Pd and Ru complexes bearing axially chiral ligands for the asymmetric hydrogenation of C=C and C=O double bonds

Article abstract of DOI:10.1016/j.cattod.2013.03.043

Complexes composed of either Pd or Ru as central metal and ligands with axial chirality in all cases were used as hydrogenation catalysts. The ligands were (R)- and (S)-6,6′-dimethyl-2,2′-diaminobiphenyl, (R)-(+)-1-1′-Bi(2-naphtylamine), (R)-2,2′-Bis(diphenylphosphino)-1, 1′-binaphthalene and (R)-2,2′-Bis(di-p-tolylphosphino)-1,1′- binaphthyl. The Pd(II) complexes had one diamine ligand and the Ru(II) complexes had one bisphosphine and one diamine ligand, forming seven member chelate rings with the metal center. The pro-chiral substrates used were itaconic acid, α-acetamidocinnamic acid and acetophenone. The Pd complexes showed 100% chemoselectivity toward the CC bond, and toward the CO bond in the case of Ru. The yield and enantiomeric excess versus time behavior was studied using a large substrate/catalyst ratio. The addition of an organic base to the reaction with Pd complexes enhanced yield and enantiomeric excess. Use of the (S)-diamine ligand in the complex favored the (R)-products. The best results with itaconic acid were 61% yield and 56% enantiomeric excess and 55% yield and 52% enantiomeric excess with α-acetamidocinnamic acid, both catalyzed by Pd(OCOCF₃)₂ ((S)-6,6′-dimethyl-2,2′- diaminobiphenyl) in 2,2,2-trifluoroethanol. In the case of the Ru catalysts, (S)-1-phenylethanol formed preferentially during hydrogenation of acetophenone. Potassium tert-butoxide stabilized the enantiomeric excess. The best result was 87% yield and 41% enantiomeric excess catalyzed by ((R)-2,2′-Bis(di-p- tolylphosphino)-1,1′-binaphthyl)-RuCl₂-((R)-(+)-1-1′- Bi(2-naphtylamine)).

Full text of DOI:10.1016/j.cattod.2013.03.043

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Contents lists available at SciVerse ScienceDirect
Catalysis Today
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Pd and Ru complexes bearing axially chiral ligands for the asymmetric
hydrogenation of C C and C O double bonds
Victor M. Rivera, J. Pablo Ruelas-Leyva, Gustavo A. Fuentes∗
Department of Process Engineering, Universidad A. Metropolitana-Iztapalapa, San Rafael Atlixco # 186, México, DF 09340, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Complexes composed of either Pd or Ru as central metal and ligands with axial chirality in all cases were
used as hydrogenation catalysts. The ligands were (R)- and (S)-6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl, (R)-
(+)-1-1ꢀ-Bi(2-naphtylamine), (R)-2,2ꢀ-Bis(diphenylphosphino)-1,1ꢀ-binaphthalene and (R)-2,2ꢀ-Bis(di-p-
tolylphosphino)-1,1ꢀ-binaphthyl. The Pd(II) complexes had one diamine ligand and the Ru(II) complexes
had one bisphosphine and one diamine ligand, forming seven member chelate rings with the metal center.
The pro-chiral substrates used were itaconic acid, ␣-acetamidocinnamic acid and acetophenone. The Pd
complexes showed 100% chemoselectivity toward the C C bond, and toward the C O bond in the case of
Ru. The yield and enantiomeric excess versus time behavior was studied using a large substrate/catalyst
ratio. The addition of an organic base to the reaction with Pd complexes enhanced yield and enantiomeric
excess. Use of the (S)-diamine ligand in the complex favored the (R)-products. The best results with
itaconic acid were 61% yield and 56% enantiomeric excess and 55% yield and 52% enantiomeric excess with
␣-acetamidocinnamic acid, both catalyzed by Pd(OCOCF3)2 ((S)-6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl) in
2,2,2-trifluoroethanol. In the case of the Ru catalysts, (S)-1-phenylethanol formed preferentially during
hydrogenation of acetophenone. Potassium tert-butoxide stabilized the enantiomeric excess. The best
result was 87% yield and 41% enantiomeric excess catalyzed by ((R)-2,2ꢀ-Bis(di-p-tolylphosphino)-1,1ꢀ-
binaphthyl)-RuCl2-((R)-(+)-1-1ꢀ-Bi(2-naphtylamine)).
