A new efficient bis(phosphinite)-ruthenium(II) catalyst system for the asymmetric transfer hydrogenation of aromatic ketones

Article abstract of DOI:10.1016/j.ica.2013.11.029

Many chiral ligands bearing phosphorus donor atoms were developed and applied in the area of metal catalyzed asymmetric catalysis. A new chiral bis(phoshinite) ligand was successfully synthesized from optically active (1S,8S)-3,6-[N-(S)-α-phenylethyl]diaza-1,8-diphenoxymethyl-1,8-octanediol in high yield. A ruthenium(II) complex generated in situ from [Ru(η⁶-p-cymene)(μ-Cl)Cl]₂, bis(phosphinite) ligand and KOH acts as an active catalyst for asymmetric transfer hydrogenation (ATH) of acetophenone derivatives in isopropyl alcohol (IPA) as both solvent and hydrogen source. The catalytic reaction proceeds cleanly to give enantiomerically enriched secondary alcohols up to 90% or 89% ee's at 50 or 82 C, respectively.

Full text of DOI:10.1016/j.ica.2013.11.029

Inorganica Chimica Acta 411 (2014) 77–82
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A new efficient bis(phosphinite)-ruthenium(II) catalyst system
for the asymmetric transfer hydrogenation of aromatic ketones
⇑
Feyyaz Durap , Murat Aydemir, Akın Baysal, Duygu Elma, Bünyamin Ak, Yılmaz Turgut
Dicle University, Science Faculty, Department of Chemistry, TR-21280 Diyarbakır, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2013
Received in revised form 25 November 2013
Accepted 28 November 2013
Available online 6 December 2013
Many chiral ligands bearing phosphorus donor atoms were developed and applied in the area of metal
catalyzed asymmetric catalysis. A new chiral bis(phoshinite) ligand was successfully synthesized from
optically active (1S,8S)-3,6-[N-(S)-
yield. A ruthenium(II) complex generated in situ from [Ru(
a
-phenylethyl]diaza-1,8-diphenoxymethyl-1,8-octanediol in high
g
6-p-cymene)(
l-Cl)Cl]2, bis(phosphinite)
ligand and KOH acts as an active catalyst for asymmetric transfer hydrogenation (ATH) of acetophenone
derivatives in isopropyl alcohol (IPA) as both solvent and hydrogen source. The catalytic reaction pro-
ceeds cleanly to give enantiomerically enriched secondary alcohols up to 90% or 89% ee’s at 50 or
82 °C, respectively.
Keywords:
Chiral
Asymmetric transfer hydrogenation
Phosphinite
Ó 2013 Elsevier B.V. All rights reserved.
Ruthenium
C2-symmetry
1. Introduction
interest to develop highly effective chiral phosphinite ligands for
asymmetric catalysis.
The design of organometallic complexes containing chiral li-
gand is of fundamental importance for asymmetric catalysis.
Improvements in reactivity and selectivity of catalyst for the asym-
metric catalysis attracted a great deal of attention. Catalytic asym-
metric reduction of unsaturated compounds such as olefins,
ketones or imines is one of the most reliable methods used to syn-
thesize the corresponding chiral saturated and enantio-enriched
products such as alkanes, alcohols or amines [1]. Generally, all
asymmetric organometallic catalysts bear one chiral ligand at least.
Up to now, a number of complexes involving metal or metal ion
and chiral organic ligand have been widely used for asymmetric
synthesis. Especially, the use of tertiary phosphorus ligands such
as phosphine, phosphinite or phosphite etc., is widespread in orga-
nometallic chemistry and homogeneous catalysis as these ligands
can be used to fine tune the metal reactivity and selectivity [2].
