Asymmetric amplification by kinetic resolution using a racemic reagent: Example in amine acetylation

Article abstract of DOI:10.1002/chem.200600026

The reaction of a racemic reagent on a mixture of enantiomers with small ee (ee = enantiomeric excess) has been studied for amine acylation. A substantial asymmetric amplification could be- realized, for example, from 67 to > 95.5 ee. The combination of asymmetric amplifications is subsequently discussed. Two sequential asymmetric amplifications, one using a racemic reagent and another using a positive nonlinear effect allowed us to start from 1.5 % ee and end with a large amount of a product of 97% ee.

Full text of DOI:10.1002/chem.200600026

FULL PAPER
DOI: 10.1002/chem.200600026
Asymmetric Amplification by Kinetic Resolution UsingA Racemic Rea ge nt:
Example in Amine Acetylation
[
a]
Tummanapalli Satyanarayana and Henri B. Kagan*
Dedicated to the memory of AndrØ Rassat
Abstract: The reaction of a racemic reagent on a mixture of enantiomers with
small ee (ee=enantiomeric excess) has been studied for amine acylation. A sub-
stantial asymmetric amplification could be realized, for example, from 67 to
Keywords: asymmetric synthesis ·
chirality · kinetic resolution · non-
linear effects · racemic reagent
>
95.5 ee. The combination of asymmetric amplifications is subsequently discussed.
Two sequential asymmetric amplifications, one using a racemic reagent and anoth-
er using a positive nonlinear effect allowed us to start from 1.5% ee and end with
a large amount of a product of 97% ee.
Introduction
The coupling (statistical or not) between enantiomers has
been used as a way to amplify their eeꢀs by removal of the
[16,17]
Asymmetric amplification of small eeꢀs is an old problem
meso dimer before cleavage of the homochiral dimers.
which still of current interest because of its relevance with
the origin and propagation of optical activity on earth.
The catalytic process is especially valuable, as the product is
generated both in larger amounts and with larger eeꢀs than
in the starting material.
[1–4]
Many processes have been proposed to initiate an imbalance
between the distribution of two enantiomers, such as sponta-
neous resolution by crystallization, spontaneous resolution
combined with in situ racemization, kinetic resolution or
Here we wish to describe a simple way to amplify in ho-
mogeneous conditions a small ee (ee ) or enantiomeric ratio
0
(er ) of a compound, without the use of an enantioenriched
0
[5–7]
asymmetric synthesis by circularly polarized light, etc.
chiral auxiliary. Some examples of multiple asymmetric am-
plifications are subsequently described.
The asymmetric amplification of eeꢀs has been envisaged by
a variety of processes. An early suggestion was the Frank
model, which involves an autocatalysis coupled with an in-
The usefulness of a racemic reagent: There are several
[8]
hibition reaction between enantiomers. In 1995 Soai dis-
covered a superb and unique chemical system which is relat-
facets for reactions involving a racemic substrate or a race-
[18,19]
mic reagent or catalyst.
The kinetic resolution of a race-
[9]
ed to the Frank model. Positive nonlinear effects (asym-
mic or enantioenriched mixture involves a chiral reagent or
[17]
metric amplification) are also a way to translate a small ee
catalyst. Usually the reagent or the catalyst are fully re-
[
18,19]
(
of the catalyst or reagent) into a larger ee (in the prod-
solved. The use of enantio-impure reagents
lysts
or cata-
in kinetic resolution has been discussed. The key
factor for a high ee for the recovered starting material (ee )
[10,11]
[11,12]
[20–23]
uct).
It can operate in the usual catalytic reactions
[13]
and in autocatalytic reactions. Many other methods exist
sm
[14]
to amplify the ee of a compound, for example kinetic resolu-
is the size of the stereoselectivity factors. A high s=krel
[
14,15]
tion
or the Horeau–Langenbeck duplication method.
will allow the recovery of quite a large amount of the initial
compound with excellent ee . If s is small (<10) then the
sm
[
a] Dr. T. Satyanarayana, Prof. H. B. Kagan
Laboratoire de Catalyse MolØculaire (UMR 8075)
Institut de Chimie MolØculaire et des MatØriaux d’Orsay
UniversitØ Paris-Sud, 91405 Orsay (France)
Fax : (+33)169 15 46 80
conversion has to be increased to get a similar ee . Mutual
kinetic resolution between an alcohol and an anhydride,
sm
each one of small ee, led to substantial asymmetric amplifi-
[24]
cations. In some special situations racemic catalysts have
been used in enantioselective reactions (in presence of
E-mail: kagan@icmo.u-psud.fr
[25]
chiral additives)
mixtures.
or in kinetic resolution of nonracemic
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[26]
Chem. Eur. J. 2006, 12, 5785 – 5789
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5785

Let us now consider the partial transformation of a mix-
ture of enantiomers by a racemic reagent. Obviously the
racemic reagent, like an achiral reagent, will be unable to
resolve a racemic mixture, but, interestingly, it is able to
modify the initial ee (ee ) of a partially resolved mixture.
