Synthesis of enantiopure β-amino alcohols via AKR/ARO of epoxides using recyclable macrocyclic Cr(III) salen complexes

Article abstract of DOI:10.1016/j.tet.2011.08.077

A series of chiral macrocyclic Cr(III) salen complexes 1-8 were synthesized and characterized. These complexes were found to be highly active, regio-, diastereo-, and enantioselective catalysts in aminolytic kinetic resolution (AKR) of racemic trans-epoxides as well as asymmetric ring opening (ARO) of prochiral meso-epoxides with various anilines as nucleophiles at room temperature in 18-24 h. Excellent yields (>99% with respect to the nucleophile) with high enantioselectivity (ee, >99%) of chiral anti-β-amino alcohols was achieved with concomitant recovery of corresponding epoxides in high ee (up to >99%). The complex 1 also catalyzed the ARO of meso-epoxides to provide corresponding syn-β-amino alcohols in high yield (99%) and ee (up to 91%). Due to built-in basic sites in the catalyst, no external base (as an additive) was required to promote AKR and ARO reactions. The catalyst 1 was conveniently recycled several times with retention of its performance. The AKR of trans-stilbene oxide with aniline was successfully demonstrated at relatively higher scale (10 mmol) using the catalyst 1.

Full text of DOI:10.1016/j.tet.2011.08.077

Tetrahedron 67 (2011) 8300e8307
Contents lists available at SciVerse ScienceDirect
Tetrahedron
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Synthesis of enantiopure b-amino alcohols via AKR/ARO of epoxides using
recyclable macrocyclic Cr(III) salen complexes
*
Rukhsana Ilays Kureshy , K. Jeya Prathap, Manish Kumar, Prasanta Kumar Bera, Noor-ul Hasan Khan,
Sayed Hasan Razi Abdi, Hari Chandra Bajaj
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR),
G. B. Marg, Bhavnagar 364 021, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2011
Received in revised form 24 August 2011
Accepted 27 August 2011
Available online 1 September 2011
A series of chiral macrocyclic Cr(III) salen complexes 1e8 were synthesized and characterized. These
complexes were found to be highly active, regio-, diastereo-, and enantioselective catalysts in aminolytic
kinetic resolution (AKR) of racemic trans-epoxides as well as asymmetric ring opening (ARO) of prochiral
meso-epoxides with various anilines as nucleophiles at room temperature in 18e24 h. Excellent yields
(>99% with respect to the nucleophile) with high enantioselectivity (ee, >99%) of chiral anti-
alcohols was achieved with concomitant recovery of corresponding epoxides in high ee (up to >99%).
The complex 1 also catalyzed the ARO of meso-epoxides to provide corresponding syn- -amino alcohols
b-amino
Keywords:
Aminolytic kinetic resolution
anti/syn-b-Amino alcohols
Asymmetric catalysis
Chiral macrocylic salen
Chromium
b
in high yield (99%) and ee (up to 91%). Due to built-in basic sites in the catalyst, no external base (as an
additive) was required to promote AKR and ARO reactions. The catalyst 1 was conveniently recycled
several times with retention of its performance. The AKR of trans-stilbene oxide with aniline was
successfully demonstrated at relatively higher scale (10 mmol) using the catalyst 1.
Ó 2011 Published by Elsevier Ltd.
trans/meso-Epoxides
1. Introduction
oxide in 40 h. Further, recent several reports have demonstrated
a cooperative role of catalytic sites in bimetallic macrocyclic/
Chiral
b
-amino alcohols are valuable intermediates for the
linear complexes12,13 in various enantioselective reactions. Taking
a clue from these findings, we have designed new recyclable
chiral macrocyclic salen complexes 1e8 bearing two chiral salen
units linked through tertiary nitrogens (basic sites) in hope of
harnessing the benefits of multiple catalytic sites (metal centers)
and built-in basic sites. This is also in line of our continued in-
terest in developing recyclable enantioselective catalysts for the
epoxide ring opening reactions.5d,6a,8c,d,14 The chiral macrocyclic
Cr(III) salen catalysts synthesized for the present study (partic-
ularly the complex 1) have demonstrated excellent performance
in asymmetric AKR of racemic trans-epoxides with different an-
preparation of various biologically active compounds1 and chiral
ligands,2 hence their preparation has attracted attention of many
research groups worldover.3e9 Aminolytic kinetic resolution8,9
and asymmetric ring opening reactions4e7 of racemic and meso-
epoxides are among the smartest strategies to prepare b-amino
alcohols in high optical purity because of the ready availability of
raw materials at a very economical prices. Jacobsen et al.10 and
Katsuki et al.11 have demonstrated the amazing ability of chiral
salen ligands in preparing various metal complexes to catalyze
numerous asymmetric organic transformations. Consequently,
Bartoli et al. revealed the use of chiral monomeric Co(III) and
Cr(III) salen complexes in catalyzing AKR of terminal and trans-
ilines to give anti-b-amino alcohols in quantitative yields with
excellent enantioselectivity(ee, >99%) and with concomitant re-
covery of corresponding epoxides in excellent optical purity (ee
up to >99%) in 18 h at room temperature. The complex 1 also
displayed its competence in catalyzing ARO of meso-epoxides
aromatic epoxides8a,b to produce
same paper also demonstrated asymmetric aminolysis of meso-
stilbene oxide to get corresponding syn- -amino alcohol in ex-
b-amino alcohols in high ee. The
b
cellent ee (90%) in the presence of monomeric Cr(III) salen
(10 mol %) as a catalyst. However, this reaction needed Et3N as an
additive to promote Cr(III) salen catalyzed AKR of meso-stilbene
with aniline to produce corresponding syn-b-amino alcohols in
quantitative yields and high ee (84e91%). Significantly, as visu-
alized, this system does not require an addition of a base additive
to promote the reaction. Notwithstanding their excellent perfor-
mance in AKR and ARO, the complex 1 has also shown very good
recyclability.
