A highly potential analogue of Jacobsen catalyst with in-built phase transfer capability in enantioselective epoxidation of nonfunctionalized alkenes

Article abstract of DOI:10.1006/jcat.2002.3558

A new analogue of Jacobsen Mn^III SALEN epoxidation catalysts having in-built phase transfer capability by means of introducing tertiary amino alkyl groups at the 5,5′-position of the Salen ligand were used as catalysts for the liquid-phase enantioselective epoxidation of 2,2-dimethyl-6-cyano chromene, indene, and styrene in the presence of O-coordinating axial bases with NaOCl as an oxidant under biphasic reaction conditions. Excellent conversions were obtained with catalyst loading in the range 0.4-2.0 mol% in all alkenes, but high chiral induction (EEs > 99%) was obtained only in the case of 2,2-dimethyl-6-cyano chromene. The enhanced activity of these complexes is attributed to the presence of t-alkyl amines in the Salen ligand, imparting in-built phase transfer capability to the catalyst.

Full text of DOI:10.1006/jcat.2002.3558

Journal of Catalysis 209, 99–104 (2002)
doi:10.1006/jcat.2002.3558
A Highly Potential Analogue of Jacobsen Catalyst with In-built Phase
Transfer Capability in Enantioselective Epoxidation
of Nonfunctionalized Alkenes
Rukhsana I. Kureshy,1 Noor-ul H. Khan, Sayed H. R. Abdi, Sunil T. Patel, Parameswar K. Iyer,
P. S. Subramanian, and Raksh V. Jasra
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
Received November 7, 2001; revised February 5, 2002; accepted February 5, 2002
the substrate molecule and the latter affecting the reactiv-
A new analogue of Jacobsen MnIII SALEN epoxidation catalysts
having in-built phase transfer capability by means of introducing
ity of the oxo intermediates formed during the reaction.
Furthermore, in MnIII SALEN-based catalysts, employing
tertiary amino alkyl groups at the 5,5ꢀ-position of the Salen ligand NaOCl as an oxidant, the addition of pyridine N-oxide
were used as catalysts for the liquid-phase enantioselective epoxida-
tion of 2,2-dimethyl-6-cyano chromene, indene, and styrene in the
presence of O-coordinating axial bases with NaOCl as an oxidant
under biphasic reaction conditions. Excellent conversions were ob-
tained with catalyst loading in the range 0.4–2.0 mol% in all alkenes,
but high chiral induction (EEs > 99%) was obtained only in the
case of 2,2-dimethyl-6-cyano chromene. The enhanced activity of
these complexes is attributed to the presence of t-alkyl amines in
the Salen ligand, imparting in-built phase transfer capability to the
derivatives improves (6) both catalyst turnover and enan-
tioselectivity, as in these cases the catalyst resides entirely
in the organic phase of the two-phase reaction medium;
water-insoluble pyridine N-oxide derivatives bearing hy-
drophobic substituents such as 4-phenylpyridine N-oxide
(4-PPyN-O) and 4-(3-phenylpropyl) pyridine-N-oxide
(4-PPPyN-O) also act as phase transfer catalysts in trans-
porting HOCl from aqueous to organic phase (6). Based
on this understanding and specific requirements for bipha-
sic epoxidation systems, we synthesized a new analogue of
Jacobsen’s catalysts, 1a–1c and 2a–2c, which have in-built
phase transfer capability. Due to this in-built feature, these
catalysts show high epoxidation activity even when PyN–O
and 1,4-dioxane are used as axial bases with NaOCl as an
oxidant, thus avoiding the requirement of expensive sub-
stituted pyridine N-oxide under biphasic conditions.
c
catalyst. ꢁ 2002 Elsevier Science (USA)
Key Words: enantioselective; chiral; epoxidation; nonfunctional-
ized alkenes; phase transfer; homogeneous catalysis; manganese.
INTRODUCTION
Chiral epoxides are used as building blocks for the syn-
thesis of many pharmaceuticals and fine chemicals (1, 2).
