Fast and efficient reductive amination of aldehydes by NaBH 4/B(OH)3 and NaBH4/Al(OH)3

Article abstract of DOI:10.1002/jccs.201300138

Reductive amination a variety of aldehydes and anilines to their corresponding secondary amines were carried out with NaBH₄/B(OH) ₃ and NaBH₄/Al(OH)₃ as new reducing systems in CH₃CN at room temperature in high to excellent yields of products (90-96%).

Full text of DOI:10.1002/jccs.201300138

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Fast and Efficient Reductive Amination of Aldehydes by NaBH4/B(OH)3 and
NaBH4/Al(OH)3
Davood Setamdideh,* Samira Hasani and Shahla Noori
Department of Chemistry, Faculty of Sciences, Mahabad Branch, Islamic Azad University, Mahabad, 59135-443, Iran
(Received: Mar. 16, 2013; Accepted: May 13, 2013; Published Online: Jul. 11, 2013; DOI: 10.1002/jccs.201300138)
Reductive amination a variety of aldehydes and anilines to their corresponding secondary amines were
carried out with NaBH4/B(OH)3 and NaBH4/Al(OH)3 as new reducing systems in CH3CN at room temper-
ature in high to excellent yields of products (90-96%).
Keywords: NaBH4; B(OH)3; Al(OH)3; Reductive amination; Carbonyl compounds.
INTRODUCTION
It is well know that amines are ubiquitous and unique
BH3,31 [Ir(cod)2]BF4,32 Hantzsch dihydropyridines,33
Zr[(BH4)2(Cl)2(dabco)2],34 benzylamine–BH3,35 Hantzsch
dihydropyridine using scandium triflate as a catalyst,36
Et3-SiH-CF3CO2H,37 PhSiH3/Bu2SnCl2,38 Polymethyl-
hydrosiloxane-Ti(OiPr)4,39 NaBH4-NiCl2,40 Bu3SnH,41
Bu2SnIH,42 t-BuMeiPrN–BH3,43 silicagel-Zn(BH4)2,44
functionalities in active pharmaceutical intermediates and
drugs. The reduction of nitrogen-containing functional
groups (nitro, cyano, azide, and carboxamide derivatives),
alkylation of amines (using alkyl halides or sulfonates) are
the most common routes for the synthesis of amines. Other
approach is the so-called reductive amination reaction in a
single operation (one-pot) which is described as direct
reductive amination (DRA) of offers significant advan-
tages. It can be achieved by reductive amination of car-
bonyl compounds by a variety of reducing systems and re-
ducing agents, such as Li/arene/THF,1 PdO-Fe3O4/Poly-
methylhydrosiloxane,2 NH4CO2H/toluene/heat,3 lewis
acid/THF then NH3BH3,4 dihydropyridines/trimethyl silyl
chloride,5 NaBH4/cellulose sulfuric Acid/EtOH,6 NaBH4-
amberlyst15,7 NaBH4-silica chloride,8 NaBH4-silica-gel-
supported sulfuric acid,9 Polymethylhydrosiloxane/tri-
fluoroacetic acid,10 [Zr(BH4)2(Ph3P)2],11 2-picoline–BH3,12
5-ethyl-2-methylpyridine borane/AcOH,13 sodiumtetrakis
Ti(O-i-Pr)4-NaBH4,45
NaBH4-wet-clay-microwave,46
NaBH(OAc)3,47 NaBH4-ZrCl4,48 pyridine-BH3,40 NaBH4-
H2SO4,50 electrochemical reductive amination reactions,51
NaBH4/Mg(ClO4)2,52 2,6-diborane–methanol53 and
NaBH3CN.54
The mentioned methods have many advantages and
some disadvantages in one way or another such as excess
amount of reagents, prolonged reaction time, higher reac-
tion temperature, acidic conditions, inert conditions, toxic
byproducts and so on. In addition, traditional methodolo-
gies for secondary amines are often problematic because of
harsh reaction conditions, overalkylation, low chemical se-
lectivity and generally poor yields. Therefore, there is a
specific interest in developing controlled synthesis of sec-
ondary amines due to its vast applications. So, in continu-
ing our efforts for the development of new reducing sys-
tems,55-58 we have carried out extensive re-examination of
the DRA reaction in an effort to develop a convenient sys-
tem. Herein, we now wish to report an efficient, facile and
mild one-pot reductive mono-N-alkylation of anilines us-
ing aldehydes by NaBH4/X(OH)3 (X: B, Al) as new reduc-
ing system in CH3CN at room temperature.
