Highly catalytic enantioselective reduction of aromatic ketones using chiral polymer-supported Corey, Bakshi, and Shibata catalysts

Article abstract of DOI:10.1016/j.tetasy.2004.03.012

A systematic study was conducted to formulate the optimal reaction parameters for polymer-supported (PS)-oxazaborolidine catalyzed enantioselective ketone reduction. The B-methylated chiral oxazaborolidine prepared in situ from the previously reported polymers by Degni et al. have been used in the enantioselective borane reduction of some substituted aromatic ketones to afford the corresponding optical active secondary alcohol products. While the linear-bound system shows low enantioselectivity, the cross-linked version affords enantioselectivities almost identical to those of the monomeric model (with up to 96% enantiomeric excesses).

Full text of DOI:10.1016/j.tetasy.2004.03.012

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 1495–1499
Highly catalytic enantioselective reduction of aromatic ketones
using chiral polymer-supported Corey, Bakshi, and Shibata
catalysts
*
Sylvestre Degni, Carl-Eric Wil ꢀe n and Ari Rosling
ꢁ
ꢁ
Laboratory of Polymer Technology, Abo Akademi University, Abo, FIN-20500 Turku, Finland
Received 26 February 2004; accepted 10 March 2004
Available online 15 April 2004
Abstract—A systematic study was conducted to formulate the optimal reaction parameters for polymer-supported (PS)-oxaza-
borolidine catalyzed enantioselective ketone reduction. The B-methylated chiral oxazaborolidine prepared in situ from the previ-
ously reported polymers by Degni et al. have been used in the enantioselective borane reduction of some substituted aromatic
ketones to afford the corresponding optical active secondary alcohol products. While the linear-bound system shows low enantio-
selectivity, the cross-linked version affords enantioselectivities almost identical to those of the monomeric model (with up to 96%
enantiomeric excesses).
ꢀ
2004 Elsevier Ltd. All rights reserved.
1
. Introduction
challenging target in asymmetric synthesis. One ap-
proach to reduce the product-specific catalyst costs is to
use the chiral catalyst repeatedly. In order to avoid
lengthly recovery and purification, several polymer-
The enantioselective reduction of prochiral ketones
leading to the corresponding optically active secondary
alcohols is a topic of current interest. One of the most
successful methods has been based on the use of chiral
1
5
bound heterogeneous catalysts have been prepared.
2;3
5e;6
1
,3,2-oxazaborolidines 2, as catalysts (Scheme 1).
In previous papers, Degni et al.
anchoring of chiral
described the facile
-prolinol derived amino alcohol
L
2
This method was developed by Itsuno and co-workers
3
ligands on mechanically ‘stable’ and chemically inert
polyethylene (PE) fibers by electron beam (EB) preir-
radiation induced graft co-polymerization. Chiral
styrenic ligand derivatives such as 3 and 4 (Fig. 1) were
successfully immobilized on the fibrous support to
generate the corresponding polymeric ligands P1 and P2
(Fig. 2) and were then employed in the reduction of
prochiral ketones with borane from different sources.
and further improved by Corey et al. (CBS reduc-
tion ). Over the past decade, numerous examples
describing the application of this method have been
3a
4
reported by several groups. Some of these catalysts
have been extensively studied with great success, how-
ever the development of cost-effective catalysts that
exhibit high reactivity and enantioselectivity is still a
Ph
Ph
OH
Ph
Ph
O
B
Borane
N
NH
R
OH
N
1
2a: R = H
b: R = CH3
2
OH
N
H
Scheme 1. Some oxazaborolidine catalysts.
3
4
*
0
Corresponding author. Tel.: +358-2-2154992; fax: +358-2-2154866;
e-mail: sdegni@abo.fi
Figure 1. Chiral styrenic ligands 3 and 4.
957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.03.012

1496
S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 1495–1499
O
OH
R2
R2
1
0 mol% Cat*
OH
+ BH .SMe2
3
R1
N
THF
R1
R or S
OH
6
7
8
: R1 = H, R2 = CH3
N
H
: R1 = OCH , R2 = CH3
3
: R1 = OCH , R2 = CH Cl
3
2
P1
P2
9: R1 = CH , R2 = CH3
3
Scheme 2. Me-CBS reduction of substituted aromatic ketones leading
to the corresponding optically active secondary alcohols.
