Regioselective ring-opening of epoxides with amines using Zn(ClO4)2-Al2O3 as a heterogeneous and recyclable catalyst

Article abstract of DOI:10.1016/j.tetlet.2008.01.044

A simple and efficient method has been developed for the synthesis of β-amino alcohols by regioselective ring-opening of epoxides with amines in the presence of zinc perchlorate-neutral alumina as a heterogeneous recyclable catalyst at room temperature in high yields.

Full text of DOI:10.1016/j.tetlet.2008.01.044

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1795–1800
Regioselective ring-opening of epoxides with amines using
Zn(ClO4)2–Al2O3 as a heterogeneous and recyclable catalyst
Muchchintala Maheswara a, Kummari Subba Venkata Krishna Rao b, Jung Yun Do a,*
a Department of Advanced Materials for Information and Display, Pusan National University, Busan 609-735, Republic of Korea
b Department of Engineering and Polymer Sciences, Pusan National University, Busan 609-735, Republic of Korea
Received 27 November 2007; revised 24 December 2007; accepted 10 January 2008
Available online 15 January 2008
Abstract
A simple and efficient method has been developed for the synthesis of b-amino alcohols by regioselective ring-opening of epoxides
with amines in the presence of zinc perchlorate–neutral alumina as a heterogeneous recyclable catalyst at room temperature in high
yields.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Epoxides; Amines; Zinc perchlorate–neutral alumina; b-Amino alcohols
1. Introduction
phenyl phosphonate,20 clays,21 polymer supported copper
sulfate,22 ionic liquids23 and rare earth metals24 have been
utilized. However, there are some limitations such as longer
reaction times, and use of expensive reagents in stoichio-
metric quantities and hazardous organic solvents and poor
regioselectivity. Zinc perchlorate Zn(ClO4)2 belongs to the
class of Lewis acids. The catalyst using zinc perchlorate
supported on neutral alumina and a related solid acid
catalyzed reaction was the field of growing interest. The
acidic or neutral solid substances like silica gel, alumina,
ion exchange resin and active carbon are suitable supports
and the more commonly being used one is neutral alumina
for various organic transformations.25
Regioselective ring-opening of epoxides with amines is
an important reaction for synthetic organic and medicinal
chemists as the resultant b-amino alcohols represent a wide
range of b-adrenergic blockers and used in the manage-
ment of cardiovascular disorders.1 The versatility of this
transformation is recognized well as it constitutes the key
step for the synthesis of b2-adrenoceptor agonists,2 anti
HIV agents,3 glycosidase inhibitor,4 4-demethoxydauno-
mycine,5 antimalarial agents,6 liposidomycin B class of
antibiotics,7 taxoid side chain,8 protein kinase C inhibitor
balanol9 and naturally occurring brassinosteroids,10 a vast
range of biologically active natural and synthetic prod-
ucts,11 unnatural amino acids12 and chiral auxiliaries for
asymmetric synthesis.13 The classical synthesis of b-amino
alcohols involves the heating of epoxide with an excess of
amines at elevated temperature.14 Recently, various meth-
ods have been reported using metal amides,15 metal salts,16
metal alkoxides,17 metal triflates18 and metal halides.19
Some heterogeneous catalysts such as zirconium sulfo-
2. Results and discussion
In this Letter, we have described a mild and efficient
method for the nucleophilic ring-opening of epoxides with
amines in the presence of heterogeneous catalyst zinc
perchlorate–neutral alumina to afford the corresponding
b-amino alcohols in excellent yields. It was observed in
the absence of catalyst, the reaction did not proceed in
dichloromethane even after prolonged reaction times
(Scheme 1). In the presence of zinc perchlorate Zn(ClO4)2
*
Corresponding author. Tel./fax: +82 51 581 2348.
E-mail address: jydo@pusan.ac.kr (J. Y. Do).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.044

1796
M. Maheswara et al. / Tetrahedron Letters 49 (2008) 1795–1800
OH
attacked at the less hindered carbon atom and the b-amino
alcohols were formed.
