Chiral separation of substituted phenylalanine analogues using chiral palladium phosphine complexes with enantioselective liquid-liquid extraction

Article abstract of DOI:10.1039/b924749a

Chiral palladium phosphine complexes have been employed in the chiral separation of amino acids and phenylalanine analogues in particular. The use of (S)-xylyl-BINAP as a ligand for the palladium complex in enantioselective liquid-liquid extraction allowed the separation of the phenylalanine analogues with the highest operational selectivity reported to date. ³¹P NMR, FTIR, FIR, UV-Vis, CD and Raman spectroscopy methods have been applied to gain insight into the binding mechanism of the amino acid substrates with the chiral palladium phosphine complexes. A complexation in a bidentate fashion is proposed.

Full text of DOI:10.1039/b924749a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Chiral separation of substituted phenylalanine analogues using chiral
palladium phosphine complexes with enantioselective liquid–liquid extraction†
Bastiaan J. V. Verkuijl, Boelo Schuur, Adriaan J. Minnaard,* Johannes G. de Vries* and Ben L. Feringa*a
a
b
a
a,c
Received 25th November 2009, Accepted 30th March 2010
First published as an Advance Article on the web 17th May 2010
DOI: 10.1039/b924749a
Chiral palladium phosphine complexes have been employed in the chiral separation of amino acids and
phenylalanine analogues in particular. The use of (S)-xylyl-BINAP as a ligand for the palladium
complex in enantioselective liquid–liquid extraction allowed the separation of the phenylalanine
3
1
analogues with the highest operational selectivity reported to date. P NMR, FTIR, FIR, UV-Vis, CD
and Raman spectroscopy methods have been applied to gain insight into the binding mechanism of the
amino acid substrates with the chiral palladium phosphine complexes. A complexation in a bidentate
fashion is proposed.
Introduction
The preparation of enantiomerically pure compounds is a major
1
contemporary challenge in the pharma and chemical industry.
The methods available to access enantiopure compounds include
resolution of racemates, the chiral pool, fermentation, asymmet-
ric catalysis and biocatalysis, with resolution being the major
2
source. The repetoir of currently available resolution methods
comprises resolution via crystallisation of diastereomeric salts,
3
4
i.e. classical resolution, kinetic resolution using enzymes or
5
chiral catalysts, or dynamic kinetic resolution. Sometimes the
undesired enantiomer can be racemised and recycled. Despite
6
the theoretical maximum of 50% yield classic resolution is still
a widely applied method in the fine chemical and pharmaceu-
tical industries. The labor intensiveness and the problems with
Fig. 1 Schematic representation of enantioselective liquid–liquid extrac-
tion. Symbols: grey blocks: (S)-substrate, black blocks: (R)-substrate, bold
brackets: host.
7
reproducibility it suffers from, provides considerable incentives to
8
find alternative solutions. Currently among the methods being
develop hosts which, for instance, separate the racemates of an
entire class of compounds.
9
explored are resolution with in situ racemization, membrane-
1
0
11
assisted separations, diastereomer separation by distillation,
Typically U-tubes or membranes are employed in separation
schemes where the chiral host is applied in a catalytic fashion. The
use of a number of membranes in series has even allowed for com-
12–14
supercritical extraction,
fractional enantioselective extraction
15,16
17
and chiral simulated moving bed (SMB).
In enantioselective liquid–liquid extraction (ELLE), an enan-
21
plete separation. Maier, Lindner and co-workers have reported
tiopure host is used as an extractant to bind enantiospecifically and
reversibly with a racemic substrate (Fig. 1). If the host is confined
to one phase in a biphasic system, an enantiomeric separation of
the substrate can take place between the two phases in a single
step. If the separation is imperfect, a fractional extraction series
the use of a centrifugal partition chromatograph containing an
MTBE solution of bis-1,4-(dihydroquinidinyl)phthalazine as the
stationary chiral host solution which was able to fully separate the
herbicide 2-(2,4-dichlorophenoxy)propionic acid (dichlorprop)
which was fed as a solution in aqueous buffer as the mobile phase.
Since these methods suffer from problems regarding scalability,
22
18,19
is required.
A minimal selectivity of 1.5 is generally perceived
as necessary to achieve separation within a reasonable number of
we have developed the use of centrifugal separators as a highly
20
fractional extraction steps. Important features of this approach
are its potential versatility and ease of operation. It is possible to
23–25
efficient method for continuous extraction.
Applying a number
26
of these in series allows for the full separation of a racemate.
