Asymmetric epoxidation of olefins by manganese (III) complexes stabilised on nanocrystalline magnesium oxide

Article abstract of DOI:10.1002/adsc.200505428

The asymmetric epoxidation of unfunctionalised olefins to epoxides is realised by using manganese(III) complexes stabilised on nanocrystalline magnesium oxide in the presence (1R,2R)-(-)-diaminocyclohexane as a chiral ligand in good yields and up to 91% enantiomeric excess.

Full text of DOI:10.1002/adsc.200505428

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DOI: 10.1002/adsc.200505428
Asymmetric Epoxidation of Olefins by Manganese(III)
A
Complexes Stabilised on Nanocrystalline Magnesium Oxide
B. M. Choudary,a,* Ujjwal Pal,b M. Lakshmi Kantam,b K. V. S. Ranganath,b
and B. Sreedharb
a
Ogene Systems (I) Pvt. Ltd.,11-6-56, GSR Estates, Moosapet, Hyderabad, India
Phone/Fax: (+91)-40-2377-5566; e-mail: bmchoudary@yahoo.com
Indian Institute of Chemical Technology, Hyderabad 500 007, India
b
Phone: (+91)-40-2719-3510, Fax: (+91)-40-2716-0921
Received: October 31, 2005; Revised: April 20, 2006; Accepted: May 8, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Theasymmteric peoxidation of unfunc-
tionalised olefins to epoxides is realised by using
and zeolites.[7e,f] Nanocrystalline metal oxides[8a] have
been efficiently used as absorbents for gases, for the
destruction of hazardous chemicals and as catalysts
for organic transformations.[8b–e] Nanomaterials with
their three-dimensional structure and defined sizes
and shapes are demonstrated to be suitable candi-
dates for proper alignment with prochiral substrates
for the unidirectional introduction of the reacting spe-
cies to induce asymmetric centres as exemplified in
asymmetric epoxidation.[9a,b] Similarly, wehypothesise
that the catalytic system comprising metal ions ex-
changed on a support having a defined shape and size
is expected to display asymmetric induction upon
using a suitable chiral auxiliary. We herein report the
manganese(III) complexes stabilised on nanocrys-
A
talline magnesium oxide in the presence (1R,2R)-
(À)-diaminocyclohexane as a chiral ligand in good
yields and up to 91% enantiomeric excess.
Keywords: asymmetric epoxidation; heterogeneous
catalysis; magnesium oxide; manganese acetylaceto-
nate; nanocrystalline materials
Asymmetric epoxidation (AE) of unfunctionalised asymmetric epoxidation of unfunctionalised olefins to
olefins is an important organic transformation since epoxides using manganese acetylacetonate stabilised
the resulting epoxides are important building blocks on nanocrystalline magnesium oxide (aerogel pre-
for the synthesis of biologically active molecules.[1] pared MgO, NAP-MgO) with a defined shape and
Although Srinivasan et al.[2a] reported Mn(II)-(salen) size in the presence of (1R,2R)-(À)-diaminocyclohex-
complexes for the epoxidation of olefins for the first ane(DAC) as a chiral ligand in good yields and up to
time, the chiral Mn(III)-(salen) complexes reported 91% enantiomeric excess[10] (Scheme 1).
[2b,c]
by thegroups of Jacobsne
and Katsuki[3] have
Various magnesium oxide crystals[8b,11,12] [commer-
emerged as extremely efficient systems for the AE of cial MgO, CM-MgO (SSA: 30 m2 /g), conventionally
the unfunctionalised olefins. The high enantiomeric prepared MgO, NA-MgO (SSA: 250 m2 /g), aerogel
excess (ee) values achieved here is ascribed to the di- prepared MgO, NAP-MgO (SSA: 590 m2 /g)] were ini-
rected path of the prochiral olefin to the metal centre, tially evaluated in the epoxidation reaction of unfunc-
while other possible paths are blocked by the intro- tionalised olefins separately in order to understand
duction of bulky substituents on salen ligands. Katsuki the relationship between the structure and reactivity.