Received 3 January 2013
Received in revised form 25 March 2013
Accepted 27 March 2013
Available online xxx
Keywords:
Asymmetric hydrogenation
Palladium complex
Ruthenium complex
Bisphosphine
Diamine
Axial chirality
© 2013 Published by Elsevier B.V.
1. Introduction
modifier [7,10]; another option is to form a complex containing
metal atoms bound to a chiral ligand [8,9], an idea that has already
Pure enantiomers are widely used in pharmaceuticals, fine
chemicals, agrochemicals and fragrances [1]. Standard chemical
syntheses of enantiomers result in racemic mixtures, but their use
is in most cases questionable because of the real or potential danger
that one of the enantiomers may pose [2,3]. Industrial production
of chiral molecules with high enantiomeric excess (ee) generally
involves chiral resolution from the racemic mixture, a method that
causes significant waste. Asymmetric catalysis, a potentially more
efficient strategy, has been gaining momentum in recent years [4].
In this area, asymmetric hydrogenation of prochiral substrates is a
powerful tool to obtain chiral compounds, either as final products,
or as key intermediates in the synthesis of important chemicals
[5]. A recent review on the scaling-up of asymmetric homogeneous
hydrogenation is that of Ager et al. [6].
reached industrial application using 2,2ꢀ-Bis(diphenylphosphino)-
1,1ꢀ-binaphthalene, BINAP, as the organic ligand [11]. BINAP
has demonstrated its versatility in asymmetric hydrogenations
and promoted the development and use of other bisphosphine
ligands [8,11,12]. As an example, in the case of Pd, square-planar
LQ-Pd-(CF3CO2)2 complexes (where LQ is a bisphosphine ligand)
have been prepared with DuPhos, SynPhos, SegPhos, BIPHEP, as
well as BINAP, and tested satisfactorily in the enantioselective
hydrogenation of functionalized ketones [13], and an assortment of
imines [14–16].
Although amine and phosphine groups are isoelectronic, and
hence can form similar compounds, the use of chiral diamine lig-
ands has received significantly less attention than bisphosphines.
Thomas and co-workers [17] reported the use of a Pd complex with
Extensive studies of the asymmetric hydrogenation of C
C
a diamine ligand to hydrogenate C C (␣-phenylcinnamic) and C O
and C O groups [7–9] have been conducted with Pd, Ru or Rh
as the active moiety of the catalyst. One alternative is to use
the metal as supported nanoparticles interacting with a chiral
(methyl benzoylformate) double bonds with good results. They
argued there was an advantage for catalysts having diamine lig-
ands stemming from their stability and low cost when compared
with bisphosphines, although this argument is applicable when
only one organic ligand is required. Other examples of the bene-
fits when using diamine ligands are found in the case of Ru. The
BINAP-Ru system was able to hydrogenate functionalized ketones,
∗
Corresponding author. Tel.: +52 55 58044648; fax: +52 55 58044900.
E-mail address: gfuentes@xanum.uam.mx (G.A. Fuentes).
0920-5861/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.cattod.2013.03.043
Please cite this article in press as: V.M. Rivera, et al., Pd and Ru complexes bearing axially chiral ligands for the asymmetric hydrogenation of
C and C O double bonds, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.03.043
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but hydrogenation of simple ketones was ineffective [18]. This lim-
itation was overcome by the addition of a diamine ligand to the
BINAP-Ru catalyst [19]. Furthermore, by changing the chelate ring
formed by the diamine with Ru, new substrates were hydrogenated
[20] and the performance of the catalyst was enhanced [21].