O
Zn(ClO4)2-Al2O3
CH2Cl2 r. t.
However, simple acyclic aliphatic amines such as pentyl
amine and hexyl amine did not give any satisfactory yields
less than 20% in the reaction of aliphatic epoxide of propyl-
ene oxide (Scheme 4).
R
NH2
+
R
Ph
Ph
N
H
Scheme 1.
The regioselective outcome can be explained by the
steric and electronic factors associated with the epoxide
and the amine. In general, the alkyl amines are harder
bases than the aromatic amines and therefore more
efficiently compete with the epoxides for the catalyst.
According to the Pearson theory (hard–soft acid–base
theory), ethers are considered hard bases, as alkyl amines.
In contrast, aryl amines are considered as borderline cases.
This being the case, the harder amines will tend to retard
the rate of the reaction, in line with our observations and
concomitantly requires slightly elevated levels of catalyst
to observe similar rates and conversions. The lack of reac-
tivity of aliphatic amines towards aminolysis of epoxides
has been reported due to the stronger complexation with
the catalyst as a consequence of their higher basicity.
In general, the methodology worked well independently
on the nature of the epoxide, furnishing the corresponding
b-amino alcohols in high yields. The catalyst system of zinc
perchlorate–neutral alumina was conveniently separated
from the reaction mixture by simple filtration. The catalyst
was recovered, activated and reused for two consecutive
times with only slight variation in the yields of the corres-
ponding products. It is clear from Table 2 that the activity
and selectivity are retained during the two recycling steps.
without using alumina only trace amount of product was
obtained even after 2 h.
Using the optimized reaction condition, styrene oxide
underwent reaction with aryl amines, in the presence of
zinc perchlorate–neutral alumina, at room temperature to
afford the corresponding b-amino alcohols in excellent
yields with high regioselectivity. The epoxide opening takes
place in a regioselective manner with preferential nucleo-
philic attack at the benzylic position. This success encour-
aged us to examine the generality of this reaction with
various epoxides and amines and the results are summa-
rized in Table 1. Interestingly, aliphatic amines on treat-
ment with styrene oxide formed the product by cleavage
at the terminal position. Thus, aryl amines, being less
nucleophilic react preferentially at the benzylic carbon of
styrene oxide, whereas in the case of aliphatic amines, the
preferential attack of the amine occurs at the terminal car-
bon of styrene oxide, because of the increased nucleophilic-
ity favouring the SN2 process.26 The cyclic aliphatic amines
such as morpholine and pyrrolidine reacted with styrene
oxide to afford the corresponding b-amino alcohols in
excellent yields (Table 1, entries e and f). We have clearly
1
observed the H NMR spectra of crude products showing
the formation of only one regioisomer in each case and
no other product could be detected.
3. Conclusion
The ring-opening of cyclohexene oxide was treated with
various amines such as aromatic, benzylic and aliphatic
amines to afford the corresponding amino alcohols in high
We have demonstrated a mild and efficient method for
the preparation of b-amino alcohols by the ring-opening
of epoxides with amines using zinc perchlorate–neutral alu-
mina at room temperature. The present method is associ-
ated with several advantages such as application of a
heterogeneous catalyst, operational simplicity, short reac-
tion times, reduction of by-products, high yields and excel-
lent regioselectivity are the notable advantages of this
protocol.
1
yields. In the H NMR spectra of the products, the cou-
pling constants of the ring protons adjacent to the –NHR
and –OH groups provided the evidence for this stereo-
chemical assignment. For example, the J values of these
two protons for 3g are 3.5, 10.0 and 10.0 Hz and 4.0,
10.0 and 10.5 Hz, suggesting the trans-configuration of
the compound (Scheme 2).
