Synthesis and electrochemical characterization of iminophosphine-based ruthenium(II) complexes and application in asymmetric transfer hydrogenation reaction as catalysts

Article abstract of DOI:10.1007/s13738-018-1569-7

A range of Ru(II) complexes have been prepared with chiral iminophosphine ligands ([(2-PPh₂)C₆H₄CH=NCH(CH₃)C₆H₅(4-R)]; R = –H, p-CH₃, p-NO₂) and characterized by ¹H, ¹³C, ³¹P{¹H} NMR and FTIR spectroscopy. The electrochemical properties of the [Ru(PN)₂Cl₂] complexes were investigated in ACN/TBAP solution with cyclic voltammetry and square wave voltammetry techniques. The use of chiral [Ru(PN)₂Cl₂] complexes as catalysts for the asymmetric transfer hydrogenation of aromatic and aliphatic ketones was studied in 2-propanol in an attempt to demonstrate the effect of substituents, which attached to the phenyl ring bonded to the nitrogen donor, on the catalytic activity and enantioselectivity. It was seen that the electronic effects of these substituents did not contribute to the catalytic efficiency of the ruthenium(II) catalysts.

Full text of DOI:10.1007/s13738-018-1569-7

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1569-7
ORIGINAL PAPER
Synthesis and electrochemical characterization of iminophosphine-
based ruthenium(II) complexes and application in asymmetric transfer
hydrogenation reaction as catalysts
Mustafa Keleş1 · Hülya Keleş1 · Mustafa Kemal Yılmaz2
Received: 28 May 2018 / Accepted: 4 December 2018
© Iranian Chemical Society 2018
Abstract
A range of Ru(II) complexes have been prepared with chiral iminophosphine ligands ([(2-PPh2)C6H4CH=NCH(CH3)C6H5(4-
R)]; R=–H, p-CH3, p-NO2) and characterized by 1H, 13C, 31P{1H} NMR and FTIR spectroscopy. The electrochemical prop-
erties of the [Ru(PN)2Cl2] complexes were investigated in ACN/TBAP solution with cyclic voltammetry and square wave
voltammetry techniques. The use of chiral [Ru(PN)2Cl2] complexes as catalysts for the asymmetric transfer hydrogenation
of aromatic and aliphatic ketones was studied in 2-propanol in an attempt to demonstrate the effect of substituents, which
attached to the phenyl ring bonded to the nitrogen donor, on the catalytic activity and enantioselectivity. It was seen that the
electronic effects of these substituents did not contribute to the catalytic efficiency of the ruthenium(II) catalysts.
Keywords Iminophosphine · Ruthenium · Electrochemistry · Asymmetric transfer hydrogenation
Introduction
It is known that the catalytic efficiency of iminophosphine
(PN) ligands increases in the catalytic reactions. Therefore,
in complex structures, the ruthenium metal coordinates with
the soft phosphorus and the hard nitrogen acts as an efficient
catalyst [13–15]. In this paper, we reported the synthesis of
PN-based chiral Ru(II) complexes and their characterization.
These complexes were used as catalyst for the asymmetric
reduction of acetophenone, substituted benzophenones and
aliphatic ketones in 2-propanol as a hydrogen source.
Application of new transition metal complexes in catalytic
reactions leading to high activity has increased in aca-
demic and industrial areas for the preparation of building
blocks of biological interest [1–3]. Asymmetric transfer
hydrogenation reaction is one of these catalytic reactions
and used to attain secondary alcohols by the reduction of
simple ketones via different ligands and transition metal
complexes as catalysts [4–6]. The synthesis of many impor-
tant chiral drugs is accomplished by reducing ketones in
basic media and using non-toxic iPrOH as hydrogen source.
During the 1990s, Noyori’s studies on asymmetric transfer
hydrogenation with C2-symmetric phosphine ligands, which
have showed high catalytic activity, increased the interest
of researchers, and researchers have incessantly designed a
series of phosphorus- and nitrogen-based ligands and tested
their catalytic effects for hydrogenation reactions [7–12].