* Corresponding author. Fax: þ91 0278 2566970; e-mail address: rukhsana93@
yahoo.co.in (R.I. Kureshy).
0040-4020/$ e see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.tet.2011.08.077

R.I. Kureshy et al. / Tetrahedron 67 (2011) 8300e8307
8301
2. Results and discussion
a representative nucleophile at room temperature (27ꢁ2 ꢂC) in
various solvents.
The synthesis of chiral macrocyclic salen complexes is presented
in Scheme 1. The dialdehyde 26 was prepared in high yield by the
reaction of 3-t-Bu-5-chloromethyl-2-hydroxy benzaldehyde15 24
with piperazine 25 in dry toluene. The macrocyclic chiral salen li-
gand 28 was synthesized by reacting stoichiometric amount of the
dialdehyde 26 with (1S,2S)-(þ)-1,2-diaminocyclohexane 27 in THF
under stirring at room temperature for 3 h. Remarkably, the
product constituted mostly dimeric macrocyclic salen ligand 28
(w97%) with a small amount of its monomeric version (w3%) as
determined by 1H NMR, LCMS, and MALDI-TOF data. A further
purification of the dimeric ligand was hampered as it strongly binds
on silica gel and various grades of alumina. Recrystallization with
various solvents was also of little consequence. The absence of re-
sidual aldehyde group in 1H NMR and IR spectra in the dimeric
macrocyclic salen ligand 28 suggested its cyclic nature. A molecular
peak at 1088.488 in MALDI-TOF (scanned up to 5000 m/z value to
look for higher molecular weight structures) for ligands 28 further
established its proposed cyclic structure (data are given in
Supplementary data). The ligand 28 on reaction with anhydrous
Cr(II) chloride followed by its auto-oxidation gave chiral macrocy-
clic Cr(III) salen complex 1. The ICP data confirmed the complete
metalation of the ligand 28 to give the complex 1. The complex 1,
when allowed to react with NH4BF4, NH4PF6, or silver salts of NOꢀ3 ,
ClOꢀ4 , OTfꢀ, SbFꢀ6 , or with KBr resulted in the formation of the
complexes 2e8, respectively.
The reaction failed in dichloromethane (DCM) possibly due to
the poor solubility of the catalyst (Table 1, entry 1). The catalyst was
completely soluble in methanol and the AKR conducted in this
solvent provided b-amino alcohol in excellent yield and fairly good
ee (entry 2). Interestingly, a mixture of DCM/MeOH (1:1 v/v) im-
proved ee of the product (entry 3). A further increase in DCMMeOH
ratios 7:1 and 9:1 gave steadily better performance (entries 4 and
5). These results encouraged us to explore other solvents viz.,
CHCl3, dichloroethane (DCE), THF, acetonitrile (ACN), and toluene in
combination with MeOH, ethanol (EtOH), and isopropanol (IPA) in
9:1 v/v ratio (entries 6e12). Overall chloroalkane in combination
with an alcoholic solvent worked better in terms of activity and
enantioselectivity of anti-b-amino alcohol (entries 5e7, 11, and 12),
best being DCM/MeOH (9:1 v/v) (entry 5). Therefore, taking this
mixed solvent system as suitable medium, the catalyst loading was
varied for the AKR of racemic trans-stilbene oxide 9 by keeping
other reaction parameters constant. On increasing the catalyst
loading from 1 mol % to 5 mol %, there was an increase in the re-
activity of the complex but a decrease in ee of the product and
unreacted epoxide was observed (entries 13 and 14). On the other
hand, 0.5 mol % catalyst loading gave improved performance in 18 h
(entry 15). A further decrease in the catalyst loading slowed down
the reaction with a slight decrease in the ee of the products (entry
16). Therefore, we took parameters in entry 15 as optimum as far as
the catalyst loading and solvent system is concerned. This reaction
N
N
O
OH
HN
NH
1 SnCl4, (HCHO)n
2 Conc. HCl, trioxane
OH
25
Dry tolune
t-Bu
O
O
Cl
HO
t-Bu
t-Bu
OH
t-Bu
24
26
NH2
NH2
THF, RT
27
2X-
N
N
N
N
N
1. CrCl2
N
N
N
O
O
t-Bu
t-Bu
t-Bu
HO
HO
OH
OH
2. Auto-oxidation
t-Bu
t-Bu
t-Bu
t-Bu
Cr
Cr
Dry THF
t-Bu
O
O
N
N
N
N
N
N
N
N
1, X- = Cl-
MeOH/ H2O
2 equi. AgX/ NaBr
N
N
28
2X-
N
N
O
O
t-Bu
t-Bu
t-Bu
Cr
Cr
t-Bu
O
O
N
N
N
N
X- = 2= BF4-, 3= PF6-, 4= NO3-, 5= ClO4
6= - OTf, 7= SbF6-, 8= Br-
,
-
Scheme 1. Synthetic route for chiral macrocyclic Cr(III) salen complexes 1e8.
