Ligand-acceleration by a chiral modifier in the enantioselective hydrogenation of methyl acetoacetate on a raney nickel catalyst: Effect of a modifier configuration

Article abstract of DOI:10.1246/bcsj.20140276

Reaction rate and enantioselectivity of asymmetric hydrogenation of methyl acetoacetate were studied over Raney nickel catalysts modified with (R,R)-tartaric acid, malic acid, or succinic acid to reveal the impacts of the modifier configuration. Catalysts comodified with two different acids were also examined to confirm the conclusions. From analysis of the enantiomer ratio of the hydrogenation product and initial reaction rate, tartaric acid (TA) was found to have dual functions as the modifier during the hydrogenation; effective suppression of the racemic catalysis on bare Ni surface and extensive enantiodifferentiating ligand acceleration by adsorbed TA. It was demonstrated that each adsorbed chiral modifier molecule independently takes part in the enantiospecific hydrogenation.

Full text of DOI:10.1246/bcsj.20140276

Ligand-Acceleration by a Chiral Modifier in the Enantioselective
Hydrogenation of Methyl Acetoacetate on a Raney Nickel Catalyst:
Effect of a Modifier Configuration
Takashi Sugimura,*1 Satoshi Nakagawa,2 Naoya Kamata,1 Takahiro Tei,1 Takashi Tajiri,1
Ryo-ichi Tsukiyama,3 Tadashi Okuyama,1 and Yasuaki Okamoto1
1Graduate School of Material Science, University of Hyogo, 3-2-1 Kohto, Kamigori, Ako-gun, Hyogo 678-1297
2Toyo Kasei Kogyo Co., Ltd., 2900 Sone, Takasago, Hyogo 676-0082
3Higashimaru Shoyu Co., Ltd., 100-3 Tominaga, Tatsuno, Hyogo 679-4193
E-mail: sugimura@sci.u-hyogo.ac.jp
Received: September 13, 2014; Accepted: October 24, 2014; Web Released: November 12, 2014
Reaction rate and enantioselectivity of asymmetric hydrogenation of methyl acetoacetate were studied over Raney
nickel catalysts modified with (R,R)-tartaric acid, malic acid, or succinic acid to reveal the impacts of the modifier
configuration. Catalysts comodified with two different acids were also examined to confirm the conclusions. From
analysis of the enantiomer ratio of the hydrogenation product and initial reaction rate, tartaric acid (TA) was found to have
dual functions as the modifier during the hydrogenation; effective suppression of the racemic catalysis on bare Ni surface
and extensive enantiodifferentiating ligand acceleration by adsorbed TA. It was demonstrated that each adsorbed chiral
modifier molecule independently takes part in the enantiospecific hydrogenation.
Enantiodifferentiating hydrogenation of prochiral ketones
and olefins over chiral organic compound modified heteroge-
neous metal catalysts have recently received extensive atten-
tion from both scientific and practical points of view.1-7 Since
the first report on the hydrogenation of methyl acetoacetate
(MAA)8 over (R,R)-tartaric acid-modified Raney nickel (TA-
MRNi) catalysts, this and analogous hydrogenation systems
have been keenly investigated by many research groups to
improve the optical yield,1,9-15 that has recently reached 98%
or even higher over MRNi catalysts doubly modified with TA
and NaBr (TA-NaBr-MRNi)16-19 as well as to obtain deeper
insights into the nature of molecular events during enantiodif-
ferentiating hydrogenation.20-25 Since the enantiodifferentiation
step of this system is the adsorption of the substrate prior to the
rate-determining hydrogenation,26-29 the kinetic effect by the
chiral modifier is hidden behind the energy barrier of the total
process, and thus the expression of the enantiodifferentiation
mode is still controversial. In the present study, the reaction rate
and enantioselectivity of asymmetric hydrogenation of MAA
were systematically studied over Raney nickel catalysts modi-
fied with (R,R)-tartaric acid (TA), malic acid (MA), or succinic
acid (SA) to reveal the effect of the modifier configuration.