Figure 2. Previously described fibrous chiral polymer P1 and P2.
For example, the fibrous ligand P1 gave 1-phenyletha-
nol in 99% yield and 5–6% ee whereas, the monomer
can be expected to increase as the enantiomeric excesses
(ees) achieved increase and the range of reactions, which
afford high ees increases.
5e
5e
analogue 3 in the presence of NaBH /Me SiCl com-
4
3
bination gave the secondary alcohol in 98% yield and
4% ee in the reduction of acetophenone.
9
2. Results and discussion
It has been reported that diborane can be generated by
5
e
7
treating NaBH
to the best of our knowledge, we report the first asym-
metric reduction of acetophenone by employing NaBH
BF ÆOEt as reducing agent, and chiral ligand 3 as the
catalyst (Table 1, entry 8).
4
with Me
3
SiCl or BF
3
ÆOEt
2
. Herein,
The syntheses of all chiral monomeric ligands, together
with chiral polymeric ligands P1 and P2 presented in
this paper are described previously. We have now
turned our attention to the preparation of their corre-
5e;6
4
/
3
2
sponding oxazaborolidine catalysts using BH ÆSMe . A
3
2
survey of the literature reveals that a variety of condi-
tions have been recommended for the preparation of
different oxazaborolidines, for example, with –BH sub-
stituent, by stirring the amino alcohol at 35 ꢁC with
To take our investigation of this general approach a step
further, we prepared, in situ, some polymer-supported
oxazaborolidines, together with their nonpolymeric
3a
analogues using BH
3
ÆSMe
2
as the borane source. The
2 equiv of BH ÆTHF followed by sublimation, reflux-
3
various catalysts were used to catalyze the asymmetric
reduction of substituted aromatic ketones (Scheme 2).
The results of this study are presented herein.
ing the amino alcohol with 2 equiv of BH ÆTHF under
moderate pressure (1.7 bar), and more recently, stirring
3
9
a mixture of amino alcohol with excess of BH ÆSMe at
3
2
10
room temperature for 10 h. Clearly the last mentioned
procedure could provide the most convenient route for
laboratory as well as an industrial scale procedure. We
thought that this procedure could work well for the
preparation of simple oxazaborolidines, but could not
The use of polymer-supported (PS) chiral catalysts to
achieve asymmetric synthesis promises to become a
major area of development, which is of particular
8
interest to the pharmaceutical industry. The interest
Table 1. Enantioselective reduction of substituted aromatic ketones at 45 ꢁC (or otherwise mentioned)
a
c
Entry
Catalyst
Borane source ‘BH
3
’
Ketone
Yield (%) of product
Ee %
b
5e
1
2
3
4
5
6
7
8
9
2a
2a
2b
2b
2b
2b
3a
3a
4a
4b
5
NaBH
NaBH
4
/Me
/BF
3
SiCl
ÆOEt
6
6
6
7
8
9
6
6
6
6
6
6
6
6
8
8
9
6
Quant.
98 (Reflux)
96
95.1
7
4
3
2
SMe
SMe
SMe
SMe
2
2
2
2
99
90
98
95
98
94
78
80
5e
NaBH
NaBH
NaBH
4
4
4
/Me
3
SiCl
98
78
94
/BF
/Me
3
ÆOEt
2
70
28
17
95
3
SiCl
>99 (Reflux)
b
10
11
12
13
14
15
16
17
18
SMe
SMe
2
Quant.
Quant.
b
2
5e
P1a
P1b
NaBH
4
/Me
3
SiCl
99 (rt)
90
6
d
SMe
SMe
SMe
SMe
SMe
SMe
2
2
2
2
2
2
95.2
d
78
ꢀ
P1b
P1b
90
Quant.
b
88
86
96
44
ꢀ
b
P1b
Quant.
P1b
P2b
57
99
a
b
c
Isolated yield.
No ketone was detected by GC or H NMR.
1
Enantiomeric excess (ee) was determined by HPLC.
Enantiomeric excess (ee) was determined by CGC.
Reuse.
d
*

S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 1495–1499
1497
be satisfactory on our
We believe that the formation of 2, which involves a
L
-prolinol-based fiber catalysts.
et al. investigation however fell short of describing ‘the
ideal’ structure of an oxazaborolidine structure.
strained [3.3.0] fused ring system, would be incomplete
under the conditions described.
