Enantioselective transfer hydrogenation of various ketones with novel efficient iridium(III) ferrocenyl-phosphinite catalysts

Article abstract of DOI:10.1016/j.jorganchem.2016.06.002

The asymmetric reduction of prochiral ketones is a pivotal reaction for the preparation of chiral alcohols which form an extremely important class of intermediates for fine chemicals and pharmaceuticals. Especially, iridium-based asymmetric reduction of ketones to enantiomerically enriched alcohols has recently attracted important attention by a number of research groups and interest in this area is growing. Therefore, a series of novel neutral mononuclear iridium(III) ferrocenyl-phosphinite complexes have been prepared and applied in the iridium(III)-catalyzed asymmetric transfer hydrogenation (ATH) of ketones to give corresponding secondary alcohols with outstanding enantioselectivities and reactivities using 2-propanol as the hydrogen source (up to 99% ee and 99% conversion). It was seen that the substituents on the backbone of the ligands resulted in a significant effect on both the activity and % enantioselectivity. Furthermore, the structural elucidation of the complexes was carried out by elemental analysis, IR and multi-nuclear NMR spectroscopic data.

Full text of DOI:10.1016/j.jorganchem.2016.06.002

Journal of Organometallic Chemistry 819 (2016) 120e128
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Enantioselective transfer hydrogenation of various ketones with novel
efficient iridium(III) ferrocenyl-phosphinite catalysts
,
, b
Nermin Meriç a *, Murat Aydemir a
a Department of Chemistry, Faculty of Science, University of Dicle, TR-21280, Diyarbakir, Turkey
b Science and Technology Application and Research Center (DUBTAM), Dicle University, Diyarbakir, TR-21280, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
The asymmetric reduction of prochiral ketones is a pivotal reaction for the preparation of chiral alcohols
which form an extremely important class of intermediates for fine chemicals and pharmaceuticals.
Especially, iridium-based asymmetric reduction of ketones to enantiomerically enriched alcohols has
recently attracted important attention by a number of research groups and interest in this area is
growing. Therefore, a series of novel neutral mononuclear iridium(III) ferrocenyl-phosphinite complexes
have been prepared and applied in the iridium(III)-catalyzed asymmetric transfer hydrogenation (ATH)
of ketones to give corresponding secondary alcohols with outstanding enantioselectivities and re-
activities using 2-propanol as the hydrogen source (up to 99% ee and 99% conversion). It was seen that
the substituents on the backbone of the ligands resulted in a significant effect on both the activity and %
enantioselectivity. Furthermore, the structural elucidation of the complexes was carried out by elemental
analysis, IR and multi-nuclear NMR spectroscopic data.
Received 10 March 2016
Received in revised form
23 May 2016
Accepted 3 June 2016
Available online 29 June 2016
Keywords:
Iridium(III)
Monodendate ferrocenyl-phosphinite
Homogeneous catalysis
Asymmetric transfer hydrogenation
Ketones
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
donating property [17]. Furthermore, their distinctive structure
allows one to design a variety of chiral ferrocenyl phosphine li-
The development of new asymmetric catalytic systems is still a
major challenge because of its importance in synthetic organic
chemistry and manufacturing fine chemicals [1,2]. An increasing
number of chiral compounds and enantiomerically pure drugs are
prepared through transition metal-catalyzed asymmetric reactions
[3e6]. Since the reactivity and stereoselectivity of an asymmetric
transformation are highly dependent on the structure of the chiral
ligand coordinated to the transition metal, the design and synthesis
of efficient chiral ligands are important in this area and have
attracted a great deal of attention from both academia and industry
[7,8]. In particular, asymmetric transfer hydrogenation of prochiral
ketones to provide chiral alcohols has received a great deal of
attention in the last decade or so [9,10]. For that reason, among
several applications such as polymers [11] to bioorganometallic
chemistry [12e14], the use of ferrocene-based chiral ligands in
asymmetric synthesis is the most prominent [15,16]. The ferrocene
moiety has been extensively explored as a backbone of chiral
phosphine ligands due to its easy modifiability and highly electron
gands, which are useful tools in metal-catalyzed asymmetric hy-
drogenation reactions [18,19].
In spite of the encouraging performance of many ferrocene
based bidendate phosphorus-chelate ligands [20e25], the past few
years have witnessed a renewed interest in the development of
chiral monodendate phosphorus ligands for use in asymmetric
hydrogenation reactions [26e29]. This resurgence in monodendate
ligands is due to the ready accessibility of a range of diverse ligand
structures, and often their lower cost compared to bidendate ones
[30]. Following the studies from the groups of Pringle [31], Reetz
[32], Feringa [33] and more recently Chan [34] and Zhao [35] a large
number of chiral monodendate phosphonite, phosphite, and
phosphoramidite ligands have been found to induce good to
excellent enantioselectivities in asymmetric hydrogenation re-
actions, comparable to or exceeding those obtained with bidendate
ligands [36,37]. Furthermore, the most important advantage of
chiral phosphinite ligands over the corresponding phosphine li-
gands is the easiness of preparation, which leads to a substantial
interest to develop highly effective chiral monodendate phos-
phinite ligands for asymmetric catalysis [38e41].
Lately, the synthesis and applications of efficient rhodium-based
catalysts have been reported in the literature [42]. However the
* Corresponding author.
E-mail address: nmeric@dicle.edu.tr (N. Meriç).
http://dx.doi.org/10.1016/j.jorganchem.2016.06.002
0022-328X/© 2016 Elsevier B.V. All rights reserved.