The complex 1 (1 mol % with respect to racemic epoxide) was
first screened as a catalyst in the AKR of racemic trans-stilbene
oxide 9 (2 equiv) as a model substrate with aniline 10a (1 equiv) as
was found to be sensitive toward temperature as well. It was ob-
served that an increase in temperature (35 ꢂC) caused decrease in
enantioselectivity (entry 17) while a decrease in temperature

8302
R.I. Kureshy et al. / Tetrahedron 67 (2011) 8300e8307
(10 ꢂC) slowed down the reaction (entry 18). Hence, 0.5 mol %
catalyst loading and solvent mixture of CH2Cl2/MeOH 9:1 v/v ratio
at room temperature was taken as optimum reaction parameters to
explore the effect of an external additives viz., triphenylphosphine,
triethylamine, (S)-2-MeO-imine,6a (ꢀ)-cinchonine, triphenylphos-
phine oxide, and pyridine N-oxide on the performance of the AKR
of racemic trans-stilbene oxide 9 with aniline 10a. It is evident from
the entries 19e24, that the presence of these additives in the AKR
reaction was counterproductive. A possible reason for this behavior
can be attributed to the built-in tertiary amine sites in the catalyst 1,
which are sufficient to promote the AKR reaction in the absence of
an external base.
the difference in enantioselectivities is not remarkably large, it is
difficult to assign a single reason for these results.
Table 2
Screening of the chiral macrocyclic Cr(III) salen complexes 1e8 in the presence of
different counter ions in the AKR of tran-stilbene oxide 9 with aniline 10a at room
temperaturea
Entry
Catalyst
Time (h)
Unreacted
b-Amino alcohol
epoxide ee (%)b
Yield (%)c
ee (%)d
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
18
22
24
19
18
20
22
20
>99
82
90
94
90
86
82
85
99
98
98
98
98
98
98
98
>99
80
92
94
92
88
84
86
Table 1
Optimization of reaction conditions for the AKR of trans-stilbene oxide 9 with
aniline 10aa
a
Entry Catalyst Temp Solvent
(mol %) (ꢂC)
Time Unreacted
b
-Amino alcohol
Conditions: epoxide 9 (0.2 mmol), RNH2 10a (0.1 mmol), 1e8 (0.5 mol %) in
DCMþMeOH at rt.
(h)
epoxide ee
Yield(%)c ee(%)d
(%)b
b
Unreacted epoxide recovered in quantitative yield after the AKR reaction.
Isolated yield with respect to nucleophile.
Based on HPLC Chiral pack OD column.
c
1
1
1
1
1
1
1
1
1
1
1
1
1
2.5
5
0.5
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
35
10
rt
rt
rt
rt
rt
rt
rt
DCM
MeOH
24
24
20
20
18
20
19
18
20
00
63
92
93
94
92
93
85
70
83
90
87
90
85
>99
97
83
>99
90
00
99
91
95
99
99
99
99
99
99
99
99
99
99
99
99
99
99
98
98
98
98
98
98
99
00
65
92
94
96
94
94
88
72
85
92
90
92
80
>99
98
85
>99
92
d
2
3e
DCMþMeOH
DCMþMeOH
DCMþMeOH
CHCl3þMeOH
DCEþMeOH
THFþMeOH
ACNþMeOH
4f
In view of the highest enantioselectivity obtained with the
complex 1, it was taken as a preferred catalyst to expand the scope
of this AKR protocol of trans-stilbene oxide 9 with a variety of an-
ilines as nucleophiles viz., aniline 10a, 2-MeO-10b, 4-MeO-10c, 4-
Me-10d, 2-Cl-10e, 4-Cl-10f, and 4-NO2-aniline 10g under the op-
5g
6
7
8
9
10
11
12
13
14
15
16
17
18
19h
20h
21h
22h
23h
24h
25i
TolueneþMeOH 24
timized reaction conditions (Table 3). Although b-amino alcohols
DCMþEtOH
DCMþIPA
20
22
16
15
18
24
15
22
18
17
24
24
16
15
19
were obtained in high ee with all the nucleophiles used in the
present study, the best results in terms of yield and enantiose-
lectivity was achieved with aniline 10a and 4-MeO aniline 10c
(entries 1 and 3). The AKR of trans-stilbene oxide 9 failed with
strongly electron deficient nucleophile, 4-NO2-aniline 10g (entry
7). In order to check general applicability of the present protocol we
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
DCMþMeOH
conducted the AKR reaction of trans-
trans-butene oxide 13 with aniline and substituted anilines
(10aeg). Agreeably, trans- -methyl styrene oxide 12 and trans-
butene oxide 13 gave excellent yield (96e99%) of respective chiral
anti- -amino alcohols. However, excellent chiral induction (ee
>99%; entry 8) and regioselectivity was obtained in the case of AKR
of trans- -methyl styrene oxide 12 with aniline 10a. The nucleo-
b-methyl styrene oxide 12 and
93
83
90
92
90
>99
95
85
90
93
92
>99
b
b
a
b
Conditions: epoxide 9 (0.2 mmol), RNH2 10a (0.1 mmol), 1(0.25e5 mol %) in
DCMþMeOH.