A fractional extraction centrifugal contactor separator cascade
(CCS cascade) is depicted in Fig. 2. The fractional extraction
a
is based on the principle of countercurrent extraction, giving the
optimal separation in as low a number of stages as possible and the
optically pure substrate in the raffinate (1). Every stage represents
a centrifugal contactor separator (CCS) which is a table top
apparatus capable of mixing and separating two liquid phases in
a continuous fashion. The extraction of the host, which is defined
as back extraction (Fig. 2, right) gives the other enantiomer of the
Laboratories for Organic Chemistry, Stratingh Institute for Chemistry, Uni-
versity of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
b
19
Eindhoven University of Technology, Department of Chemistry and Chemi-
cal Engineering, Den Dolech 2, 5600 MB, Eindhoven, The Netherlands
c
DSM Innovative Synthesis BV, P.O. Box 18, 6160, MD Geleen, The
Netherlands
†
Electronic supplementary information (ESI) available: Experimental and
analytical details. See DOI: 10.1039/b924749a
This journal is © The Royal Society of Chemistry 2010
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Fig. 2 Fractional extraction cascade scheme.
substrate in the raffinate (2). The host acts in a reversible manner
and remains in a closed cycle. As a result, the separation of the
substrate occurs as a continuous process.
The relationship between the minimal amount of required
fractional extraction steps (Nmin) for full separation of both
enantiomers of the substrate, the desired ee of the substrate and
the operational selectivity (aop) of a single extraction, is described
27
by the Fenske equation (eqn (1)).
(
1)
The relationship between Nmin and aop is shown in Fig. 3a. At
an aop of 1.5, Nmin is still 25. Nmin drops rapidly as aop increases,
leaving only 10 steps when aop = 3.0. Nmin subsequently lowers
more slowly and 5 extraction steps remain at aop = 8.0. It can
be concluded, that full separation is already possible at moderate
levels of aop by using a fractional extraction CCS cascade with a
reasonable number of fractional steps. A further increase in aop
provides only a minor decrease in Nmin. The relationship between
the desired ee and Nmin is depicted in Fig. 3b. For an aop = 7.0,
only 4 steps are required to obtain 95% ee. Higher ee is dependent
on the number of extraction steps. To achieve 99% ee, 6 steps are
required. An additional step provides 99.8% ee.
Fig. 3 Minimal number of fractional extraction steps needed (Nmin) versus
a
op where ee = 99% (a) and ee where aop = 7.0 (b).
Over and above the technological aspects, the development
of improved chiral host compounds is essential. In particular
the enantioselective extraction of underivatized amino acids is
one of the key challenges within the field of ELLE. The work
of Cram et al. demonstrates that high selectivities could be
achieved for a range of amino acid perchlorate salts using crown
28;29
Fig. 4 General extraction scheme.
ethers with a functionalized BINOL backbone.
Subsequently,
De Mendoza and co-workers combined a chiral guanidinium
group with a crown-ether to extract tryptophan enantioselectively
complexes as hosts. Furthermore, several spectroscopic methods
are used to gain insight into the structure of the complex, which
results in the structure presented in this article.
30
(
Fig. 4). Metal complexes as hosts have been employed including
31
20,32
lanthanide b-diketonate and copper-proline complexes.
cently we have shown that the use of the metal complex PdCl
Re-
2
((S)-
Results and discussion
BINAP) as a host results in the highest operational selectivity
for underivatized tryptophan in an ELLE system using metal
Ligands
33
complexes reported to date. This system meets all the necessary
requirements regarding selectivity, control of distribution and
reversibility for future application in a CCS cascade.
The effect of structural variation of the ligand which is complexed
to the palladium-ion was explored using BINAP analogues. In
addition, the extraction behavior of the palladium (S)-BINAP and
(R)-BINAP complexes was tested. The complexes were formed
in situ by stirring a solution of 1.1 eq. of ligand overnight with
Here, we report the use of the (S)-xyl-BINAP ligand for
the palladium host, which results in the highest selectivities for
a range of phenylalanine derivatives reported to date and the
highest selectivity for phenylalanine achieved in ELLE using metal
1.0 eq. of the palladium precursor [PdCl
2
(CH
3
CN) ] and the
2
3
046 | Org. Biomol. Chem., 2010, 8, 3045–3054
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Table 1 Ligand screening
Table 2 Solvent screening
a
b
a
Entry Ligand
Substrate
Trp
Trp-Na
Trp
Trp-Na
Trp
Trp-Na
Trp
Trp-Na
Trp
Trp-Na
Trp
[host]
D(org/aq)
a
op
Entry
Solvent
P(org/aq)
D(org/aq)
a
op
1
2
3
4
5
6
7
8
9
1
1
1
(R)-BINAP
5.0
5.0
1.0
1.0
5.0
5.0
1.0
1.0
5.0
5.0
1.0
1.0
0.2
2.0
0.1
0.5
0.2
2.6
0.1
0.5
0.2
2.9
0.1
0.6
2.2
2.3
2.2
2.4
1.7
1.5
1.8
1.6
3.0
3.1
3.1
3.2
1
2
3
4
5
6
7
8
1-Octanol
Dichloromethane
Chloroform
1,2-Dichloroethane
Toluene
Chlorobenzene
Carbon tetrachloride
1-Butanol
rac-2-Butanol
(R)-2-Butanol
(S)-2-Butanol
Octafluoro-pentanol
Dodecafluoroheptanol
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.6
0.6
0.7
0.7
0.1
0.0
0.5
0.5
0.6
0.5
0.5*
0.2
0.5*
2.6
2.9
2.7
2.3
1.3
1.4
3.3
3.0
2.4
3.1
1.6
2.8
1.3
1.6
1.7
1.7
1.6
2.2
1.8
a
(R)-BINAP
(R)-BINAP
(R)-BINAP
(S)-tol-BINAP
(S)-tol-BINAP
(S)-tol-BINAP
(S)-tol-BINAP
(S)-xylyl-BINAP
(S)-xylyl-BINAP
(S)-xylyl-BINAP
(S)-xylyl-BINAP
9
0
1
2
10
11
12
Trp-Na
1
3
a
. Conditions: T = 6 ◦C, organic phase:
Trp-Na = Trp + 1 eq. NaHCO
3
DCM, aqueous phase: dd water; [substrate] = 2.0 mM. [a] The distribution
Conditions: host: PdCl
2
((S)-xylyl-BINAP), Substrate: Trp-Na, [host] =
◦
D(org/aq) of the substrate (AA) over the two phases is defined as the
1.0 mM, [substrate] = 2.0 mM, T = 6 C, aqueous phase: dd water,
a
ratio of the concentration of the substrate over the two phases (D =
(*)precipitation/suspension. The physical partition (P) is defined as the
b
[
AA]org/[AA]aq). The operational selectivity (aop) is defined as the ratio
distribution (D) of the substrate in absence of a host.