Experimental
General methods and materials
All the synthesis and catalytic reactions were carried out
under an argon atmosphere using conventional Schlenk
glassware. Solvents were dried using established proce-
dures and then immediately distilled under argon prior to
use [16]. The (S)-(−)-α-methylbenzylamine, (S)-(−)-α,4-
dimethylbenzylamine and (S)-α-methyl-4-nitrobenzylamine
hydrochloride were purchased from Sigma-Aldrich Chemie
GmbH (Steinheim, Germany) and used without further
purification. 2-(Diphenylphosphino)benzaldehyde and
[Ru(dmso)4Cl2] were synthesized as described in literature
[17, 18].
* Mustafa Keleş
mkeles@osmaniye.edu.tr
1
2
Department of Chemistry, Faculty of Arts and Sciences,
Osmaniye Korkut Ata University, 80000 Osmaniye, Turkey
Department of Chemistry, Faculty of Arts and Sciences,
Mersin University, 33343 Mersin, Turkey
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Elemental analysis was performed using a LECO CHNS
932 instrument. Infrared spectra were recorded on Per-
kin Elmer RX1 Spectrophotometer in the range between
4000 and 650 cm−1. All 1H, 13C NMR and 31P{1H} spectra
were recorded at 25 °C with deuteriated CDCl3 in a Bruker
Avance-400 NMR spectrometer. 31P{1H} NMR spectra
were recorded with complete proton decoupling and refer-
enced with 85% H3PO4 as external standard. The catalytic
products were analyzed with a Perkin Elmer Clarus 500
series gas chromatograph equipped with a flame ionization
detector and β-Dex (30 m × 0.25 mm × 0.25 µm) capillary
column. Enantiomeric excesses and conversions were also
determined by HPLC analysis using a Hitachi L 2300 with
a Chiralcel OD-H column. Thin-layer chromatography was
performed to monitor the reactions.
Preparation of bis(1‑(2‑(diphenylphosphino)
phenyl)‑N‑(1‑phenylethyl)methanimine)
dichlororuthenium(II), [Ru((2‑PPh2)
C6H4CH=NCH(CH3)C6H5)2Cl2] (1b)
A mixture of cis-[Ru(dmso)4Cl2] (0.200 g, 0.420 mmol)
and 1-(2-(diphenylphosphino)phenyl)-N-(1-phenylethyl)
methanimine (1) (0.394 g, 1 mmol) in 30 mL of dry THF
was heated under reflux for 6 h. The resulting brown solu-
tion was then evaporated to dryness, and the resulting solid
was dissolved in CH2Cl2 (ca. 10 mL). The crude solution
was triturated with diethyl ether (ca. 50 mL) to afford a red
solid, which was filtered off and dried under high vacuum.
Yield: 0.346 g (86%). [ꢀ]25=39.1 (c=0.003 M CHCl3). 1H
NMR (400.2 MHz), CDCDl3: δ 8.93 (s, 2H, CH=N, H7), 8.39
(dd, J=7.3, 3.0 Hz, 2H, Ar), 7.63–7.31 (m, 18H, Ar), 7.21
(d, J=5.4 Hz, 14H, Ar), 7.12 (dd, J=8.3, 6.3 Hz, 2H, Ar),
6.99–6.93 (m, 2H, Ar), 4.49 (q, J = 6.3 Hz, 2H, H8), 1.00
(d, J=6.8 Hz, 6H, H9). 13C NMR (100.6 MHz, CDCl3): δ
172.5 (C7), 144.5 (C10), 143.3, 136.8, 136.6, 133.1, 132.6,
132.0, 131.6 (s), 130.4, 129.4, 128.4, 127.9, 127.3, 126.9,
68.3 (C8), 15.3 (C9). 31P NMR (162.0 MHz, CDCl3): δ
(ppm) 45.90 (s). FT-IR, (KBr, cm−1) υ: 1626 (–C=N), 3051
(ArCH). Anal. calcd. for C54H48Cl2N2P2Ru: C, 67.64; H,
5.05; N, 2.92%. Found: C, 66.49; H, 5.49; N, 3.12%.
