Asymmetric transfer hydrogenation reaction in water: Comparison of chiral proline amide/amine ruthenium(II) complexes

Article abstract of DOI:10.1016/j.jorganchem.2014.12.023

Chiral proline amide/amine ligands (2, 3), synthesized by multi-step reaction starting from l-proline (1), were evaluated as catalyst generated in situ from [RuCl₂(p-cymene)]₂ for asymmetric transfer hydrogenation of aromatic ketones in the presence of sodium formiate and sodium dodecyl sulfate (SDS). The results revealed that efficiencies and enantioselectivities strongly depend on the N-substituents.

Full text of DOI:10.1016/j.jorganchem.2014.12.023

Journal of Organometallic Chemistry 779 (2015) 62e66
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Asymmetric transfer hydrogenation reaction in water: Comparison
of chiral proline amide/amine ruthenium(II) complexes
,
*
a
b
b
a
Serpil Denizaltı a , Deniz Mercan , Betül S¸ en , Aytaç Gürhan Gokçe , Bekir Çetinkaya
€
a
_
Ege University, Department of Chemistry, 35100 Bornova, Izmir, Turkey
Dokuz Eylül University, Department of Physics, 35160 Buca, Izmir, Turkey
b
_
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral proline amide/amine ligands (2, 3), synthesized by multi-step reaction starting from L-proline (1),
Received 8 November 2014
Received in revised form
11 December 2014
Accepted 17 December 2014
Available online 26 December 2014
were evaluated as catalyst generated in situ from [RuCl2(p-cymene)]2 for asymmetric transfer hydro-
genation of aromatic ketones in the presence of sodium formiate and sodium dodecyl sulfate (SDS). The
results revealed that efficiencies and enantioselectivities strongly depend on the N-substituents.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
L-proline
Ruthenium
Asymmetric transfer hydrogenation
Introduction
isopropyl alcohol (IPA) [6]. In this study, we have evaluated the
catalytic efficiency of proline-derived chiral amides/diamines (2, 3)
Transition metal-catalyzed transfer hydrogenation of carbonyl
compounds is one of the most important reactions in pharmaceu-
tical and chemical industries [1]. Whereas direct hydrogenation of
unsaturated compounds is more widely applied, transfer hydro-
genation (TH) is a powerful alternative in view of its easy handling,
the ready availability of hydrogen donor, low cost of reducing
agents and safety [1,2]. Therefore, the asymmetric transfer hydro-
genation (ATH) of prochiral ketones using various chiral ligands is
highly efficient method to obtain enantiomerically enriched alco-
hols. For this purpose, several catalysts such as chiral diamines have
been used [1a,b,3]. Over the last two decades, chiral proline amide
ligands and their derivatives (IeIV) have been applied in many
asymmetric catalyzes, such as asymmetric aldol, Michael addition
reaction and high enantioselectivities have been obtained [4].
Moreover, the catalytic activities of amide ligands in ATH reaction
were reported by several groups (ee's up to 95%) (Fig. 1) [5]. Karim
and co-workers also published the synthesis of proline-derived
chiral diamine ligands (VeVI) containing phenyl or naphthyl
group on nitrogen atom (Fig. 1). Their catalytic activities in ATH
revealed that the substitution of phenyl by naphthyl group was
increased the activity and enantioselectivity (ee's up to 95%) in
in ATH reaction. To the best of our knowledge, these chiral ligands
(3) have not been evaluated in ATH, except 2a [5d], although they
are known in the literature [7]. For this purpose, herein, we
compared the proline-derived chiral diamine ligands with amide
analogs and also investigated the steric effect of aryl ring in the
enantioselectivity.
Results and discussion
Synthesis and characterization of proline-derived chiral amide/
diamine ligands (2, 3) and ruthenium complexes (4b, 5b)
Chiral proline amides/amines (2, 3) were prepared by three/
four-step reactions, starting from the commercially available
proline (Scheme 1).
L-
NMR spectra of the products were in accordance with the
literature values [8]. The complex (5b) consists of [RuCl(C14H22N2)
(ɳ6eC10H14)]þ cation, the charge being balanced by one interstitial
chloride anion (Fig. 2). The structure exhibits a typical three legged
piano-stool geometry (a description commonly used for half-
esandwich compounds) with RuII coordinated by two N atoms of
the organic ligand, which acts as bidentate and a terminal Cl atom.