phile aniline also worked well with trans-butene oxide 13 by giving
high ee (89%; entry 15) in the product.
b
c
Unreacted epoxide was recovered in quantitative yields after the AKR reaction.
Isolated yield with respect to the nucleophile.
Other nucleophiles also fared well with both epoxides (entries
9e13 and 16e20) except for strongly electron withdrawing nitro-
aniline 10g, which failed to open the epoxide ring (entries 14 and
21). In all the catalytic runs, the (S)-form of chiral recyclable mac-
rocyclic Cr(III) salen complexes converted the epoxides 9 and 12
d
e
f
Based on HPLC Chiral pack OD column.
Reaction was conducted in the presence of 1:1 DCMþMeOH.
Reaction was conducted in the presence of 7:1 DCMþMeOH.
Reaction was conducted in the presence of 9:1 DCMþMeOH.
Reaction was conducted in the presence of 10 mol % additives viz., triphenyl-
g
h
phosphine, triethylamine, (S)-2-MeO-imine,6a (ꢀ)- cinchonine, triphenylphosphine
into predominantly (S)-anti-b-amino alcohols as determined by
oxide, pyridine N-oxide, respectively.
i
comparing the HPLC profiles reported in the literature for these
Reaction was conducted at a 10 mmol scale of 9 and 5 mmol of 10a by keeping
products.8a,c,14a However, trans-butene oxide 13 favored the for-
other reaction conditions as per entry 15.
mation of (R)-anti-b-amino alcohol as dominant enantiomer. The
reasons behind the inversion of the configuration in the case of the
products from epoxide 13 is not known but this kind of behavior
has been observed earlier.9c,14c,14d
The catalyst 1 was also explored for its use in the ARO of dif-
ferent meso-epoxides viz., meso-stilbene oxide 16, cis-butene oxide
17, cyclohexene oxide 18, and cyclopentene oxide 19 with aniline
10a. Different catalyst loadings (0.25e0.75 mol %) were screened
for the ARO of meso-stilbene oxide 16 taken as a model substrate
and aniline 10a as a nucleophile at room temperature. The reaction
The catalyst 1 has Clꢀ as counter ion in macrocyclic Cr(III) salen
complex and counter ions are known to influence the activity and
enantioselectivity in the AKR reaction due to difference in their
basicity.8c Therefore, we have varied the counter anions viz., BFꢀ4 ,
PFꢀ6 , NO3ꢀ, ClO4ꢀ, OTfꢀ, SbF6ꢀ, Brꢀ and carried out AKR of racemic
trans-stilbene oxide 9 with aniline 10a under the above optimized
reaction conditions (Table 2, entry 1). The reactivity of the catalysts
1e8 was nearly same (w99%) but there was some variation in the
enantioselectivity (ee, 80e99%), highest being with catalyst 1 having
Clꢀ as counter ion (entry 1). Among others, catalyst 3, 4, and 5 having
PFꢀ6 , NOꢀ3 , and ClOꢀ4 ions, respectively, gave similar enantioselectiv-
ities (ee 93ꢁ1%) (entries 3e5), which are closely behind the catalyst
1. The lowest enantioselectivity (ee, 80%) amid these was however,
obtained with catalyst 2 having BFꢀ4 as counter ion (entry 2). Since
proceeded well and gave excellent yield (99%) of b-amino alcohol
20a (Table 4, entry 1) with 84% ee in 24 h with 0.5 mol % catalyst
loading. The catalyst 1 (loading of 0.5 mol %) was further explored
in the ARO of cis-butene oxide 17, cyclohexene oxide 18, and
cyclopentene oxide 19 with aniline 10a as nucleophile at room
temperature (entries 6e8). Although all the meso-epoxides gave

R.I. Kureshy et al. / Tetrahedron 67 (2011) 8300e8307
8303
Table 3
Enantioselective AKR of racemic trans-epoxides with different anilinesa
Entry
1
Trans epoxide
Amine
Time
(h)
Unreacted
epoxide ee
(%)b
b-Amino alcohol
Yield (%)c
ee (%)d
10a
18
>99
>99
>99e
2
3
4
5
6
7
10b
10c
10d
10e
10f
18
18
18
20
24
24
80
85
82
70
73
Racemic
99
98
98
92
96
d
81e
87e
84e
71e
78e
d
9
10g
8
10a
18
>99
>99
>99f
9
10b
10c
10d
10e
10f
18
24
30
24
20
18
80
69
62
67
52
Racemic
99
98
96
96
98
d
81f
69f
64f
68f
53f
d
10
11
12
13
14
12
10g
15
10a
20
85
96
86g
16
17
18
19
20
21
10b
10c
10d
10e
10f
18
19
19
24
24
24
70
74
42
56
53
Racemic
98
96
95
98
98
d
70g
76g
45g
58g
50g
d
13
10g
a
Conditions: epoxide 9, 12, 13 (0.2 mmol), RNH2 10a (0.1 mmol), 1 (0.5 mol %) in DCMþMeOH at rt.
