Microwave-assisted asymmetric ring opening of meso-epoxides with aromatic amines catalyzed by a Ti-S-(-)-BINOL complex

Article abstract of DOI:10.1016/j.tetlet.2006.05.150

Catalytic asymmetric ring opening of cyclohexene oxide and meso-stilbene oxide with anilines was catalyzed by a Ti-(S)-(-)-BINOL complex to afford β-amino alcohols in high yield (up to 95%) and good enantioselectivities (ee up to 55%) under microwave irra

Full text of DOI:10.1016/j.tetlet.2006.05.150

Tetrahedron Letters 47 (2006) 5277–5279
Microwave-assisted asymmetric ring opening of meso-epoxides
with aromatic amines catalyzed by a Ti-S-(À)-BINOL complex
*
Rukhsana I. Kureshy, Surendra Singh, Noor-ul H. Khan, Sayed H. R. Abdi,
Santosh Agrawal, Vishal J. Mayani and Raksh V. Jasra
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), Bhavnagar,
3
64 002 Gujarat, India
Received 12 March 2006; revised 18 May 2006; accepted 24 May 2006
Available online 13 June 2006
Abstract—Catalytic asymmetric ring opening of cyclohexene oxide and meso-stilbene oxide with anilines was catalyzed by a Ti-(S)-
(
À)-BINOL complex to afford b-amino alcohols in high yield (up to 95%) and good enantioselectivities (ee up to 55%) under micro-
wave irradiation. The reaction using a microwave was found to be 10 times faster than traditional oil-bath heating with retention of
enantioselectivity.
Ó 2006 Elsevier Ltd. All rights reserved.
Chiral transition metal complex catalyzed asymmetric
ring opening of meso-epoxides with aromatic amines
meso-epoxides with aromatic amines using Ti-(S)-BI-
NOL at ambient temperature, we report for the first
1
,2
21
is of particular interest because the chiral b-amino alco-
time asymmetric ring opening of meso-stilbene oxide
and cyclohexene oxide with aniline and substituted ani-
lines catalyzed by a Ti-(S)-BINOL complex to give
enantioenriched syn-b-amino alcohols and trans-b-ami-
no alcohols in high yields (up to 95%) with up to 55%
enantiomeric excess under microwave (MW) irradiation
at 60 °C. The enantiomeric excess (ee) was comparable
with the values obtained at room temperature.
hol products have wide applications in the synthesis of
3
pharmaceutically active compounds and chiral auxilia-
ries/ligands. In recent years, attempts have been made to
ring open meso-epoxides with alkyl/aryl amines to gen-
4
–8
erate b-amino alcohols using various lanthanides with
9
10
(
R)/(S)-BINOL, Cr(Salen) and Sc(bipyridine). Micro-
wave irradiation is used for the ring opening of vinyl
1
1,12
epoxides and terminal epoxides by NH OH,
ols, pyrazole and imidazole and amines;
thi-
4
1
3
14
15–17
how-
Complex 1 was generated in situ by the reaction of equi-
i
ever, the area of transition metal catalyzed asymmetric
molar quantities of (S)-BINOL, Ti(O Pr) and water in
4
1
8
epoxide ring opening reactions using microwaves has
scarcely been studied, probably due to the difference in
activation energy of the two enantiomers involved in
the asymmetric reaction which is insignificant in com-
dry toluene (Scheme 1). NMR and MS data for complex
1 suggested the existence of a catalytically active dimeric
species, which was in equilibrium with the correspond-
ing monomeric species and this observation was in
1
9
22,23
parison with the energy supplied by microwaves. Con-
sequently, the enantioselectivity of the reactions could
be affected in a negative way by the use of microwaves.
Recently, Jacobs et al. reported the asymmetric ring
agreement with previous reports.
In our earlier
report on asymmetric ring opening of meso-stilbene oxide
with aniline at room temperature, we observed that
complex 1 was superior in performance to other Ti(IV)
2
1
opening of epoxides using TMSN as the nucleophile
complexes and toluene was the solvent of choice.
3
2
0
under microwave irradiation. As part of our conti-
nuing efforts towards asymmetric ring opening of
The asymmetric ring opening (ARO) of meso-cyclo-
hexene oxide with aniline was carried out using
10 mol % of complex 1 under microwave irradiation as
a test reaction in toluene. The asymmetric ring opening
under microwave irradiation at 60 °C showed a compa-
rable enantiomeric excess (49% ee, Table 1, entry 3) to
that of the reaction conducted at 60 °C in an oil bath
(54% ee, Table 1, entry 5). The turnover frequency
Keywords: Microwave irradiation; Ring opening; Epoxides; Aniline;
Ti-BINOL complex.
*
Corresponding author. Tel.: +91 278 567760; fax: +91 0278
566970; e-mail: rukhsana93@yahoo.co.in
2
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.150