Achievement of the high optical yield means that the active
nickel surface for the hydrogenation is uniformly modified. TA
molecules cover the nickel surface and the substrate is adsorbed
on the catalyst where the interaction with the TA molecule is
possible for enantiodifferentiation and for hydrogenation to
take place. Thus, the uncovered nickel surface should remain
around the TA molecule to keep the catalyst activity, although
the uncovered surface is a potential source of a racemic
product. Recently, it was suggested that the highest enantio-
selectivity was achieved at a 20% fractional coverage of TA
over a Ni catalyst reduced at 1373 K30 and Ni/SiO2 supported
catalysts.31-33 Many researchers have tried to solve this basic
issue, but no reasonable conclusion could be obtained.9-14 In
this report, we will show that the strict stereocontrol ability of
TA-NaBr-MRNi is not due to a delicate balance of the covered
and uncovered surfaces, but due to the dual functions of the
TA modifier; minimization of the racemic catalysis on bare
Ni surface and enantio-differentiating ligand acceleration by
adsorbed TA.
To look into phenomena occurring prior to the transition
state of the overall process, we have planned to construct two
reaction sites on a catalyst by comodification with two kinds of
modifier acids. When such a comodified catalyst is employed
for the hydrogenation, kinetic effects of the adsorbed acids can
be determined by the product analysis in the enantiomer ratio.
This system is effective because the two kinds of enantio-
differentiating sites formed by the coexisting modifiers work
under absolutely the same reaction conditions, and thus, the
result is not affected by other factors including reproducibility
of the catalyst.
Experimental
Catalyst Preparation. Raney nickel (RNi) was prepared
from a Ni/Al alloy (42/58, Kawaken Fine Chemicals, Ltd.
Japan) by W-1 type development followed by washing with
water under ultrasonic irradiation.16-18 An aliquot of the alloy
(1.0 g) was treated in an alkaline solution of NaOH (4.5 g) in
deionized water (20 mL) at 373 K for 1 h. The modification was
Bull. Chem. Soc. Jpn. 2015, 88, 271–276 | doi:10.1246/bcsj.20140276
© 2014 The Chemical Society of Japan | 271

performed to prepare MRNi (0.4 g) by heating the RNi at
373 K for 1 h in a solution, including the modifier (TA, MA, or
SA, 6.6 mmol) or a homo-stereochemical combination of the
modifiers (TA-MA, TA-SA, or MA-SA, 3.3 mmol each) on
comodification in the simultaneous presence of NaBr (5 g) in
water (50 mL), after adjustment of the pH with NaOH at 3.2.
After the modification, the solution was removed by decant-
ation, followed by thorough washing with water, then with
methyl alcohol, and finally with tetrahydrofuran (THF).16-18
Unless otherwise noted, MRNi denotes RNi catalysts doubly
modified with both the organic acid and NaBr hereafter. The
BET surface area was determined by N2-adsorption at a liquid
N2-temperature after evacuation at 393 K for 1 h (<ca. 10¹5 Pa).
Analysis of Adsorbed Acid on the Catalyst. MRNi (0.4 g)
was heated in a NaOH solution (1.25 M, 20 mL) at 373 K for
1 h. The water layer was collected by decantation. Amounts
of the acids dissolved in this solution were determined by a
Shimadzu LC-10AD organic acid analyzer with a Shim-pack
SCR-102(H) column and a CCD-6A detector. Retention times
eluted with 5 mM p-toluenesulfonic acid (0.4 mL min¹1) are
18.0 min for TA, 19.5 min for MA, and 22.7 min for SA.
Hydrogenation and Analysis of the Product. The catalyst
(0.4 g) and a solution of methyl acetoacetate (MAA, 2 g) in
THF (20 mL) were placed in a 100 mL autoclave. Hydrogen
was charged into the autoclave under the initial pressure of
10 MPa, and the autoclave was heated to 333 « 1 K under
reciprocating shaking until the end of the hydrogen uptake.