Electrochemical studies of iminophosphine ligands [19]
and their Ru(II) complexes were carried out in acetonitrile
(ACN) solution under Ar atmosphere. 0.1 M tetrabutylam-
monium perchlorate (TBAP) was used as the supporting
electrolyte and the concentration of the complexes and free
ligands was 1 mM. Cyclic voltammetry (CV) and square
wave voltammetry (SWV) techniques were utilized using
CHI 6094D electrochemical analyzer equipped with three-
electrode cell system. A glassy carbon electrode (GCE)
with 3.0 mm diameter used as working electrode, a Pt
wire auxiliary electrode and all potentials were referred to
(Ag/Ag+) reference electrode. The solution was degassed
with a continuous flow of Ar gas before and during the
scanning.
Preparation of complex 2b and 3b
Cyclic voltammetry measurements were performed in
the potential range of −1.5 and 1.5 V with 100 mV/s scan
rate and SWV settings where: step potential 4 mV; ampli-
tude 25 mV at a frequency of 15 Hz.
(Bis(1-(2-(diphenylphosphino)phenyl)-N-(1-(p-tolyl)
ethyl)methanimine)dichloro) ruthenium(II), [Ru((2-PPh2)
C6H4CH=NCH(CH3)C6H4(4-CH3))2Cl2] (2b) and bis(1-
(2-(diphenylphosphino)phenyl)-N-(1-(4-nitrophenyl)
ethyl)methanimine)dichloro ruthenium(II), [Ru((2-PPh2)
C6H4CH=NCH(CH3)(4-NO2)C6H4(4-NO2)2Cl2] (3b) were
prepared by a procedure similar to that used for the prepa-
ration of 1b.
Ligand 1, 2 and 3 were synthesized according to the
methods of Frauenlob and Yılmaz [19, 20] (Scheme 1).
[Ru((2-PPh2)C6H4CH=NCH(CH3)C6H4(4-CH3))2Cl2]
(2b). Yield 302 mg (73%). [ꢀ]25 = 22.0 (c = 0.003 M
D
CHCl3). H NMR (400.2 MHz, CDCl3): δ 8.99 (s, 2H7),
1
8.47 (s, 2H, H), 7.72–7.58 (m, 16H), 7.45–7.23 (m, 14H),
7.10 (d, J = 7.9 Hz, 4H), 3.48 (q, J = 7.0 Hz, 2H, H8),
2.31 (s, 6H, H14), 1.08 (d, J = 6.6 Hz, 6H, H9). 13C NMR
(100.6 MHz, CDCl3): δ 171.6 (C7), 142.8, 140.9, 136.2
(C10), 134.6, 131.1, 129.0, 129.9, 128.9, 128.1, 128.0,
127.6, 126.9, 126.3, 67.7 (C8), 24.1 (C9), 18.1 (C14). 31P
NMR (162.0 MHz, CDCl3): δ (ppm) 46.02 (s). FT-IR,
(KBr, cm−1) υ: 1688 (-CH=N), 3055 (ArCH). Anal. calcd.
for C56H52Cl2N2P2Ru: C, 68.15; H, 5.31; N, 2.84%. Found:
C, 68.31; H, 5.59; N, 2.89.
[Ru((2-PPh2)C6H4CH=NCH(CH3)(4-NO2)C6H4(4-
NO2)2Cl2] (3b). Yield 302 mg (73%). [ꢀ]25 = 5.0
D
Scheme 1 Structure of iminophosphine ligands
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Journal of the Iranian Chemical Society
1
(c = 0.003 M CHCl3). H NMR (400.2 MHz, CDCl3):
displacement of imine carbon signal from upfield in non-
coordinated ligands (1: 158.0, 2: 158.0, and 3: 159.4 ppm)
to the downfield in the ruthenium(II) complexes (1b: 172.5,
2b: 171.6, and 3b: 167.2 ppm) also suggested the coordi-
nation of imine nitrogen to the ruthenium(II) center. The
chemical shift and coupling constant of H8 were split into
quartet [23] [4.49 ppm, J=6.3 Hz (1b), 3.48 ppm, J=7.0 Hz
(2b), 4.58 ppm, J=6.5 Hz (3b)] due to the coupling of H9
proton. The 1H-NMR spectrum showed that a doublet signal
was observed at δ = 1.00 ppm (J = 6.6 Hz, 1b), 1.08 ppm
(J=6.6 Hz; 2b), and 1.23 ppm (J=6.4 Hz, 3b) which was
assigned to protons of the CH group bonded to the methyl
group. Meanwhile, the signal of aromatic protons of the
complexes appeared between 8.39 and 6.93 ppm.