5278
R. I. Kureshy et al. / Tetrahedron Letters 47 (2006) 5277–5279
i
Dry toluene
O Pr
O
O
O
O
O
O
O
O
OH
OH
Ti
Ti
Ti
i
i
O Pr
Ti(O Pr) , water
4
1
Scheme 1. Preparation of complex 1.
a
Table 1. Asymmetric ring opening of cyclohexene oxide with anilines catalyzed by complex 1 under microwave irradiation
R2
i
Ti(O Pr) (10 mol%)
4
R2
OH
S-BINOL (10 mol%)
O
+
R1
N
H
Dry toluene, H O (10 mol%)
2
R1
NH2
TPP (10 mol%), MW, 60 °C
2
1
2
3
3
3
3
3
a; R = H, R = H
4a-e
1
2
b; R = H, R = OMe
1
2
c; R = OMe, R = H
1
2
d; R = H, R = Me
1
2
e; R = H, R = Cl
b
c
d
e
Entry
Amine
Temperature
Reaction time
Isolated yield 4a–e (%)
ee (%)
TON
TOF
4.6
960
1080
1020
93
1140
112
1080
1104
1140
1128
1
2
3
4
5
6
7
8
9
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
27 (RT)
2 h
30 s
30 s
30 s
6 min
30 s
5 min
30 s
30 s
30 s
30 s
92
80
90
85
93
95
94
90
92
95
94
i
39
50
49
17
54
26
55
46
50
47
48
9.2
8.0
9.0
8.5
9.3
9.5
9.4
9.0
9.2
9.5
9.4
40 (MW)
60 (MW)
60 (MW)
f
60 (oil-bath heating)
70 (MW)
70 (oil-bath heating)
60 (MW)
60 (MW)
60 (MW)
1
1
0
1
60 (MW)
a
A mixture of complex 1 (generated in situ using 10 mol % S-(À)-BINOL and 10 mol % Ti(O Pr)
4
in 150 lL toluene), 10 mol % TTP, epoxide
(
0.5 mmol) and aniline (0.5 mmol) was irradiated for 30 s in a microwave oven at 60 °C.
b
c
d
e
f
Isolated yield after chromatographic separation.
Determined by HPLC using OJ and OD columns. The absolute configuration was found to be 1S,2S by comparison with the literature.
TON, turnover number (number of substrate molecules converted to product per catalyst molecule).
TOF, turnover frequency (TON per unit of time) h
Reaction carried out in the absence of TPP.
8,21
À1
.
(
TOF) of complex 1 was also increased 10-fold under
and the HPLC profile with the literature, and details
are given in the Supplementary data.
microwave irradiation (Table 1, entry 3) compared to
traditional heating at 60 °C (Table 1, entry 5) and was
2
35 times that of the reaction conducted at room tem-
Furthermore, we have studied the ARO of cyclohexene
oxide with aniline under microwave irradiation whilst
varying the temperature (Table 1, entries 2, 3 and 6).
The TOF and ee were highest at 60 °C as shown in Fig-
ure 1. On increasing the temperature up to 70 °C, the
TOF was increased but the enantioselectivity was
decreased (Table 1, entry 6). On the other hand, when
the temperature was decreased to 40 °C, there was a
decrease in TOF along with a slight improvement in
enantioselectivity (entry 2).
perature (Table 1, entry 1).
Further it has been reported in the literature3,21 that the
use of additives improves the ee of the amino alcohols.
We conducted similar experiments in the presence of
triphenylphosphine (TPP) and in the absence of TPP
(
(
Table 1, entry 4), and the results showed that TPP
Table 1, entry 3) increased the ee of trans-b-amino alco-
hol 4a probably due to the weak interaction of TPP with
complex 1. These results are significantly superior com-
pared to those obtained using the previously reported
lanthanide iodo binaphtholate8 (yield 55%, ee 20%,
The ARO of cylohexene oxide with different anilines
under microwave irradiation at 60 °C was also studied.
The isolated yields and ee’s of the products 4b–e were
in the range of 90–95% and 46–50%, respectively (entries
8–11). The ARO of cyclohexene oxide with 4-methoxy-
aniline gave product 4b (ee 46%, yield 90%, TON = 9
1
8 h) and the enantiomeric excess was comparable with
1
0
Sc(Bipyridine), but the reactivity was found to be bet-
ter in the case of complex 1 than both the other sys-
8
,10
tems.
The absolute configuration was determined
À1
by comparing the optical rotation (found to be 1S,2S)
and TOF = 1080 h ), which is an improvement on that

R. I. Kureshy et al. / Tetrahedron Letters 47 (2006) 5277–5279
5279
Enantiomeric excess
TOF
Acknowledgements
S.S., CSIR (SRF) and R.I.K. are thankful to DST and
CSIR Network on catalysis for financial assistance and
are also thankful to Dr. P. K. Ghosh, the Director of
the Institute, for providing instrumentation facilities.
6
5
4
3
2
1
0
0
0
0
0
0
0
120
10
100
1
Supplementary data
90
A typical experimental procedure for the asymmetric
ring opening of an epoxide by an aromatic amine,
80
4
0
60
Temperature ( C)
70
1
13
H and C NMR data and a HPLC chromatogram
˚
are provided as supplementary data. Supplementary
data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2006.05.150.
Figure 1. Effect of temperature on the enantiomeric excess and TOF of
the asymmetric ring opening of cyclohexene oxide with aniline.
References and notes
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