Enantioselective epoxidation of nonfunctionalized alkenes catalyzed by recyclable new homochiral dimeric Mn(III) salen complexes

Article abstract of DOI:10.1016/j.jcat.2004.02.037

Two recyclable dimeric Mn^III salen complexes with geminal methyl groups on the linking carbon atom at the 5,5′ position of the salen units were synthesized. They gave a high epoxide yield (>99%) and enantiomeric excess up to >99% in 2 to 11 h a

Full text of DOI:10.1016/j.jcat.2004.02.037

Journal of Catalysis 224 (2004) 229–235
www.elsevier.com/locate/jcat
Enantioselective epoxidation of nonfunctionalized alkenes catalyzed
by recyclable new homochiral dimeric Mn(III) salen complexes
Rukhsana I. Kureshy,∗ Noor-ul H. Khan, Sayed H.R. Abdi, Surendra Singh, Irshad Ahmad,
Raksh V. Jasra, and Amish P. Vyas
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar-364 002, Gujarat, India
Received 22 September 2003; revised 5 January 2004; accepted 26 February 2004
Available online 16 April 2004
Abstract
III
ꢀ
Two recyclable dimeric Mn salen complexes with geminal methyl groups on the linking carbon atom at the 5,5 position of the salen
units were synthesized. They gave a high epoxide yield (> 99%) and enantiomeric excess up to > 99% in 2 to 11 h at 2 mol% catalyst loading
in the enantioselective epoxidation of nonfunctionalized alkenes using NaOCl as the oxidant in the presence of Py N–O as the axial base.
The epoxidation reaction was successful even at a catalyst loading of 0.4 mol%, but the reaction took longer. The catalyst was recovered
easily and recycled five times in a simple separation process. To understand the mechanism of the catalytic reaction, the kinetic investigation
was carried out with different concentrations of the catalyst, the oxidant, and the substrate and with styrene as the representative substrate.
The epoxidation of styrene was first order with respect to the catalyst and the oxidant but did not depend on the initial concentration of the
substrate. An appropriate mechanism of the epoxidation reaction is proposed.
2004 Elsevier Inc. All rights reserved.
III
Keywords: Enantioselective; Homochiral; Dimeric Mn salen; Nonfunctionalized alkenes; Kinetics; Mechanism; NaOCl
1. Introduction
we used new, recyclable dimeric MnIII salen complexes in
which the two MnIII salen units are linked by a carbon
atom with geminal methyl groups. These catalysts were re-
covered easily after precipitation with hexane. They were
reused several times and their activity and enantioselectiv-
ity were higher than those of dimeric MnIII salen [14,15],
polymeric MnIII salen [16,17], and monomeric Jacobsen
MnIII salen complexes [18–20]. Furthermore, kinetic inves-
tigations were carried out to elucidate the mechanism of the
epoxidation reaction with the dimeric complexes as catalysts
and styrene as the representative substrate.
Significant progress has been made in the design and syn-
thesis of homogeneous metal complexes with chiral ligands.
They are used as enantioselective catalysts for diverse ap-
plications [1,2]. However, separation of the catalyst from
the reaction mixture and reuse are still problematic [3]. To
address this issue, several groups have tried to immobilize
MnIII salen complexes on polymer supports [4–6], on zeo-
lites, clays, or siloxane membranes [7,8], on ordered meso-
porous silica like MCM-41 [9–12], and in ionic liquids [13].
Although the immobilized catalysts can be separated, the
catalytic efficiency was lower than that of MnIII salen com-
plexes under homogeneous conditions. We reported [14] an
alternative strategy: the solubility of the catalyst was de-
creased by increasing its molecular weight, thereby making
it easier to isolate the product and recover the catalyst. In
an attempt to develop an efficient recyclable catalyst for the
enantioselective epoxidation of nonfunctionalized alkenes,
2. Experimental methods
Manganese acetate (SD Fine Chem. Ltd.), indene, and
styrene (both from Fluka) were passed through a pad of neu-
tral alumina before use. (1R,2R)-(−)-cyclohexanediamine
was resolved from a technical grade cis–trans mixture ac-
cording to procedure in Ref. [21]. Synthesis of 3 (Scheme 1)
was carried out according to a modified procedure [22].
