One-pot reductive amination of acetals with aromatic amines using decaborane (B10H14) in methanol

Article abstract of DOI:10.1081/SCC-120023997

Aldehyde acetals and ketone ketals were reductively aminated in one pot with aromatic amines to give the corresponding amines in methanol or aqueous methanol in good to high yields. This direct method might be important because acetals and ketals are used as a popular protecting group for aldehyde and ketones. The advantage of our method is effective even in an aqueous solution and in the application to selective reaction.

Full text of DOI:10.1081/SCC-120023997

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One-Pot Reductive Amination of Acetals with Aromatic
Amines Using Decaborane (B10H14) in Methanol
Eun Soo Park a , Ji Hee Lee a , Soo Jung Kim a & Cheol Min Yoon a
a Graduate School of Biotechnology, Korea University, Seoul, South Korea
Available online: 15 Aug 2006
To cite this article: Eun Soo Park, Ji Hee Lee, Soo Jung Kim & Cheol Min Yoon (2003): One-Pot Reductive Amination of Acetals
with Aromatic Amines Using Decaborane (B10H14) in Methanol, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:19, 3387-3396
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 19, pp. 3387–3396, 2003
One-Pot Reductive Amination of Acetals
with Aromatic Amines Using Decaborane
(B10H14) in Methanol
Eun Soo Park, Ji Hee Lee, Soo Jung Kim,
*
and Cheol Min Yoon
Graduate School of Biotechnology, Korea University,
Seoul, South Korea
ABSTRACT
Aldehyde acetals and ketone ketals were reductively aminated in
one pot with aromatic amines to give the corresponding amines in
methanol or aqueous methanol in good to high yields. This direct
methodmight be important because acetals andketals are usedas a
popular protecting group for aldehyde and ketones. The advantage
of our methodis effective even in an aqueous solution andin the
application to selective reaction.
*Correspondence: Cheol Min Yoon, Graduate School of Biotechnology, Korea
University, Sungbookgu Anamdong 1, 5-ka, Seoul, 136-701, South Korea;
Fax: þ82-2-3290-3433; E-mail: cmyoon@korea.ac.kr.
3387
DOI: 10.1081/SCC-120023997
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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3388
Park et al.
Key Words: One-pot reaction; Reductive amination; Decaborane;
Acetals; Ketals.
The direct conversion of protected functional groups to other
functional groups is very important since it provides time and cost
savings by avoiding the deprotection step and tedious work-up.[1]
Acetals and ketals are widely used as protecting groups for aldehydes
and ketones due to their easy preparation, deprotection and stability in
[2]
neutral andbasic condition.
The reductive amination reaction is a
popular methodfor the preparation of secondary amines from the primary
amines andcarbonyls. [3,4] A large number of reagents for the deprotection
of acetals and for reductive amination have been developed. To the best of
our knowledge, one pot reductive amination from acetals and amines to
corresponding secondary amines has not been developed because two
reagents for deprotection and reductive amination are not comaptible in
the same solution. The other reason is the reaction condition. Efficient
reductive amination needs an anhydrous condition, while
generally deprotection needs aqueous conditions. The best way to carry
out the direct reductive amination of acetals is development of the
new reagent compatible for two consecutive reactions; deprotection and
reductive amination.
Recently, we reported reductive amination using decaborane having
dual roles as a catalyst as well as a reducing agent, which can be easily
[5,6]
removedby a simple work-up.
In connection with the studies on
decaborane, we tried the direct reductive amination of acetals and
ketals (andenol ethers) with aromatic amines in methanol as well as in
aqueous methanol (Sch. 1) andthe results are shown in Tables 1 and2.
The reductive amination of various conjugated acetals, ketals and
enol ethers with anilines having electron withdrawing groups (nitro or
ester) in methanol using decaborane gave the corresponding products in
high yields as shown in Table 1. The optimum amount of decaborane in
the reaction was 30 mol%, the amount of which was determined by the
reaction of benzaldehyde dimethy acetal with 4-nitroaniline as a model.
X
B10H14
MeO OMe
X
3
NH2
+
HN
R1
R2
2
1
R1 R2
Scheme 1.

