Reusable chiral dicationic chromium(III) salen catalysts for aminolytic kinetic resolution of trans-epoxides

Article abstract of DOI:10.1002/adsc.201000428

A series of new recyclable chiral dicationic chromium(III) salen complexes 1-10 bearing different substituents, viz., hydrogen, methyl, tert-butyl, triphenylphosphinomethyl, triethylaminomethyl, methylimidazolium, methylpyridinium, methyl-N,N-dimethylpyri

Full text of DOI:10.1002/adsc.201000428

FULL PAPERS
DOI: 10.1002/adsc.201000428
Reusable Chiral Dicationic Chromium(III) Salen Catalysts for
G
Aminolytic Kinetic Resolution of trans-Epoxides
Rukhsana I. Kureshy,a,* K. Jeya Prathap,a Tamal Roy,a Nabin Ch. Maity,a
Noor-ul H. Khan,a Sayed H. R. Abdi,a and Hari C. Bajaja
a
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
Fax : (+91)-0278-256-6970; e-mail: rukhsana93@yahoo.co.in
Received: June 1, 2010; Published online: November 17, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000428.
Abstract: A series of new recyclable chiral dicationic With the use of catalyst 3, anti-b-amino alcohols
chromium
ent substituents, viz., hydrogen, methyl, tert-butyl, spect to the nucleophile) and enantioselectivities
triphenylphosphinomethyl, triethylaminomethyl, (ee>99%) with the concomitant recovery of corre-
methylimidazolium, methylpyridinium, methyl-N,N- sponding epoxides in high optical purity (ee up to
dimethylpyridinium at the 3,3’- and 5,5’- positions of >99%) and quantitative yields in 12 h. The catalyst
ACHTUNGTRENNUNG(III) salen complexes 1–10 bearing differ- were obtained in excellent yields (>99% with re-
the salen unit with (1S,2S)-(+)-1,2-diaminocyclohex- 3 is recyclable in the aminolytic kinetic resolution
ane, (1S,2S)-(À)-1,2-diphenyl-1,2-diaminoethane, and process and worked well up to six cycles with reten-
(S)-(À)-1,1’-binaphthyl-2,2’-diamine collars have tion of enantioselectivity.
been synthesized and characterized by various physi-
co-chemical methods. These complexes were used as
catalysts for the highly enantioselective aminolytic Keywords: anti-b-amino alcohols; aminolytic kinetic
kinetic resolution of racemic trans-epoxides with dif- resolution; asymmetric catalysis; chromium
ferent anilines as nucleophiles at room temperature. salen complexes; dicationic salen; trans-epoxides
ACHTUNGTRENNUNG(III)
Introduction
different nucleophiles. In spite of high activity and
enantioselectivity of these monomeric salen com-
Enantiopure b-amino alcohols are valuable building plexes, there is a nagging issue of separation and recy-
blocks for biologically active compounds[1] and as cling of the catalyst. As a result, the last decades have
chiral auxiliaries in asymmetric synthesis.[2] Among witnessed an avid research activity in heterogenizing
the various efficient catalytic methods developed[3–11] chiral homogeneous catalysts.[14]
for their synthesis, the chiral metal complexes-cata-
Another strategy for making the chiral catalyst re-
lyzed aminolytic kinetic resolution (AKR) of racemic cyclable is to modify the solubility of the catalyst. In
epoxides with amine is an important strategy. AKR this strategy one can still carry out the catalytic reac-
not only affords enantiomerically enriched b-amino tion under homogenous condition, but at the end of
alcohols, in a single step, but also this strategy simul- the reaction the catalyst is precipitated out by the ad-
taneously produces valuable enantiomerically en- dition of a solvent in which the catalyst is insoluble
riched epoxides from racemic terminal and trans-ep- while the reactants and products remain in the solu-
ACTHNUTRGNEUNG
oxides.[10] Since the landmark discovery of the Mn(III) tion.[15] The monomeric salen complexes are highly
salen complex as enantioselective epoxidation catalyst soluble in most of the organic solvents. In order to
by Jacobsen et al., salen-type complexes with differ- make them less soluble in some of the non-polar or-
ent metal ions have also found application in different ganic solvents, e.g., n-hexane, we have earlier report-
asymmetric organic transformations.[12] such as hydro- ed the dimeric and polymeric version of salen com-
lytic kinetic resolution (HKR) of terminal epox- plexes.[7f,g,h,j,l] However, in the present study we have
ACHTUNGTRENNUNG
ides,[13a,b] asymmetric ring opening (ARO) of meso- introduced bulky groups as well as charges (akin to
[13d]