We are interested in understanding and in making use of axial
chirality, or atropoisomerism, present in some simple diamine lig-
ands. Axial chirality involves a rotational barrier, in contrast to
the requirement of a stereogenic atom present in the majority
of the chiral diamine ligands used so far. We have done previ-
ous work with such a ligand, 6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl
(MAB) [22]. It forms a seven member chelate ring with the metal,
similar to the structure formed by most of the main bisphos-
phine ligands. MAB is also an efficient ligand in the Pd-catalyzed
Suzuki–Miyaura and Mizoroki–Heck coupling reactions [23] and in
the CuI-catalyzed Sonogashira coupling reaction [24]. We report
here the syntheses of several new Pd and Ru complexes containing
MAB and other axially chiral ligands and their catalytic evaluation
in the asymmetric hydrogenation of substrates containing C C or
Fig. 2. Bisphosphine/diamines Ru complexes used as catalysts in the asymmetric
hydrogenation of acetophenone.
C
O double bonds.
The solution was heated and kept under reflux during 8 h before
cooling to room temperature. The solution was concentrated under
vacuum and the Pd complex precipitated by adding hexane. Hex-
ane was then removed under vacuum atm room temperature to
obtain a light brown powder. The yield of the synthesis was 93%.
No additional purification of the powder was performed.
2. Experimental
To characterize the complexes we used Nuclear Magnetic Res-
onance (NMR) and Infrared (IR) spectroscopies. 1H and 31P NMR
was performed in a Bruker spectrometer (Avance III 500, 1H reso-
nance frequency of 500 MHz). The experiments were carried out in
the liquid phase using CDCl3 as solvent for the Ru complexes and
DMSO-d6 for the Pd complexes. The IR spectra were acquired in a
Bruker FTIR spectrometer (Tensor 27). The samples were diluted in
KBr and then compressed to form tablets.
During synthesis, all the manipulations were carried out with
Schlenk equipment inside a glove bag purged several times with N2.
(R)-2,2ꢀ-Bis(diphenylphosphino)-1,1ꢀ-binaphthalene ((R)-BINAP),
(R)-2,2ꢀ-Bis(di-p-tolylphosphino)-1,1ꢀ-binaphthyl ((R)-Tol-BINAP),
RuCl2(C6H6) dimer, PdCl2·(CH3CN)2, Pd(OCOCF3)2, (R)-(+)-1-
1ꢀ-Bi(2-naphtylamine) ((R)-DABN), acetophenone, itaconic acid
(IA), ␣-acetamidocinnamic acid (AA), potassium tert-butoxide
(t-BuOK), potassium hydroxide (KOH), benzylamine (BA), N,N-
dimethylformamide (DMF), isopropyl alcohol (IPA), hexane,
acetone, methanol (MeOH), 2,2,2-trifluoroethanol (TFE), and
dichloromethane (DCM) were purchased from Aldrich with the
highest purity available and/or anhydrous. MAB was synthesized
and then resolved into (R) and (S) enantiomers according to previ-
ous reports [25,26].
2.1.2. Pd(OCOCF3)2 ((S)-6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl) or
S-MF
We followed
a procedure similar to the one used in
[13]. Pd(OCOCF3)2 (0.1 mmol), (S)-MAB (0.12 mmol) and acetone
(15 mL) were poured in a Schlenk Flask. The solution was stirred
for 1 h at room temperature. The solvent was removed under vac-
uum at room temperature to give a yellow powder. The yield was
also 93%. No further purification of the complex was made.
2.1.3. ((R)-2,2ꢀ-Bis(diphenylphosphino)-1,1ꢀ-binaphthalene)-
RuCl2-((R)-(+)-1-1ꢀ-Bi(2-naphtylamine)) or
R-BD
Synthesis of all the Ru complexes was done adapting the tech-
niques reported in [20,28]. (R)-BINAP (0.3 mmol) and RuCl2(C6H6)
dimer (0.15 mmol) were mixed in a Schlenk flask and DMF (12 mL)
was then added. The solution was heated to 115 ◦C during 3 h
and then cooled to room temperature. To this mixture, (R)-DABN
(0.3 mmol) was added and left under stirring overnight. DMF was
removed under vacuum at 40 ◦C. A dark red powder was obtained
and no further purification of the complex was performed. The yield
was 90%.