et al. also reported the asymmetric epoxidation of ole- They are not active as such for the epoxidation of un-
fins by using a combination of achiral Mn-salen com-
plex and chiral amine.[3e]
The immobilisation of transition metal complexes
on solid supports can provide catalysts that are easier
to handle and sometimes exhibit improved activities
and enantioselectivities induced by the support.[4] AE
has been reported by using chiral Mn-(salen) complex
immobilised on mesoporous materials,[5,6] polymer
Scheme 1. Asymmetric epoxidation of olefins using Mn-
supports,[7a,b] layered double hydroxides,[7c] clays,[7d]
(acac)n-NAP-MgO in the presence of a chiral ligand.
1038
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Asymmetric Epoxidation of Olefins by Manganese(III) Complexes
A
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Table 1. Asymmetric epoxidation of styrene using different catalysts at À208C.
Entry
Catalyst
Time[h]
Yield [%]
ee [%][e]
1
2
3
4
5
NAP-MgO
24
24
24
24
24
0, 0,[a] 0[b]
10
0
0
0
Mn
Mn
U
10
Mn (acac)n-NAP-MgO
MnO2-NAP- MgO
70, 20,[c] 0[d]
0
42, 0[c]
0
[a]
NA-MgO.
[b] CM-MgO.
[c]
Mn
ACHTREUNG
[d] Mn
ACHTREUNG
[e]
Absoluteconfiguration was found to be( R). The ee was determined by HPLC using a Diacel Chiralcel OD-H column
(flow rate: 1.0 mLminÀ1, 3% isopropyl alcohol in hexane).
functionalised olefins (Table 1, Entry 1). All the MgO dant was found to be superior over NA-MgO and
samples were then treated with Mn
samples of Mn(acac)n-MgO.
The ion exchange capability of NAP-MgO is well using theMn
established.[13] Theexchangecapability of Mn
(acac)3 and assorted crystals of Mn
on thevarious MgO supports showed that themaxi- tively (Table 1). The AE reactions catalysed by Mn-
mum amount of Mn(acac)n is loaded on NAP-MgO, (acac)n-NAP-MgO conducted in THF using 6-cyano-
ACHTREUNG
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
when compared to the other MgO samples, which is chromene, indene, styrene and 4-methylstyrene gave
attributed to the presence of large surface ionic charg- the chiral epoxides in good yields and ees (Table3).
es on NAP-MgO. In the process of optimisation of
The catalyst was recycled for three times. The de-
the AE reaction, we explored the various MgO sam- crease in yield in the recycling experiments is due to
ples (Table 1) using different oxidants (Table 2). Mn leaching into solution from the support during the
Among the MgO samples screened for the AE reac- reaction as confirmed by AAS (Table 4). The catalyst
tion, theMn (acac)n-NAP-MgO using TBHP as oxi- was recovered by filtration, while the chiral ligand
A
Table 2. Effect of oxidant in the asymmetric epoxidation of styrene[a] at À208C.
[b]
Entry
Oxidants
Time[h]
Yield [%]
ee [%][c]
1
2
3
4
TBHP
m-CPBA
NaOCl (aqueous)
H2O2 (aqueous)
24
15
24
24
70
90
0
42
0
0
0
0
[a]
Reaction conditions: olefin (1 mmol), dry THF (3 mL), TBHP (1 mL, 3.5 mmol), catalyst (0.100 g), chiral ligand (DAC)
(0.025 g).
1
[b] Yields based on H NMR.
The ee was determined by HPLC using a Diacel Chiralcel OD-H column (flow rate: 1.0 mLminÀ1, 3% isopropyl alcohol
[c]
in hexane).
Table 3. Asymmetric epoxidation of unfunctionalised alkenes with Mn
A
[a}
Entry
ld [%h]]
SubstrateYTieme
ee [%][c]
1
2
3
4
5
Styrene
24
24
24
18
18
70, 55[b]
78
66
90
85
42, 41[b]
40
24
91
84
4-Methylstyrene
trans-Stilbene
6-Cyanochromene
Indene
[a]
1
Yields based on H NMR.