The coordination geometry around RuII atom is distorted octahe-
dron with three sites occupied by the p-cymene ligand (with an ƞ6
coordination mode) while the remaining three sites occupied by
* Corresponding author. Tel.: þ90 232 311 17 80; fax: þ90 232 388 82 64.
E-mail address: serpil.denizalti@ege.edu.tr (S. Denizaltı).
http://dx.doi.org/10.1016/j.jorganchem.2014.12.023
0022-328X/© 2014 Elsevier B.V. All rights reserved.

S. Denizaltı et al. / Journal of Organometallic Chemistry 779 (2015) 62e66
63
Table 1
Selected geometric parameters for 5b (Å, ꢁ).
Ru1eCl1
Ru1eN1
2.402(2)
2.235(6)
Ru1eC5
Ru1eC6
2.161(9)
2.178(10)
Ru1eN2
Ru1eC1
Ru1eC2
Ru1eC3
Ru1eC4
2.129(6)
2.231(11)
2.204(12)
2.181(12)
2.242(8)
Cl1eRu1eN1
Cl1eRu1eN2
N1eRu1eN2
83.24(19)
87.0(2)
76.4(2)
ring (Ru1\N1\N2\C14\C15) adopts an envelope conformation with
atom C14 displaced by ꢀ0.3328 Å from the mean plane of the other
ring atoms (Table 2).
Fig. 1. Chiral proline-derived chiral amide/diamine ligands used for ATH of
acetophenone (yield/ee).
The six-membered ring of the p-cymene is almost planar, with a
maximum deviation of 0.033 (8) Å (C1) and an overall r.m.s. devi-
ation of 0.0201 Å. The dihedral angle between the p-cymene ring
(C1eC6) and the isopropyl group (C8eC9) planes is 70.86 (0.82)ꢁ.
The distance between the RuII ion and the least-squares plane of the
p-cymene aromatic ring is 1.684 (4) Å, which is very close to that
reported in other RuII arene complexes [12,13]. Defining X as the
centroid of the aromatic ring, the Cl1eRu1eX, N1eRu1eX and
N2eRu1eX angles are 125.95 (18), 135.2 (2) and 130.9 (2)ꢁ,
respectively. The RueCl bond distance is normal and also agrees
well with those in related complexes [14,15]. The two RueN bond
distances are substantially different [2.129 (6) and 2.235 (6) Å],
longer than those reported in other complexes, such as in
[{RuCl(ɳ6ecym)}2(
m
-1,6,7,12-Tetraazaperylene)](PF6)2
[16]
(2.105(2) and 2.105(2) Å) and [(ɳ6epePriC6H4Me)RuCl(2,20-bipyr-
imidine)][PF6] [17] (2.084 (2) and 2.080 (3) Å).
The non-coordinated chloride anions interconnect the
[RuCl(C10H14)(C14H22N2)]þ cations via hydrogen bonds into a chain
running along [010] (Fig. S3 in ESI). Besides, the compound contains
intra-molecular CeH/Cl type weak interaction (C14eH14/Cl1)
generating an S(5) ring motif [18].
Scheme 1. Synthesis of proline-derived chiral amide/diamine ligands (2, 3) and
ruthenium complexes (4b, 5b). Reaction conditions: i) (Boc)2O, Et3N, CH2Cl2; ii) EtO-
COCl, Et3N, Ar-NH2, THF; iii)TFA, CH2Cl2; iv) LiAlH4, THF; v) [RuCl2(p-cymene)]2,
toluene.
