Magnetic nano Fe3O4 catalyzed solvent-free stereo- and regioselective a-aminolysis of epoxides by amines; A green method for the synthesis of β-amino alcohols

Article abstract of DOI:10.1055/s-0033-1340844

We report the use of magnetic nano Fe₃O₄ as a mild heterogeneous catalyst for the aminolysis of epoxides with amines. The approach constitutes a green method for the formation of a variety of β-amino alcohols with very high stereo- and regioselectivity under solvent-free and ambient reaction conditions. The aminolysis of chiral epoxides with amines gave the corresponding chiral β-amino alcohols with complete inversion of stereochemistry. The magnetic nano Fe₃O₄ catalyst can be easily recovered and recycled. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1340844

LETTER
▌835
lMettear gnetic Nano Fe3O4 Catalyzed Solvent-Free Stereo- and Regioselective
Aminolysis of Epoxides by Amines; a Green Method for the Synthesis of
β-Amino Alcohols
Regio- and Stereoselective Aminolysis of Epoxides
Amit Kumar, Ramarao Parella, Srinivasarao Arulananda Babu*
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Manauli P.O., Sector 81, SAS Nagar,
Mohali, Knowledge City, Punjab 140306, India
Fax +91(172)2240266; E-mail: sababu@iisermohali.ac.in
Received: 11.12.2013; Accepted after revision: 28.01.2014
type reactions using nano Fe3O4 and CuFe2O4 as cata-
Abstract: We report the use of magnetic nano Fe3O4 as a mild het-
lysts.8
erogeneous catalyst for the aminolysis of epoxides with amines.
The approach constitutes a green method for the formation of a va-
riety of β-amino alcohols with very high stereo- and regioselectivity
under solvent-free and ambient reaction conditions. The aminolysis
of chiral epoxides with amines gave the corresponding chiral β-ami-
no alcohols with complete inversion of stereochemistry. The mag-
netic nano Fe3O4 catalyst can be easily recovered and recycled.
To the best of our knowledge, the aminolysis of epoxides
with amines by using iron-based magnetic nanoparticles
has not been reported. In a continuation of our recent
work, we report herein the use of magnetically separable
nano Fe3O4 as an efficient, heterogeneous catalyst for the
aminolysis of epoxides, leading to the formation of β-ami-
no alcohols with very high stereo- and regioselectivity un-
der solvent-free conditions at ambient temperature.
Key words: amino alcohols, epoxides, green chemistry, nano mag-
netite, regioselectivity, stereoselective synthesis
Initially, optimization reactions with epoxide 1a and ani-
line 2a were undertaken (Table 1). The reaction of epox-
ide 1a with aniline 2a in the absence of any catalyst, at
either room temperature or 80 °C, under solvent-free con-
ditions afforded the product 3a in only 30 and 38% yields,
respectively (Table 1, entries 1 and 2). The reaction of 1a
with 2a in the presence of nano Fe3O4 (5 or 10 mol%, par-
ticle size of 50 nm or less) at 80 °C without solvent gave
3a in 95 and 99% yields, respectively (Table 1, entries 3
and 4). Carrying out the same reaction at ambient temper-
ature for 12 and 20 hours furnished 3a in 92 and 99%
yields, respectively (Table 1, entries 5 and 6). Use of other
heterogeneous catalysts, such as powdered Fe3O4 or nano
Fe2O3 or nano CuFe2O4, gave 3a in low to moderate yields
(35–56%; Table 1, entries 7–9).9,10
The β-amino alcohol unit is present in numerous natural
products and bioactive molecules,1–3 and such compounds
can be synthesized by heating epoxides with excess
amine.4 However, this transformation is associated with
limitations such as poor efficiency and regioselectivity,
requirement for elevated reaction temperatures when an
unreactive amine is used, and polymerization/isomeriza-
tion of epoxides under the experimental conditions. To
address these limitations and because of the importance of
β-amino alcohols in medicinal chemistry, several methods
have been developed for aminolysis of epoxides with
amines by using a range of catalysts/activators/promoters
including metal triflates, metal halides, and Lewis acid
catalysts.4–6 However, in some of the existing methods,
the separation, recovery and recycling of moisture-sensi-
tive homogenous catalysts is not possible or high temper-
atures or strong Lewis acidic catalysts are required. Low
yields and poor regioselectivity can sometimes result be-
cause the strongly acidic catalysts can cause polymeriza-
tion or rearrangement of epoxides. A limited number of
reports are available that deal with the aminolysis of ep-
oxides involving an efficient catalyst recovery and, hence,
there is scope for an improved protocol for this important
reaction.
The generality of the nano Fe3O4 catalyzed aminolysis of
the meso epoxides 1a and 1b was tested with an array of
anilines 2a–h under solvent-free conditions and all gave
the corresponding β-amino alcohols 3a–e, 4a and 4b with
trans stereochemistry in very good yields (91–99%; Table
2, entries 1–5, 8 and 9). In the reaction of the epoxide 1a
with secondary amines 2f and 2g, β-amino alcohols 3f and
3g were obtained in 58 and 81% yields, respectively (Ta-
ble 2, entries 6 and 7). Styrene oxide 1c underwent cleav-
age by anilines 2a–c and 2e, 2g, 2i, and 2j with
preferential attachment at the benzylic position and selec-
tively furnished the corresponding primary alcohols 5a–g
in 65–85% yields (Table 2, entries 10–16).9,10
In recent years, the development of greener protocols and
catalytic methods with recyclability of catalyst has been
considered to be an important task for the synthetic chem-
istry community. In this regard, magnetic nanoparticle
based heterogeneous catalysts have attracted significant
attention.7 Recently, we have reported Friedel–Crafts-
In contrast, with secondary amines 2k–n, 1c underwent
selective cleavage by the amine attacking at the less hin-
dered position, selectively furnishing the corresponding
secondary alcohols 5h–k in 73–99% yield (Table 3, en-
tries 1–4). Similarly, treatment of aliphatic epoxides such
as 2-(phenoxymethyl)oxirane (1d) and 2-[(o-tolyl-
oxy)methyl]oxirane (1e) with amines 2a and 2o in the
SYNLETT 2014, 25, 0835–0842
Advanced online publication: 06.03.2014
0
9
3
6
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5
2
1
4
1
4
3
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2
0
9
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DOI: 10.1055/s-0033-1340844; Art ID: ST-2013-D1129-L
© Georg Thieme Verlag Stuttgart · New York