2,2-Dimethylchromene and its derivatives were synthesized
*
Corresponding author. Fax: +91 0278 2566970.
E-mail address: salt@csir.res.in (R.I. Kureshy).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.02.037
2004 Elsevier Inc. All rights reserved.

230
R.I. Kureshy et al. / Journal of Catalysis 224 (2004) 229–235
Scheme 1. Synthesis of complexes 1a and 1b.
as described in Ref. [23]. All the solvents were purified be-
fore use [24].
styrene and indene was 70 to 150 ◦C, while for chromene
the isothermal temperature was 150 ◦C. Synthetic standards
of the products were used to determine conversions by com-
paring the peak height and area. The ee of styrene oxide
was determined by GC on a chiral capillary column (Chi-
raldex GTA). For the chromenes and indene epoxides, the ee
was determined by H NMR using the chiral shift reagent
Eu(hfc)3 and by HPLC (Shimadzu SCL-10AVP) using a
Chiralcel column (OD, OJ/OB).
Microanalysis of the catalyst was carried out on Perkin
Elmer CHN Analyzer 2400. 1H NMR spectra of the dimeric
ligands were recorded in CDCl3 (Bruker F113V, 200 MHz).
FTIR spectra were recorded on a Perkin Elmer Spectrum GX
spectrophotometer in a KBr/nujol mull. Electronic spectra of
the dimeric complexes were recorded in dichloromethane on
a Hewlett-Packard Diode Array spectrophotometer (Model
8452A). Molar conductance was measured at room tempera-
ture on a Digisun Electronic Conductivity Bridge DI-909 in-
strument. The optical rotation in dichloromethane was mea-
sured on an Atago polarimeter, (Japan). The melting points
were determined with a capillary apparatus and are reported
without correction.
1
Scheme 1 and the following section describe, the com-
plete synthesis of complexes 1a and 1b, used as catalysts.
2.1. Synthesis of 5,5ꢀ-di-tert-butyl-6,6ꢀ-dihydroxy-3,3ꢀ-
(propane-2,2-diyl)dibenzaldehyde 3
A solution consisting of 2,2ꢀ-di-tert-butyl-4,4ꢀ-(propane-
2,2-diyl)diphenol (32.6 mmol), SnCl4 (19.5 mmol), 2,6-
lutidine (78.1 mmol), and 200 ml toluene was stirred in
a nitrogen atmosphere at room temperature for 45 min.
Paraformaldehyde (260 mmol) was then added and the mix-
ture heated while refluxing. The progress of the reaction
was monitored by TLC. The reaction mixture was cooled to
room temperature and 200 ml water and 200 ml diethyl ether
The purity of the solvent and alkenes and the analysis
of the epoxide product were determined by gas chromatog-
raphy (GC) using a Shimadzu GC 14B instrument with a
stainless-steel column (2 m long, 3 mm inner diameter, 4 mm
outer diameter) packed with 5% SE30 (mesh size 60–80)
and equipped with an FID detector. Ultrapure nitrogen was
the carrier gas (rate 30 ml/min) and the injection port tem-
perature was 200 ◦C. The temperature of the column for

R.I. Kureshy et al. / Journal of Catalysis 224 (2004) 229–235
231
were added. The resulting emulsion was filtered through a
pad of celite, the organic layer was washed with water and
brine and dried over anhydrous sodium sulfate. A light yel-
low solid (mp 130 ◦C) of 5,5ꢀ-di-tert-butyl-6,6ꢀ-dihydroxy-
3,3ꢀ-(propane-2,2-diyl)dibenzaldehyde 3 was obtained (85%
yield), which was purified by flash column chromatography
using hexane: ethyl acetate (95:5) as an eluent. Anal. calcd.
for C25H32O4: C, 75.82; H, 8.14; N, 3.93. Found: C, 75.79;
4H, HCN), 7.25 (s, 4H, aromatic), 7. 17 (s, 10H), 6.97 (s,
4H, aromatic), 4.72 (s, 4H), 1.42 (s, 36H), 1.22 (s, 18H):
[α]D = −327 (c = 0.12, CH2Cl2).