[b] 3rd cycle.
[c]
The ee values were determined by HPLC using a Diacel Chiralcel OD-H column (flow rate: 1.0 mLminÀ1, 3% isopropyl
alcohol in hexane).
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B. M. Choudary et al.
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Table 4. Reusability of Mn
A
[b]
Entry
cataClynystctlien[mMtmhneoclo/gn]te
Yield [%][c]
ee [%][d]
1
2
3
1
2
3
0.369
0.332
0.258
70
66
55
42
42
41
[a]
Reaction conditions: olefin (1 mmol), dry THF (3 mL), TBHP (1 mL, 3.5 mmol), catalyst (0.100 g), chiral ligand (DAC)
(0.025 g).
[b] Mn content determined by AAS analysis.
[c]
1
Yields based on H NMR.
[d] The ee values were determined by HPLC using a Diacel Chiralcel OD-H column (flow rate: 1.0 mLminÀ1, 3% isopropyl
alcohol in hexane).
was recovered by column chromatography. When the surface ionic charges per unit area. The presence of
reaction was conducted with the obtained filtrate, no corner and edge sites on the surface of NAP-MgO
product formation was observed. This result infers could approach 20%, whileon NA-MgO they amount
that the Mn leached into solution is inactive in the re- to less than 0.5% and on CM-MgO essentially 0%.
action and that theMn complex bound to MgO is the
only active species in this reaction.
For example, an edge or even more a corner O anion
is coordinatively unsaturated and seeks Lewis acids to
To understand the relation between structure and help stabilise and delocalise its positive charge. Con-
reactivity in AE, it is essential to know the structure versely, an Mg2+ ion on an edge or corner seeks
and nature of the reactive sites of NAP-MgO. NAP- Lewis bases (acac) to stabilise and delocalise its posi-
MgO has a three-dimensional polyhedral structure, tivecharge(Scheme2).
showing the presence of high surface concentrations
of edge/corner and various exposed crystal planes Mg2+ ions readily accept incoming Lewis acids and
(such as 002, 001, 111), which leads to inherently high Lewis bases, respectively. Thus, Mn(acac)n-NAP-MgO
Therefore, these coordinatively unsaturated OÀ and
AHCTREUNG
Scheme 2. Preparation of the Mn(acac)n-NAP-MgO catalyst.
G
Scheme 3. Proposed mechanism for the Mn(acac)n-NAP-MgO-catalysed asymmetric epoxidation of olefins.
G
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Asymmetric Epoxidation of Olefins by Manganese(III) Complexes
A
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were carried out using an Autospec M (Micromass, Man-
chester, UK) mass spectrometer of EBE geometry, equipped
with an OPUS V3.1X data system. Ionisation was effected
by a 2.2 mA primary beam of caesium ions accelerated to
25 keV and the ion source was operated at an acceleration
voltageof 8 kV. High performanceliquid chromatography
(HPLC) was performed using an AGILENT-1100 series
liquid chromatograph equipped with a single pump and UV
detector (fixed at 216 nm) using a CHIRACEL-OD-H capil-
lary column with isopropyl alcohol/hexane as eluting agent.
For these studies, all reaction products were taken as
column purified. Optical rotations were obtained on an au-
tomated JASCO P-1020 Polarimeter, and the values are re-
ported in absolute reactions: [a]tDemperature [concentration c=
g/100 mL of solvent].
did indeed display the highest activity compared to
NA-MgO and CM-MgO. Conversely, the MnO2-NAP-
MgO formed by calcination of Mn(acac)n-NAP-MgO
A
is inactive for this epoxidation reaction. This may be
dueto a disturbanceof themolecular chemistry. In
the proposed mechanism for the epoxidation of ole-
fins such as styrene, the Mn(III) on nanomagnesium
oxide(A) is complexed with a chiral DAC ligand and
=
subsequently oxidised to form a metal-oxo [Mn(IV)
O] (B) species with the oxidant, t-BuOOH
(Scheme 3); this is indicated by XPS analysis (see Fig.
