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1,2-DISUBSTITUTED BENZIMIDAZOLES AND QUINOXALINES
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Scheme 1. 1-(4-Sulfonic acid)butyl-3-methylimidazolium hydrogen sulfate [(CH2)4SO3HMIM][HSO4]
as an efficient and recyclable catalyst for synthesis of 1,2-disubstituted benzimidazole and quinoxaline
derivatives.
ignored.[8] Consequently, the necessity of designing novel and particularly green
methods for synthesis of these precious heterocyclic compounds is recognized.
Synthesis of 1,2-disubstituted benzimidazoles is based mainly on condensation
of 1,2-diaminoarene derivatives with carboxylic acid derivatives,[9] aldehydes,[10] aryl
halides,[11] and orthoesters.[12] Alternatively, the palladium- and copper-catalyzed
intramolecular N-arylation starting from o-haloanilines has been used.[13] The most
common method for the synthesis of quinoxalines is the reaction of 1,2-diaminoar-
ene derivatives with 1,2-dicarbonyl compounds in different conditions. Recently,
various protocols have been introduced that achieve this goal efficiently.[14]
Organic synthesis in water attracts great attention because of cost, safety, and
environmental concerns. It is the best substitute for volatile and toxic organic
solvents, especially chlorinated hydrocarbons, which are broadly used in organic
synthetic procedures. Additionally, water exhibits special properties as compared
to commonly employed organic solvents. For example, several reactions accelerate
in water.[15] Aside from these advantages, when a water-soluble catalyst, such as
Brønsted acid ionic liquid (BAIL), is used, the insoluble products can be separated
by simple filtration, and the catalyst system can be recycled.
In this work, besides an ecofriendly moiety in the reaction, nontoxic BAIL,
1-(4-sulfonic acid)butyl-3-methylimidazolium hydrogen sulfate [(CH2)4SO3-
HMIM][HSO4] (Scheme 1), has been used, which possesses both green and efficient
catalytic effects in organic reactions.[16]
Generally, ionic liquids (ILs) are defined as fused salts with a melting point less
than 100 ꢀC, whereas salts with higher melting points are referred to as molten
salts.[17] Special properties of ILs, such as polarity, high thermal stability, immisci-
bility with a number of organic solvents, negligible vapor pressure, and recyclability,
are remarkable. Among different ILs, BAILs, which are related to protic ILs, possess
Brønsted acidity and have catalyst roles for many organic syntheses. In this class,
imidazolium salts have their own specific effects to their counterion.
Herein, [(CH2)4SO3HMIM][HSO4] has been introduced as a desirable IL that
catalyzes reaction of o-phenylenediamine with different aromatic aldehydes and also
the reaction of 1,2-diaminoarene derivatives with 1,2-diketones to synthesis 1,2-
disubstituted benzimidazoles and quinoxalines in water, respectively.
RESULTS AND DISCUSSION
In continuation of our efforts to synthesize 1,2-disubstituted benzimidazoles[10b]
and qunoxalines,[14a–d] and also achieve organic reactions in water,[14b,18] we have
developed an environmentally friendly, simple, and efficient method for the synthesis
of various 1,2-disubstituted benzimidazole and quinoxaline derivatives (Schemes 2