These ligands were tested in the ATH reaction of aromatic ke-
tones (Table 3). As can be seen from Table 3, the best yield was
obtained with the ligand 2a. However, the amine ligand 3c did not
show activity. The results showed that the amides ligands (2aec)
had the better yield than the amine counterparts (3aec). Besides,
the enantioselectivity was improved but the yield was decreased
when the bulky chiral ligands were used. In this respect, the chiral
bidentate ligand to form a five-membered chelate ring and Cl
ligand. The distorted octahedral geometry is evident from the bond
angles given in Table 1. The coordination environment yields a
chiral ruthenium center with S configuration, according to the
ligand priority sequence ɳ6epecymene > Cl > N1 > N2 [9e11]. N1,
N2 and C14 atoms are also chiral centers with the absolute
configuration of S, R and S, respectively. The five membered chelate
Fig. 2. The molecular structure of 5b, with the atom labeling. Displacement ellipsoids are shown at the 40% probability level. The CH2Cl2 solvent molecules have been omitted for
clarity.

64
S. Denizaltı et al. / Journal of Organometallic Chemistry 779 (2015) 62e66
Table 2
Table 4
Crystallographic data for C25H38N2Cl4Ru.
ATH of ketones using ligand 2c.
Crystal data
Entry
Ketone
T (ꢁC)
t (h)
Yield (%)
ee (%)
Chemical formula
Formula weight
Temperature (K)
Space group
C25H38N2Cl4Ru
609.44
293(2)
P21
Monoclinic
9.2088(6), 12.1642(6), 12.6561(11)
90.00, 100.322(6), 90.00
1394.76(17)
1
2
3
4
5
6
7
8
PhCOCH2CH3
80
80
40
80
40
80
80
40
40
80
80
80
40
2
0.5
4
0.5
4
0.5
2
4
12
2
2
0.5
4
68
99
92
93
83
60
96
35
47
92
87
99
63
99
91
96
92
97
91
90
99
98
94
99
91
94
20Cl-PhCOCH3
20Cl-PhCOCH3
40Cl-PhCOCH3
Crystal system
a, b, c (Å)
40Cl-PhCOCH3
40MeO-PhCOCH3
40MeO-PhCOCH3
40MeO-PhCOCH3
40MeO-PhCOCH3
30,4’e(CH3)2ePhCOCH3
2-Acetylnaphthone
PhCOCH2Cl
a
,
b
,
g
(ꢁ)
Cell volume (Å3)
Formula unit cell Z
rcalc (g/cm3)
2
9
1.451
628.0
0.961
10
11
12
13
F(000)
Absorption coefficient
Crystal size (mm3)
Data collection
Diffractometer
Temperature (K)
m
(mmꢀ1
)
0.4719 ꢂ 0.1871 ꢂ 0.1342
PhCOCH2Cl
Xcalibur, Eos
293(2)
Reactions conditions: 5% mol ligand, 2.5% mol [RuCl2(p-cymene)]2, 0.5 mmol ketone,
5 mmol HCO2Na, SDS (2% mol), H2O.
Radiation/wavelength (Å)
Reflections measured
MoKa/0.71070
6334
Independent/observed reflections 4700/4031
chloroacetophenone derivatives were used, the yield was >90%
after 0.5 h at 80 ꢁC. The decrease of the reaction temperature from
80 to 40 led to extend the reaction time and to slightly increase the
enantioselectivity (Table 4, Entry 2e4). We also used 2-
acetylnaphthalene and 2-chloroacetophenone as substrate for the
transfer hydrogenation reaction, affording excellent enantiose-
lectivities. Although the ligand 2a showed better yield than the
others, chiral ligand 2c gave the excellent enantioselectivity (99%).
The enantioselectivity increased changing the phenyl group by 2,6-
diisopropylphenyl (Table 4).
Range of h, k, l
Refinement
ꢀ11 ꢃ h ꢃ 9, ꢀ15 ꢃ k ꢃ 14, ꢀ13 ꢃ l ꢃ 15
Data/Restraints/Parameters
4700/1/295
Final R indexes [I ꢄ 2
s
(I)]
R1 ¼ 0.0588, wR2 ¼ 0.1577
R1 ¼ 0.0703, wR2 ¼ 0.1724
1.047
1.54/ꢀ0.73
Final R indexes [all data]
Goodness-of-fit on F2
Largest diff. peak/hole (e Åꢀ3
)
ligand 2c gave the best enantioselectivity (90%) for acetophenone
although the yield of the product was moderate (Table 3, Entry 3).
