Aqueous micellar and non-micellar effects during the asymmetric hydrogenation of dehydroamino acid derivatives: Influence of amphiphiles on enantioselectivity and α-CH/CD exchange

Article abstract of DOI:10.1016/S0022-328X(00)00777-4

The effect of different amphiphiles on the CH/CD exchange in the homogeneously catalysed asymmetric hydrogenation/deuteration of methyl (Z)-α-acetamidocinnamate or methyl α-acetamidoacrylate in an aqueous micellar medium has been investigated in connectio

Full text of DOI:10.1016/S0022-328X(00)00777-4

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 621 (2001) 158–165
Aqueous micellar and non-micellar effects during the asymmetric
hydrogenation of dehydroamino acid derivatives: influence of
amphiphiles on enantioselectivity and a-CH/CD exchange
Ingrid Grassert, Gu¨nther Oehme *
Institut fu¨r Organische Katalyseforschung an der Uni6ersita¨t Rostock e.V., Buchbinderstr. 5-6, D-18055 Rostock, Germany
Received 16 August 2000
Dedicated to Professor Dr H. Brunner on the occasion of his 65th birthday
Abstract
The effect of different amphiphiles on the CH/CD exchange in the homogeneously catalysed asymmetric hydrogenation/deuter-
ation of methyl (Z)-a-acetamidocinnamate or methyl a-acetamidoacrylate in an aqueous micellar medium has been investigated
in connection with the effect of amphiphiles on the enhancement of enantioselectivity. In comparison with the exchange of
a-CH/CD in water, the amphiphiles inhibit the reaction in the order: cationicBzwitterionicꢀanionic. In mixtures of cationic
(cetyltrimethylammonium hydrogen sulfate, C16H33N(CH3)+3 HSO−4 ) and anionic amphiphiles (sodium dodecyl sulfate, SDS) the
H/D-exchange amount is low in the presence of an excess of SDS, but it increases rapidly near a CTA+HSO4− mole fraction of
0.5 to give a high level of exchange. The enantioselectivity drops to a minimum in the 1:1 mixture because of the low solubility
of the cationic–anionic aggregates and the absence of micelles. The results obtained with mixed micelles of Brij 35 (polyethyle-
neoxide(23) monododecylether, C12E23) and SDS are quite different. This mixture is dispersible and able to form micelles over the
entire range of mole fractions (0 to 1). As a consequence, the isotope exchange is almost constant from a mole fraction of 0.3–0.9
of SDS. The enantioselectivity is nearly constant over the whole range. The inhibition of H/D exchange in the presence of
long-chain alkyl sulfates seems to be caused by a specific interaction with the catalytic rhodium complex. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Amphiphiles; Asymmetric hydrogenation; Deuteration; Hydrogen–deuterium exchange; Mechanistic investigations; Micelles
tion. The question of whether an E–Z isomerization
takes place during the reduction of a-acyl amino cin-
namic acid derivatives was answered by this method.
Ojima et al. [5] used this deuteration method to gain
mechanistic insight into the very effective asymmetric
induction when chiral phosphino-pyrrolidines were em-
ployed as ligands. A relationship between variations in
the structure of the substrate and the effect of deutera-
tion was published by Brown and Parker [6]. They used
hydrogen deuteride (HD) to determine the isotope ef-
fect of the reaction. Later, this method of deuterium
labelling was used to compare the mechanisms of hy-
drogenation and transfer hydrogenation of a,b-unsatu-
rated carboxylic acids [7], where the favoured substrate
was itaconic acid. A new phase of development in
asymmetric hydrogenation began with the synthesis of
1. Introduction
The homogeneously catalysed asymmetric hydro-
genation of a-amino acid precursors is one of the best
established enantioselective reactions [1]. In particular,
the mechanism of the rhodium(I)-complex-catalysed hy-
drogenation has been investigated by several groups,
but was ultimately defined by Halpern [2]. The first
results about the stereochemistry of CꢀH bond forma-
tion during the asymmetric reaction were reported inde-
pendently by Kagan and co-workers [3] and Koenig
and Knowles [4] in 1978 by means of deuterium addi-
* Corresponding author. Tel.: +49-381-4669330; fax: +49-381-
4669324.
E-mail address: guenther.oehme@ifok.uni-rostock.de (G. Oehme).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00777-4

