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Scheme 2. Formation of hydrogen bond between carbonyl compound and ChCl/Urea.
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product in the reaction with benzaldehyde and NaBH4 (Table 1, entry 8).
By applying these conditions to ketones such as acetophenone and
phenylacetone, the corresponding products were obtained in good yields.
The results showed that the nature of the applied reagents and the type of
substituent on the aromatic ring has no profound influence on the reac-
tivity. In all reactions secondary amines were isolated with high yield in
the short reaction time.
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According to the previously reported ChCl/Urea catalyzed reac-
tions [35], it seems that this reagent catalyze the reaction through the
formation of hydrogen bonds with carbonyl compound and followed
with imine results in increased electrophilicity of carbon (Scheme 2).
In conclusion, the reductive amination catalyzed by biodegradable
ammonium deep eutectic solvent based on choline chloride and urea
as the catalysts provides an efficient and green method for the synthesis
of secondary amines. The use of metal-free and non-toxic catalytic
system and the formation of products with high yields provide a good
and practical alternative to existing methods.
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Acknowledgments
(2003) 70.
[34] Typical procedure for the reductive amination of aldehydes/ketones in the presence of
ChCl/Urea: To a stirred solution of aldehyde or ketone (1 mmol) and amine
(1 mmol) in 2 mL of methanol was added ChCl/Urea (50 mg). The mixture was
stirred for 5 min and then NaBH4 (1 mmol) was added to it. The progress of the re-
action was followed by TLC (eluent: n-hexane/EtOAc 4:1). After completion, water
(10 mL) was added and the reaction mixture was extracted with CH2Cl2 (2 × 20 mL).
The combined organic layers were dried over anhydrous MgSO4 and evaporated. The
crude product was purified by silica gel column chromatography with n-hexane/
EtOAc as the eluent to afford the pure amine.
We are thankful to Tarbiat Modares University for partial support of
this work.
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