Synthesis of optically pure terminal epoxide and 1,2-diol via hydrolytic kinetic resolution catalyzed by new heterometallic salen complexes

Article abstract of DOI:10.1080/00397910600640354

The inactive chiral (salen)Co complex is easily activated by InCl ₃ and TlCl₃ Lewis acids by forming heterometallic salen complexes. These complexes show very high catalytic activity for the synthesis of enantiomerically enriched terminal epoxides (>99% ee) and 1,2-diols simultaneously via hydrolytic kinetic resolution. Strong synergistic effects of different Lewis acids, Co-In and Co-Tl, were exhibited in the catalytic process. The system described is very simple and efficient. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600640354

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Synthesis of Optically Pure Terminal Epoxide
and 1,2‐Diol via Hydrolytic Kinetic Resolution
Catalyzed by New Heterometallic Salen
Complexes
Santosh Singh Thakur a , Shu‐Wei Chen a , Wenji Li a , Chang‐Kyo Shin a ,
Yoon‐Mo Koo b & Geon‐Joong Kim a
a Department of Chemical Engineering , Inha University , Incheon, South
Korea
b Department of Biological Engineering, ERC for Advanced Bioseparation
Technology , Inha University , Incheon, South Korea
Published online: 22 Sep 2006.
To cite this article: Santosh Singh Thakur , Shu‐Wei Chen , Wenji Li , Chang‐Kyo Shin , Yoon‐Mo Koo
& Geon‐Joong Kim (2006) Synthesis of Optically Pure Terminal Epoxide and 1,2‐Diol via Hydrolytic
Kinetic Resolution Catalyzed by New Heterometallic Salen Complexes, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:16, 2371-2383, DOI:
10.1080/00397910600640354
To link to this article: http://dx.doi.org/10.1080/00397910600640354
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Synthetic Communicationsw, 36: 2371–2383, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600640354
Synthesis of Optically Pure Terminal
Epoxide and 1,2-Diol via Hydrolytic Kinetic
Resolution Catalyzed by New
Heterometallic Salen Complexes
Santosh Singh Thakur, Shu-Wei Chen, Wenji Li,
and Chang-Kyo Shin
Department of Chemical Engineering, Inha University, Incheon,
South Korea
Yoon-Mo Koo
Department of Biological Engineering, ERC for Advanced Bioseparation
Technology, Inha University, Incheon, South Korea
Geon-Joong Kim
Department of Chemical Engineering, Inha University, Incheon,
South Korea
Abstract: The inactive chiral (salen)Co complex is easily activated by InCl3 and TlCl3
Lewis acids by forming heterometallic salen complexes. These complexes show very
high catalytic activity for the synthesis of enantiomerically enriched terminal epoxides
(.99% ee) and 1,2-diols simultaneously via hydrolytic kinetic resolution. Strong
synergistic effects of different Lewis acids, Co-In and Co-Tl, were exhibited in the
catalytic process. The system described is very simple and efficient.
Keywords: Asymmetric catalysis, chiral catalyst, heterometallic complexes, hydro-
lytic kinetic resolution, terminal epoxides
Received in Japan January 13, 2006
Address correspondence to Geon-Joong Kim, Department of Chemical Engineer-
ing, Inha University, Incheon, South Korea. E-mail: kimgj@inha.ac.kr
2371

