A new catalytic enantioselective reducing reagent system from (-)-α,α-diphenylpyrrolidinemethanol and 9-borabicyclo[3.3.1]nonane, especially effective for hindered and substituted aralkylketones

Article abstract of DOI:10.1016/S0040-4020(01)01213-3

New catalytic enantioselective reduction systems were prepared from aminoalcohols and dialkylboranes, for the enantioselective reductions of prochiral aromatic ketones. Among these, the system prepared from (-)-α,α-diphenylpyrrolidinemethanol with 9-borabicyclo[3.3.1]nonane proved especially promising for such reductions. This complex catalyzes the reduction of prochiral aralkyl ketones to the corresponding alcohols with BH₃-THF, with enantioselectivities 82-99.2%. Also, this catalyst is particularly effective for the more hindered and substituted aralkyl ketones. Various modifications in this new catalytic reduction system, such as changing reaction conditions, reducing agent and dialkylborane, were also examined.

Full text of DOI:10.1016/S0040-4020(01)01213-3

TETRAHEDRON
Pergamon
Tetrahedron 58 '2002) 1069±1074
A new catalytic enantioselective reducing reagent system
from ꢀ2)-a,a-diphenylpyrrolidinemethanol and
-borabicyclo[3.3.1]nonane, especially effective for hindered
9
q
and substituted aralkylketones
p
Josyula V. B. Kanth and Herbert C. Brown
Department of Chemistry, H. C. Brown Center for Borane Research, Purdue University, 1393 Brown Building, West Lafayette,
IN 47907, USA
Received 2 June 2001; accepted 12 June 2001
AbstractÐNew catalytic enantioselective reduction systems were prepared from aminoalcohols and dialkylboranes, for the enantioselective
reductions of prochiral aromatic ketones. Among these, the system prepared from '2)-a,a-diphenylpyrrolidinemethanol with 9-borabi-
cyclo[3.3.1]nonane proved especially promising for such reductions. This complex catalyzes the reduction of prochiral aralkyl ketones to the
corresponding alcohols with BH ±THF, with enantioselectivities 82±99.2%. Also, this catalyst is particularly effective for the more hindered
3
and substituted aralkyl ketones. Various modi®cations in this new catalytic reduction system, such as changing reaction conditions, reducing
agent and dialkylborane, were also examined. q 2002 Elsevier Science Ltd. All rights reserved.
Asymmetric reduction of prochiral ketones to obtain the
corresponding optically active alcohols constitutes a very
important step in the synthesis of several medicinally
important compounds. The discovery of oxazaborolidines
as catalytic reagents for enantioselective reduction of
prochiral ketones by borane in tetrahydrofuran has been
an important milestone in organic synthesis. The `enzyme
likeꢀ nature of these reagents has attracted the attention of
chemists all over the world and several new reagents
In continuation of our efforts in asymmetric organic syn-
thesis, we became interested in developing a more conve-
nient reagent system for such catalytic asymmetric
reductions. The 9-borabicyclo[3.3.1]nonane '9-BBN) is a
very useful bulky dialkylborane reagent. Its usage in
conjunction with some chiral auxiliaries produced excellent
w 4
asymmetric reduction agents, such as Alpine Borane .
Though oxazaborolidines, prepared from aminoalcohols
and alkylboronic acids, have proven to be excellent for
asymmetric reductions, care must be taken to remove
water completely from such condensations to avoid
1
derived from various amino alcohols have been reported.
The aminoalcohol '2)-a,a-diphenylpyrrolidinemethanol
5
'
1), derived from l-proline has been widely accepted as
undesired effects. It occurred to us that the catalyst
prepared from aminoalcohol '1) and 9-BBNwould elimi-
nate hydrogen, not water, and thereby avoid these problems.
2
,3
2
superior. However, the high synthetic utility of these
oxazaborolidines in asymmetric synthesis necessitates the
development of a reagent system that would serve as a
general enantioselective reducing system for various classes
3
of prochiral ketones.
ꢀ
1†
q
Reaction of '2)-a,a-diphenylpyrrolidinemethanol '1 equiv.)
with 9-BBN'0.5 equiv., dimer), rapidly evolves 1 equiv. of
hydrogen. The B NMR examination of the reaction
mixture showed the disappearance of the peak due to
Communication number 16 from H. C. Brown Center for Borane
Research, Purdue University.
This work was presented as part of a paper 'ORGN125) presentation at
the 217th National meeting of the American Chemical Society, Anaheim,
CA, March 22, 1999.
1
1
9-BBN'at d 127 ppm) and the appearance of a new peak
at d 18.3 ppm 'singlet). Evaporation of the solvent gave a
Keywords: chiral aminoalcohol; dialkylborane; enantioselective reductions;
aromatic ketones; borane±Lewis base complexes.
1
13
white solid, mp 120±1258C, with the spectral data ' H,
NMR and mass) in accordance with the above structure 3.
C
p
Corresponding author. Tel.: 11-765-494-5316; fax: 11-765-494-0239;
e-mail: hcbrown@purdue.edu
This compound is stable in the presence of BH ±THF with
3
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020'01)01213-3