N. Meriç, M. Aydemir / Journal of Organometallic Chemistry 819 (2016) 120e128
121
interest in iridium catalysts [43], which have been successfully used
for the asymmetric transfer hydrogenation, is rapidly growing in
recent years. Iridium has the advantage to be much less expensive
than rhodium [44]. Since the pioneering work of Mestroni et al. on
the use of iridium complexes in the transfer hydrogenation of ke-
tones [45,], numerous iridium-based catalytic systems have been
studied [46]. The different catalytic systems will be presented ac-
cording to the coordination pattern of the ligands involved. One of
the first reports on the iridium-based ATH of ketones was by Gra-
zani and co-workers [47]. Dichlorobis(1,4-cyclooctadiene)diiridium
was used as a precatalyst in the presence of the chiral phosphines
for the asymmetric transfer reduction of ketones. In addition,
contemporarily, Bakos et al. found that in-situ prepared iridium
complexes of phosphinites could also catalyze the ATH of aromatic
ketones [48,49]. Comparable studies were carried out by numerous
P-based ligands by many researchers. Even though ferrocenyl-
phosphine ligands have found widespread applications in transi-
tion metal catalyzed asymmetric transformations [50e52], the
analogous phosphinites provide different chemical, electronic and
structural advantages compared to phosphines. For instance, the
metal-phosphorus bond is often stronger in phosphinites
compared to the related phosphines due to the presence of
values previously observed for similar compounds [72e75]. The
structures for these ferrocene based chiral amino alcohols are
consistent with the data obtained from a combination of multi-
nuclear NMR spectroscopy, IR spectroscopy and elemental analysis.
The whole reactions of [Ir(
phosphinite ligands, 17e24 are shown in Scheme 1. Treatment of
[Ir( -Cl)Cl]2 with ferrocenyl-phosphinites 9e16 in ½:1
5-C5Me5)(
h m-Cl)Cl]2 with ferrocenyl-
5-C5Me5)(
h
m
molar ratio in CH2Cl2 resulted the formation of mononuclear
complexes 17e24 as crystalline solids. All neutral iridium(III)
complexes were readily synthesized in good yields, which are air
stable and orange microcrystalline powders. The ferrocenyl-
phosphinite ligands were expected to cleave the [Ir(h m-
5-C5Me5)(
Cl)Cl]2 dimer to give the corresponding complexes, 17e24 via
monohapto coordination of the phosphinite group. All complexes
were isolated as indicated by singlets in the 31P-{1H} NMR spectra
at approximately
d 74 ppm, with a coordination shift of approxi-
mately 50 ppm (see supporting information, SI) attributed to
d
formation of the corresponding iridium(III) ferrocenyl-phosphinite
complexes. The assignment of the 1H chemical shifts was derived
from 2D HH-COSY spectra and the appropriate assignment of the
13C chemical shifts from DEPT and 2D HMQC spectra. Furthermore,
elemental analyses of the complexes are also consistent with the
suggested molecular formulas. The absorption bands correspond-
ing to ferrocenyl-phosphinite ligands in Ir(II) complexes in the IR
spectra of complexes do not exhibit significant differences with
respect to those of free ligands (see Experimental section).
electron-withdrawing P-OR group. In addition, the empty
s*-
orbital of the phosphinite P(OR)R2 is stabilized, making the phos-
phinite a better acceptor [53].
We have recently shown that chiral monodendate ferrocenyl-
phosphinites ligands, which contain sterically and electronically
different ligating fragments, are able to ensure high enantiose-
lectivities in a variety of transition-metals [54]. Furthermore,
considering the advantage of phosphinites, in recent years our
research group has reported the synthesis [55e57], characteriza-
tion and coordination properties of this kind of ligands [58e60].
With an aim to design the efficient catalysts for asymmetric
transfer hydrogenation (ATH) of ketones, herein, we describe the
synthesis and characterization of novel neutral iridium(III)
ferrocenyl-phosphinite complexes. As far as we know, there are not
so many reports on asymmetric transfer hydrogenation of ketones
by using this kind of Iridium-complexes as catalyst. Thus, these
iridium complexes have been employed successfully as catalysts in
the asymmetric transfer hydrogenation of various ketones.
2.2. Asymmetric transfer hydrogenation of prochiral ketones with
2-propanol
The usage of
p-arene metal complexes as catalysts for asym-
metric transfer hydrogenation from an appropriate donor (usually
2-propanol or formic acid) has been a subject of ongoing research
for some decades. It is well-known that hydrogen gas presents
considerable safety hazards especially for large scale reactions
[76,77]. The use of a solvent that can donate hydrogen overcomes
these difficulties. 2-propanol is a popular reactive solvent for
transfer hydrogenation reactions since it is easy to handle and
relatively non-toxic, environmentally benign, and inexpensive. The
volatile acetone by-product can also be easily removed to shift
unfavourable equilibrium.
2. Results and discussion
It was found that ferrocenyl-phosphinites demonstrate
outstanding air stability and their design does not need any
extreme conditions, and thus their preparation is rather easy [78].
Thus, this kind of ligands combines a number of characteristics
making uniquely attractive for asymmetric catalysis [79e83].
Encouraged by our recent success in the development of new chiral
and highly active catalysts [84,85, and references therein], we
initiated a study of the synthesis of a series of iridium(III)
ferrocenyl-phosphinite complexes in the asymmetric transfer
hydrogenations.
Firstly, complexes 17e24 were evaluated as precursors for the
catalytic asymmetric transfer hydrogenation of acetophenone by 2-
propanol and the results were summarized in Table 1. Catalytic
experiments were carried out under argon atmosphere using
standard Schlenk-line techniques. To an 2-propanol solution of
Ir(III) ferrocenyl-phosphinite complex, an appropriate amount of
acetophenone and KOH/2-propanol solutions were added, at room
temperature. The solution was stirred, and then examined with
capillary GC analysis. At room temperature, transfer hydrogenation
of acetophenone occurred very slowly [86], with low conversion
(up to 20%, 24 h) and moderate to high enantioselectivity (up to 92%
ee) (Table 1, entries 1e8). As a result of the reversibility at room
temperature prolonging the reaction time (72 h) led to a decreasing
of enantioselectivity, as indicated by the catalytic results collected
2.1. Synthesis and characterization of the iridium(IIII) ferrocenyl-
phosphinites complexes
We have had an ongoing interest in the synthesis and use of
optically active ligands in asymmetric catalysis, especially
ferrocenyl-phosphinites. The synthesis of
phenylalaninol, -, -valinol, -leucinol and
accomplished in one step from -, -phenylglycine,
anine, -, -valine, -leucine or -isoleucine, respectively, according
D
-,
L
-phenylglycinol,
-isoleucinol were
-, -phenylal-
D-, L-
D
L
L
L
D
L
D
L
D
L
L
L
to the procedures described in the literature [61,62]. The ferrocene
based amino alcohols were synthesized by the condensation re-
action between ferrocenecarboxaldehyde [63] and amino alcohols
in the presence of the base catalyst [64]. The synthetic process for
the preparation of the ferrocenyl-phosphinites [65,66] is shown in
Scheme 1. Ferrocenyl-phosphinite ligands, 9e16 were prepared by
a hydrogen abstraction from the described ferrocene based chiral
amino alcohols 1e8, by a base (Et3N) and the subsequent reaction
with one equivalents of Ph2PCl, in anhydrous toluene under inert
argon atmosphere (Scheme 1). The progress of this reaction was
conveniently followed by 31P-{1H} NMR spectroscopy. The 31P-{1H}
NMR spectra of compounds, 9e16 show single resonances due to
phosphinite at approximately
d 115 ppm [67e71] in line with the

122
N. Meriç, M. Aydemir / Journal of Organometallic Chemistry 819 (2016) 120e128
Scheme 1. Reagents and conditions (i) 1 equiv. Ph2PCl, 1 equiv. Et3N, toluene for 9e16 (ii)1/2 equiv. Ir(h m-Cl)Cl]2, CH2Cl2 for 17e24.