I. Grassert, G. Oehme / Journal of Organometallic Chemistry 621 (2001) 158–165
159
2. Results and discussion
water-soluble phosphines and the successful application
of aqueous two-phase systems [8]. Substitution of H2O
by D2O resulted in a partial exchange of CH for CD,
indicating that water participates as a reactant in asym-
metric hydrogenation [9,10]. The extent of deuterium
incorporation depends on the nature of the phosphine,
the organic solvent and the amount of water present
[11]. Scrambling of D or H during the hydrogenation of
unsaturated acids or their methyl esters in the presence
of D2O, or alternatively in the presence of H2O for
deuteration, implied that rapid b-elimination competes
with a slower reductive elimination process. The idea of
using this method of deuteration for the synthesis of
labelled compounds was developed by Hardick et al.
[12].
Changing from a liquid–liquid two-phase system to a
micro-heterogeneous micellar system yielded improve-
ments with respect to activity and enantioselectivity in
the asymmetric hydrogenation of a-amino acid precur-
sors [13]. Usually, no water-soluble catalytic system is
necessary for the solubilization in micelles, but Joo et
al. [14] were able to show that micelles can be effective
for water-soluble Ru and Rh systems with monosul-
fonated triphenylphosphine (TPPMS) ligands. Incorpo-
ration of D on the basis of D2/H2O or H2/D2O was
clearly retarded in the presence of sodium dodecyl
sulfate (SDS) as the micelle-forming amphiphile.
We began to investigate the effect of different am-
phiphiles on the CH/CD exchange during the asymmet-
ric hydrogenation of a-acetamidoacrylic acid methyl
ester and (Z)-a-acetamidocinnamic acid methyl ester in
aqueous media. In addition to exploring the influence
of the micelle formation on activity and enantioselectiv-
ity, we wanted to elucidate the mechanism of am-
phiphile–reactant interactions by means of isotope
labelling.
Two systems were investigated, the hydrogenation in
the presence of D2O, leading to a CH/CD exchange in
the a-position, and the deuteration in H2O as the
medium, leading to a preferential CD/CH exchange in
the a-position (Scheme 1).
All experiments were carried out using the N-tert-
butoxycarbonyl-4-diphenylphosphino-2-diphenylphos-
phinomethylpyrrolidine (BPPM ) ligand [15] together
with a cationic rhodium–(cod)- complex. Two sub-
strates were used: methyl (Z)-a-acetamidocinnamate
(Table 1) and methyl a-acetamidoacrylate (Table 2).
1
The extent of CH/CD exchange was measured by H-
NMR in the regions between 4.82–4.92 ppm (a-posi-
tion) and 3.01–3.20 ppm (b-position) and by MS
(chemical impact; CI) in the molecular ion region. All
experimental data were consistent. All tables contain
data for the rate of hydrogenation, the enantioselectiv-
ity and for CD (or alternatively CH) exchange in the
a-position, with respect to different solvent systems and
different surfactants in aqueous media.
The concentration of surfactants was above the criti-
cal micelle concentration (CMC) in all examples, except
for the non-micellar sodium hexyl sulfate (SHS) and the
sodium decyl sulfate (SDeS), which has a relatively high
CMC. Whereas SHS gave no useful CH/CD exchange
data due to rhodium precipitation, the effect of SDeS
was poor for both the activity and enantioselectivity
but high for the inhibition of CH/CD exchange (Table
1). This seems to be important to get an idea from the
influence of the micelle formation.
Table 2 contains selected results of CH and CD
exchange experiments with the water-soluble substrate
methyl a-acetamidoacrylate. It is obvious that the activ-
ity in water or D2O is relatively high (because of the
solubility), but the enantioselectivity is very low, and
with the (S)-configuration and the CH/CD exchange is
Scheme 1. Asymmetric hydrogenation of unsaturated amino acid derivatives in water.