Aliquots of the reaction mixture were periodically withdrawn
for analysis. When MAA was hydrogenated over the modified
RNi catalysts, methyl 3-hydroxybutyrate was obtained in a
quantitative yield. Enantiomeric ratios of (R)- and (S)-methyl
3-hydroxylbutyrates were determined by GLC equipped with
a CP-Chirasil DEX CB capillary column (25 m, 0.25 mm id,
GL Science, Japan, flow rate: 30 cm s¹1). After acetylation with
acetic anhydride and pyridine, the retention times of the
product (363 K) are 7.1 min for (S) and 7.5 min for (R). Enantio
excess (ee) is defined here as
Furthermore, Humblot et al.20 demonstrated with RAIRS and
STM the formation of bitartrate surface species even at 300 K
with Ni(110) surface. It was also reported that the presence of
NaBr has no effect on the adsorption mode of TA.22 In these
studies using the surface science techniques, TA was adsorbed
on Ni surface in a vacuum, in contrast to the modification of
RNi with TA and NaBr in an aqueous solution. Taking into
consideration the high dielectric constant of H2O, it is reason-
able to expect enhanced double deprotonation of TA to form
bitartrate species on Ni metal. Since the modification temper-
ature was 373 K in the present study, it is considered that TA
molecules are adsorbed as bitartrate species with two bonding
to the Ni metal surface of RNi and/or as monosodium bitartrate
surface species (sodium nickel tartrate), that was originally
proposed by Tai et al.34 and later by Osawa et al.13 to explain
the positive effect of Na+ ions. The molecular cross section of
bitartrate surface species was calculated to be 0.68 © 0.46 nm2
or 0.31 © 10¹18 m2.20 Assuming this cross section, the frac-
tional coverage of TA molecules on TA-MRNi is estimated to
be 0.12. With RNi catalysts prepared from a 42/58 Ni/Al-alloy
as in the present study, Okamoto et al.35 showed with XPS that
the fraction of Ni in the surface was 0.7 when the alloy was
activated at 373 K. It is then estimated that under the present
modification conditions the coverage of TA on Ni metal surface
is 0.17, the value which has been suggested to be optimum
for the highest enantioselectivity.30-33 This may be one of the
reasons why the maximum selectivity (86% enantio excess)
was achieved over the TA-MRNi catalyst in the present study
(vide infra).
The amounts of adsorption of MA and SA are very close to
that of TA, as presented in Table 1. The adsorption geometries
of MA and SA on Ni metal surface have rarely been studied
with spectroscopic techniques. With Cu(110) surface, RAIRS
and STM studies showed that bisuccinate surface species were
formed on the adsorption of SA at >350 K, as bitartrate species
for TA/Cu(110),36,37 with a subtle second-order effect of the
OH groups on finer details of the self-assembled structure.
Similarly, it was demonstrated with RAIRS, XPS, and STM
that MA was adsorbed on Cu(110) surface to form bimalate
structure.38 Thus, it is evident that TA, MA, and SA form
essentially the same bicarboxylate structure by double depro-
tonation on the adsorption on Cu(110). It is rational to assume
that Ni metal adsorbs these organic acids to form similar
surface bicarboxylate species with a different number of OH
ee ð%Þ ¼ 100ð½Rꢀ ꢁ ½SꢀÞ=ð½Rꢀ þ ½SꢀÞ
ð1Þ
where [R] and [S] denote the amounts of (R)- and (S)-methyl
3-hydroxylbutyrates.
Results and Discussion
The adsorbed acids on MRNi catalysts were extracted into a
hot NaOH solution, and the amounts of the acids in the solution
were directly determined by ion chromatography. This proce-
dure is essentially the same used by Osawa et al.30 Table 1
summarizes the adsorbed amounts of the modifiers thus deter-
Table 1. BET Surface Area of Raney Nickel Catalysts and
the Adsorbed Amount of the Modifiera)
Adsorbed amount
mined as well as the BET surface areas. The surface area
of the modifier
/¯mol g
¹1
was decreased to 25-30% of the original RNi (102 m2 g
)
BET surface area
Modifier
¹1
¹1
/m2 g
by the modification with the organic acids. The TA amount
adsorbed on TA-MRNi is 19 « 2 ¯mol (g-cat)¹1 or 0.66 « 0.07
¯mol m¹2. The adsorption mode of TA on Ni{111} surface,
which is thermodynamically the most stable surface for Ni
metal particles, was studied by Jones and Baddeley25 by means
of reflection absorption infrared spectroscopy (RAIRS) and
scanning tunneling microscopy (STM) techniques, showing
that monotartrate surface species is relatively stable at 300 K,
while bitartrate species are predominantly formed at > 350 K.
