The synthesis of benzimidazoles and quinoxalines from aromatic diamines and alcohols by iridium-catalyzed acceptorless dehydrogenative alkylation

Article abstract of DOI:10.1002/chem.201400400

Benzimidazoles and quinoxalines are important N-heteroaromatics with many applications in pharmaceutical and chemical industry. Here, the synthesis of both classes of compounds starting from aromatic diamines and alcohols (benzimidazoles) or diols (quinoxalines) is reported. The reactions proceed through acceptorless dehydrogenative condensation steps. Water and two equivalents of hydrogen are liberated in the course of the reactions. An Ir complex stabilized by the tridentate P^N^P ligand N²,N ⁶-bis(di-isopropylphosphino)pyridine-2,6-diamine revealed the highest catalytic activity for both reactions. Aromatic diamines were reacted with alcohols and diols to form benzimidazoles or quinoxalines, respectively (see scheme). In the course of the reactions, water and two equivalents of hydrogen gas were eliminated/liberated. An Ir complex stabilized by a tridentate PNP ligand was found to be an efficient catalyst in these reactions.

Full text of DOI:10.1002/chem.201400400

DOI: 10.1002/chem.201400400
Communication
&
Sustainable Synthesis
The Synthesis of Benzimidazoles and Quinoxalines from Aromatic
Diamines and Alcohols by Iridium-Catalyzed Acceptorless
Dehydrogenative Alkylation
[
a]
Toni Hille, Torsten Irrgang, and Rhett Kempe*
The acceptorless dehydrogenation is an elegant catalytic
[
17]
Abstract: Benzimidazoles and quinoxalines are important
N-heteroaromatics with many applications in pharmaceuti-
cal and chemical industry. Here, the synthesis of both
classes of compounds starting from aromatic diamines
and alcohols (benzimidazoles) or diols (quinoxalines) is re-
ported. The reactions proceed through acceptorless dehy-
drogenative condensation steps. Water and two equiva-
lents of hydrogen are liberated in the course of the reac-
tions. An Ir complex stabilized by the tridentate P^N^P
synthesis method.
It generates a valuable and easy to
remove by-product namely H . By doing so, a shift of the exist-
2
ing equilibriums towards product formation can be facilitated.
An especially attractive version of acceptorless dehydrogena-
tion is its combination with condensation steps (acceptorless
dehydrogenative condensations–ADC). Here, the liberation of
H is combined with the elimination of water (condensation).
2
Recently, ADC has been used to react alcohols and amines to
[18]
imines and (synthetically very useful) to react alcohols and/
or diols and amines or amino alcohols to selectively substitut-
2
6
ligand
N ,N -bis(di-isopropylphosphino)pyridine-2,6-di-
[19]
amine revealed the highest catalytic activity for both
reactions.
ed N-heteroaromatic compounds, for instance, pyrroles and
[20]
pyridines (Scheme 2).
Benzimidazole and its derivatives are important com-
[
1–2]
pounds.
They are applied as pharmaceuticals [for example,
the anthelmintic thiabendazole A (Scheme 1)] and as fungi-
[
3]
cides. Furthermore, they are the basis of special chemicals for
[4]
industrial applications such as chemical UVB filters, pig-
Scheme 1. Examples of an important benzimidazole (A) and quinoxaline (B).
[
5]
[6]
ments, optical brighteners for coatings, and thermo stable
[
7]
membranes for fuel cells. Similar applications are known for
quinoxalines. Quinoxaline derivatives are effective antineoplas-
[
8]
[9]
[10]
[11,12]
tics, antivirals, antidepressants, antibiotics,
as well as
[
13]
coccidiostats
medicine and biocides . In addition, they are used as dyes
(sulfaquinoxaline B, Scheme 1) in veterinary
[
14]
[15]
Scheme 2. State of the art in N-heterocycle synthesis through acceptorless
dehydrogenative condensation reactions and the benzimidazole and qui-
noxaline synthesis introduced here.
[
16]
and organic semi-conductors.
[
a] T. Hille, Dr. T. Irrgang, Prof. Dr. R. Kempe
Lehrstuhl fꢀr Anorganische Chemie II, Universitꢁt Bayreuth
Universitꢁtsstrasse 30, NW I, 95440 Bayreuth (Germany)
Fax: (+49)921552157
Here, we report on an Ir-catalyzed synthesis of benzimida-
zoles and quinoxalines by ADC starting from benzene-1,2-dia-
mines and aliphatic alcohols or 1,2-diols, respectively. Two
equivalents of dihydrogen are eliminated in the course of the
E-mail: kempe@uni-bayreuth.de
[21]
reactions. Recently, several synthesis protocols for the syn-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400400.