Direct oxygen atom transfer to alkenes employing homo-
geneous inorganic metal-complex-based catalysts is emerg-
ing as an important strategy for the synthesis of chiral epox-
ides. The choice of an appropriate chiral ligand and metal
is central to the design of efficient catalysts for this ap-
plication. In this context, the reactivity and selectivity of
MnIII SALEN systems (3–5) are observed to be remark-
able. These catalyst systems are known to be sensitive to a
dissymmetric diamine bridge derived from a C2 symmetric
1,2-diamine as well as bulky substituents at the 3,3ꢀ-position
of the salicylide ligand. It is further reported (6) that the
presence and nature of substituents at 5,5ꢀ-positions of the
salicylide ligand can also influence enantioselectivity dur-
ing epoxidation. Substituents on the ligand influence cata-
lyst performance through steric and electron effects, with
the former determining the enantioselective pathway for
EXPERIMENTAL METHODS
1H NMR spectra were recorded in CDCl3 using a Bruker
F113V, 200-MHz spectrometer. IR spectra were recorded
on a Biorad FTS-40 spectrophotometer in KBr/nujol mull.
Electronic spectra were recorded in dichloromethane us-
ing a UV–vis spectrometer (HP Diode Array Spectropho-
tometer model 8452A). The optical rotation (Atago, Japan)
and CD spectra (Jasco Machine J-20, Japan) were recorded
in dichloromethane. Melting points were determined with
capillary apparatus and are reported without further cor-
rection.
All the solvents used were procured from Merck and
purified by known procedures (7). Indene and styrene
were passed through a pad of neutral alumina before use.
2,2-Dimethyl-6-cyano chromene (8) and 3,5-di-t-Bu salicy-
laldehyde were synthesized by reported procedure (9). The
purity of the solvents, alkenes, and reaction products was
1 To whom correspondence should be addressed. Fax: +91-0278-566970.
E-mail: salt@csir.res.in.
99
0021-9517/02 $35.00
c
ꢁ 2002 Elsevier Science (USA)
All rights reserved.

100
KURESHY ET AL.
1
determined by gas chromatography (GC) using a Shimadzu and H NMR data of the sample are in agreement with
GC model 14B having a stainless steel column (2 m long, those reported earlier (11, 12).
3 mm i.d., 4 mm o.d.) packed with 5% SE30 (mesh size
60–80) and FID detector. Ultrapure nitrogen was used as
Synthesis of 1ꢀꢀa–1ꢀꢀc. An appropriate secondary amine
(20 mmol) in 20 ml of benzene was added dropwise to a
stirring solution of 3TBSCMB (4.53 g; 20 mmol) in 20 ml
carrier gas with a flow rate of 30 ml/min and the injection
port temperature was kept at 200◦C. For styrene and indene
of dry benzene. The resulting hazy solution was allowed to
analysis, the column temperature was programmed to be-
reflux with stirring for 6 h. The products 1ꢀꢀa and 1ꢀꢀc were
tween 70 and 150◦C while for chromenes it was kept isother-
obtained as a white crystalline solid while 1ꢀꢀb was a viscous
mal at 150◦C. Synthetic standards of the products were used
oil. These products were characterized and the data are
to determine yields by comparison of peak height and area.
given in Table 1.
The optical yield of the product was determined by GC us-
Synthesis of chiral Schiff base ligands 1ꢀa–1ꢀc and 2ꢀa–2ꢀc.
To a solution of 1ꢀꢀa–1ꢀꢀc (10 mmol, 2 equivalent) in absolute
EtOH (10 ml) was added a 10-ml solution of (1S,2S)(+)1,2-
diaminocyclohexane/(1R,2R)(+) diphenyldiaminoethane
(5 mmol, 1 equivalent). The resulting mixture was allowed
to reflux for 7–8 h. The progress of the reaction was mon-
itored on TLC. The solvent was partially removed under
reduced pressure on a rotary evaporator and the yellow
products 1ꢀa–1ꢀc and 2ꢀa–2ꢀc were precipitated by hexane.
Analytical data for these compounds are given in Table 1.