2.1. Syntheses of the complexes
The same procedure was followed to synthesize the remaining
Ru complexes, by just exchanging the corresponding ligands.
In Figs. 1 and 2 we show the Pd and Ru complexes synthesized
in this paper.
2.1.4. ((R)-2,2ꢀ-Bis(di-p-tolylphosphino)-1,1ꢀ-binaphthyl)-RuCl2-
((R)-(+)-1-1ꢀ-Bi(2-naphtylamine)) or
R-TD
2.1.1. PdCl2 ((S)-6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl) or S-MCl
The synthesis of this complex was adapted from [27].
PdCl2·(CH3CN)2 (0.1 mmol) and (S)-MAB (0.12 mmol) were mixed
in a Schlenk flask and then dichloromethane was poured in (10 mL).
Ligands: (R)-Tol-BINAP and (R)-DABN. The yield was 92%.
2.1.5. ((R)-2,2ꢀ-Bis(diphenylphosphino)-1,1ꢀ-binaphthalene)-
RuCl2-((R)-6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl) or
R-BM
Ligands: (R)-BINAP and (R)-MAB. The yield was 87%.
2.1.6. ((R)-2,2ꢀ-Bis(di-p-tolylphosphino)-1,1ꢀ-binaphthyl)-RuCl2-
((R)-6,6ꢀ-dimethyl-2,2ꢀ-diaminobiphenyl) or
R-TM
Fig. 1. Pd complexes synthesized for the asymmetric hydrogenation of prochiral
Ligands: (R)-Tol-BINAP and (R)-MAB. The yield was 88%.
C
C bonds.
Please cite this article in press as: V.M. Rivera, et al., Pd and Ru complexes bearing axially chiral ligands for the asymmetric hydrogenation of
C and C O double bonds, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.03.043
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Table 1
1H NMR and IR signals for the diamine ligand, (S)-MAB, and the Pd-MAB complexes.
Ligand and complex
1H NMR (ı, ppm)
IR (cm−1
N–H
)
N–H2
C2–H
C3–H
C4–H
C–H3
(S)-MAB
S-MCl
S-MF
3.28
3.5
3.62
6.7
6.72
6.72
7.06
7.08
7.09
6.62
6.63
6.64
1.95
1.97
1.97
3348, 3435
3173, 3320
3179, 3319
the diamine. The results are summarized in Table 1. The 1H NMR
peaks of the Pd complexes were shifted downfield when compared
to (S)-MAB, in agreement with the deshielding effect caused by the
bonding of the amine groups with Pd [29].
2.2. Asymmetric hydrogenation of the prochiral substrates
All reactions were performed in a glass reactor (100 ml Mini-
clave, Büchi) as described in the next sections. The products of
reaction were analyzed by gas chromatography (HP 5890 with
FID detector, Agilent) using a chiral column (cp-chirasildex-cb
7502). The product configuration was determined by compari-
son with standards. (R)- and (S)-enantiomers of 1-phenylethanol,
methylsuccinic acid and 2-acetamido-3-phenylpropanoic acid
were purchased from Aldrich.
The IR vibrations at 3348 and 3435 cm−1 of (S)-MAB were
attributed to N–H stretching modes [28]. These signals were also
observed in the complexes, shifted to 3173 and 3320 cm−1 in S-
MCl and to 3179 and 3319 cm−1 in the case of S-MF, and confirm
the attachment of (S)-MAB to Pd.
