Substrate dependent ligand acceleration in enantioselective hydrogenation of (E)-2,3-diarylpropenoic acid on cinchonidine-modified Pd/C

Article abstract of DOI:10.1016/j.cattod.2014.03.075

The hydrogenation of substituted 2,3-diarylpropenoic acids over cinchonidine-modified Pd/C showed obvious ligand acceleration effects with the substrates yielding over 90% enantiomeric excess (ee). 2,3-Di(methoxyphenyl)propenoic acid exhibited profound ligand acceleration, and its hydrogenation rate relative to the rate on unmodified Pd/C was 370%, yielding 91%ee. 3-(p-Fluorophenyl)-2-(o-methoxyphenyl)propenoic acid showed 92%ee with an activity increase of 230%. It is considered that the difference is caused by the ortho-methoxy substituent on the α-phenyl ring. The substituent effect of the ortho-substituent on the α-phenyl ring demands the presence of an electron-donor substituent on the β-phenyl ring. A remarkable substituent effect was observed with 3-(p-methoxyphenyl)-2-(o-methoxyphenyl)propenoic acid, which attained up to 93%ee. The factors affecting enantioselectivity results are analyzed for each substituent.

Full text of DOI:10.1016/j.cattod.2014.03.075

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Substrate dependent ligand acceleration in enantioselective
hydrogenation of (E)-2,3-diarylpropenoic acid on
cinchonidine-modified Pd/C
Takuya Mameda, Kengo Nakai, Tomonori Misaki, Yasuaki Okamoto, Takashi Sugimura∗
Graduate School of Material Science, University of Hyogo, 3-2-1, Kohto, Kamigori, Ako-gun, Hyogo 678-1297, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydrogenation of substituted 2,3-diarylpropenoic acids over cinchonidine-modified Pd/C showed
obvious ligand acceleration effects with the substrates yielding over 90% enantiomeric excess (ee).
2,3-Di(methoxyphenyl)propenoic acid exhibited profound ligand acceleration, and its hydrogenation
rate relative to the rate on unmodified Pd/C was 370%, yielding 91%ee. 3-(p-Fluorophenyl)-2-(o-
methoxyphenyl)propenoic acid showed 92%ee with an activity increase of 230%. It is considered that
the difference is caused by the ortho-methoxy substituent on the ␣-phenyl ring. The substituent effect
of the ortho-substituent on the ␣-phenyl ring demands the presence of an electron-donor substituent
on the ␤-phenyl ring. A remarkable substituent effect was observed with 3-(p-methoxyphenyl)-2-(o-
methoxyphenyl)propenoic acid, which attained up to 93%ee. The factors affecting enantioselectivity
results are analyzed for each substituent.
Received 9 December 2013
Received in revised form 25 March 2014
Accepted 31 March 2014
Available online xxx
Keywords:
Enantioselective hydrogenation
Chiral modification
␣,␤-Unsaturated acid
Pd/C
Modifier concentrations
© 2014 Published by Elsevier B.V.
1. Introduction
of the modified vs. unmodified hydrogenation as shown by the
following formula (1)
Cinchonidine (CD)-modified Pd/C is a heterogeneous catalyst
that produces optically active ␣-chiral carboxylic acids by enan-
tioselective hydrogenation of 2,3-unsaturated acids [1–6]. Pd/C
catalysts pretreated at 353 K in H2 show excellent enantiose-
lectivity, 82% enantiomeric excess (ee) for the hydrogenation of
2,3-(E)-diphenylpropenoic acid (␣-phenylcinnamic acid, PCA) in
wet dioxane (2.5% H2O) at atmospheric hydrogen pressure at 296 K
in the presence of benzylamine (1 equiv.) [7–9] (Scheme 1).
It is considered that an acid group of the substrate interacts with
the basic quinuclidine (1-azabicyclo[2,2,2]octane) part of the chiral
surface modifier CD through a hydrogen bond during the reaction.
Thus prochirality around the olefin is stereocontrolled [10,11] to
give the hydrogenation product enantioselectively. We proposed
that the CD-substrate interactions specific to the substrate deter-
mine its intrinsic stereoselectivity (i%) during hydrogenation [12].
In addition, the reaction rate of enantioselective hydrogenation of
the CD-modified sites relative to that of the corresponding racemic
hydrogenation can be a dominant factor to determine the prod-
uct ee. Thus, the product enantioselectivity is determined by two
independent factors; intrinsic stereoselectivity i% and the rate ratio
vm
(vm + vu)
ee% = i% ×
(1)
Here, vm denotes the hydrogenation rate on the Pd surface mod-
ified with CD, and vu denotes the hydrogenation rate on the Pd
surface in the absence of CD.