1H NMR data are supplied as supplementary material.
ing a Chiraldex GTA chiral capillary column, by 1H NMR
using a chiral shift reagent, (+)Eu(hfc)3, or by HPLC (Shi-
madzu SCL-10AVP) using a Chiralcel column OJ.
Magnetic susceptibility measurements were carried out
as per the procedure reported by Evans (10). The inner
tube (∼2.5 mm i.d.) was filled with the known concentra-
tion of sample solution in CDCl3 + ∼2% TMS, while the
outer tube was filled with CDCl3 + 2% TMS. A paramag-
netic shift observed in a TMS resonance line was used to
calculate χM using Eq. [1].
Synthesis of chiral MnIII Schiff base complexes 1a–1c and
2a–2c. An ethanolic solution of (10 ml) Mn(CH3COO)2 ·
4H2O (10 mmol) was added to a solution of 1ꢀa–1ꢀc/2ꢀa–
2ꢀc (5 mmol) in absolute ethanol (50 ml) under N2 atmo-
sphere. The resulting brown mixture was refluxed for 7–
8 h, following which the reaction mixture was cooled to
room temperature and solid LiCl (0.64 g, 15 mmol) was
added to it and stirred for an additional 4 h while the re-
action mixture was exposed to air and filtered. The solvent
was completely evaporated from the filtrate and the result-
ing residue was extracted with dichloromethane (100 ml).
The organic phase was washed with water and dried over
Na2SO4. Thedryingagent wasremovedby filtrationandthe
solution concentrated to yield the desired complex. Char-
acterization of catalysts was done by CHN analysis, opti-
cal rotation, magnetic susceptibility, and FAB mass spec-
tral analysis (Table 1). Electronic and CD spectral data are
supplied as supplementary materials.
χM = 3ꢀf/2π fm + χ0 + χ0(d0 − ds)/m,
[1]
where ꢀf = frequency separation between the TMS lines,
fm = frequency at which the proton resonance is studied,
and m = mass of the substance. The magnetic moment was
calculated from χM using Eq. [2].
√
µeff
=
χM T.
[2]
Synthesis of Metal Complex Catalysts
A four-step synthetic strategy was followed for the
synthesis of catalysts 1a–1c and 2a–2c (Scheme 1). 3-t-Bu-
2-hydroxy benzaldehyde, 3THB, was chloromethylated
by modified procedure to yield 3TB5CMB, which was
allowed to react with an appropriate secondary amine to
yield 1ꢀꢀa–1ꢀꢀc. On condensation of 1ꢀꢀa–1ꢀꢀc with (+)(1S,
2S)1,2-diaminocyclohexane/(+)(1R, 2R)1,2-diphenyl-1,2-
diaminoethane in 2 : 1 molar ratio, ligands 1ꢀa–1ꢀc and
2ꢀa–2ꢀc were formed which were then complexed with
MnIII to give 1a–1c and 2a–2c, respectively.
Enantioselective Epoxidation Reaction
Synthesis of 3-t-Bu-5-(chloro-methyl)-2-hydroxy ben-
Enantioselective epoxidation reactions were performed
zaldehyde (3TBSCMB). 3TBSCMB was synthesized by according to the established procedure (13) using 2 mol%
modifying the procedure reported in the literature (11, 12). of the complexes 1a–1c and 2a–2c with 6-cyano-2,2-
3-t-Bu-2-hydroxy benzaldehyde (26.8 g; 150 mmol) and dimethylchromene, styrene, and indene (1.29 mmol) as a
paraformaldehyde (10.0 g; 333 mmol) was added to pre- substrate in 1 ml of dichloromethane in the presence of O-
cooled (5–10◦C) concentrated hydrochloric acid (100 ml) coordinating ligands, namely, pyridine N-oxide (PyN-O),
and the resulting emulsion was stirred for 48 h with slow 4-phenyl pyridine N-oxide (PPyN-O), 4-(3-phenyl propyl
bubbling of HCl gas for 6 h, following which the reac- pyridine N-oxide) (PPPyN-O), morpholine N-oxide
tion mixture was extracted with Et2O. The organic phase (MorNO), 1,4 dioxane (1,4-D), and dimethyl sulphoxide
was washed carefully with saturated aqueous NaHCO3 and (DMSO) (0.13 mmol) using NaOCl (2.75 mmol) in four
brineanddriedoverNa2SO4. Thiswasfurtherconcentrated equal portions as an oxidant at 0◦C. The progress of the
in vacuum to get 3TBSCMB as a white crystalline solid reaction was monitored by GC analysis of the products.
