Asymmetric hydrolytic kinetic resolution with recyclable polymeric Co(iii)-salen complexes: A practical strategy in the preparation of (S)-metoprolol, (S)-toliprolol and (S)-alprenolol: Computational rationale for enantioselectivity

Article abstract of DOI:10.1039/c4cy00594e

A series of chiral polymeric Co(iii)-salen complexes based on a number of achiral and chiral linkers were synthesized and their catalytic performances were assessed in the asymmetric hydrolytic kinetic resolution of terminal epoxides. The effects of the linker were judiciously studied and it was found that in the case of the chiral BINOL-based polymeric salen complex 1, there was an enrichment in catalyst reactivity and enantioselectivity of the unreacted epoxide, particularly in the case of short as well as long chain aliphatic epoxides. Good isolated yields of the unreacted epoxide (up to 46% compared to 50% theoretical yield) along with high enantioselectivity (up to 99%) were obtained in most cases using catalyst 1. Further studies showed that catalyst 1 could retain its catalytic activity for six cycles under the present reaction conditions without any significant loss in activity or enantioselectivity. To show the practical applicability of the above synthesized catalyst we have synthesised some potent chiral β-blockers in moderate yield and high enantioselectivity using complex 1. The DFT (M06-L/6-31+G??//ONIOM(B3LYP/6-31G?:STO-3G)) calculations revealed that the chiral BINOL linker influences the enantioselectivity achieved with Co(iii)-salen complexes. Further, the transition state calculations show that the R-BINOL linker with the (S,S)-Co(iii)-salen complex is energetically preferred over the corresponding S-BINOL linker with the (S,S)-Co(iii)-salen complex for the HKR of 1,2-epoxyhexane. The role of non-covalent C-H?π interactions and steric effects has been discussed to control the HKR reaction of 1,2-epoxyhexane.

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Asymmetric hydrolytic kinetic resolution with
recyclable polymeric Co(III)-salen complexes: A
practical strategy in the preparation of (S)-
Metoprolol, (S)-Toliprolol and (S)-Alprenolol:
Computational rationale for enantioselectivity
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Tamal Roy,[a] Sunirmal Barik,[b,c] Manish Kumar,[a,b] Rukhsana I.
Kureshy,*[a,b] Bishwajit Ganguly,*[b,c] Noor-ul H. Khan,[a,b] Sayed H. R.
Abdi,[a,b] Hari C. Bajaj[a,b]
A series of chiral polymeric Co(III) salen complexes based on a number of achiral and chiral linkers were
synthesized and their catalytic performances were assessed in asymmetric hydrolytic kinetic resolution
of terminal epoxides. The effect of the linker were judiciously studied and it was found that in case of
chiral BINOL based polymeric salen complex 1, there was certain enrichment in catalyst reactivity and
enantioselectivity of the unreacted epoxide particularly in the case of shor t as well as long chain
aliphatic epoxides. Good isolated yield of the unreacted epoxide (up to 46% out of 50% theoretical
yield) along with high enantioselectivity (up to >99%) were obtained in most of the cases using catalyst
1. Further studies exhibited that the catalyst 1 could retain its catalytic activity for six cycles under the
present reaction conditions without any significant loss in activity and enantioselectivity. To show the
practical applicability of the above synthesized catalyst we have carried out synthesis of some potent
chiral
-blockers using complex 1 in moderate yield and high enantioselectivity. The DFT (M06-L/6-
31+G**//ONIOM(B3LYP/6-31G*:STO-3G)) calculations revealed that the chiral BINOL linker influences
the enantioselectivity with Co(III)salen complexes. Further, the transition state calculations show that
the R-BINOL linker with (S,S)-(salen)Co(III) complex is energetically preferred over the corresponding S-
BINOL linker with (S,S)-(salen)Co(III) complex for HKR of 1,2-epoxyhexane. The role of non-covalent C-
H… π interaction and steric effects has been discussed to control the HKR reaction of 1,2 -epoxyhexane.
Katsuki5 to develop manganese-salen complex for the
asymmetric epoxidation of non-functionalized olefins. Besides
numerous exciting developments have been made in the area of
organo-catalytic asymmetric epoxidation
reaction.6
Nevertheless it is the seminal work of Jacobsen et al. on
asymmetric hydrolytic kinetic resolution (HKR) of terminal
epoxides that makes the synthesis of enantiopure 1,2-epoxides
convenient, which was otherwise difficult to obtain by the
earlier methods.7 Good yield, high enantioselectivity for both
the unreacted epoxide and product 1,2-diol, high atom
efficiency and extraordinary Krel value makes the process of
particular interest mainly in total synthesis of bioactive
compounds and in pharmaceutical industry.8 A number of
works have been reported afterwards to improve on the reaction
Introduction
Presence of strained three member ring unit makes epoxides a
unique and one of the most significant molecule in synthetic
organic chemistry.1 Additionally, stereospecific ring opening of
epoxides, in particular chiral epoxides, leads to several
biologically and medicinally active compounds together with
some of the most important building block in asymmetric
synthesis.2 Over the last three decades methodologies to
synthesize epoxides in high optical purity has become a prime
area of interest amongst the scientific community worldwide.