1070
J. V. B. Kanth, H. C. Brown / Tetrahedron 58 ꢀ2002) 1069±1074
Scheme 1.
1
1
6
no N!B bond dissociation noted in B NMR examination.
'4) with 9-BBNreadily evolved 1 equiv. of hydrogen. The
B NMR examination showed the presence of a mixture of
both the N±B coordinated cyclic oxazaborolidine type `ateꢀ
complex 8 '16.8 ppm) and the dialkyloxyborane 9
'156 ppm), indicating incomplete formation of the desired
1
1
Upon the addition of acetophenone to the complex '3) in the
presence of BH ±THF, the ketone was reduced to the corre-
sponding alcohol within 5 min and the enantiomeric purity
of the 'R)-alcohol obtained was 83% ee 'Eq. '2)).
3
catalyst 'Scheme 1). This mixture when treated with H B±
3
THF and acetophenone, gave the corresponding alcohol in
only 68% ee.
The reaction of 9-BBNwith N-isopropyl-'1R,2S)-nore-
phedrin '5) gave the N±B coordinated ate complex essen-
tially quantitatively ' B NMR: 112.5). However, with the
'2)
1
1
addition of H B±THF, the N±B coordination completely
3
1
disappeared, as revealed by B NMR examination, which
1
showed signals corresponding to sN±BH 'q, 220.4 ppm),
3
dialkyloxyborane '156 ppm) and the signal at 112.5
completely vanished. The reduction of acetophenone with
the resulting reagent system is very slow '48 h) and the
enantioselectivity of the alcohol obtained is only 64.3%.
Similar reactions with S'2)-N-benzyl-a,a-diphenylpyrro-
lidinemethanol '7) also gave 2-phenylethanol of 52.4% ee.
The enantioselectivity obtained in the reduction of aceto-
phenone with 'S)-prolinol '6), 9-BBNand H 3B±THF
system is comparable to that obtained for '2)-a,a-diphe-
In order to achieve improved enantioselectivities, various
modi®cations were made on the reagent system. Initially,
several other amino alcohols '4±7) were tested as chiral
auxiliaries.
nylpyrrolidinemethanol, 9-BBNand H B±THF, 83% ee.
3
However, with the 'S)-prolinol system, the reduction is
very slow and it is only 90% complete even after 48 h at
room temperature. The results obtained with various amino
alcohols, are summarized in Table 1.
With all the above amino alcohols, active asymmetric
reduction catalysts were prepared by mixing the amino
alcohol and 9-BBNin THF and the formation of the desired
borinate complex, con®rmed by measuring the hydrogen
gas evolved and examining the B NMR spectrum of the
reaction mixtures. The reaction of S-'2)-diphenylvalinol
From these initial results, it is clearly evident that the
reagent system prepared from '2)-a,a-diphenylpyrrolidine-
methanol and 9-BBNdiffers from the other derivatives in
existing as the oxazaborolidine complex '3). Moreover, it is
1
1
Table 1. Enantioselective reduction of acetophenone with H B/THF in the
presence of catalyst prepared from various aminoalcohols and 9-BBN
3
stable to H B±THF. Consequently, we selected this system
3
for further study and possible optimization.
a
S. No
Amino-alcohol
Reaction conditions
Ee '%)
Initially, a number of experiments were carried out to see
whether a change in reaction temperature would enhance
the enantioselectivity. Unfortunately, no improvement in
enantioselectivity was noted with change in temperature.
Only the reduction time changed with the temperature
change, with more time required at lower temperatures.
1
2
3
4
5
1
4
5
6
7
Rt/0.05 h
Rt/0.05 h
Rt/48 h
Rt/48 h
Rt/0.5 h
83.2
68.2
59.0
83.0
52.4
b
To THF solution of amino alcohol '1 equiv.), 9-BBN'0.5 equiv., dimer)
was added and H B±THF '0.6 equiv.) was added after the hydrogen evolu-
tion is complete '2±3 h) followed by acetophenone '1 equiv.).
3
Also, in order to examine the catalytic ef®ciency of the
present reagent system, the reduction of acetophenone
a
Enantiomeric excess was established using chiral HPLC analysis of the
alcohol product on Chiralcel-OD column.
Reduction is only 90% complete after 48 h.
'1 equiv.) was carried out in the presence of varying
amounts of the catalyst 3 with BH ±THF. In all these
b
3