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Reductive Amination Using Decaborane
Table 1. Direct reductive alkylation of aromatic amines.
3389
a
Entry
1
Cabonyl
Amine
Yield(%)
96
Time (h)
1
OCH3
O2N
NH2
OCH3
O2N
2
91
2
NH2
O
H3CO
O2N
3
4
90
96
1.5
1
NH2
OCH2CH3
OCH2CH3
NH2
O
H3CO
O2N
5
6
86
95
8
NH2
OCH3
OCH3
1/6
NH2
O
O
O
H3CO
O2N
7
8
94
93
1
NH2
H3CO
OCH3
0.5
NH2
O
H3CO
O2N
9
10
11
97
100
93
1.5
1
NH2
H3CO
OCH3
NH2
MeO
O
1
NH2
H3CO
H3CO
OCH3
12
13
99
96
0.5
1
O2N
O
NH2
NH2
H3CO
OCH3
14
15
99
95
1
1
O2N
NH2
NH2
O
H3CO
O2N
16
17
56
12
24
O
NH2
NH2
OCH3
OCH3
No rex
O2N
aIsolated yield and products were characterized by spectroscopic methods.

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3390
Park et al.
Table 2. Direct reductive alkylation of 3-toluidine with various acetals and ketals.
Acetal
(1.1 equiv.)
Reaction time
(h)b
Yielda
(%)
Entry
1
Solvent
OCH3
OCH3
THF:H2O ¼ 4:1
1 (1)
83
H3CO
OCH3
2
3
THF:H2O ¼ 4:1
1 (12)
1 (1)
40
84
OCH3
MeOH:H2O ¼ 10:1
OCH3
OCH3
H3CO
4
5
MeOH:H2O ¼ 10:1
MeOH:H2O ¼ 10:1
1 (12)
1 (1)
80
95
OCH3
O
O
OCH3
H3CO
OCH3
6
MeOH:H2O ¼ 10:1
1 (4)
90
MeO
H3CO
OCH3
7
8
MeOH:H2O ¼ 10:1
MeOH:H2O ¼ 10:1
1 (4)
1 (4)
88
85
OCH3
9
O
MeOH:H2O ¼ 10:1
1 (12)
20
aIsolatedyield.
bThe reaction time for acetal hydrolysis and the reaction time in parenthesis for
reductive amination.
The imine intermediate in the reaction seemed to be formed after hydro-
lysis of acetals (or ketals) to aldehyde (or ketones). Decaborane in the
reaction seemedto act as a catalyst in the hydrolysis of acetals andthe
imine formation as well as a reducing agent of imine. One point one
equivalent of acetals andketals was usedin the reaction due to possible
side reactions such as reduction of carbonyls[7] andreductive etherifica-
tion.[8] The reaction of 4-nitroaniline andmethyl 4-aminobenzoate with
acetals andketals was complete within 1h to give the corresponding
products in high yields (Entries 1–13). Whereas the reaction of 2-meth-
oxypropene with 4-nitroaniline (Entry 14) andmethyl 4-aminobenzoate
(Entry 15) gave the corresponding products within 1 h in almost quanti-

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Reductive Amination Using Decaborane
3391
NO2
4-nitroaniline B10H14
MeOH
CH(OMe)2
+
CH(OMe)2
N
H
96%
Scheme 2.
X
B
H
10 14
N
H
CH(OMe)
+
H N
2
X
2
aqueous MeOH
X = COOMe or NO2
X = NO2 : 93%, X = COOMe : 91%
Scheme 3.
tative yield, the reaction of n-butyl vinyl ether with 4-nitroaniline gave
the product in 56% yield due to its slow hydrolysis rate to aldehyde
(Entry 16). The reductive amination of dihydrocinnamaldehyde acetal
with 4-nitroaniline (Entry 17) did not proceed at all even in extended
reaction time due to its stability against hydrolysis to carbonyl under
the reaction condition.
Our reaction condition can be applied to selective reductive
amination between hydrolyzable acetal and nonhydrolyzable acetal. One
example is shown in Sch. 2. When a solution of benzaldehyde dimethyl
acetal and dihydrocinnamaldehyde acetal in the presence of 1 equiv. of 4-
nitroaniline in methanol was exposedto our reaction condition, benzal-
dehyde acetal underwent the selective reductive amination to give benzyl
4-nitrophenylamine within 1 h in 96% yield. However, dihydrocinnam
aldehyde dimethyl acetal was recovered quantitatively (Sch. 2).
One pot reductive amination also was tried in aqueous methanol.
The reaction of benzaldehyde acetal with methyl 4-aminobenzoate
and4-nitroaniline in a solution of MeOH andwater (10:1) gave the
corresponding amines within 1 h in 91% and 93% isolated yields
respectively (Sch. 3). These results are similar to those of reductive
aminations in anhydrous methanol (Entries 1 and 3 of Table 1).
The reductive amination of benzaldehyde acetal with 3-toluidine in
MeOH using decaborane was failed in one-pot, because 3-toluidine is too
basic for decaborane to act as a Lewis acid catalyst in the hydrolysis of
acetals. Therefore, the consecutive procedure was employed. Acetals
(or ketals or enol ethers) were hydrolyzed to aldehydes (or ketones) in
the presence of catalytic amount of decaborane (2 mol%) followed
by addition of 3-toluidine and 30% decaborane. While the reductive