Most significant breakthrough was achieved by Sharpless et al.3
for the synthesis of enantio-enriched epoxy alcohols from
allylic alcohol, followed by pivotal work of Jacobsen4 and
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parameters such as catalyst loading, reaction time, epoxides were obtained with most of the 1,2-epoxides used in
enantioselectivity of the products and catalyst recyclability as the present reaction condition sparing a very little amount of
well as to understand the mechanistic behaviour of the catalyst 1. Furthermore the catalyst 1 was highly recyclable in
reaction.9 In quest of synthesizing highly recyclable catalyst, the present reaction conditions and could be reused for six
immobilization of the chiral Co(III) salen complex was successive cycles just by performing regeneration of the
achieved by means of solid support such as mesoporous catalyst once with acetic acid. To show general applicability of
silica,9e,10 zeolites,11 polymers,9e,12 gold colloids.13 Fluorous complex
biphasic catalytic system14 and ionic liquid15 have also been three potent chiral
1
, we have checked its efficacy in the synthesis of

- blockers namely (S)-Metoprolol, (S)-
explored in the asymmetric HKR of terminal epoxides. Toliprolol and (S)-Alprenolol in moderate yield (up to 44%
Alternatively, with the advent of dimeric,16 oligomeric7,9b and w.r.t starting phenol) and high enantioselectivity (99%).
polymeric17 ligands, the catalyst recyclability was significantly
improved even in homogeneous reaction condition along with
additional benefit of improved reactivity than their monomeric
Results and Discussion
counter parts, as mechanistically it is evident that HKR reaction
An extra element of chirality together with multiple
proceeds via a bimetallic pathway. Our constant interest in the
catalytically active sites often increases the catalyst
field of HKR16b-d as well as in development of recyclable salen
performance in asymmetric catalysis. Further, salen ligand has
based complexes in several other asymmetric organic
proven role in numerous asymmetric transformations including
transformations,18 on the basis of high molecular weight and
HKR.19-22 Keeping these simple facts in mind achiral/chiral
linker based polymeric salen ligands 1’-8’ were synthesized in
two steps; i.e. initial formation of bis-aldehydes by interaction
of 5-chloromethyl-3-tert-butyl salicylaldehyde with
diamines18b/ diols19-21 followed by condensation using chiral
trans 1,2-diaminocyclohexane as per earlier reported
procedure.22 In turn metalation of these ligands was furnished
with Co(OAC)2.4H2O under anaerobic condition followed by
auto-oxidation in air with the help of an appropriate counter ion
source (Scheme 1) to achieve polymeric cobalt salen complexes
less solubility of these catalysts in non-polar solvents, led us to
the synthesis of a series of polymeric Co(III)-salen complexes
based on some chiral (S-BINOL, R-BINOL, diethyl tartrate)
and achiral (trigol, piperazine) linkers. Subsequently we have
tested the synthesized catalysts for asymmetric hydrolytic
kinetic resolution of terminal epoxides. Preliminary results
showed that in case of catalyst
1, i.e. catalyst with chiral
BINOL linker, there was some additive effect of auxiliary
chirality of BINOL particularly in the case of short as well as
long chained aliphatic 1,2-epoxides. Good isolated yield (up to
46%) and high enantioselectivity (up to >99%) of the unreacted
1-8 (Figure 1).
Scheme 1. Synthetic steps for the preparation of chiral polymeric Co(III) salen complexes.
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Both polymeric ligands and their corresponding metal
complexes were characterized by dint of suitable physico-
chemical techniques like IR-, 1H- and 13C NMR, UV-Vis
spectroscopy, elemental analysis, GPC and optical rotation (See
supporting information). Once the above synthesized metal
complexes are aptly characterized these are employed for
asymmetric hydrolytic kinetic resolution of 1,2-epoxyhexane as
a test substrate.
100
80
60
40
20
0
Yield (h) Ee (%)
Figure 2. Study of effect of counter ion in asymmetric HKR of 1,2-epoxyhexane.
Reaction Condition: 1,2-epoxyhexane (10 mmol), Catalyst (0.05 mol %), H2O (5
mmol), Room temperature.
This phenomenon could possibly be explained by considering
the mechanistic aspects of asymmetric HKR reaction as
proposed by Jacobsen et al.9k where the reaction is believed to
proceed in a bimetallic pathway with in situ generation of an
active Co(III)-OH species (Scheme 2).
Figure 1. Structures of the polymeric Co(III) salen complexes 1-8.
To check the influence of counter ion in HKR of 1,2-
epoxyhexane, polymeric Co(III) salen complexes with diverse
counter ions were prepared via treatment of the Co(II) salen
complex of ligand 1’ with a suitable source such as, acetic acid,
trichloroacetic acid, trifluoroacetic acid, para-nitro benzoic
acid, lutidine 2,6-dimethyl pyridiniumtosylate (LPTS)
ferroceniumhexafluorophosphate and ferroceniumtetrafluoro
borate in an appropriate solvent. No general trend in terms of
activity and enantioselectivity, was obtained from this study,
however it has been found that Co(III) salen complex obtained
with the use of acetic acid was most suitable (Figure 2), reason
for which is not clearly known but are in tandem with some of
the earlier literature reports.7a,b Besides the data obtained from
above study clearly suggest that counter ions having a covalent
bond with the metal centre were better performed in terms of
reactivity in HKR compare to those where there is an ionic
interaction between metal centre and the counter ion.
Scheme 2. Proposed bimetallic mechanism of Co(III) salen complex catalyzed
asymmetric HKR of terminal epoxides.
So it would be fair to expect that the counter ions that are
attached to the metal centre via an ionic interaction are non-
transferable and could not generate the active Co(III)-OH
species as faster compare to the formers. Moreover, strong
acidic counter ions increased the Lewis acidity of the metal
centre to such an extent that a majority of the reaction
proceeded in
a
non-chiral pathway to give less
enantioselectivity of the unreacted 1,2-epoxyhexane.
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With optimization of the counter ion, next to better understand
the effect of the linker connecting two salen units, we have
employed all the above synthesized polymeric Co(III) salen
100
80
60
40
20
0
complexes
1-8 in asymmetric HKR reaction of (+/-)-1,2-
epoxyhexane (Table 1).