The glycidyl ethers also reacted smoothly with aromatic,
benzylic and aliphatic amines to afford the corresponding
b-amino alcohols in excellent yields with high regioselectiv-
ity. The reaction did not proceed under solvent-free condi-
tions even after prolonged reaction times (Scheme 3).
In the glycidyl ethers reactions, the main product was b-
amino alcohols, where the nucleophile strongly favoured
attack at the less hindered carbon atom of the epoxide ring.
Presumably, this is because the zinc perchlorate–neutral
alumina formed a chelate structure with the oxygen atoms
of the glycidyl ether. This is also probably due to the
decreased Lewis acidic effect that the metal would usually
have on the internal carbon atom, allowing steric effects
to play a more dominant role. Therefore the nucleophile
4. Experimental
4.1. The preparation of Zn(ClO4)2–Al2O3
Ten grams of neutral alumina (BIO-RAD, Brockman
activity, grade I, 2–44 lm) was impregnated with 11.4 g
of Zn(ClO4)2Á6H2O (Aldrich) dissolved in 10 g of distilled
water, and the mixture was stirred at room temperature
and the precatalyst was then dried for 16 h at 120 °C,
and then calcined for 4 h at 250 °C under air. The resulting
powdered catalyst was stored under argon for future use.
General procedure for the synthesis of b-amino alcohols:
To a solution of an epoxide (1 mmol) in dichloromethane
(5 mL), amine (1 mmol) and zinc perchlorate–neutral

M. Maheswara et al. / Tetrahedron Letters 49 (2008) 1795–1800
1797
Table 1
Zinc perchlorate–Al2O3 catalyzed ring-opening of epoxides with amines
Entry
Epoxide 1
Amine 2
Product 3
Time (h)
1.0
Yield (%)
OH
NH2
O
a
95
N
H
NH2
OH
OMe
O
b
1.5
92
N
H
OMe
NH2
OH
O
c
2.0
90
N
H
OH
O
O
H
N
d
e
2.5
3.5
89
94
NH2
OH
O
O
N
HN
OH
O
NH
f
3.5
1.5
91
93
N
NH2
OH
g
O
N
H
NH2
OH
OMe
h
O
2.0
91
N
H
OMe
NH2
OH
i
2.5
89
O
N
H
(continued on next page)

1798
M. Maheswara et al. / Tetrahedron Letters 49 (2008) 1795–1800
Table 1 (continued)
Entry
j
Epoxide 1
Amine 2
Product 3
Time (h)
3.0
Yield (%)
87
OH
O
NH2
N
H
OH
NH2
O
O
O
O
O
k
1.5
2.0
2.5
3.0
90
91
88
85
O
O
O
O
N
H
Ph
Ph
Ph
Ph
NH2
OMe
OH
OH
OH
O
O
O
l
N
H
OMe
NH2
m
N
H
n
NH2
N
H
The structures of the products were established from their spectral (1H NMR and MS) data.
The spectral (IR, 1H NMR and MS) data of the
b-amino alcohols (Table 1) are given below.
OH
Zn(ClO4)2-Al2O3
CH2Cl2 r. t.
+
O
R
NH2
Compound 3a: IR (KBr): mmax 3341, 3267, 3047, 3051,
2972, 2847, 1605, 1543, 1438, 1361, 1232, 1128, 1045,
R
N
H
1
878, 746 cmÀ1; H NMR (CDCl3, 200 MHz): d 3.78 (dd,
Scheme 2.
1H, J = 5.0, 10.5 Hz), 3.90 (dd, 1H, J = 4.0, 10.5 Hz),
4.55 (dd, 1H, J = 6.5, 10.8 Hz), 6.40 (d, 2H, J = 7.5 Hz),
6.80 (t, 1H, J = 7.8 Hz), 6.95 (d, 2H, J = 8.0 Hz), 7.30–
7.45 (m, 5H); EIMS: m/z (%): 213 (M+), 195 (18), 185
(10), 107 (100), 91 (35), 77 (28), 57 (40).
alumina [10 mol % (37 mg)] were added. The reaction
mixture was stirred at room temperature and reaction was
monitored by TLC. The resulting mixture was filtered and
then washed with dichloromethane (2 Â 5 mL). The filtrate
was concentrated under vacuo and the products were puri-
fied by silica gel column chromatography eluted by an
EtOAc and hexane to afford the corresponding pure b-
amino alcohols in excellent yields.