(yield, 33.2 g, 97%; m.p. 63–65◦C). The CHN analysis, IR, After completion of reaction the solvent was removed, the

A JACOBSEN CATALYST ANALOGUE WITH PHASE TRANSFER CAPABILITY
101
O
OH
O
OH
HCHO, HCl, rt, 48h
ClCH2
97%
3TB5CMB
3THB
RR'NH, benzene
reflux, 6h
Yield
95-98%
O
R
OH
NCH2
. HCl
R'
NH2
NH2
Ph
H
H
2N
Ph
H
NH2
1"a, 1"b and 1"c
EtOH,
reflux, 7-8 h
Yield, 80-95%
EtOH,
reflux, 7-8 h
Yield, 86-95%
Ph
Ph
N
H
N
H
N
N
R
N
R' R'
R
R
R
OH HO
CH2
NCH
2
OH HO
CH2N
. HCl
NCH2
. HCl
R'
. HCl
R'
. HCl
1'a, 1'b and 1'c
Mn(OAc)2
2'a, 2'b and 2'c
Mn(OAc)2
Yield
Yield
86-90 %
EtOH, 10-12 h
LiCl, reflux
EtOH, 10-12 h
80-95 %
LiCl, reflux
Ph
N
Ph
H
N
H
N
N
Mn
R
CH2 N
R
R
CH2N
R'
Mn
R
O
O
NCH2
Cl
O
O
NCH2
R'
Cl
R'
R'
2a, 2b and 2c
1a, 1b and 1c
NRR' ; a=
N
; b=
; c=
N
N
O
SCHEME 1. Synthesis of complexes 1a–1c and 2a–2c.
product was separated by a short silica gel (60–120 mesh) (36–54%); nevertheless, the catalysts 2a–2c were found to
column using a hexane:dichloromethane mixture as an give higher EE values than 1a–1c. In all the catalytic runs,
eluent, and enantiomeric excesses (EEs) were determined. configuration of the dominant enantiomer of the product is
the same as that of the catalyst used.
A Jacobsen catalyst with 2 mol% catalyst loading using
4-PPyN-O as axial base is reported (13–15) to give 96%
conversion with 97% EEs for 6-cyano-2,2-dimethyl-2H-
chromene in 9 h. However, the catalysts 1a–1c and 2a–2c
take only 6–7 h to give >99% conversion and 97–99% EEs
under similar reaction conditions even with simple PyN-O
(Table 2, entries 7–12) as axial base. It is to be noted that
the catalysts reported here do not require expensive hy-
drophobic4-PPyN-O, possiblyduetoin-builtphasetransfer
capability of the catalyst rendered by the amino alkyl group
in the salicylide ligand. Furthermore, a catalyst loading of
RESULTS AND DISCUSSION
Product yields, EEs, and turnover frequency for MnIII
SALEN complexes, 1a–1c and 2a–2c, with substrates 6-
cyano-2,2-dimethyl-2H-chromene, indene, and styrene are
given in Table 2. Quantitative yields were obtained in all
the alkenes studied with these complexes in 1.5–7 h. The
results obtained with 6-cyano-2,2-dimethyl-2H-chromene
are encouraging, with >99% yield and 97–99% EEs. The
catalyst 2b was observed to give the highest EEs (84%) for
indene. However, of styrene, the EEs were not encouraging

102
KURESHY ET AL.
TABLE 1
Physicochemical Characterization Data of the Compounds
% Microanalysis found (calcd)
H
[α]2D7
(CH2Cl2)
FAB mass
(m/z)
Compounda
M.P. (◦C)
Yield (%)
µeff (B.M.)