2.4. Synthesis of 5,5-(2ꢀ,2ꢀ-dimethylpropane)-di-[(R,R)-
{N-(3-tert-butylsalicylidine)-Nꢀ-(3ꢀ,5ꢀ-di-tert-butylsalicyli-
dine)}-1,2-cyclohexanediaminato(2-)manganese(III)
chloride (1a) and 5,5-(2ꢀ,2ꢀ-dimethylpropane)-di-[(S,S)-
{N-(3-tert-butylsalicylidine)-Nꢀ-(3ꢀ,5ꢀ-di-tert-butylsalicyli-
dine)}-1,2-di-phenylethylenediaminato(2-)manganese (III)
chloride (1b)
27
1
H, 8.09; N, 3.89%: H NMR (CDCl3, 200 MHz); δ ppm
11.72 (s, 2H, OH), 9.84 (s, 2H, HCO), 7.30 (bs, 4H, aro-
matic), 1.70 (s, 6H, methyl), 1.34 (s, 18H, tert-butyl): IR
(KBr), cm−1; 3473, 2963, 2870, 1646.
The synthesis of N-(2-hydroxy-3,5-di-tert-butylbenzal-
dehyde)-1-amino-2-cyclohexaneimine (1ꢀꢀa) was carried out
according to the method in Ref. [15].
The dimeric homochiral Schiff base (1ꢀa) and (1ꢀb)
(0.001 mol) was dissolved in 40 mL dichloromethane:meth-
anol (1:1) and 0.002 mol Mn(CH3COO)2 ·4H2O was added
in an inert atmosphere. The reaction mixture was refluxed
for 8 h and the reaction was monitored by TLC using
hexane:ethyl acetate (6:4) as the mobile solvent. The re-
action mixture was cooled to room temperature, lithium
chloride (0.006 mol) was added, and the mixture was stirred
for 5 h in air and filtered. The solvent was removed from the
filtrate and the residue was extracted with dichloromethane.
The organic layer was washed with water and brine and dried
over sodium sulfate. After partial removal of the solvent the
desired complexes 1a and 1b were precipitated upon the
addition of petroleum ether (40–60). 1a: Yield 80%, anal.
calcd. for C67H92Mn2N4O4 Cl2: C, 67.16; H, 7.74; N, 5.92.
Found: C, 67.01; H, 7.68; N, 5.85: IR (KBr) (cm−1) 3476
(br), 2957 (s), 2865 (s), 1612 (s), 1587 (s), 1528 (s), 1440
(sh), 1388 (w), 1328 (w), 1274 (w), 1241 (w), 1215 (m),
1176 (m), 1136 (w), 1011 (w), 879 (w): UV–vis. (CH2Cl2),
λmax(∈), 506 (2740), 436 (6510), 321 (22040), 239 (59700):
2.2. Synthesis of N-(2-hydroxy-3,5-di-tert-butylbenzaldehy-
de)-1-amino-1,2-diphenylethaneimine (1ꢀꢀb)
3,5-Di-tert-butylsalicylaldehyde 2 (0.001 mol) dissolved
in 10 mL chloroform reacted slowly with 0.001 mol of
1S,2S-(−)-1,2-diphenylethylenediamine in 50 mL of cold
chloroform; the reaction mixture was stirred for 48 h at 0 ◦C.
The progress of the reaction was checked on TLC using a
hexane:ethyl acetate (9:1) mixture. 1ꢀꢀb: Yield 89%: anal.
calcd. for C29H36N2O: C, 81.32; H, 8.40; N, 6.53. Found: C,
81.25; H, 8.36; N, 6.48%: 1H NMR. (CDCl3, 200 MHz): δ
ppm 13.60 (s, 1H, OH exchangeable with D2O), 8.46 (s, 1H,
H–C=N), 6.87–7.30 (bs, 12H, aromatic), 4.72 (d, 1H), 4.29–
4.44, (q, 1H), 1.66 (2H, s, br), NH2 proton D2O exchange-
able) 1.47 (9H, s), 1.29 (9H, s): IR (KBr), cm−1 3448, 2958,
2868, 1626, 1598, 1453, 1414, 1391, 1249, 1173, 1047, 879.