Table 1. Catalyst screening in asymmetric HKR of 1,2-epoxyhexane at room
temperature under solvent free condition.a
0
2
4
6
8
10
Time (h)
Co(III)salen
complex
Catalyst
loading
(mol %)
0.05
0.05
0.05
0.05
0.05
0.1
Time (h)
Unreacted epoxide
Yield (%) ee (%)
Catalyst 1
Catalyst 4
Catalyst 7
Catalyst 2
Catalyst 5
Catalyst 8
Catalyst 3
Catalyst 6
6
6
6
6
6
8
8
6
42
99b
1
2
3
4
5
6
7
8
42
41
42
40
41
42
43
86c
86b
99c
72c
95c
89c
90c
50
40
30
20
10
0
0.1
0.1
aReaction conditions: 1,2-epoxyhexane (10 mmol), Catalyst (0.05 to 0.1
mol%), H2O (5 mmol), Room temperature. bEpoxide configuration (S),
bEpoxide configuration (R).
From the mechanism proposed for the reaction it can be
anticipated that the linker might have some role on the outcome
of the reaction due to their ability to provide the necessary
arrangement to the two metal centres involved in the reaction
pathway. Preliminary results suggested that all these complexes
0
2
4
6
8
10
Time (h)
except complex
5 (ee, 72%) were showing good activity and
Catalyst 1
Catalyst 4
Catalyst 7
Catalyst 2
Catalyst 5
Catalyst 8
Catalyst 3
Catalyst 6
moderate to good enantioselectivity in asymmetric HKR of 1,2-
epoxyhexane. A possible explanation for less reactivity of
complex
5
was provided by us in an earlier report,22 as the
distortions (random twists) in the structure of the polymeric
salen ligand in case of ligand 5’ prepared from racemic BINOL
where both R- and S-BINOL motifs may perhaps present
randomly in a single polymeric chain.
Figure 3. The catalytic activity of Co(III)salen complexes 1-8 in the asymmetric
HKR 1,2-epoxyehxane at RT a) ee value of the epoxide versus time (above); and
b) epoxide consumed in percentage versus time (below). Reaction conditions:
1,2-epoxyhexane=10 mmol, H2O=5 mmol, catalyst=0.05-0.1 mol%.
Besides the complexes
7 and 8 with an achiral linker joining the
Straight chain aliphatic epoxides are particularly important
class of compounds and have their occurrence in many
biologically active compounds. These epoxides are hard to get
in good yield and high enantioselectivity and often requires
either higher catalyst loading or use of an extra volume of water
(0.6-0.7 equivalent) under HKR condition, which results in
diminishing enantioselectivity for the corresponding 1,2-diol.
When employed in asymmetric HKR of 1,2-epoxyhexane (with
0.5 equivalent of water), we found best activities (Table 1,
salen units gave comparatively inferior results (ee 89% and
90% respectively) when employed in HKR of 1,2-epoxy
hexane. An intermediate result (ee 95% of unreacted epoxide)
was obtained under similar reaction condition with complex
6
having diethyl tartrate as a chiral linker (Table 1, Figure 3). The
enantioselectivity of the unreacted 1,2-epoxyhexane could be
increased up to 99% with the use of extra equivalent of water
(0.7 equivalent) in case of complex
7 and 8 as reported earlier
by Jacobsen et al., at the cost of reduced ee of the
corresponding diol (78%).
Figure 3) with complexes
1 and 4 (ee of unreacted epoxide is
99% in both cases), having different absolute configuration of
the linker and of the diamine collar [(R,S,S) or (S,R,R)].
Enantioselectivity values obtained with these two catalysts
were significantly higher than the other catalysts, where there is
incompatible chirality at those two points (ee 86% in both
cases).
From the results obtained in HKR of 1,2-epoxyhexane it could
be concluded that though the diamine collar in the salen motif
was mainly responsible for governing the enantioselectivity in
products, still the distance and the orientation between the two
salen units as provided by the achiral/chiral linkers also played
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an important role. In an earlier work by us where we have used P), were used with success in HKR condition using complex
1
the above set of complexes, we have recorded CD spectra of and the results are summarized in Table 2.
complexes 1-3 in DCM which showed similarity in spectra
except to obtain opposite cotton effect for the complexes The reaction was performed under solvent free condition in
having opposite chiral arrangement at the diamine collar, most of the cases except few where either the substrate epoxide
confirming that the configuration of the product is solely was solid, or the catalyst had poor solubility in the substrate
controlled by the diamine collar.22
epoxide. When compared our catalytic data with some of the
benchmark monomeric/oligomeric/polymeric Co(III) Salen
complexes reported so far in the literature, the results are either
in parallel or even better particularly in case of long chain
aliphatic epoxides at the expense of a lower catalyst loading
(0.05 to 0.15 mol %) (See supporting information, Table 4). A
series of reactions performed with 4-substituted phenylglycidyl
ethers showed no effect of the electronic character or the
bulkiness of the substituent on the outcome of the reaction.
Table 2. Asymmetric HKR of terminal epoxides using BINOL based
polymeric Co(III) salen complex 1.a
Catalyst
loading
(mol %)
Time
(h)
Unreacted
epoxide
Yield(%) ee (%)b
1,2-Diol
Substrate
Additionally we have isolated the other product i.e. 1,2-diols
also in most of the cases with a good isolated yield (up to 45%)
and enantioselectivity (up to 96%). However, when employed
Yield (%) ee (%)
A
B
C
D
E
F
G
H
I
0.01
0.05
0.1
4
5
43
46
45
45
42
45
45
45
44
46
42
43
41
44
45
42
99
99
99
99
99
99
93
92
99
99
98
99
99
99
99
99
44
45
43
43
43
45
42
43
44
42
42
43
41
42
43
42
96
96
95
96
96
96
92
90
94
95
95
96
94
96
96
95
in asymmetric HKR of terminal arene epoxide the catalyst
1
gives moderate results likened with Jacobsen’s oligomeric
Co(III) salen complex, reason for which is unknown to us.
6
0.1
8
Next 1,2-epoxyhexane due to its less volatility was selected as a
model substrate for the study of recyclability using complex 1.