δ 9.00 (s, 2H, H7), 8.40–8.32 (m, 2H, Ar), 8.06 (d,
J = 8.6 Hz, 4H, Ar), 7.65–7.34 (m, 16H, Ar), 7.32–7.20
(m, 12H, Ar),7.01 (dd, J = 10.1, 8.2 Hz, 2H, Ar), 4.58
(q, J = 6.5 Hz, 2H, H8), 1.23 (d, J = 6.4 Hz, 6H, H9). 13C
NMR (100.6 MHz, CDCl3): δ 167.2 (C7), 141.4 (C10),
131.9, 130.3, 129.5, 129.3, 129.0, 128.6, 128.5, 127.4,
126.1, 123.9, 123.7, 123.5, 65.2 (C8), 27.0 (C9). 31P NMR
(162.0 MHz, CDCl3): δ (ppm) 45.15 (s). FT-IR, (KBr,
cm−1) υ: 1630 (–CH=N), 3049 (ArCH). Anal. calcd. for
C54H46Cl2N4O4P2Ru: C, 61.84; H, 4.42; N, 5.34%. Found:
C, 62.31; H, 4.76; N, 4.91.
General procedure for asymmetric transfer
hydrogenation reactions
In the 31P NMR spectra, the signals from phosphorus
atom in the ruthenium complexes (1: −15.92; 2: −12.46;
3: −12.36 ppm), which show only one singlet, have shifted
from upfield to the downfield after complexation (1b: 45.90;
2b: 46.02; 3b: 45.15 ppm). Besides, In the FTIR spectrum,
displacement of C=N stretching frequencies from 1634
to 1645 cm−1 in the free iminophosphine ligands to lower
values of 1607–1619 cm−1 in the [Ru(PN)2Cl2] complexes
indicates the coordination of imine nitrogen to the ruthenium
center [24, 25]. Also, micro analyses for C, H, and N of the
complexes proved that the iminophosphine–Ru complexes
were formed with 2:1 ratio. It was found that 1b, 2b and 3b
were very soluble in dichloromethane, chloroform, ethanol
and acetone; however, they were insoluble in n-hexane and
diethyl ether.
Under an argon atmosphere, the [Ru(PN)2Cl2] complexes
i
(0.1 mmol%), PrOH (3.0 mL), base (1.2 mmol), and the
substrate (1.0 mmol) were introduced into a Schlenk tube,
respectively. The reaction mixture was stirred at appropri-
ate temperature for 24 h. Next, the mixture was cooled and
diluted with Et2O and chromatographed on silica gel. Con-
versions and enantiomeric excesses were determined by GC
and HPLC analysis.
Results and discussion
The reaction of chiral iminophosphine ligands (1–3) with
[Ru(dmso)4Cl2] resulted in the formation of PN bidentate
chelate complexes, [Ru(PN)2Cl2] (1b–3b), as shown in
Scheme 2. The [Ru(PN)2Cl2] complexes were fully char-
acterized by elemental analysis, NMR and FTIR spectros-
Electrochemical behavior of compounds
1
copy. The H NMR spectra showed that the signal due to
The electrochemical properties of the synthesized Schiff
base ligands (1, 2, 3) and their ruthenium(II) complexes (1b,
2b, 3b) were studied by the CV and SWV techniques. The
obtained potential data are summarized in Table 1.