[3,5-di(trifluoromethyl)phenyl]borate (NaBArF )/hydri-
4
oiridium(III) complex/H2O,14 FeII/EDTA-Na2,15 NaBH4-
Silica phosphoric acid,16 5-ethyl-2-methylpyridine–BH3,17
NaBH4-H3PW12O40,18 NaBH4/guanidine hydrochloride/
H2O,19 Cu(PPh3)2BH4, MeOH/NH2SO3H,20 NaBH4/Bronsted
acidic ionic liquid (1-butyl-3-methyl imidazolium tetra-
fluoroborate [(BMIm)BF4]),21 NaBH4 or LiAlH4/LiClO4/
diethyl ether,22 HCOONH4/Pd-C,23 triazole-derived irid-
ium-(I) carbine complexes, 24 zinc borohydride N-methyl
piperidine,25 water-soluble transition metal catalysts,26 hy-
drogen in the presence of metal catalysts,27 dibutylchlo-
rotin hydride complex,28 ammonium chloride/Ru and Pd,29
NaBH4-PhCO2H, NABH4/P-TsOH,30 dimethylamine–
RESULTS AND DISCUSSION
Reductive amination will be successful if it relies on
the rapid imine formation and selective reduction. Imine
formation is usually the rate-determining step for reductive
* Corresponding author. E-mail: davood.setamdideh@gmail.com
J. Chin. Chem. Soc. 2013, 60, 1267-1271
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Setamdideh et al.
Table 1. Optimization of the reductive amination of benzaldehyde (1 mmol) and aniline (1 mmol) to
benzylaniline with NaBH4/B(OH)3 or Al(OH)3 at room temperature
Entry NaBH4 (mmol)
Co-reactant (mmol)
Solvent (3 mL)
Time (min)
Conversiona (%)
1
1
1
B(OH)3(1)
B(OH)3(1)
B(OH)3(1)
B(OH)3(1)
B(OH)3(1)
B(OH)3(1)
B(OH)3(1)
B(OH)3(0.5)
B(OH)3(2)
B(OH)3(1)
Al(OH)3(1)
Al(OH)3(1)
Al(OH)3(1)
Al(OH)3(1)
Al(OH)3(1)
Al(OH)3(1)
Al(OH)3(1)
Al(OH)3(2)
THF
30
3
100
100
2
CH3CN
EtOH
3
1
60
60
60
40
60
60
1
<100
<100
<100
100
4
1
CH3OH
CH2Cl2
Et2O
5
1
6
1
7
0.5
1
CH3CN
CH3CN
CH3CN
CH3CN
THF
<100
<100
100
8
9
1
10
11
12
13
14
15
16
17
18
2
1
100
1
300
5
100
1
CH3CN
EtOH
100
1
60
60
60
50
3
<100
<100
<100
<100
100
1
CH3OH
CH2Cl2
Et2O
1
1
2
CH3CN
CH3CN
1
3
100
a Conversion refer to benzylaniline and were monitored by TLC (eluent; CCl4/Et2O: 5/2).
aminations therefore addition of co-reactants is desirable.