C
N
1ꢀꢀa
1ꢀꢀb
1ꢀꢀc
1ꢀa
1ꢀb
1ꢀc
2ꢀa
2ꢀb
2ꢀc
1a
189–191
—
98
97
95
85
95
88
90
86
90
88
92
89
92
91
90
—
—
—
—
—
—
—
—
—
—
—
—
4.80
4.85
4.85
4.80
4.80
4.85
—
—
—
—
—
—
—
—
—
720
1028
717
818
1126
814
61.28 (61.24)
71.80 (71.87)
65.40 (65.49)
64.65 (64.68)
73.46 (73.45)
68.43 (68.47)
68.70 (68.74)
75.57 (75.61)
72.05 (72.09)
63.26 (63.30)
72.36 (72.34)
70.69 (70.72)
67.39 (67.45)
74.52 (74.57)
70.70 (70.72)
7.68 (7.66)
10.40 (10.70)
8.30 (8.35)
8.21 (8.23)
10.88 (10.86)
8.84 (8.85)
7.42 (7.47)
10.03 (10.08)
7.97 (8.01)
7.48 (7.50)
10.27 (10.31)
7.32 (7.37)
6.80 (6.84)
9.54 (9.59)
7.33 (7.37)
4.44 (4.47)
3.02 (3.00)
4.43 (4.49)
7.92 (7.94)
5.49 (5.53)
7.93 (7.99)
6.92 (6.97)
5.02 (5.04)
6.98 (7.01)
7.77 (7.75)
5.40 (5.45)
6.84 (6.88)
6.79 (6.84)
4.93 (4.97)
6.86 (6.88)
224–226
176–178
190–191
177–179
135
138–139
196–198
240b
+340 (c = 0.07)
+158 (c = 0.10)
+342 (c = 0.07)
−125 (c = 0.08)
−110 (c = 0.07)
−129 (c = 0.08)
+339 (c = 0.04)
+240 (c = 0.10)
+218 (c = 0.07)
−287 (c = 0.04)
−123 (c = 0.10)
−193 (c = 0.08)
1b
1c
2a
2b
190
240b
240b
182
238
2c
a(1ꢀꢀa) 3-t-Bu-2-hydroxy-5-(methylene-N morpholino) benzaldehyde hydrochloride, (1ꢀꢀb) 3-t-Bu-2-hydroxy-5-(methylene-N,N-dioctylamino) ben-
zaldehyde hydrochloride, (1ꢀc) 3-t-Bu-2-hydroxy-5-(methylene-N piperidino) benzaldehyde hydrochloride, (1ꢀa) (S,S)-[[2,2ꢀ-][(1,2-cyclohexanediyl)
bis(nitrilomethylidyne)] bis[4-(methylene-N-morpholino)-6-(1,1-dimethylethyl)phenol]] dihydrochloride, (1ꢀꢀa) 3-t-Bu-2-hydroxy-5-(methylene-N
morpholino) benzaldehyde hydrochloride, (1ꢀꢀb) 3-t-Bu-2-hydroxy-5-(methylene-N,N-dioctylamino) benzaldehyde hydrochloride, (1ꢀꢀc) 3-t-Bu-2-
hydroxy-5-(methylene-N piperidino) benzaldehyde hydrochloride, (1ꢀa) (S,S)-[[2,2ꢀ-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-
(methylene-N-morpholino)-6-(1,1-dimethylethyl)phenol]] dihydrochloride, (1ꢀb) (S,S)-[[2,2ꢀ-][(1,2-cyclohexanediyl) bis(nitrilomethyli-dyne)] bis[4-
(methylene-N,Nꢀ-dioctylamino)-6-(1,1-dimethylethyl)phenol]] dihydrochloride, (1ꢀc) (S,S)-[[2,2ꢀ-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)]
bis[4-(methylene-N-piperidino)-6-(1,1-dimethylethyl)phenol]] dihydrochloride, (2ꢀa) (R,R)-[[2,2ꢀ-][(1,2-diphenyl 1,2-ethanediyl] bis(nitrilo-