Compound 3b: IR (KBr): mmax 3385, 3271, 3049, 2937,
2851, 1614, 1546, 1438, 1361, 1293, 1202, 1138, 1045,
1
1065, 878, 741 cmÀ1; H NMR (CDCl3, 200 MHz): d 3.76
(dd, 1H, J = 4.5, 10.0 Hz), 3.90 (dd, 1H, J = 4.0,
10.0 Hz), 3.93 (s, 3H), 4.50 (dd, 1H, J = 6.0, 10.5 Hz),
6.45 (d, 2H, J = 8.1 Hz), 6.90 (d, 2H, J = 8.0 Hz),
OH
O
Zn(ClO4)2-Al2O3
O
O
R
R
NH2
+
CH2Cl2 r. t.
Ph
Ph
N
H
Scheme 3.

M. Maheswara et al. / Tetrahedron Letters 49 (2008) 1795–1800
1799
OH
J = 4.0, 10.0, 10.5 Hz), 6.65–7.10 (m, 5H); EIMS: m/z
(%): 191 (M+), 173 (10), 147 (10), 131 (15), 117 (20), 105
(12), 92 (20), 81 (100), 77 (31), 52 (41), 41 (18).
O
Zn(ClO4)2-Al2O3
H
N
R
NH2
+
R
CH2Cl2 r. t.
5h
Compound 3h: IR (KBr): mmax 3361, 3274, 3052, 2943,
2851, 1608, 1569, 1506, 1452, 1315, 1243, 1205, 1122,
Amine
Yield (%)
Pentyl amine
HExyl amine
18
15
1062, 1015, 987, 857, 743 cmÀ1
;
1H NMR (CDCl3,
200 MHz): d 1.08–1.30 (m, 1H), 1.38–1.48 (m, 4H), 1.75–
1.83 (m, 2H), 2.20–2.30 (m, 2H), 3.20 (ddd, 1H, J = 3.5,
10.0, 10.0 Hz), 3.40 (ddd, 1H, J = 4.0, 10.0, 10.0 Hz),
3.80 (br s, 1H), 3.90 (s, 3H), 6.85 (d, 2H, J = 7.0 Hz),
7.30 (d, 2H, J = 7.0 Hz); EIMS: m/z (%): 221 (M+), 206
(18), 190 (56), 174 (10), 114 (100), 99 (10), 92 (25), 82
(15), 77 (68), 63 (18), 51 (24), 43 (20).
Scheme 4.
Table 2
Yields (%) of catalyst recycling reactions
Product
Yields (%)
Initial
1st
2nd
Compound 3i: IR (KBr): mmax 3487, 3261, 3124, 2963,
2847, 1605, 1581, 1508, 1462, 1325, 1246, 1215, 1120,
reaction
recycle
recycle
1059, 1005, 981, 853, 736 cmÀ1
;
1H NMR (CDCl3,
OH
200 MHz): d 1.06–1.28 (m, 1H), 1.35–1.45 (m, 2H), 1.73–
1.82 (m, 2H), 2.21–2.30 (m, 1H), 3.24 (ddd, 1H, J = 3.5,
10.0, 10.0 Hz), 3.41 (ddd, 1H, J = 4.0, 10.0, 10.0 Hz),
3.85 (br s, 1H), 3.91 (s, 2H), 4.10 (br s 1H), 4.80 (s, 2H),
7.20–7.50 (m, 5H); EIMS: m/z (%): 205 (M+), 187 (12),
174 (10), 114 (100), 107 (25), 99 (10), 92 (25), 82 (15), 77
(60), 63 (18), 51 (28), 43 (20).