J. V. B. Kanth, H. C. Brown / Tetrahedron 58 ꢀ2002) 1069±1074
1071
Table 2. Effect of temperature and amount of catalyst on the reduction of acetophenone using H
3
B/THF in the presence of catalyst 'S)-3
a
S. No
Amount of catalyst '3) equiv.
Reaction temperature '8C)
Reaction time 'h)
Ee '%)
1
2
3
4
5
6
7
8
1.0
1.0
1.0
1.0
0.5
0.25
0.1
0.05
278
215
0
25
0
0
0
0
2.0
1.5
83.2
82.8
83.1
83.5
83.2
82.5
82.7
79.2
0.25
0.05
0.25
0.25
0.25
0.25
3
To THF solution of 3 '0.1 equiv.), H B±THF '0.6 equiv.) was added followed by acetophenone '1.0 equiv.).
a
Enantiomeric excess was established using chiral HPLC analysis of the alcohol product on Chiralcel-OD column.
cases, the reductions are instantaneous with up to 10 mol%
of the catalyst, the enantioselectivity achieved was in the
neighborhood of 82%. However, lowering the amount of
catalyst further affected the enantioselectivity only margin-
ally. Table 2 summarizes the results.
ꢀ
3†
Consequently, in all of these experiments, the enantio-
selectivity achieved for the acetophenone reduction is at
best moderate. Unexpectedly we discovered that the reduc-
tion of certain modi®ed aralkyl ketones gave considerably
improved enantioselectivities when compared to acetophe-
none under optimized conditions. For example, the rela-
tively hindered isobutyrophenone 'entry 3, Table 3) gave
0
Table 4. Effect of electronic and steric factors on asymmetric reductions
using the catalyst 'S)-3
8
9.3% ee and 1 -acetonaphthone 'entry 7, Table 3) gave
9.2% ee.
9
a
Ee '%) 'con®g)
b
S. No
1
Aralkyl ketone
95.5 'S)
93.2 'R)
93.1 'S)
94.7 'R)
91.0 'R)
91.5 'R)
Table 3. Effect of steric factor on asymmetric reductions using the catalyst
S)-3
'
a
Ee '%) 'con®g)
b
S. No
1
Aralkyl ketone
2
3
4
5
6
83.2 'R)
82.1 'R)
89.3 'R)
91.5 'S)
87.5 'R)
2
3
4
5
7
8
91.2 'R)
73.2 'R)
6
7
94.5 'R)
99.2 'R)
To THF solution of 3 '0.1 equiv.), H
followed by aralkyl ketone '1.0 equiv.) at 08C.
3
B±THF '0.6 equiv.) was added
3
To THF solution of 3 '0.1 equiv.), H B±THF '0.6 equiv.) was added
followed by aralkyl ketone '1.0 equiv.) at 08C.
Enantiomeric excess was established using chiral HPLC analysis of the
a
a
Enantiomeric excess was established using chiral HPLC analysis of the
alcohol product on Chiralcel-OD column.
alcohol product on Chiralcel-OD column.
Con®guration was established by comparing sign of rotation with litera-
ture reports.
b
b
Con®guration was established by comparing sign of rotation with litera-
ture reports.