epoxides.[13c] and C C bond forming reaction
with ionic liquids) in the monomeric salen complexes in
À
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view of harnessing the virtues of homogeneous cataly- and triphenylphosphinomethyl groups as in the case
sis at the same time make the catalysts easily recover- of complex 4, the ees of the anti-b-amino alcohol and
able due to their altered solubility in organic solvents. unreacted epoxide declined sharply (entry 4). More-
Accordingly we are reporting here the synthesis of over, this catalyst also took 35 h for completion of the
new chiral dicationic Cr
having different substituents at the 3,3’- and 5,5’-posi- ee were achieved with catalyst 3, for our next AKR
tions of the salen unit. The chiral dicationic Cr(III) experiments we fixed the t-Bu group at the 3,3’-posi-
ACHTUNGTNER(NUNG III) salen complexes 1–10 AKR reaction. As best results in terms of yield and
AHCTUNGTRENNUNG
salen complexes were used as catalysts for the first tions and triphenylphosphinomethyl groups at the
time in enantioselective AKR of various trans-epox- 5,5’-positions and changed to the 1,2-diamino collars
ides, although Co, Mn and Ru complexes of dication- originating from (1S,2S)-(À)-1,2-diphenyl-1,2-diami-
ic-salen have also been applied for achiral[16] and noethane and (S)-(À)-1,1’-binaphthyl-2,2’-diamine
chiral organic transformations.[17] Excellent yields (up (catalysts 5 and 6). In both the case we achieved ex-
to >99%) and enantioselectivity (ee up to >99%) of cellent conversion of the anti-b-amino alcohol in 12–
chiral anti-b-amino alcohols and remaining epoxides 15 h but, the ees were low (entries 5 and 6). Having
in high optical purity (ee, up to >99%) were achieved established that the enantioselectivity is higher for the
in 12 h at room temperature with the use of the recy- complexes derived from (1S,2S)-(+)-1,2-diaminocy-
clable chiral dicationic Cr
alyst. The most active and enantioselective dicationic the 3,3’-positions, we kept these positions fixed and
Cr(III) salen complex 3 was also subjected to catalyst changed the groups at the 5,5’-positions (complexes
recycling experiments where the complex 3 retained 7–10). These complexes 7–10 having different amino-
ACHTUNGTNER(NUNG III) salen complexes as cat- clohexane and salicylaldehyde having t-Bu groups at
ACHTUNGTRENNUNG
its catalytic performance over six cycles.
alkyl groups, viz., triethylaminomethyl, methylimida-
zolium, methylpyridinium, methyl-N-N-dimethylpyri-
dinium at the 5,5’-positions of salen ligands gave the
anti-b-amino alcohol in excellent yield in 24 h with
58–80% ee (entries 7–10) but none could match the
Results and Discussion
Chiral Cr
ACHTUNGTRENNUNG
by the reaction of respective dicationic chiral salen li-
gands 1’–10’ with chromium(II) chloride under an and enantioselective catalyst at 2 mol% loading, next
inert atmosphere followed by autooxidation we used this catalyst to optimize the reaction parame-
(Scheme 1). All chiral dicationic metal complexes ters such as catalyst loading, temperature and solvent
were characterized by microanalysis, FT-IR, UV/Vis variations for the AKR reaction of trans-stilbene
spectroscopy and optical rotation (see Supporting In- oxide 11 as model substrate and aniline 12a as nucleo-
formation). The substituents on the salen unit of the phile. A catalyst loading ranging over 1–5 mol% was
catalysts 1–10 were chosen in such a way that at the explored and the data are given in Table 2 (entries 1–
end of the present study there emerge some under- 3). A catalyst loading of 1 mol% gives the product
standings on structure-activity-relationships. Accord- anti-b-amino alcohol 13a with an ee of 95% in 20 h
ingly, the 3,3’- and 5,5’-positions and diamine collar (entry 2), whereas 5 mol% of the catalyst was able to
were subjected to change.
produce the desired product with ee >99% in 8 h
The catalytic activity and selectivity of catalysts 1– (entry 3). Looking at the ee (>99%) of the product
10 (2 mol%) were explored for the AKR of trans-stil- obtained with 2 mol% catalyst loading and the time
bene oxide 11 (2.0 mmol) as a model test substrate taken for the completion of the reaction (12 h) vis-ꢀ-
with aniline 12a (1.0 mmol) as nucleophile at room vis 5 mol% catalyst loading, it was decided to take
temperature and the data are summarized in Table 1. 2 mol% catalyst loading as the optimum (entry 1).