TA
19
MA
SA
TA-NaBr
MA-NaBr
SA-NaBr
TA-MA-NaBr
TA-SA-NaBr
MA-SA-NaBr
29
33
®
23
23
25
18
9
20
7
7
10
7
8
a) TA: tartaric acid, MA: malic acid, SA: succinic acid.
272 | Bull. Chem. Soc. Jpn. 2015, 88, 271–276 | doi:10.1246/bcsj.20140276
© 2014 The Chemical Society of Japan

40
30
20
10
0
reaction over TA-MRNi, the conversion reaction time relation
is described as a more or less concave curve, that is, the
incremental increase in the conversion is enhanced as the
reaction proceeds. In addition, it is evident that the curvature is
enlarged as the increase in the initial reaction rate (the linear
lines in Figure 1); TA > TA-MA > TA-SA µ MA > MA-SA.
Such reaction behavior is reasonably understood on the basis
of a Langmuir-Hinshelwood (L-H) formalism; the reaction
rate r can be expressed as, assuming that the substrate MAA
and hydrogen are competitively adsorbed on the Ni surface and
that the first addition of a hydrogen atom to adsorbed MAA is
the rate-determining step of the hydrogenation,
TA
TA-MA
TA-SA
MA
1=2 2
r ¼ k0KSCSKH1=2PH1=2=ð1 þ KSCS þ KH1=2PH
Þ
ð2Þ
MA-SA
where k0 is a real rate constant, KS and KH the adsorption
constants of MAA and H2, CS the concentration of MAA, and
PH the pressure of H2. When KS is large and/or the concen-
tration of MAA is high (KSCS º 1 + KH1/2PH1/2), r can be
reduced to
0
1
2
3
4
5
Reaction Time / h
Figure 1. Conversions of methyl acetoacetate (MAA) as a
function of the reaction time for the asymmetric hydro-
genation of MAA over a Raney nickel catalyst (RNi)
doubly modified with NaBr and (R,R)-tartaric acid (TA)
(blue circle) or (S)-malic acid (MA) (yellow triangle) and
over a RNi catalyst comodified with TA-MA (green
circle), TA-succinic acid (SA) (orange circle), or MA-SA
(blue triangle). Reaction conditions: 10 MPa of H2 at
333 K.
1=2
r ¼ k0KSmCSmKH1=2PH
ðꢁ1 < m < 0Þ
ð3Þ
When KS is not very high and/or CS is relatively low, m in eq 3
is positive. Keane39 reported that m = 0.3 at 343 K over TA-
NaBr-Ni/SiO2. However, the concave curve in Figure 1 clearly
shows that the reaction order m is negative with respect to the
substrate MAA under the present reaction conditions (333 K,
10 MPa, 0.86 M of MAA). It is deduced from the L-H rate
equation that the negative order is caused by relatively strong
adsorption of MAA on the modified catalysts and that the
adsorption strength of MAA increases in the order, SA < MA-
SA < MA µ TA-SA < TA-MA < TA. It is noteworthy that
the initial reaction rate increases as the adsorption strength
or the adsorption amount of MAA on the MRNi catalyst
increases. With the reaction on SA-MRNi (not shown in
Figure 1), the conversion of MAA linearly increased up to 55%
after 45 h reaction time, indicating that the reaction is zeroth
order (m = 0) with respect to MAA over SA-MRNi. It is
deduced that MAA is more weakly adsorbed on SA-MRNi than
on TA-MRNi or MA-MRNi.
groups. Thus, it is presumed that the fractional coverage of
adsorbed MA (bimalate) and SA (bisuccinate) species on Ni
metal surface of RNi are essentially the same as that of TA,
around 20% on the basis of the molecular cross section.
Table 1 also summarizes the amounts of the adsorbed
modifier for the comodified catalysts. A TA-SA-MRNi como-
dified catalyst was prepared by our standard procedure18 except
for the use of the modifying solution containing both TA
¹1
and achiral SA in equimolar amounts. 7 « 1 ¯mol g of TA
and 10 « 1.5 ¯mol g of SA are adsorbed on TA-SA-MRNi
(Table 1). The total amount of TA and SA is 17 ¯mol g and
¹1
¹1
approximately identical to the amount of the modifier adsorbed
on TA-MRNi or SA-MRNi, showing that about half of the TA-
adsorbed species on TA-MRNi is replaced by the SA-adsorbed
species on TA-SA-MRNi and vice versa. With TA-MA- and
MA-SA-MRNi catalysts, the total amounts of the adsorbed
acids are 16 and 15 ¯mol g¹1, respectively, and comparable
with the other catalysts including individually modified ones.