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I. Grassert, G. Oehme / Journal of Organometallic Chemistry 621 (2001) 158–165
Table 1
Deuteration (hydrogenation) of methyl (Z)-a-acetamidocinnamate in H2O (D2O) using [Rh(cod)2]BF4 and BPPM. Effect of different surfactants
on the H/D exchange. Reaction conditions: 25°C; 0.1 MPa D2 or H2; 7.5 ml H2O or D2O; 0.5 mmol methyl (Z)-a-acetamidocinnamate;
0.005 mmol [Rh(cod)2]BF4; 0.005 mmol BPPM; in situ preparation of catalyst; 0.1 mmol surfactant
System
Amphiphilea
t/2 (min)
e.e. R (%)
Exchange
%CH D2/H2O
%CD
H2/D2O
D2/H2O
H2/D2O
D2/CH3OH
H2/CH3OD
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
H2/D2O
H2/D2O
D2/H2O
H2/D2O
Without in H2O
Without in D2O
Without in CH3OH
Without in CH3OD
R4N+HSO−4
R4N+HSO−4
R4N+CF3SO−3
R4N+CF3SO−3
R4N+BF−4
180 Rh¡
8 h Rh¡
2
3
4
6
130
265
110
140
4
3
3
5
3
12
88 Rh¡
100 Rh¡
25
24
2
5
5
8
6
7
4
83
78
92
93
97
94
80
77
81
78
95
94
94
91
95
94
82
80
73
78
95
90
93
89
97
96
94
92
41
2
30
2
40
43
43
44
33
29
25
16
15
16
19
34
27
29
31
23
22
23
R4N+BF−4
Brij 35
Brij 35
R4N+[CH3OSO3]−
R4N+[CH3OSO3]−
HDAPS
HDAPS
SHS
SHS
SDeS
SDeS
SDS
SDS
SHDS
b
–
11
9
SHDS
9
Na-dodecylsulfonate
Na-dodecylsulfonate
Na-palmitoylprolinate
Na-palmitoylprolinate
10
17
15
a HDAPS: hexadecyldimethylammoniumpropanesulfonate; SHDS: sodium hexadecylsulfate; R4N+: (hexadecyltrimethylammonium)+
b Not possible to analyse.
.
Table 2
Deuteration (hydrogenation) of methyl a-acetamidoacrylate in H2O (D2O) using [Rh(cod)2]BF4 and BPPM. Effect of different surfactants on the
H/D exchange. Reaction conditions: 25°C; 0.1 MPa D2 or H2; 7.5 ml H2O or D2O; 0.5 mmol methyl a-acetamidoacrylate; 0.005 mmol
[Rh(cod)2]BF4; 0.005 mmol BPPM, in situ preparation of catalyst, 0.1 mmol surfactant
System
Amphiphilea
t/2 (min)
e.e. R (%)
Exchange
%CH D2/H2O
%CD
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
D2/H2O
H2/D2O
Without in H2O
Without in D2O
R4N+HSO−4
R4N+HSO−4
SDS
18 Rh¡
17 Rh¡
6 (S)
6 (S)
74
64
79
68
83
63
–
54
9
–
35
5
3
3
3
3
3
3
SDS
Na-dodecylsulfonate
Na-dodecylsulfonate
17
10
a R4N+: (hexadecyltrimethylammonium)+
.
the enantiomeric excess (e.e.) value to 80% e.e. (system
D2/H2O) directed to the (R)-configuration. CD or CH
exchange was more significant than without the am-
not reliable because of the precipitation of rhodium
during the reaction. Addition of 20 mol% of
cetyltrimethylammonium hydrogen sulfate increased

I. Grassert, G. Oehme / Journal of Organometallic Chemistry 621 (2001) 158–165
161
phiphile, and we propose a reaction with water or D2O
being activated on the surface of the micelle. The
influence of SDS on activity and enantioselectivity is
quite similar, but the CH and CD exchange is much
lower (9% CH and 5% CD), showing a specific effect of
sulfate as an anionic amphiphile. Sodium dodecanesul-
fonate, in comparison, gave a large increase in enan-
tioselectivity, as well as a slightly higher exchange of
CH or CD during the hydrogenation. The exceptional
position of SDS as an isotope-exchange-inhibiting am-
phiphile is more evident with methyl a-acetamido-acry-
late as the substrate than with methyl (Z)-a-acetamido-
cinnamate (see Tables 1 and 2).
an enhancement of the enantioselectivity. Comparison
