Chiral phosphinites as efficient ligands for enantioselective Ru(II), Rh(I) and Ir(III)-catalyzed transfer hydrogenation reactions

Article abstract of DOI:10.1007/s11243-017-0140-1

Abstract: Metal-catalyzed enantioselective transfer reduction of ketones to enantiomerically enriched chiral alcohols has recently attracted attention. Therefore, a series of methyl alkyl or alkyl/aryl ketones have been reduced by using Ru(II), Rh(I) and Ir(III) catalysts based on C₂-symmetric chiral ferrocenyl phosphinite ligands. The corresponding optically active secondary alcohols were obtained in excellent conversions and moderate-to-good enantioselectivities. The best results were obtained with an iridium catalyst, giving up to 98% conversion and 80% ee.

Full text of DOI:10.1007/s11243-017-0140-1

Transit Met Chem
DOI 10.1007/s11243-017-0140-1
Chiral phosphinites as efficient ligands for enantioselective Ru(II),
Rh(I) and Ir(III)-catalyzed transfer hydrogenation reactions
Akın Baysal1 Duygu Elma Karakas¸2 Nermin Meric¸1 Bu¨nyamin Ak1
•
•
•
•
Murat Aydemir1 Feyyaz Durap1
•
Received: 27 January 2017 / Accepted: 23 March 2017
Ó Springer International Publishing Switzerland 2017
Abstract Metal-catalyzed enantioselective transfer reduc-
tion of ketones to enantiomerically enriched chiral alcohols
has recently attracted attention. Therefore, a series of
methyl alkyl or alkyl/aryl ketones have been reduced by
using Ru(II), Rh(I) and Ir(III) catalysts based on C2-sym-
metric chiral ferrocenyl phosphinite ligands. The corre-
sponding optically active secondary alcohols were obtained
in excellent conversions and moderate-to-good enantiose-
lectivities. The best results were obtained with an iridium
catalyst, giving up to 98% conversion and 80% ee.
though transition metal-mediated enantioselective hydro-
genation of pro-chiral ketones using hydrogen is still the
most powerful method available, asymmetric transfer
hydrogenation represents an alternative to molecular
hydrogenation because of its operational simplicity and a
number of cheap and safe chemicals that can be used as
hydrogen donors, such as 2-propanol or the azeotrope of
formic acid and triethylamine (5:2) [3]. Thus, the enan-
tioselective transfer hydrogenation of alkyl ketones is one
of the most cost-effective approaches to the synthesis of
enantiomerically pure secondary alcohols [4].
Even after many decades of intensive research, the
synthesis of chiral ligands is still a key issue for obtaining
both high catalytic activity and selectivity in enantiose-
lective catalysis [5]. Phosphorus-based ligands such as
phosphines, phosphinites [6] and aminophosphine phos-
phinites [7] have become increasingly important for such
applications [8]. Thus, a variety of ruthenium, rhodium and
iridium [9] catalysts containing phosphorus donor ligands
have been synthesized and proved to be highly effective for
this transformation [10]. However, applications of transi-
tion metal complexes containing phosphinite ligands [11]
in homogeneous catalysis are relatively less studied [12].
Ferrocene has been widely used as a skeleton in chiral
ligand catalysis [13]. In particular, chiral ferrocenyl phos-
phines or phosphinites possessing C2-symmetric structure
represent an important class of ligands used in homoge-
neous asymmetric catalysis with transition metals [14].
Ferrocenyl phosphines are effective ligands for transi-
tion metal complexes in asymmetric catalysis, often pro-
viding high enantioselectivity [15, 16]. Because of their
great success in catalytic asymmetric reactions [17], chiral
ferrocenyl-phosphine ligands have attracted considerable
attention in recent years [18]. These ligands are very effi-
Introduction
Optically active alcohols are very important building
blocks for the synthesis of physiologically active pharma-
ceutical compounds, agrochemicals, fragrances, flavor
ingredients and fine chemicals [1]. The growing demand
for chiral products has made necessary the development of
efficient methods that favor the formation of a specific
enantiomer or diastereomer [2]. Among the various
strategies employed for this purpose, asymmetric catalysis
offer significant advantages over other procedures. Even
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-017-0140-1) contains supplementary
material, which is available to authorized users.