It is concluded that the two acids in the comodified catalysts
are competitively adsorbed on Ni metal surface and that the
total fractional coverage is limited to about 20% in the present
modification procedure. It appears that the modifier acids have
a similar ability to modify the surface of RNi. However, scruti-
nizing the amount of each modifier on the co-modified MRNi
catalysts in Table 1, it seems that the adsorbed amount of the
modifier slightly decreases in the order SA > MA µ TA. This
order may be due to the order of slightly increasing van der
Waals cross section of the adsorbed modifier molecule and/or
to a slightly different self-assembled surface structure.
To evaluate the hydrogenation activity of MRNi catalysts,
the initial reaction rate (r mmol h¹1 g¹1) was calculated from
the initial conversion (<10%) of MAA and reaction time as
presented in Figure 1. Table 2 compares the initial reaction
rate and ee of the enantioselective hydrogenation of methyl
acetoacetate (MAA) over a Raney Ni catalyst modified with
TA, MA, or SA. The reaction rate r can be divided into two
components, rR and rS corresponding to the formation rates of
(R)- and (S)-configurations, respectively.
r ¼ rR þ rS
ee ð%Þ ¼ 100ðrR ꢁ rSÞ=ðrR þ rSÞ
ð4Þ
ð5Þ
The values of rR and rS were calculated using the reaction rate
and selectivity, enantiomeric excess (ee). Table 2 summarizes
r, rR, and rS for the relevant catalyst systems.
The initial reaction rate r of the MAA hydrogenation over
MA-MRNi is 4.1 fold increased relative to that on SA-MRNi.
While no enatiodifferentiating hydrogenation is observed with
SA-MRNi as expected, enantioselectivity is induced to the
hydrogenation to provide 60% ee by replacing SA with MA,
Figure 1 shows the conversion of MAA as a function of
reaction time for the enantioselective hydrogenation over the
modified RNi catalysts. As apparently exemplified by the
Bull. Chem. Soc. Jpn. 2015, 88, 271–276 | doi:10.1246/bcsj.20140276
© 2014 The Chemical Society of Japan | 273

Table 2. Enantioselectivities and Initial Reaction Rates of the Hydrogenation of MAA over Modified Raney Nickel Catalysts
Modifiera)
TA-NaBr
MA-NaBr
SA-NaBr
TA-MA-NaBr
TA-SA-NaBr
MA-SA-NaBr
ee/%d)
86
13
4.3
4.0
0.31
60
4.0
1.2
0.96
0.24
0
1
0.29
0.15
0.15
81 (79)
9
2.4 (2.5)
2.2
86 (78)
13
1.7 (1.9)
1.6
50 (52)
3
0.73 (0.73)
0.55
rR/rS
r/mmol h¹1 g
¹1 b)
rR/mmol h¹1 g
¹1
rS/mmol h¹1 g
0.24
0.12
0.18
¹1
a) Abbreviation of the modifiers; TA: tartaric acid, MA: malic acid, SA: succinic acid. b) Values in parentheses are calculated assuming
independent contributions from the respective modified sites.
which has a single hydroxy group as a chiral origin. That is,
rR is increased 6.4-fold compared to that of SA-MRNi, while
rS only 1.6-fold. A comparison of rR and rS between MA-
MRNi and SA-MRNi reveals that the modification of Ni metal
surface with MA obviously induces “ligand acceleration”40,41
in the hydrogenation of MAA over chiral-modified Ni cata-
lysts. Keane39 noted a promotion of the reaction rate as well
as a decrease in the activation energy in the enantioselective
hydrogenation of MAA (ee < 30%) by the modification of Ni/
SiO2 with TA.