11
According to the literature and our results, the conclu-
sion may be drawn that the enantioselectivity of oxaz-
aborolidine-catalyzed borane reduction of ketones is
dependent on both catalyst structure and the tempera-
ture of the reduction (Table 1, entries 12 and 13). It is
difficult to provide a straightforward explanation in the
case of 4b and P2b (Table 1, entries 10 and 18, respec-
tively), because in general, the enantioselectivity of this
asymmetric reduction is likely to be a result of many
The reaction of a homochiral 1,2-substituted amino
alcohol in the presence of excess BH is likely to proceed
3
as described in Scheme 3. The excess borane renders the
nitrogen atom acidic and cyclization takes place easily.
Such a mode of cyclization explains why the interme-
diate ‘ate’-complex obtained by using 1 equiv of borane
12
requires prolonged heating at 100 ꢁC to cyclize,
whereas the cyclization is facile in the presence of excess
15a
complex factors.
10
borane.
15b
Zhou et al. have prepared and used amino alcohols of
squaric acid as examples of bifunctional chiral catalysts
for the reduction of prochiral ketones to the corre-
sponding optical active secondary alcohols. Those amino
alcohols of squaric acid could be considered as another
substituted pattern of 1 and 5 with the characteristics of
having in the substituent groups, heteroatoms that could
effectively coordinate with BH . We therefore believed
3
that the second functional group (the ester group) in 5
(Fig. 3) could also preferentially coordinate with borane
At the outset, we decided to use a 2 M solution of
BH ÆSMe in THF. The choice was based on the con-
3
2
sideration that the fiber-supported ligand swelled well in
THF. As we have mentioned earlier, the temperature at
which the reduction should be conducted, remains
4
c;13
controversial,
study dealing with the preparation of –BMe and –BPh
although there has been a definitive
14
derivatives of oxazaborolidine.
When we stirred the amino alcohol ( -prolinol deriva-
tives) with an excess of BH ÆSMe at 45 ꢁC for 16–22 h
L
and lead to an intramolecular delivery of hydride to a
carbonyl group in a selective manner.
3
2
(monomers) and 24 h (polymers). The polymer catalyst
P1b thus obtained, reduced acetophenone 6 providing
the corresponding secondary alcohol in 90% yield and
9
1
5.2% ee and ketone 9 in 57% yield and 96% ee using
0 mol % of the supported-oxazaborolidine catalyst
OH
N
(
Table 1, entries 13 and 17, respectively) compared to
O
the soluble analogue 2b (99% yield, 98% ee for ketone 6
and 80% yield, 94% ee for ketone 9 (Table 1, entries 3
and 6, respectively). Thus, the aforementioned chiral
oxazaborolidines, easily prepared by the reaction of the
O
5
Figure 3. Chiral amino alcohol 5.
corresponding amino alcohols with BH
3
2
ÆSMe complex
according to our optimized reaction conditions, reduced
the substituted aromatic ketones 6–9 in high yields and
high enantioselectivities.
Thus applying the same reaction conditions as men-
tioned above, 10 mol % of 5b prepared using 5 in the
presence of BH
3
2
ÆSMe , was found to reduce acetophen-
one, 6 to the corresponding secondary alcohol in
quantitative yield and 95% ee. This result implies that
the oxygen atoms in the N-substituent of 5 has effec-
Catalysts 2b and 4b (prepared, respectively, from 2 and 4
with BH
3
2
ÆSMe ) were generated in situ and used as
models in order to test the influence of substitution on
the pyrrolidine ring nitrogen on the enantioselectivity of
the reduction. As shown in Table 1 (entries 3 and 10),
the enantioselectivity obtained with 2b (98% ee) is far
higher than that reached with 4b (17% ee). Obviously the
substitution on the nitrogen atom has created an unfa-
3
tively coordinated with BH and has directed the
hydride to approach selectively one of the prochiral
faces of the carbonyl group in the substrate ketone.
We are not the first to work toward a polymer-bound
oxazaborolidine catalysts. Itsuno et al. have pub-
4f
5a;b
vorable interaction in the case of ligand 4. This would
lead to a weaker coordination of borane (BH ) to the
nitrogen atom and hence slower catalysis. Quallich
lished
their preparation of several ligands covalently
3
bonded to various polymer supports and subsequent
generation of oxazaborolidine catalysts for the reduc-
tion of ketones, imines, and oximes. But as we men-
tioned earlier, here we describe the first attempt to
develop such an optimal catalyst by preparing a poly-
ethylene (PE)-bound oxazaborolidine.