In the first set of screening of complexes 1–3, we al- Next, we varied the reaction temperature from 0 to
tered the substituents (H, Me and t-Bu) at 3,3’-posi- 408C (entries 4–6) and found that room temperature
tions of the salen unit by fixing the substituent (tri- (288C) is just right for the AKR reaction (entry 1).
phenylphosphinomethyl) at the 5,5’-positions and the This protocol worked well in terms of yield of anti-b-
diamine collar originating from (1S,2S)-(+)-1,2-diami- amino alcohol even at a relatively higher scale
nocyclohexane. The results (entries 1–3) clearly show (10 mmol) in 13 h (Table 2, entry 7). The nature of
that as we increase the size of the substituent (H< the solvents is known to influence the reactivity and
Me<t-Bu) there is a steady increase in the ee of the enantioselectivity of the AKR of racemic epoxide.[7g]
desired anti-b-amino alcohol 13a (ee, 58<70<99%, Hence, we conducted the AKR in different solvents,
respectively).
viz., THF, CH3CN, CHCl3, toluene and MeOH (en-
However, the yield of the product was quantitative tries 8–12) keeping other optimized parameters con-
(~99%) in all the cases. The ee of the unreacted epox- stant (CH2Cl2, entry 1). However, best results were
ide 11’ followed same pattern of H<Me<t-Bu=56< obtained with the use CH2Cl2 as solvent which was
65<99%. On exchanging the positions of the t-Bu used as solvent of choice in our subsequent studies.
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Reusable Chiral Dicationic ChromiumACTHUNTGRNEUNG(III) Salen Catalysts for Aminolytic Kinetic Resolution
Scheme 1. Synthetic route for chiral dicationic CrACHTUNTRGNEUNG(III) salen complexes. Conditions: a) concentrated HCl, trioxane, 45–508C,
72 h (yield: 90%); b) TPP, dry benzene, 4–6 h, 608C (yield: 90–91%); c) (1S,2S)-(+)-1,2-diaminocyclohexane, dry methanol,
reflux, 4-5 h (yield: 90–98%); d) dry THF, anhydrous CrCl2/autooxidation (yield: 90–95%); e) (1S,2S)-(À)-1,2-diphenyl-1,2-
diaminoethane, dry methanol, reflux, 4–5 h (yield: 95%); f) (S)-(À)-1,1’-binaphthyl-2,2’-diamine, dry methanol, reflux, 4–5 h
(yield: 85%); g) TEA, dry benzene, 4-6 h, 608C (yield: 90%); h) imidazole, dry benzene, 4–6 h, 608C (yield: 92%); i) pyri-
dine, dry benzene, 4–6 h, 608C (yield: 93%); j) N,N-dimethylamine, dry benzene, 4–6 h, 608C (yield: 95%).
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Table 1. Screening of the catalysts 1–10 for enantioselective
AKR of trans-stilbene oxide 11 with aniline 12a at room
temperature.[a]
Entry Complex Time Unreacted epoxide b-Amino alco-
[h]
ee [%][b]
hol
Yield
ee
[%][d]
[%][c]
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
24
24
12
35
12
15
24
24
24
24
56
65
>99
28
30
43
78
56
72
69
99
99
>99
99
99
98
98
98
98
98
58
70
>99
28
35
12
80
58
70
65
Figure 1. Yield [%] and ee [%] of chiral anti-b-amino alco-
hols with the catalysts 1–10.
10
10
[a]
observed in the case of the respective products 13a,
13b and 13e obtained from the ring opening of trans-
stilbene oxide with 12a, 12b and 12e in 10–18 h
(Table 3, entries 1, 2 and 5) (Figure 2). However, the
ring opening reaction of 11 with 4-chloroaniline 12f
gave a low enantioselectivity (entry 6) while 4-nitro-
Conditions: epoxide 11 (0.2 mmol), RNH2 12a
(0.1 mmol), 1–10 (2 mol%) in CH2Cl2.