It is clearly demonstrated in Table 2 that TA-MRNi exhibits
even more significant ligand acceleration effects than MA-
MRNi. The initial reaction rate r is 15-fold increased by replac-
ing SA with TA. In particular, rR is 27-fold enhanced with
respect to SA-MRNi, while rS is only doubled, resulting in the
very high enantioselectivity, 86% ee. When compared with
MA-MRNi, the value of r is 3.6-fold increased. rR is 4.2-fold
enhanced by replacing MA with TA, while rS is only slightly
increased. It is concluded that the enatiodifferentiating hydro-
genation is induced not only by poisoning the racemic sites
with the modifier molecules and NaBr but also more predom-
inantly by a great increase of rR by specific interactions of
MAA with the surface chiral modifier, that is, by the ligand
acceleration effects induced by the chiral modifier. In the above
discussion on the relative magnitudes of rR and rS, a possible
contribution of the reaction over non-enantioselective sites
(N-sites) to the reaction rates is neglected, since the fraction of
the reaction on the N-sites was previously estimated to be 4%
over TA-MRNi.18
The ee greatly increases as the number of hydroxy groups
of the chiral modifier increases: nil, one, and two for SA
(0%), MA (60%), and TA (86%), respectively, as presented in
Table 2. This is rationally accounted for in terms of the inter-
action modes between the hydroxy groups and MAA: two-
point interaction mode or two pairs of hydrogen bonding
between the surface bitartrate (possibly monosodium bitartrate
surface species in the presence of NaBr)13,34 and MAA for
highly enantiodifferentiating reaction to occur and one-point
interaction mode or a single hydrogen bonding between
adsorbed bimalate and MAA, leading to less selective hydro-
genation. From the kinetics analysis, rR increases several times
as the number of hydroxy group increases by one: 6.4-fold
from nil to one and 4.2-fold from one to two (ligand accel-
eration). On the other hand, the corresponding rS values
increase only 1.6- and 1.3-fold, respectively. These increases in
the reaction rates, which reflect both the true rate constant k0
directly connected with the surface reactions and the adsorp-
tion constant KS of MAA (eqs 2 and 3), are induced by the
interactions between the surface modifier and the substrate.
RAIRS and STM provided such direct interactions between
surface bitartrate and MAA with Ni(110)21 and Ni{111}.24 It is
surmised that the adsorption constant KS increases as the
number of the hydroxy groups increases from nil through two
and thus as the interaction mode of MAA changes from van der
Waals interaction with surface bisuccinate, one hydrogen-
bonding interaction with surface bimalate, and two-point
hydrogen-bonding interaction with surface bitartrate, resulting
in the increase in the reaction rate. However, it is expected from
the increase of rS by replacing SA with TA that the increase
of the reaction rate due to the increase of KS is limited to, at
most, twofold. Thus it is concluded that the great increase in
rR is due to the increase in k0 induced by the interactions with
deprotonated surface MA or TA molecules (ligand acceler-
ation). It is evident that the increasing magnitude in the
difference between rR and rS from SA through TA is induced
by the increase in the intrinsic enantiodifferentiating ability,
i,11,42 due to the change in the modifier-substrate interaction
modes from dipole or van der Waals interaction (SA), one-point
hydrogen bonding (MA), and two-point hydrogen bonding
(TA) interactions.11,13 This was previously envisaged by the
findings that the addition of pyvaric acid reduces the enantio-
selectivity of TA-MRNi for the enantioselective hydrogenation
of various ketoesters and 2-octanone.43 It is concluded that the
increase in the number of the hydroxy groups of the modifier
not only enhances the enantiodifferentiating ability of the chiral
modifier but also causes a significant increase in the reaction
rate (ligand acceleration) through the changes in the interaction
mode and strength, accompanying the increase in the adsorp-
tion strength of the substrate. The interaction between the
surface modifier and MAA is clearly evidenced by the findings
in Figure 1 that the magnitude of the negative reaction order
with respect to MAA, that is, the interaction strength increases
in the order; SA < MA < TA.