836
A. Kumar et al.
LETTER
presence of nano Fe3O4 afforded the respective secondary
alcohols 6–8 (Table 3, entries 5–7) in 70 and 81% yields,
respectively. 2-Methyl-2-vinyloxirane (1f) underwent
cleavage by the primary amine 2a in the presence of nano
Fe3O4 to give primary alcohol 9 (78%; Table 3, entry 8).
Treatment of epichlorohydrin (1g) and ethyl 3-phenylgly-
cidate (1h, mixture of trans and cis) with aniline afforded
the respective amino alcohols 10 and 11 in 60 and 75%
yields, respectively (Table 3, entries 9 and 10).9,10
Table 1 Nano Fe3O4 Catalyzed Aminolysis of Epoxide 1a
OH
catalyst (x mol%)
H2N
O
+
NH
neat condition
r.t. to 80 °C, time (h)
open atmosphere
1a
3a
(1.5 mmol)
2a (1 mmol)
Entry
Catalyst
(x mol%)
Temp.
(°C)a
Time
(h)
Yield of
3a (%)
The generality of this method was further tested by per-
forming double aminolysis reactions (Table 4). The nano
Fe3O4catalyzed reaction of epoxides 1a and 1b with 4,4′-
methylenedianiline (2p) gave amino alcohols 12 and 13
(Table 4, entries 1 and 2). Similarly, double aminolysis of
epoxide 1i with various amines furnished the amino alco-
hols 14–16 in 50–75% yields (Table 4, entries 3–5).9,10
1
2
none
r.t.
80
80
80
r.t.
r.t.
24
24
24
6
30
38
95
99
92
99
50
35
56
(0)b
none
3
nano Fe3O4 (5)
nano Fe3O4 (10)
nano Fe3O4 (10)
nano Fe3O4 (10)
4
5
12
20
24
24
24
24
12
We then extended the scope of this protocol by producing
chiral β-amino alcohols. In this regard, we performed the
stereoselective ring opening of chiral epoxides 1j (R) and
1k (S) with amines under the optimized conditions (Table
5). The nano Fe3O4 catalyzed aminolysis of chiral epox-
ides 1j (R) and 1k (S) with anilines 2a–c, 2e and 2j gave
the respective chiral amino alcohols 5l–r in very good
yields with excellent enantioselectivity (99% ee; Table 5,
entries 1–7). Similarly, the aminolysis of chiral epoxides
1k (S) with 2° amine 2k gave the respective chiral amino
alcohol 5s in very good yield with excellent enantioselec-
tivity (99% ee; Table 5, entry 8). Notably, chiral epoxides
1j (R) and 1k (S) gave the chiral β-amino alcohols
(99% ee) with complete inversion of stereochemistry,
which indicated the operation of a clean SN2-type mecha-
nism in the aminolysis process.
6
7
powder Fe3O4 (10)
nano Fe2O3 (10)
nano CuFe2O4 (10)
nano Fe3O4 (5)
r.t.
8
r.t.
9
r.t.
10
11
r.t.
nano Fe3O4 (10)
reflux
(63)b
(80)c
(32)d
a In all the cases, r.t. refers to the temperature range 28–32 °C.
b MeCN (2 mL) was used as solvent.
c CH2Cl2 (2 mL) was used as solvent.
d Toluene (2 mL) was used as solvent.
Table 2 Nano Fe3O4 Catalyzed Ring Opening of Epoxides9a–c
OH
O
nano Fe3O4
(10 mol%)
H2N
+
n
NH
neat condition
r.t., 20 h
open atmosphere
n
1a n = 2
1b n = 1
R
R
2a–i
3a–h n = 2
4a,b n = 1
(y mmol)
(x mmol)
Entry
Epoxide
Amine
Product 3
Yield (%)
99
NH2
OH
O
1
2
3a
3b
N
H
1a (1.5 mmol)
2a (1 mmol)
OH
NH2
O
N
H
92
1a (1 mmol)
2b (1 mmol)
H2N
OH
3
4
5
3c; R = Me
3d; R = OMe
3e; R = Cl
98
99
99
O
R
R
N
H
1a (2 mmol)
2c–e (1 mmol)
Synlett 2014, 25, 835–842
© Georg Thieme Verlag Stuttgart · New York