1072
J. V. B. Kanth, H. C. Brown / Tetrahedron 58 ꢀ2002) 1069±1074
Scheme 2.
Accordingly, we undertook a systematic study of the effect
of the steric factor in these enantioselective reductions with
increasing bulk of both the alkyl and the aryl groups
would give improved selectivities. Thus, in a bid to ®nd a
better reducing agent, various other borane reagents were
tried in the reduction of acetophenone in the presence of
catalyst 3.
'
Eq. '3)). The results are presented in Table 3. Also,
changing the electronic factors on the aralkyl ketone also
has considerable effect on the enantioselectivity. Halogen
substitution on the alkyl side chain 'entries 1±3, Table 4) or
on the aromatic ring 'entries 5±7, Table 4) achieves con-
siderable improvement in enantioselectivity. Only 2,3,4,5,6-
penta¯uoroacetophenone, F C COCH , gave poorer results.
The reduction of acetophenone using N,N-diethylaniline-
borane is very slow at 08C. When H B±THF was replaced
3
with N,N-diethylaniline±borane in catalytic enantio-
selective reductions using the catalyst 3, the reduction is
complete in 6 h and the corresponding alcohol was obtained
in slightly improved ee '89%). However, with other slow
reducing amine±boranes, such as triethylamine±borane and
N,N-diisopropylethylamine±borane, the reduction is very
slow at 08C. When the reductions were carried at higher
temperatures, the enantioselectivity realized was poor.
5
6
3
These results are summarized in Table 4.
1
. Asymmetric reduction of acetophenone using various
reducing reagents in the presence of catalyst 3
Fortunately, dimethyl sul®de±BH gave the best results,
3
As mentioned earlier, the reduction of acetophenone using
H B±THF in the presence of the catalyst prepared from
providing the 2-phenylethanol of 95.3% ee. The results
are summarized in Table 5.
3
a,a-diphenylpyrrolidinemethanol and 9-BBN, give only
8
3% ee under varying conditions. It was thought that a
better understanding of the mechanism for this reaction
might provide clues for the improvement of this reagent
We have also tried to improve the catalyst by changing the
dialkylborane. Thus, catalysts were prepared from diphenyl-
prolinol and several other dialkylborane reagents in addition
to 9-BBN. Dicyclohexylborane and both isomers of diiso-
pinocamphylborane were used in these studies.
1
1
system. Accordingly, we have carried out several
B
NMR studies during the course of the reaction at various
stages. From these studies, the following may be a plausible
path for the reaction 'Scheme 2).
Both '1) and '2) isomers of diisopinocamphylborane
readily reacted with the aminoalcohol 1. However, the
catalyst thus obtained is unstable and a-pinene is elimi-
nated. Addition of H B±THF followed by acetophenone
3
provided the alcohol in only 65% ee. On the other hand,
reaction with dicyclohexylborane gave a stable catalyst and
The formation of an intermediate wherein borane is
complexed with the nitrogen atom '3%) of the diphenyl-
prolinol may provide a handle for the interaction of the
carbonyl group with the boron atom of the 9-BBNstructure.
The hydride may be delivered internally from the amine±
reduction of acetophenone with H
of this catalyst gave 2-phenylethanol in 87% ee 'Eq. '4)).
B±THF in the presence
3
borane or externally from H B±THF present in the system.
3
In the case of other aminoalcohols, such as diphenylvalinol
'4) and prolinol '6), the interaction between the the nitrogen
and boron atom of the of 9-BBNmoiety is disturbed and the
major product obtained after the addition of H B±THF to
3
the catalyst is the sN±BH adduct, which reduced aceto-
3
ꢀ4†
phenone very slowly. The faster reaction for the catalyst
derived from diphenylprolinol indicates external hydride
delivery from H B±THF. The low enantioselectivity in
3
the case of acetophenone is probably due to the competitive
uncatalysed reduction of acetophenone using H B±THF. It
3
was of interest to see whether other less reactive reducing
agents and other borane reagents having single hydride
From the current study it is quite evident that the reagent

