Zn(ii)-catalyzed desymmetrization of meso-epoxides by aromatic amines in water

Article abstract of DOI:10.1055/s-2008-1078410

The Zn(OTf)2-SDS-bipyridine 1 system has proved to be an effective catalyst in water for the desymmetrization of ep-oxides 2a-d by anilines 3a-d. After obtaining comparable results by using Sc(III)-SDS and Zn(II)-SDS systems in the case of 2a and 3a, we h

Full text of DOI:10.1055/s-2008-1078410

1574
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Zn(II)-Catalyzed Desymmetrization of meso-Epoxides by Aromatic Amines in
Water
D
esymmetriza
i
tionof
m
m
eso-Epoxides ona Bonollo, Francesco Fringuelli,* Ferdinando Pizzo,* Luigi Vaccaro
Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italia
Fax +39(075)5855560; E-mail: frifra@unipg.it
Received 27 December 2007
Dedicated to Professor Gianlorenzo Marino on the occasion of his 80th birthday
To realize this process many Lewis acid catalysts in com-
Abstract: The Zn(OTf)2–SDS–bipyridine 1 system has proved to
be an effective catalyst in water for the desymmetrization of ep-
oxides 2a–d by anilines 3a–d. After obtaining comparable results
by using Sc(III)–SDS and Zn(II)–SDS systems in the case of 2a and
bination with chiral ligands have been used,10 and gener-
ally, the use of an organic reaction medium has been
adopted to reach high level of enantioselectivity. Highly
3a, we have explored the enantioselective ring opening of small ep- enantioselective processes in sole water are difficult to be
realized and examples in this field are rare.1,5
oxides for which water has never been used as reaction medium. Up
to date the 85% ee obtained in the case of cyclohexene oxide 2b
with amines 3a and 3d are very good results and are the highest val-
Recently, in this context, excellent results have been ac-
complished by Kobayashi et al. by using Sc(DS)3 as
Lewis acid surfactant combined catalyst (LASC) and
enantiomerically pure (S,S)-6,6¢-bis(1-hydroxy-2,2-di-
methyl propyl)-2,2¢-bipyridine in water.5 This protocol al-
lowed to reach in the aminolysis of meso-epoxides by
anilines, levels of enantioselectivity superior to those ob-
tained by using organic solvents.5 The higher efficiency
found by using water has been justified by invoking hy-
drophobic interactions existing among the reactants and
the LASC–bipyridine (1) complex.
ues obtained in water as reaction medium.
Key words: aqueous medium, aminolysis, meso-epoxides, Zn(II)–
SDS, desymmetrization
Water has proven to be an excellent reaction medium to
realize environmentally friendly chemical processes
reaching high chemically efficiency.1–3 We have been in-
vestigating the use of water as a reaction medium for
many years,1a,2,3a,b,4 and more in detail, we have shown
that aqueous medium is one of the most appropriate for
the nucleophilic ring opening of epoxides.2 We found that
Although the results achieved are brilliant, this work has
been essentially focused on the aminolysis of cis-stilbene
oxide (2a) and its derivatives and on the use of Sc(DS)3.
in the case of charged nucleophiles (such as N3 , Br–, I–)
–
the best results are obtained by using Cu(II), Al(III), and
In(III) salts not depending on their counterions.
We have decided to investigate this process in more detail
by extending the study to more Lewis acids [other than
Sc(III)] and to a wider range of meso-epoxides and
amines.
Recently, the aminolysis of epoxides in water has been
investigated3,5 and excellent results have been accom-
plished.
Considering that hydrophobic interactions play a key role
in the efficiency of this process,3a,5 we intended to verify
how the concentration of the reactants in water can influ-
ence the enantioselectivity of the aminolysis of meso-ep-
oxides. We have focused our attention on inexpensive
transition-metal catalysts [Zn(II), Cu(II), Ni(II), and
Co(II), which are interesting biocatalysts generally used
by nature to regulate catalytic processes11] combined with
sodium dodecyl sulfate (SDS), to promote the aminolysis
of cis-stilbene oxide (2a) with aniline (3a) in water by us-
ing (R,R)-bipyridine 1 as chiral ligand. The results ob-
tained are illustrated in Table 1.