We determined the proper substrate/catalyst ratio to facilitate
sampling as a function of time. This had two purposes: to determine
global reaction kinetics, and to follow catalyst deactivation. When
small ratios are used, conversion of the substrate occurs too fast to
give meaningful kinetic information. Deactivation is commonly dis-
regarded because 100% conversions are frequently reached because
of catalyst overload. Both topics have practical and fundamental
relevance for the use of chiral catalysis.
3.1.2. Ru complexes
Table 2 summarizes 1H and 31P NMR, as well as IR results for
the Ru-based complexes. Through 1H NMR the presence of the
NH2 group in the complexes was ascertained. The NMR peaks
shifted to lower field compared to those of the free diamine ligand
because of the interaction with the metal center [29]. Infrared spec-
troscopy showed not only that the NH2 group was conserved, but
also that there were absorption bands that could be assigned to
Ru-N bonding. The IR signals at 3330 and 3230 cm−1 have been
attributed to the N–H stretching vibrations [28], whereas the sig-
nals around 500–420 cm−1 have been assigned to ꢀ(Ru–N) [30].
Both techniques confirmed bonding of the diamine ligand to Ru.
In the case of the bisphosphine, the 31P NMR of all the complexes
had only one signal between 51 and 48 ppm, which is characteris-
tic in the bisphosphine/diamine Ru complexes [31,32]. The IR band
observed between 525 and 530 cm−1 corresponds to ꢀ(Ru–P) [33]
and, together with the 31P NMR results, confirm the preservation
of the bisphosphine ligand and its attachment to the Ru atom.
2.2.1. ˛-Acetamidocinnamic and itaconic acids (C
C
hydrogenation)
The corresponding Pd complex (0.05 mmol) was dissolved in
the reactor with the proper amount of MeOH or TFE. The prochiral
substrate (10 mmol), IA or AA, was then added to the reactor. In
some cases BA (0.05 mmol) was used as an additive. Finally, the
reactor was flushed 5 times with pure H2 before setting the pressure
to 100 psi. Samples (0.15 mL) were drawn from the reactor to follow
the reaction as a function of time.
2.2.2. Acetophenone (C O hydrogenation)
3.2. Catalytic asymmetric hydrogenation
Typically, the Ru complex (0.01 mmol) and the base (t-BuOK
or KOH; 0.025 mmol) were poured in the reactor followed by the
required amounts of IPA and then acetophenone (1 mmol). The
reactor was purged 5 times with pure H2 before the pressure was
set at 100 psi. Several samples (0.15 mL) were acquired during the
reaction to obtain the yield vs. reaction time data.
The reaction results that we report for both C O and C C hydro-
genations show the sensitivity of the global kinetics to the complex
used as a catalyst as well as to the solvent and base present. They
affect reactivity, enantioselectivity and deactivation. As mentioned
before, the substrate/complex ratio was adjusted in order to allow
data acquisition as a function of reaction time, including the effect
of deactivation. Most reports in the literature are concerned only
with high yield data, commonly a single point at a fixed reaction
time. If we increase the amount of catalyst loaded in our reactor the
final yield is 100%, but we lose what we believe is important infor-
mation. The analysis of the hydrogenation kinetics and deactivation
of our complexes is presented in a forthcoming paper.
3. Results and discussion
3.1. Characterization
3.1.1. Pd complexes
All the Pd complexes and the (S)-MAB ligand had only five 1H
NMR signals stemming from the equivalence of the two halves of
Table 2
1H and 31P NMR and IR signals of the bisphosphine/diamine Ru complexes.
Complex
NMR (ı, ppm)
1H
IR (cm−1
N–H
)
31P
Ru–N
Ru–P
CH3
NH2
(R)-MAB
(R)-Tol-BINAP
R-BD
R-TD
R-BM
R-TM
–
–
1.98
1.99
–
3.71
3.75
3.64
3.69
50.5
49.0
50.3
48.4
3334, 3237
3334, 3238
3339, 3241
3334, 3236
482, 438, 426
481, 451, 424
482, 439, 429
481, 450, 440
528
529
528
527
2.26, 1.81
–
2.27, 1.80
–: not present in complex.