of distribution of enantiomers (aop = DD-AA/DL-AA).
Table 3 Octane/Octanol mixing experiments
resulting light yellow solution was directly applied in extraction
experiments.
The palladium (R)-BINAP complex as expected shows dis-
tributions and operational selectivities (Table 1, entries 1–4)
that correspond well with the palladium (S)-BINAP complex
Entry
Octane-%
D(org/aq)
a
op
1
2
3
4
5
6
0
0.5
0.4
0.3
0.3
0.0*
0.0*
3.3
3.3
3.4
3.4
—
20
40
60
80
100
(
2
corresponding conditions entry 4: D(org/aq) = 0.5, aop
=
—
33
.4). The RP-HPLC data showed that the palladium (R)-BINAP
complex extracts L-Trp selectively, whereas D-Trp is extracted by
the palladium (S)-BINAP complex.
Entries 5–8 and 9–12 (Table 1) show the results for the
Host: [PdCl
2
((S)-xylyl-BINAP)], Substrate: TRP-Na, [host] = 1.0 mM,
◦
[
substrate] = 2.0 mM, T = 6 C, aqueous phase: dd water, (*)precipita-
tion/suspension.
(
S)-tol-BINAP and the (S)-xylyl-BINAP palladium complexes,
respectively. In these cases, a preference is shown for the D-Trp.
The distributions are essentially constant for all of the palladium
complexes examined. For example, D = 0.5 for the extraction of
Trp-Na (entries 4, 8 and 12). The aop for the palladium (S)-tol-
BINAP complex is significantly lower (selectivities < 1.8) than
for the (S)-BINAP complex. In contrast, the (S)-xylyl-BINAP
palladium complex shows a significantly higher aop (> 3.0). This
increase in selectivity is attributed to the increased steric hindrance
of the (S)-xylyl-BINAP ligand.
dichloroethane (Table 2, entry 1), allowing for a non-halogenated
solvent as 1-octanol to be applied.
The P(org/aq) is 0.6–0.7 for the substrate with the different
butanol solvents used as the organic phase (Table 2, entries 8–11)
and this can be attributed to the partial miscibility of the butanol
solvents and water (e.g. 7.7 g 1-butanol per 100 mL water). As
noted, physical partition of the substrate reduces the achievable
a
op. Fluorinated alcohols (Table 2, entries 12 and 13) provide little
to no physical partition, but nevertheless show lower aop than
-octanol and the halogenated solvents. This lowering of the aop
1
is probably due to increased polarity of perfluorinated alcohols
compared to alkyl alcohols, which has been attributed to strong
hydrogen bonding.
The relation between organic solvent polarity and aop was
examined by varying the amount of octane mixed with 1-octanol.
As the percentage of octane is increased (Table 3, entries 1 to
Solvent dependence
35
33
As noted in our previous publication, the organic solvent
employed has a major influence on selectivity, as a consequence
of differences in solvation contributions in enantioselective host–
guest complexations (Table 2).
The extra methyl groups present in the xylyl moiety of the
34
4
), the distribution decreases compared to neat 1-octanol. The
decreased polarity of the organic solvent disfavors formation of
the complex. However, the aop increases slightly to 3.4. At 80%
octane and higher, immediate precipitation of the host occurred
(
S)-xylyl-BINAP, make the palladium (S)-xylyl-BINAP complex
soluble in a wider range of solvents compared to the palladium (S)-
BINAP complex. However, the complex formed upon extracting
Trp-Na into toluene or carbon tetrachloride precipitated (Table 2,
entries 5 & 7). The extractions using (S)-xylyl-BINAP follow the
same trend as observed for (S)-BINAP in the solvents tested with
the palladium (S)-BINAP complex, showing high operational
selectivities for dichoromethane and 1,2-dichloroethane (Table 2,
entries 2 and 4). It should be emphasized that the use of 1-octanol
as a solvent provides a higher aop compared to DCM and 1,2-
(
Table 3, entries 5 and 6).