Compared with PCA (1), 2,3-di(methoxyphenyl)propenoic
acid
(2,
DMPCA)
and
3-(p-fluorophenyl)-2-(o-
methoxyphenyl)propenoic acid (3, FMPCA) showed high
enantioselectivity to give corresponding products of 91%ee
[13] and 92%ee [14,15], respectively under reaction conditions
similar to CD-modified Pd/C catalysis. In a preliminary commu-
nication, we showed that the hydrogenation rates of DMPCA and
FMPCA display anomalous acceleration by cinchona modification
(ligand acceleration), and concluded that the rate acceleration is
primarily the origin of these high ee% values [16]. Here, the rate
acceleration can be induced by the substituents of an ␣-phenyl
and/or ␤-phenyl ring. Thus by using substrates (1–9) we tried to
clarify which substituent causes significant ligand acceleration
resulting in enhanced enantioselectivity.
∗
Corresponding author. Tel.: +81 791580168.
E-mail address: sugimura@sci.u-hyogo.ac.jp (T. Sugimura).
http://dx.doi.org/10.1016/j.cattod.2014.03.075
0920-5861/© 2014 Published by Elsevier B.V.
Please cite this article in press as: T. Mameda, et al., Substrate dependent ligand acceleration in enantioselective hydrogenation of
(E)-2,3-diarylpropenoic acid on cinchonidine-modified Pd/C, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.075

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Scheme 1. Enantioselective hydrogenation of PCA with CD-modified Pd/C.
Table 1
HPLC conditions.
Substrate
Columna
Lengthb
Eluent (Hex:IPA:TFA)
(R)-productc
(S)-productc
1
2
3
4
5
6
7
8
9
OJ-H
AD-3
OJ-H
OJ-3
AD-3
OJ-3
OJ-H
OJ-3
OJ-3
25 cm
5 cm
25 cm
5 cm
5 cm
5 cm
25 cm
5 cm
5 cm
95/5/0.1
95/5/0.1
95/5/0.1
95/5/0.1
97.5/2.5/0.1
95/5/0.1
95/5/0.1
95/5/0.1
95/5/0.1
15.4
7.1
12.8
5.6
18.7
2.7
17.4
5.6
24.4
8.1
15.7
6.9
20.7
6.4
29.5
7.0
5.6
8.4
a
Obtained from Daicel Corporation.
The inside diameter of chiral column is 4.6 mm.
The retention time [min] at the flow rate is 1.0 mL/min with the exception of 0.6 mL/min for 5.
b
c
2. Experimental
A 5%Pd/C catalyst was obtained from N.E. Chemcat (5% STD type)
in a 51% wet form, and stored at 280 K until use. PCA (1) was
used after recrystallization from acetone, and all the other sub-
strates (2–9) were prepared by Perkin reaction using a method
reported [15]. The hydrogenation conditions were as follows.
5%Pd/C (23 mg calculated as a dry form, 10 ␮mol of Pd) was pre-
treated at 353 K in wet dioxane (dioxane containing v/v 2.5% water)
under atmospheric hydrogen for 30 min. After cooling to 296 K,
CD (0.05–100 ␮mol) in wet dioxane was added to the catalyst.
A substrate (0.5 mmol) and then benzylamine (1.0 equiv.) in wet
dioxane were added (total amount of the solvent = 10 ml) to the CD-
modified catalyst with stirring. Within 3–6 h the substrate was fully
converted to the expected products at 296 K under 105 Pa of hydro-
gen. The product ee% were determined by chiral high performance
liquid chromatography (HPLC). The relative initial hydrogenation
rates were determined at 25% conversion by measuring hydrogen
consumption and normalized to the corresponding rates at 0-M of
CD (relative initial rate = observed initial rate over the modified cat-
alyst/the initial rate over the unmodified catalyst). HPLC conditions
are shown in Table 1.
Fig. 1. Substrates used for the enantioselective hydrogenation over CD-modified
Pd/C.
Table 2
Substrate substitution effect on the hydrogenation rate for unmodified Pd/C.
a
b
c
Substrate
␣-Substituent
␤-Substituent
vu
v2u
v1/2u
1
2
3
4
5
6
7
8
9
Ph
Ph
96
26
17
73
46
33
16
21
24
93
27
18
79
40
33
16
21
24
94
26
16
79
43
34
12
24
20
p-MeO-Ph
o-MeO-Ph
p-MeO-Ph
o-MeO-Ph
Ph
o-MeO-Ph
Ph
p-MeO-Ph
p-MeO-Ph
p-F-Ph
Ph
Ph
p-MeO-Ph
p-MeO-Ph
p-F-Ph
p-F-Ph
3. Results and discussion
a
Hydrogen uptake rate (vu/mmol g−1 h−1) at 25% substrate conversion of the sub-
The CD modification usually reduces the hydrogenation rate
because of the prevention of the substrate-adsorption on the Pd
surface by the adsorbed modifier (relative initial rate < 1.0) [17].