azomethine protons (N=CH) appeared at 8.93 (1b), 8.99
(2b) and 9.00 (3b) ppm as a singlet after coordination to the
Ru(II) center [21, 22]. In addition, from 13C-NMR spectra,
Scheme 2 Synthesis of chiral
[Ru(PN)2Cl2] complexes
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Journal of the Iranian Chemical Society
Table 1 Electrochemical parameters for Ru(II) complexes in 0.1 M
TBAP/ACN solution
b
Compound Redox couple Epc (V) Epa (V) aΔEp (mV) E1/2 (V)
1b
2b
Ru+2/Ru+1
Ru+2/Ru+1
Ru+3/Ru+2
Ru+2/Ru+1
Ru+3/Ru+2
−1.311
–
–
–
−1.304 −1.292 12
−0.358 −0.264 94
−1.244 −1.157 87
−1.298
−0.311
−1.201
0.108
3b
0.128 0.088
40
aΔEp =Epa −Epc
bE1/2 =(Epa +Epc)/2, ν=100 mV/s
Fig. 2 Cyclic voltammograms of 1 mM of Schiff base ligand 2 (dot-
ted line) and Ru(II) complex 2b (blue line) in 0.1 M TBAP/ ACN
on GC electrode with 0.1 V s−1 scan rate (inset: SWV of 2 and 2b,
amplitude=25 mV; frequency=15 Hz; step potential=4 mV)
Fig. 1 Cyclic voltammograms of 1 mM of Schiff base ligand 1 (dot-
ted line) and Ru(II) complex 1b (blue line) in 0.1 M TBAP/ ACN
on GC electrode with 0.1 V s−1 scan rate (inset: SWV of 1 and 1b,
amplitude=25 mV; frequency=15 Hz; step potential=4 mV)
Fig. 3 Cyclic voltammograms of 1 mM of Schiff base ligand 3 (dot-
ted line) and Ru(II) complex 3b (blue line) in TBAP/ACN on GC
electrode with 0.1 V s−1 scan rate (inset: SWV of 3 and 3b, ampli-
tude=25 mV; frequency=15 Hz; step potential=4 mV)
All the Schiff base ligands exhibited several oxidation
peaks at anodic potentials, when the potential scanned from
the positive to negative direction (starting from 1.5 to 0.0 V).
These peaks are also observed in the CV responses of the
synthesized Ru(II) complexes and were not interpreted
further. We have focused on the metal-based responses
of the Ru(II) complexes at negative potentials of the
voltammograms.
process observed for 2b complex is metal centered. The inset
diagram in Fig. 2 shows the square wave voltammograms
of 2 and 2b. A couple of reduction and oxidation peaks at
Epc =−0.358 V and Epa =−0.264 V in 2b differently from
SWV of 2 were observed. This reductive response indicates
that the reduction is metal centered and assigned as Ru(III)
to Ru(II). Ru(III) formation can be explained as the result of
applying +1.5 V potential in the beginning of the potential
scanning. Ru(III)/Ru(II) reduction response is comparable to
previously reported CV studies of ruthenium complex [29].
Figure 3 represents the voltammetric responses of 3
and 3b. In the case of Schiff base ligand 3, several reduc-
tive peaks at the negative potential region differ from the
other two ligands. The complex 3b also showed revers-
ible redox processes during scan of the negative potential
range. One of the reduction peaks belonging to the ligand
3 overlapped with the peaks of 3b and this overlapping
did not permit easy evaluation. However, a metal-centered
reduction peak is observable in CV with E1/2 =−1.201 V,
which corresponds to Ru(II) to Ru(I) conversion with reduc-
tive and oxidative peak potentials at Epc = − 1.244 V and
Figure 1 shows the cyclic voltammogram responses of
ligand (1) and its Ru (II) complex (1b) on GCE. The cyclic
voltammogram of 1b exhibited one irreversible metal-based
reduction peak at Epc =−1.311 V due to Ru(II)/Ru(I) redox
couple. The reduction of Ru(II)→Ru(I) is clearly observed
on the square wave voltammogram at the inset diagram
in Fig. 1. 1b did not show any oxidation response in the
positive direction of the scan; however, the Epc value of
the Ru(II)/Ru(I) reduction is comparable with the values
reported earlier [26, 27]. The reason for irreversible reduc-
tion of the complex may be due to short-lived reduced state
of the metal or oxidative degradation of the ligands [28].