We have contributed to the realization of this process and
showed that: a) in water, by regulating appropriately the
pH, amines readily react with epoxides,3b b) at pH 5.0 zir-
conium dodecyl sulfate [Zr(DS)4] is one of the most effec-
tive catalysts but proved to promote poorly the
desymmetrization of cis-stilbene oxide (2a) with aniline
(3a).3a
Optically active b-amino alcohols are important mole-
cules that find wide use as starting materials for the prep-
aration of target molecules6–8 and also as chiral auxiliary
or ligands in asymmetric processes.9 An elegant access
route to this class of molecules in a catalytic and enantio-
selective manner is given by the asymmetric aminolysis of
epoxides (i.e. desymmetrization and kinetic resolu-
tion).5,10
In the reaction of 2a with 3a (1.0 M in water) at 30 °C and
in the presence of 1 mol% of Zn(OTf)2, 2 mol% of SDS,
and 1.2 mol% of bipyridine 1, after 30 hours the conver-
sion to (S,S)-4 was 20% only and with low ee (53%;
Table 1, entry 1). It is noteworthy that by using Sc(III)–
SDS system as catalyst the enantioselectivity of the reac-
tion is opposite.5a
SYNLETT 2008, No. 10, pp 1574–1578
x
x
.x
x
.2
0
0
8
A better result was obtained when 5 mol% of Zn(OTf)2
and 1 were used in the presence of 10 mol% of SDS, in
Advanced online publication: 16.05.2008
DOI: 10.1055/s-2008-1078410; Art ID: Y02507ST
© Georg Thieme Verlag Stuttgart · New York

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Desymmetrization of meso-Epoxides
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Table 1 Enantioselective Aminolysis of cis-Stilbene Oxide (2a) with Aniline (3a)
N
N
HO
Ph
Ph
Ph
Ph
OH
OH
1
PhNH2
+
O
H2O, r.t., 30 h
(1.0 equiv)
NHPh
2a
3a
(+)-(S,S)-4
Entry
Catalyst (mol%)
SDS (mol%)
1 (mol%)
Concentration (M)a Conversion (%)b ee (%)c
1
2
Zn(OTf)2 (1)
Zn(OTf)2 (5)
Zn(OTf)2 (5)
Zn(OTf)2 (5)
Zn(OTf)2 (5)
Zn(OTf)2 (5)
Zn(OTf)2 (5)
Cu(OTf)2 (5)
CoCl2 (5)
2
10
10
5
1.2
5
1.0
20
97
97
97
50
–
53
91d
90
90
87
–
1.0
3
5
0.25
0.25
0.025
0.25
0.25
0.25
0.25
0.25
4
5
5
5
5
6
–
5
7
–
5e
5
54
90
15
8
80
87
51
43
8
10
10
10
9
5
10
NiCl2 (5)
5
a Formal concentration calculated by considering that reactants are completely soluble.
b Conversion measured by GLC analyses, the remaining material was the unreacted 2a.
c Enantiomeric excess measured by chiral HPLC analyses.
d Isolated yield: 93%.
e In CH2Cl2.
fact, b-amino alcohol (+)-(S,S)-4 was isolated in 93% the amount of SDS used in the desymmetrization reac-
yield and 91% ee (Table 1, entry 2). When the concentra- tions in order to minimize the contribution of the not ste-
tion of 2a and 3a was reduced to 0.25 M similar results in reoselective reaction pathways. Therefore, we have
terms of reaction rate and enantioselectivity were ob- considered the use of 5 mol% of Zn(OTf)2–SDS–1 in
tained both when 5 mol% or 10 mol% of SDS were used equimolar amounts at 0.5 M concentration (Table 1, entry
(Table 1, entries 3 and 4). When the concentration of 2a 4) as the best catalytic conditions. Under these highly het-
and 3a was reduced to 0.025 M the aminolysis slowed sig- erogeneous reaction conditions, the amount of Zn(OTf)2,
nificantly, and also the enantioselectivity was slightly re- SDS, and bipyridine 1 that combine to form the true cata-
duced (Table 1, entry 5).
lytic species are presumably significantly smaller than 5
mol%.