More than a thousand of phosphorus-based chiral nonracemic li-
gands including striking examples dissymmetric phosphine ligand
BINAP [3], DIOP [4], DuPHOS [5], JOSIPHOS [6], CHIRAPHOS [7],
phosphinite ligand BINAPO [8], phosphonite [9], and phosphate
[10] ligands have already been synthesized. The ligand structure
plays a key role in the performance of such phosphorus-based cat-
alysts. It was stated that the phosphinites are among the most
important phosphorus based ligands because they have different
steric and electronic properties [11–13]. Thus, it is a considerable
Among the unsaturated compounds, ketones are the most used
compounds in asymmetric catalysis. Therefore, much effort has
been devoted to their reduction into secondary alcohols via cata-
lytic hydrogenation [14]. This process has been exploited in pro-
duction of a variety of secondary alcohols which is an important
class of intermediates for numerous fine chemicals and pharma-
ceuticals [2,15]. Catalytic asymmetric transfer hydrogenation of
ketones is promising alternative for catalytic H2-hydrogenation.
Isopropyl alcohol (IPA) and formic acid/triethylamine (TEAF) are
the most used hydrogen sources in transfer hydrogenation reac-
tions [16]. The advantages of asymmetric transfer hydrogenation
using IPA as a hydrogen source are well documented in the litera-
ture [17–19]. Metal-based reduction of ketones to alcohols has re-
cently attracted attention by a number of research groups [1] and
references therein. Main group metals such as aluminium have his-
torically been used in the transfer hydrogenation of ketones [2,20].
Since the pioneering work of Noyori on the use of Ru(II) complexes
in the transfer hydrogenation of ketones [21], numerous metal-
based catalytic systems have been reported. Nowadays the metal
complexes ruthenium [22], rhodium [23] or iridium [24] are
mainly catalysts of choice for this type of reaction. Among them,
ruthenium catalysts attracted considerable interests due to their
superior performance [22] and cost-effectiveness of ruthenium
compared to other metals used in asymmetric hydrogenation.
Therefore, phosphinite-based ruthenium complexes [25–27]
encouraged us to develop new chiral phosphinite-Ru(II) catalytic
system.
⇑
Corresponding author. Tel.: +90 412 248 8550; fax: +90 412 248 8300.
E-mail address: fdurap@dicle.edu.tr (F. Durap).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.11.029

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F. Durap et al. / Inorganica Chimica Acta 411 (2014) 77–82
Recently, we reported the synthesis of new C2-symmetric chiral
stirred at room temperature for an hour and the triethylammo-
nium chloride was removed by filtration under argon atmosphere
and the remaining organic phase dried in vacuo to produce a white
phosphinite ligands derived from amino alcohols and their applica-
tion in Ru-catalyzed asymmetric transfer hydrogenation [28]. Next,
we prepared a new C2-symmetric chiral phosphinite ligand namely
viscous oily compound (1S, 8S)-3,6-[N-(S)-a-phenyl ethyl]diaza-
(1S, 8S)-3,6-[N-(S)-
a
-phenyl ethyl]diaza-1,8-diphenoxymethyl-
1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite) (Yield:
1,8-octanebis(diphenylphosphinite) as an extension of ongoing
project and employed it in ruthenium catalyzed asymmetric trans-
fer hydrogenation of prochiral aromatic ketones (acetophenone
derivatives) with isopropyl alcohol (IPA) under varying conditions.
151 mg, 94%).
1H NMR (CDCl3) d(ppm): 1.44 (d, 6H, J = 7.1 Hz), 2.49–2.63 (m,
5H), 2.79–2.86(m, 3H),3.85–4.10 (m, 8H), 6.91–7.81 (m, 40H); 13C
NMR (CDCl3) d(ppm):13.95, 49.80, 53.62, 59.45, 67.66, 70.13,
114,70, 120.94, 127.73, 127.91, 128.23, 128.30, 129.50, 131.7,
133.31, 136.73, 142.22, 158.90; 31P-{1H} NMR (CDCl3) d(ppm):
114.17.
2. Experimental
2.1. General methods and materials
3. Results and discussion
Unless otherwise stated, all reactions were carried out under in-
ert atmosphere (argon) using conventional Schlenk glass-ware, sol-
vents were dried using established procedures and distilled under
argon or nitrogen immediately prior to use. Analytical grade and
deuterated solvents were purchased from Merck or Sigma–Aldrich.