& Murat Aydemir
aydemir@dicle.edu.tr
1
Department of Chemistry, Science Faculty, Dicle University,
21280 Diyarbakir, Turkey
2
Science and Technology Application and Research Center
(SUBTAM), Siirt University, 56000 Siirt, Turkey
cient for
a
wide range of reactions, including
123

Transit Met Chem
1
2
1
2
R
R
1
2
R
R
R
R
4
4
R
4
R
R
N
N
N
H
H
H
3
3
3
R
R
OPPh Rh
2
R
OPPh
OPPh
Ir
Ir
2
Ru
Cl
2
Cl
Cl
Cl
Cl
Cl
Cl
Fe
Fe
Fe
Cl
Ru
Cl
4
OPPh
Cl
4
4
OPPh Rh
2
OPPh
R
R
2
R
2
H
N
H
N
H
N
3
3
3
R
R
R
2
1
2
1
2
1
R
R
R
R
R
R
1-5
11-15
6-10
Scheme 1 Complexes 1–15 used as pre-catalyst in asymmetric transfer hydrogenation of methyl alkyl and alkyl aryl ketones. R1: benzyl, R2: H,
R3: H, R4: H, 1, 6, 11; R1: Ph, R2: H, R3: H, R4: H, 2, 7, 12; R1: i-butyl, R2: H, R3: H, R4: H, 3, 8, 13; R1: sec-butyl, R2: H, R3: H, R4: H, 4, 7,
14; R1: H, R2: Ethyl, R3: H, R4: H, 5, 10, 15
hydrogenation [19], hydrosilylation [20], allylic alkylation
[21], Grignard reactions [22] and cyclopropanation [23].
Although ferrocenyl phosphines have found widespread
applications in transition metal-catalyzed asymmetric
transformations [24], the analogous phosphinites provide
different chemical, electronic and structural properties
compared to phosphines [25]. Hence, in recent years our
research group has reported the synthesis [3], characteri-
zation and coordination properties of such ligands [26]. In a
search for efficient phosphinite ligands for asymmetric
transfer hydrogenation (ATH) of ketones, we previously
reported that Ru(II), Rh(I) or Ir(III) complexes [27, 28] of
C2-symmetric bis(phosphinite) ligands [29] are efficient
catalysts for asymmetric reduction of aromatic ketones [3].
Encouraged by these observations, we now report the
asymmetric transfer hydrogenation of alkyl/methyl or
alkyl/aryl ketones by Ru(II), Rh(I) and Ir(III) complexes
based on C2-symmetric chiral ferrocenyl phosphinite
ligands (Scheme 1).
(400.1 MHz), 13C (100.6 MHz) and 31P {1H} NMR
(162.0 MHz) spectra were recorded on a Bruker AV400
1
spectrometer, with TMS as an internal reference for H
NMR and 13C NMR and 85% H3PO4 as an external ref-
erence for 31P {1H} NMR.
GC parameters
GC analyses were performed on a Shimadzu GC 2010 Plus
Gas Chromatograph equipped with a cyclodex B (Agilent)
capillary column (30 m 9 0.32 mm I.D. 9 0.25 lm film
thickness). The GC parameters for product analysis were as
follows: initial temperature, 50 °C; initial time, 1.1 min;
solvent delay, 4.48 min; temperature ramp, 1.3 °C/min; final
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 temperature, 200 °C, injection volume, 2.0 lL.
Experimental
General procedure for the transfer hydrogenation
of ketones
Materials and methods
The standard procedure for the catalytic hydrogen-transfer
reaction was as follows. A solution of the corresponding
ferrocenyl phosphinite complexes 1–15 (0.01 mmol), KOH
(0.05 mmol) and the methyl alkyl ketones or alkyl/aryl
ketones (1 mmol) in isopropanol (10 mL) was refluxed
until completion of the reaction. A sample of the reaction
mixture was taken, diluted with acetone and analyzed
immediately by GC. Conversions obtained are related to
the residual unreacted methyl/alkyl ketones or alkyl/aryl
ketones.
Unless otherwise mentioned, all manipulations were per-
formed under an atmosphere of argon using conventional
Schlenk glassware. Organic solvents were dried by stan-
dard methods and distilled under argon just prior to use.