2372
S. S. Thakur et al.
INTRODUCTION
Homo- and heterodinuclear complexes have enormous potential to revolutio-
nize asymmetric catalysis. They can activate both partners of bimolecular
reaction simultaneously, overcome entropy barriers associated with bringing
the two reagents together, minimize the energy barrier that arises from
solvent shell rearrangements during the reaction, and recognize prochiral
faces or groups within the reagent through predetermination of the reaction
trajectory.[1] Quite recently, several groups have made significant progress
toward the design and use of chiral homo- and heterobimetallic complex for
asymmetric catalysis.[2–7]
The high selectivity factors (krel values) across a wide range of terminal
epoxides provide efficient enantioselective kinetic resolution of terminal
epoxides with water (i.e., hydrolytic kinetic resolution, HKR).[8] In the asym-
metric ring opening of epoxides catalyzed by chiral [(salen)Co] complexes,[9]
the reaction involved two separate chiral salen–metal species in the rate-
limiting step of the reaction, and subsequently, very efficient oligomeric
catalysts were developed based on this principle.[10] However, complicated
multistep synthesis of oligomeric catalysts[10] and solubility problems
during HKR make it a less attractive strategy. With this approach, we
thought an easily synthesized heterometallic chiral salen complex might
provide a better catalyst, which would exhibit a faster rate of reaction and
high enantioselectivity. As part of our ongoing research toward the design
of di- and multinuclear chiral (salen)Co catalysts,[11] herein we report the
catalytic activity of new, easily synthesized, heterometallic chiral (salen)Co-
MCl3 catalyst (Scheme 1) for the enantioselective hydrolytic kinetic resol-
ution of terminal epoxides. Comparisons with their monomeric analogue
have also been discussed.
RESULTS AND DISCUSSION
The formation of a heterometallic new complex can be viewed from a UV-vis
spectrophotometer. The complexes show the new characteristic UV-vis
absorption band at 370 nm, and a band at 420 nm, due to precatalyst
(salen)Co, disappeared. Vibrational circular dichroism (VCD) spectroscopy
has been applied to elucidate the stereochemistries of chiral molecules,
including the accurate estimation of enantiomeric excess and their absolute
configurations.[12] In the present study, VCD spectra were used to elucidate
the stereochemistries of chiral 1a–2a complexes. The shift in peak position
of 1a and 2a shows the existence of two independent systems with the
chiral (salen)Co precatalyst (Fig. 1). The FAB-mass spectra of 1a and
dinuclear 2a provide direct evidence of formation of two distinct
complexes. Compound 2a showed molecular mass of more than 1418 and
monomer complex 1a of more than 780.

Synthesis of Optically Pure Epoxide and 1,2-Diol
2373
Scheme 1.
Figure 1. Monitoring of formation of 1a and 2a by vibration circular dichroism
(VCD) spectra and comparison with Co(salen) complex.

2374
S. S. Thakur et al.
Many authors have been reported that oxygen atoms of the metal
complexes of the Schiff bases are able to coordinate to the transition and
group 13 elements to form bi- and trinuclear complex.[13] On these bases,
possible structure of chiral (salen)Co-In/Co-Tl complexes is depicted in
Scheme 1. The catalytic activities of the heterometallic catalysts 1a and 2a
for of the representative diverse racemic terminal epoxides are shown in
Table 1. The HKR of propylene and butylene oxide proved to be an efficient
reaction, requiring only 0.1 mol% catalyst under solvent-free conditions
(entries 1 and 2). Glycidol (entry 3) needs solvent THF to optimize the
HKR. Epichlorohydrin (entry 4) is a readily available C-3 unit, which is
widely employed in organic and polymer synthesis.[14] Glycidyl methyl
ether, glycidyl isopropyl ether, glycidyl a-naphthyl ether (entries 5, 6, and
11), and glycidyl phenyl ether and their m-CH3 and m-Cl derivatives (entries
7, 12, and 13) underwent resolution in excellent yield, employing 0.2 mol%
of the catalyst. The kinetic resolutions of glycidyl propionate and glycidyl
butyrate (entries 9 and 10) were effected in a straightforward manner.
Styrene oxide (entry 8) and its derivatives are among the most useful
terminal epoxides from a synthetic standpoint and are therefore particularly
important candidates for the HKR reaction. Resolution of styrene oxide was
possible using water-miscible solvents such as THF and 0.5 mol% of catalyst
for a longer reaction time. The HKR of the styrene oxide might be plagued
by conflicting steric and electronic factors influencing regioselectivity in the
epoxide ring-opening reaction; consequently, a high catalyst loading amount
and a longer reaction time required. The order of reactivity for the heterome-
tallic complexes were found to be 2a . 2b . 1a . 1b. It is quite obvious
from Figures 2 and 3 that the bimetallic complex 2a or 2b catalyst amount
(0.05 mol%) is half of the monomeric complex 1a or 1b (0.1 mol%), but it
shows twice the activity of their monomeric analogue. This fact clearly
indicates that 2a or 2b is not a physical mixture of the monomeric Co-In/
Co-Tl and precatalyst but that it possesses real dimeric nature.
.
The catalyst prepared from hydrated MCl3 nH2O exhibited similar
activity and enantioselectivity for the present study. In a control experiment,
the heterometallic complexes, treated with excess of water and worked up,
are found to have similar reactivity and selectivity (Scheme 2). This reveals
that there is no dissociation of Co-In/Tl complex during HKR reaction. It
appears that Cl2 ions are also coordinated to In/Tl metals during synthesis
of 1a–2b and consequently formed stable complexes. The UV-vis spectrum
of the catalyst during the HKR reaction is also in favor of this fact (Fig. 4).
Neither precatalyst (salen)Co nor In/TlCl3 alone induce any HKR reaction.
However, 2a or 2b could be generated readily in situ by suspension of the
precatalyst in epoxide or epoxide/solvent and addition of monomer
(salen)Co-MCl3 complex.
Kinetic experiment of the HKR of epichlorohydrin (ECH) was carried
out as a model reaction (Table 2). The two-term rate equation involving
both intra- and intermolecular components [Eq. (1)] could be