0
There will be an increase in the ee of the recovered starting
material (ee
A
C
H
T
R
E
U
N
G
(final)>ee (initial)) if each of the enantiomers
A
T
E
N
sm
0
of the racemic reagent displays a high enantiodifferentiation
large stereoselectivity factor s=k ). Ugi et al. have per-
(
rel
formed calculations on bimolecular reactions between two
[27]
chiral reactants of various initial eeꢀs. With the assumption
of a first-order reaction in each reactant, the kinetic treat-
ment on an equimolar mixture of reactants allowed the au-
thors to set tables of data for various krel and conversions.
The special case of a partially resolved compound versus a
racemic reagent has also been treated, clearly showing an in-
crease of eesm with conversion, with a correlation to the size
of s. The influence of the initial ee of a reagent on the kinet-
ic resolution of a racemic mixture was briefly revisited by us
[20]
in 1999. Plotted in Figure 1 are some curves computed as
in reference [20] with s=30 (initial eesubstrate =10%) and two
substrate/reagent initial ratios. The general trends are seen
on these curves: the ee of the substrate is amplified by the
racemic reagent, simultaneously the racemic reagent is pro-
gressively resolved by the substrate. For example, in Figur-
e 1a for which the initial ratio [substrate]/[racemic re-
agent]=1:1 at 75.8% conversion of substrate, its initial
1
0% ee is now equal to 20.1%, while the reagent is weakly
resolved (eereagent =17.5%). In Figure 1b for which initially
substrate]/[reagent]=1:0.5 and ee =10%, at 49.4%
[
ACHTREUNG
substrate
substrate conversion and consequently 98.7% reagent con-
version, one finds eesubstrate =17.7% and eereagent =54.4%. To
our knowledge there are no reports in the literature on the
intriguing possibility of an increase in the imbalance be-
tween enantiomer distribution by a racemic reagent. Thus,
we investigated this (see below).
Figure 1. Amplification of 10% ee substrate with initial molar ratio sub-
strate over reagent of 1.0:1.0 (a) and 1.0:0.5 (b). Computed plots accord-
ing to reference [20], for reactions that are first order with respect to sub-
strate and reagent.
Use of a racemic reagent in acylation reactions: As a model
case we selected the convenient acylation of a racemic
amine 3 by the monoacetyl bistriflamide 2 (Scheme 1a)
[28]
which has been recently described by Mioskowski et al.
For example racemic and (100% ee) in DMPU
DMPU=N,N’-dimethyl-N,N’-propylene urea) gave 2 with
3
2
and conversion C (Table 1). Encouraged by these results
and in order to evaluate the generality of this amplification
strategy, we examined the alternate reaction of N-acetylbis-
triflamide 2 of small ee with racemic amine 3 (Scheme 1c,
Table 2). When 1.0 equiv of acylating agent (1S,2S)-2
(1.5% ee) was treated with 0.93 equiv of racemic amine 3
and kept until the complete consumption of rac-3 (for exam-
ple, 93% conversion of 2), the remaining unreacted acylat-
ing agent (1S,2S)-2 was isolated with 15.5% ee. The evalua-
tion of the asymmetric amplification was conveniently done
by isolation of 2 by flash chromatography and chiral HPLC
or by hydrolysis of 2 into 1, the ee of which was measured
(
84% ee for 50% conversion. The authors calculated from
these data that s=krel =30. We easily reproduced this ex-
periment. Then we allowed amine 3 (R) with 67% ee (ini-
0
tial) to react with less than one equivalent of racemic re-
agent 2 (Scheme 1b, Table 1). When 1.0 equiv of amine 3
(
2
R, 67% ee) was treated with 0.53 equiv of racemic reagent
and kept until the complete consumption of rac-2 (for ex-
ample, 53% conversion of 3), the isolated residual amine 3
R) had an eesm of 95%, in agreement with calculations
(
made in reference [28] and the value of s=30; the N-acetyl-
amine 4 (product) was obtained in 37% ee (eeproduct, R). A
set of asymmetric amplifications of initial eeꢀs of amine 3
under various experimental conditions confirmed the good
correlation between eesm (initial and final) and the size of s
1
9
by F NMR in presence of quinidine. The results of Tables 1
and 2 confirm that a racemic reagent can easily enhance
the ee of its partner if the stereoselectivity factor is not too
small.