Unreacted epoxide was recovered in quantitative yield after the AKR reaction.
Isolated yield is with respect to nucleophile.
Based on HPLC using Chiral pack OD column.
Confign. of the product (1S,2R).
b
c
d
e
f
Confign. of the product (1S,2R).
Confign. of the product (2R,3S).
g
corresponding
b-amino alcohols 20ae23a in quantitative yields
gave chiral syn-b-amino alcohol in 99% yield with 84% ee in 24 h.
(99e95%), the best result in terms of enantioselectivity was ach-
ieved with cis-butene oxide 17 (ee, 91%, entry 6).
These results are even better than the results obtained with poly-
meric Cr(III) salen complexes reported by us,8c possibly due the
built-in basic sites and better cooperation between the two salen
sub-units in the case of dimeric complex 1.
Recyclabilityof the catalyst 1 in the AKR reaction of trans-stilbene
oxide 9 with aniline 10a under the optimized reaction conditions
was carried out and the results are given in Table 5. After the first
catalytic run, the products were extracted with n-hexane/diethyl
For the sake of comparison, the ARO reaction was also con-
ducted with monomeric Cr(III) salen complex8a (0.5 mol %) in the
presence and absence of triethylamine (as an additive) using meso-
stilbene oxide 16 as a representative substrate with aniline 10a
under the optimized reaction conditions (Table 4, entries 4 and 5).
The product chiral syn-b-amino alcohol was obtained in quantita-
tive yield with 60% ee in the absence of triethylamine in 32 h. The
addition of triethylamine to the above reaction resulted in an in-
crease in the ee (73%) in 48 h for similar conversion. Significantly,
results obtained under the identical reaction conditions with the
chiral dimeric macrocyclic complex 1 (no triethylamine added)
ether (70:30). The products anti-b-amino alcohol 11a and enan-
tioenriched epoxide 9a were recovered from the organic layer and
separated by column chromatography. The recovered catalyst was
then dried overnight in a vacuum desiccator and used directly in the
next catalytic run. It is evident from the results that the performance

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R.I. Kureshy et al. / Tetrahedron 67 (2011) 8300e8307
Table 4
Product yield and ee of enantioselective ring opening reaction of different meso-epoxides with aniline as nucleophile catalyzed by complex 1 under optimized reaction
conditiona
Entry
1
Meso-epoxides
Nucleophile
Product
Catalyst
loading
(mol %)
Time
(h)
Yield
(%)c
ee
(%)b
0.50
24
99
84f
2
0.25
0.75
0.50
0.50
40
20
32
48
95
99
99
99
82f
78f
60f
73f
3
4d
5e
NH2
16
20a
6
0.50
0.50
0.50
28
24
28
99
98
98
91f
75f
30f
NH2
NH2
NH2
17
21a
O
7
18
22a
8
19
23a
a
Catalyst 1 (0.5 mol %), meso-epoxide (1 mmol), anilines (1 m mol) in DCMþMeOH at rt.
Based on HPLC using Chiral pack OD column.
Isolated yield with respect to the nucleophile.
Reaction was conducted in the absence of triethylamine with monomeric Cr(III) salen complex (0.5 mol %).
Reaction conducted in the presence of triethylamine with monomeric Cr(III) salen complex (0.5 mol %).
Confign. of the product (1R,2R).
b
c
d
e
f
Table 6
of the catalyst does not change over four reuse catalytic runs. Sim-
ilarly ARO of meso-stilbene oxide 16 with aniline 10a by using
complex 1 showed excellent recyclability over four recycle catalytic
runs (Table 6). The isolation of the complex from the reaction mix-
ture and reuse protocol remained the same as given above.
Enantioselective ARO of 16 with 10a using recovered complex 1 in DCMþMeOHa
Run
1
2
3
4
5
Time (h)
Yield (%)b
ee (%)c
24
99
84
24
99
84
24
98
84
24
98
84
24
97
84
a
Catalyst 1 (0.5 mol %), meso-epoxide 16(1 mmol), anilines 10a(1 mmol) in
Table 5
DCMþMeOH at rt.