methylidyne)] bis[4-(methylene-N-morpholino)-6-(1,1-dimethylethyl)phenol]] dihydrochloride, (2ꢀb) (R,R)-[[2,2ꢀ-][(1,2-diphenyl-1,2-ethanediyl)
bis(nitrilomethylidyne)] bis[4-(methylene-N,Nꢀ-dioctylamino)-6-(1,1-dimethylethy) phenol]] dihydrochloride, (2ꢀc) (R,R)-[[2,2ꢀ-][(1,2-diphenyl
1,2-ethanediyl] bis(nitrilomethylidyne)] bis[4-(methylene-N-piperidino)-6-(1,1-dimethylethyl)phenol]] dihydrochloride, (1a) chloro-(S,S)-[[2,2ꢀ-]
[(1,2-cyclohexanediyl) bis(nitrilomethylidyne) bis[4-(methylene-N-morpholino)-6-(1,1-dimethylethyl)phenolato]]-[N,Nꢀ,O,Oꢀ]manganese(III), (1b)
Chloro-(S,S)-[[2,2ꢀ-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)] bis[4-(methylene-N,Nꢀ-dioctylamino)-6-(1,1-dimethylethyl)phenolato]]-[N,Nꢀ,O,
Oꢀ]manganese(III), (1c) chloro-(S,S)-[[2,2ꢀ-][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)-bis[4-(methylene-N-piperidino)-6-(1,1-dimethylethyl)
phenolato]]-[N,Nꢀ,O,Oꢀ]manganese(III), (2a) chloro-(R,R)-[[2,2ꢀ-][(1,2-diphenyl 1,2-ethanediyl) bis(nitrilomethylidyne)] bis[4-(methylene-N-
morpholino)-6-(1,1-dimethylethyl)-phenolato]]-[N,Nꢀ,O,Oꢀ]manganese(III), (2b) chloro-(R,R)-[[2,2ꢀ-][(1,2-diphenyl 1,2-ethanediyl) bis(nitrilo-
methylidyne)] bis[4-(methylene-N,N-dioctylamino)-6-(1,1-dimethylethyl)-phenolato]]-[N,Nꢀ,O,Oꢀ]manganese(III), (2c) chloro-(R,R)-[[2,2ꢀ-][(1,2-
diphenyl-1,2-ethanediyl) bis(nitrilomethylidyne)] bis[4-(methylene-N-piperidino)-6-(1,1-dimethylethyl)phenolato]][-N,Nꢀ,O,Oꢀ]manganese(III).
b Decomposed.
0.4 mol% is sufficient to achieve reported conversion and phase transfer capability into the catalyst itself, hydropho-
selectivity within 6 h. However, reduction in catalyst load- bic axial base ligands such as 4-PPyN-O and 4-PPPyN-O
ing (0.2 mol%) caused an increase in overall reaction time are no longer needed. A water-soluble O-coordinating ax-
(24 h) with a marginal decrease in EEs. This shows that ial base such as 1,4-dioxane (Table 3, entries 27 and 28) is
amino alkyl groups at 5- and 5ꢀ-positions on the salicylide good enough to enhance the epoxidation reaction rate with
ligand have electronic and steric features favorable to con- marginally or no decrease in enantioselectivity.
V==
version and enantioselectivity.