Additionally, it was known that the surfactants give advantages in
terms of yield and enantioselectivity in ATH [5,19]. Therefore, in
order to increase the reaction yield, sodium dodecyl sulfate (SDS)
used as phase transfer catalyst (PTC) for ATH reaction of aceto-
phenone in the presence of ligand 2c (Table 3, Entries 6e8). When
2 mol% of SDS was used, the yield was considerably improved.
Increasing the amount of SDS led to higher yield but the lower
ee (Table 3, Entries 6e8). The ruthenium complexes 4b and 5b were
also synthesized and tested in ATH. The results are comparable to
those obtained by in situ, so no isolation and purification of the
ruthenium complexes was required.
Conclusion
In conclusion, chiral proline-derived chiral amide/diamine li-
gands 2 and 3 used as in situ prepared catalysts with [RuCl2(p-
cymene)]2. The complexes 4b, 5b were also synthesized. The crystal
structure of 5b was solved by X-ray analysis. According to the X-ray
data, the ruthenium center with R configuration was confirmed.
The catalytic activities of the ligands were evaluated for ATH. Chiral
proline amide ligands having more acidic NeH gave better yields as
compared with the corresponding amine derivatives. Moreover, it
was found that the bulk of the aryl substituents on the ligand
increased the enantioselectivity. The ligand 2c with 2,6-diisopropyl
groups on the phenyl provided the excellent yield and enantiose-
lectivity. The protocol with the facile synthesis of the ligand rep-
resents a practical and green approach to various functionalized
aromatic ketones in water in high yield and ee values.
Under the optimized reaction conditions, we extended the
scope of the reaction using a variety of ketones with the ligand 2c
(Table 4). The electron-withdrawing substituents on acetophenone
gave better yield than donating groups (Table 4, Entry 2e7). When
Table 3
ATH of acetophenone.
Experimental
General considerations
All reactions were performed in air unless otherwise stated.
Reactions involving air-sensitive components were performed by
using Schlenk-type flasks under argon atmosphere and high
vacuum-line techniques. The solvents were analytical grade and
distilled under argon atmosphere from sodium (diethyl ether,
tetrahydrofuran, toluene, hexane), P2O5 (dichloromethane). Re-
agents were purchased from Aldrich, Merck, Acros Organics, Alfa
Aesar, Fluka, and were used as received. Chiral proline amides/
amines (2, 3) were synthesized according to the literature [4a,d,8].
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured
on Varian AS 400 Mercury spectrometers and tetramethylsilane
(TMS) was used as the internal standard. All coupling constants (J
values) were reported in Hertz (Hz). Elemental analyses were
performed on a PerkineElmer PE 2400 elemental analyzer. Optical
rotations were taken on a Rudolph Research Analytical Autopol I
Entry
Ligands/complexes
Yield (%)
ee (%)
1
2
3
2a(3a)
2b(3b)
2c(3c)
2b
3b
2c
2c
2c
4b
5b
99(60)
69(30)
30(<5)
94
50
50
54
86
95
55
28(14)
80(71)
90(n.d.)
82
62
94
94
89
81
64
4a
5a
6a
7b
8c
9a
10a
a
2% mol SDS.
5% mol SDS.
10% mol SDS was used.
b
c

S. Denizaltı et al. / Journal of Organometallic Chemistry 779 (2015) 62e66
65
automatic polarimeter with a wavelength of 589 nm; the concen-
tration ‘c’ has units of g/100 mL. The conversion and enantiomeric
excess (ee) were determined by a chiral GC column Supelco b-Dex
225.
reflections and 295 parameters. All non-hydrogen atoms were
refined anisotropically. The crystal structure positions of hydrogen
atoms were treated as riding atoms. Geometrical calculations were
performed using PLATON [23] The figures were made using ORTEP
[24] and PLATON [23] The summary of the crystal data, experi-
mental details and refinement results are listed in Table 3.
General procedure for the synthesis of Ru(II) complex (4b, 5b)
[RuCl2(p-cymene)]2 (0.23 mmol) was added to the solution of
2b/3b (0.46 mmol) in dry toulene under argon. The mixture was
stirred at room temperature for 24 h. The orange precipitate was
filtered and then recrystallized from CH2Cl2/hexane.