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3392
Park et al.
1) 2 m ol % B10H14in MeOH: H2O (10:1)
2) 30 mol % B10H14 and 3 -toluidine
MeO OMe
R1
HN
R2
R1 R2
3
1
Scheme 4.
amination of benzaldehyde acetal and acetophenone acetal with
3-toluidine in an aqueous THF (THF:water ¼ 4:1) gave the products
within 1 h in 83% and40% yield(Entries 1 and2) respectively under
the consecutive reaction condition, that of benzaldehyde acetal and acet-
ophenone acetal with 3-toluidine in aqueous methanol (methanol:
water ¼ 10:1) gave the corresponding amines in 84% and 80% yields
(Entries 3 and4). Therefore, aqueous methanol was a choice solution
for the reaction. When water ratio was increased, the reaction became
inefficient because of the low solubility of decaborane. The reductive
amination of various acetals with 3-toluidine in aqueous methanol gave
the corresponding amines in high yield (Entries 3–8). The reductive ami-
nation of n-butyl vinyl ether gave secondary amine in 20% yield due to
the stability against its hydrolysis to aldehyde under the reaction condi-
tion (Entry 9).
In conclusion, acetals andketals were reductively aminatedwith
anilines using decaborane as a catalyst and a reagent in methanol or in
aqueous methanol in high yield in one pot way. However, direct reductive
amination of some enol ethers with aniline having electron withdrawing
groups was not efficient and gave the reductive amination product in low
yield. For basic 3-toluidine, the reductive amination reaction with various
acetals in aqueous methanol (methanol:water ¼ 10:1) in consecutive way
gave the corresponding amines generally in high yield.
EXPERIMENTAL
All solvents andreagents were usedas purchase.d Silica gel 60
(230–400 mesh, Merck) was usedfor column chromatography, and
silica gel 60F254 plates (0.25 mm, Merck) were usedfor TLC. 1H NMR
spectra were recorded on a VARIAN at 300 MHz.
Two typical procedures depending on the basicity of anilines are as
follows:
Method 1 (Table 1). To a homogeneous solution of 4-nitroaniline
(92 mg, 0.67 mmol) andacetals (112 mg, 0.74 mmol, 1.1 equiv.) in metha-