Ru(II) complex 2b showed a reversible reduction wave
with E1/2 =−1.298 V (Fig. 2). The Schiff base ligand, 2, is
not reversibly reduced within the negative potential limit (0.0
to −1.5 V). Consequently, it is considered that the reduction
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Journal of the Iranian Chemical Society
Epa =−1.157 V. A redox couple is better noticeable in the
inset diagram of Fig. 3. These peaks correspond to Ru(III/II)
redox couple. The peak at E1/2 =0.108 V with Epc =0.128 V
and Epa =0.088 V couple was attributed to Ru(III)→Ru(II)
conversion [30]. The reasons for the changes in the oxidation
and reduction potential of the ruthenium(II) complexes are
generally attributed to the stabilizing effect of the ligand by
a combination of σ and π effects [31].
hand, good product conversion (67%) but very low enan-
tioselectivity (<5) was observed with the using of K2CO3
as base (Table 2, entry 4). Thus, NaOH was identified as
the most effective base for our catalyst system. The effect
of temperature on the performance of the reduction reac-
tion using catalyst 1b was then examined. The relevant data
(Table 2, entry 3) show that on lowering the temperature to
25 °C, the catalyst activity decreases sharply (24%) and no
significant beneficial effect on enantioselectivity (40%) is
observed. I also added Table 3 to the system.
Asymmetric transfer hydrogenation
Next, we aimed to investigate the catalytic activity of
1b–3b catalysts for the reduction of substituted benzophe-
nones which have electron-donating and electron-withdraw-
ing groups and aliphatic ketones as substrate under the opti-
mized conditions determined in the preliminary studies for
1b (0.1 mmol% catalyst, NaOH, 82 °C). According to the
results in Table 3, the best product conversion (87%) was
achieved with catalyst 1b using 3-chlorobenzophenone as a
substrate but in relatively low ee (27%) (entry 2). Under the
same reaction conditions, 65% conversion and 22% ee were
reached within 24 h with the complex 3b, while only 7%
conversion and very poor enantioselectivity were achieved
with the complex 2b after 24 h (entry 2). On the other hand,
with catalyst 3b, the asymmetric reduction of 3-fluorobenzo-
phenone can be carried out with good conversion (83%) with
only 46% ee in 24 h (entry 3). All of the results indicate that
the selected substituted benzophenones could only be cata-
lyzed with 46% ee by our catalyst system. So, we thought
that the lower enantioselectivity is related to the high reac-
tion temperature. Therefore, we tried to decrease the tem-
perature of the reaction to 50 and 25 °C to see the influence
of the temperature on the enantioselectivity, but decreasing
the temperature from 82 °C to room temperature dramati-
cally reduced the catalyst conversion and did not affect the
catalyst enantioselectivity. The catalytic results show that
generally moderate to good conversions were achieved with
substituted benzophenones having chloro, fluoro and nitro
groups. This suggests that, the electron-withdrawing effect
of these substituents reduce the electron density on the
carbonyl group and so accelerate the reduction of benzo-
phenone to the corresponding secondary alcohols [32–34].
Besides that, poor enantioselectivities were obtained with
our catalyst systems and the lower ees were not significantly
affected by the reaction temperature. Next, the performance
of the Ru(II) catalysts on the enantioselective reduction of
aliphatic ketones was further investigated for 2-heptanone
and 2-pentanone under the optimized conditions. Fortu-
nately, as seen from entries 5 and 6, conversion was 88%
at 82 °C for 24 h of the reaction when 2-pentanone was
used as substrate, whereas it reached 94% with 2-heptanone
under the same conditions. However, comparing the results
obtained from our catalysts, only 52% enantioselectivity was
achieved with the use of aliphatic ketones.
The catalytic activity of the 1b, 2b, and 3b complexes were
evaluated in iPrOH, which was used as a hydrogen source,
for the asymmetric reduction of aromatic and aliphatic
ketones to the corresponding alcohols as shown in Scheme 3
and the data were presented in the Tables 2 and 3.