27
ΛM(MeOH): 5 ꢀ−1 cm−1 mol−1: [α]D = −88 (c = 0.11,
2.3. Synthesis of 5,5-(2ꢀ,2ꢀ-dimethylpropane)-di-[(R,R)-{N-
(3-tert-butylsalicylidine)-Nꢀ-(3ꢀ,5ꢀ-di-tert-butylsalicylidine)}-
1,2-cyclohexanediamine] (1ꢀa) and 5,5-(2ꢀ,2ꢀ-dimethylpro-
pane)-di-[(S,S)-{N-(3-tert-butylsalicylidine)-Nꢀ-(3ꢀ,5ꢀ-di-
tert-butylsalicylidine)}-1,2-diphenylethylene diamine (1ꢀb)
CH2Cl2).
1b: Yield 80%, anal. calcd. for C84H96Cl2Mn2N4O4; C,
71.49; H, 7.08; N, 4.02. Found; C, 71.18; H, 7.05; N, 3.95:
IR (KBr) (cm−1): 3471 (br), 2957 (s), 2869 (s), 1606 (s),
1534 (s), 1456 (sh), 1429 (s), 1388, 1311 (s), 1249 (s),
1177 (m), 1135 (w), 1027 (w), 850 (w): UV–vis: (CH2Cl2),
λmax(∈) (nm) 328 (73085), 442 (23550), 509 (8270), 535
A solution of 1ꢀꢀa/1ꢀꢀb (0.002 mol) in dichlorometh-
ane and 5,5ꢀ-di-tert-butyl-6,6ꢀ-dihydroxy-3,3ꢀ-(propane-2,2-
diyl)dibenzaldehyde 3 (0.001 mol) in ethanol was refluxed
for 6 to 8 h. The progress of the reaction was checked on
TLC. After completion of the reaction the resulting solution
was concentrated to give the desired ligands 1ꢀa/1ꢀb. Yield,
89%: 1ꢀa: anal. calcd. for C67H96N4O4: C, 78.78; H, 9.47;
N, 5.48. Found: C, 78.50; H, 9.30; N, 5.38%: IR (KBr):
27
(6611): ΛM(MeOH): 6 ꢀ−1 cm−1 mol−1: [α]D = −139.8
(c = 0.14, CH2Cl2).
2.5. Enantioselective epoxidation of nonfunctionalized
alkenes
Enantioselective epoxidation reactions were performed
using 2 mol% of complexes 1a and 1b with 2,2-dimethyl-
chromene (CR), 6-cyano-2,2-dimethylchromene (CNCR),
6-methoxy-2,2-dimethylchromene (MeOCR), spiro[cyclo-
hexane-1,2ꢀ-[2H][1]chromene] (CyCR), indene (IND), and
styrene (STR) (1.29 mmol) was the substrates. One milli-
liter of dichloromethane was added in the presence of Py
N–O (0.13 mmol) as an axial ligand using buffered NaOCl
(2.75 mmol) (pH 11.3) as an oxidant and added in four equal
portions at 0 ◦C. The epoxidation reaction was monitored
1
1628 cm−1 ν(H–C=N); H NMR (CDCl3, 200 MHz): δ
ppm 13.70 (bs, 4H exchangeable with D2O, OH), 8.29 (s,
4H, HCN), 7.25 (s, 4H, aromatic), 6.97 (s, 4H, aromatic),
3.29 (m, 4H), 1.90 (s, 6H), 1.40 (s, 36H), 1.23 (s, 18H):
27
[α]D = −238 (c = 0.10, CH2Cl2).
1ꢀb: anal. calcd. for C83H100N4O4: C, 81.87; H, 8.28;
N, 4.26. Found: C, 81.69; H, 8.18; N, 4.23%; IR (KBr):
1
1626 cm−1 ν(H–C=N); H NMR (CDCl3, 200 MHz): δ
ppm 13.58 (bs, 4H exchangeable with D2O, OH), 8.31 (s,

232
R.I. Kureshy et al. / Journal of Catalysis 224 (2004) 229–235
by GC with n-tridecane (0.1 mmol) as the GLC internal
standard for product quantification. After the reaction, the
product was extracted with CH2Cl2, washed with water, and
dried over sodium sulfate. The catalyst was separated from
the product epoxide by precipitation with 2 mL hexane and
used as such for further catalytic runs.