We selected B(OH)3 and Al(OH)3 as co-reactants and per-
formed the reductive amination reaction in CH3CN using
NaBH4 as the reducing reagent. It is notable, in the absence
of co-reactants, imine formation does not occur and the al-
dehyde is reduced to benzyl alcohol. Reductive amination
of benzaldehyde with aniline was chosen as the model reac-
tion. This reaction was adopted in different solvents, differ-
ent molar ratio of the substrates/co-reactants/reducing
agent for the selection of appropriate conditions at room
temperature as shown in Table 1. Among the tested protic
and aprotic solvents such as C2H5OH, CH3OH, CH2Cl2,
Et2O, THF, CH3CN the reaction was most facile and pro-
ceeded to give the highest yield in CH3CN. The optimiza-
tion reaction conditions showed that using 1 molar equiva-
lents of NaBH4 and 1 molar equivalents of B(OH)3 or
Al(OH)3 in CH3CN (Table 1, entries 2 & 12) were the best
conditions to complete the reductive amination of benz-
aldehye (1 mmol) and aniline (1 mmol) to N-benzylaniline.
Our observation reveals that reductive amination com-
pletes within 3-5 min with 94-95% yields of product as
shown in Scheme I, (Table 2, entry 1). In order to evaluate
the generality of the process a variety of aldehydes and ani-
lines were ground with NaBH4 in the presence of B(OH)3
or Al(OH)3 in CH3CN at room temperature. In this ap-
proach, the corresponding secondary amines were obtained
in quantitative yield (90-96%) and within appropriate
times (3-7 min). The results have been reported in Table 2.
1
The purity of the products was determined by H-NMR,
which showed the exclusive formation of the correspond-
ing benzylanilines. The products was determined from the
1H-chemical shift of the CH2 group which appeared around
4.22-4.47 ppm as a singlet. Also the NH stretching fre-
quency in FT-IR spectrum appeared around 3380-3427
cm-1.
Scheme I
Since the insolubility of B(OH)3 or Al(OH)3 in THF,
the reaction takes place under heterogeneous conditions.
The mechanism for the influence of B(OH)3 or Al(OH)3 is
not clear, but as shown in Scheme II, we think that with the
addition of B(OH)3 or Al(OH)3 (as lewis acid) to the reac-
tion mixture, aldehyde slowly being coordinated with
B(OH)3 or Al(OH)3 (Scheme II, step I), therefore, carbonyl
group can be more readily to attack the nitrogen of aniline.
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JOURNAL OF THE CHINESE
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Amination of Aldehydes by NaBH4/B(OH)3 & NaBH4/Al(OH)3
Table 2. Reductive amination of aldehydes (1 mmol) with anlines (1 mmol) by NaBH4 (1 mmol) in the presence of B(OH)3 (1 mmol) or
Al(OH)3 (1 mmol) in CH3CN (3 mL) at room temperature
Substrates
Aldehydes
Time (min)
Al(OH)3 B(OH)3 Al(OH)3 B(OH)3
Yields (%)a
d CH2 n NH
(ppm) (cm-1)
Entry
Products
Anilines
1
benzaldehyde
benzaldehyde
aniline
N-benzyl aniline
4.31
4.28
4.28
4.29
4.29
4.24
4.22
4.24
4.23
4.22
4.22
4.24
4.47
4.29
4.29
4.27
3418
3427
3411
3413
3408
3421
3384
3419
3416
3390
3413
3410
3427
3388
3419
3410
5
7
4
5
5
5
5
6
6
5
6
6
5
5
6
6
3
3
5
4
3
3
3
5
5
5
5
5
3
4
5
3
95
94
93
92
91
94
95
93
90
90
93
92
93
94
94
95
94
96
92
90
92
95
90
90
95
93
94
93
92
90
91
95
2
4-bromoaniline
4-methoxyaniline
4-methylaniline
aniline
N-benzyl-4-bromoaniline
3
benzaldehyde
N-benzyl-4-methoxyaniline
N-benzyl-4-methylaniline
4
benzaldehyde
5
4-bromobenzaldehyde
N-(4-bromobenzyl)aniline
6
4-bromobenzaldehyde 4-methoxyaniline
N-(4-bromobenzyl)-4-methoxyaniline
N-(4-methylbenzyl)-4-bromoaniline
N-(4-methylbenzyl)aniline
7
4-methylbenzaldehyde
4-methylbenzaldehyde
4-methylbenzaldehyde
4-bromoaniline
aniline
8
9
4-methylaniline
N-(4-methylbenzyl)- 4-methylaniline
10
11
12
13
14
15
16
4-methylbenzaldehyde 4-methoxyaniline N-(4-methylbenzyl)-4-methoxyaniline
4-methoxybenzaldehyde 4-methylaniline
N-(4-methoxybenzyl)-4-methylaniline
N-(4-methoxybenzyl)aniline
4-methoxybenzaldehyde
4-nitrobenzaldehyde
aniline
aniline
N-(4-nitrobenzyl)aniline
2-methoxybenzaldehyde 4-bromoaniline
4-methoxybenzaldehyde 4-methylaniline
N-(2-methoxybenzyl)-4-bromoaniline
N-(4-methoxybenzyl)-4-methylaniline
N-(4-bromobenzyl)-4-methylaniline
4-bromobenzaldehyde
4-methylaniline
aYields refer to isolated pure products.