Initially, in order to explore the catalytic activity of the
ruthenium(II) catalysts for the asymmetric transfer hydro-
genation reactions, the reduction of acetophenone to 1-phe-
nylethanol was chosen as the model reaction to study the
efficiency of different bases such as KtOBu, NaOH, K2CO3,
and Et3N with catalyst 1b at 82 °C, as shown in Table 2. The
results show that, no product was detected in the absence of
base (entry 5). Also the use of Et3N as base gave no con-
version (Table 2, entry 4). However, 1-phenylethanol was
obtained in 79% conversion and 39% enantioselectivity with
NaOH whilst KtOBu gives 77% conversion and lower enan-
tioselectivity (<5) (Table 2, entries 1 and 2). On the other
Scheme 3 The catalytic transfer hydrogenation of acetophenone
Table 2 The effect of base and temperature on the asymmetric trans-
fer hydrogenation of acetophenone with catalyst 1b in 2-propanol
Entry
Base
Conversion (%)a
ee (%)
1
2
3
4
5
KtOBu
NaOH
K2CO3
Et3N
–
77
<5
79 (24)b
38 (40)b
67
–
–
<5
–
–
Reaction conditions: acetophenone (1.0 mmol), 1b (0.1 mmol %),
i
base (1.2 mmol), PrOH, 3.0 mL, 82 °C, 24 h and substrate to metal
ratio (S/C) was 1000:1
aConversions and enantiomeric excesses (ee) were determined by gas
chromatography
b25 °C
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Journal of the Iranian Chemical Society
Table 3 Asymmetric transfer hydrogenation of substituted ketones with catalyst 1b–3ba
1b
2b
3b
Entry
Substrate
Product
Conv.
(%)
ee
(%)
Conv.
(%)
ee
(%)
Conv.
(%)
ee
(%)
OH
O
18
13
20
28
1
(16)c
(7)d
(36)c
(40)d
nd
nd
(18)c
(9)d
(22)c
(6)d
*
H
H3CO
3CO
OH
*
O
87
27
7
<5
65
22
2
3
(40)c
(6)d
(27)c
(25)d
(nd)c
(nd)d
(nd)c
(nd)d
(48)c
(15)d
(26)c
(18)d
Cl
F
Cl
F
OH
*
O
59
18
18
17
83
46
(37)c
(16)d
(17)c
(11)d
(nd)c
(nd)d
(nd)c
(nd)d
(62)c
(24)d
(38)c
(9)d
OH
*
O
60
8
<5
<5
61
36
4
5
(40)c
(6)d
(nd)c
(nd)d
(nd)c
(nd)d
(nd)c
(nd)d
(37)c
(22)d
(29)c
(16)d
N
O2
N
O2
94
52
46
42
86
47
OH
O
(58)c
(28)d
88
(46)c
(43)d
48
(18)c
(nd)d
28
(39)c
(nd)d
18
(46)c
(28)d
66
(44)c
(43)d
34
*
OH
O
6
(54)c
(32)d
(44)c
(35)d
(16)c
(nd)d
(16)c
(nd)d
(46)c
(18)d
(34)c
(32)d
*
nd not detected
i
aReaction conditions: substrate (1.0 mmol), [RuL2Cl2] (0.1 mmol %), NaOH (1.2 mmol), PrOH (3.0 mL), 82 °C, 24 h. and substrate to metal
ratio (S/C) was 1000:1
bConversions and enantiomeric excesses (ee) were determined by HPLC analyses
c50 °C
d25 °C
the Ru(II)-catalyzed asymmetric transfer hydrogenation of
ketones, the catalytic activities and enantioselectivity were
not significantly affected. Probably, the electronic effects
of these substituents which are quite far away from nitro-
gen donor did not contribute to the catalytic performance
effectively.
Conclusions
We have successfully synthesized a series of new chiral
iminophosphines–Ru(II) complexes and characterized with
spectroscopic, optical and electrochemical techniques. To
determine their catalytic activities for asymmetric trans-
fer hydrogenation reactions, complex 1b was applied in
the reduction of acetophenone to detect the optimal ATH
conditions. Then, all complexes were applied as catalyst for
the asymmetric reduction of aliphatic ketones and substi-
tuted benzophenones having electron-withdrawing (-fluoro,
-bromo and -nitro) and -donating groups (-methoxy). Under
our experimental ATH conditions, these Ru(II) complexes
efficiently catalyzed the asymmetric transfer hydrogenation
of electron-withdrawing substituted benzophenones with
relatively low ees. Additionally, with the use of a variety of
ligands with different substituents (R: –H, –CH3, –NO2) for
Acknowledgements We thank Osmaniye Korkut Ata University
and the Scientific and Technological Research Council of Turkey
(TUBİTAK, Project number: 115Z829) for financial support.
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