As discussed above, the coverage of the modifier bicarbox-
ylate on the Ni metal surface is estimated to be about 20% from
the molecular cross section. In spite of rather low coverages
of the modifiers, high ee values are observed, in particular
with TA-MRNi, as presented in Table 2. It is proposed that the
preadsorbed modifier molecules strongly suppress the hydro-
genation activity by blocking the Ni metal ensemble sites
required for racemic reactions but the preadsorbed modifier
molecule having appropriate interactions with the substrate
forms a novel ensemble site including Ni metals for highly
enantiodifferentiating hydrogenation. The loss of the ensemble
sites for the hydrogenation may result from surface reconstruc-
tion by the adsorption of modifier molecules and from the
274 | Bull. Chem. Soc. Jpn. 2015, 88, 271–276 | doi:10.1246/bcsj.20140276
© 2014 The Chemical Society of Japan

formation of ordered surface structure. Actually, Humblot
et al.20 showed with STM and DFT calculations for TA/
Ni(110) that the formation of the surface bitartrate accompanies
reconstruction of surface Ni atoms, the size of which amounts
to 1.25 © 1.55 nm2 or 1.9 © 10¹18 m2, six times larger than the
molecular cross section of bitartrate (0.31 © 10¹18 m2). Ordered
adlayers of tartrate species are formed on Ni{111}, depending
on the modification temperature.25 Self-assembled structures
of bisuccinate were observed for SA/Cu(110).36
high enantiodifferentiating ability of TA-MRNi is induced by
the high ligand acceleration effects due to the chiral modifier
as well as effective poisoning of nonselective hydrogenation
sites. The interaction strength between the surface modifier and
MAA increases in the order; SA < MA < TA. MRNi catalysts
comodified with two different acids were also examined to
confirm the conclusions. It was demonstrated that each adsorb-
ed chiral modifier molecule independently takes part in the
enantiospecific hydrogenation.
We studied the enantioselective hydrogenation over comodi-
fied MRNi catalysts to confirm the reaction rates and ee
results, obtained separately on the MRNi catalysts individually
modified with TA, MA, or TA, and to obtain deeper insights, if
possible, into the generation of enantiospecific hydrogenation
on the chiral modified RNi. Table 2 summarizes the initial
reaction rates and ee values for the comodified MRNi catalysts.
The reaction rates and ee values on the comodified catalysts are
intermediate between the values of the respective individually
modified MRNi catalysts. It is reasonable to estimate the reac-
tion rates and ee values from the adsorbed amounts of the
modifiers in Table 1 and the reaction results in Table 2, assum-
ing that the modifier molecules independently take part in
the asymmetric hydrogenation. Table 2 compares the observed
and thus estimated reaction rates and ee values. It is obvious
that estimated values are essentially identical to the observed
ones within experimental accuracy, confirming that the reaction
rates and enantioselectivity in Table 1 have been measured
accurately enough for the above discussion on the effects of the
modifier configuration. The coincidence in Table 2 furthermore
strongly indicates that the surface chiral modifier bicarboxylate
species independently take part in the enantioselective hydro-
genation of MAA. This suggests that with TA-MRNi, the for-
mation of 1:1 surface interaction species between the bitatrate
and MAA via two-point hydrogen-bonding interactions is a
predominant origin to produce (R)-methyl 3-hydroxylbutyrate,
as tentatively proposed by several research groups.1,7,11
The authors thank Professors Yoshiharu Izumi (Osaka
University, Emeritus Professor) and Akira Tai (Himeji Tech-
nical University, Emeritus Professor) for their encourage-
ment. They also thank Professor Yuriko Nitta for her helpful
comments.
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Conclusion
The reaction rate and enantioselectivity of the asymmetric
hydrogenation of MAA were studied over Raney nickel cata-
lysts modified with (R,R)-tartaric acid, malic acid, or succinic
acid to reveal the impacts of the modifier configuration. The
surface coverage of the modifier was about 20% irrespective
of the configuration. From the enantiomer ratio of the hydro-
genation product and initial reaction rate, it is concluded that
surface modifier molecules have dual functions during the
hydrogenation; effective suppression of the racemic catalysis
on bare Ni surface and extensive enantiodifferentiating ligand
acceleration through the hydrogen-bonding interactions with
the substrate molecule.
One difference in the three modifiers is the number of
hydroxy groups. The adsorption of the modifier poisons much
wider area of the nickel surface than the molecular cross
section. Replacement of a proton on the adsorbed SA with a
hydroxy group in making a chiral center on the modifier MA
accelerates the hydrogenation 4.1-fold (ligand acceleration),
and it is the origin of the enantiodifferentiation. A further
replacement of the hydrogen of MA with a hydroxy group
enhances more the rate by 3.6 times. It is concluded that the
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