Table 1
a
Product yields and ee for enantioselective epoxidation of nonfunctional-
ized alkenes catalyzed by complexes 1a and 1b with pyridine N–O and
NaOCl as oxidants
b
c
−3 k
Entry
Catalyst Substrate Yield Time (h) ee
TOF × 10
(%)
(%)
d
1 (2)
1a (1b) IND
> 99
2 (5)
3 (6)
77
6.87 (2.75)
e
e
g
g
g
g
i
(> 99)
(66)
2.6. Epoxidation reaction for kinetic measurements
∗
∗ l
d
1
(2 )
87
(71)
The catalysts 1a and 1b (0.20 × 10−2–0.84 × 10−2 M) in
1 mL CH2Cl2 were stirred with Py N–O (2.09 × 10−2 M)
and STR (10.41 × 10−2–41.64 × 10−2 M) at 0 ◦C; the re-
f
3 (4)
5 (6)
7 (8)
9 (10)
1a (1b) CyCR
1a (1b) CR
> 99
(96)
11 (7)
91
1.25 (1.90)
2.75 (2.50)
3.43 (2.75)
2.75 (2.29)
4.58 (4.58)
(96)
f
> 99
(> 99)
5 (5.5) 83
sulting solution was treated with NaOCl (22.49 × 10−2
–
(88)
f
1a (1b) MeOCR > 99
(> 99)
4 (5)
5 (6)
3 (3)
99
89.96 × 10−2 M) and stirred constantly. To determine the
rates of epoxidation, aliquots were drawn from the reaction
mixture every 2 min, quenched with triphenylphosphine, and
analyzed on GC.
(99)
f
1a (1b) CNCR
> 99
(> 99)
> 99
(> 99)
92
99
(99)
h
11 (12) 1a (1b) STR
47
(62)
j
h
13
14
15
A
B
STR
STR
STR
11
7.5
3.5
48
1.16
1.83
3.92
j
h
> 99
> 99
35
3. Results and discussion
j
h
1a
47
a
Reactions conditions: catalyst (2 mol% in 1 ml CH Cl ), substrate
2
2
Product yield and ee data (Table 1) show that complexes
1a and 1b exhibit excellent epoxidation activity (> 99%
conversion to epoxide) with all the substrates (entries 1–12)
after 2 to 11 h and gave > 99% ees with chromenes having
the polar substituents, MeOCR (entries 7, 8) and CNCR (en-
tries 9, 10). The ee was moderate to good for IND (entries
1, 2), (CyCR) (entries 3, 4), and CR (entries 5, 6). At −5 ◦C
the epoxidation of IND took a longer time to reach comple-
tion but the ee was higher (Table 1, entries 1∗, 2∗). Complex
1b (entry 2) induced higher enantioselectivity for STR than
for 1a, while the over all performance of catalyst 1a was bet-
ter than 1b for most of the alkenes (Table 1). Furthermore,
the epoxidation reaction was faster for all the alkenes with
improved enantioinduction except in case of the electron-
deficient CNCR where the reaction was slower than that of
the previously reported dimeric MnIII complex (B) [14] (Ta-
ble 1). The increase in enantioselectivity is attributed to two
methyl groups on the linking carbon atom of the homochi-
ral dimeric complexes 1a, 1b which may cause the substrate
to follow a chiral path; in the case of the previously reported
dimer [14] there were two hydrogen atoms on the linking
carbon atom. In all catalytic runs, the configuration of the
dominant enantiomer of the product was the same as that of
the catalyst.
(1.29 mmol), pyridine N-oxide (0.13 mmol), NaOCl (2.75 mmol).
b
Determined on GC.
Chiral capillary column GTA-type/By H NMR using chiral shift
c
1
reagent (+)Eu(hfc) /chiral HPLC column OJ, OD, OB.
3
d
Epoxide configuration, 1R,2S.
Epoxide configuration, 1S,2R.
Epoxide configuration, 3R,4R.