LETTER
Regio- and Stereoselective Aminolysis of Epoxides
837
Table 2 Nano Fe3O4 Catalyzed Ring Opening of Epoxides9a–c (continued)
OH
O
nano Fe3O4
(10 mol%)
H2N
+
n
NH
neat condition
r.t., 20 h
open atmosphere
n
1a n = 2
1b n = 1
R
R
2a–i
3a–h n = 2
4a,b n = 1
(y mmol)
(x mmol)
Entry
Epoxide
Amine
Product 3
Yield (%)
58
NHMe
OH
O
6
7
3f
N
Me
1a (1.5 mmol)
2f (1 mmol)
OH
O
N
H
3g
81
N
1a (1.5 mmol)
2g (1 mmol)
NH2
R
OH
O
8
9
4a; R = H
4b; R = Cl
93
91
R
N
1b (1.5 mmol)
H
2a (1 mmol)
2h (1 mmol)
O
OH
H2N
10a
11
12
13
5a; R = H
5b; R = Me
5c; R = Cl
5d; R = NO2
83
66
70
65
NH
R
R
2a,c,e,i (1 mmol)
1c (2 mmol)
OH
NH
NH2
O
14
5e
75
1c (2 mmol)
2b (1 mmol)
NH2
Br
O
OH
NH
Br
15
16
5f
85
76
1c (2 mmol)
2j (0.8 mmol)
O
OH
N
N
H
5g
2g (0.8 mmol)
1c (2 mmol)
a Epoxide 1a (1.5 mmol) and amine 2 (1.0 mmol) was used.
The magnetic nano Fe3O4 can be easily separated9b and re- the nano Fe3O4 was stable, and the FT-IR spectra and
cycled. Thus, the aminolysis of epoxide 1a and 2a was XRD patterns (see the Supporting Information) indicated
carried out by using recovered magnetic nano Fe3O4 (up the absence of any impurities in the recycled nano Fe3O4.
to the ten runs), which gave the product 3a in 85% yield We also performed the aminolysis of epoxide 1l with iso-
in the 10th run of the catalyst (Table 6). Examination of propyl amine in the presence of nano Fe3O4, which gave
the HRTEM images of the recovered nano Fe3O4 (see the (R,S)-propranolol in 85% yield (Table 6).
Supporting Information) showed that the morphology of
© Georg Thieme Verlag Stuttgart · New York
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A. Kumar et al.
LETTER
Table 3 Nano Fe3O4 Catalyzed Aminolysis of Epoxides9a–c,a
Entry
Epoxide
Amine
Product
Yield (%)
X
X
O
N
1
2
5h; X = O
5i; X = CH2
99
87
N
H
OH
2k (1 mmol)
2l (1 mmol)
1c (1.5 mmol)
O
N
NH
3
4
5j
73
78
OH
2m (1.5 mmol)
1c (1 mmol)
O
N
N
N
N
N
5k
NH
OH
1c (1.5 mmol)
2n (1 mmol)
NH2
O
O
O
NH
OH
O
5
6
7
6
7
8
73
81
70
O
O
1d (1.5 mmol)
2a (1 mmol)
NH
OH
O
NH2
2o (2 mmol)
1d (1 mmol)
NH2
NH
OH
O
O
Me
1e (1.5 mmol)
Me
2a (1 mmol)
NH2
O
8
9
78
NH
1f (1.5 mmol)
2a (1 mmol)
HO
NH2
OH
O
Cl
Cl
9
10
60
75
HN
1g (1.5 mmol)
2a (1 mmol)
NH2
O
OEt
Ph
NH COOEt
OH
10
11
O
Ph
1h (1.5 mmol)
2a (1 mmol)
a All the reactions were carried out by using nano Fe3O4 (10 mol%) without solvent and open to the atmosphere for 20 h.
Synlett 2014, 25, 835–842
© Georg Thieme Verlag Stuttgart · New York