3.1. Synthesis of chiral C2-symmetric bis(phosphinite) ligand and its
corresponding Ru(II) complex
The C2-symmetric chiral diols derivated from amino alcohols
have led to the synthesis of several chiral bis(phosphinites) as li-
gands for asymmetric catalysis [28]. Firstly, we prepared the chiral
C2-symmetric amino alcohol (1S,8S)-3,6-[N-(S)-a-phenyl ethyl]-
diaza-1,8-diphenoxymethyl-1,8-octanediol according to the litera-
ture procedure [29]. We then synthesized C2-symmetric chiral
bis(phosphinite) ligand (1S,8S)-3,6-[N-(S)-
1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite)
the chiral C2-symmetric amino alcohol by the subsequent reaction
with two equivalents of chlorodiphenylphosphine (Ph2PCl) and
Et3N in freshly distilled toluene under argon atmosphere as seen
in Scheme 1. It is well known that the phosphinites are unstable
in the solid state and decompose rapidly on exposure to air or
moisture. In order to avoid oxidation of the phosphinite groups,
the synthesis and isolation of the phosphinites must be carried
out under argon atmosphere. The formation of bis(phosphinite) li-
gand was followed by 31P-{1H} NMR spectroscopy in which the sig-
nal of the Ph2PCl at d = 81.0 ppm disappeared and new singlet
appeared due to diphenylphosphinite moiety at 114.17 ppm (in
line with the values previously observed for similar compounds
[30–33]) due to the equivalent phosphorus nuclei [33,28a] of C2-
symmetric bis(phosphinite) ligand as seen Fig. 1.
The starting material (1S,8S)-3,6-[N-(S)-a-phenylethyl]diaza-1,8-
diphenoxymethyl-1,8-octanediol was prepared according to the
literature [29]. 1H (400.1 MHz), 13C NMR (100.6 MHz) and
31P-{1H} NMR (162.0 MHz) spectra were recorded on a Bruker AV
400 spectrometer, with d referenced to external TMS and 85%
H3PO4 respectively. GC analyses were performed on a Shimadzu
GC 2010 plus gas chromatograph equipped with cyclodex B (Agi-
a
-phenyl ethyl]diaza-
from
lent) capillary column (30 m  0.32 mm I.D.  0.25
lm film thick-
ness). The GC parameters for asymmetric transfer hydrogenation
reactions were as follows; initial temperature, 50 °C; initial time
1.1 min; solvent delay, 4.48 min; temperature ramp 1.3 °C/min; fi-
nal temperature, 150 °C; initial time 2.2 min; temperature ramp
2.15 °C/min; final temperature, 250 °C; initial time 3.3 min; final
time, 44.33 min; injector port temperature, 200 °C; detector tem-
perature, 200 °C, injection volume, 2.0 lL.
2.2. General procedure for the asymmetric transfer hydrogenation of
ketones
According to the typical experimental procedure; a flame dried
Schlenk flask was charged with [Ru(
(0.01 mmol), (1S, 8S)-3,6-[N-(S)- -phenyl ethyl]diaza-1,8-diphen-
g l-Cl)Cl]2
6-p-cymene)(
The ammonium salt was separated by filtration and the ligand
was obtained by removing the solvent under reduced pressure in
94% yield at the end of the reaction. Furthermore, solution of the
phosphinite ligand in CDCl3 or common organic solvents is unsta-
ble, decomposes gradually to give corresponding oxide. We cir-
cumvented this problem by preparing ruthenium complex via an
in situ way. The corresponding ruthenium(II) complex was pre-
pared from the interaction of chiral C2-symmetric bis(phosphinite)
a
oxymethyl-1,8-octanebis(diphenylphosphinite) (0.02 mmol) and
a stir bar. To these components were added IPA (5 mL) and the
resultant solution was stirred at room temperature until the com-
plex was formed (reaction was followed by 31P-NMR). A solution of
ketone (0.5 mmol) in IPA (5 mL) was then added to catalyst mix-
ture and the solution was heated to the desired temperature (gen-
erally, 25, 50 or 82 °C). The reaction was then initiated with the
addition of a solution of KOH (2.5 mL, 0.1 M IPA). Catalytic reac-
tions were periodically monitored by GC. As soon as the reaction
completed, an aliquot of the catalytic solution (1 mL) was removed
via syringe and evaporated under reduced pressure. The resultant
oil was subjected to flash chromatography (silica gel-60, Et2O)
and subsequent evaporation under reduced pressure to yield clear
liquid. Subsequently a sample of the reaction mixture was taken
off, diluted with acetone and analyzed immediately by GC, conver-
sions obtained are related to the residual unreacted ketone.
ligand and [Ru(g l-Cl)Cl]2 (molar ratio 2:1) and then
6-p-cymene)(
used as a catalyst precursor in asymmetric transfer hydrogenation
reactions (new singlet appeared due to diphenylphosphinite-Ru(II)
moiety at 112.14 ppm, Fig. 1).