Enantioselective AKR of 9 with 10a using recovered complex 1 in DCMþMeOHa
b
Isolated yield of syn-
b
-amino alcohol with respect to nucleophile.
c
ee of syn-b-amino alcohol based on HPLC Chiral pack OD column.
Run
1
2
3
4
5
Time (h)
Yield(%)b
ee (%)c
18
99
99
18
99
99
18
99
99
18
98
99
18
97
99
3. Conclusions
In conclusion, an efficient synthetic strategy for the preparation
of new chiral macrocyclic Cr(III) salen complexes with piperazine
linker has been demonstrated. Among all the complexes prepared
complex 1 showed excellent activity and enantioselectivity in the
a
Conditions: epoxide 9 (0.2 mmol), RNH2 10a (0.1 mmol), 1 (0.5 mol %) in
DCMþMeOH at rt.
b
Isolated yield of trans-
ee of trans-
b
-amino alcohol with respect to nucleophile.
c
b-amino alcohol based on HPLC Chiral pack OD column.

R.I. Kureshy et al. / Tetrahedron 67 (2011) 8300e8307
8305
AKR of racemic trans-epoxides and ARO of meso-epoxides with
4.5. Characterization data of macrocyclic salen ligand (28)
anilines as nucleophile in the absence of an external base as an
additive at room temperature. Overall the results are found to be
superior than the chiral monomeric and polymeric Cr(III) salen
complexes reported earlier for these reactions. Additionally, the
macrocyclic Cr(III) salen complex 1 was efficiently recovered and
reused several times and scaled up to 10 mmol batch size.
Yellow solid; Yield: 90%. Mp 236e238 ꢂC; [Found: C, 74.86; H,
8.79; N, 10.20. C68H96N8O4 requires C, 74.96; H, 8.88; N, 10.28%];
½
a 2D7
ꢄ
þ760 (c 0.083, CHCl3); nmax (KBr): 2937, 2863, 2806, 2364,
1629, 1441, 1385, 1319, 1265, 1206, 1159, 1095, 1009, 926, 878, 821,
772, 707 cmꢀ1
dH (200 MHz, CDCl3) 13.79 (s, 4H), 8.26 (s, 4H), 7.11
;
(br s, 4H,), 6.96 (s, 4H), 3.34e3.22 (br s,12H), 2.41 (s,16H),1.96e1.64
(m, 16H), 1.37 (s, 36); dC (200 MHz) 167.3, 161.2, 138.6, 132.3, 131.8,
128.7, 120.1, 74.3, 64.4, 54.7, 36.6, 35.0, 31.3, 26.2; ESI mass spectra
(MþH) 1089(55), 831(10), 319(100); MALDI-TOF found m/z
1088.488 C68H96N8O4 required 1088.
4. Experimental section
4.1. General
Anhydrous chromium chloride, racemic trans-epoxides viz.,
trans-stilbene oxide 9, trans-butene oxide 12, aniline 10a, 2-MeO-
10b, 4-MeO-10c, 4-Me-10d, 2-Cl-10e, 4-Cl-10f 4-NO2-aniline 10g,
and TBME were purchased from Aldrich. 3-t-Bu-5-chloromethyl-2-
hydroxy benzaldehyde 24 was synthesized by the reported meth-
4.6. Synthesis of chiral macrocyclic Cr(III) salen complex (1)
To a solution of 28 (1.0 mmol) in dry degassed THF (25 mL) was
added solid anhydrous Cr(II) chloride (1.1 mmol) in a glove box and
the resulting solution (dark brown in color) was stirred for 4 h
under an inert atmosphere at room temperature. Subsequently, the
resulting reaction mixture was exposed to air with stirring for 3 h
for auto-oxidation. The dark brown solution was diluted with TBME
(30 mL) to give chiral macrocyclic Cr(III) salen complex 1 as a brown
precipitate. The precipitate thus obtained was washed with TBME
(3ꢃ20 mL) and dried overnight under vacuum. (For characteriza-
tion please see the Supplementary data).
od.15b Racemic epoxide of trans-
b-methyl styrene was prepared by
its oxidation with m-CPBA.8c The solvents were dried by standard
procedures, distilled, and stored under nitrogen. HPLC traces were
compared with racemic samples prepared with the use of racemic
[Cr(salen)Cl] catalyst.8a
4.2. Preparation of ligand precursors
4.7. Synthesis of chiral macrocyclic Cr(III) salen complexes 2e8
4.2.1. Synthesis of 5,50-(piperazine-1,4-diylbis(methylene))-bis-(3-t-
Bu-2-hydroxybenzaldehyde) (26). A solution of chloromethylsali
cylaldehyde15b 24 in dry toluene (40 mmol, 20 mL) was added
drop-wise to a stirring solution of piperazine 25 in dry toluene
(20 mmol, 20 mL) at room temperature. The resulting yellow so-
lution was allowed to heat at 80 ꢂC with vigorous stirring for 6 h to
give dialdehyde dichloride as a white precipitate, which was
washed with solvent ether (3ꢃ10 mL) and toluene (3ꢃ10 mL) to
remove unreacted starting materials. The resulting solid was
treated with saturated sodium bicarbonate solution (20 mL) and
the desired dialdehyde 26 was extracted with CH2Cl2 (4ꢃ10 mL).