Synthesis of Optically Pure Epoxide and 1,2-Diol
2375
Table 1. HKR of terminal epoxides catalyzed by the heterometallic catalyst
Catalyst/catalyst
loading (mol%)b
Time
(h)
% Yield
(ee)c
Entry
1
Recovered epoxidesa
1a
2a
0.1
0.1
4
2
44 (99.3)
45 (99.5)
2
1a
2a
0.1
0.1
7
3
45 (99.2)
48 (99.6)
3d
4
1a
2a
0.1
0.1
11
6
42 (98.5)
44 (98.9)
1a
2a
0.1
0.1
3
2
43 (99.8)
47 (99.9)
5
6
1a
2a
0.2
0.2
4
2
45 (99.4)
49 (99.6)
2a
0.3
5
43 (99.5)
7
2a
0.2
3
42 (99.7)
8d
9
2a
2a
0.5
0.2
5
3
41 (98.8)
44 (99.6)
10
2a
2a
2a
2a
0.2
0.2
0.2
0.2
3
5
2
2
45 (99.5)
42 (96.7)
43 (99.4)
45 (99.1)
11e
12e
13e
aIsolated yield is based on racemic epoxides (theoretical maximum ¼ 50%).
bLoading on a per [Co] basis with regard to racemic epoxides.
cee percentage was determined by chiral GC or chiral HPLC.
dTHF was used as a solvent.
eSolvents CH2Cl2:THF ¼ 2:1.

2376
S. S. Thakur et al.
Figure 2. Comparison of catalytic activity of the heterometallic catalyst with
Jacobsen’s on the HKR of (a) epoxy propane and (b) epichlorohydrine. Catalyst
loading amount is 0.1 mol% per [Co] unit for the heterometallic catalyst.
implemented.[11a,15] Analysis of such plots
2
rate / kintra½catalystꢀ þ kinter½catalystꢀ
ð1Þ
with rate data obtained with heterometallic catalysts 1a–2b explicitly lead to
the linear correlations with positive slopes and nonzero y-intercepts, consist-
ent with participation of both inter- and intramolecular pathway for the HKR
(Fig. 5). Similar analyses of rate data obtained with the reported Co-Al
monomeric catalyst,[11a] Co-Ga monomeric catalyst,[11d] and Jacobsen’s
catalyst[8] revealed y-intercepts of zero, reflecting the absence of any

Synthesis of Optically Pure Epoxide and 1,2-Diol
2377
Figure 3. Catalytic activity of heterometallic complexes 2a and 2b for representative
terminal epoxides using catalytic amount of 0.1–0.5 mol% (per Co unit) at ambient
temperature.
first-order pathway for these complexes. With Co-In catalyst 2a, a maximum
value for intramolecular and intermolecular rate constants was obtained
showing highest reactivity and enantioselectivity.
Lewis acids play a crucial role in the asymmetric catalysis.[1b,16] In our
present finding Co-In 1a and Co-Tl 1b act as heterometallic complexes,
exhibiting two different Lewis acid centers, Co and In[17a–c] and Co and
Tl,[17d] which show a strong synergistic effect. The intramolecular
pathway for the catalysts 1a and 1b could be explained on the basis of
the proposed Scheme 3, which may be similar to the enantioselective ring
opening of epoxides with 4-methoxyphenol catalyzed by Ga heterobimetal-
lic reported by Iida et al.[2c] The central metal atom Co appears to activate
and control the orientation of epoxide and stereoselectively bind only one
enantiomer, and the latter seems to activate and control the orientation of
Scheme 2.