Unreacted epoxide recovered in quantitative yield after
AKR reaction.
Based on an HPLC Chiral pack OD column.
Isolated yield with respect to nucleophile.
[b]
[c]
[d]
AHCTUNGTRENNUNG
entry 1) were then applied for the synthesis of a series and 11’e were recovered in quantitative yield with ex-
of anti-b-amino alcohols with catalyst 3 using other cellent optical purity (>99%) (Table 3, entries 1 and
anilines as nucleophile, viz., aniline (12a), 2-MeO- 2). We further explored the AKR reaction of trans-b-
(12b), 4-MeO- (12c), 4-Me- (12d), 2-Cl- (12e), 4-Cl methylstyrene 14 and trans-butene oxide 15 with ani-
(12f) and 4-NO2-aniline (12g) with trans-stilbene line and substituted anilines (12a–g). Ironically, trans-
oxide 11 (Scheme 2). A good to excellent yield (82– b-methylstyrene oxide and trans-butene oxide gave
99%) for anti-b-amino alcohol was achieved for all good to excellent yields (84–98%) of the respective
anilines while high enantioselectivity (ee >99%) was chiral anti-b-amino alcohols, but high chiral induction
Table 2. Optimization of reaction condition for enantioselective AKR of trans-stilbene oxide 11 with aniline 12a in the pres-
ence of complex 3.[a]
Entry
Catalyst [mol%]
Temp. [8C]
Solvent
Time [h]
Unreacted epoxide ee [%][b]
b-Amino alcohol
Yield [%][d]
ee [%][c]
1
2
3
4
2
1
5
2
2
2
2
2
2
2
2
2
r.t.
r.t.
r.t.
0
10
40
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
CH3CN
CHCl3
Toluene
MeOH
12
20
8
30
24
8
13
28
30
20
24
24
>99
99
>99
99
99
82
99
50
42
78
>99
99
>99
99
99
99
99
99
99
99
>99
95
>99
99
99
80
99
40
36
80
5
6
7[e]
8
9
10
11
12
00
00
00
00
00
00
[a]
Conditions: epoxide 11 (0.2 mmol), RNH2 12a (0.1 mmol), 3 (2 mol%) in different solvents
Unreacted epoxide recovered in quantitative yield after AKR reaction.
Based on an HPLC Chiral pack OD column.
Isolated yield with respect to nucleophile.
Reaction conducted at 10-mmol scale of 11 and 5 mmol of 12a in CH2Cl2 keeping other conditions as per entry 1.
[b]
[c]
[d]
[e]
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Reusable Chiral Dicationic ChromiumACTHUNTGRNEUNG(III) Salen Catalysts for Aminolytic Kinetic Resolution
Scheme 2. AKR of racemic trans-epoxides with different anilines.
(ee >99%, 95%) was obtained only in the case of meric CrACTHNUTRGNEUNG
(III) salen complexes[10] for similar trans-ep-
ring opening of butene oxide 15 with 2-MeO-aniline oxides under homogeneous system. In all catalytic
12b and 2-Cl-aniline 12e (Table 3, entries 16 and 19). runs, the (S)-form of chiral recyclable dicationic Cr-
In fact trans-b-methyl styrene oxide 14 as substrate
ACHTUNGTREN(NGNU III) salen complexes converted all epoxides into pre-
has undergone the AKR reaction well in terms of re- dominantly (R)-anti-b-amino alcohols as determined
activity with several anilines (entries 8–13) albeit with by comparing the HPLC profiles reported in the liter-
moderate enantioselectivity. It is to be noted that re- ature for these products.[7g,k,10]
sults obtained in the present study are significantly su-
In addition to high reactivity and enantioselectivity
perior in term of yields (99%, 12 h) and enantioselec- in the AKR reaction, the catalyst 3 is recyclable as
tivity of the products chiral anti-b-amino alcohols evidenced by the recycling experiments (Table 4). The