5-C5Me5)(
with catalysts, 17e24 (Entries 1e4, [c]) [87,88]. Furthermore, as can
be inferred from the Table 1 (Entries 9e16) the presence of base is
necessary to observe appreciable conversions. It is well-known that
the base facilitates the formation of ruthenium alkoxide by
abstracting proton of the alcohol and subsequently alkoxide un-
dergoes
b-elimination to give ruthenium hydride, which is an
Table 1
Transfer hydrogenation of acetophenone with 2-propanol catalyzed by iridium(III) ferrocenyl-phosphinite complexes (17e24).
g
Entry
Complex
S/C/KOH
Time
Conv. (%)d
% eee
Conf.f
TOF (hꢀ1
)
1
2
3
4
5
6
7
8
17a
18a
19a
20a
21a
22a
23a
24a
17b
18b
19b
20b
21b
22b
23b
24b
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1
100:1
100:1
100:1
100:1
24 h (72 h)c
24 h (72 h)c
24 h (72 h)c
24 h (72 h)c
24 h
24 h
24 h
24 h
1 h
1 h
1 h
1 h
1 h
1 h
1 h
1 h
17 (52)c
20 (54)c
14 (38)c
18 (39)c
12
13
11
12
<5
<5
<5
<5
<5
<5
<5
<5
86 (82)c
R
S
R
S
R
S
S
<3
<3
<3
<3
<3
<3
<3
<3
e
92 (89)c
78 (74)c
83 (80)c
70
72
63
66
e
S
9
e
e
e
e
e
e
e
e
10
11
12
13
14
15
16
e
e
e
e
e
e
e
e
100:1
100:1
100:1
e
e
e
e
e
e
Reaction conditions.
a
At room temperature; acetophenone/Ir(III)/KOH, 100:1:5.
b
c
d
e
f
Refluxing in 2-propanol; acetophenone/Ir(III), 100:1, in the absence of base.
At room temperature; acetophenone/Ir(III)-complex/KOH, 100:1:5, (72 h).
Determined by GC (three independent catalytic experiments).
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column.
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values, (S) or (R) configuration was obtained in all experiments.
g
TOF ¼ (mol product/mol Cat.) ꢁ hꢀ1
.

N. Meriç, M. Aydemir / Journal of Organometallic Chemistry 819 (2016) 120e128
123
active species in this reaction. This is the mechanism proposed by
several research groups on the studies of ruthenium catalyzed
transfer hydrogenation reaction by metal hydride intermediates
[89e92]. Specifically, role of the base is to generate a more nucle-
ophilic alkoxide ion which would rapidly attack the ruthenium
complex responsible for dehydrogenation of 2-propanol.
range of acetophenone derivatives can be hydrogenated with good
enantioselectivities. It has also been shown that the catalytic ac-
tivities in the studied hydrogen transfer reactions were
generally much higher for [(2S)-2-(ferrocenylmethylamino)-2-
phenylethyldiphenylphosphinito(dichloro(ɳ5-
pentamethylcyclopentadienyl)iridium(III))] (18) than for the com-
plex, 17.
As can be inferred from Table 2, reduction of acetophenone into
(S)- or (R)-1-phenylethanol could be achieved in high yield by
increasing the temperature to 82 ꢂC (Entries 1e8). Furthermore,
optimization studies of the catalytic reduction of acetophenone in
2-propanol showed that good activity was obtained with a base/
ligand ratio of 5:1 (Table 2, entries 9e16). In addition, the choice of
base, such as KOH and NaOH, had little influence on the conversion
and enantioselectivity (Table 2, entries 1e4,[b]). It is well-known
that the NH functional moiety in ligand plays an important role
in catalytic system and similar tendency was reported in earlier
studies [93e95]. These ferrocenyl based monodendate phosphinite
ligands with amino (NH) moiety show much higher activity and
enantioselectivity. Higher activity and enantioselectivity of amino
containing phosphinite ligand also may be due to the fact that NH
moiety can stabilize the catalytic transition state [96]. As expected,
it is noteworthy that the catalytic systems, 17e24 display the dif-
ferences in reactivity. These results indicate that the structure of
the monodenate ferrocenyl-phosphinite ligands have a crucial in-
fluence on rate of the reaction. Compared to the other complexes,
[(2S)-2-(ferrocenylmethylamino)-2-phenylethyldiphenyl phosphi-
nito(dichloro(ɳ5-pentamethylcyclopentadienyl)iridium(III))] (18)
appears to provide a more effective chiral environment around
iridium. In the context of these results, it could be reasonably
argued that the absolute configuration of the product is governed
by the carbon centered chirality.