3.2. Asymmetric transfer hydrogenation of aromatic ketones
Asymmetric transfer hydrogenation reactions are one of the
most basic, but important, transformations in organic chemistry.
Synthesis of chiral secondary alcohols via transition metal
catalyzed reactions has also drawn a great interest. Numerous chi-
ral secondary alcohol formed by asymmetric catalysts have been
described in the literature [34]. Efficient catalysts for the asymmet-
ric transfer hydrogenation of ketones include chiral Sm(III)
complexes [1,34b] with high enantioselectivity and Noyori’s
ruthenium complexes containing arene and TsDPEN (Ts:
p-toluensulfonyl) ligands [34a,35]. The ligands used in asymmetric
2.3. Synthesis of C2-symmetric (1S, 8S)-3,6-[N-(S)-a-phenyl
ethyl]diaza-1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite)
Ph2PCl (0.352 mmol) was added to solution of (1S,8S)-3,6-[N-
(S)-
a-phenyl
ethyl]diaza-1,8-diphenoxymethyl-1,8-octanediol
(0.176 mmol) and triethylamine (0.352 mmol) in toluene (25 mL)
at room temperature with vigorous stirring. The mixture was

F. Durap et al. / Inorganica Chimica Acta 411 (2014) 77–82
79
Scheme 1. Synthesis of chiral C2-symmetric (1S,8S)-3,6-[N-(S)-a-phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite), ligands.
Fig. 1. The 31P-{1H} NMR spectra of (a) (1S,8S)-3,6-[N-(S)-
a-phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite) and (b) in situ formed
bis(phosphinite)-Ru(II) catalyst system.
transfer hydrogenation reactions contain various combinations of
nitrogen, oxygen, phosphorus, sulfur and arsenic as the donor atom
and they can also be classified as anionic or neutral [16]. Among
these ligands, phosphinites are the most important phosphorus
based ligands with different steric and electronic properties
[11–13]. Intensive studies have been carried out both in industry
and academia to the development of bis(phosphinite) or bis(phos-
phine) ligands that promote enantioselective ruthenium catalyzed
asymmetric transfer hydrogenation of pro-chiral aromatic ketones
[1,27].
The catalytic performance (conversion and enantioselectivity)
of a novel chiral C2-symmetric bis(phosphinite) ligand-ruthe-
nium(II) catalyst system was explored in the asymmetric transfer
hydrogenation of acetophenone by isopropyl alcohol (IPA),
wherein IPA is the hydrogen source as seen in Scheme 2. This
catalyst system catalyzed the reduction of acetophenone to the
(S)-1-phenylethanol via hydrogen transfer from IPA (Scheme 2)
and the results are summarized in Fig. 2.
Employed acetophenone/Cat/KOH molar ratios and reaction
temperatures were varied in order to find optimal transfer hydro-
genation conditions. The optimization of acetophenone/Cat/KOH
molar ratio and reaction temperature were to be essential to
achieve high enantioselectivity or conversion (Fig. 2). The presence
of base (KOH or NaOH) is absolutely necessary to observe apprecia-
ble conversions because of the fact that base presumably facilitates
catalysis by promoting formation of intermediates (possibly [Ru]-H
from the [Ru]-Cl precatalyst) [16,36]. The transfer hydrogenation
of acetophenone (acetophenone/Cat/KOH; 100:1:5 ratios)
proceeded very slowly and the (S)-1-phenylethanol was formed
with a low conversion (up to 15%) and moderate enantioselectivity
(up to 76% ee) in all reactions at room temperature. As shown in
Fig. 2, optimization studies of the catalytic reduction of
Scheme 2. Hydrogen transfer from IPA to acetophenone by in situ prepared [Ru(g l-Cl)Cl]2/(1S,8S)-3,6-[N-(S)-a-phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-
6-p-cymene)(
octanebis(diphenylphosphinite), catalyst.