Substituent effects of the substrate
Phenylalanine analogues. The substrate scope was examined
using a series of Phe analogues and PdCl ((S)-BINAP) as a host
2
in a number of solvents. The relation between the electronic effects
of the substituents in the substrate Phe and aop, were investigated
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Table 4 Relationship between host and substrate
Table 6 Phenylalanine analogues in 1-octanol
Entry
Substrate
P(org/aq)
D(org/aq)
a
op
Entry
Substrate
4CF -Phe
4Br-Phe
4Cl-Phe
4F-Phe
Phe
P(org/aq)
D(org/aq)
a
op
a
1
2
3
4
4Cl-Phe
0.0
0.0
0.0
0.0
0.7
0.5
0.4
0.5
3.3
2.9
2.5
2.4
1
2
3
4
5
6
3
0.5
0.3
0.1
0.0
0.0
0.0
1.9
6.0
0.7
0.5
0.4
0.5
1.2
1.7
1.1
2.1
1.9
1.8
4F-Phe
Phe
4MeO-Phe
Conditions: Host: [PdCl
2
((S)-BINAP)], [host] = 1.0 mM, [substrate] =
◦
a
3
4MeO-Phe
2
.0 mM, organic phase = DCM, T = 6 C, 1 eq. NaHCO
. 4X-Phe
represents an underivatized phenylalanine with the X substituent at the
Conditions: Host = PdCl
2
((S)-xylyl-BINAP), [host] = 0.25 mM, [sub-
◦
para-position.
strate] = 2.0 mM, org = 1-octanol, aq = dd., T = 6 C, 1 eq. NaHCO3.
using Phe derivatives substituted with electron donating and
electron withdrawing groups at the para-position. All of the Phe
analogues have a distribution between 0.4 and 0.7 (Table 4).
The aop however, shows an increase with increasing electron
withdrawing groups at the para-position. The aop increases from
for Phe using chiral metal complexes as hosts. The aop of the
other analogues represent the highest selectivities reported to date
and the aop of 7.0 for 4Br-Phe allows full separation of both
enantiomers in a 6 stage CCS cascade.
A correlation between the polarity index P’, which is a measure
2
.4 for the electron-donating methoxy group (entry 4) to 3.3 for
the electron-withdrawing chloride group (entry 1).
With the PdCl ((S)-xylyl-BINAP) complex as the host, a wider
36
of the polarity of the solute–solvent interactions, and aop is
observed. Dichloromethane has the lowest polarity of 3.4, while
chloroform and 1,2-dichloroethane have a polarity index of 3.7
and 4.4, respectively. The lower polarity of dichloromethane
appears to benefit the selectivity of the extraction, as the tested
Phe analogues, all show the same trend of increasing aop with
decreasing polarity.
2
substrate scope of Phe analogues was extracted (Table 5) in a
number of halogenated organic solvents. As with tryptophan, the
palladium complex with the (S)-xylyl-BINAP ligand shows higher
a
op than with the (S)-BINAP ligand (Table 4). For example, in the
case of Phe, the aop is 2.5 in the case of (S)-BINAP (Table 4, entry
) whereas it is increased to 4.4 with (S)-xylyl-BINAP (Table 5,
entry 6).
As with (S)-BINAP, a relationship is observed between the
When 1-octanol is used as the organic phase (Table 6) the
3
physical partitioning of the CF
3
-, Br and Cl-Phe analogues (entries
1
–3) should be noted. This physical partition is not observed using
halogenated solvents (Table 5). The observed physical partition
with 1-octanol as an organic solvent also has a dramatic effect
on the distribution, which is up to 6.0 for 4Br-Phe (entry 2). Phe
shows a distribution of 0.4 (entry 5) which is comparable to the
distributions observed in halogenated solvents (Table 5, entries 6,
electron-withdrawing or donating property of the substituent on
the para position of the phenyl ring of the Phe analogue and the
a
op. The Phe analogues bearing an electron withdrawing group
show selectivities above 5.7 in DCM (entries 1–5), whereas Phe
and 4MeO-Phe show aop = 4.4 (entries 6, 7). The selectivity of
1
3, 20). The observed aop in 1-octanol does not correspond to
4
.4 observed for Phe is the highest selectivity observed to date
its polarity index of 3.2, which is presumably due to additional
interactions of 1-octanol with the substrate, as observed in entries
Table 5 The ELLE of Phe analogues with PdCl ((S)-xylyl-BINAP) and
2
1–3.