LETTER
Regio- and Stereoselective Aminolysis of Epoxides
839
Table 4 Nano Fe3O4 Catalyzed Double Ring Opening of Epoxides9a–c,a
Entry
Epoxide
Amine
Product
Yield (%)
H2N
NH2
OH
HO
O
HN
NH
1
99
99
1a (4 mmol)
2p (1 mmol)
12
H2N
NH2
OH
HO
O
HN
NH
2
1b (4 mmol)
2p (1 mmol)
13
OH
H
N
O
O
NH2
O
OH
H
N
3
60
O
O
O
2a (4 mmol)
1i (1 mmol)
14
OH
Me
O
O
N
O
NHMe
OH
Me
N
4
75
O
O
O
2f (4 mmol)
1i (1 mmol)
15
16
OH
N
O
O
O
O
O
N
O
5
50
N
H
OH
O
O
O
2k (4 mmol)
1i (1 mmol)
a All reactions were carried out using nano Fe3O4 (10 mol%) without solvent and open to the atmosphere for 20 h.
Table 5 Nano Fe3O4 Catalyzed Aminolysis of Chiral Epoxides9a–c
Entry
Epoxide
Amine
Product
Yield (%) [ee (%)]
99 (99)
OH
NH
O
H2N
(S)
1a
R
R
2a (1 mmol)
1j (R) (2 mmol)
5l; R = H
2
3
2c (1 mmol)
2e (1 mmol)
5m; R = Me
5n; R = Cl
75 (99)
75 (99)
© Georg Thieme Verlag Stuttgart · New York
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A. Kumar et al.
LETTER
Table 5 Nano Fe3O4 Catalyzed Aminolysis of Chiral Epoxides9a–c (continued)
Entry
4
Epoxide
Amine
Product
Yield (%) [ee (%)]
OH
O
H2N
NH
(R)
93 (99)
2a (0.8 mmol)
1k (S) (2 mmol)
5o
5p
5q
OH
Br
Br
Br
O
H2N
NH
(S)
5
6
87 (99)
87 (99)
1j (R) (2 mmol)
2j (0.8 mmol)
OH
Br
O
H2N
(R)
NH
1k (S) (2 mmol)
2j (0.8 mmol)
OH
O
H2N
(S)
NH
7
8
70 (99)
85 (99)
1j (R) (2 mmol)
2b (1 mmol)
5r
5s
O
OH
O
HN
O
N
(S)
2k (0.8 mmol)
1k (S) (2 mmol)
a The reaction was carried out using 1j (2.0 mmol) and amine 2a (0.8 mmol).
Table 6 Catalyst Recycling and Synthesis of (RS)-Propranolol9a–c
NH2
nano Fe3O4
OH
(10 mol%)
O
+
neat condition
r.t., 20 h, open atmosphere
NH
3a
1a
2a
(1 mmol)
(1.5 mmol)
Run
Initial
1
2
3
4
5
6
7
8
9
10
Yield of 3a (%)
99
20
98
20
93
20
92
20
93
24
85
24
85
24
89
30
89
26
87
26
85
24
Reaction time (h)
Synthesis of (RS)-propranolol
OH
NH
O
NH2
O
OH
Cl
O
2o
O
nano Fe3O4
(10 mol%)
neat condition
r.t., 20 h
K2CO3, MeCN
reflux,16 h
(RS)-propranolol
17 85%a
1l 80%
a The reaction was carried out using 1l (1.0 mmol) and 2o (2.0 mmol).
Synlett 2014, 25, 835–842
© Georg Thieme Verlag Stuttgart · New York