0.15*
0.005
0.1
8
5
A recyclability study carried out for six successive cycles
witnessed a diminished catalytic activity, which reflected in an
increase of the reaction time without sacrificing the
enantioselectivity of the unreacted epoxide. However, a catalyst
regeneration step, as per earlier literature reports, after four
catalytic cycles shows the similar kinetic behaviour for the
HKR of 1,2-epoxyhexane as observed in fresh catalytic reaction
(Figure 4).
12
12
12
6
0.1
0.05
0.05
0.05
0.05
0.05*
0.05*
0.05
0.1*
J
K
L
6
6
M
N
O
P
8
8
6
10
aReaction conditions: racemic epoxide (10 mmol), H2O (5 mmol), Room
temperature. * THF was used as a solvent. bEpoxide configuration (S)
Figure 4. Study of catalyst recyclability using complex
epoxyhexane (10 mmol), catalyst (0.05 mmol), water (5 mmol), RT.
1. Reaction Condition: 1,2-
After having established complex
1 as the best choice for
asymmetric hydrolytic kinetic resolution of 1,2-epoxyhexane,
we have employed this catalyst with other terminal epoxides. A
wide variety of terminal epoxides namely, straight chain
aliphatic epoxides (A-E), halogenated epoxide (F), arene
epoxides (G, H), aromatic epoxide (I) and aryloxy epoxides (J-
Chiral -blockers are important class of bioactive compounds
and are potent drug, used in the treatment of several
cardiovascular diseases like hypertension, angina, acute
myocardial infarction and congestive heart failures. In most of
these

-blockers, (S)-enantiomer is potentially more active than
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its (R)-form and hence administered as a single isomer for Co(III) salen with R- BINOL, respectively.23 Therefore, we
better activity. Asymmetric HKR of the representative glycidyl have also examined the stability of the (S,S)-Co(III) salen
ether is one of the simplest routes for the synthesis of the above complex with chiral R- and S- BINOL, respectively. The
class of compounds in moderate yield and high binding energy calculated for (S,S)-Co(III) salen with R-BINOL
enantioselectivity. We have carried out the synthesis of three using B3LYP/ 6-31G* level of theory suggests that it is
enantiopure
and (S)-Alprenolol by means of HKR of the corresponding S-BINOL linker (Figure 5).
glycidyl ethers using catalyst in moderate yield (up to 44%
w.r.t phenols) and high enantioselectivity (>99%) (Scheme 3).
An alternative procedure for the synthesis of these -blockers
 -blockers namely (S)-Metoprolol, (S)-Toliprolol energetically 1.0 kcal/mole more stable than the corresponding
1

starting from optically pure epichlorohydrin however, gave the
end product with a maximum enantioselectivity of 92%.
The experimental studies suggested that the enantioselectivity
in the HKR of aliphatic terminal 1,2-epoxides is governed by
the chiral and stepped conformational nature of Co(III) salen
complexes. The importance of chiral BINOL linker in the
generation of enantiomeric excess (ee) of aliphatic terminal 1,2-
epoxides has been observed (Table 2). We have performed the
DFT calculations to unravel the origin of the generation of ee
for the studied aliphatic terminal 1,2-epoxide. The salen
R-BINOL
with
S-BINOL with (S,S)-
(S,S)-Co(III) salen
Co(III) salen
complexes
enantioselectivity as shown in Figure 3. Therefore, we have
considered salen complex for our computational study.
1 and 4 are enantiomers and hence they yield similar
B3LYP/6-31G*
ΔE(kcal/mol)
[0.0]
[1.0]
1
The earlier reports suggest that both the enantiomers (S,S)-
Co(III) salen with R-BINOL and (R,R)-Co(III) salen with S-
BINOL shows the high enantio-selectivity compared to the
corresponding (S,S)-Co(III) salen with S-BINOL and (R,R)-
Figure 5. The optimized structures of R-BINOL with (S,S)-Co(III) salen and S- BINOL
with (S,S)-Co(III) salen with B3LYP/6-31G* level of theory.
Scheme 3. Synthesis of chiral -blockers using complex 1.
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The (S,S)-Co(III) salen complex with chiral R-BINOL linker transition state (TS-1) for R-1,2-epoxyhexane and the transition
has been used to examine the formation of ee for 1,2- state (TS-2) for S-1,2-epoxyhexane is shown in Figure 6. The
epoxyhexane. We have determined the two possible transition activation energy barriers calculated for the TS-1 and TS-2
states of bimetallic (S,S)-Co(III) salen complex with R- and S- suggest that the TS-1 is energetically favoured as compared to
1,2-epoxyhexane. Due to the bigger size of the two salen the TS-2 (Figure 6). Single-point energy calculations performed
complexes, ONIOM (B3LYP/6-31G*:STO3G) method was with M06-L/6-31+G** level of theory using the ONIOM-
employed. Recent studies have shown that two salen complexes optimized geometries also corroborate the experimental
are necessary to induce the ee in substituted 1,2-epoxides.24 The observations.
TS-1
TS-2
ONIOM(B3LYP/6-31G*:STO3G)
ΔE(kcal/mol)
[0.0]
[0.0]
[2.30]
[3.21]
ΔG(kcal/mol)
M06-L/6-31+G**//ONIOM (B3LYP/6-31G*:STO3G)
ΔE(kcal/mol) [0.0]
[7.83]
Figure 6. Transition state structures of (S,S)-(salen)Co(III) with chiral R- BINOL towards the ring opening of (R)-1,2-epoxyhexane and (S)-1,2-epoxyhexane calculated
with ONIOM(B3LYP/6-31G*:STO-3G) level of theory. Single point energy calculations are also given at M06-L/6-31+G** level of theory.