2378
S. S. Thakur et al.
Figure 4. The UV-vis spectral analysis of the catalyst during and after the HKR of
racemate methyl glycidyl ether.
nucleophlie H2O by enabling an enantioselective ring opening of epoxides
with nucleophiles. The intermolecular mechanism for 1a–2b may follow
the earlier report.[11a,15] Although the HKR reaction is easily carried out
with Co(II) complexes, it appears that the reactive species is in fact
Co(III).[18]
In summary, readily available chiral (salen)Co, the presence of multiple
Lewis acid centers, and a low catalyst loading amount are some of
the special features of the heterometallic salen catalyst used in the present
investigation. The role of group 13 elements for the activation of the
precatalyst, catalyst recyclability, scalability, heterogenization, and use of
Table 2. Kinetic data for the HKR of racemic ECH catalyzed by 1a–2b
No. of
(salen)Co unit
kintra
(min21ꢁ1022 a
kinter
(M21ꢁmin21 a
Catalyst
)
)
Co-Al
1a
2a
1
1
2
1
2
1.0
15.2
22.0
12.1
21.0
49.8
66.0
49.3
61.3
1b
2b
aCalculated from Fig. 5. using Eq. 1.

Synthesis of Optically Pure Epoxide and 1,2-Diol
2379
Figure 5. Initial rate kinetics for the asymmetric HKR of the ECH catalyzed by the
1a–2b complexes.
Scheme 3. Possible working model for the HKR of terminal epoxides catalyzed by
Co-In and Co-Tl heterometallic complexes.

2380
S. S. Thakur et al.
the catalyst for other asymmetric catalytic reactions are focus of our ongoing
studies.
EXPERIMENTAL
General
1
All H NMR and 13C NMR data were recorded using a 400-MHz FT NMR
spectrophotometer (Varian Unity Nova 400) at ambient temperature.
Optical rotation measurements were conducted using a Jasco DIP 370
digital polarimeter. Gas chromatography analyses were performed on
Hewlett-Packard 5890 series II instruments equipped with FID detectors
using chiral column (Chiraldex G-TA and A-TA, 20-m ꢁ 0.25-mm id,
Astec) and HP 3396 integrators with HP Chem Station software for data
analysis. Chiral HPLC analyses were performed on Younglin instrument
using a Chiralcelw OD column (24-cm ꢁ 0.46-cm i d; Chiral Technologies,
Inc.) and Regis (S,S)Whelk-O1 at 254 nm. UV spectra were recorded on
UV-vis spectrophotometer (Optizen 2120 UV) interfaced with a PC using
Optizen View 3.1 software for data analysis. VCD was measured in
ChiralirTM (ABB Bomem Inc.) with a pathlength of 94 m set at 8-cm21
resolution and a spectral collection time of 1 h using Bomem GRAMS-32
software. All reagents were purchased from Aldrich, Fluka, and TCI.
(+)-(a-Naphthyl) glycidyl ether was prepared by a known method[19]
and confirmed by 1H NMR,13C NMR, and elemental analysis. The
general method for the HKR and kinetic analysis are the same as cited
literature.[8,11a]
Synthesis of the Heterometallic Complexes 1 and 2
(R,R)-SalenCo (5.0 g, 8.28 mmol, 1.0 equiv.) was added to a solution of
hydrated Indium chloride (2.428 g, 8.28 mmol, 1.0 equiv.) in tetrahydrofuran
(30 mL), and the mixture was stirred for 1 h in an open atmosphere at
room temperature. As soon as the chiral (salen)Co was added, the color of
the solution changed from brick red to dark olive green. The resulting
solution was concentrated under reduced pressure. The crude solid was
worked up with H2O and CH2Cl2. Yield was 98–99% as a dark green solid
powder. UV-vis shows strong absorbance at 370 nm. The characteristic
absorption band of precatalyst (salen)Co(II) at 420 nm disappeared. For
complex 1a, FAB-mass was calculated (779.83) and observed (780.6).
Complex 2a was prepared by stirring 2:1 equiv. of the precatalyst
(salen)Co(II) and hydrated Indium chloride respectively in THF solvent for
1 h. FAB-mass was calculated (1418) and observed (1419). A similar
procedure has been adopted for the synthesis of 1b and 2b.

Synthesis of Optically Pure Epoxide and 1,2-Diol
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