Epoxide configuration, 3S,4S.
Epoxide configuration, R.
Epoxide configuration, S.
Catalyst (1 mol%).
e
f
g
h
i
j
k
Turnover frequency is calculated by the expression, [product]/
−1
([catalyst] × time) (s ).
l
∗
∗
◦
1
(2 ) reaction conducted at −5 C.
Jacobsen catalyst (A) (2 mol%), the –CH2– linked dimeric
complex (B) (1 mol%) [14], and the CH3–C–CH3-linked
complex 1a (1 mol%) were reacted under identical reaction
conditions; pyridine N-oxide (5 mol%) was the axial base
and NaOCl the oxidant. These catalysts gave epoxide con-
versions of 92%; (ee 48%) in 9 h (Table 1, entry 13), > 99%
(ee 35%) in 7.5 h (entry 14), and > 99% (ee 47%) in 3.5 h
(entry 15), respectively. Moreover, the initial rate constants,
kobs, during the initial 2 h of this reaction suggest that the
two catalytic units do not operate in isolation but interact.
The cause of this behavior is unknown.
The literature shows that axial bases have a pronounced
effect on the reactivity and selectivity of the enantioselective
epoxidation reaction. In the case of chiral monomeric MnIII
salen complexes, 4-phenylpyridine N-oxide (4-PhPyNO) is
reported to (a) stabilize the formation of catalytically active
metal oxo species and (b) act as a phase-transfer reagent in
transporting HOCl from the aqueous to the organic phase
under biphasic reaction conditions. We therefore studied
the effect of axial bases such as 4-(3-phenylpropyl)pyridine
N-oxide (4-PPPyNO), 4-PhPyNO, and DMSO on the epoxi-
dation of STR as a model substrate in dichloromethane using
A
tertiary carbon-linked polysalen–MnIII complex
(4 mol%) was reported [16] to give 86% yield and 37%
ee for styrene oxide using 4-phenylpyridine N-oxide (20
mol%); the catalyst was recycled several times without loss
of enantioselectivity. The dimeric version of this catalyst
(complex 1a) gave > 99% conversion and 47% ee with
styrene as the substrate under similar reaction conditions as
with Py N–O (5 mol%) (entry 11).
We compared the catalytic activity of the monomeric
[18–20] and dimeric MnIII salen complex 1a, by conduct-
ing experiments to determine the epoxidation of styrene. The

R.I. Kureshy et al. / Journal of Catalysis 224 (2004) 229–235
233
Table 2
Table 4
Effect of different axial bases on enantioselective epoxidation of (STR) cat-
Enantioselective epoxidation of (STR) with recycled catalysts 1a and 1b
alyzed by complexes 1a and 1b with NaOCl
and PyNO as the axial base with NaOCl
−3
Run
1
2
3
4
5
Entry Catalyst Axial base Conversion Time (h) ee (%) TOF × 10
(%)
Time (h)
Conversion
ee
3 (3)
5.5 (5)
7 (8)
80 (99)
47 (60)
8 (9)
70 (60)
47 (59)
10 (12)
62 (50)
47 (59)
100 (99)
47 (62)
100 (99)
47 (59)
16
1b
—
20
7
52
0.39
17 (18) 1a (1b) PyNO
> 99
(> 99)
3 (3)
47 (62) 4.62 (4.62)
Results in parenthesis are for catalyst 1b.
19 (20) 1a (1b) 4-PhPyNO > 99
2.5 (2.5) 45 (61) 5.50 (5.50)
2 (1.6) 46 (60) 6.87 (8.59)
(> 99)
21 (22) 1a (1b) 4-PPPyNO > 99
(> 99)
23 (24) 1a (1b) DMSO
50 (80)
—
6 (5)
24
46 (57) 1.16 (2.22)
25
—
PyNO
—
—
Table 3
Effect of solvent on epoxidation of styrene using catalyst 1b in the presence
of Py N–O as axial base
Solvent
CH Cl
Time (h)
Conversion (%)
ee
3
6
2.5
4
> 99
> 99
> 99
> 99
62
36
46
57
2
2
◦
Fig. 1. Time-dependent plot of the formation of epoxide at 0 C for cata-
CH OH
3
−2
−2
M,
lysts 1a and 1b [catalyst] = 0.42 × 10
M, [STR] = 20.82 × 10
CH CN
3
−2
[oxidant] = 89.96 × 10 M.