a number of organic solvents
Entry
Substrate
Solvent
P(org/aq)
D(org/aq)
a
op
N-Acetyl protected amino acids
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
4NO
4CF
4Br-Phe
4Cl-Phe
4F-Phe
Phe
2
-Phe
-Phe
DCM
DCM
DCM
DCM
DCM
DCM
DCM
1,2-dce
1,2-dce
1,2-dce
1,2-dce
1,2-dce
1,2-dce
1,2-dce
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.8
1.9
1.7
0.9
0.7
0.6
0.7
0.7
1.4
1.3
0.8
0.5
0.5
0.6
0.9
2.5
2.3
0.9
0.8
0.7
0.8
6.7
6.3
7.0
6.4
5.7
4.4
4.4
5.8
5.3
6.0
6.0
5.2
4.3
4.2
4.2
5.0
4.9
4.3
4.3
3.3
3.2
The distribution of acetyl protected amino acids is shown in
Table 7. The free carboxylate group is necessary for the binding
to the palladium complex. The distribution changes from 0.5
3
for unprotected Trp (Table 4.1, entry 7) to 1.1 for the N-acetyl
+
4MeO-Phe
protected Trp. The pK
a
7
of the dissocation constant of the NH
3
3
4NO
4CF
2
-Phe
-Phe
group of Trp is 9.39, whereas the pK of N-acetyl protected
amino acids is around 3.6. As a result, the concentration of the
a
3
38
0
1
2
3
4
5
6
7
8
9
0
1
4Br-Phe
4Cl-Phe
4F-Phe
Phe
substrate in its anionic form will be higher in the case of N-Ac-Trp
at the pH conditions of the experiments.
Comparison of the aop of the protected Trp with the unprotected
Trp are compared, and a significant decrease is observed. The aop
is 1.6 for N-Ac-Trp-Na (entry 3) compared to aop = 2.4 for Trp-
4MeO-Phe
4NO
4CF
2
-Phe
-Phe
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
3
3
3
3
3
3
3
3
33
4Br-Phe
4Cl-Phe
4F-Phe
Phe
Na. The free amine group of Trp is supposed to interact with
39
the palladium to induce operational selectivity. There is a clear
relationship between the host/substrate stoichiometry and the aop
,
4MeO-Phe
with aop increasing with higher concentrations of host relative to
the substrate. This indicates that a different extraction mechanism
is involved. Typically, host/substrate stoichiometry dependent aop
behaviour is indicative for a homogeneous mechanism rather than
Conditions: host = PdCl
2
((S)-xylyl-BINAP), [host] = 1.0 mM, [sub-
◦
strate] = 2.0 mM, T = 6 C, 1 eq. NaHCO
3
3
048 | Org. Biomol. Chem., 2010, 8, 3045–3054
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Table 7 ELLE of N-Ac protected amino acids
Temperature Dependence studies
Entry
Substrate
[host]/mM
D(org/aq)
a
op
The distribution and aop of the ELLE of Trp using PdCl
2
((S)-
BINAP) as a host were determined in the temperature interval
1
2
3
4
5
6
7
8
9
N-Ac-Trp
5.0
1.0
5.0
1.0
5.0
1.0
5.0
1.0
5.0
1.0
5.0
1.0
1.1
0.5
2.3
0.5
1.4
0.5
1.8
0.5
1.6
0.5
3.0
0.5
1.5
1.1
1.6
1.0
1.2
1.0
1.3
1.1
1.2
1.1
1.3
1.0
◦
◦
from T = 6 C to 60 C. 1-Octanol was chosen as the organic
N-Ac-Trp
◦
phase, due to its high boiling point (195 C).
N-Ac-Trp-Na
N-Ac-Trp-Na
N-Ac-Pge
◦
Below T = 30 C, the distribution increases slightly from 0.5 to
◦
0
1
.6. Above T = 40 C, the distribution increases to a value above
N-Ac-Pge
.0 (Fig. 5a). A distribution above 1.0 at a host concentration
N-Ac-Pge-Na
N-Ac-Pge-Na
N-Ac-Phe
of 1.0 mM indicates that the concentration of substrate in the
41
organic layer exceeds the concentration of host. This overloading
1
1
1
0
1
2
N-Ac-Phe
N-Ac-Phe-Na
N-Ac-Phe-Na
of substrate with respect to the host most probably involves the
complexation of a second amino acid on the palladium.
◦
Conditions: host = PdCl
2
((S)-BINAP), [substrate] = 2.0 mM, T = 6 C,
1
eq. NaHCO
3
.
20
an interface mechanism. This implies that the N-acetyl protected
substrate will diffuse into the organic phase prior to complexation.
Counterion, temperature, concentration and kinetic studies
Counterion effect. To evaluate the effect of the counterions of
the palladium host, in situ experiments with palladium salts and
three halogen anions, nitrate and acetate as counterions were per-
formed. Data from single extraction experiments are summarized
in Table 8. A strong relationship between the nature of the halide
anion and the distribution is revealed. The distribution drops
from 0.8 with the chloride (entry 1) to 0.1 with the iodide anion
(
entry 3). This effect is attributed to the lipophilicity and hence
the increased polarizability of the halide anions with increasing
atomic number. The lipophilicity of iodide and its subsequent low
aqueous solubility compared to chloride shifts the equilibrium of
the complex formation of the substrate (Trp) with the palladium
in favour of the complex. This is a recurrent phenomenon in the
40
field of phase-transfer catalysis. The increased aop of the host
with the bromide anion (entry 2) could be an indication for an
improved interaction of the substrate with the host. Unfortunately,
the distribution of the iodide anion complex (entry 3) was too low
to determine the aop
.