TS-1
TS-2
Figure 7. ONIOM(B3LYP/6-31G*:STO3G) optimized geometries of the two transition states TS-1 and TS-2 and the C-H… π distances are given in (ꢀ)
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The transition state structures show that the C-H….π and their derivatives chiral HPLC was performed on Shimadzu
interactions are present in both the TS-1 and TS-2, respectively SCL-10AVP by using a Chiralcel OD column.
(Figure 7). It appears that the TS-1 is more stabilized via non-
Preparation of polymeric salen ligands 1’-8’
covalent C-H… π interactions as the distances in this case is
much shorter compared to the TS-2 structure. Further, the Di-aldehydes
geometrical analysis reveal that the alkyl chain of 1,2- diylbis(oxy))-bis(methylene)-bis(3-tert-butyl-2-
epoxyhexane in the TS-1 orients in the trans-fashion, however, hydroxybenzaldehyde)19
/ (S)-5,5'-(1,1'-binaph-thyl-2,2'-diyl-
A-E
viz.,
(R)-5,5'-(1,1'-binaphthyl-2,2'-
A
the alkyl chain in TS-2 adopts a gauche conformation and bis(oxy))-bis(methylene)-bis(3-tert-butyl-2-
hence there is energy penalty in the later conformation.25
hydroxybenzaldehyde)19
5-formyl-4-hydroxybenzyloxy) succinate20
1,4-diylbis-(methylene))-bis(3-tert-butyl-2-hydroxy
benzaldehyde)18b
and 5,5'-(2,5,8,11-tetraoxadodecane-1,12-
diyl)-bis(3-tert-butyl-2-hydroxylbenzalde-hyde)21
(1 mmol)
were dissolved in dry THF: EtOH mixture (2:1) at 65 C to
B
, (2S,3S)-diethyl-2,3-bis(3-tert-butyl-
, 5,5'-(piperazine-
C
D
Experimental
E
Materials and Methods

which (1R,2R)/(1S,2S)-diaminocyclohexane (0.136 g, 1.2
mmol) in EtOH were added slowly under nitrogen atmosphere.
The resulting solutions were stirred for 4 h under refluxing
condition (TLC checked) and the solvent was partially removed
from the each reaction mixture on a rotary evaporator that gave
yellow precipitate. The solid obtained after filtration were
washed with hexane:DCM (50:1) to get the yellow coloured
(1R,2R)-(-)-1,2-diaminocyclohexane, counter ion sources,
epoxides
epoxyhexane,
(1,2-epoxypropane,
1,2-epoxyoctane,
1,2-epoxybutane,
1,2-epoxydecane,
1,2-
1,2-
epoxydodecane, epichlorohydrin, 1,2-epoxy-5-hexene, 1,2-
epoxy-8-nonene, styrene oxide, phenylglycidyl ether, 2-methyl
phenylglycidyl ether, 4-chloro phenylglycidyl ether, 4-tert-
butyl phenylglycidyl ether, 4-methoxy phenylglycidyl ether,
benzylglycidyl ether) and isopropyl amine were purchased from
Sigma-Aldrich, Across and Alfa-Aesar and were used without
any further purification. Cobalt acetate was purchased from SD
Fine Chemicals India and was used as received.
Naphthylglycidyl ether was prepared as per the procedure
depicted in the literature.26 The solvents used for present study
were dried prior to use. Microanalysis of the intermediates,
ligand and catalysts was carried out on a Perkin Elmer 2400
CHNS analyzer. All the melting points reported here were
determined on a Mettler Toledo-FB62 and were uncorrected. 1H
&13C NMR spectra were recorded on Bruker 200 MHz or 500
MHz spectrometer at ambient temperature and referenced
against TMS as an internal standard. FTIR spectra were
recorded as KBr pellet on a Perkin Elmer Spectrum GX
spectrophotometer. Optical rotation of the chiral polymeric
salen ligands were recorded on an automatic polarimeter
(Digipol 78, Rudolph) instrument. High-resolution mass spectra
were obtained with a LC-MS (Q-TOFF) LC (Waters), MS
(Micromass), MALDI-TOF, Model make Ultra flex TOF/TOF,
Burker Daltonics, Germany instruments. All the products were
purified either by distillation or via flash chromatography using
silica gel 60-200 mesh purchased from SD Fine-Chemicals
Limited, Mumbai (India). The purity of the solvents, epoxides
and the analysis of the products were carried out by gas
chromatography (GC) on 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 used as
carrier gas (rate 30 mL/min). Injection port and detector
desired polymeric ligands 1’-8’ (Scheme 1).
Preparation of polymeric salen catalysts 1-8
In 50 mL three neck RBF (equipped with a magnetic bar) under
nitrogen atmosphere, the polymeric salen ligands 1’-8’ (1
mmol) were dissolved in deoxygenated toluene (10 mL). In
another 25 mL vial Co(OAc)2.4H2O (1.5 mmnol) was dissolved
in deoxygenated methanol (10 mL) under nitrogen for 10 min
to ensure complete deoxygenation. The solution of
Co(OAc)2.4H2O in MeOH (purple) was transferred under
nitrogen via cannula to the solution of polymeric salen ligand
(yellow), affording a dark red precipitate. The mixture was
stirred for 2 h at RT and then the solvent was removed under
vacuum, dissolved the residue in DCM (50 mL) and passed
through celite-545 pad to remove the excess of metal ions. The
filtrate was evaporated by vacuum afforded a dark red powder.
To get the desired active catalysts, Co(II) chiral polymeric salen
complexes were dissolved in DCM (2 mL) under dry oxygen
atmosphere and an appropriate counter ion source (1.2 mmol)
was added to them and the resulting solutions were stirred for 5
h to convert Co(II) chiral polymeric salen complexes to Co(III)
chiral polymeric salen complexes 1-8 (Scheme 1).