Analytical grade and deuterated solvents and chemicals
were purchased from Merck or Sigma-Aldrich and used as
received. Ru(II) 1–5 [34], Rh(I) 6–10 [35] and Ir(III) 11–15
[36] complexes of C2-symmetric ferrocenyl phosphinites
1
were prepared according to the literature procedures. H
123

Transit Met Chem
Table 1 Asymmetric transfer hydrogenation of methyl ethyl ketone with isoPrOH catalyzed by Ru(II) (1–5), Rh(I) (6–10) and Ir(III) (11–15)-C2
symmetric ferrocenyl phosphinite complexes
OH
OH
O
O
Cat.
+
+
*
Entry
Catalyst
Time (h)
Conv. (%)e
% eef
TOF (h1)g
Conf.h
1
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
11a
12a
13a
14a
15a
1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
11b
12b
13b
14b
15b
72
72
72
72
72
72
72
72
72
72
72
72
72
72
72
1
48 (52)c
45 (60)c
47 (41)c
50 (40)c
49 (42)c
25 (67)c
27 (75)c
25 (59)c
29 (55)c
26 (58)c
22 (59)c
21 (70)c
23 (59)c
20 (51)c
22 (51)c
98 (96)d
97 (92)d
99 (96)d
98 (93)d
99 (95)d
99 (93)d
97 (94)d
99 (95)d
99 (93)d
99 (95)d
97 (93)d
98 (96)d
99 (93)d
98 (94)d
99 (95)d
51 (46)c
60 (54)c
41 (36)c
40 (36)c
42 (37)c
60 (63)c
71 (72)c
50 (54)c
52 (50)c
51 (52)c
59 (53)c
71 (66)c
50 (44)c
50 (46)c
51 (45)c
57 (53)d
66 (63)d
47 (41)d
48 (46)d
46 (42)d
66 (63)d
75 (72)d
56 (54)d
56 (50)d
57 (52)d
65 (63)d
77 (73)d
55 (52)d
56 (50)d
55(52)d
\1
\1
\1
\1
\1
\1
\1
\1
\1
\1
\1
\1
\1
\1
\1
98
R
S
2
3
R
S
4
5
R
R
S
6
7
8
R
S
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
a
R
R
S
R
S
R
R
S
1
97
1
99
R
S
1
98
1
99
R
R
S
2
50
2
49
2
50
R
S
2
50
2
50
R
R
S
5/2
5/2
5/2
5/2
5/2
39
39
40
R
S
39
40
R
At room temperature; methyl ethyl ketone/Cat./KOH, 100:1:5 (refluxing in isoPrOH; methyl ethyl ketone/Cat., 100:1, in the absence of base;
conversions were \10% for all complexes)
a
Refluxing in isoPrOH; methyl ethyl ketone/Ru/KOH, 100:1:5
b
At room temperature; methyl ethyl ketone/Cat./KOH, 100:1:5 (144 h)
c
Refluxing in isoPrOH; methyl ethyl ketone/Cat./NaOH, 100:1:5
d
Determined by GC (three independent catalytic experiments)
e
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column
TOF = (mol product/mol Cat.) 9 h-1
f
g
Determined by comparison of the retention times of the enantiomers on the GC traces with the literature values, (S) or (R) configuration was
obtained in all experiments
123

Transit Met Chem
Table 2 Asymmetric transfer hydrogenation results for alkyl methyl ketones or alkyl/aryl ketones catalyzed by Ru(II) catalysts 1–5
OH
OH
O
O
Cat.
+
+
R2
R1
*
R2
R1
Entry
Cat.