Acknowledgments
The financial support from The Turkish Academy of Sciences
(TUBA) and Ege University is acknowledged. The authors also
acknowledge Dokuz Eylul University for the use of the Agilent
Xcalibur Eos difractometer (purchased under University Research
Grant No: 2010.KB.FEN.13). We also thank Dr. Zeynep Tas¸ çı for
helpful discussion.
4b: Yield: 69%; mp177e180 ꢁC. Anal. Cal. For C24H34Cl2N2ORu: C,
53.53; H, 6.36; N, 13.17. Found: C, 53.47; H, 6.33; N, 13.14.
27.4
[
a]
ꢀ44.78 (c.0.134 in CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS,
D
25 ꢁC, ppm): 1.10 (d, J ¼ 7.2 Hz, 3H, CH(CH3)2); 1.17 (d, J ¼ 7.2 Hz, 3H,
CH(CH3)2); 1.85e2.13 (m, 4H, CH2); 2.02, 2.20, 2.23 (s, 9H, CH3;
C6H2(CH3)3); 2.59e2.68 (m, 1H, CH(CH3)2); 2.77e2.81 (bs, 1H, NH);
3.40e3.48 (m, 2H, NCH2); 4.41e4.43 (m, 1H, CH); 5.62 (m, 3H, p-
cymene-Ar-H); 5.74 (d, J ¼ 6.0 Hz, 1H, p-cymene-Ar-H); 6.79 (s, 2H,
C6H2(CH3)3); 9.51 (bs, 1H, NH). 13C NMR (100 MHz, CDCl3, TMS,
25 ꢁC, ppm): 17.9 (CH(CH3)2); 18.3, 21.0 (C6H2(CH3)3); 21.7
(CH(CH3)2); 22.5 (CH3); 28.0 (CH(CH3)2); 30.9, 31.6, 52.3 65.9 (pro-
C); 78.7, 81.1, 81.9, 84.5, 92.7, 101.9 (p-cymene-Ar-C); 128.7; 129.9;
135.9; 137.3 (Ar-C); 181.1 (CO).
Appendix A. Supplementary material
CCDC 1008774 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
Appendix. BSupplementary data
5b: Yield: 73%; mp 105e108 ꢁC. Anal. Calc. for C24H36Cl2N2Ru: C,
54.96; H, 6.92; N, 13.52. Found: C, 54.85; H, 6.88; N, 13.49.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.12.023.
ꢀ69.23 (c.0.26 in CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS,
21.3
[
a]
D
25 ꢁC, ppm): 1.00 (d, J ¼ 6.8 Hz, 3H, CH(CH3)2); 1.23 (d, J ¼ 6.8 Hz,
3H, CH(CH3)2); 1.71e2.09 (m, 4H, CH2); 2.17, 2.25, 2.33 (s, 9H,
C6H2(CH3)3); 2.21e3.40 (m, 5H, NCH2CH; NCH2;CH(CH3)2); 2.64 (s,
3H, CH3); 3.65e3.68 (m, 1H, CH); 4.78 (d, J ¼ 5.6 Hz, 1H, p-cymene-
Ar-H); 4.90 (d, J ¼ 5.6 Hz, 1H, p-cymene-Ar-H); 5.46 (bs, 1H, NH);
5.89 (d, J ¼ 5.2 Hz, 1H, p-cymene-Ar-H); 5.95 (d, J ¼ 5.2 Hz, 1H, p-
cymene-Ar-H); 6.86 (s, Hz, 1H, C6H2(CH3)3); 6.96 (s, Hz, 1H,
C6H2(CH3)3); 9.21 (bs, 1H, NH). 13C NMR (100 MHz, CDCl3, TMS,
25 ꢁC, ppm): 18.6 (CH(CH3)2); 19.2 (C6H2(CH3)3); 20.5 (CH(CH3)2);
21.8 (C6H2(CH3)3); 21.9 (C6H2(CH3)3); 23.6 (CH3); 23.9 (CH(CH3)2);
24.9; 29.3; 48.4; 50.8 (pro-C); 62.7 (NCH2); 76.5, 79.5, 84.9, 85.4,
102.1, 103.4 (p-cymene-Ar-C); 128.9, 129.5, 130.8, 132.3, 136.2, 145.8
(Ar-C).
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