Typical Procedure for asymmetric HKR of terminal epoxides
An appropriate amount of pre-catalyst (0.005-0.15 mol %
1-8
w.r.t. substrate) was taken in DCM in a vial and treated with 2
equivalent of suitable counter ion source and the resulting
solution was stirred for 1 h. After 1 h the solvent was
evaporated and the epoxide (10 mmol) was introduced into the
reaction vial either in neat condition or in presence of THF as a
solvent. Water (5 mmol) was added drop wise to this solution
temperature was kept at 200 C. The ee of aliphatic epoxides
and their corresponding diols were determined on GC using a
chiral capillary column (Chiraldex ATA, Chiraldex BTA or
Chiraldex GTA). For ee determination of aromatic epoxides
placing the vial at 0 C. Subsequently, the reaction mixture was
allowed to warm gradually to RT and stirred at this temperature
for required amount of time. After completion of the reaction
the catalyst was precipitated out from the reaction medium with
8 | J. Name., 2012, 00, 1-3
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Journal Name
Catalysis Science & Technology
ARTICLE
the addition of hexanes washed several times with hexane:ethyl achieve higher catalytic activity and enantioselectivity in
acetate (90:10), dried overnight in a desiccator under vacuum hydrolytic kinetic resolution of terminal epoxides. In case of
and subsequently reused in next cycle. The epoxide was chiral linker all possible permutation and combinations of
separated from the mixture either by distillation or by column linker configuration vis-à-vis salen configuration were screen in
chromatography and checked on GC or HPLC for order to understand the role of additional element of chirality in
enantioselectivity. Enantio-purity of the remaining diol was influencing the configuration of the product. The experimental
determined as trifluoroacetate derivative on GC or HPLC.
results clearly indicate that, while the configuration of the salen
unit decides the configuration of the product, the configuration
For kinetics study of the reaction an aliquot was withdrawn of linker does effect enantioselectivity. Out of several catalysts
from the reaction mixture at certain interval, quenched and after used in the present study, catalyst with opposite absolute
1
separation of the catalyst by precipitation checked on GC for configuration of BINOL linker and the diamine collar of the
conversion and enantioselectivity of the remaining 1,2- salen moiety gives best results in terms of activity and
epoxyhexane.
enantioselectivity as compared to the catalysts based on other
chiral or achiral linker. Moreover, catalyst can be effectively
reused in the present reaction condition for six successive runs
1
General procedure for the synthesis of chiral -blockers
To a stirred solution of an apt phenol 9a-c (20 mmol) and at the expense of only one catalyst regeneration step.
K2CO3 (30 mmol, 4.14 g) in dry acetone (100 mL), Furthermore, a gram scale synthesis of -blockers (S)-

epichlorohydrin (30 mmol, 2.4 mL) was added and the reaction Metoprolol, (S)-Toliprolol and (S)-Alprenol in their desired
mixture was allowed to reflux until all of the phenol had been enantiomeric forms suggests practical applicability of the
consumed (checked on TLC). Subsequently the reaction catalyst. The DFT calculations suggest that the R-BINOL linker
mixture was filtered and the residue was purified by column preferably bind with (S,S)-Co(III) salen complex compared to
chromatography using ethyl acetate and hexane as eluent to the S-BINOL linker. The computational analysis suggests that
obtain the glycidyl ethers (10a-c) in pure form.
the enantioselective ring opening of terminal 1,2-epoxyhexane
is governed by the non-covalent C-H… π interaction and steric
Next the above synthesized glycidyl ether (10 mmol) was effects induced in the transition state geometries.
subjected to HKR using catalyst to get the (S)-glycidyl ethers
10’ a-c) in an enantiopure form. The enantiopure epoxides
1
(
obtained thus were purified by column chromatography and
consequently reacted with isopropyl amine (2 equivalents) in
Acknowledgements
presence of Na--zeolite as a catalyst and isopropanol as a
solvent to obtain optically pure (S)-Metoprolol (11a), (S)-
Toliprolol (11b) and (S)-Aleprenolol (11c).
CSIR-CSMCRI Communication No. 072/2014.The authors are
thankful to DST and CSIR network project CSC-0135 for
financial assistance. Tamal Roy and Sunirmal Barik are
thankful to CSIR and UGC respectively for their SRF and JRF.
The authors are also thankful to analytical science discipline for
providing instrumental facilities.
Computational Methods
The ground state and transition state geometries were fully
optimized using ONIOM (B3LYP/6-31G*:STO-3G) method.27
Single-point energy calculations were performed at M06-L/6-
31+G** level of theory using the optimized structures of
ground and transition states with ONIOM method.28 To reduce
Notes and references
the computational cost, the wire-frame moieties in the a Discipline of Inorganic Materials and Catalysis, CSIR, Central Salt and
Marine Chemicals Research Institute (CSMCRI), G. B. Marg, Bhavnagar
364 021, Gujarat, India. E-mail: rkureshy@csmcri.org; Fax: (+91) 0278-
optimized structures were treated with lower level B3LYP
/STO-3G level in ONIOM calculations. The pseudo-potential
combined with valence double zeta basis set, LANL2DZ has
been used for effective core potential of cobalt29 and 6-31G*
basis set30 for the other atoms (C, H, O, N) in the high level
2566970
b
Academy of Scientific and Innovative Research, CSIR-CSMCRI,
Bhavnagar, Gujarat, India
Analytical Discipline and Centralised Instrumental Facility, CSIR,
c
Central Salt and Marine Chemicals Research Institute (CSMCRI), G. B.