5786
www.chemeurj.org
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 5785 – 5789

Asymmetric Amplification
FULL PAPER
Scheme 1. a) Kinetic resolution of rac-3 by enantiopure-2. b) Enantiomeric amplification of nonrac-3 by rac-2. c) Enantiomeric amplification of nonrac-2
by rac-3.
[
a,b]
Table 1. Amplification of the ee of 3 by the use of racemic reagent 2.
Entry Amine 3 Acylating Conversion Recovered Amide
Initial ee [%] agent rac 2
[c]
[d]
[e]
of 3 [%]
amine 3
ee(final)
%]
4
(mol equiv)
[mol equiv]
A
H
E
N
eeproduct
[%]
[
1
2
3
4
5
6
7
85.8 (1.0)
67.0 (1.0)
67.0 (1.0)
67.0 (1.0)
67.0 (1.0)
4.2 (1.0)
0.23
0.28
0.53
0.64
0.83
0.50
0.90
23.0
28.0
53.0
64.0
83.0
50.0
90
98.0
46.3
25.0
37.0
54.0
60.8
0.15
1.8
83.0
95.5
98.0
>99.5
8.2
Figure 2. Sequential asymmetric amplifications. R
product isolated after the first asymmetric amplification, it could be
structurally identical to the starting material R , S , or of a different
structure (if R , S is a reagent or catalyst giving the product R , S ). er
eri-1 (or ee
1 1
and S refer to the
4.2 (1.0)
22.6
[
a] Reactions were performed at À208C for 20–48 h in DMPU until com-
0
0
plete consumption of racemic acyl transfer reagent 2. [b] Compounds 3
and 4 have an R configuration. [c] Conversion calculated from eeꢀs of 3
and 4 or measured by GC. [d] Measured by HPLC (OD-H) on the corre-
sponding amide obtained on acetylation. [e] Measured by HPLC (OD-H).
0
0
1
1
i
>
i
>eei-1).
modest initial ee. This is illustrated in Figure 2. The enantio-
meric amplification index, a (>1), in the first step can be ex-
pressed by the ratio of er values a=er /er or by the ratio of
Table 2. Amplification of the ee of the acyl-transfer agent 2 by the use of
racemic amine 3.
[
a,b]
1
0
eeꢀs a=ee /ee (for the relative advantages of both defini-
1
0
Entry Initial ee [%] of acyl-
transfer agent 2
Mol
Conversion
ee [%] of re-
covered 2
[
c]
[d]
tions see reference [12]). The overall amplification in the ith
step will be a =er (or ee )”a ”a ”···”a . The sequential er
i
equiv of of 2 [%]
racemic
(1 mol equiv)
i
0
0
1
2
i
reagent
or ee are of increasing values. Here we do not discuss the
i
A
H
R
U
G
numerical values of this approach (some details are given in
3
[29]
the Supporting Information), instead we have illustrated
1
2
3
20.0
5.0
1.5
0.65
0.80
0.93
55.0
80.0
93.0
28.0
31.0
15.5
these below with some experiments.
We undertook the project to couple a first asymmetric
amplification by a racemic reagent (rac-3) to a subsequent
enantioselective catalytic reaction in which there is a posi-
tive nonlinear effect with respect to a catalyst (1).
Bistriflamide 1 is a classical and useful asymmetric cata-
[
a] Reactions were performed at À208C for 20–48 h in DMPU until com-
plete consumption of racemic amine 3. [b] Compounds 1 and 2 have an
1S,2S) configuration. [c] Complete conversion of 3 was assumed, except
in entry 1. [d] Measured after isolation and hydrolysis into 1, followed by
(
1
9
F NMR with quinidine as the shift reagent; in entry 3 ee was also meas-
lyst for the addition of organozincs on aldehydes, when used
ured on recovered 2 by HPLC.