However, a properly structured substrate can show ligand accelera-
tion by a certain interaction with the adsorbed CD on Pd/C (relative
initial rate > 1.0). The new substrates (4–9) shown in Fig. 1 were
investigated to reveal the reason for the high enantioselectivity
with DMPCA (2) and FMPCA (3) by measuring the ligand accelera-
tion effect.
CD concentration was varied in a wide range to examine the
balance of the deceleration/acceleration effects of the adsorbed
CD modifier. Unmodified catalyst were prepared through the same
modification process, except for the absence of CD [17]. The initial
hydrogenation rates (vu) of 1–9 on the unmodified catalyst under
standard reaction conditions are presented in Table 2. The reaction
strate at 296 K in the absence of CD under substrate/benzylamine/5%Pd/C/2.5% wet
dioxane = 0.5 mmol/0.5 mmol/21 mg/10 ml.
b
v2u (mmol g−1 h−1) was obtained similarly to vu except for the use of twice the
amount of substrate and benzylamine (1.0 mmol each).
c
v1/2u (mmol g−1 h−1) was obtained similarly to vu except for the use of half the
amount of substrate and benzylamine (0.25 mmol each).
rate varied from 16 to 96 (mmol/hg), which can be attributed
either to a substrate adsorption constant or the reactivity [18].
To further investigate the unmodified catalysts, the amount of the
substrate was varied; standard amount (0.5 mmol, vu), twice the
amount (1 mmol, v2u), or half the amount (0.25 mmol, v1/2u). Equal
amounts of benzylamine and the substrate were used. Regardless of
substrate concentration (25–100 M), the hydrogenation rates were
essentially identical, which lead to the conclusion that the substrate
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Fig. 2. (a) The product ee of the hydrogenation of 1, 4 and 5 as a function of logarithmic CD concentration (mol/L). (b) The product ee of the hydrogenation of 3 and 7 as a
function of logarithmic CD concentration. (c) The product ee of the hydrogenation of 2, 6, 8 and 9 as a function of logarithmic CD concentration.
Fig. 3. (a) Relative initial hydrogenation rates of 1, 4 and 5 as a function of logarithmic CD concentration (mol/L). (b) Relative initial hydrogenation rates of 3 and 7 as a
function of logarithmic CD concentration. (c) Relative initial hydrogenation rates of 2, 6, 8 and 9 as a function of logarithmic CD concentration.
fully covered the Pd surface under all substrate concentration con-
ditions in the absence of CD. The saturated adsorption amounts of
substrates 1–9 on the Pd surface are presumed to be almost iden-
tical because the structure and cross-section of the substrates are
similar. Thus, it is presumed that the difference in the initial rates
on the unmodified catalyst (vu) is because of the difference of the
intrinsic reactivity of the individual substrate. It is clear from Table 2
that the reaction rate is decreased by ␣- or ␤-substitution. Further-
more, comparisons between 2 and 7, 3 and 9, and 4 and 5 Table 2,
indicate that the hydrogenation rates of the substrates with ␣-o-
MeO-Ph substituent are slower than those of the substrates with
␣-p-MeO-Ph. Therefore, it is considered that the intrinsic reactivity
of the substrate is determined by an electronic effect and/or a steric
effect due to the ␣-o-MeO-Ph substituent.
exchange between the product and CD is faster than that between
the product and substrate at a high CD concentration, because
the adsorption strength of CD with quinoline (1-azanaphtalene)
is stronger than that of the substrate. On the other hand, with
insufficient amounts of the CD modifier, the hydrogenation of the
substrate preferentially occurred on the unmodified sites and the
ee% value and the rate acceleration declined. The vertical dotted
lines at log[modifier] = –3.5 in Figs. 2 and 3 denote the maximum
point of the relative initial rates for 2,3-diaryl unsaturated acids.
Below this point, the ee% value starts to decrease for most sub-
strates when CD and BA are used as a chiral modifier and an
additive, respectively. Thus, the maximum point may be the CD
concentration at which the competitive adsorption between the
substrate and CD is balanced over the Pd surface.