The organic layer was washed with water (2ꢃ10 mL) and brine
(2ꢃ10 mL), and dried over anhydrous Na2SO4. The removal of the
organic solvent gave dialdehyde 26 in high yield, which was used as
such without further purification for the preparation of macrocyclic
salen ligand 28.
4.7.1. Synthesis of chiral macrocyclic Cr(III) salen complex
(2). Complex 2 was prepared by the reaction of methanolic solu-
tion of dimeric complex 1 (0.500 mg, 0.397 mmol, 20 mL) with an
aqueous solution of NH4BF4 (0.83 mg, 0.8 mmol, 0.4 mL). The
resulting suspension was stirred for 12 h. After that the reaction
mixture was centrifuged at 4500 rpm for 30 min. The solvent from
the supernatant was removed to dryness to yield the complexes 2.
(Data is given in Supplementary data).
4.7.2. Synthesis of chiral macrocyclic Cr(III) salen complex
(3). Complex 3 was prepared by the reaction of methanolic solu-
tion of dimeric complex 1 (0.500 mg, 0.397 mmol, 20 mL) with an
aqueous solution (0.130 mg, 0.8 mmol, 0.4 mL) of NH4PF6. The
resulting suspension was processed similarly as for the preparation
of complex 2 given above to get the complexes 3. (Data is given in
Supplementary data).
4.3. Characterization data of dialdehyde (26)
4.7.3. Synthesis of chiral macrocyclic Cr(III) salen complex
(4). Complex 4 was prepared by the reaction of methanolic solu-
tion of dimeric complex 1 (0.500 mg, 0.397 mmol, 20 mL) with an
aqueous solution of AgNO3 (0.136 mg, 0.8 mmol, 0.4 mL). The
resulting suspension was stirred for 6 h and the white precipitate of
AgCl was removed by centrifugation as described above. The re-
moval of the solvent from supernatant gave the complexes 4 in
quantitative yield. (Data is given in Supplementary data).
White solid; Yield: 90%. Mp 140e143 ꢂC; [Found: C, 72.10; H,
8.23; N, 6.04. C28H38N2O4 requires C, 72.07; H, 8.21; N, 6.00%]; nmax
(KBr): 3138, 2953, 2812, 2312, 1930, 1805, 1782, 1652, 1596, 1434,
1380, 1320, 1291, 1260, 1233, 1203, 1154, 1008, 965, 926, 888, 800,
753, 708, 619, 550, 529 cmꢀ1
; dH (200 MHz, CDCl3) 11.70 (s, 2H),
9.85 (s, 2H), 7.44 (s, 2H), 7.35 (s, 2H), 3.48 (s, 4H), 2.48 (s, 8H),1.41 (s,
18H); dC 199.0, 162.2, 139.9, 137.1, 133.9, 130.2, 122.2, 64.1, 54.9, 36.7,
31.2; ESI mass spectra (Mþ1) 467, (MþNa) 489.
4.7.4. Synthesis of chiral macrocyclic Cr(III) salen complex
(5). Complex 5 was prepared by the reaction of methanolic solu-
tion of dimeric complex 1 (0.500 mg, 0.397 mmol, 20 mL) with an
aqueous solution of AgClO4 (0.165 mg, 0.8 mmol, 0.4 mL). The
resulting suspension was stirred for 6 h and the reaction mixture
was processed as described above to yield the complexes 5. (Data is
given in Supplementary data).
4.4. Synthesis of chiral macrocyclic ligand (28)
A solution of dialdehyde 26 (2 mmol, 2 equiv) in THF (1.2 mL)
was added to a solution of (1S,2S)-(þ)-1,2-diaminocyclohexane27
(2 mmol, 2 equiv) in THF (0.8 mL) and the resulting mass was
stirred for 3 h at room temperature (checked on TLC). After the
completion of the reaction, the solvent was completely removed
under reduced pressure on a rotary evaporator to give chiral mac-
rocyclic salen ligand 28 in high yield, which was used as such for
the preparation of the complex 1 without further purification.