The role of water and of SDS in this process is crucial. In
fact, in aqueous medium and in the absence of SDS the Zinc(II) has been recognized as one of the most important
conversion of 2a to 4 was completely inhibited (entry 6) Lewis acid in metal-enzyme-catalyzed reactions in bio-
and when Zn(OTf)2 and bipyridine 1 were used in dichlo- logical systems,12 and we believe that Zn(II)–SDS system
romethane this transformation was much slower and the is a promising eco-friendly catalyst that merits to be fur-
enantiocontrol was less effective (Table 1, entry 7 vs. 4).
ther exploited in organic reactions in water.
Copper triflate was only slightly less effective than Considering the results obtained with cis-stilbene oxide
Zn(OTf)2 (Table 1, entry 8) while CoCl2 and NiCl2 gave (2a) with 3a we have decided to develop a complementary
unsatisfactory results in terms of reaction conversion and study and to extend the use of Zn(II)–SDS system in the
enantioselectivity (Table 1, entries 9 and 10).
aminolysis of rarely studied meso-epoxides and for which
the enantioselectivity is generally low.
These preliminary results showed that among the classic
transition-metal Lewis acids – Zn(II), Cu(II), Co(II), and We have studied the aminolysis of epoxides 2b–d with
Ni(II) – only Zn(II) was truly effective and gave results anilines 3a–d and the results obtained are reported in
quantitatively comparable but with opposite enantioselec- Tables 2–4. Only the aminolysis of 2b–d with 3a has been
tivity to those obtained by using Sc(III).5
previously studied in an organic solvent and with general-
ly low enantioselectivity, while all these processes have
never been investigated in water.
Considering that SDS alone can significantly promote the
aminolysis of epoxides in water,3a it is important to reduce
Synlett 2008, No. 10, 1574–1578 © Thieme Stuttgart · New York

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S. Bonollo et al.
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Initially, we have taken under consideration the aminoly-
sis of cyclohexene oxide (2b) with aniline (3a) (Table 2).
Table 3 Enantioselective Aminolysis of Epoxides 2b–d with
anilines 3a–c at 0.50 M Concentration in Water at 4 °Ca
Zn(OTf)2, SDS, 1
A significant influence of the temperature has been found,
in fact by lowering the reaction temperature from 30 °C to
4 °C, the enantiomeric excess goes from 34% to 52%
(Table 2, entry 1 vs. 2). Moreover, the concentration ef-
fect is impressive and by varying from 0.125 M to 0.50 M
the concentration of epoxide 2b and aniline (3a), b-amino
alcohol 5 is obtained in 52% and 85% ee, respectively
(Table 2, entries 2 and 3). By further increasing the con-
centration to 1.50 M and 2.50 M, the enantiomeric excess
was 80% and 77%, respectively (Table 2, entries 4 and 5).
R
R
OH
R
R
(5.0 mol%)
ArNHR
+
O
H2O
NHAr
(+)-(S,S)
(1.0 equiv)
2b–d
3
Entry Epoxide Aniline
Time
(h)
Product
Yield ee
(%)b (%)c
OH
Me
O
1
70
90 66
NH
N
Me
From these data it is clear that there is an optimal concen-
tration where the invoked hydrophobic interactions exist-
ing between the Zn(OTf)2–SDS–bipyridine 1 complex
and the reactants are more effective in increasing the reac-
tion rate and maximizing the transfer of the chiral mes-
sage.