of the isotope exchange in water and methanol shows
that there is an extreme solvent effect, which has been
discussed by Sinou and co-workers for 1:1 mixtures of
water and different organic solvents [10]. We were
interested in the investigation of the CH/CD exchange
within a systematically varied mixture of CH3OD–
D2O, the results of which are given in Fig. 2.
The graphical presentation shows a non-linearity be-
tween isotope incorporation and the mole fraction of
D2O in O-deuterated methanol. In the region with a
low CH3OD or D2O content the D incorporation is
significant, and in the large region between 30 and
80 mol% of D2O the exchange is almost constant. We
conclude that the change of the hydrogen bonding
structure in water is extreme, with either a low content
of methanol and, vice versa, in methanol with a low
content of water. This should influence the activity and
reactivity of water in isotope-exchange experiments.
Additionally, the curve of enantioselectivities is approx-
imately constant within the mixtures and drops dramat-
ically only from 10 mol% of methanol to pure D2O,
most probably due to the low solubility of the reactants
in water.
Fig. 3 displays the results of D-labelling experiments
in mixed micelles with different ratios of SDS and
CTA+HSO−4 . The isotopic exchange is almost constant
with an excess of SDS and rapidly increases at a 1:1
mixture to give a high-level exchange, which is charac-
teristic for CTA+HSO4−. The enantioselectivity ob-
served has a minimum at the 1:1 mixture. It is well
known that mixtures of anionic and cationic surfactants
give a precipitation of ion pairs [17]. The concentration
of micelle-forming surfactants should be extremely low
in 1:1 mixtures, with an obvious effect on the enantiose-
lectivity and a less clear effect on the CH/CD exchange.
Comparing CD and CH exchange, we observed an
isotopic effect [16], meaning that the incorporation of
H is faster than the incorporation of D. A graphical
presentation of this effect is given in Fig. 1.
The CH or CD exchange in the catalysed hydrogena-
tion is coupled with the hydrogenation process and not
observable when the educts are substituted by the prod-
ucts (methyl a-acetamidophenylalaninate or methyl a-
acetamidoalaninate) under the reaction conditions.
The ratio of incorporation is noted for each system in
parentheses and was found to be between 1.3 and 1.7.
This is comparable to the isotope effect of 1.22 reported
by Brown and Parker [6] as the ratio of hydrogenation
to deuteration for the reaction of (Z)-a-acetamidocin-
namic acid with HD in methanol using the pre-
catalyst (1.2-bicyclo[2.2.1]heptadiene)(1.2-bis(diphenyl-
phosphino)ethane)rhodium(I) tetrafluoroborate.
Both Table 1 and Fig. 1 show a dependence of the
CH/CD exchange on the type of the amphiphile, with a
decrease of incorporation in the order cationic, non-
ionic, zwitterionic and anionic. Low rates of exchange
occur especially with SDS. In comparison with the
control value in water, the addition of surfactants led to
Fig. 1. Comparison of the H/D exchange within the H2O/D2 and D2O/H2 systems during the hydrogenation of methyl (Z)-a-acetamidocinnamate
in the presence of different amphiphiles.