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Fig. 3. Asymmetric hydrogenation of itaconic acid (IA).
Fig. 4. Asymmetric hydrogenation of ␣-acetamidocinnamic acid (AA).
Fig. 6. Catalytic hydrogenation of ␣-acetamidocinnamic acid in methanol with the
S-MCl complex: 0.01 mmol catalyst, 2 mmol ␣-acetamidocinnamic acid, 0.01 mmol
benzylamine (when used), 100 psi of H2, room temperature.
3.2.1. C C hydrogenation
The substrates used to study chiral C C hydrogenation were
itaconic and ␣-acetamidocinnamic acids. They are shown in
Figs. 3 and 4. The two new Pd complexes synthesized in this work
were used as hydrogenation catalysts with or without the addition
of benzylamine. All the hydrogenations were 100% chemoselec-
tive to C C and the enantiomeric excess remained constant as the
reaction progressed. The (R) enantiomer was predominant in all
cases.
Figs. 5–8 show the results of the asymmetric hydrogenation of
itaconic and ␣-acetamidocinnamic acids using either MeOH or TFE
as solvent. In both cases, the yields during hydrogenation of itaconic
acid were larger than those obtained with ␣-acetamidocinnamic
acid, in the presence or absence of benzylamine. The lower yield
with AA may be caused by steric restrictions since it is a larger
molecule than IA. The use of benzylamine as an additive favors
the enantiomeric excess and the initial rate in both reactions, in
agreement with the results of Szöllosi et al. [34] and Nitta [35].
According to these authors, benzylamine facilitates decoordination
of the product from the catalyst.
The catalytic complexes deactivate differently depending on
the reaction solvent used. In the case of S-MCl (Figs. 5 and 6) MeOH
is the best solvent. However, when S-MF is used (Figs. 7 and 8),
the reaction in TFE provides the best results. Deactivation is
Fig. 7. Catalytic hydrogenation of itaconic acid in TFE with the S-MF complex:
0.01 mmol catalyst, 2 mmol itaconic acid, 0.01 mmol benzylamine (when used),
100 psi of H2, room temperature.
accompanied by the appearance of
characteristic of metallic Pd.
a dark gray precipitate,
Fig. 8. Catalytic hydrogenation of ␣-acetamidocinnamic acid in TFE with the S-MF
complex: 0.01 mmol catalyst, 2 mmol ␣-acetamidocinnamic acid, 0.01 mmol ben-
zylamine (when used), 100 psi of H2, room temperature.
Fig. 5. Catalytic hydrogenation of itaconic acid in methanol with the S-MCl complex:
0.01 mmol catalyst, 2 mmol itaconic acid, 0.01 mmol benzylamine (when used),
100 psi of H2, room temperature.
Please cite this article in press as: V.M. Rivera, et al., Pd and Ru complexes bearing axially chiral ligands for the asymmetric hydrogenation of
C and C O double bonds, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.03.043
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Fig. 9. Asymmetric hydrogenation of acetophenone.
The solvent effect observed with the catalysts in this study
may be associated with the solvent-metal center affinity, and the
replacement of the Cl and OCOCF3 ligands in S-MCl and S-
MF, respectively, during the formation of the active (catalytic) Pd
species. The stability of the active species depends upon the sol-
vent used, according to Milani et at. [29]. In the case of the results
using TFE, the larger ee values determined (Figs. 7 and 8) may be
caused by the larger size of the TFE molecules compared to those of
MeOH. The active Pd species formed with TFE may have more steric
restrictions, leading to higher ee together with a slight decrease in
activity. These arguments are still speculative, as there is hardly
any discussion in the literature of the catalytic cycle involving Pd
complexes.
Fig. 11. Catalytic hydrogenation of acetophenone with the R-TM complex: effect
of t-BuOK and KOH. 20 mL IPA, 0.01 mmol catalyst, 0.025 mmol base, 1 mmol ace-
tophenone, 100 psi of H2, room temperature.