[30]
in presence of stoichiometric amounts of Ti(OiPr) . The
A
T
E
G
4
original procedure for the catalyst preparation was devised
[31]
by Ohno and Kobayashi and was subsequently modified
[
32]
Multiple asymmetric amplifications: It may be interesting to
combine (one-pot when possible) various sequential process-
es of asymmetric amplification to get a final high ee from a
by Walsh. In the Ohno and Kobayashi method, 1 and Ti-
(OiPr) were first heated in toluene at 408C for 20 min, then
AHCTREUNG
4
diethylzinc was added at À788C, followed by benzaldehyde.
Chem. Eur. J. 2006, 12, 5785 – 5789
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5787

H. B. Kagan and T. Satyanarayana
The alternate procedure of Walsh et al. was to combine 1
Conclusion
and Et Zn at 238C in toluene, this was followed by the se-
2
quential addition at À508C of Ti
A
T
E
N
(OiPr) and benzaldehyde.
We established experimentally on several examples the pos-
sibility to amplify a small ee by using a racemic reagent. If
the stereoselectivity factor is large enough the process can
be quite efficient. The coupling of this type of asymmetric
amplification with a second amplification process in a cata-
lytic reaction has been realized. By this approach a small in-
itial ee (in the range of 1–5%) can give rise to a product in
large amounts with ee >95–97%. We are currently investi-
gating the scope and various aspects of the use of racemic
reagents in the kinetic resolution and strategy of sequential
asymmetric amplifications.
4
In 2002 we found that the Ohno and Kobayashi protocol
gave a strong positive nonlinear effect in contrast with the
[33]
Walsh procedure where there was a strict linearity. For ex-
ample, catalyst 1 with 10% ee provided 6a (Scheme 2) in
51% ee. We reinvestigated these experiments and combined
it with the asymmetric amplification of 2 by racemic amine
3
.
Racemic 3 amplified the low ee, ee , of a sample of N-
acetyl bistriflamide 2 until a value of ee (Table 2) had been
reached. 2 was easily recovered by flash chromatography
0
1
from the reaction mixture and quantitatively hydrolyzed
into bistriflamide 1 (ee ) which was taken as catalyst (20%
1
molequiv) in the addition of Et Zn on benzaldehyde 5 ac-
2
cording to the Ohno–Kobayashi protocol or to a slightly
modified protocol, both affording an asymmetric amplifica-
tion. The results are collected in Table 3 and highlight the
Experimental Section
Apparatus and solvents: All the apparatus used were flame-dried and
cooled under an argon atmosphere. Toluene was freshly distilled over
2
CaH . DMPU was distilled and stored over molecular sieves (4 Š).
Table 3. Asymmetric amplification in the formation of 6.
Determination of enantiomeric excess:
[
b]
Entry
ee of catalyst 1 [%]
Amount of catalyst
ee of (S)-6 [%]
For amide (4): The ee was determined by using HPLC on Chiralcel OD-
[
a]
À1
[mol equiv]
H
(n-pentane/ethanol 98.7:1.3, flow=1.0 mLmin
, retention times=
4
7 min (R), 55 min (S)).
1
2
3
4
5
6
7
8
9
10.0
10.0
10.0
10.0
15.0
15.0
31.0
100
100
0.02
0.20
0.40
0.20
0.20
0.20
0.02
0.02
0.02
51 6a
89 6a
87 6a
93 6b
88 6a
97 6b
84 6a
For amine (3): The ee was determined on the corresponding amide (4)
obtained by acylation following similar analysis conditions to those de-
scribed above.
For acylating agent (2): A method was setup in which ee was determined
by HPLC on a (S,S)-ULMO column (n-hexane/iPrOH 95:5, flow=
0.5 mLmin , UV detection at both 254 and 215 nm, retention times=
[
c]
À1
98 6a
>99.5 6b
10 min (1S,2S), 12 min (1R,2R)).
1
9
For bistriflamide (1): The ee was determined by F NMR employing qui-
nidine as chiral-shift reagent (bistriflamide (1)/quinidine 1:ꢀ2) and
[
a] The catalyst precursor (1R,2R)-1 was heated in toluene with Ti
A
H
R
U
G
4
CDCl
3
. Chemical shifts (CF
3
)=À77.34 ppm (1S,2S), À77.90 ppm (1R,2R).