4f
et al. have done a nice study dealing with the oxaz-
aborolidine structure and enantioselectivity. Even
though, the pyrrolidine moiety appears to be the ideal
substitution for oxazaborolidine catalysts, the Quallich
*
*
*
+
HN
slow
O
fast
-H
+
-H
cat.*
H N
2
OH +2BH
3
3
B. H
N
2
OBH
2
B
3
B
H
Scheme 3. Mechanism of the catalyst formation.

1498
S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 1495–1499
5
a;b
Though the results obtained by Itsuno et al. and Caze
distilled over sodium benzophenone ketyl. All other
commercially available chemicals and solvents used were
of puriss p.a. quality, or purified and dried according to
standard methods. TLC: precoated silica gel 60 F254
5c
et al. are comparable to ours, our approaches are not
related, in terms of the support, polymer synthesis and
polymer-bound oxazaborolidine catalyst preparation.
(Merck); visualization by irradiation with UV light. Flash
chromatography (FC): silica gel 60 (0.04–0.063, Merck).
H NMR spectra were recorded on Bruker AC 250 MHz
Our goal was to develop an environmental-friendly sta-
ble PE-ligand with potential for industrial use (scale-up
application) as well as university laboratories applica-
tions (small scale) concerned with simplicity of recovery.
1
at rt with a broad band probe. Enantiomeric excesses
(ees) were determined either by capillary gas chroma-
tography (CGC) on an HP-5 GC (Crosslinked 5% PH
ME siloxane, column: 15 m ꢁ 0.53 mm ꢁ 1.5 lm film b-
cyclodextrin, 30 m · 0.25 mm · 1.5 lm film thickness) or
by HPLC using an HP 1090 Liquid chromatograph sys-
tem equipped with a Chiralcel OD column (Daicel
Chemical Industries), 5% isopropanol in hexane mixture
as mobile phase and detection by UV–vis detector at
254 nm. GC/MS analyzes were performed using an HP-
5890 SERIES II Gas chromatograph equipped with a
3. Conclusion
In conclusion, we have prepared and examined the
performance of what we believe are the first two poly-
ethylene-grafted CBS catalysts. The readily accessible
crosslinked fibrous P1 is clearly superior to the N-
substituted pendant-linked P2 ligand and comparable to
the conventional solution-phase ligands 1, 2, and 5 in its
ability to direct stereoselective ketone reduction. While
several factors may contribute to these observed differ-
ences, the substitution on the nitrogen atom character-
istic of P2 relative to P1 would create unfavorable
interaction and lead to weaker coordination of borane
5971
A
mass
selective
detector
(column:
30 m ꢁ 0.25 mm ꢁ 0.25 lm-HP-1MS).
4
.2. General procedure for the soluble CBS–oxazaborol-
idine-catalyzed borane reduction of subsubstituted aro-
matic prochiral ketones
(
3
–BH ) to the nitrogen atom and hence slower catalysis.
Probably it is for this reason, the unsubstituted N-pyr-
rolidine moiety appears to be the ideal substitution for
an oxazaborolidine catalyst. Also, catalyst structure as
well as temperature effects are non-neglectable factors in
the reduction systems. We were surprised with results
obtained using the monomer chiral ligand 4 compared
with its polymeric analogue P2. We hope that our
ongoing work will allow us to explain these observa-
tions.
To a solution of the soluble ligand (10 mol %) in THF
(
(
10 mL) was added a solution of borane-dimethyl sulfide
BH ÆSMe 2 M in THF solution, 1.3 equiv) at rt. The
3
2
solution was stirred at 45 ꢁC for 16–18 h under argon
atmosphere. To this mixture was added slowly a solu-
tion of the ketone (1 equiv in 5 mL of THF) at 45 ꢁC for
3
h. After the addition, the reaction mixture was stirred
for 50 min, then cooled to rt. The solution was then
cautiously quenched at 0 ꢁC with saturated ammonium
chloride solution (15 mL) and extracted with diethyl
ether (3 ꢁ 20 mL). The extracts were combined and
washed with brine (2 ꢁ 10 mL). The solvent was evapo-
rated under reduced pressure. All crude residues were
further purified by flash column chromatography on
5e;6
As we have already demonstrated in earlier work, the
chiral ligands grafted on the polyethylene fiber P1 and
P2 are recyclable and reusable without significant loss of
activity and selectivity. As we aimed to improve the use
of these catalysts for continuous asymmetric reactions,
we once more demonstrated the reusability of, especially
P1 in repetitive enantioselective reduction of substituted
aromatic ketones to synthetically very important chiral
secondary alcohol intermediates.