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I. Grassert, G. Oehme / Journal of Organometallic Chemistry 621 (2001) 158–165
Fig. 2. CH/CD exchange and enantioselectivity with mixtures of CH3OD/D2O during the hydrogenation of methyl (Z)-a-acetamidocinnamate.
Fig. 3. Influence of SDS–CTAHSO4 mixtures on the H/D exchange and the enantioselectivity during the hydrogenation or deuteration of methyl
(Z)-a-acetamidocinnamate in D2O or H2O.
Fig. 4. Influence of SDS–Brij 35 mixtures on the H/D exchange and the enantioselectivity during the hydrogenation or deuteration of methyl
(Z)-a-acetamidocinnamate in D2O or H2O.
6×10−5 mol l−1 at 25°C in water) [18]. As a rule the
In order to be sure that we really had a mixed micelle
composition of the micelle depends on the ratio CMC2/
CMC1, and it is to be expected that Brij 35 is enriched
in micelles [19]. No precipitation was observed during
system, we used different mixtures of SDS (CMC 8.1×
10−3 mol l−1 at 25°C in water) and Brij 35 (poly-
oxyethylene(23)
dodecyl
ether,
C12E23;
CMC

I. Grassert, G. Oehme / Journal of Organometallic Chemistry 621 (2001) 158–165
163
Taking all the evidence into account, we can develop
the following mechanistic argument. On the basis of the
hydrogenation mechanism developed by Halpern [2],
and with respect to the results submitted by Sinou and
co-workers [10] obtained in D2O using a two-phase
system, we conclude that the H/D exchange occurs in
the rhodium–alkyl intermediate (4) (Scheme 2) between
the RhꢀH bond and the coordinated D2O (S).
The inhibition of the exchange could be caused by
competition between the coordination of D2O and the
alkyl sulfate anion. The effect is optimal with a long-
chain sulfate. The transfer of H or D from RhꢀH(D) to
the s-bound substrate is most likely to be responsible
for the isotope effect.
the mixing of the surfactants. As shown in Fig. 4, both
enantioselectivity and CH/CD exchange in the hydro-
genation are dependent on the surfactants added.
As expected, the enantioselectivity is rather constant:
between 92 and 94% e.e., similar to values observed in
micelles of pure Brij 35 or pure SDS [20]. In contrast to
this, the CH/CD exchange in D2O drops from pure Brij
35 micelles (22% CD in a-position) to pure SDS mi-
celles (8% CD in a-position), and stays practically
constant between 30 and 90 mol% of SDS. There is a
great similarity to the exchange in CH3OD/D2O mix-
tures (Fig. 2), but the distinction between these is the
deviation of the micelle composition from the stoi-
chiometry of the mixture. In the beginning the micelles
contain only very small amounts of SDS and the ratio
is almost constant over a large range. Only in the last
10 mol% of added SDS is a significant decrease of
nearly 6% CH/CD exchange observed. The non-linear-
3. Conclusions
ity could be
composition.
a result of the non-linear micelle
Amphiphiles influence the asymmetric hydrogenation
of dehydroamino acid derivatives in an aqueous
medium by enhancing activity and enantioselectivity.
The effect is observed only above the CMC and seems
to be typical for a reaction within a micelle. By chang-
ing the aqueous phase to D2O the a-hydrogen was
partially exchanged for deuterium. This incorporation
of deuterium is also influenced by amphiphiles, and,
indeed, below the CMC. Anionic amphiphiles, like
sodium alkyl sulfates or sodium alkanesulfonates in
particular, inhibit the exchange significantly. Addition-
ally, investigations into systems with amphiphile mix-
tures show clearly that a non-micellar reaction is
responsible for the exchange. The step at which ex-
change and competition in coordination between the
isotopic solvent and the sulfate- or sulfonate-containing
amphiphile take place has been proposed within the
Halpern mechanism. Amphiphiles exert a stabilizing
effect on the catalytic system, and in their absence the
exchange of H/D is high, but often there is a precipita-
tion of rhodium.
Moreover, we have to expect a mixture of mixed
micelles and a few SDS micelles in the region of a high
mole fractions of SDS. The results in mixed micelles
show a sort of solvent effect in the environment of
amphiphiles, but a specific interaction between the am-
phiphilic sulfate anion and rhodium complex is not
clear [21].
To check the connection between micelle formation
and CH/CD exchange we used sodium decyl sulfate
(SDeS) as the amphiphile, which has a relatively high
CMC (3.3×10−2 mol l−1 at 25°C in water [18]). The
results are presented in Fig. 5.
Activity and enantioselectivity show a significant
change between Rh/SDeS ratios of 1:20 and 1:50 (in the
region of the CMC). In comparison with that the
CH/CD exchange is much less variable, and we found a
clear inhibition (cf. entries 1 and 2 in Table 1) below
the CMC, indicating non-micellar specific interactions
of the sulfate with the catalyst [22].
Fig. 5. Influence of different SDeS concentrations on the enantioselectivity and the H/D or the D/H exchange during the hydrogenation or
deuteration of methyl (Z)-a-acetamidocinnamate in D2O or H2O.

164
I. Grassert, G. Oehme / Journal of Organometallic Chemistry 621 (2001) 158–165
Scheme 2. Mechanism of the hydrogenation (deuteration) with the H/D exchange within step (4).
4. Experimental
consume half of the theoretical amount of hydrogen
(half-life of the reaction) was taken as a measure for the
activity. After finishing the experiment, the mixture was
extracted with 5 ml of chloroform. The deuteration was
performed by exactly the same method, but the system
D2/H2O was used instead of H2/D2O. The e.e. of the
product was determined by GLC as described above.
The H/D exchange was estimated quantitatively by
comparison of the H-NMR signals of the hydrogena-
tion product and checked by MS (CI; parent peak). All
results are in good accordance.
4.1. General procedures
NMR spectra were recorded on a Bruker AC 250
instrument. Chemical shifts of H are reported in parts
1
per million and referenced to Me4Si as standard. The
mass spectra were recorded on an AMD 402 instrument
(Fa. Intectra).
The e.e. was determined by GLC on a Hewlett–
Packard chromatograph 5880 A equipped with a 10 m
capillary column (column ID: 0.2 mm), fused silica,
1
XE-60 L-valine-tertbutylamide (FID, split 1:60, 150°C).
All detergents and BPPM were purchased from com-
mercial sources and used as obtained. Tween and sulfo-
betaines were obtained from Sigma GmbH, BPPM
from Merck, CTAHSO4, SDS and Brij-compounds
from Fluka. The complexes [Rh(cod)2]BF4 [23],
[Rh(cod)(bppm)]BF4 [24,25] and methyl (Z)-a-acetami-
docinnamate [26,27] were prepared in accordance with
the literature.
Acknowledgements
We thank Mrs A. Lehmann for carrying out the
hydrogenations and syntheses, Mrs K. Kortus for the
gas chromatographic analyses, Professor M. Michalik
and Mrs Harzfeld for NMR investigations and Mrs H.
Baudisch for MS investigations. We thank the BMBF
and also the Fonds der Chemischen Industrie for finan-
cial support of this research.
4.2. General procedure for hydrogenation (deuteration)
Hydrogenation was performed by an isobaric method
at 25°C under air-free conditions in thermostatically
controlled apparatus [20]. A suspension of 1 mmol
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