The steric bulk of the R group in the ferrocenyl-ligand controls
both activitity and enantioselectivity of the catalysts. That’s to say,
this different behavior in enantioselectivities can be clarified on the
basis of aromatic moiety (phenyl) near chiral carbon center in the
ligand backbone responsible for the optimization molecular rigid-
ity. These results have also shown that enantiomeric purity of the
product can be affected by electronic and steric factors of the
substituents on the ligand. The examination of the results indicates
clearly that with each of the tested complexes, the highest enan-
tioselectivity was found for transfer hydrogenation of o-methox-
yacetophenone (99% ee) when [(2S)-2-(ferrocenylmethylamino)-2-
phenylethyldiphenylphosphinito(dichloro(ɳ5-pentam-
ethylcyclopentadienyl)iridium(III))] (18) was used as the catalyst
precursor. As already stated, electronic properties (the nature and
position) of the substituents on the phenyl ring of the ketone
caused the changes in the reduction rate. Therefore, the introduc-
tion of electron withdrawing substituents to the para position of
the aryl ring of the ketone decreased the electron density of the C]
O bond so that the activity was improved giving rise to easier hy-
drogenation [97,98]. The introduction of electron-withdrawing
substituents, such as F or NO2, to the para positions of the aryl
ring of the ketone, resulted in improved activity with good enan-
tioselectivity (Table 3, entries 1e8). As expected, the lowest enan-
tioselectivity was observed in transfer hydrogenation of p-
methoxyacetophenone. From the results, the introduction of an
electron-donating group such as methoxy group to the p-position
decelerates the reaction, but that to the o-position increases the
rate and improves the enantioselectivity.
As seen from Table 2, the catalytic activities in the studied
hydrogen transfer reactions were generally much higher for the
17e18 than those for the other complexes. So, these two complexes
were extensively investigated with acetophenone derivatives. The
catalytic reduction of acetophenone derivatives was tested with the
conditions optimized for acetophenone and the results are sum-
marized in Table 3 which illustrates conversions of the reduction
performed in a 0.1 M of 2-propanol solution containing 17e18 and
KOH (Ketone:Cat.:KOH ¼ 100:1:5). The results demonstrate that a
Table 4 summarizes the asymmetric transfer hydrogenation of
various aryl ketones in 2-propanol at 82 ꢂC. Other functionalized
ketones could also be hydrogenated using complexes 17 and 18 as
catalysts to afford the products. It was found that catalytic activity
and enantioselectivity are sensitive to the structure of ketones. In
Table 2
Transfer hydrogenation of acetophenone with 2-propanol catalyzed by Ir(III) ferrocenyl based monodendate phosphinite complexes (17e24).
f
Entry
Catalyst
S/C/KOH
Time
Conv. (%)c
% eed
Conf.e
TOF (hꢀ1
)
1
2
3
4
5
6
7
8
17a
18a
19a
20a
21a
22a
23a
24a
17
17
17
17
18
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:3
100:1:5
100:1:7
100:1:9
100:1:3
100:1:5
100:1:7
100:1:9
1/3 h (1/3 h)b
1/3 h (1/3 h)b
1/2 h (1/2 h)b
1/2 h (1/2 h)b
1/2 h
1/2 h
1/2 h
1/2 h
1/3 h
1/3 h
1/3 h
1/3 h
1/3 h
1/3 h
1/3 h
1/3 h
98 (94)
99 (95)
99 (96)
97 (93)
97
98
97
97
95
98
92
91
93
99
94
91
92 (87)
98 (92)
83 (78)
88 (76)
75
76
69
72
85
92
88
84
91
98
90
89
R
S
R
S
R
S
S
S
R
R
R
R
S
S
S
S
294 (282)
297 (285)
198 (192)
194 (186)
194
196
194
194
285
294
276
273
279
297
282
273
9
10
11
12
13
14
15
16
18
18
18
Reaction conditions.
a
Refluxing in 2-propanol; acetophenone/Ir(III)-complex/KOH, 100:1:5.
Refluxing in 2-propanol; acetophenone/Ir(III)-complex/NaOH, 100:1:5.
Determined by GC (three independent catalytic experiments).
b
c
d
e
f
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column.
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values, (S) or (R) configuration was obtained in all experiments.
TOF ¼ (mol product/mol Cat.) ꢁ hꢀ1
.

124
N. Meriç, M. Aydemir / Journal of Organometallic Chemistry 819 (2016) 120e128
Table 3
Asymmetric Transfer Hydrogenation results for substituted acetophenones catalyzed by iridium(III)-ferrocenyl based monodendate phosphinite complexes, (17e18).a
d
Entry
Catalyst
Substrate
4-F
Time
Conv. (%)b
% eec
TOF (hꢀ1
)
Config.e
1
2
3
4
5
6
7
8
17
18
17
18
17
18
17
18
17
18
17
18
1/4 h
1/4 h
1/3 h
1/3 h
1/3 h
1/3 h
1/4 h
1/4 h
1/2 h
1/2 h
1 h
99
98
98
97
99
98
99
98
98
97
99
98
89
95
86
92
83
91
88
96
95
99
77
82
396
392
294
291
297
294
396
392
196
194
99
R
S
R
S
R
S
R
S
R
S
R
S
4-Cl
4-Br
4-NO2
2-MeO
4-MeO
9
10
11
12
1 h
98
a
Catalyst (0.005 mmol), substrate (0.5 mmol), 2-propanol (5 mL), KOH (0.025 mmol %), 82 ꢂC, the concentration of acetophenone derivatives are 0.1 M.
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), yields are based on aryl ketone.
b
c
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column (30 m ꢁ 0.32 mm I.D. ꢁ 0.25
mm film thickness).
d
e
TOF ¼ (mol product/mol Cat.) ꢁ hꢀ1
.
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values.
that case, high conversion yields (97e99%) were obtained for 17
and 18 and the highest enantioselectivity, up to 99% ee, was ob-
tained for 18. As seen from Table 4, the best result in terms of
enantioselectivity was observed with 1-naphtyl methyl ketone (up
to 99% ee, entries 9e10). The reactivity and enantioselectivity were
also dependent on the bulkiness of the alkyl group (Table 4, Entries
1e8). For example, the reaction was remarkably slowed by
increasing the bulkiness of the alkyl group, whereas the enantio-
selectivity only slightly changed. Furthermore, the hydrogenation
of ketones including cyclohexyl group was very slow and the
enantioselectivities were remarkable lower (Table 4, Entries
13e16).