2b
2b
3b
3c
(+)-(S,S)-6
OH
F
2
3
4
62
90 68
85 52
88 46
NH2
NH2
N
H
F
(+)-(S,S)-7
Table 2 Enantioselective Aminolysis of Cyclohexene Oxide (2b)
with Aniline (3a)
OH
O
Zn(OTf)2, SDS, 1
160
120
OH
N
H
(5.0 mol%)
2c
2c
3a
3b
PhNH2
+
O
(+)-(S,S)-8
H2O
NHPh
(+)-(S,S)-5
(1.0 equiv)
OH
2b
3a
N
Me
Entry Concentration T
Time
Yield
(%)b
ee
(M)a
0.125
0.125
0.50
(°C)
(h)
20
65
50
25
18
(%)c
(+)-(S,S)-9
OH
1
2
3
4
5
30
4
90
50
90
90
90
34
52
85
80
77
5
6
7
2c
3c
3a
3b
120
70
81 55
90 51
89 48
N
H
F
4
(+)-(S,S)-10
1.50
4
OH
O
2.50
4
N
H
a Formal concentration calculated by considering that reactants are
completely soluble.
2d
2d
(+)-(S,S)-11
b Isolated yield.
OH
c Enantiomeric excess measured by chiral HPLC analyses.
160
N
Me
(+)-(S,S)-12
OH
A similar study was then extended to anilines 3a–c and
also to epoxides 2c–d. Those substrates display a hydro-
philicity roughly similar to that of 2b and 3a and the best
enantioselectivity results were achieved at a 0.5 M formal
concentration of the reactants. The results are summarized
in Table 3.
8
2d
3c
160
79 59
N
H
F
(+)-(S,S)-13
On this basis we have considered that by using a highly
hydrophobic amine, such as a-naphthylamine (3d), good
enantioselectivity in the desymmetrization of ‘small’ ep-
oxides such as 2b–d could be obtained.
a Formal concentration calculated by considering that reactants are
completely soluble.
b Isolated yield.
c Enantiomeric excess measured by chiral HPLC analyses.
The data illustrated in Table 4 support this hypothesis.
The enantioselective aminolysis of 2b with a-naphthyl-
amine (3d) confirmed the close dependency of the enan-
tiocontrol of the process on the concentration of the
reactants. The best efficiency was achieved with a formal
concentration of 2b and 3d of 0.125 M and in the presence
of 10 mol% of Zn(OTf)2–SDS–1 catalytic system
(Table 4, entry 2 vs. 1 and 3). Under these reaction condi-
tions, the rate of the process is acceptable and the corre-
sponding b-amino alcohol 14 has been obtained in a very
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Desymmetrization of meso-Epoxides
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Table 4 Enantioselective Aminolysis of Epoxides 2b–d with a-Naphthylamine (3d) in Water at 4 °C
NH2
Zn(OTf)2, SDS, 1
R
R
OH
R
R
(10.0 mol%)
+
O
H2O
N
H
(1.0 equiv)
(+)-(S,S)
2b–d
3d
Entry
Epoxide
Concentration (M)a Time (h)
Product
Yield (%)b
ee (%)c
OH
1
2
3
0.50
0.125
0.025
48
50
114
91
92
90
76
85
77
2b
2c
2d
N
H
(+)-(S,S)-14
OH
4
5
0.125
0.125
160
180
90
90
71
70
N
H
(+)-(S,S)-15
OH
N
H
(+)-(S,S)-16
a Formal concentration calculated by considering that reactants are completely soluble.
b Isolated yield.
c Enantiomeric excess measured by chiral HPLC analyses.
10 and extracted with EtOAc (3 × 2 mL). The combined organic
layers were dried over anhydrous Na2SO4, the solvent was evapo-
rated and the crude product was charged on a Et3N pretreated SiO2
column chromatography [Et2O–PE (1:2); SiO2–sample (30:1)].
Pure (2S,3S)-3-(2¢-Fluoro-phenylamino)butan-2-ol (13) was isolat-
ed as a colourless oil (79% yield, 59% ee).
good yield and enantiomeric excess (92% and 85%, re-
spectively).