(
5
GC conditions for determination of conversions for the kinetic resolution
at À208C. See details in Experimental Section. [b] Measured by HPLC
of amine: Column: DB1701 (Phase: 14% cyanopropylphenyl 86% meth-
(
(
OD-H), All products are of S-configuration except entry 7. [c] Sample
1S,2S)-1 obtained by hydrolysis of 2, itself isolated as in Table 2, entry 2.
ylpolysiloxane, dimensions: 30 m”0.32 mm”0.25 mm). Conditions: 708C
À1
isotherm for 10 min, 108Cmin
1
rate, 1508C isotherm for 10 min,
À1
08Cmin rate, 2508C isotherm for 15 min.
Kinetic resolution of amine (3)
efficiency of the combinations of asymmetric amplifications
Scheme 2). By taking an initial compound 2 of 1.5% ee and
after an amplification by racemic amine 3, which generates
Detailed experimental procedure for entry 5 (Table 1): (R)-1-Methylbenzyl
amine 3 (67% ee, 0.80 mmol, 0.097 g) was added to a racemic acylating
agent 2 (0.66 mmol, 0.277 g) dissolved in freshly distilled DMPU (2 mL)
at À208C. The reaction mixture was stirred at the same temperature with
regular monitoring by TLC and GC. After complete consumption of the
acylating agent 2 (in 2 d), the reaction mixture, as such, was loaded onto
a silica-gel-packed column and flashed with 30–100% EtOAc in pentane
to provide 101 mg (77.7%) of pure amide 4 (30% EtOAc in pentane)
and 9.1 mg (9.4%) of pure amine 3 (100% EtOAc).
(
Amide 4: 60.8% ee (R) (determined by HPLC).
Recovered amine 3: 99.5% ee (R) (converted to amide 4 by acylation
and the ee was then determined by HPLC).
Kinetic resolution of acylatinga ge nt (2)
Detailed experimental procedure for entry 3 (Table 2): rac-1-Methylbenzyl
amine 3 (0.93 mmol, 0.112 g) was added to (1S,2S)-acylating agent 2
Scheme 2. Catalyzed asymmetric addition of diethylzinc.
(
1.5% ee, 1.0 mmol, 0.42 g) dissolved in freshly distilled DMPU (2 mL)
at À208C. The reaction mixture was stirred at the same temperature for
0 h and then at 08C for another 8 h to ensure the complete consumption
4
of rac-3. After this time, the reaction mixture, as such, was loaded onto a
silica-gel-packed column and flashed with 3–15% EtOAc in pentane to
provide the isolated (1S,2S)-acylating agent 2 (3% EtOAc in pentane)
and bistriflamide 1 (15% EtOAc in pentane).
the catalyst 1 of 15% ee, this reagent was able to produce
large quantities of carbinol 6b in >97% ee (Table 3,
entry 6).
5788
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ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 5785 – 5789

Asymmetric Amplification
FULL PAPER
Recovered 2: 3.6% (0.015 g, isolated yield); 15.5% ee (1S,2S) (deter-
mined by HPLC).
[11] C. Girard, H. B. Kagan, Angew. Chem. 1998, 110, 3088–3127;
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[
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Bistriflamide (1): 91% (0.344 g, isolated yield); ~0% ee (determined by
1
9
[13] K. Soai, T. Shibata, I. Sato, Acc. Chem. Res. 2000, 33, 382–390.
F NMR).
Experimental procedure for asymmetric amplification in diethylzinc addi-
tion to aromatic aldehydes: Freshly dried toluene (1.0 mL) and Ti(OiPr)
1.5 mmol, 0.426 g) were added to bistriflamide 1 (15% ee (1R,2R),
[
[
14] H. B. Kagan, J.-C. Fiaud, Top. Stereochem. 1988, 18, 249–330.
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A
H
R
U
G
4
(
[
[
16] W. Langenbeck, G. Triem, Z. Phys. Chem. Abt. A 1936, 177, 401–
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.25 mmol, 0.0945 g, 20 mol%) in a flame-dried Schlenk under an argon
4
08.
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atmosphere. This mixture was stirred at 55–578C for several hours and
Zn (1.5 mmol, 1.5 mL of
then cooled to À788C. After the addition of Et
2
1
a 1.0m solution in hexane) and aromatic aldehyde (1.25 mmol), the reac-
tion mixture was left to warm, while stirring, slowly to À208C. This mix-
ture was then stirred for 12–18 h. The reaction was quenched by adding
[
[
[
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1
n HCl and the product was extracted into diethyl ether. The solvent
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