3. Conclusions and perspectives
In summary, we designed a series of ferrocenyl-phosphinite li-
gands and applied them to the Ir(III)-catalyzed asymmetric transfer
hydrogenation of various ketones. A wide range of ketones could be
Table 4
Asymmetric Transfer Hydrogenation results for various ketones catalyzed by Iridium(III)-ferrocenyl based monodendate phosphinite complexes, (17e18).a
d
Entry
Cat.
R1
R2
Time
Conv. (%)b
Ee (%)c
TOF (hꢀ1
)
Conf.e
1
2
3
4
5
6
7
8
17
18
17
18
17
18
17
18
17
18
17
18
17
18
17
18
CH3
CH3
CH3
CH2CH3
C2H5
CH2CH2C6H5
1/2 h
1/2 h
1 h
1 h
2 h
98
99
99
98
99
98
98
97
99
99
98
99
98
99
98
99
86
93
82
88
78
85
77
83
93
99
76
84
65
73
71
78
196
198
99
98
50
49
39
39
297
297
33
33
49
50
25
25
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
CH3
CH3
CH3
CH3
CH3
C6H5
CH(CH3)2
CH2CH(CH3)2
1-naphthyl
n-C4H9
2 h
5/2 h
5/2 h
1/3 h
1/3 h
3 h
3 h
2 h
2 h
4 h
9
10
11
12
13
14
15
16
C6H11
C6H11
4 h
a
Catalyst (0.005 mmol), substrate (0.5 mmol), 2-propanol (5 mL), KOH (0.025 mmol %), 82 ꢂC, the concentration of acetophenone derivatives are 0.1 M.
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), yields are based on aryl ketone.
b
c
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column (30 m ꢁ 0.32 mm I.D. ꢁ 0.25
mm film thickness).
d
e
TOF ¼ (mol product/mol Cat.) ꢁ hꢀ1
.
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values.

N. Meriç, M. Aydemir / Journal of Organometallic Chemistry 819 (2016) 120e128
125
hydrogenated enantioselectively to afford the corresponding opti-
cally active alcohols in high isolated yields and with good to
excellent enantioselectivity. The chirality of the carbon center in
the ligand backbone is of particular importance for asymmetric
induction offering an obvious target for further optimization. The
simplicity and efficiency clearly make it an excellent choice of
catalyst for the practical preparation of highly valued alcohols via
the catalytic asymmetric transfer hydrogenation of ketones.
for 30 min at room temperature. The volume was concentrated to
ca. 1e2 mL under reduced pressure and addition of n-hexane
(20 mL) gave the corresponding complex as an orange microcrys-
talline solid. The product was collected by filtration and dried in
vacuo.
4.2.2. [(2R)-2-(ferrocenylmethylamino)-2-
phenylethyldiphenylphosphinito(dichloro(ɳ5-
Furthermore, the development of
a
practical synthesis of
pentamethylcyclopentadienyl)iridium(III))] (17)
20
ferrocenyl-phosphinites and demonstration that they are compe-
tent auxiliaries for catalysis opens up a neglected vein in the rich
chemistry of phosphorus ligands. Further modification of the aryl/
alkyl group on ligand backbone and phosphorus atom is currently
underway.
Yield: 234 mg, 84.9%; mp:131e133 ꢂC; [
a
]
¼ ꢀ25.4ꢂ(c 1,
D
CH2Cl2). 1H NMR (CDCl3, ppm):
d
1.38 (d,15H, 4J ¼ 2.4 Hz, CH3 of Cp*
(C5Me5)), 3.23 (d, J ¼ 13.0 Hz, 1H, CH2NH, (a)), 3.43 (d, J ¼ 13.1 Hz,
1H, CH2NH, (b)), 3.79 (br, 1H, CHNH), 3.90e3.95 (m, 2H, CH2OP),
4.09 (s, 5H, C5H5), 4.12 (s, 3H, C5H4), 4.18 (s, 1H, C5H4), 7.35e7.38 (m,
6H, m- and p-protons of phenyls þ5H, C6H5), 7.92 (m, 4H, o-protons
4. Experimental section
of phenyls); 13C NMR (CDCl3, ppm):
d 8.21 (CH3 of Cp*(C5Me5)),
46.26 (CH2NH), 62.14 (d, 3J ¼ 7.0 Hz, CHNH), 67.65, 67.90, 68.04,
68.40, 68.75, 70.64 (C5H4þC5H5þCH2OP), 86.06 (i-C5H4), 94.11 (d,
2J ¼ 3.0 Hz, C5Me5), 127.75, 127.85, 127.94, (C6H5), 128.47 (s, carbons
of phenyls), 130.95 (d, J ¼ 6.7 Hz, m-carbons of phenyls), 133.10 (d,
J ¼ 11.5 Hz, o-carbons of phenyls), 135.75 (d, J ¼ 60.3 Hz, i-carbons
4.1. Materials and methods
Unless otherwise mentioned, all reactions were carried out
under an atmosphere of argon using conventional Schlenk glass-
ware, solvents were dried using established procedures and
distilled under argon immediately prior to use. Analytical grade and
deuterated solvents were purchased from Merck. The starting
of phenyls), 139.78 (i-C6H5); 31P-{1H} NMR (CDCl3, ppm):
d
74.08 (s,
O-P(Ph)2); IR (KBr pellet in cm¡1
) y: (C-Cp): 3054, (C]C-Cp):
1451, (P-Ph): 1436, (OeP): 1023; Anal. Calcd for [C41H45NOP-
FeIrCl2] (917.76 g/mol): C, 53.66; N,1.53; H, 4.94; Found: C, 53.29; N,
1.41; H, 4.49.