95
91
86
N
H
OH
93
90
89
87
84
80
Compound 3j: IR (KBr): mmax 3386, 3271, 2845, 1684,
1610, 1573, 1508, 1491, 1426, 1395, 1321, 1276, 1208,
1169, 1124, 1081, 1012, 993, 915, 874, 831, 786,
N
H
734 cmÀ1 1H NMR (CDCl3, 200 MHz): d 0.95 (t, 3H,
;
OH
J = 7.0 Hz), 0.98–1.05 (m, 2H), 1.25–1.45 (m, 8H), 1.72–
1.76 (m, 2H), 2.20–2.40 (m, 1H), 2.50–2.60 (m, 1H), 2.80
(ddd, 1H, J = 11.5, 9.0, 3.0 Hz), 3.20 (ddd, 1H, J = 11.5,
9.0, 3.0 Hz), 4.05 (br s, 2H); EIMS: m/z (%): 171 (M+),
153 (10), 142 (15), 114 (10), 99 (100), 69 (12), 56 (45), 43
(20).
O
N
H
7.35–7.45 (m, 5H); EIMS: m/z (%): 243 (M+), 195 (18), 185
(10), 107 (100), 91 (35), 77 (28), 57 (40).
Compound 3k: IR (KBr): mmax 3392, 3261, 3057, 2926,
2871, 1612, 1542, 1505, 1495, 1456, 1356, 1312, 1296,
Compound 3c: IR (KBr): mmax 3293, 3057, 2941, 2858,
1606, 1580, 1452, 1373, 1256, 1208, 1182, 1106, 1025,
1245, 1110, 1079, 1015, 972, 841, 743 cmÀ1 1H NMR
;
(CDCl3, 200 MHz): d 3.22–3.26 (m, 1H), 3.38–3.42 (m,
1H), 3.60–3.65 (m, 2H), 3.98–4.05 (m, 2H), 4.25 (br s,
1H), 6.64–6.68 (m, 2H), 6.70–6.75 (m, 2H), 6.88–6.91 (m,
2H), 7.05–7.25 (m, 2H); EIMS: m/z (%): 229 (M+), 195
(10), 166 (42), 133 (28), 106 (61), 94 (100), 77 (30), 65
(20), 57 (40), 51 (15).
859, 736 cmÀ1; H NMR (CDCl3, 200 MHz): d 2.70–2.85
1
(m, 2H), 3.30 (br s, 2H), 3.75–3.85 (m, 2H), 4.75–4.82
(m, 1H), 7.35–7.48 (m, 10H); EIMS: m/z (%): 227 (M+),
209 (10), 185 (10), 136 (65), 118 (25), 91 (100), 77 (22), 57
(35), 51 (18).
Compound 3d: IR (KBr): mmax 3379, 3246, 3051, 2943,
2862, 1634, 1589, 1512, 1473, 1410, 1396, 1318, 1295,
Compound 3l: IR (KBr): mmax 3260, 3054, 2939, 2865,
1638, 1605, 1596, 1508, 1495, 1446, 1352, 1302, 1269,
1169, 1086, 1015, 868, 749 cmÀ1
;
1H NMR (CDCl3,
1235, 1170, 1079, 1016, 982, 823, 751 cmÀ1 1H NMR
;
200 MHz): d 0.95 (t, 3H, J = 6.5 Hz), 1.30–1.40 (m, 2H),
1.45–1.55 (m, 2H), 2.35 (br s, 2H), 2.55–2.65 (m, 2H),
3.80–3.90 (m, 2H), 4.25 (d, 1H, J = 6.0 Hz), 7.30–7.45
(m, 5H); EIMS: m/z (%): 193 (M+), 175 (28), 164 (20),
146 (42), 136 (10), 107 (15), 103 (100), 91 (35), 78 (22), 53
(35).