LETTER
Regio- and Stereoselective Aminolysis of Epoxides
841
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In summary, we have disclosed a new catalytic protocol
for the aminolysis of epoxides by using magnetically sep-
arable nano Fe3O4 as the catalyst. The aminolysis of chiral
epoxides gave the β-amino alcohols with complete inver-
sion of stereochemistry. The reaction is carried out in the
absence of solvent, and the nano Fe3O4 can be easily re-
covered and recycled.
Acknowledgment
We thank IISER-Mohali for funding and the NMR, HRMS and X-
ray facilities of IISER-Mohali. R.P. thanks the CSIR, New Delhi,
for a fellowship.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SuppInigfor
m
o
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(9) (a) Generally, the reactions were carried out by using 1 (1.5
mmol), amine 2 (1 mmol) and nano Fe3O4 (10 mol%)
without solvent and open to the atmosphere for 20 h. See the
respective tables for individual entries. (b) General
Experimental Procedure: Epoxide (1.5–2 mmol), amine
(1.0 mmol) and nano Fe3O4 (10 mol%, 23 mg, particle size
= <50 nm) were stirred at r.t. open to the atmosphere for
20 h. EtOAc (1–2 mL) was then added and the mixture was
stirred for 1–2 min. A magnet was then externally applied to
the flask and the catalyst was allowed to accumulate at the
walls of the flask; the resulting clear solution was transferred
to a fresh flask by using a pipette. This step was repeated
twice more, and the combined organic layers were dried over
anhydrous Na2SO4, filtered, and the solvent evaporated.
Purification by column chromatography on silica gel
(EtOAc–hexanes, 20:80) gave the pure products. (c) All the
reactions were carried out at r.t (range 28–32 °C) unless
otherwise mentioned. (d) When one of the substrates
(epoxide or amine) was a solid, to facilitate homogeneous
stirring, the reaction was carried out by using 1 mmol of the
solid sample and the other corresponding liquid sample in
excess (2 mmol) and the yield was calculated based on the
limiting reagent.
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 835–842

842
A. Kumar et al.
LETTER
5e, 5h, and 5k. Their crystallographic data have been
deposited at the Cambridge Crystallographic Data Centre
(5c; CCDC-976219, 5e; CCDC-976220, 5h; CCDC-976221
and 5k; CCDC-976222). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
(10) All the reactions were regioselective and gave only a single
regioisomer. We were not able to isolate any of the
corresponding minor regioisomers for characterization. The
identity of all compounds obtained in this work was
established by comparison of their spectroscopic data with
reported data and we also confirmed the structure of
representative regioisomers from the X-ray structures of 5c,
Synlett 2014, 25, 835–842
© Georg Thieme Verlag Stuttgart · New York

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