When nitrate is used as a counterion (entry 4), distributions and
selectivities are comparable to those employing chloride (entry 1)
are observed. In the case of an acetate counterion (entry 5), a
comparable distribution to the chloride counterion is achieved,
Fig. 5 The relation between temperature and the distribution (a) and the
but with a much lower aop
.
2
aop (b) of the ELLE of Trp. Conditions: PdCl ((S)-BINAP) (1.0 mM),
Trp-Na (2.0 mM), n-octanol, dd. water. Lines are a guide to the eye.
Table 8 The effect of the halogen anion of the host
The aop decreases with increasing temperature. Over the tem-
Entry
Metal precursor
D(org/aq)
a
op
◦
perature range 6–40 C, the distribution remains below 1.0 and the
42
a
op decreases with increasing temperature (Fig. 5b). When the
1
2
3
4
5
PdCl
PdBr
PdI
Pd(NO
Pd(OAc)
2
0.8
0.4
0.1
0.7
0.9
2.2
2.7
—
2.4
1.3
2
distribution is above 1.0, the aop decreases below 1.2 and the host-
substrate interaction becomes essentially nonselective. Optimal
2
3
)
2
selectivity is achieved at lower temperatures. However, below T =
2
◦
6
C, extraction becomes toodifficultdue toanincrease in viscosity
Conditions: ligand: (S)-BINAP, substrate = Trp-Na, [metal precursor] =
of the solution and the decrease of host and substrate solubility.
◦
1
.0 mM, [ligand] = 1.1 mM, [substrate] = 2.0 mM, T = 6 C, org = DCM,
◦
Therefore in this case, T = 6 C is considered to be the optimal
aq = dd.
temperature for enantioselective extraction.
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Table 9 The ELLE of Trp at various concentrations
Entry
[host]
[substrate]
D(org/aq)
a
op
1
2
3
4
5
2.0
4.0
6.0
8.0
10.0
4.0
8.0
12.0
16.0
20.0
0.42
0.42
0.76
0.71
0.76
2.3
2.4
2.2
2.4
2.3
Conditions: PdCl
DCM, aqueous phase = dd. Water, T = 6 C.
2
((S)-BINAP), substrate = Trp-Na, organic solvent =
◦
Concentration dependence and kinetics
The concentration of both the host and the substrate can be
increased compared to the standard ELLE experiments performed
in this study. The behaviour of the extraction system at increased
concentrations is of special importance in application of ELLE to
a CCS cascade.
The concentration dependence on distribution and aop are
shown in Table 9. The increase of the distribution with increasing
concentration (from D = 0.42 for chost = 2.0 mM to D = 0.76 for
c
host = 10.0 mM) is due to the increase in pH, which is a result of
the increase of concentration of sodium-Trp. As expected, the aop
remains constant over the concentration range examined.
The time of equilibration was determined using a reaction vessel
as depicted in Fig. 6. Stirring was done at rotation speeds of
8
00, 900 and 1,000 rpm. This resulted in fully disperse conditions
and represents the conditions in a CCS, with equilibration being
achieved within 2 min. This time interval is well within the hold-up
time in a single stage of a CCS cascade and hence the extraction
system is fast enough for application in a CCS cascade.
Fig. 7 Temperature dependent studies on the PdCl((S)-BINAP)-D-Trp
(
-
a) and PdCl((S)-BINAP)-L-Trp (b) complex. Spectra 1–10 represent T =
◦
◦
◦
50 C to +40 C at 10 C intervals.
absorption (Fig. 7a, spectrum 10), where the PdCl((S)-BINAP)-L-
Trp shows a broad ill-defined absorption (Fig. 7b, spectrum 10).
◦
The temperature was decreased with 10 C intervals in the case
of PdCl((S)-BINAP)-D-Trp (Fig. 7a), the singlet absorption splits
up into an AB-system with the two absorptions at 29.50 and
◦
2
8.55 ppm and a coupling constant of J = 31.4 Hz at T = -40 C
(
spectrum 2).