Hydrogenation of both prochiral substrates using S-MCl in TFE
was not successful, giving practically no ee and low yield. This
appears to be caused by the instability of the active species in TFE,
leading to a facile decoordination of the chiral ligand, (S)-MAB, and
hence to loss of activity and of chiral induction. In the case of the
S-MF complex in MeOH there was also poor stability of the active
species, but in this case deactivation appears to stem from decoor-
dination of the OCOCF3 ligand, as there is poor yield, but with ee
of 45 and 38% for IA and AA, respectively.
KOH over the yield was noticeable at short reaction times, since
at large reaction times the yield values were similar. The main dif-
ference between R-BM and R-TM was in the enantiomeric excess.
The initial ee for R-BM was 32% and 41% when R-TM and t-BuOK
were used. Changing the base to KOH led to a decrease in the ini-
tial ee to 26% with R-BM and 33% with R-TM. Independently of
the base added, decay in ee was observed but it was slower with
t-BuOK.
When (R)-MAB was used as the diamine ligand, the best results
in yield and ee were obtained when the largest bisphosphine,
(R)-Tol-BINAP, was used to form the Ru complex (R-TM). The intro-
duction of a bulkier diamine ligand, (R)-DABN, (complexes R-BD
and R-TD, Figs. 12 and 13 respectively), decreased the activity. R-
BD and R-TD showed similar behavior in yield independently of the
base used (R-BD 65% and R-TD 64% in yield). For the R-BD complex,
exchanging t-BuOK for KOH resulted in a decrease in the initial ee
from 31 to 22%, whereas for the R-BD complex the initial ee varied
from 29 to 22%.
3.2.2. C O hydrogenation
The hydrogenation of the carbonyl group was studied with ace-
tophenone as the substrate and the Ru complexes as catalysts
(Fig. 9). In all cases, the only product detected was 1-phenylethanol,
and the (S) enantiomer was preferred. The addition of a base was
required in order to have catalytic activity. The performance of the
catalyst was studied by adding either t-BuOK or KOH.
Figs. 10 and 11 show the catalytic results with the complexes
containing (R)-MAB (R-BM and R-TM) and their behavior while
using t-BuOK or KOH as an additive. The impact of t-BuOK or
Other authors have analyzed the influence of the base on the
performance of the catalyst. Hartman and Chen [36] studied the
Fig. 10. Catalytic hydrogenation of acetophenone with the R-BM complex: effect
of t-BuOK and KOH. 20 mL IPA, 0.01 mmol catalyst, 0.025 mmol base, 1 mmol ace-
tophenone, 100 psi of H2, room temperature.
Fig. 12. Catalytic hydrogenation of acetophenone with the R-BD complex: effect
of t-BuOK and KOH. 50 mL IPA, 0.01 mmol catalyst, 0.025 mmol base, 1 mmol ace-
tophenone, 100 psi of H2, room temperature.
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V.M. Rivera et al. / Catalysis Today xxx (2013) xxx–xxx
complexes synthesized with (R)-DABN. (S)-1-Phenylethanol
was the preferred product. There was an important dependence
of the ee on the nature of the base used, t-BuOK being preferred.
Decay of the ee was slower also when t-BuOK was present. The
best result was 87% yield and 41% ee to (S)-1-phenylethanol
catalyzed by ((R)-2,2ꢀ-Bis(di-p-tolylphosphino)-1,1ꢀ-binaphthyl)-
RuCl2-((R)-(+)-1-1ꢀ-Bi(2-naphtylamine)) in the presence of
t-BuOK.
Acknowledgements
V.M. Rivera and J.P. Ruelas-Leyva acknowledge the graduate
study fellowships granted by CONACYT (Mexico). The authors
acknowledge the financial support of UAM-Iztapalapa.
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Please cite this article in press as: V.M. Rivera, et al., Pd and Ru complexes bearing axially chiral ligands for the asymmetric hydrogenation of
C and C O double bonds, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.03.043
C