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F. Durap et al. / Inorganica Chimica Acta 411 (2014) 77–82
Fig. 2. Transfer hydrogenation of acetophenone with IPA catalyzed by [Ru(g l-Cl)Cl]2/(1S,8S)-3,6-[N-(S)-a-phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-
6-p-cymene)(
octanebis(diphenylphosphinite). Reaction conditions: (a) Acetophenone/Cat/KOH, 100:1:5, at 25 °C; (b) Refluxing in iso-PrOH; acetophenone/Cat, 100:1, in the absence of
KOH, (c) Refluxing in iso-PrOH; acetophenone/Cat/KOH, 100:1:5, in the open air, (d) Refluxing in iso-PrOH; acetophenone/Cat/KOH, 500:1:5 (e) Refluxing in iso-PrOH;
acetophenone/Cat/KOH, 1000:1:5, (f) Refluxing in iso-PrOH; acetophenone/Cat/KOH, 100:1:5, conversions determined by GC with three independent catalytic experiments,
using a chiral cyclodex B (Agilent) capillary column and ee’s determined by comparison of the retention times of the enantiomers on the GC traces.
Fig. 3. Asymmetric transfer hydrogenation results for substituted acetophenones with the catalyst systems prepared from [Ru(
bis(phosphinite) ligand, (1S,8S)-3,6-[N-(S)- -phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite), at 50 °C. Reaction conditions: catalyst
g l-Cl)Cl]2 and C2-symmetric
6-p-cymene)(
a
(0.005 mmol), substrate (0.5 mmol), IPA (5 mL), KOH (0.025 mmol), 50 °C, under Ar atmosphere, the concentration of acetophenone is 0.1 M. Purity of compounds is
checked by NMR and GC with three independent catalytic experiments, yields are based on aryl ketone; conversion was determined by capillary GC analysis using a chiral
cyclodex B (Agilent) capillary column (30 m  0.32 mm I.D.  0.25
lm film thickness); ee values were determined by comparison of the retention times of the enantiomers on
the GC traces.

F. Durap et al. / Inorganica Chimica Acta 411 (2014) 77–82
81
Fig. 4. Asymmetric transfer hydrogenation results for substituted acetophenones with the catalyst systems prepared from [Ru(
g
6-p-cymene)(
l
-Cl)Cl]2 and C2-symmetric
bis(phosphinite) ligand, (1S,8S)-3,6-[N-(S)-a-phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite), at 82 °C. Reaction conditions: catalyst
(0.005 mmol), substrate (0.5 mmol), IPA (5 mL), KOH (0.025 mmol %), 82 °C, under Ar atmosphere, the concentration of acetophenone is 0.1 M. Purity of compounds is
checked by NMR and GC with three independent catalytic experiments, yields are based on aryl ketone; conversion was determined by capillary GC analysis using a chiral
cyclodex B (Agilent) capillary column (30 m  0.32 mm I.D.  0.25
lm film thickness); ee values were determined by comparison of the retention times of the enantiomers on
the GC traces.
Table 1
Comparing catalytic activity of various C2-symmetric bis(phosphinite)-Ru(II) complexes have been tested for the asymmetric transfer hydrogenation of acetophenone.
