledgements
We gratefully acknowledge the Natural Science Foundation of China (no.
21042007, 21172162), the Young National Natural Science Foundation of
China (no. 21202111), the Young Natural Science Foundation of Jiangsu
Province (BK2012174), the Key Laboratory of Organic Synthesis of
Jiangsu Province (KJS1211), PAPD, and Soochow University for finan-
cial support.
Keywords: cobalt · cross-coupling · diversity · heterocycles ·
isocyanide insertion
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Chem. Eur. J. 2013, 00, 0 – 0
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Received: January 22, 2013
Revised: February 25, 2013
Published online: && &&, 0000
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Cobalt-Catalyzed Oxidative Isocyanide Insertion
COMMUNICATION
Heterocycles
T.-H. Zhu, S.-Y. Wang,* G.-N. Wang,
S.-J. Ji* ......................... &&&& — &&&&
Cobalt-Catalyzed Oxidative Isocyanide
Insertion to Amine-Based Bisnucleo-
philes: Diverse Synthesis of Substi-
tuted 2-Aminobenzimidazoles, 2-
Aminobenzothiazoles, and 2-Amino-
benzoxazoles
Cobalt catalysis: Synthesis of substi-
tuted 2-aminobenzimidazoles, 2-amino-
benzothiazoles, and 2-aminobenzoxa-
zoles was achieved by using cobalt(II)
acetate catalyzed isocyanide insertion
to o-diaminobenzene, 2-aminoben-
tives in 1,4-dioxane (see scheme). It
was found that the reaction proceeded
efficiently to give the desired products
À
in up to 95% isolated yields by C N
À
and C S (O, N) formation in a single
step.
ACHTUNGTRENNUNGzenethiol, and 2-aminophenol deriva-
Chem. Eur. J. 2013, 00, 0 – 0
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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