R1
R2
Time (h)
Conv. (%)a
ee (%)b
TOF (h-1 c
)
Conf.d
1
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
C2H5
1
98
97
99
98
99
97
98
99
96
97
96
98
97
97
99
97
99
99
98
98
99
98
97
99
96
99
98
99
98
96
99
98
97
98
97
97
99
99
98
57
66
47
48
46
48
55
40
41
42
50
59
41
39
40
47
58
40
41
40
40
47
33
33
31
53
63
45
44
42
62
70
51
52
50
43
50
35
36
98
97
99
98
99
32
33
33
32
32
24
25
24
24
25
10
20
20
20
20
50
49
48
50
48
50
49
50
49
48
198
196
194
196
194
24
25
25
25
R
S
2
1
3
1
R
S
4
1
5
1
R
R
S
6
n-C4H9
3
7
3
8
3
R
S
9
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
3
R
R
S
CH(CH3)2
4
4
4
R
S
4
4
R
R
S
CH2CH(CH3)2
5
5
5
R
S
5
5
R
R
S
C6H11
2
2
2
R
S
2
2
R
R
S
CH2CH2C6H5
1-Naphthyl
C6H11
2
2
2
R
S
2
2
R
R
S
‘
1/2
1/2
1/2
1/2
4
R
S
R
R
S
4
4
R
S
4
123

Transit Met Chem
Table 2 continued
Entry
40
Cat.
R1
R2
Time (h)
4
Conv. (%)a
99
ee (%)b
TOF (h-1 c
)
Conf.d
5
35
25
R
Catalyst (0.005 mmol), substrate (0.5 mmol), isoPrOH (5 mL), KOH (0.025 mmol%), 82 °C, the concentration of pro-chiral ketones is 0.1 M
a
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), and yields are based on ketone
b
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column (30 m 9 0.32 mm I.D. 9 0.25 lm film thickness)
TOF = (mol product/mol Cat.) 9 h-1
c
d
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values
slightly decreased both the reaction rate and enantiose-
lectivity (Table 1, entries 16–30d). The data in Table 1
reveal that the reactivities of the ruthenium complexes
(1–5) were higher than those of the rhodium and iridium
complexes. The corresponding optically active alcohols
were produced in moderate-to-high enantioselectivities.
By using complex 12 as a pre-catalyst under otherwise
identical conditions, the hydrogenation of methyl ethyl
ketone was accomplished with 98% conversion and an ee
of 77% (Table 1, entry 27).
Results and discussion
We have recently reported a series of half-sandwich metal
complexes, based on ligands with C–O–P backbones,
which proved to be active catalysts for the reduction of
aromatic ketones [30 and references therein]. The excellent
catalytic performance and structural permutability of
phosphinite-based transition metal catalysts [31] prompted
us to investigate the catalytic reduction of various ketones
[32]. The most important advantage of chiral phosphinite
ligands over the corresponding phosphine ligands is their
ease of preparation [33].
It is known that the catalytic asymmetric transfer
hydrogenation of aliphatic ketones is less facile than that of
aromatic ketones. Therefore, using the optimized reaction
conditions, a series of methyl/alkyl ketones and alkyl/aryl
ketones were tested using iPrOH both as the solvent and the
hydrogen source, in the presence of KOH as the base at
reflux temperature.
To evaluate the effectiveness of our Ru(II) [34, 35]
Rh(I) and Ir(III) [36] complexes (1–15) as pre-catalysts in
asymmetric transfer hydrogenation of ketones, we first
investigated the optimal conditions starting with 2-bu-
tanone as a standard test reaction. Catalytic experiments
were carried out under argon using standard Schlenk-line
techniques. To an iPrOH solution of each complex,
appropriate amounts of 2-butanone and KOH/iPrOH
solution were added at room temperature. The solution
was stirred, and the reaction was monitored by GC. The
results of these reactions are summarized in Table 1. The
reactions proceeded to give (R) or (S)-2-butanol in
20–50% conversion with 40–71% ee’s after stirring at
room temperature for 72 h. An increase in the reaction
time to 144 h resulted in 40–75% conversion with
36–72% ee’s. Due to the reversibility at room tempera-
ture, the prolonged reaction time led to a slight decrease
in enantioselectivity. However, when the reactions were
carried out at 82 °C for less than 3 h, all complexes
showed satisfying catalytic activities since almost con-
versions to the total corresponding alcohol were observed
(Table 1, entries 16–30). Although the lower reaction
temperatures should have a beneficial effect on the
enantioselectivities, slightly higher asymmetric inductions
(46–77% ee’s) were observed at 82 °C. This may attrib-
uted to the shorter reaction times. The complexes were
very active catalysts, leading to quantitative conversions
of (R) or (S)-2-butanol with a catalyst/base ratio of 1:5. A
control experiment in the absence of base did not lead to
meaningful conversion. Replacing KOH with NaOH
It was observed that the Ru(II) complexes were better
than the Rh(I) or Ir(III) complexes in terms of reaction
rates. However, as can be seen from Tables 2, 3 and 4, the
ruthenium catalyst is less efficient than both the rhodium
and iridium catalysts in terms of asymmetric induction.