(CDCl3, 200 MHz): d 3.20–3.28 (m, 1H), 3.35–3.45 (m,
1H), 3.63 (br s, 2H), 3.79 (s, 3H), 4.05–4.10 (m, 2H), 4.28
(br s, 1H), 6.70 (d, 2H, J = 8.5 Hz), 6.85 (d, 2H,
J = 8.5 Hz), 6.90–7.10 (m, 3H); EIMS: m/z (%): 259
(M+), 258 (10), 242 (35), 195 (10), 166 (23), 137 (36), 107
(41), 93 (40), 91 (68), 77 (22), 65 (20), 57 (40), 51 (25).
Compound 3m: IR (KBr): mmax 3305, 3272, 3061, 2953,
2846, 1632, 1598, 1583, 1510, 1490, 1453, 1368, 1305,
Compound 3g: IR (KBr): mmax 3352, 3295, 3068, 2935,
2857, 1604, 1543, 1502, 1498, 1450, 1430, 1321, 1243,
1156, 1102, 1031, 1008, 976, 891, 743 cmÀ1
;
1H NMR
1246, 1215, 1126, 1078, 1012, 983, 869, 751 cmÀ1 1H
;
(CDCl3, 200 MHz): d 1.02–1.43 (m, 4H), 1.50 (br s, 1H),
1.70–1.75 (m, 2H), 2.12–2.18 (m, 2H), 3.05 (br s, 1H),
3.15 (ddd, 1H, J = 3.5, 10.0, 10.0 Hz), 3.40 (ddd, 1H,
NMR (CDCl3, 200 MHz): d 2.75–2.85 (m, 1H), 2.95 (br
s, 1H), 3.80 (d, 2H, J = 2.0 Hz), 3.95 (d, 2H, J = 5.0 Hz),
4.05–4.15 (m, 1H), 6.85–6.95 (m, 3H), 7.30–7.40 (m, 7H);

1800
M. Maheswara et al. / Tetrahedron Letters 49 (2008) 1795–1800
EIMS: m/z (%): 243 (M+), 180 (100), 160 (34), 142 (20), 117
(56), 91 (42), 88 (10), 77 (62), 65 (20), 57 (28), 51 (10).
Compound 3n: IR (KBr): mmax 3312, 3256, 3072, 2967,
2853, 1605, 1586, 1508, 1492, 1446, 1358, 1315, 1296,
Hoveyda, A. H. J. Am. Chem. Soc. 1998, 120, 8340–8347; (c) Chng, B.
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Med. Chem. 1989, 32, 1217–1230.
1241, 1173, 1109, 1085, 1035, 1026, 986, 873, 749 cmÀ1
;
12. (a) O’Brien, P. Angew. Chem., Int. Ed. 1999, 38, 326–329; (b) Li, G.;
Chang, H. T.; Sharpless, K. B. Angew. Chem., Int. Ed. 1996, 35, 451–
454.
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4255–4258.
1H NMR (CDCl3, 200 MHz): d 1.05 (t, 3H, J = 6.0 Hz),
1.85–1.95 (m, 4H), 2.70–2.90 (m, 4H), 3.90 (s, 2H), 4.01–
4.05 (m, 1H), 6.80–6.90 (m, 3H), 7.25–7.35 (m, 2H); EIMS:
m/z (%): 209 (M+), 194 (10), 166 (15), 146 (20), 137 (18),
130 (100), 93 (42), 91 (42), 88 (10), 77 (25), 74 (10), 65
(20), 57 (28), 51 (18).
16. (a) Zhao, P. Q.; Xu, L. W.; Xia, C. G. Synlett 2004, 846–850; (b)
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Acknowledgement
17. Sagawa, S.; Abe, H.; Hase, Y.; Inaba, T. J. Org. Chem. 1999, 64,
4962–4965.
This work was supported for two years by the Pusan
National University, Research Grant.
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Chini, M.; Croti, P.; Favero, L.; Macchia, M.; Pineschi, M.
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