(>99%) and unreacted epoxides (ee >99%) as com- recyclability experiments were carried out by using
pared to the earlier reported non-recyclable mono- trans-stilbene oxide 11 (2.0 mmol) as a model sub-
Table 3. Enantioselective AKR of racemic trans-epoxides with different anilines.[a]
Entry
trans-Epoxide
Amine
Time [h]
Unreacted epoxide[b]
ee [%]
b-Amino alcohol
Yield [%][d]
ee [%][c]
1
2
3
4
5
6
7
12a
12b
12c
12d
12e
12f
12g
12
18
12
15
10
10
24
11’a
11ꢀb
11ꢀc
11ꢀd
11ꢀe
11ꢀf
11ꢀg
>99
>99
92
89
97
13a
13b
13c
13d
13e
13f
13g
>99
>99
99
88
86
99
50
–
98
96
96
82
96
–
46
racemic
8
9
12a
12b
12c
12d
12e
12f
12g
20
20
20
20
24
24
24
14’a
14ꢀb
14ꢀc
14ꢀd
14ꢀe
14ꢀf
14ꢀg
60
67
58
65
40
46
racemic
16a
16b
16c
16d
16e
16f
16g
97
96
98
96
96
98
–
62
71
62
68
45
49
–
10
11
12
13
14
15
16
17
18
19
20
21
12a
12b
12c
12d
12e
12f
12g
20
20
20
24
24
24
24
15’a
15ꢀb
15ꢀc
15ꢀd
15ꢀe
15ꢀf
15ꢀg
48
98
50
51
92
48
racemic
17a
17b
17c
17d
17e
17f
17g
95
98
94
95
98
84
–
50
>99
52
54
95
50
–
[a]
Conditions: epoxides 11, 14, 15 (0.2 mmol), RNH2 12a–g (0.1 mmol), 3 (2 mol%) in CH2Cl2.
Unreacted epoxide recovered in quantitative yield after AKR reaction.
Based on an HPLC Chiral pack OD column.
[b]
[c]
[d]
Isolated yield with respect to nucleophile.
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Rukhsana I. Kureshy et al.
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activity and enantioselectivity. The products from the
AKR of trans-stilbene oxide, viz., anti-b-amino alco-
hols (ee >99%) and epoxides (ee >99%) were effi-
ciently separated from the precipitated catalyst in n-
hexane by simple filtration. To the best of our knowl-
edge, there are no other reports on the AKR reaction
of trans-epoxide using a recyclable monomeric dicat-
ionic CrACTHNUTRGNEUG(N III) salen complex under homogeneous con-
ditions which circumvent the need of a multi-step pro-
cess of anchoring the homogeneous catalyst on vari-
ous solid supports for recycling purposes.
Experimental Section
Figure 2. Yield [%] and ee [%] of chiral anti-b-amino alco-
hols with different anilines using the catalyst 3.
Preparation of Chiral Ligand Precursors (1’’-4’’and
7’’–10’’)
An appropriate chloromethylsalicylaldehyde A–D (see Sup-
porting Information) (2 mmol) in 20 mL benzene was added
dropwise to a stirring solution of triphenylphosphine, tri-
Table 4. Enantioselective AKR of 11 with 12a using recov-
ered complex 3 in dichloromethane.[a]
Run
1
2
3
4
5
6
AHCTUNGTREGeNNUN thylamine, imidazole, pyridine and N,N-dimethylaminopyr-
idine, (2 mmol) in 20 mL of dry benzene. The resulting
cloudy solution was allowed to reflux with vigorous stirring
for 6 h to give the products (3-formyl-4-hydroxybenzyl)tri-
phenylphosphonium chloride 1’’, (3-formyl-4-hydroxy-5-
methylbenzyl)triphenylphosphonium chloride 2’’, (3-tert-
butyl-5-formyl-4-hydroxybenzyl)triphenylphosphonium chlo-
ride 3’’, (5-tert-butyl-3-formyl-2-hydroxybenzyl)triphenyl-
phosphonium chloride 4’’, N-(3-tert-butyl-5-formyl-4-
Time [h]
Yield [%][b]
ee [%][c]
12
99
>99
12
99
>99
12.5
99
>99
12.5
98
>99
12.5
98
>99
13
97
>99
[a]
Conditions: epoxides 11 (0.2 mmol), RNH2 12a
(0.1 mmol), 3 (2 mol%) in CH2Cl2. Unreacted epoxide
recovered in quantitative yield after AKR reaction.
Isolated yield with respect to nucleophile.