materials
leucine, -isoleucine, PPh2Cl and Et3N were purchased from Fluka
and used as received. Ferrocenecarboxaldehyde, [68] and [Ir(h5
C5Me5)( -Cl)Cl]2 [99] were prepared according to the literature
D-, L-phenylglycine, D-, L-phenylalanine, D-, L-valine, L-
L
-
m
4.2.3. [(2S)-2-(ferrocenylmethylamino)-2-
procedures. 1H (at 400.1 MHz), 13C (at 100.6 MHz) and 31P-{1H}
NMR (at 162.0 MHz) spectra were recorded on a Bruker AV400
spectrometer, with TMS (tetramethylsilane) as an internal refer-
ence for 1H NMR and 13C NMR or 85% H3PO4 as external reference
for 31P-{1H} NMR. The IR spectra were recorded on a Mattson 1000
ATI UNICAM FT-IR spectrometer. Specific rotations were taken on a
Perkin-Elmer 341 model polarimeter. Elemental analysis was car-
ried out on a Fisons EA 1108 CHNS-O instrument. Melting points
were recorded by a Gallenkamp Model apparatus with open
capillaries.
phenylethyldiphenylphosphinito(dichloro(ɳ5-
pentamethylcyclopentadienyl)iridium(III))] (18)
20
Yield: 237 mg, 86.0%; mp:131e133 ꢂC; [
a
]
¼ þ25.9ꢂ (c 1,
D
CH2Cl2). 1H NMR (CDCl3, ppm):
d
1.38 (d,15H, 4J ¼ 2.0 Hz, CH3 of Cp*
(C5Me5)), 3.25 (d, J ¼ 12.6 Hz, 1H, CH2NH, (a)), 3.46 (br, 1H, CH2NH,
(b)), 3.87 (br, 1H, CHNH), 3.95 (br, 2H, CH2OP), 4.09 (s, 5H, C5H5),
4.13 (s, 3H, C5H4), 4.23 (s, 1H, C5H4), 7.31e7.39 (m, 6H, m- and p-
protons of phenyls þ5H, C6H5), 7.90 (d, J ¼ 7.5 Hz, 4H, o-protons of
phenyls); 13C NMR (CDCl3, ppm):
d 8.26 (CH3 of Cp* (C5Me5)), 46.17
(CH2NH), 62.06 (d, 3J ¼ 8.0 Hz, CHNH), 67.82, 68.10, 68.12, 68.17,
68.45, 68.62 (C5H4þC5H5þCH2OP), 86.61 (i-C5H4), 94.14 (d,
2J ¼ 3.0 Hz, C5Me5), 127.72, 127.82, 127.93, (C6H5), 128.57 (s, carbons
of phenyls), 131.06 (d, J ¼ 15.1 Hz, m-carbons of phenyls), 132.95 (br,
o-carbons of phenyls), 135.70 (d, J ¼ 61.4 Hz, i-carbons of phenyls),
GC analyses were performed on a Shimadzu GC 2010 Plus Gas
Chromatograph equipped with cyclodex B (Agilent) capillary col-
umn (30 m ꢁ 0.32 mm I.D. ꢁ 0.25
mm film thickness). Racemic
samples of alcohols were obtained by reduction of the corre-
sponding ketones with NaBH4 and used as the authentic samples
for ee determination. The GC parameters for asymmetric transfer
hydrogenation of ketones were as follows; initial temperature,
50 ꢂC; initial time 1.1 min; solvent delay, 4.48 min; temperature
ramp 1.3 ꢂC/min; final temperature, 150 ꢂC; initial time 2.2 min;
temperature ramp 2.15 ꢂC/min; final temperature, 250 ꢂC; initial
time 3.3 min; final time, 44.33 min; injector port temperature,
141.54 (i-C6H5); 31P-{1H} NMR (CDCl3, ppm):
d
74.24 (s, O-P(Ph)2);
IR (KBr pellet in cm¡1
) y: (C-Cp): 3058, (C]C-Cp): 1451, (P-Ph):
1436, (OeP): 1023; Anal. Calcd for [C41H45NOPFeIrCl2] (917.76 g/
mol): C, 53.66; N, 1.53; H, 4.94; Found: C, 53.23; N, 1.40; H, 4.45.
4.2.4. [(2R)-2-(ferrocenylmethylamino)-3-
phenylpropyldiphenylphosphinito(dichloro(ɳ5-
200 ꢂC; detector temperature, 200 ꢂC, injection volume, 2.0
mL.
pentamethylcyclopentadienyl)iridium(III))] (19)
20
Yield: 238 mg, 85.3%; mp:125e126 ꢂC); [
a
]
¼ þ19.6ꢂ (c 1,
D
4.2. General procedure for the transfer hydrogenation of ketones
CH2Cl2). 1H NMR (CDCl3, ppm):
d
1.38 (d,15H, 4J ¼ 2.1 Hz, CH3 of Cp*
(C5Me5)), 2.82 (br, 1H, CH2C6H5 (a)), 2.86 (m, 1H, CH2C6H5 (b)), 3.03
(br, 1H, CHNH), 3.38 (br, 1H, CH2NH, (a)), 3.46 (m, 1H, CH2NH, (b)),
3.83 (br, 2H, CH2OP), 4.01 (s, 5H, C5H5), 4.07 (s, 3H, C5H4), 4.12 (s,1H,
C5H4), 7.12e7.40 (m, 6H, m- and p-protons of phenyls þ5H,
CH2C6H5) 7.95e8.02 (m, 4H, o-protons of phenyls); 13C NMR (CDCl3,
Typical procedure for the catalytic hydrogen transfer reaction: a
solution of iridium complexes 17e24 (0.005 mmol), KOH
(0.025 mmol) and the corresponding ketone (0.5 mmol) in
degassed 2-propanol (5 mL) was refluxed until the reaction
completed. Then, a sample of the reaction mixture is taken off,
diluted with acetone and analyzed immediately by GC, conversions
obtained are related to the residual unreacted ketone.