Mn O complexes are often proposed (4, 16–19) as
In order to evaluate the performance of our catalyst intermediates in MnIIISALEN-catalyzed epoxidation of
with various O-coordinating axial bases, epoxidation of alkenes. Collins and Gordon-Wylie (20) have reported sta-
styrene (as a representative substrate) was carried out un- ble monomeric MnV[η4-HMPA-B]−4[O] which show well-
der the above-described reaction conditions. In contrast resolved 1H NMR spectra in diamagnetic region. In order
to reported findings (6), 4-PPyN-O and 4-PPPyN-O axial to trap the intermediate oxo species in the case of cata-
bases (Table 3, entries 21–24) slow down (42–60% conver- lysts used in the present study, we oxidized 1b with ozone
sion in 24–30 h) the epoxidation reaction significantly with- both at −78◦C and at room temperature in CH2Cl2. This
out affecting EEs values. On the other hand, in O-donor turned into green complex [A] within 10 min but failed
ligands, PyN-O (Table 3, entries 19, 20, and 25–30) with to give a diamagnetic NMR. A UV–vis spectrum of [A]
catalysts 1a and 2a gave epoxidation conversion >99% of shows the formation of a band at 656 nm (Fig. 1). The com-
styrene in 5–7 h. Therefore, it is inferred that by introducing plex [A] is paramagnetic and is stable (no color or UV–vis

A JACOBSEN CATALYST ANALOGUE WITH PHASE TRANSFER CAPABILITY
103
TABLE 2
Product Yield, EEs, and TOF for Enantioselective Epoxidation of Nonfunctionalized Alkenes Catalyzed by 1a–1c and 2a–2ca
Catalyst 1a–1c
and 2a–2c
−−−−−−−−−−→
CH2Cl2 · PyNO
0◦C
Entry
Catalyst
Substrate
Time
Product
Yield (%)b
EE (%)c
TOFd × 10−3
1 (2)
3 (4)
5 (6)
1a (2a)
1b (2b)
1c (2c)
6.0 (5.0)
5.0 (4.5)
6.5 (7.0)
99 (99)
97 (98)
>99 (99)
69e (68) f
71e (84) f
53e (68) f
2.29 (2.75)
2.69 (3.02)
2.11 (1.96)
7 (8)
9 (10)
11 (12)
1a (2a)
1b (2b)
1c (2c)
6.0 (7.0)
6.0 (6.5)
6.5 (7.0)
>99 (99)
>99 (99)
>99 (99)
>99g (97)h
99g (98)h
99g (97)h
2.29 (1.96)
2.29 (2.11)
2.11 (1.96)
13 (14)
15 (16)
17 (18)
1a (2a)
1b (2b)
1c (2c)
5.0 (5.5)
4.5 (1.5)
5.5 (6.0)
98 (99)
99 (99)
99 (99)
39i (50)j
36i (51)j
39i (54)j
2.72 (2.50)
3.06 (9.17)
2.50 (2.29)
a Reactions were performed in CH2Cl2 (1 ml) with catalyst, 2 mol%; substrate, 1.29 mmol, pyridine N-oxide, 0.13 mmol; NaOCl, 2.75 mmol.
b Determined by GC.
c By 1H NMR using chiral shift reagent (+)Eu(hfc)3/chiral capillary column GTA-type/chiral HPLC column OJ.
d Turnover frequency is calculated by the expression (product)/(catalyst) × time mol−1 cat−1 s−1
.
e Epoxide configuration, 1S,2R.
f Epoxide configuration, 1R,2S.
g Epoxide configuration, 3S,4S.
h Epoxide configuration, 3R,4R.
i
Epoxide configuration, S.
j Epoxide configuration, R.
spectral change) for over a week at −78◦C, for 24 h at 4◦C,
and for 3 h at room temperature. The green solution turns
brown on the addition of styrene, which gave a spectrum
TABLE 3
(Fig. 1) very similar to the original complex 1b (Fig. 1), sug-
Effect of Different Axial Bases on Enantioselective Epoxidation
IV==
gestingtheinvolvementofaparamagneticMn
Ospecies
by Complexes 1aa (2a)
in the oxygen atom transfer step.
To further support the paramagnetic nature of the inter-
IV==
solutions undergoing these transformations. It is reported
Catalyst 1a/2a
−−−−−−−−−−→
axial base i–vi
mediate Mn
O, magnetic moment was measured for the
Axial
bases
Conversion
(%)b
Time
(h)
EE
Entry
(%)c
TOF × 10−3
19 (20) PyN-O
21 (22) 4-PPyN-O
23 (24) PPPyN-O
25 (26) MorN-O
27 (28) 1,4-D
>99 (99)
42 (60)
60 (50)
>99 (99)
98 (99)
98 (99)
5.0 (5.5) 39 (50)
2.75 (2.50)
0.19 (0.35)
0.35 (0.29)
2.12 (1.96)
2.09 (1.96)
2.47 (2.50)
30.0 (24)
24.0 (24)
6.5 (7)
40 (51)
48 (55)
36 (50)
38 (50)
6.5 (7)
29 (30) DMSO
5.5 (5.5) 38 (49)
a Reactions were carried out in CH2Cl2 (1 ml) with catalyst (2 mol%),
substrate (1.29 mmol), axial bases (0.13 mmol), NaOCl (2.75 mmol), 0◦C.
b Determined on GC.