J. V. B. Kanth, H. C. Brown / Tetrahedron 58 ꢀ2002) 1069±1074
1073
Table 5. Reduction of acetophenone using various reducing reagents in the
presence of catalyst 3
chemical shifts are in d relative to boron tri¯uoride-diethyl
etherate. HPLC analyses were carried out using Chiralcel-
w
OD column. Optical rotations were measured on a polari-
meter.
S. No.
1
Reducing agent
BH /THF
Reaction conditions
Ee '%)
83.2
3
08C/10 min
2
¯
.1.1. Preparation of the catalyst ꢀ3). An oven-dried RB
ask provided with a septum inlet and stirring bar, was
2
3
4
08C/6 h
08C/4 h
89.2
62.5
62
cooled under dry nitrogen gas. Into the ¯ask was placed,
aminoalcohol 1 '10 mmol) and 9-BBN'5 mmol, dimer) in
dry THF '10 mL). The outlet of the ¯ask was connected to a
gasimeter. The contents were stirred at room temperature
for 1 h, by which time the evolution of one equivalent
3 2 2
'CH ) S±BH Cl
a
11
508C/2 h
of hydrogen is complete and B NMR examination
'17.8 ppm, singlet) showed the clean formation of the
catalyst 3, white solid, mp 120±1258C. The IR spectrum
didnꢀt show the characteristic hydroxy stretch around
b
5
6
Et
BH
3
N±BH
±DMS
3
508C/2 h
08C/5 h
52
95.3
3
21
13
3350 cm , for starting aminoalcohol. C N MR 'd ): 24.2,
24.7, 24.8, 28.4, 31.4, 32.1, 32.9, 33.3, 33.7, 47.4, 69.7,
82.5, 126.0, 126.1, 126.5, 127.7, 127.8, 147.3, 150.0.
7
8
9
08C/5 h
75.2
72
Mass '70 eV, EI CI): 374 'M11, 64.82%), 373 'M1,
6.98%), 372 '31.81%), 254 'base peak, 100%).
2
08C/2 h
08C/2 h
The catalyst 3, thus obtained is inde®nitely stable under
inert atmosphere. It is moderately air and moisture sensitive.
For the catalytic asymmetric reductions, the catalyst does
not require any further puri®cation. Pre-formed catalyst '3)
as well as in situ generated catalyst '3) gave comparable
results in the asymmetric reduction of acetophenone.
Accordingly, in majority of the cases it was prepared in
situ and used immediately for the reductions. However,
with the other amino alcohols '4±7), reaction with 9-BBN
gave mixture of products 'as described in Scheme 2) and the
mixture as such was used as the catalyst.
86
To THF solution of 3 '0.1 equiv.), H
by acetophenone '1.0 equiv.) at 08C.
3
B/LB '0.6 equiv.) was added followed
a
At 08C, the reduction is very slow and incomplete even after 48 h.
The reduction does not occur at 08C or even at room temperature.
b
system prepared form '2)-a,a-diphenylpyrrolidine-
methanol '1), 9-BBN, H B±THF gives good results with
both hindered aralkyl ketones and aralkyl ketones having
electron-withdrawing groups either on the aromatic ring or
on the alkyl side chain. In the case of acetophenone reduc-
tion, the use of DMS±BH in place of H B±THF provides
3
2.2. General reduction procedure
3
3
An oven-dried RB ¯ask provided with a septum inlet and
stirring bar, was cooled to 08C under a continuous ¯ow of
dry nitrogen gas. The ¯ask was charged with the amino-
alcohol 1 '0.1 equiv.) and 9-BBN'0.05 equiv.) in anhydrous
THF '10 mL). The contents were stirred at room tempera-
ture for 1 h. The reaction ¯ask was cooled to 08C and BH3±
THF '1 M, 0.6 equiv.) was added. The aralkyl ketone
'1 equiv.) in THF '20 mL) was slowly added during
good enantioselectivities. Also, the easy preparation of this
structurally well-de®ned catalyst should facilitate use of this
reagent by synthetic chemists for the enantioselective reduc-
tion of such ketones.
2
. Experimental
1
5 min at 08C. The reduction is complete as soon as the
11
2
.1. General
addition of ketone is complete, as observed by B NMR,
which shows the disappearance of the peak due to borane
'10.3 ppm) and the appearance of a new peak due to
dialkoxyborane '121 ppm). The reaction was quenched
with water and dil. HCl '2 M) and n-pentane were added
to precipitate the catalyst. The clear organic layer was
separated by ®ltration and dried over anhydrous magnesium
sulfate. Evaporation of the organic layer provided essen-
tially pure alcohol, which was further puri®ed by passing
through a silica gel pad. The enantiomeric purity of the
alcohol was established by HPLC analysis on a Chiralcel-
All glassware was oven-dried for several hours at 1208C,
assembled while hot and cooled in a stream of dry nitrogen
gas. Syringes were assembled and ®tted with needles while
hot and cooled under nitrogen gas. Techniques for handling
air-sensitive compounds described elsewhere were fol-
lowed. All manipulations and reactions with air-sensitive
compounds were carried out under nitrogen atmosphere.
The amino alcohols, diphenylvalinol, prolinol, '2)-N-
benzyl-a,a-diphenylpyrrolidinemethanol, '2)-a,a-diphenyl-
pyrrolidinemethanol were prepared following the literature
procedures. 9-BBNis a commercial product and dicyclo-
hexylborane, diisopinocamphylborane, were prepared
according to the reported procedures. All the borane±
Lewis base complexes and aralkyl ketones utilized are
7
1
c
8
2
w
OD column.
7
,9
9
Acknowledgements
1
13
11
commercial products. H, C and B NMR spectra were
recorded on a 300 MHz multinuclear instrument. The
Financial support for this work from the Purdue Borane
Research Fund is gratefully acknowledged.
1
1
B

1074
J. V. B. Kanth, H. C. Brown / Tetrahedron 58 ꢀ2002) 1069±1074
4. 'a) Midland, M. M.; Greer, S.; Tramontano, A.; Zderic, S. A. J.
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. We did not observe any reaction of the catalyst 3 with BH ±
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1
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3
7
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999, 55, 5991.
'
f) Singh, V. K. Synthesis 1992, 605.