Entry
L⁄
Temp. (°C)
Time
Conv. (%)
ee (%)
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
[a]
[a]
[a]
[b]
[b]
[b]
[c]
[c]
[d]
[d]
[e]
[e]
[f]
rt
24 h
24 h
1 h
24 h
24 h
30 min
24 h
2 h
24 h
30 min
24 h
1 h
52 h
6 h
15
41
95
33
94
99
30
99
15
99
32
97
98
96
76
86
85
82
88
84
80
88
81
76
16
16
69
67
This study
This study
This study
[28c]
[28c]
[28c]
[28b]
[28b]
[28b]
[28b]
50
82
rt
50
82
rt
82
rt
82
50
82
50
82
[28a]
[28a]
[25a]
[25a]
[f]
[a] (1S,8S)-3,6-[N-(S)-a-phenyl ethyl]diaza-1,8-diphenoxymethyl-1,8-octanebis(diphenylphosphinite),
[b]
L
-isoleusinol derivative C2-symmetric bis(phosphinite),
[c]
L-alaninol derivative C2-symmetric bis(phosphinite),
[d] (S)-Valinol derivative C2-symmetric bis(phosphinite),
[e] N,N, 0-bis[(1S)-1-benzyl-2-O-(diphenylphosphinite)ethyl]ethanediamide,
[f] (2R)-2-[benzyl{(2-((diphenylphosphanyl)oxy)ethyl)}amino]butyldiphenylphosphinite
⁄
Acetophenone/Cat/Base, 100:1:5.

82
F. Durap et al. / Inorganica Chimica Acta 411 (2014) 77–82
acetophenone in IPA showed that good activity was obtained with
a base/catalyst ratio of >5:1 and increasing the substrate-to-cata-
lyst ratio does not lower the conversions of the product in most
cases. It should be noted that the transfer hydrogenation of aceto-
phenone could be achieved up to 95% conversion and enant-
ioselectivty was still high even if the substrate-to-catalyst ratio
reached to 1000:1. The transfer hydrogenation of acetophenone
proceeded enantioselectivity with a slow rate (but conversion did
not change) when the reaction performed in air.
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Encouraged by the good catalytic activity (enantioselectivities
and conversions) obtained from these optimization studies, we ex-
tended our investigation to include asymmetric transfer hydroge-
nation of acetophenone derivatives at different temperatures (50
or 82 °C). Only low conversions (36–46%) but high ee’s (82–89%)
were observed after 24 h for acetophenone derivatives (Fig. 3) at
50 °C. The catalytic activity of chiral bis(phosphinite)-Ru(II) cata-
lyst system was significantly enhanced by increasing the reaction
temperature from room temperature to 82 °C, giving almost quan-
titative conversions and high enantioselectivity of acetophenone
derivatives (p-fluoroacetophenone, p-chloroacetophenone, p-bro-
moacetophenone, p-methoxyacetophenone, o-methoxyacetophe-
none) in range of 90–98% and 80–89% in 1 h, respectively (Fig. 4).
A significant higher conversion could be achieved at 82 °C, which
led to only a slight decrease in enantioselectivity in comparison
to the reaction at 50 °C (Fig. 3 and Fig. 4) [37]. It is well known that
electronic properties of the substituents on the phenyl ring of the
acetophenone also caused changes in the reduction rate. The intro-
duction of F, Cl or Br (known as electron-withdrawing group) to the
p-position of the aryl ring of the ketone was improved the reduc-
tion rate giving rise to easier hydrogenation [25,38]. As shown in
Fig. 3 or Fig. 4, the introduction of a methoxy group (known as
electron-donating group) to the p-position of the aryl ring of the
ketone decelerates the reaction, but that to the o-position increases
the rate and improves the enantioselectivity. p-methoxyacetophe-
none is known to be a challenging substrate thus, the introduction
of a methoxy group to the p-position decelerates the reaction
probably because of its low redox potential, but addition to the
o-position increases the rate and improves the enantioselectivity
[37]. Among all selected ketones, the best results were obtained
in the reduction of o-methoxyacetophenone giving 90% or 89% ee
at 50 or 82 °C, respectively.
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Table 1 shows the various C2-symmetric chiral bis(phosphi-
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Compared to catalyst systems given in Table 1, it is worth to note
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hydrogenation of acetophenone.
4. Conclusion
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A new and efficient chiral C2-symmetric bis(phosphinite)-Ru(II)
catalyst system was used for asymmetric transfer hydrogenation of
pro-chiral and substituted aromatic ketones. We found that this
catalyst system shows moderate to high conversion and also shows
high enantioselectivity at different temperatures.
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Acknowledgements
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_
_
Financial support from TÜBITAK (Project number: 108T061) is
(b) I. Özdemir, S. Yasßar, Trans. Met. Chem. 30 (2005) 831.
gratefully acknowledged.