ppm):
d 8.22 (CH3 of Cp* (C5Me5)), 37.97 (CH2Ph), 46.51 (CH2NH),
58.66 (d, 3J ¼ 7.0 Hz, CHNH), 67.84, 68.40, 68.63, 68.80, 69.07, 69.66
(C5H4þC5H5 þCH2OP), 86.51 (i-C5H4), 94.10 (d, 2J ¼ 3.0 Hz, C5Me5),
126.51, 128.60, 129.31, (CH2C6H5), 127.81 (d, J ¼ 11.1 Hz, m-carbons
of phenyls), 132.61 (d, J ¼ 11.1 Hz, p-carbons of phenyls), 133.69 (d,
J ¼ 11.1, o-carbons of phenyls), 136.17 (br, i-carbons of phenyls),
4.2.1. General procedures for synthesis of ferrocene based
iridium(III) ferronenyl-phosphinites complexes (17e24)
[Ir(h
5-C5Me5)(
m-Cl)Cl]2 (0.15 mmol) and ferrocene based phos-
138.16 (i-CH2C6H5); 31P-{1H} NMR (CDCl3, ppm):
d
73.72 (s, O-
phinite (0.30 mmol) were dissolved in 20 mL of CH2Cl2 and stirred
P(Ph)2); IR (KBr pellet in cm¡1
) y: (C-Cp): 3058, (C]C-Cp): 1451,

126
N. Meriç, M. Aydemir / Journal of Organometallic Chemistry 819 (2016) 120e128
(P-Ph): 1436, (OeP): 1023; Anal. Calcd for [C42H47NOPFeIrCl2]
(931.78 g/mol): C, 54.14; N, 1.50; H, 5.08; Found: C, 54.00; N,1.38; H,
5.00.
4.2.8. [(2S)-2-(ferrocenylmethylamino)-4-
methylpentyldiphenylphosphinito(dichloro(ɳ5-
pentamethylcyclopentadienyl)iridium(III))] (23)
(Yield: 229 mg, 85.1%; mp: 140e141 ꢂC); [
20
a
]
¼ þ37.5ꢂ (c 0.5,
D
CH2Cl2). 1H NMR (400.1 MHz, CDCl3, ppm)
d: 0.87 (d, J ¼ 4.8 Hz, 6H,
4.2.5. [(2S)-2-(ferrocenylmethylamino)-3-
CH(CH3)2), 1.27e1.36 (m, 15H,CH3 of Cp* (C5Me5)þ2H,CH2CHþ1H,
CH(CH3)2), 2.89 (br, 1H, CHNH), 3.67 (br, 2H, CH2NH), 3.78 (br, 2H,
CH2OP), 4.07e4.31 (m, 5H, C5H5þ4H, C5H4), 7.41e7.44 (m, 6H, m-
and p-C6H5P), 7.87e7.96 (m, 4H, o-C6H5P); 13C NMR (100.6 MHz,
phenylpropyldiphenylphosphinito(dichloro(ɳ5-
pentamethylcyclopentadienyl)iridium(III))] (20)
20
Yield: 229 mg, 81.9%; mp:129e130 ꢂC; [
a]
¼ ꢀ19.2ꢂ (c 1,
D
CH2Cl2). 1H NMR (CDCl3, ppm):
d
1.38 (d,15H, 4J ¼ 1.3 Hz, CH3 of Cp*
CDCl3, ppm) d: 8.32 (CH3 of Cp* (C5Me5)), 22.19, 22.98 (CH(CH3)2),
(C5Me5)), 2.77e2.82 (br, 2H CH2C6H5), 3.05 (br, 1H, CHNH), 3.48 (br,
2H, CH2NH), 3.84 (br, 2H, CH2OP), 4.02 (s, 5H, C5H5), 4.08 (s, 3H,
C5H4), 4.14 (s, 1H, C5H4), 7.11e7.40 (m, 6H, m- and p-protons of
phenyls þ5H, CH2C6H5) 7.93e8.01 (m, 4H, o-protons of phenyls);
24.57 (CH(CH3)2), 39.31 (CH2CH), 45.88 (CH2NH), 55.08 (d,
J ¼ 6.0 Hz, CHNH), 65.48, 68.64, 68.85, 69.06, 69.32, 69.73
(C5H5þC5H4þ CH2OP), 84.62 (i-C5H4), 94.16 (d, J ¼ 3.0 Hz, C5Me5),
127.89 (t, J ¼ 9.1 Hz, m-C6H5P), 130.83 (s, p-C6H5P), 133.67 (d,
J ¼ 12.1 Hz, o-C6H5P); 135.74 (d, J ¼ 17.1 Hz, i-C6H5P); 31Pe{1H}
13C NMR (CDCl3, ppm):
d 8.23 (CH3 of Cp* (C5Me5)), 38.06 (CH2Ph),
46.53 (CH2NH), 58.65 (d, 3J ¼ 6.0 Hz, CHNH), 67.82, (2J ¼ 7.0 Hz,
CH2OP), 67.91, 68.40, 68.62 68.79, 69.66 (C5H5þC5H4), 86.65 (i-
C5H4), 94.10 (d, 2J ¼ 2.0 Hz, C5Me5), 126.51, 128.60, 129.31,
(CH2C6H5), 127.80 (d, J ¼ 11.1 Hz, m-carbons of phenyls), 132.63 (d,
J ¼ 11.1 Hz, p-carbons of phenyls), 133.68 (d, J ¼ 12.1, o-carbons of
phenyls), 136.17 (br, i-carbons of phenyls), 138.17 (i-CH2C6H5); 31P-
NMR (162.0 MHz, CDCl3, ppm)
d: 72.13 (s, O-P(Ph)2); IR (KBr pellet
in cm¡1
) y: (C-Cp): 3057, (P-Ph): 1436, (C C-Cp): 1449, (O P): 1026;
Anal. Calc. for [C39H49NOPFeIrCl2] (897.77 g/mol): C 52.18, N 1.56, H
5.50; found: C 52.00, N 1.26, H 5.24%.
4.2.9. [(2S-3S)-2-(ferrocenylmethylamino)-3-
{1H} NMR (CDCl3, ppm):
d 73.85 (s, O-P(Ph)2; IR (KBr pellet in
methylpentyldiphenylphosphinito(dichloro(ɳ5-
cm¡1
)
y: (C-Cp): 3054, (C]C-Cp): 1449, (P-Ph): 1436, (OeP): 1023;
pentamethylcyclopentadienyl)iridium(III))], (24)
Anal. Calcd. for [C42H47NOPFeIrCl2] (931.78 g/mol): C, 54.14; N,
1.50; H, 5.08; Found: C, 53.98; N, 1.37; H, 4.99.