To disclose the balance of the deceleration/acceleration of the
hydrogenation rate, the concentration of the modifier was stud-
ied in detail. The chiral modification was carried out with 5%
Pd/C (21 mg as a dry form, 10 ␮mol as Pd) in a wide range
(100–0.01 ␮mol) of CD, i.e., 10−2–10−6 M. The product ee and the
relative initial hydrogenation rate were observed for each CD con-
centration and are plotted in Figs. 2 and 3, respectively. The relative
hydrogenation rate increased as CD concentration increased. After
taking a maximum of [CD] = 0.3 mM, the relative rate decreased
with increased CD concentration regardless of the substrate. It is
considered that at very high CD concentrations, the ligand acceler-
ation is suppressed by the competitive adsorption of the substrate
with CD during hydrogenation. It is presumed that the surface
The maximum of the modified rates (vm), maximum of the rela-
tive initial rates (rvm/vu) and ee%max are summarized in Table 3 for all
substrates used in the present study. On the basis of the relative ini-
tial rate, substrates (1–9) are classified into three groups. PCA type
contains 1, 4, and 5 shown in Figs. 2(a) and 3(a), FMPCA type con-
tains 3 and 7 shown in Figs. 2(b) and 3(b), and DMPCA type contains
2, 6, 8 and 9 shown in Figs. 2(c) and 3(c). Correlation between ligand
acceleration and enantioselectivity is evident for all the substrates.
PCA type shows relatively low eemax values (81–83%ee). In addi-
tion, the dependences of ee% on CD concentration is steep, and the
ee% values at low CD concentration are relatively poor. With FMPCA
type, the eemax values of 3 and 7 are the highest in all substrates
(92–93%ee). The CD concentration dependence of ee% is similar for
Please cite this article in press as: T. Mameda, et al., Substrate dependent ligand acceleration in enantioselective hydrogenation of
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Table 3
Substitution effect of the substrate on the maximum values of hydrogenation rate (vm), relative rate (rvm/vu), and ee%max over CD-modified Pd/C.
b
c
d
Substrate
␣-Substituent
␤-Substituent
Groupa
Max of vm
Max of rvm/vu
ee%max
1
2
3
4
5
6
7
8
9
Ph
Ph
a
c
b
a
a
c
b
c
c
116
96
40
90
72
120
43
74
130
370
230
123
150
370
260
360
400
83
91
92
81
82
91
93
88
87
p-MeO-Ph
o-MeO-Ph
p-MeO-Ph
o-MeO-Ph
Ph
o-MeO-Ph
Ph
p-MeO-Ph
p-MeO-Ph
p-F-Ph
Ph
Ph
p-MeO-Ph
p-MeO-Ph
p-F-Ph
p-F-Ph
96
a
The substrates are classified into (a) PCA type, (b) FMPCA type, and (c) DMPCA type on the basis of the maximum relative initial rate (rvm/vu).
vm (mmol g−1 h−1) is the hydrogenation rate under varying CD concentrations (log[modifier]/M = −6.0 to −2.0).
rvm/vu [%] is the value of relative initial rate at a varying CD concentration (log[modifier] = −6.0 to −2.0).
b
c
d
ee%max [%] is the maximum value of enantioselectivity at varying CD concentrations (log[modifier] = −6.0 to −2.0).
3 and 7. As shown in Fig. 2(c), the eemax values for the DMPCA
the substituent. The o-methoxy substituent on the ␣-phenyl does
not exerts the steric effect rather than the electronic effect through
the ␲-electron of the phenyl ring because p-methoxy substituent
on ␣-phenyl does not contribute to the ee value or ligand acceler-
ation. The o-methoxy substituent on the ␣-phenyl also contributes
to the twisting of the ␣-phenyl ring from the olefinic plane. It is
considered that the conjugation of ␤-arene-olefin-carboxylic acid
is enhanced because the conjugation of ␣-arene-olefin-␤-arene
becomes weak due to the twisted ␣-phenyl ring. In addition, the
oxygen atom may interact with carboxylic acid using hydrogen to
control the conformation of the substrate.
It is considered that the high enantioselectivity with 2, 6 and
9 in group c (Table 3) is obtained by high ligand acceleration; the
vm values are greatly enlarged as shown in Fig. 3(c). On the other
hand, the high enantioselectivity with 3 and 7 in group c (Table 3)
are achieved by higher intrinsic stereoselectivity (i%), because the
ee% values of DMPCA type and FMPCA type are almost equal despite
the lower acceleration of FMPCA type. The hydrogenation of 7 was
obtained at 93%ee because of the synergism effect between the ␣-o-
MeO, which improves intrinsic stereoselectivity and the ␤-p-OMe
substituent, which enhances ligand acceleration. The enantioselec-
tivity of FMPCA and DMPCA types is affected by different factors.
type substrates are satisfactory (87–91%ee), and the ee% values at
a low CD concentration are higher than those with the other sub-
strates. The magnitude of the CD concentration dependence of ee%
increases in the following order: PCA type < FMPCA type < DMPCA
type.