10 in theSupporting Information). Mn(IV) is indeed
found as an activespecies in Jacobsen–Katsuki cata-
lytic epoxidations.[14] The complex (B) thus generated
transfer oxygen to the interactive olefin in a selected
path to give the intermediate (C) which, on reductive
elimination, affords the chiral epoxide. The high facial Preparation of Mn(acac)n-NAP-MgO
G
selectivity of the complex is due to its surface chemi-
cal properties.[15] The very good result for the enan-
tioselectivities is significant using the simple chiral
auxiliary (1R,2R)-(À)-diaminocyclohexane as a chiral
ligand, specially designed for directing a prochiral
olefin unidirectionally to metal centre-supported
nanocrystallinemagnesium oxide, in theplaceof the
widely used chiral salen ligand,[2,3] to facilitatethefor-
mation of chiral epoxides.[9a,b] Similarly, thepropre
alignment of MnIV complex (B) supported on three-
dimensional nanocrystalline magnesium oxide with
the unfunctionalised olefin allowed the unidirectional
delivery of oxygen to afford the chiral epoxide.[9a]
The MgO samples were heated under vacuum at 5008C
before use. Mn(acac)n-NAP-MgO was prepared by treating
vacuum-dried NAP MgO (1.0 g) with Mn(acac)3 ( 0.200 g)
in THF (8 mL) under a nitrogen atmosphere at room tem-
perature with stirring for 24 h by a method similar to that
reported by Klabunde and co-workers.[15] Then the slurry
was filtered off, washed with THF and vacuum dried to give
AHCTREUNG
AHCTREUNG
Mn
A
colour. The IR spectrum shows evidence for the adsorbed
Mn
A
ACHTREUNG
À1
was found to be0.369 mmolg
troscopy (AAS).
In conclusion, Mn
A
as a catalyst for theasymmetric epoxidation of un-
General Procedure for the Epoxidation of Olefins
functionalised olefins. The nanomaterials with their under Heterogeneous Conditions
three-dimensional structure and defined size and
A mixtureof styrnee(1 mmol, 0.104 g), TBHP (1 mL,
shape as supports for metal complexes have been
demonstrated to be suitable candidates for proper
alignment with prochiral substrates for the unidirec-
tional introduction of the reacting species to induce
an asymmetric centre.
3.5 mmol), catalyst (0.100 g), chiral ligand (DAC) (0.025 g)
in dry THF (3 mL) was stirred at À208C for theappropriate
time (Entry 1, Table 3) under a nitrogen atmosphere. After
completion of the reaction as indicated by TLC, the reaction
mixture was centrifuged to separate the catalyst and the cat-
alyst was reused after vacuum drying for another cycle. The
reaction mixture was concentrated under reduced pressure
to afford thecrudeproduct and theresulting product was
purified by column chromatography on silica gel with ethyl
acetate and n-hexane as eluent.
Experimental Section
General Remarks
Nanocrystalline MgO samples were obtained from Nano-
Scale Materials Inc., Manhattan, Kansas, USA. All chemi-
cals were purchased from Aldrich and used as received. All
solvents were purchased as analytical grade and used as re-
ceived from Merck India Pvt. Ltd. Dry THF was used for all
reactions. Commercial column chromatography grade silica
gel (100–200 mesh) was used. All reactions were conducted
at À208C in dry THF under a nitrogen atmosphere. The
1H NMR spectra of the samples were recorded on Varian-
Unity 400 MHz, and Bruker Avance 300 MHz spectrometers
using TMS as an internal standard in CDCl3. IR spectra
were recorded on a Perkin–Elmer instrument. All the liquid
secondary ion mass spectromertric (LSIMS) experiments
Acknowledgements
We wish to thank the CSIR for financial support under the
Task Force Project CMM-0005. U. Pal and K.V.S.R thanks to
Council of Scientific and Industrial Research, India, for re-
search fellowships. Nanocrystalline MgO catalysts were ob-
tained from NanoScale Materials Inc., Manhattan, Kansas,
USA.
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B. M. Choudary et al.
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