While the conversions were quite similar, the iridium cat-
alysts (11–15) induced slightly higher enantioselectivities
than the rhodium catalysts.
The results show that the required reaction times are
strongly dependent on the linear chain length and steric hin-
drance of the ketones. Increasing the reaction time and linear
chain length of the ketone caused decreases in both reactivity
and enantioselectivity. Ir-catalyzed reduction of 2-butanone
(Table 4, entries 1–5) and n-butyl methyl ketone (Table 4,
entries 6–10) was carried out at 82 °C for 5/2 and 7 h,
respectively, giving almost complete conversions and ee val-
uesof 55–77and42–65%, respectively. Theenantioselectivity
generally decreases as the bulkiness of the alkyl group
increases. It may be assumed that the steric hindrance around
the carbonyl group is the primary factor leading to low
asymmetric induction. The reduction of cyclohexyl phenyl
ketone also proceeded in 97–99% conversion and with about
35–58% ee (Tables 2, 3, 4). On the other hand, replacement of
a naphthyl group by a methyl led to an increase in the enan-
tioselectivity. Catalyst 12 proved to be the best catalyst, giving
123

Transit Met Chem
Table 3 Asymmetric transfer hydrogenation results for alkyl methyl ketones or alkyl/aryl ketones catalyzed by Rh(I) catalysts 6–10
OH
OH
O
O
Cat.
+
+
R2
R1
*
R2
R1
Entry
Cat.
R1
R2
Time (h)
Conv. (%)a
ee (%)b
TOF (h-1 c
)
Conf.d
1
6
7
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
C2H5
2
2
2
2
2
7
7
7
7
7
7
7
7
7
7
8
8
8
8
8
4
4
4
4
4
4
4
4
4
4
1
1
1
1
1
6
6
6
6
6
99
97
99
99
97
97
98
98
97
97
99
98
97
97
99
99
99
97
98
98
98
98
98
99
98
99
99
99
98
98
98
99
97
99
99
97
98
99
98
98
66
75
56
56
57
55
64
44
42
43
58
69
50
51
50
60
71
50
51
50
47
54
38
36
38
62
72
52
53
52
69
77
57
60
59
51
58
43
40
43
50
49
50
50
49
14
14
14
14
14
14
14
14
14
14
12
12
12
12
12
25
25
25
25
25
25
25
25
25
25
99
99
97
99
99
16
16
17
16
16
R
S
2
3
8
R
S
4
9
5
10
6
R
R
S
6
n-C4H9
7
7
8
8
R
S
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
10
6
R
R
S
CH(CH3)2
7
8
R
S
9
10
6
R
R
S
CH2CH(CH3)2
7
8
R
S
9
10
6
R
R
S
C6H11
7
8
R
S
9
10
6
R
R
S
CH2CH2C6H5
1-Naphthyl
C6H11
7
8
R
S
9
10
6
R
R
S
7
8
R
S
9
10
6
R
R
S
7
8
R
S
9
10
R
Catalyst (0.005 mmol), substrate (0.5 mmol), isoPrOH (5 mL), KOH (0.025 mmol%), 82 °C, the concentration of pro-chiral ketones is 0.1 M
a
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), and yields are based on ketone
b
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column (30 m 9 0.32 mm I.D. 9 0.25 lm film thickness)
TOF = (mol product/mol Cat.) 9 h-1
c
d
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values
123

Transit Met Chem
Table 4 Asymmetric transfer hydrogenation results for alkyl methyl ketones or alkyl/aryl ketones catalyzed by Ir(III) catalysts 11–15
OH
OH
O
O
Cat.
+
+
R2
R1
*
R2
R1
Entry
Cat.