CH COOC H
2 5
3
catalysts 1a and 1b (Table 2). Of all the axial bases used,
4-PPPyNO had the highest turnover frequency with catalysts
1a and 1b (entries 21, 22) followed by 4-PhPyNO (entries
19, 20), and Py N–O (entries 17, 18) while DMSO (23, 24)
was least effective. We reported the same trend with the
dimeric catalyst [14], which is in agreement with the MnIII
salen complexes [25]. In the absence of an axial base the
conversion of STR was only 20% after 7 h (entry 16) (Ta-
ble 2), indicating a significant contribution of the axial bases
in the catalytic reactions. Furthermore, the epoxidation of
STR in the presence of Py N–O (no catalyst added) with
NaOCl did not occur (entry 25).
◦
Fig. 2. Plot of catalysts [1a, 1b] versus
20.82 × 10 M and [oxidant] = 89.96 × 10 M.
k
at 0 C, [STR] =
obs
−2
−2
The choice of solvent has a significant effect on the
activity and the selectivity of the chiral MnIII salen com-
plexes [26,27]. We studied the effect of various solvents on
the epoxidation of STR using complex 1b as a represen-
tative catalyst (Table 3). Although complete conversion to
the product epoxide was achieved in all cases, the solvent
dichloromethane had the best effect on the reaction time and
enantioselectivity.
The recovered catalysts, 1a and 1b, were reused with the
NaOCl and Py N–O system with STR as the substrate (Ta-
ble 4). After each use, the catalyst was precipitated from the
reaction mixture by the addition of hexane. In subsequent
catalytic runs, the precipitated catalyst, dried in vacuum, was
used without further purification. However, the epoxidation
reaction proceeded only after the addition of fresh Py N–O
(axial base). Furthermore, the activity of the recycled cat-
alyst after successive epoxidation indicates degradation of
the catalyst and/or physical loss during the recovery process.
However, the ee of the product did not decrease.
substrate, STR, and catalysts 1a and 1b in the presence of
NaOCl as the oxidant and Py N–O as the axial base; the
concentrations of the catalysts, the oxidant, and STR were
varied. At the beginning of epoxidation reaction, the kinetic
runs showed a liner plot; saturation was achieved by the end
of the reaction (Fig. 1). Based on these results the initial rate
constants kobs (up to the linear portion of the graph) were
determined by estimating the amount of epoxide formed by
the end of the reaction.
The kinetic data for the epoxidation of STR with var-
ious concentrations of catalysts 1a and 1b gave linear
plots of the logkobs versus log[catalyst], with unit slopes
(d logkobs/d log[catalyst] = 1) suggesting that the epoxi-
dation of STR is first order with respect to the concen-
trations of the catalysts (Fig. 2). Further, the plots of the
rate constants (kobs) versus the concentration of oxidant
(d logkobs/d log[oxidant] ∼ 1) also showed first-order de-
pendence of the reaction on the oxidant concentration
(Fig. 3). Whereas, zero-order dependence was observed for
the initial concentration of STR (10.41–41.64 × 10−2 M) at
To understand the mechanism of the epoxidation reaction,
kinetic investigations were carried out with a representative

234
R.I. Kureshy et al. / Journal of Catalysis 224 (2004) 229–235
Fig. 4. UV–vis spectra with 0.2 mM solution of 1b in CH Cl with Py N–O
2
2
(X), with oxidant (Y), on addition of substrate (STR) (Z).