The same reaction conditions were extended to the reac-
tions of 2c and 2d with 3d, and the corresponding b-amino
alcohols 15 and 16 were isolated in satisfactory yield and
enantiomeric excess (Table 4, entries 4 and 5).
(1S,2S)-2-(2¢-Fluorophenylamino)-1-cyclopentanol (10)
In conclusion, Zn(II)–SDS system combined with bipyri-
dine 1 has proved to be an effective catalyst in water for
the desymmetrization of meso-epoxides. In this study we
have explored the desymmetrization by aminolysis with
anilines 3a–d by focusing our attention on substrates 2b–
d, for which generally rare and poor results have been re-
ported, never by using water as reaction medium. Up to
date the 85% ee obtained in the case of cyclohexene oxide
(2b) and aniline (3a) is a very good result and is the high-
est value obtained in water as reaction medium.
Isolated in 81% yield; colorless oil; chromatography on SiO2, elu-
ent: Et2O–PE (1:2). 1H NMR (400 MHz, CDCl3): d = 1.35–1.50 (m,
1 H), 1.55–1.70 (m, 1 H), 1.70–1.90 (m, 2 H), 1.90–2.05 (m, 1 H),
2.05–2.20 (m, 1 H), 2.20–2.35 (m, 1 H), 3.55–3.65 (m, 1 H), 3.70–
3.95 (br s, 1 H), 4.05–4.10 (m, 1 H), 6.55–6.65 (m, 1 H), 6.82 (t,
J = 8.3 Hz, 1 H), 6.90–7.00 (m, 2 H). 13C NMR (100.6 MHz,
CDCl3): d = 151.4, 136.2, 124.6, 116.7, 114.4, 112.9, 78.2, 61.7,
32.9, 31.1, 21.0. GC-MS (EI): m/z (%) = 195 (51) [M+], 151 (13),
150 (100), 137 (13), 136 (10), 130 (11), 124 (52), 122 (18), 111
(24), 109 (10), 95 (10). Anal. Calcd for C11H14FNO: C, 67.67; H,
7.23; F, 9.73; N, 7.17. Found: C, 67.30; H, 7.01; F, 9.76; N, 7.20.
The ee was determined by HPLC using a Daicel Chiralpak OD-H
column [hexane–i-PrOH (95:5); flow rate 0.5 mL/min; l = 254 nm,
21
The chiral bipyridine ligand 1 was prepared according to the report-
ed procedure.13 b-Amino alcohols 4–9, 11, 12, 14, and 15 are known
compounds while b-amino alcohols 10, 13, 16 are new compounds
and their characterization data are reported below.
tR (1R,2R) = 26.3 min, tR (1S,2S) = 30.7 min]; ee = 55%. [a]D
+11.1 (c 0.86, CH2Cl2).
(2S,3S)-3-(2¢-Fluorophenylamino)butan-2-ol (13)
Isolated in 79% yield; colorless oil; chromatography on SiO2, elu-
ent: Et2O–PE (1:2). 1H NMR (400 MHz, CDCl3): d = 1.17 (d,
J = 6.4 Hz, 3 H), 1.26 (d, J = 6.2 Hz, 3 H), 2.45–2.80 (br s, 1 H),
3.34 (quint, J = 6.3 Hz, 1 H), 3.60–3.80 (br s, 1 H), 3.70 (quint,
J = 6.2 Hz, 1 H), 6.60–6.70 (m, 1 H), 6.79 (t, J = 8.4 Hz, 1 H), 6.90–
7.05 (m, 2 H). 13C NMR (100.6 MHz, CDCl3): d = 152.2, 136.1,
124.6, 117.4, 114.7, 113.6, 71.2, 55.6, 19.4, 17.3. GC-MS (EI): m/z
(%) = 183 (22) [M+], 139 (17), 138 (100), 91 (13). Anal. Calcd for
C10H14FNO: C, 65.55; H, 7.70; F, 10.37; N, 7.64. Found: C, 65.87;
Zn(II)-Catalyzed Desymmetrization of meso-Epoxides –Typical
Procedure for the Aminolysis of 2d with 3c
In a screw-capped vial equipped with a magnetic stirrer, Zn(OTf)2
(0.025 mmol), SDS (0.025 mmol), and chiral bipyridine ligand 1
(0.025 mmol) were stirred in H2O (1 mL) for 5 min at r.t. 2-Fluoro-
aniline (3c, 0.5 mmol) was added and, after a few minutes, 2,3-ep-
oxybutane (2d, 0.5 mmol) was added at 0 °C. The reaction mixture
was stirred for 160 h at 4 °C, then basified with aq 5 M NaOH to pH
Synlett 2008, No. 10, 1574–1578 © Thieme Stuttgart · New York