[22]
thesis of benzimidazoles, based on o-diamines and primary
Chem. Eur. J. 2014, 20, 1 – 5
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alcohols were developed. These protocols require additives
latter as the better solvent. The amount of base, which acceler-
ates the alcohol oxidation and mediates the condensation
step, was optimized towards an efficient benzimidazole forma-
tion. In the absence of base, a yield of only 20% of 3a was ob-
served.
[
23–27]
acting as hydrogen acceptors.
Furthermore, a photo-cata-
[
28]
lytic version of the reaction was reported.
As a useful test reaction to find the optimal reaction condi-
tions, we first investigated the formation of 2-phenyl-1H-ben-
zo[d]imidazole (3a) from benzene-1,2-diamine and benzyl alco-
hol (Scheme 3). Benzene-1,2-diamine (1.0 equiv), benzyl alcohol
To make sure we were using the best catalyst, several P^N-
ligand stabilized Ir-complexes and commercially available Ir-
precursors and their combination with various phosphanes
were investigated as catalysts. The Ir-precursors [IrOMe(cod)]2
and [IrCl(cod)]2 (cod=1,5-cyclooctadiene) themselves and in
combination with several phosphane ligands did not give ac-
ceptable yields of the product (Table 1, entries 11–18). Similarly,
Scheme 3. Test reaction for the optimization of the reaction conditions.
[
a]
Table 2. Substrate scope (3a–l) benzimidazole synthesis.
(1.1 equiv), and KOtBu (1.0 equiv) in diglyme under nitrogen at-
mosphere (pressure tube closed with a bubble counter) were
used as starting parameters. A yield of 50% within 24 h at
a temperature of 1108C was observed with a catalyst loading
of 1.0 mol% (catalyst 1a, Table 1). In order to avoid dialkylation
of the diamine, the diamine to alcohol ratio was altered and
an excess of 1.43 equivalents of diamine could increase the
yield up to 88%. A comparison of THF (semipermeable mem-
[b]
Entry
1
Product
Alcohol
Yield [%]
85
3a
3b
3c
[
19a]
brane)
and diglyme (pressure tube closed with a bubble
counter) both under nitrogen gas atmosphere revealed the
2
3
56
68
[
a]
Table 1. Catalyst optimization for the benzimidazole synthesis.
4
3d
81
5
6
7
8
9
3e
3 f
3g
3h
3i
96
95
95
89
78
70
91
[
b]
Entry
Cat. complex
Yield [%]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1 f
1g
1h
2a
2b
88
47
38
44
75
48
68
65
31
26
21
18
9
10
3j
10
1
1
[IrOMe(cod)]
2
12
13
14
15
16
17
18
[IrCl(cod)]
[IrCl(cod)]
[IrCl(cod)]
[IrCl(cod)]
[IrCl(cod)]
[IrCl(cod)]
[IrCl(cod)]
2
2
2
2
2
2
2
11
3k
+1 equiv PPh
+2 equiv PPh
+3 equiv PPh
+1 equiv P(C
+2 equiv P(C
+3 equiv P(C
3
3
3
9
8
24
31
34
6
H
11
)
11
)
11
)
3
3
3
12
3l
93
6
6
H
H
[a] Reaction conditions: benzene-1,2-diamine (8.0 mmol), benzyl alcohol
(5.6 mmol), KOtBu (8.0 mmol), catalyst loading 1.4 mol%, 1108C, 24 h.
[b] Determined by GC analysis.
[a] Reaction conditions: benzene-1,2-diamine (8.0 mmol), alcohol
(5.6 mmol), 1a (1.4 mol% ), KOtBu (8.0 mmol ), diglyme (3 mL). [b] Isolat-
ed yields.
&
&
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Communication
Ir complexes stabilized by bi-dentate P^N-ligands, which are
very efficient in borrowing hydrogen (or hydrogen auto-trans-
[
a]
Table 4. Substrate scope (4a–f) quinoxaline synthesis.
[
29]
[b]
fer) reaction, failed as catalysts (Table 1, entries 9 and 10).