Similar results are obtained with the PdCl((S)-BINAP)-L-Trp
complex. The ill-defined absorption splits up into an AB-system
with the two absorptions at a shift of 29.25 and 28.78 ppm and
a coupling constant of J = 31.2 Hz. The coupling constant in
both diastereomeric complexes is almost the same, whereas the
difference in chemical shift of the complexes is ddd = 0.48 ppm. The
larger change in the chemical shift is observed with the PdCl((S)-
BINAP)-D-Trp complex. This is another indication of the more
stable host-substrate complex formed by the palladium host and
the D-Trp enantiomer. The difference in chemical shift observed for
the AB-systems of both diastereomeric complexes could imply a
method for direct determination of the diastereomeric excess of the
Fig. 6 Reaction vessel scheme. Conditions: host: [PdCl ((S)-BINAP)],
2
substrate = D-Trp-Na, [host] = 0.1 mM, [substrate] = 0.2 mM, org =
DCM, aq = dd, T = rT, V(org) = 10 mL, V(aq) = 10 mL.
3
1
P-NMR Spectroscopy
3
1
P-NMR spectroscopic studies were carried out using PdCl ((S)-
2
BINAP) in CDCl
3
at c = 5.0 mM and AA-Na at c = 50.0 mM.
The P-NMR spectra of the PdCl ((S)-BINAP) complex upon
titration of the enantiomers of Trp has been reported by us
3
1
2
43;44
host-substrate complexes.
The attempt to direct diastereomer
33
31
31
previously. Here a temperature dependence P-NMR study is
determination by P-NMR is depicted in Fig. 8.
presented (Fig. 7). (S)-BINAP acts as an internal standard. At
Unfortunately, no accurate analysis could be made, since the
absorptions overlap, even at lower temperature (Fig. 8). Other
amino acids (Phe, Pro, Val, Asp) were tested, but did not provide
◦
higher temperatures (T = 40 C) the two complexes show distinct
absorptions: the PdCl((S)-BINAP)-D-Trp complex, which is the
3
1
preferred diastereomer formed in ELLE, shows a single, defined
well resolved P-NMR absorptions. On the contrary, in the case of
3
050 | Org. Biomol. Chem., 2010, 8, 3045–3054
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3
1
Fig. 8
P-NMR spectra of the organic phase of the ELLE of DL-Trp-Na,
◦
◦
taken at T = -40 C (bottom) and +20 C (top). Conditions ELLE: host =
◦
[
PdCl
2
((S)-BINAP)], [host] = 10.0 mM, [substrate] = 20.0 mM, T = 6 C.
Cys, excellent separated absorptions were observed even at room
temperature, presumably as a result of the coordination of the thiol
45
functionality of Cys to the palladium metal centre. Fig. 9 shows
3
1
the P-NMR spectrum of the PdCl ((S)-BINAP) complex upon
2
titration of Cys. Two sets of two doublets can be seen, the first set
of doublets is at d = 32.1 and 16.9 ppm with J = 35.3 Hz. The
second set is at d = 31.1 and 17.6 ppm with J = 32.4 Hz. The two
sets can be identified as two AB-systems, which represent the two
diastereomers of D-Cys and L-Cys bound to the host. The ratio by
NMR integration is 1.4, which represents an ee of 20% for D-Cys
in the organic phase. This corresponds to aop = 2.2 if D = 1.0.
Fig. 10 UV-Vis spectra of PdCl
2
((S)-BINAP) upon titration of D-Trp
(
(
0.0–10.0 mM) (left) and UV-Vis differential spectra (right) of D-Trp
black) and L-Trp (grey) at 1.0 (solid) and 10.0 mM (dashed).
affinity of the PdCl ((S)-BINAP) for the D-Trp enantiomer. At
2
higher concentration (c = 10.0 mM), only a minor difference
at l = 417 nm (Fig. 11b) is observed, whereas the difference in
the absorption in the UV-Vis spectra is observed over the entire
measured wavelength interval.
3
1
Fig. 9
P-NMR spectra of the organic phase of the ELLE of DL-Cys-Na,
◦
taken at T = -40 C. Conditions ELLE: host = PdCl
2
((S)-BINAP),
◦
[
host] = 10.0 mM, [substrate] = 20.0 mM, T = 6 C.
These results suggest the binding of both the Trp enantiomers
to the palladium complex is similar, resulting in a similar type
of geometry of the palladium complexes. The slight difference in
ellipticity at l = 417 nm is the only indication of the observed
preference of the D-Trp enantiomer over the L-Trp enantiomer
observed in the ELLE experiments vide supra.
UV-Vis and CD Spectroscopy
UV-Vis and CD spectroscopy were employed to study the binding
of Trp to PdCl ((S)-BINAP) by titration of the Trp enantiomers.
2
The titration involved the increase of concentration of a Trp
enantiomer in the aqueous phase in an extraction experiment.
The UV-Vis spectra of the titration of D-Trp to the PdCl
2
((S)-
FTIR Spectroscopy. FTIR spectra were taken for the
BINAP) complex shows an isosbestic point at 328 nm and a
decrease in absorption above 328 nm (Fig. 10a). The differential
spectra (Fig. 10b) reveal two minima in absorption, at l = 335 nm
and l = 365 nm. A difference in the maximum is observed at c =
PdCl
complex (Fig. 12).