R1
R2
Time (h)
Conv. (%)a
ee (%)b
TOF (h-1 c
)
Conf.d
1
11
12
13
14
15
11
12
13
14
15
11
12
13
14
15
11
12
13
14
15
11
12
13
14
15
11
12
13
14
15
11
12
13
14
15
11
12
13
14
15
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
C2H5
5/2
5/2
5/2
5/2
5/2
7
97
98
99
98
99
99
98
98
97
99
97
98
98
97
99
98
99
97
98
99
99
98
97
99
99
98
99
98
98
99
99
99
97
99
98
99
98
99
97
98
65
77
55
56
55
57
65
46
44
42
55
68
48
46
47
56
67
48
51
50
44
53
39
36
36
61
73
52
52
53
71
80
57
58
60
50
56
43
41
42
39
39
40
39
40
14
14
14
14
14
14
14
14
14
14
12
12
12
12
12
20
20
20
20
20
20
20
20
20
20
99
99
97
99
98
12
12
12
12
12
R
S
2
3
R
S
4
5
R
R
S
6
n-C4H9
7
7
8
7
R
S
9
7
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
7
R
R
S
CH(CH3)2
7
7
7
R
S
7
7
R
R
S
CH2CH(CH3)2
8
8
8
R
S
8
8
R
R
S
C6H11
5
5
5
R
S
5
5
R
R
S
CH2CH2C6H5
1-Naphthyl
C6H11
5
5
5
R
S
5
5
R
R
S
1
1
1
R
S
1
1
R
R
S
8
8
8
R
S
8
8
R
Catalyst (0.005 mmol), substrate (0.5 mmol), isoPrOH (5 mL), KOH (0.025 mmol%), 82 °C, the concentration of pro-chiral ketones is 0.1 M
a
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), and yields are based on ketone
b
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column (30 m 9 0.32 mm I.D. 9 0.25 lm film thickness)
TOF = (mol product/mol Cat.) 9 h-1
c
d
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values
123

Transit Met Chem
4. Abdur-Rashid K, Faatz M, Lough AJ, Morris RH (2001) J Am
Chem Soc 123:7473
5. Wang X-B, Zhou Y-G (2008) J Org Chem 73:5640
6. Jayasinghe D, Kraatz H-B (2006) Inorg Chim Acta 359:3054
7. Guo R, Elpett C, Chen X, Song D, Morris RH (2005) Chem
Commun 24:3050
8. Balakrishna MS, Rao S, Choubey B (2012) J Chem Sci 124:1191
9. Aydemir M, Baysal A, Ozkar S, Yıldırım LT (2011) Inorg Chim
Acta 367:166
10. Ok F, Aydemir M, Durap F, Baysal A (2014) Appl Organomet
Chem 28:38
11. Adhikary A, Guan HR (2015) ACS Catal 5:6858
12. Toma S, Csizmadiova J, Meciarova M, Sebesta R (2014) Dalton
Trans 43:16557
13. Zhu D-Y, Chen P, Xia J-B (2016) ChemCatChem 8:68–73
14. Rafikova K, Kystaubayeva N, Aydemir M, Kayan C, Ocak YS,
Temel H, Zazybin A, Gurbuz N, Ozdemir I (2014) J Organomet
Chem 758:1
(S)-(-)-1-(1-naphthyl)ethanol in 98% conversion and 80% ee
(Table 4, entry 32). Higher enantioselectivity for the hydro-
genation of methyl 1-naphthyl ketone by all of these catalysts
can be attributed to the lack of C(sp2)-H/p attractive interac-
tions, which are generally observed between an arene ligand
and aromatic ketones in the transition state [37]. Compared to
our previous report [38], catalysts 1–15 appear to provide less
effective chiral environments around the ruthenium, rhodium
or iridium centers. These results demonstrate that the structure
of the ligand backbone is a significant factor in terms of con-
trolling the enantioselectivity. It can be seen that when the
stereogenic center is near the metal center, the enantiomeric
excess is high.