FIG. 1. Absorption spectra obtained with 1 mM solution in CH2Cl2 of
1b (dashed line), with O3 bubbled in 1b solution (green complex) (dotted
line), and after the addition of styrene in green solution (solid line).
c From comparing RT of the authentic sample of R-styrene oxide on
GC using a γ -type chiral capillary column.

104
KURESHY ET AL.
TABLE 4
Magnetic Susceptibility Data for the Complex 1b under Different Conditions
µeff (B.M.) with respect to timea
Entry
Experiment
Immediately 10 min 30 min 180 min 24 h
31
32
33
34
1b + NaOCl
4.88
4.66
5.21
3.98
4.27
4.64
4.31
3.97
3.88
4.29
4.24
3.80
2.91
3.87
4.19
3.00
N.D.
N.D.
4.12
2.15
1b + NaOCl + PyNO
1b + styrene + PyNO + NaOCl
1b + PyNO + NaOCl 3 h + styrene
a In the sequence they were added.
that (20–22) in a MnIIISALEN-catalyzed epoxidation re- of PyN-O as axial base. Also, by using magnetic moment
action the central metal ion undergoes several changes in studies by the Evans method, paramagnetic NMR, and UV–
its oxidation state, i.e., MnIII, MnIV, MnV, dimeric species vis spectral analysis, we have successfully demonstrated the
IV==
in the presence of PhIO, NaOCl as oxidant, and PyN-O formation of catalytically active Mn
as axial base.The magnetic moment of the complex 1b epoxidation reaction.
upon addition of NaOCl (no green color observed) de-
O species during
creased from 4.88 to 4.27, 3.88 and 2.91 B.M. at 10, 30, and
180 min, respectively (Table 4, entry 31), indicating the ini-
IV==
ACKNOWLEDGMENTS
One of the author (RIK) is thankful to DST for financial assistance.We
thank Dr. P. K. Ghosh, Director, of the Institute for providing instrumen-
tation facility.
tial formation of catalytically active monomeric Mn
O
species which get dimerized (Table 4, entry 31, 2.91 B.M.)
to catalytically inactive MnIV–O–MnIV. A similar obser-
vations were reported by Adam et al. (21). The low mag-
netic moment value of 2.91 B.M. can be explained due to
antiferromagnetic coupling in a high-spin MnIV–O–MnIV
dimer. The presence of PyN–O in 1b and a NaOCl mix-
ture slows down the formation of the MnIV–O–MnIV dimer
(Table 4, entry 32). Further, after the addition of NaOCl
to the mixture of 1b + styrene + PyN-O, there is an ini-
tial increase in µeff value (Table 4, entry 33, 5.21 B.M.) and
then it remains constant (4.31–4.12 B.M.) over a period of
24 h under refrigeration (Table 4, entry 33). The initial rise
in the magnetic moment is attributed to the formation of
MnIII–O–MnIV, as also reported earlier (22), with use of
EPR techniques, which later disproportionate to form high-
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IV==
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O species. Therefore, it further
strengthens the view that the use of PyN-O with NaOCl
prevents the formation of the catalytically inactive MnIV–
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(16). The complete conversion of styrene to styrene ox-
IV==
ide strengthens our view that Mn
O is involved in the
catalytic cycle. However, if the substrate is added 3 h af-
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completion, with a marked decrease in magnetic moment
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CONCLUSION
We have described the synthesis of chiral MnIIISALEN
complexes having in-built phase transfer capability, which
are very active and selective catalysts for enantioselective
epoxidation of styrene, indene, and 6-cyano-2,2-dimethyl-
2H-chromene using NaOCl as an oxidant in the presence