Fig. 3. Plot showing linear dependence of k with respect to the concentra-
literature [32] establishes that Py N–O (A) occupies the ax-
ial position in the MnIII salen complex, and that the resulting
complex reacts with the oxidant to form an oxo complex,
(L(A)Mn=O) in the rate-determining step. The formation of
the (L(A)Mn=O) complex is proposed on the basis of spec-
troscopic measurements as well as on our kinetic results,
which show first-order dependence on the catalyst and ox-
idant concentrations. The interaction of alkene with the oxo
complex occurs relatively fast to selectively yield the epox-
ide through oxygen atom transfer at the olefinic bond. The
alkene interacts with (L(A)Mn=O) and probably follows the
route of manganaoxetane formation [33], (enantioselective
pathway a, Scheme 2) to selectively give the epoxide and
the catalyst (L(A)Mn) in its original form. However, in the
absence of absolute chiral induction with the substrate STR,
the racemic pathway (b) through the formation of a rad-
ical intermediate (c), which is largely responsible for the
racemization due to the rotation of the C–C bond, cannot
be ruled out. This pathway has been proposed especially
in the case of conjugated alkenes, resulting in cis/trans iso-
merization [34]. Racemization may also occur via a radical
pathway, which may result in the formation of a carboca-
tionic intermediate (d), which is largely responsible for the
formation of an aldehyde [35]. In all our catalytic epoxida-
tion reactions, the formation of epoxide only (with almost
absolute enantioinduction in case of substituted chromenes)
and the absence of aldehyde suggest that pathway (a) is
probably operative in this system.
obs
◦
−2
M
tion of oxidant for epoxidation of styrene at 0 C, [1a, 1b] = 0.42×10
−2
and [STR] = 20.82 × 10 M.
Spectroscopic investigations were carried out to show the
formation of (L(A)Mn=O) as a catalytically active species
in the MnIII salen-catalyzed epoxidation of alkene. Fig. 4
shows a stepwise overlay of UV–vis spectra for complex 1b
in CH2Cl2 at 0 ◦C. (X) is the spectrum of catalyst 1b with
Py N–O, with a peak at 515 nm, typical of MnIII salen com-
plexes [36]. Upon the addition of NaOCl as the oxidant to
this solution, the solution becomes dark brown, and a new
absorption band develops around 650 nm; this is commonly
reported with NaOCl [35] and other oxidants such as PhIO,
t-BuOOOH [36]. This spectral change is attributed to the
formation of (L(A)Mn=O) (Y). After the addition of the
substrate STR, a spectrum (Z), appeared which is similar to
the original complex 1b (X). This supports our observation
that the L(A)Mn=O species is involved in the oxygen atom
Scheme 2. Proposed mechanism for Mn(III) salen complexes 1a and 1b.
constant concentration of the other reactants and the physical
conditions. Zero-order kinetics in the catalytic epoxidation
of alkenes at higher alkene concentrations and a first-order
dependence for alkene at lower concentrations [30,31] have
been reported for the MnIII salen complexes as catalysts and
NaOCl [28] and HOCl [29] as oxidants. In our kinetic runs,
neat styrene epoxide was found without side products, sup-
porting the zero-order dependence on substrate at reasonably
high concentrations.
Based on the kinetic measurements and product distri-
bution, Scheme 2 represents the probable mechanism. The

R.I. Kureshy et al. / Journal of Catalysis 224 (2004) 229–235
235
transfer and is consistent with earlier reports on MnIII salen
complexes [28,36].
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4. Conclusion
We found new recyclable homochiral dimeric MnIII salen
complexes, in which two MnIII salen units are linked by a
carbon atom bearing geminal methyl groups. These com-
plexes were used as epoxidation catalysts for nonfunction-
alized alkenes. Excellent conversions were obtained for all
the alkenes, but > 99% chiral induction was obtained only
with the methoxy and cyano chromenes. The catalysts were
reused up to five times without purification, the reactivity of
the catalyst decreased gradually with successive use, possi-
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during recovery and epoxidation. The catalyst loading was
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on the activity and selectivity, suggesting that the two units
in the dimer work together. The kinetic investigations of a
representative substrate STR show first-order dependence on
the concentrations of the catalysts 1a and 1b, the oxidant,
but no dependence on the initial substrate concentration.
During epoxidation, the catalysts oxidized to form catalyt-
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step; which, upon interaction with alkene, selectively give
the epoxide product with moderate to high chiral induction
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Acknowledgments
R.I. Kureshy is grateful to DST, CSIR task force project,
and S. Singh to CSIR (JRF) for financial support and to
P.K. Ghosh, the Director, of the Institute, for providing ac-
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