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Reductive Amination Using Decaborane
3393
nol (5 mL, HPLC grade), decaborane (24 mg, 0.2 mmol, 30 mol%) was
added at rt under nitrogen. The resulting solution was stirred until the
amine disappeared on TLC using a solution of ethyl acetate and hexane
(1:4). During the reaction, a yellow solidwas formed. After 1 h, the
reaction was cooled, filtered, washed with ice cold methanol and dried
to give a yellow crystalline solidin 96% yield(147 mg).
1
N-Benzyl-4-nitroaniline. H NMR (CDCl3, 300 MHz): 8.09 (d, J ¼
5.1 Hz, aromatic 2H), 7.32–7.38 (m, aromatic 5H), 6.58 (d, J ¼ 5.1 Hz,
aromatic 2H), 4.85 (s, 1H), 4.43 (d, J ¼ 3.9 Hz, 2H).
N-Benzyl-3-nitroaniline. 1H NMR (CDCl3, 300 MHz): ꢀ 7.52–7.65
(m, aromatic 1H), 7.45 (t, J ¼ 2.1 Hz, aromatic 1H), 7.29–7.38 (m,
aromatic 6H), 6.86–6.9 (m, aromatic 1H), 4.39 (s, 3H).
1
Methyl 4-N-benzylaminobenzoate. H NMR (CDCl3, 300 MHz): 7.86
(d, J ¼ 8.7 Hz, aromatic 2H), 7.29–7.36 (m, aromatic 5H), 6.59 (d, J ¼
8.7 Hz, aromatic 2H), 4.49 (s, 1H), 4.39 (s, 2H), 3.85 (s, 3H).
N-Piperonyl-4-nitroaniline. 1H NMR (CDCl3, 300 MHz): ꢀ 8.09 (d, J ¼
9.3 Hz, aromatic 2H), 6.81 (d, J ¼ 2.4 Hz, aromatic 3H), 6.56 (d, J ¼
9.3 Hz, aromatic 2H), 5.97 (s, 1H), 4.79 (s, 1H), 4.33 (d, J ¼ 5.4 Hz, 2H).
1
Methyl 4-N-piperonylaminobenzoate. H NMR (CDCl3, 300 MHz): ꢀ
7.86 (d, J ¼ 8.7 Hz, aromatic 2H), 6.79–6.83 (m, aromatic 3H), 6.58 (d, J ¼
8.7 Hz aromatic 2H), 5.95 (s, 2H), 4.29 (s, 2H), 3.85 (s, 3H).
1
N-(4-Nitrophenyl)-1-phenethylamine. H NMR (CDCl3, 300 MHz): ꢀ
8.00 (d, J ¼ 9.3 Hz, aromatic 2H), 7.24–7.37 (m, aromatic 5H), 6.46 (d,
J ¼ 9.3 Hz, aromatic 2H), 4.88 (s, 1H), 4.60 (d, J ¼ 6.3 Hz, 1H), 1.58 (d,
J ¼ 6.6 Hz, 3H).
Methyl
4-(N-1-phenethylamino)-benzoate.
1H NMR
(CDCl3,
300 MHz): ꢀ 7.78 (d, J ¼ 8.7 Hz, aromatic 2H), 7.23–7.34 (m, aromatic
5H), 6.47 (d, J ¼ 9 Hz, aromatic 2H), 4.55–4.57 (m, 1H), 4.48 (s, 1H), 3.81
(s, 3H), 1.55 (d, J ¼ 9.0 Hz, 3H).
N-[4-(4-Methoxyphenyl)-2-butyl]-4-nitroaniline. 1H NMR (CDCl3,
300 MHz): ꢀ 8.05 (d, J ¼ 9.3Hz, aromatic 2H), 7.17 (d, J ¼ 8.7 Hz, aromatic
2H), 6.83 (d, J ¼ 8.7 Hz, aromatic 2H), 6.41 (d, J ¼ 9.3 Hz, aromatic 2H),
4.29 (d, J ¼ 8.1 Hz, 1H), 3.8 (s, 1H), 3.53–3.58 (m, 1H), 2.66 (t, J ¼ 6.6 Hz,
2H), 1.79–1.89 (m, 2H), 1.26 (d, J ¼ 6.6 Hz, 3H).
Methyl N-4-[4-(4-methoxyphenyl)-2-butylamino]-benzoate. 1H NMR
(CDCl3, 300 MHz): ꢀ 7.83 (d, J ¼ 8.7 Hz, aromatic 2H), 7.08 (d, J ¼
8.4 Hz, aromatic 2H), 6.83 (d, J ¼ 8.4 Hz, aromatic 2H), 6.46 (d, J ¼
8.7 Hz, aromatic 2H), 3.94 (d, J ¼ 6.9 Hz, 1H), 3.85 (d, 3H), 3.79 (s, 3H),
3.54 (m, 1H), 2.66 (t, J ¼ 7.8 Hz, 2H), 1.82 (m, 2H), 1.23 (d, J ¼ 6.3 Hz, 3H).
N-Isopropyl-4-nitroaniline. 1H NMR (CDCl3, 300 MHz): ꢀ 8.08
(d, J ¼ 9.3 Hz, aromatic 2H), 6.49 (d, J ¼ 9.3 Hz, aromatic 2H), 4.35
(s, 1H), 3.7–3.77 (m, 1H), 1.27 (d, J ¼ 6.3 Hz, 6H).