1578
S. Bonollo et al.
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H, 7.72; F, 10.32; N, 7.66. The ee was determined by HPLC using
a Daicel Chiralpak AD-H column [hexane–i-PrOH (95:5; flow rate
1.0 mL/min; l = 254 nm, tR (2R,3R) = 9.4 min, tR (2S,3S) = 11.4
min]; ee = 59%. [a]D21 +55.5 (c 0.57, CH2Cl2).
(4) (a) Fioroni, G.; Fringuelli, F.; Pizzo, F.; Vaccaro, L. Green
Chem. 2003, 5, 425. (b) Fringuelli, F.; Pizzo, F.; Tortoioli,
S.; Vaccaro, L. J. Org. Chem. 2003, 68, 8248.
(c) Amantini, D.; Fringuelli, F.; Pizzo, F.; Tortoioli, S.;
Vaccaro, L. Synlett 2003, 2292. (d) Fringuelli, F.; Pizzo, F.;
Vaccaro, L. Tetrahedron Lett. 2001, 41, 1131.
(2S,3S)-3-(Naphth-1¢-ylamino)butan-2-ol (16)
Isolated in 90% yield; colorless oil; chromatography on SiO2, elu-
ent: Et2O–PE (1:2). 1H NMR (400 MHz, CDCl3): d = 1.25–1.30 (br
s, 2 H), 1.27 (d, J = 6.4 Hz, 3 H), 1.34 (d, J = 6.2 Hz, 3 H), 3.58
(quint, J = 6.4 Hz, 1 H), 3.88 (quint, J = 6.2 Hz, 1 H), 6.80 (d,
J = 7.3 Hz, 1 H), 7.27–7.40 (m, 2 H), 7.40–7.50 (m, 2 H), 7.75–7.85
(m, 1 H), 7.85–7.90 (m, 1 H). 13C NMR (100.6 MHz, CDCl3):
d = 142.1, 134.5, 128.7, 126.4, 125.8, 125.0, 124.1, 120.0, 118.5,
106.8, 71.3, 55.9, 19.8, 16.9. GC-MS (EI): m/z (%) = 215 (24) [M+],
171 (15), 170 (100), 155 (13), 154 (15), 129 (13), 128 (15), 127
(15). Anal. Calcd for C14H17NO: C, 78.10; H, 7.96; N, 6.51. Found:
C, 77.71; H, 7.99; N, 6.48. The ee was determined by HPLC using
a Daicel Chiralpak AD-H column [hexane–i-PrOH (95/5); flow rate
1.0 mL/min; l = 254 nm, tR (2S,3S) = 10.2 min, tR (3R,3R) = 11.6
min]; ee = 70%. [a]D21 +77.0 (c 0.58, CH2Cl2).
(e) Amantini, D.; Fringuelli, F.; Pizzo, F.; Vaccaro, L. Green
Chem. 2001, 3, 229. (f) Amantini, D.; Fringuelli, F.; Pizzo,
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Acknowledgment
We gratefully acknowledge the Ministero dell’Università e della
Ricerca (MiUR) and the Università degli Studi di Perugia (COFIN
2006) and FONDAZIONE Cassa di Risparmio di Perugia for finan-
cial support (Metodologie sintetiche non convenzionali per processi
a basso impatto ambientale – 2007).
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