However, Ir complexes stabilized by tri-dentate P^N^P-li-
gands gave significantly better conversions (Table 1, entries 1–
Entry
1
Product
Diol
Yield [%]
89
4a
4b
4c
4d
4e
8). Variation of ligand substituents revealed 1a as the best cat-
alyst for the benzimidazole synthesis discussed here. The re-
leased gas was identified as hydrogen. After optimization of
the reaction conditions and finding the best catalyst, we
became interested in exploring the substrate scope of the re-
action (Table 2). Various functionalized benzyl alcohols and ali-
phatic alcohols were used. Olefinic groups at the aliphatic alco-
hol can be tolerated (Table 2, entry 8). With chloro-substituted
benzyl alcohol a decrease of the yield is observed due to
minor dehalogenation. Particularly good to very good isolated
yields were observed using aliphatic alcohols. Furthermore, we
were able to access methyl and methoxy-substituted diamines
2
3
4
87
83
81
61
[
c,d]
e]
5
6
[
4 f
77
(
Table 2, entries 9 and 10). The use of mono N-alkylated di-
1
[a] Reaction conditions: benzene-1,2-diamine (8.0 mmol), 1,2-diol
amines leads to N -alkylated benzimidazoles in isolated
yields> 90% (Table 2, entries 11 and 12). Based on the condi-
tions we optimized, 1,2-diols were reacted with benzene-1,2-
diamine. Instead of 2-hydroxyalkyl functionalized benzimida-
(8.8 mmol), 1a (0.06 mol%), KOtBu (16.0 mmol), THF (26 mL), 908C, 24 h.
[b] Isolated yields. [c] 1,2-diol (12.8 mmol). [d] 1a (0.3 mol%); [e] 1a
(0.1 mol%).
[
22e–g,r,30]
zoles, quinoxalines
were formed. The catalyst system ef-
ficiently oxidized both OH-groups of the diol, which condense
much faster than benzimidazole formation is observed. In con-
trast to earlier described transition metal-catalyzed synthetic
sired products can be obtained in good to very good yields.
Higher catalyst loadings were needed for disubstituted 1,2-
diols (Table 4, entry 5) and monosubstituted 1,2-diols carrying
a bulky substituent (Table 4, entry 6).
[
31]
routes starting from o-phenylenediamines and diols or a-hy-
[
32]
droxy ketones which only work in the presence of a hydro-
gen acceptor, we present a protocol with liberation of molecu-
lar hydrogen to give quinoxalines. This new reaction was again
optimized regarding the reaction parameters. The performance
of selected catalysts is shown in Table 3.
In conclusion, we developed a novel benzimidazole synthe-
sis. Aromatic diamines and alcohols are connected by a con-
densation step and by the liberation of two equivalents of H2.
Iridium complexes stabilized by tridentate P^N^P-ligands are
the best catalysts for this reaction. If 1,2-diols are used, the oxi-
dation of both alcohol functions and subsequent condensation
forming quinoxalines is significantly faster than benzimidazole
formation.
The following parameters resulted from the optimization:
a benzene-1,2-diamine to butane-1,2-diol ratio of 1.0 to 1.1,
THF as solvent and 2.0 equivalents of KOtBu. Interestingly,
a very low catalyst loading (0.06 mol% of 1a) is needed. The
catalysis runs nicely at 908C for 24 h in a reaction tube con-
[
19a]
nected with a semi permeable membrane.
The semi perme-
Acknowledgements
able membrane selectively liberates H and allows working in
2
THF above the boiling point of the solvent. This synthesis
route permits the use of a variety of 1,2-diols (Table 4). The de-
This work was supported by the Deutsche Forschungsgemein-
schaft (KE 756/23-1).
Keywords: acceptorless dehydrogenation
benzimidazoles · iridium · quinoxalines
·
alcohols
·
Table 3. Catalyst screening for the quinoxaline synthesis.
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[
[
21] Formation of H was confirmed by GC analysis. Please see the Support-
2
ing Information for details.
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5
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Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 5
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
&
Sustainable Synthesis
T. Hille, T. Irrgang, R. Kempe*
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The Synthesis of Benzimidazoles and
Quinoxalines from Aromatic Diamines
and Alcohols by Iridium-Catalyzed
Acceptorless Dehydrogenative
Alkylation
Aromatic diamines were reacted with
alcohols and diols to form benzimid-
azoles or quinoxalines, respectively (see
scheme). In the course of the reactions,
water and two equivalents of hydrogen
gas were eliminated/liberated. An Ir
complex stabilized by a tridentate
P^N^P ligand was found to be an effi-
cient catalyst in these reactions.
Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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These are not the final page numbers! ÞÞ