3
eluent. All the secondary alcohols were obtained in rel-
0 g of silica gel using diethyl ether–n-pentane (40:60) as
1
16;17
atively good yields (see Table 1). H NMR data
matched with authentic samples. Enantiomeric excesses
of all alcohols were determined by HPLC analysis using
Chiralcel OD chiral column, isopropanol–n-hex-
17
ane ¼ 5:95; 0.5 mL/min, UV 254 nm.
4. Experimental
18
As a general rule, the use of the (R)-enantiomer of the
catalysts gives the (S)-configured alcohol and vice versa.
4
.1. General
4c;5f;19
According to published work,
(R)-configured
All reactions were carried out under argon atmosphere
using flame-dried glasswares. Borane-dimethyl sulfide
alcohols are obtained for the reduction of ketones 6, 7,
and 9; subsequently, the (S)-configured alcohol is
obtained for the reduction of 4-methoxy-a-chloroace-
(
BH
Acros Organics, NaBH
8% BF ) were purchased from Acros Organics and used
3
ÆSMe
2
a 2 M solution in THF) was purchased from
4
powder 98% and BF ÆOEt (ca.
3
2
1
;7;8;19;20
tophenone
when using the (S)-enantiomer of the
4
3
catalysts derived from (S)-prolinol.
3
as received. Me ClSi, 99% was purchased from ABCR
and used as received. The chiral reference amino alcohol 1
was purchased from Fluka. Synthesis of 3, 5, and P1 have
been described in our previous paper. The synthesis of
ligands 4 and P2 is reported in Ref. 6. Ketones 6 (99%), 7
4.3. General procedure for the PS CBS–oxazaborolidine-
catalyzed borane reduction of subsubstituted aromatic
prochiral ketones
5e
(
98%), 8 (96.5%) were purchased from Acros Organics
and used as received. Ketone 9 (97%) was purchased from
Riedel-de H €a en and used as received. THF was freshly
To a solution of the PS ligand (10 mol %) in THF
(15 mL) was added a solution of Borane-dimethyl sulfide

S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 1495–1499
1499
(
BH
3
ÆSMe
2
2 M in THF solution, 1.3 equiv) at rt. The
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3
h. After the addition, the reaction mixture was stirred
for 50 min, then cooled to rt. The solution was then
cautiously quenched at 0 ꢁC with methanol solution
(
15 mL) and filter (glass filter no 4). The fiber was
washed with methanol (50 mL). The filtrate was evapo-
rated and the residue was dissolved in diethyl ether
(
20 mL). The ether phase was washed saturated ammo-
nium chloride and extracted with diethyl ether
3 ꢁ 20 mL). The extracts were combined and washed
with brine (2 ꢁ 10 mL) then dried over anhydrous
MgSO The solvent was evaporated under reduced
5. (a) Itsuno, S.; Sakurai, Y.; Shimizu, K.; Ito, K. J. Chem.
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1
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4
3
pressure. The alcohols were purified and identified as
above mentioned.
2
2
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(
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. Degni, S.; Wilen, C.-E.; Leino, R. Tetrahedron: Asymme-
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100 mL), methanol (100 mL) and dried under high
vacuum at rt for several hours, then reused in repetitive
runs.
6
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8
. Hu, J.-B.; Zhao, G.; Ding, Z.-D. Angew. Chem., Int. Ed.
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This work was supported by Svenska Tekniska
Vetenskapsakademin. S.D. is grateful to Svenska
Tekniska Vetenskapsakademin for a postgraduate
fellowship. The authors thank Robert Peltonen,
Kenneth Ekman, and Mats Sundell (Smoptech Ltd) for
supplying the polymer fibers and for carrying out the
preirradiation grafting experiments. Andrei Pranovich is
thanked for his skillful assistance with HPLC analyzes.
1
1
1
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