With DMPCA type substrates, the relative initial rates are
obviously higher(360–400%) than the other types, as shown in
Fig. 3(c). However the ee% values for 8 and 9 are slightly lower than
90%ee despite the high ligand acceleration. In contrast to the high
ee%max values, the ligand accelerations with 3 and 7 are not as high
(230–260%). PCA type shows relatively low ligand accelerations
(120–150%).
The ligand acceleration and intrinsic stereoselectivity can be
explained in term of the effect of substituents on ␣- and ␤-phenyl
rings. From a comparison of the ligand acceleration and ee%max of
PCA type with those of the other types, it is considered that the
methoxy substituent on the ␣-phenyl ring is independent of enan-
tioselectivity and ligand acceleration in the absence of a ␤-phenyl
ring substituent. On the other hand, the p-methoxy and p-fluorine
substituent on the ␤-phenyl ring influenced the ligand accelera-
tion. The advantage of the substituents on the ␤-phenyl ring can
be explained in terms of the mesomeric electron-donor effect. The
effect causes the conjugation of ␤-arene-olefin-carboxylic acid,
and the substrate conformation becomes flat [13]. It is considered
that the flatter conformation results in the high ee value and the
enhanced ligand acceleration. In fact, the substrates with a sub-
stituent on the ␤-phenyl ring show remarkable ligand acceleration.
Thus, these substrates exhibit high ee values by decreasing the con-
tribution of racemic hydrogenation on a CD-unmodified surface.
Comparison of 2, 6, 8, and 9 shows that the ee%max values of 2 and
6 were higher than those of 8 and 9 (Table 3). It is considered that
because the fact that the p-methoxy substituent is a stronger con-
jugative electron-donor than the p-fluorine substituent. In fact, the
ee values of 8 and 9 do not reach 90%ee. Moreover, the p-methoxy
substituent on the ␣-phenyl ring does not affect enantioselectivity
in the presence of the ␤-phenyl electron-donor substituent.
Interestingly, in contrast to the p-methoxy substituent on the ␣-
phenyl ring the o-methoxy substituent on the ␣-phenyl ring plays a
role for enantioselectivity in the presence of the ␤-phenyl electron-
donor substituent. As shown in Table 3, a comparison between 3
and 8 suggests that the ee% value with 3 is improved from 88%ee
to 92%ee by introducing the o-methoxy substituent on ␣-phenyl
ring. Similarly, the ee%max value of 7 is slightly improved compared
with 6 and 7. On the other hand, the introduction of the o-methoxy
substituent on the ␣-phenyl ring reduced the reaction rate. Hence
it is considered that the o-methoxy substituent on the ␣-phenyl
ring improves intrinsic stereoselectivity and reduces the hydro-
genation rate on CD-modified sites, and the synergism of some
substituents is varies in relation to the position and the property of
4. Conclusion
In summary, the correlation between ligand acceleration, ee%,
and substituent effects was analyzed in relation to the enan-
tioselective hydrogenation of 2,3-diaryl unsaturated acids over a
CD-modified Pd/C catalyst. The p-methoxy and p-fluorine sub-
stituents on the ␤-phenyl ring contribute to improved ligand
acceleration, resulting in the high product ee%. In addition, the o-
methoxy substituent on the ␣-phenyl ring was observed to improve
intrinsic stereoselectivity. However, the effect of the o-substituent
on the ␣-phenyl ring requires the p-electron-donor substituent on
the ␣-phenyl ring. The mesomeric effect of the p-electron-donor
substituent caused the conjugation of ␤-arene-olefin-carboxylic
acid, and the substrate conformation became flat. The o-methoxy
substituent on the ␣-phenyl also contributes to the twist of ␣-
phenyl ring from the olefinic plane. The effect of the substituents
on stereoselectivity was the sum of the individual substituents, and
the synergism effect varied. Among various factors, ligand acceler-
ation was shown to be the most important for high ee% values.
Overall the substrate with the two factors showed the highest ee%
value. The results will be applied to the other substrates to further
clarify in this catalysis system.
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Please cite this article in press as: T. Mameda, et al., Substrate dependent ligand acceleration in enantioselective hydrogenation of
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Please cite this article in press as: T. Mameda, et al., Substrate dependent ligand acceleration in enantioselective hydrogenation of
(E)-2,3-diarylpropenoic acid on cinchonidine-modified Pd/C, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.075