4.7.5. Synthesis of chiral macrocyclic Cr(III) salen complex
(6). Complex 6 was prepared by the reaction of methanolic solu-
tion of dimeric complex 1 (0.500 mg, 0.397 mmol, 20 mL) with an
aqueous solution (0.206 mg, 0.8 mmol, 0.4 mL) of AgOTf. The

8306
R.I. Kureshy et al. / Tetrahedron 67 (2011) 8300e8307
resulting suspension was stirred for 6 h and the reaction mixture
was processed as mentioned above to yield the complexes 6. (Data
is given in Supplementary data).
monitored on TLC. At the end of the reaction the solvent was
completely removed and the residue was repeatedly extracted with
n-hexane/diethyl ether (70:30, 5ꢃ6¼30 mL). The residue left was
the catalyst and the organic layer contained the products trans-b-
4.7.6. Synthesis of chiral macrocyclic Cr(III) salen complex
(7). Complex 7 was prepared by the reaction of methanolic solu-
tion of dimeric complex 1 (0.500 mg, 0.397 mmol, 20 mL) with an
aqueous solution (0.275 mg, 0.8 mmol, 0.4 mL) of AgSbF6. The
resulting suspension was stirred for 6 h and centrifuged. The sol-
vent removal from the supernatant yielded the complex 7. (Data is
given in Supplementary data).
amino alcohol (1S,2R)-11a and the unreacted epoxides (2R,3R)-9,
recovery of which was done by column chromatography in quan-
titative yields using hexaneþethyl acetate (9:1) as an eluent.
4.11. Recycling of the catalyst 1
Catalyst recycle experiments were carried out at the 5 mmol
scale of the substrates trans-stilbene oxide 9 (0.981 g) and aniline
10a (0.227 mL) with 1 mol % of the catalyst 1 (0.031 g) under the
above optimized reaction conditions. At the end of the catalytic run
(checked on TLC) the solvent was completely removed under re-
duced pressure. The residue was extracted with n-hexane/diethyl
ether (70:30) (3ꢃ5 mL) and the remaining solid (catalyst) was
further washed with hexane (2ꢃ5 mL). The recovered solid was
dried under the reduced pressure for 1e2 h and was used as re-
covered catalyst for recycle experiments of AKR reaction of trans-
stilbene oxide. The products aminoalcohol and enantioenriched
epoxide were obtained in the manner described in preceding par-
agraph. Similar scale and procedure was followed for the catalyst 1
recycle experiment conducted for the ARO of meso-stilbene oxide
16 with aniline 10a.
4.7.7. Synthesis of chiral macrocyclic Cr(III) salen complex (8). To
prepare complex 8, a solution of complex 1 (0.500 mg, 0.397 mmol)
in methanol containing 5% water (20 mL), which was passed slowly
over a bed (1 cmꢃ10 cm) of KBrover 3 h. The solvent was then
removed completely and the residue was dried in a desiccator to
get the complex
8 in quantitative yield (Data is given in
Supplementary data).
4.8. Typical procedure for the AKR of racemic trans-epoxides
In a small vial equipped with a magnetic stirring bar, the chiral
macrocyclic Cr(III) salen complexes 1e8 (1 mol % based on mono-
meric unit) were taken in DCMþMeOH (9:1, 0.4 mL) and the
resulting solution was stirred for 5 min followed by the addition of
an appropriate epoxide (0.2 mmol). The resulting mass was stirred
for 10 min followed by the addition of desired aniline as a nucleo-
phile (0.1 mmol) at room temperature. The progress of the catalytic
reaction was monitored on TLC. At the end of the reaction the re-
action mixture was repeatedly extracted with n-hexane/diethyl
Acknowledgements
K.J.P. and R.I.K. are thankful to CSIR-SRF, DST, and CSIR Network
Project on Catalysis for financial assistance. The authors also
thankful to ‘Analytical Science Discipline’ for providing in-
strumental facilities.
ether (70:30, 2ꢃ5 mL). The product trans-
b-amino alcohols 11aeg,
14aeg, 15aeg and the unreacted epoxides (2R,3R)-9, (2R,3R)-12,
(2S,3S)-13 were recovered by column chromatography. The re-
covered catalyst was dried under vacuum and stored in a desiccator
for its use in the subsequent catalytic runs.
Supplementary data
Characterization data of chiral macrocyclic ligand, its precursor
and chiral Cr(III) salen complexes including 1H, 13C NMR, FT-IR,
CHN, ICP, mp, Optical rotation, ESI-MS, MALDI-TOF, and HPLC pro-
file. Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.tet.2011.08.077.
4.9. Typical experimental procedure for ring opening of
meso-epoxides
To a 5 mL round bottom flask equipped with rubber septum and
a magnetic stirring bar, the chiral macrocyclic Cr(III) salen complex
1 (0.5 mol %) was taken in DCMþMeOH (9:1, 0.4 mL) and the
resulting solution was stirred for 5 min followed by the addition of
an appropriate meso-epoxide (0.2 mmol). Subsequently, after
20 min, aniline (0.2 mmol) was added and the reaction mixture was
further allowed to stir for the specified time at room temperature.
The progress of the reaction was checked on TLC using hexane/ethyl
acetate (8/2) as mobile phase. After the completion of the reaction,
solvent was removed under vacuum and the product was purified
by column chromatography using silica gel 100e200 mesh as sta-
tionary phase and hexane/ethyl acetate (8:2) as mobile phase. All
the products were characterized by appropriate spectroscopic
techniques, microanalysis, LCMS, and optical rotation, which were
found to be in consonance with the reported values.5c,5d,6a,6b
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