these ONIOM calculations. The stationary points were Marg, Bhavnagar 364 021, Gujarat, India. E-mail: ganguly@csmcri.org
characterized by frequency calculation in order to verify that
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
the transition structures had one, and only one, imaginary
frequency and ground state geometries were characterized with
no imaginary frequency. All calculations were performed with
the Gaussian 09 suite of program.31
1
2
K. Yudin, Aziridines and Epoxides in Organic Synthesis (2006) ISBN: 978-
3-527-60748-8
Conclusions
(a) E. N. Jacobsen, in E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.),
Comprehensive Organometallic Chemistry II, Vol. 12, Pergamon, New
York, 1995, pp. 1097; (b) E. N. Jacobsen and M.H. Wu, in E. N. Jacobsen,
A. Pfaltz and H. Yamamoto (Eds.), Comprehensive Asymmetric Catalysis,
A series of chiral polymeric Co(III) salen complexes with
several chiral and achiral linkers were synthesized so as to
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DOI: 10.1039/C4CY00594E
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Page 10 of 10
ARTICLE
Journal Name
Vol. II, Springer, New York, 1999, pp. 649; (c) T. Katsuki, in I. Ojima (Ed.),
Catalytic Asymmetric Synthesis, 2nd edn., Wiley–VCH, New York, 2000,
Tetrahedron Lett., 2003, 44, 7081; (f) B. M. Rossbach, K. Leopold and R.
Weberskirch, Angew. Chem., 2006, 118, 1331.
pp. 287.
18 (a) R. I. Kureshy, T. Roy, N. H. Khan, S. H. R. Abdi, A. Sadhukhan and H.
C. Bajaj, J. Catal., 2012, 286, 41; (b) R. I Kureshy, K. J. Prathap, M. Kumar,
P. K. Bera, N. H. Khan, S. H. R. Abdi and H. C. Bajaj, Tetrahedron, 2011,
67, 8300; (c) R. I. Kureshy, N. H. Khan, S. H. R. Abdi and R. V. Jasra, in
Lawrence P. Bevy (Ed.), Catalysis Research at the Cutting Edge, Nova
Science Publishers Inc., New York, 2005, pp115; (d) N. H. Khan, R. I.
Kureshy, S. H. R. Abdi, S. Agrawal and R. V. Jasra, Coord. Chem. Rev.,
2008, 252, 593.
3
4
T. Katsuki and K. B. Sharpless, J. Am. Chem. Soc., 1980, 102, 5974.
(a) W. Zhang, J. L. Loebach, S. R. Wilson and E. N. Jacobsen, J. Am.
Chem. Soc., 1990, 112, 2801; (b) E. M. McGarrigle and D. G. Gilheany,
Chem. Rev., 2005, 105, 1563.
5
6
7
(a) R. Irie, K. Noda, Y. Yto, N. Matsumoto and T. Katsuki, Tetrahedron Lett.,
1990, 31, 7345; (b) T. Katsuki, Coord. Chem. Rev., 1995, 140, 189.
19 M. Kumar, R. I. Kureshy, D. Ghosh, N. H. Khan, S. H. R. Abdi and H. C.
(a) Y. Shi, Acc. Chem. Res., 2004, 37, 488; (b) W. Adam, C. R. Saha-Moller
and P. A. Ganeshpure, Chem. Rev., 2001, 101, 3499.
Bajaj, ChemCatChem, 2013, 5, 2336.
20 S. Saravanan, N. H. Khan, P. K. Bera, R. I. Kureshy, S. H. R. Abdi, P.
(a) M. Tokunaga, J. F. Larrow, F. Kakiuchi and E. N. Jacobsen, Science,
1997, 277, 936; (b) S. E. Schaus, B. D. Brandes, J. F. Larrow, M.
Tokunaga, K. B. Hansen, A. E. Gould, M. E. Furrow and E. N. Jacobsen, J.
Am. Chem. Soc., 2002, 124, 1307; (c) J. M. Ready and E. N. Jacobsen, J.
Am. Chem. Soc., 2001, 123, 2687; (d) J. M. Ready and E. N. Jacobsen,
Kumari and H. C. Bajaj, ChemCatChem, 2013, 5, 1374.
21 N. H. Khan, A. Sadhukhan, N. C. Maity, R. I. Kureshy, S. H. R. Abdi, S.
Saravanan and H. C. Bajaj, Tetrahedron, 2011, 67, 7073.
Angew. Chem., 2002, 114, 1432; Angew. Chem. Int. Ed., 2002, 41, 1374; 22 M. Kumar, R. I. Kureshy, A. K. Shah, A. Das, N. H. Khan, S. H. R. Abdi and
(e) D. A. Annis and E. N. Jacobsen, J. Am. Chem. Soc., 1999, 121, 4147. H. C. Bajaj, J. Org. Chem., 2013, 78, 9076.
8
9
(a) P. Kumar, V. Naidu and P. Gupta, Tetrahedron, 2007, 63, 2745; (b) P. 23 P. Yan and H. W. Jing, Adv. Synth. Catal., 2009, 351, 1325.
Kumar, P. S. Chowdhury and M. Pandey, Adv. Synth. Catal., 2013, 9, 1719;
24 (a) D. D. Ford, L. P. C. Nielsen, S. J. Zuend and E. N. Jacobsen, J. Am.
(c) B. Saikia, T. J. Devi and N. C. Barua, Tetrahedron, 2013, 69, 2157; (d)
Chem. Soc., 2013, 135, 15595; (b) K. B. Hansen, J. L. Leighton and E. N.
U. Nookaraju, A. Harbindu, A. D. Bhise, B. M. Sharma and P. Kumar, RSC
Jacobsen, J. Am. Chem. Soc., 1996, 118, 10924; (c) E. N. Jacobsen, Acc.
Adv., 2012, 2, 11231; (e) R. S. Reddy, P. V. Chouthaiwale, G. Suryavanshi,
Chem. Res., 2000, 33, 421.
V. B. Chavan and A. Sudalai, Chem. Commun., 2010, 5012.
25 J. E. McMurray, Organic Chemistry 6e, Brooks Cole, 2003.
(a) B. M. Rossbach, K. Leopold and R. Weberskirch, Angew. Chem. Int.
Ed., 2006, 45, 1309; (b) X. Zheng, C. W. Jones and M. Weck, J. Am. Chem.