Also, B(OH)3 or Al(OH)3 able to absorb H2O that produce
IV).
This procedure does not enable the reductive amina-
in situ by condensation of aldehyde and aniline.
tion of ketones. Therefore, this reducing system can act as a
chemoselective system for the DRA of aldehydes over ke-
tones. In order to show the chemoselectivity of the system,
we performed the reductive amination of one molar equiva-
lents of benzaldehyde in the presence of one molar equiva-
lents of acetophenone with 1 molar equivalents NaBH4 in
the presence of 1 molar equivalents of B(OH)3 or Al(OH)3
at room temperature, as shown in Scheme III. The selectiv-
ity ratio for the reductive amination of aldehyde with re-
spect to ketones was 100%. This is a general trend for the
reductive amination of various aldehydes in the presence of
ketones.
Scheme II
Scheme III
This process causes to protonate nitrogen of imine as
intermediate for more reducibility by reducing agents
(Scheme II, step III). On the other hands, we observed so-
dium borohydride is decomposed and hydrogen gas slowly
is liberated in situ. Hydrogen gas generation seems to be di-
rectly related to interaction of B(OH)3 or Al(OH)3 and
NaBH4 (Scheme II, step II). Consequently, the synergisti-
cally generated molecular hydrogen combines with more
easily hydride attack to protonated imine intermediate, thus
accelerates the rate of reduction reaction (Scheme II, step
EXPERIMENTAL
General. All substrates and reagents were purchased from
commercially sources with the best quality and used without fur-
1
ther purification. IR and H NMR spectra were recorded on
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Setamdideh et al.
PerkinElmer FT-IR RXI and 400 MHz Bruker spectrometers, re-
spectively. The products were characterized by their 1H NMR or
IR spectra and comparison with authentic samples (melting or
boiling points). Organic layers were dried over anhydrous sodium
sulfate. All yields referred to isolated pure products. TLC was ap-
plied for the purity determination of substrates, products and re-
action monitoring over silica gel 60 F254 aluminum sheet.
Reductive amination of benzaldehyde and aniline with
NaBH4/B(OH)3, Atypical procedure. In a round-bottomed flask
(10 mL) equipped with a magnetic stirrer, a solution of benz-
aldehyde (0.106 g, 1 mmol), aniline (0.093 g, 1 mmol) and
B(OH)3 (0.062, 1 mmol) in CH3CN (3 mL) was prepared. The re-
sulting mixture was stirred for 5 min at room temperature. Then
the NaBH4 (0.036 g, 1 mmol) was added to the reaction mixture
and stirred at room temperature. TLC monitored the progress of
the reaction (eluent; CCl4/Ether: 5/2). The reaction was filtered
after completion within 3 min. Evaporation of the solvent and
short column chromatography of the resulting crude material over
silica gel (eluent; CCl4/Ether: 5/2) afforded the N-benzylaniline
(0.l72 g, 94% yield, Table 2, entry 1).
ACKNOWLEDGEMENTS
The authors gratefully appreciated the financial sup-
port of this, work by the research council of Islamic Azad
University branch of Mahabad.
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