20
(Yield: 234 mg, 86.8%; mp: 131e132 ꢂC); [
a
]
¼ þ34.0ꢂ (c 0.5,
D
CH2Cl2). 1H NMR (400.1 MHz, CDCl3, ppm)
d 0.69 (br, 3H, CH2CH3),
0.83 (d, J ¼ 6.6, 3H,CH(CH3)), 1.01 (m, 2H, CHCH2(CH3)), 1.14 (m, 1H,
CHCH2(CH3)), 1.43 (s, 15H, CH3 of Cp* (C5Me5)), 2.79 (br, 1H, CHNH),
4.84 (br, 2H, CH2NHþ2H,CH2OP), 4.17e4.33 (m, 5H, C5H5þ4H,
C5H4), 7.31e7.45 (m, 6H, m- and p-C6H5P), 7.86e7.97 (m, 4H, o-
4.2.6. [(2R)-2-(ferrocenylmethylamino)-3-
methylbutyldiphenylphosphinito(dichloro(ɳ5-
pentamethylcyclopentadienyl)iridium(III))] (21)
C6H5P); 13C NMR (100.6 MHz, CDCl3, ppm)
d: 8.33 (CH3 of Cp*
20
Yield: 230 mg, 86.8%; mp: 138e139 ꢂC); [
a]
¼ ꢀ29.5ꢂ (c 0.5,
D
CH2Cl2). 1H NMR (400.1 MHz, CDCl3, ppm)
d: 0.81 (d, J ¼ 6.6 Hz, 6H,
(C5Me5)), 11.68 (CH2CH3), 14.59 (CHCH3), 25.84 (CHCH2CH3), 34.47
(CHCH2CH3), 45.96 (CH2NH), 60.31 (d, J ¼ 9.1 Hz, CHNH), 68.51,
68.74, 68.93, 69.09, 69.16 (C5H5þC5H4þCH2OP), 84.62 (i-C5H4),
94.24 (C5Me5), 127.82 (t, J ¼ 10.1, m-C6H5P), 130.82 (s, p-C6H5P),
133.02 (d, J ¼ 23.7, o-C6H5P), 135.78 (d, J ¼ 30.3 Hz, i-C6H5P); 31Pe
CH(CH3)2), 1.44 (s, 15H, CH3 of Cp* (C5Me5)), 1.84 (m, 1H, CH(CH3)2),
2.58 (br, 1H, CHNH), 3.55 (br, 2H, CH2NH), 3.75 (br, 2H, CH2OP),
4.12e4.22 (m, 5H, C5H5þ4H, C5H4), 7.33e7.40 (m, 6H, m- and p-
C6H5P), 7.97 (br, 4H, o-C6H5P); 13C NMR (100.6 MHz, CDCl3, ppm)
d:
{1H} NMR (162.0 MHz, CDCl3, ppm)
d
: 72.89 (s, O-P(Ph)2); IR (KBr
8.32 (CH3 of Cp* (C5Me5)), 18.52 (CH(CH3)2), 28.60 (CH(CH3)2),
46.93 (CH2NH), 62.12 (d, J ¼ 7.0 Hz, CHNH), 67.98, 68.06, 68.52,
68.82, 68.97, 69.43(C5H5þC5H4þCH2OP), 84.84 (i-C5H4), 94.11
(C5Me5), 127.75 (t, J ¼ 10.6 Hz, m-C6H5P), 130.96 (s, p-C6H5P), 133.27
(d, J ¼ 12.6 Hz, o-C6H5P), 135.55 (d, J ¼ 23.1 Hz, i-C6H5P); 31Pe{1H}
pellet in cm¡1
) y: (C-Cp): 3056, (C]C-Cp): 1451, (P-Ph): 1435,
(OeP): 1026; Anal. Calc. for [C39H49NOPFeIrCl2] (897.77 g/mol): C
52.18, N 1.56, H 5.50; found: C 51.97, N 1.28, H 5.22.
Acknowledgements
NMR (162.0 MHz,CDCl3, ppm)
d
: 74.33 (s, O-P(Ph)2); IR (KBr pellet
in cm¡1
)
y: (C-Cp): 3057, (C]C-Cp): 1448, (P-Ph): 1435, (OeP):
Partial support from Dicle University (Project number:
FEN.15.021 and FEN.15.022) is gratefully acknowledged.
1024; Anal. Calc. for [C38H47NOPFeIrCl2] (883.74 g/mol): C 51.65, N
1.59, H 5.36; found: C 51.51, N 1.48, H 5.21%.
Appendix A. Supplementary data
4.2.7. [(2S)-2-(ferrocenylmethylamino)-3-
methylbutyldiphenylphosphinito(dichloro(ɳ5-
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2016.06.002.
pentamethylcyclopentadienyl)iridium(III))] (22)
20
Yield: 227 mg, 85.7%; mp: 132e134 ꢂC); [
a
]
¼ þ29.2ꢂ (c 0.5,
D
CH2Cl2). 1H NMR (400.1 MHz, CDCl3, ppm)
d: 0.81e0.89 (m, 6H,
References
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(br, 1H, CHNH), 3.54 (br, 2H, CH2NH), 3.75 (br, 1H, CH2OP),
4.12e4.21 (m, 5H, C5H5þ4H,C5H4), 7.33e7.40 (m, 6H, m- and p-
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46.95 (CH2NH), 62.15 (d, J ¼ 5.0 Hz, CHNH), 67.91, 68.02, 68.13,
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C6H5P), 133.19 (d, J ¼ 22.1 Hz, o-C6H5P), 135.58 (d, J ¼ 28.2 Hz, i-
C6H5P); 31Pe{1H} NMR (162.0 MHz, CDCl3, ppm)
d
: 73.72 (s, O-
P(Ph)2); IR (KBr pellet in cm¡1
) y: (C-Cp): 3057, (C]C-Cp): 1448,
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(883.74 g/mol): C 51.65, N 1.59, H 5.36; found: C 51.53, N 1.43, H
5.24%.
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