Soc., 2007, 129, 1105; (c) K. Venkatasubbaiah, C. S. Gill, T. Takatani, C.
26 D. S. Bose, A. V. N. Reddy and S. V. Chavhan, Synthesis, 2005, 2345.
D. Sherrill and C. W. Jones, Chem. Eur. J., 2009, 15, 3951; (d) D. H. Kim, 27 (a) S. Dapprich, I. Komáromi, K. S. Byun, K. Morokumaand M. J. Frisch, J.
U. S. Shin and C. E. Song, J. Mol. Cat. A: Chem., 2007, 271, 70; (e) C. S.
Gill, K. Venkatasubbaiah, N. T. S. Phan, M. Weck and C. W. Jones, Chem.
Eur. J., 2008, 14, 7306; (f) S. Kemper, P. Hrobárik, M. Kaupp and N. E.
Schlörer, J. Am. Chem. Soc., 2009, 131, 4172; (g) S. Jain, K.
Venkatasubbaiah, C. W. Jones and R. J. Davis, J. Mol. Cat. A: Chem.,
2010, 316, 8; (h) J. Park, K. Lang, K. A. Abboud and S. Hong, Chem. Eur.
J., 2011, 17, 2236; (i) Y Liu, J. Rawlston, A. T. Swann, T. Takatani, C. D.
Sherrill, P. J. Ludovice and M. Weck, Chem. Sci., 2011, 2, 429; (j) C. Zhu,
G. Yuan, X. Chen, Z. Yang and Y. Cui, J. Am. Chem. Soc., 2012, 134,
8058; (k) L. P. C. Nielsen, S. J. Zuend, D. D. Ford and E. N. Jacobsen, J.
Org. Chem., 2012, 77, 2486.
Mol. Struct. (Theochem)., 1999, 461-462, 1; (b) F. Maseras and K.
Morokuma, J. Comput. Chem., 1995, 16, 1170; (c) S. Humbel, S. Sieber
and K. Morokuma, J. Chem. Phys., 1996, 105, 1959; (d) M. Svensson, S.
Humbel, R. D. J. Froese, T. Matsubara, S. Sieber and K. Morokuma, J.
Phys. Chem., 1996, 100, 19357; (e) M. Svensson, S. Humbel and K.
Morokuma, J. Chem. Phys., 1996, 105, 3654; (f) T. Vreven, K. Morokuma,
Ö. Farkas, H. B. Schlegel and M. J. Frisch, J. Comput. Chem., 2003, 24,
760.
28 Y. Zhao and D. G. Truhlar, J. Chem. Phys., 2006, 125, 194101/1.
29 (a) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270; (b) P. J. Hay
and W. R. Wadt, J. Chem. Phys., 1985, 82, 284; (c) P. J. Hay and W. R.
Wadt, J. Chem. Phys., 1985, 82, 299.
10 (a) G. J. Kim and D. W. Park, Catal. Today, 2000, 63, 537; (b) S. S. Thakur,
S. W. Chen, W. J. Li, C. K. Shin, S. J. Kim, Y. M. Koo and G. J. Kim, J.
Organomet. Chem., 2006, 691, 1862.
30 (a) W. J. Hehre, R. Ditchfield and J. A. Pople, J. Phys. Chem., 1972, 56,
2257; (b) P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28,
213.
11 (a) H. Q. Yang, L. Zhang, L. Zhong, Q. H. Yang and C. Li, Angew. Chem.,
2007, 119, 6985; Angew. Chem. Int. Ed., 2007, 46, 6861; (b) H. Q. Yang,
L. Zhang, W. G. Su, Q. H. Yang and C. Li, J. Catal., 2007, 248, 204.
31 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J.
R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H.
Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G.
Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O.
Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski and D. J. Fox Gaussian 09, Revision B.01, Gaussian, Inc.,
Wallingford CT, 2009.
12 K. Matkiewicz, A. Bukowska and W. Bukowski, J. Mol. Cat. A: Chem.,
2013, 368–369, 43.
13 T. Belser and E. N. Jacobsen, Adv. Synth. Catal., 2008, 350, 967.
14 (a) I. Shepperson, M. Cavazzini, G. Pozzi and S. Quici, J. Fluorine Chem.,
2004, 125, 175; (b) M. Cavazzini, S. Quici and G. Pozzi, Tetrahedron, 2002,
58, 3943.
15 R. Oh, D. J. Choo, W. H. Shim, D. H. Lee, E. J. Roh, S. Lee and C. E.
Song, Chem. Commun., 2003, 1100.
16 (a) R. G. Konsler, J. Karl and E. N. Jacobsen, J. Am. Chem. Soc., 1998,
120, 10780; (b) A. Sadhukhan, N. H. Khan, T. Roy, R. I. Kureshy, S. H. R.
Abdi and H. C. Bajaj, Chem. Eur. J., 2012, 18, 5256; (c) R. I. Kureshy, S.
Singh, N. H. Khan, S. H. R. Abdi, I. Ahmad, A. Bhatt and R. V. Jasra,
Chirality, 2005, 17, 590; (d) R. I. Kureshy, N. H. Khan, S. H. R. Abdi, S. T.
Patel and R. V. Jasra, J. Mol. Catal. A: Chem., 2002, 179, 73.
17 (a) X. Zheng, C. W. Jones and M. Weck, Adv. Synth. Catal., 2008, 350,
255; (b) M.Holbach and M. Weck, J. Org. Chem., 2006, 71, 1825; (c) X.
Zheng, C. W. Jones and M. Weck, Chem. Eur. J., 2006, 12, 576; (d) Y.
Song, X. Yao, H. Chen, C. Bai, X. Hu and Z. Zheng, Tetrahedron Lett.,
2002, 43, 6625; (e) Y. Song, H. Chen, X. Hu, C. Bai and Z. Zheng,
10 | J. Name., 2012, 00, 1-3
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