[b]
[c]
hydroxyACHTNUTRGNEbNUG enzyl)-N,N,N-triethylammonium chloride 7’’, 3-(3-
Based on HPLC Chiral pack column.
tert-butyl-5-formyl-4-hydroxybenzyl)-1H-imidazol-3-ium
chloride 8’’, 1-(3-tert-butyl-5-formyl-4-hydroxybenzyl) pyridi-
nium chloride 9’’, 1-(3-tert-butyl-5-formyl-4-hydroxybenzyl-)-
4-(dimethylamino)pyridinium chloride 10’’; yields: 92–95%
(see Supporting Information).
strate with aniline 12a (1.0 mmol) as nucleophile in
dichloromethane at room temperature using the com-
plex 3 (2 mol%) as catalyst. After completion of the
catalytic reaction, the products were extracted with n-
hexane. The products anti-b-amino alcohol and enan-
tioenriched epoxide were recovered from the organic
layer and separated by column chromatography. The
recovered complex was dried under vacuum and was
used as such for the subsequent catalytic runs, which
worked well up to 6 catalytic runs with retention of
the reactivity and enantioselectivity. From the recy-
cling experiments it is evident that the dicationic
Preparation of Dicationic Chiral Salen Ligands 1’–10’
To an ethanolic solution of an appropriate aldehyde 1’’–10’’
(2 mmol) was added the chiral diamine, viz., (1S,2S)-(+)-
1,2-diaminocyclohexane, (1S,2S)-(À)-1,2-diphenyl-1,2-diami-
noethane, or (S)-(À)-1,1’-binaphthyl-2,2’-diamine (1 mmol)
and the resulting mass was allowed to reflux for 7–8 h. The
solvent was partially removed under reduced pressure on a
rotary evaporator, and the yellow product of 1’–10’ was pre-
cipitated by n-hexane. The chiral ligands 1’–10’ were filtered
and dried under vacuum (see Supporting Information).
CrACHTUNGTRENNUNG(III) salen complexes are fairly stable and do not
deteriorate during the course of the AKR reaction.
Preparation of Dicationic Chiral Cr
Complexes 1–10
ACHTUNGTRENUN(NG III) Salen
Conclusions
A 100-mL, 2-necked, round-bottom flask with a nitrogen
inlet and outlet was charged with a solution of 1’–10’ in dry
degassed THF (26 mL). To the yellow solution, anhydrous
chromium(II) chloride was added and the solution turned
into dark brown which was stirred for 4 h under a blanket of
nitrogen and then exposed to air for a further 3 h. The dark
brown solution was diluted with TBME (t-butyl methyl
ether) resulting in the precipitation of the complexes 1–10.
In summary, chiral dicationic CrACHTUNTRGNEUNG(III) salen complexes
1–10 having different substituents at the 3,3’- and 5,5’-
positions of salen unit were prepared and used for the
AKR of racemic trans-epoxides with anilines at room
temperature. Among all the complexes used in the
present study the complex 3 having triphenylphosphi-
nomethyl groups at the 5,5’-positions and t-Bu groups
at the 3,3’-positions worked efficiently in term of re- The complexes were filtered and washed with saturated
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Reusable Chiral Dicationic ChromiumACTHUNTGRNEUNG(III) Salen Catalysts for Aminolytic Kinetic Resolution
NH4Cl solution and brine to remove the excess of chromium
chloride, the complexes were dried over night under
vacuum (see Supporting Information).
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Asymmetric Aminolytic Kinetic Resolution (AKR) of
Racemic trans-Epoxides
In a small vial equipped with a magnetic stirring bar, the di-
cationic CrACHTUNGTRENNUNG(III) salen complexes 1–10 (2 mol%) were taken
in dichloromethane (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 nucleo-
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monitored on TLC. At the end of the reaction the reaction
mixture was repeatedly extracted with n-hexane/diethyl
ether (70:30). The product trans-b-amino alcohols 13a–g,
16a–g, 17a–g and the unreacted epoxides 11’a–g, 14’a–g,
15’a–g were recovered by column chromatography. The re-
covered catalyst was dried under vacuum and stored in des-
iccator for its use in subsequent catalytic runs.
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Recycling of the Catalyst 3
At the end of the catalytic run (checked on TLC) the sol-
vent was completely removed under reduced pressure. The
residue was extracted with hexane, and the remaining solid
was further washed with n-hexane/diethyl ether (70:30)
(10 mL). The recovered solid was dried under reduced pres-
sure for 1–2 h and was used as catalyst for recycle experi-
ments of the AKR reaction of trans-stilbene oxide 11 as rep-
resentative substrate with aniline 12a as nucleophile.
Acknowledgements
K. J. Prathap and RIK 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 instrumental facilities.
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