NaBH 4 / C : A Convenient System for Reductive Amination of Aldehydes

In this context, NaBH4 in the presence of activated charcoal has been used for thereductive aminationofa varietyof aldehydes withanilines. The reductive amination reactions have been performed within 60-100 min in THFunder reflux conditionsin high to excellent yields of products (85-90%).


INTRODUCTION
The reductive aminationreaction is a suitable procedure for the preparation of amines from their corresponding of aldehydes.Because, other approaches such as: the reduction of nitro, cyano, azide, carboxamide compounds or the alkylation of amines are often problemssuch as: harsh reaction conditions, overalkylation, low chemical selectivity and generally poor yields.The reductive aminationreactionhas been carried out by sodium borohydridewith different reducing system 1- 15 .But, in continuing our efforts for the development of new reducing systems [16][17][18][19][20][21][22][23][24][25][26][27][28] ,in this context, we have reported the reductive amination reaction of aldehydes with anilines by NaBH 4 /C system in THF.
In the past, we have distributed some application of activated charcoal as catalyst in different reducing system such as: NaBH 4 /C for the reduction of carbonyl compounds 27 and the reduction of nitro arenes 28 .Recently, we have reported Zn(BH 4 ) 2 /C is an efficient reducing system for the reduction of carbonyl compounds 20 .Here in, we have used NaBH 4 /C as convenient system for reductive amination of aldehydes with anilines.
For this goal, the model reaction has been perfor med by reductive amination of benzaldehydeand aniline.This reaction was carried outwith different molar ratio of the benzaldehyde/ aniline/charcoal/NaBH 4 in different solvents for the HASANLOIE & SETAMDIDEH, Orient.J. Chem., Vol.30(1), 341-344 (2014) optimazition reaction conditions.Our experiments have been shown that using 0.5 gof activated charcoalin THF (5 mL) under reflux conditions is the best conditions to complete the reductive amination of benzaldehye (1 mmol) and aniline (1 mmol) to N-benzylaniline.The reductive aminationwas completed within 60 min with 85% yields of product as shown in scheme 1.

Scheme 1
The various structurally different aldehydes and anilines have been used by this reducing system.Experiments have been shown the correspondingsecondary amines were obtained in excellent yields (85-90%) within 60-100 min(Table 1); therefore the efficiency of this protocol was further examined for the reductive amination of aldehydes.The influence of activated charcoal is not clear but we have reported [26][27] sodium borohydride is slowly decomposed by activated charcoal.Consequently, it is liberated hydrogen gas in situ.Thus, the generated molecular hydrogen accelerate the reduction reaction.

ExpERIMENTAL
The products were characterized by their 1 H NMR (400 MHzBruker)or IR (PerkinElmer FT-IR RXI) and comparison with authentic samples (melting or boiling points).TLC was applied for the purity determination of substrates, products and reaction monitoring over silica gel 60 F 254 aluminum sheet.

CONCLUSION
In this research, we have shown that the NaBH 4 /charcoalis convenient system for the reductive amination of a variety of aldehydes and anilines to their corresponding secondary amines.The reduction reactions were accomplished with NaBH 4 (1 mmol) and activated charcoal (0.5 g) in THFunder reflux conditions.High efficiency of the reduction reactions and easy work-up procedure makes as an attractive new protocol for reductive amination of aldehydes.