15. Arrayas RG, Adrio J, Carretero JC (2006) Angew Chem Int Ed
45:7674
Conclusions
16. Sutcliffe OB, Bryce MR (2003) Tetrahedron Asymmetry 14:2297
17. Blaser HA, Spindler F (1997) Chimia 51:297
18. Burk MJ, Gross MF (1994) Tetrahedron Lett 35:9363
19. Kang J, Lee JH, Ahn SH, Choi JS (1998) Tetrahedron Lett
39:5523
In conclusion, C2-symmetric ferrocenyl bis(phosphinite)-
Ru(II), Rh(I) and Ir(III) complexes are homogeneous cat-
alysts for the asymmetric transfer hydrogenation of alkyl/
i
20. Sawamura M, Kuwano R, Ito Y (1994) Angew Chem Int Ed Engl
33:111
methyl or alkyl/aryl ketones using PrOH as the primary
hydrogen source. In all cases, high conversions and mod-
erate-to-good enantioselectivities are observed. The activ-
ities of the Ru(II) complexes were higher than those of the
Rh(I) and Ir(III) catalysts. However, similar enantioselec-
tivities were observed in the presence of Rh(I) or Ir(III)
catalysts in hydrogenation of alkyl/methyl or alkyl/aryl
ketones, which were higher than those of Ru(II) catalysts.
The iridium catalysts induced slightly higher enantiose-
lectivities than the rhodium catalysts.
21. Sawamura M, Ito Y (1992) Chem Rev 92:857
22. Kılıc¸ A, Durap F, Aydemir M, Baysal A, Tas¸ E (2008) J Orga-
nomet Chem 693(17):2835
23. Song J-H, Cho D-J, Jeon S-J, Kim Y-H, Kim T-J (1999) Inorg
Chem 38:89
24. Patti A, Pedotti S (2003) Tetrahedron Asymmetry 14:597
25. Galka PW, Kraatz H-B (2003) J Organomet Chem 674:24
˘
26. Aydemir M, Durap F, Baysal A, Meric N, Buldag A, Gu¨mgu¨m B,
¨
Ozkar S, Yıldırım LT (2010) J Mol Catal A Chem 326:75
27. Durap F, Aydemir M, Baysal A, Elma D, Ak B, Turgut Y (2014)
Inorg Chim Acta 411:77
28. Ak B, Aydemir M, Ocak YS, Durap F, Kayan C, Baysal A, Temel
H (2014) Inorg Chim Acta 409:244
29. Ak B, Elma D, Meric N, Kayan C, Is¸ık U, Aydemir M, Durap F,
Baysal A (2015) Tetrahedron Asymmetry 20:1257
30. Ak B, Elma D, Meric¸ N, Kayan C, Is¸ık U, Aydemir M, Durap F,
Baysal A (2013) Tetrahedron Asymmetry 24:1257
31. Alvarez M, Lugan N, Mathieu R (1994) J Chem Soc Dalton Trans
2755. doi:10.1039/DT9940002755
Supplementary data
All additional information pertaining to characterization of
the previously reported compounds using NMR technique
is given in the supporting information.
32. Gao JX, Ikariya T, Noyori R (1996) Organometallics 15:1087
33. Kuroki Y, Asada D, Sakamaki Y, Iseki K (2000) Org Lett
41:4603
34. Ak B, Durap F, Aydemir M, Baysal A (2015) Appl Organomet
Chem 29:764
˙
Acknowledgements Partial supports of this work by TUBITAK
(Project Number: 111T419) and Dicle University (Project Numbers:
¨
FEN.16.010 (NMR), DUBAP FEN.15.021 and FEN.15.022) are
gratefully acknowledged.
35. Ak B, Aydemir M, Durap F, Meric¸ N, Baysal A (2015) Inorg
Chim Acta 438:42
36. Ak B, Aydemir M, Durap F, Meric¸ N, Elma D, Baysal A (2015)
Tetrahedron: Asymmetry 26:1307
References
37. Yamakawa M, Yamada I, Noyori R (2001) Angew Chem Int Ed
40:2818
38. Durap F, Elma Karakas¸ D, Ak B, Baysal A, Aydemir M (2016) J
Organomet Chem 818:92
1. Malacea R, Poli R, Manoury E (2010) Coord Chem Rev 254:729
2. Wang Z, Ding K, Uozumi Y, Ding K, Uozumi Y (eds) (2008)
Handbook of asymmetric heteregeneous catalysis. Wiley-VCH,
Weinheim, p 1
3. Elma D, Durap F, Aydemir M, Baysal A, Meric N, Ak B, Turgut
Y, Gu¨mgu¨m B (2013) J Organomet Chem 729:46
123