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3394Park et al.
Methyl 4-N-isopropylamino-benzoate. 1H NMR (CDCl3, 300 MHz): ꢀ
7.84 (d, J ¼ 9.0 Hz, aromatic 2H), 6.52 (d, J ¼ 9.0 Hz, aromatic 2H), 3.84
(s, 3H), 3.69 (m, 1H), 1.23 (d, J ¼ 6 Hz, 6H).
N-Ethyl-4-nitroaniline. 1H NMR (CDCl3, 300 MHz):
ꢀ
8.09
(d, J ¼ 9.3 Hz, aromatic 2H), 6.52 (d, J ¼ 9.3 Hz, aromatic 2H), 4.40
(s, 1H), 3.22–3.31 (m, 2H), 1.31 (t, J ¼ 8.7 Hz, 3H).
Method 2 (Table 2). To acetal (112 mg, 0.74 mmol, 1.1 equiv.) in 5 mL
of a solution of MeOH andwater (10:1) we addeda catalytic amount of
decaborane (1.6 mg, 2 mol%) and the resulting solution was sirred for 1h
at rt. Andthen, amine (74 mg, 0.67 mmol) anddecaborane (22.8 mg,
28 mol%) was added and the resulting solution was stirred for 1 h. The
reaction was monitoredon TLC using a solution of ethyl acetate and
hexane (1:4). After concentration, the concentratedstuff was chromato-
graphedon silica gel using a solution of ethyl acetate andhexane (1:4 to
1:10). The concentration gave the expected reductive amination product
as syrup in 73% yield(96 mg).
1
N-Benzyl-m-toluidine. H NMR (CDCl3, 300 MHz): ꢀ 7.18–7.27 (m,
aromatic 5H), 6.98 (t, J ¼ 7.5 Hz, aromatic 1H), 6.46 (d, J ¼ 7.5 Hz,
aromatic 1H), 6.36 (d, J ¼ 10.2 Hz, aromatic 2H), 4.21 (s, 2H), 3.85
(s, 1H), 2.81 (s, 3H).
N-(1-Phenethyl)-m-toluidine. 1H NMR (CDCl3, 300 MHz): ꢀ 7.23–7.40
(m, aromatic 5H), 6.99 (t, J ¼ 7.8 Hz, aromatic 1H), 6.49 (d, J ¼ 7.8 Hz,
aromatic 1H), 6.38 (s, aromatic 1H), 6.32 (d, J ¼ 8.4 Hz, aromatic 1H),
4.50 (m, 1H), 3.97 (s, 1H), 2.22 (s, 3H), 1.52 (d, J ¼ 6.6 Hz, 3H).
1
N-Piperonyl-m-toluidine. H NMR (CDCl3, 300 MHz): ꢀ 7.06 (t, J ¼
7.5 Hz, aromatic 1H), 6.67–6.86 (m, aromatic 3H), 6.54 (d, J ¼ 7.2 Hz,
aromatic 2H), 6.44 (d, J ¼ 8.4 Hz, aromatic 1H), 5.94 (s, 2H), 4.22 (s, 2H),
2.27 (s, 3H).
N-[4-(4-Methoxy-phenyl)-2-butyl]-m-toluidine. 1H NMR (CDCl3,
300 MHz): ꢀ 7.11 (d, J ¼ 8.7 Hz, aromatic 2H), 7.04 (t, J ¼ 7.5 Hz, aromatic
1H), 6.84 (d, J ¼ 8.7 Hz, aromatic 2H), 6.50 (d, J ¼ 7.5 Hz, aromatic 1H),
6.36 (d, J ¼ 5.7 Hz, aromatic 2H), 3.80 (s, 3H), 3.44–3.50 (m, 1H), 2.67
(t, J ¼ 7.8 Hz, 2H), 2.26 (s, 3H), 1.71–1.86 (m, 2H), 1.21 (d, J ¼ 6.3 Hz,
3H).
N-Isopropyl-m-toluidine. 1H NMR (CDCl3, 300 MHz): ꢀ 7.04–7.09
(m, aromatic 1H), 6.51 (d, J ¼ 3.6 Hz, aromatic 2H), 6.41 (d, J ¼ 6.9 Hz,
aromatic 2H), 3.61–3.65 (m, 1H), 2.28 (s, 3H), 1.21 (d, J ¼ 6.3 Hz, 6H).
N-Ethyl-m-toluidine. 1H NMR (CDCl3, 300 MHz): ꢀ 7.08 (t, J ¼
8.7 Hz, aromatic 1H), 6.53 (d, J ¼ 7.8 Hz, aromatic 1H), 6.44 (d, J ¼
7.2 Hz, aromatic 2H), 3.09–3.20 (m, 2H), 1.26 (m, 3H).

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Reductive Amination Using Decaborane
ACKNOWLEDGMENT
3395
This work was supportedby Korea Research Foundation Grant
(KRF-2001-015-000000).
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Receivedin Japan November 20, 2002