The strong absorption band at 1654 cm can be assigned to the
2
((S)-BINAP) complex and the PdCl((S)-BINAP)-D-Trp
-
1
46
carboxylate anion complexed to the palladium. The absorption
-
1
1
0.0 mM for both substrate enantiomers: D-Trp shows the largest
band at 3309 cm can be assigned to the stretching vibrations
of the primary amine group coordinated to the palladium metal
change in absorption.
47
The CD spectra of PdCl
2
((S)-BINAP) upon titration of D-Trp
center.
(
3
Fig. 11a) show a bathochromic shift from l = 363 nm to l =
To confirm the coordination of the amine to the palladium,
a comparison between the tryptophan palladium complex and
75 nm. A hypsochromic shift is observed for the maximum at
l = 345 nm, which shifts to l = 343 nm upon titation of D-Trp. A
the tryptophan sodium complex (H
by FTIR spectrscopy. The FTIR spectra of the complexes are
depicted in Fig. 13. The stretching band of the primary amine of
the sodium salt of Trp appears at 3382 cm , whereas the corre-
sponding band of the palladium complex appears at 3309 cm as
mentioned before. This difference in wavenumber is an additional
2
N-CHR-CO Na) was made
2
bathochromic shift is observed at l = 325 nm, which shifts to l =
3
29 nm upon titration of D-Trp.
-
1
The CD differential spectra reveal corresponding ellipticities
for the titration of both Trp enantiomers (Fig. 11b). The higher
ellipticity at l = 340 nm for D-Trp can be explained by the higher
-
1
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Fig. 13 FTIR spectra of PdCl((S)-BINAP)(D-Trp) (grey) and Trp-Na
(
black). Arrows indicate absorption bands assigned to the stretching
vibrations of the primary amine.
Fig. 11 CD spectra of PdCl
0.0–10.0 mM) (a) and CD differential spectra (b) of D-Trp (black) and
L-Trp (grey) at 1.0 mM (solid) and 10.0 mM (dashed).
2
((S)-BINAP) upon titration of D-Trp
(
Fig. 14 FT-FIR spectra of PdCl ((S)-BINAP) (black) and PdCl((S)-
2
BINAP)-D-Trp (dashed).
in the spectrum of the PdCl((S)-BINAP)-D-Trp complex, whereas
-
1
the band at 290 cm has decreased in intensity and shifted to
-
1
2
88 cm (Fig. 15b).
Structure of the complex
As is shown in our previous publication, the amino acid substrate
binds to the palladium complex in a 1 : 1 ratio at neutral pH
conditions, proven by extraction experiments, ESI-MS, NMR
Fig. 12 FTIR spectra of PdCl
BINAP)(D-Trp) (black).
2
((S)-BINAP) (grey) and PdCl((S)-
33
spectroscopy and the calculation of association constants.
The results of the Far IR and Raman spectroscopy suggest
the dissociation of at least one Pd–Cl bond. The other Pd–Cl
bond is supposed to be either noncoordinating (indicated by the
loss of absorption in the Raman spectrum), or less coordinating
(indicated by the decrease in intensity in the FT-FIR spectrum).
The FT-IR spectrum shows a shift in the bands of the amine. This
could indicate a coordination to the palladium.
indication for the primary amine coordinating to the palladium
39,48
metal center.
FT-FIR and Raman Spectroscopy. For PdCl
Pd–Cl stretching bands were found at wavenumbers corresponding
2
((S)-BINAP),
-
1
49
to those in the literature: 306 and 292 cm (Fig. 14). The
spectra of both PdCl ((S)-BINAP) and PdCl((S)-BINAP)-D-Trp
2
All this evidence points to a coordination of the amino acid
in a bidentate fashion to the palladium complex as proposed in
Fig. 16. The bidentate complexation is a valid explanation for the
-
1
were normalized at 504 cm . The aforementioned Pd–Cl bands
decrease in intensity in the spectrum of PdCl((S)-BINAP)-D-Trp
-
1
51
and shift to 314 and 302 cm .
observed high operational selectivities using metal complexes.
52
The Raman spectrum of the PdCl
2
((S)-BINAP) (Fig. 15a)
The three point interaction rule (TPI) is commonly used in ELLE
-
1
31,32
shows absorptions at 302 and 290 cm which can be assigned to
to explain the mechanism behind selective binding.
The first
the Pd–Cl stretching band. The band at 302 cm-1 is not present
50
two interactions are attracting interaction and are provided by the
3
052 | Org. Biomol. Chem., 2010, 8, 3045–3054
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ature and the nature of the substrate. The combined spectroscopic
evidence suggests a bidentate complexation of the substrate to the
host and a three-point interaction causing the enantioselectivity.
For future application in a CCS cascade, the system meets all the
required properties such as a high selectivity (which dramatically
limits the number of required stages), reversibility, versatility,
easy distribution control and sufficient concentration and kinetic
properties.
Acknowledgements
We thank Dr W.R. Browne for useful discussions. This research
was financially supported by the Netherlands Organisation for
Scientific Research (NWO) through the Separation Technology
program in cooperation with DSM and Schering-Plough.
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3
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This journal is © The Royal Society of Chemistry 2010
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