Synthesis of aminomethyl quinazoline based ruthenium (II) complex and its application in asymmetric transfer hydrogenation under mild conditions

Article abstract of DOI:10.1016/j.jorganchem.2016.07.001

The new chiral aminomethyl quinazoline (amq) type ligand derived from L-phenylalanine was synthesized and coordinated with [RuCl₂(PPh₃)dppb] to obtain ruthenium(II) complex. This catalyst displayed considerable reactivity (up to 97% ee and 99% conversion) in the asymmetric transfer hydrogenation of ketones using 2-propanol as a hydrogen source in the presence of NaOⁱPr.

Full text of DOI:10.1016/j.jorganchem.2016.07.001

Journal of Organometallic Chemistry 819 (2016) 189e193
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of aminomethyl quinazoline based ruthenium (II) complex
and its application in asymmetric transfer hydrogenation under mild
conditions
,
*
Ahmet Agac a, Idris Karakaya b, Irfan Sahin a, Sedat Emir a, Semistan Karabuga a
,
Sabri Ulukanli b
a Department of Chemistry, Faculty of Science and Letters, Kahramanmaras Sutcu Imam University, 46100, Kahramanmaras, Turkey
b Department of Chemistry, Faculty of Science and Letters, Osmaniye Korkut Ata University, 80000, Osmaniye, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
The new chiral aminomethyl quinazoline (amq) type ligand derived from L-phenylalanine was synthe-
Received 12 April 2016
Received in revised form
14 June 2016
Accepted 4 July 2016
Available online 14 July 2016
sized and coordinated with [RuCl2(PPh3)dppb] to obtain ruthenium(II) complex. This catalyst displayed
considerable reactivity (up to 97% ee and 99% conversion) in the asymmetric transfer hydrogenation of
ketones using 2-propanol as a hydrogen source in the presence of NaOiPr.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Asymmetric transfer hydrogenation
NN ligand
Quinazoline
Ruthenium
1. Introduction
excellent selectivities and reactivities for the asymmetric reduction
of ketones have been attained through ruthenium complexes
The enantioselective hydrogenation of ketones is crucial for
synthesis of chiral secondary alcohols which are important motifs
in the preparation of the pharmaceuticals and fine chemicals. As a
result, tremendous efforts in recent years have been centered
around identifying better catalytic systems with transition metal
complexes for asymmetric transfer hydrogenation (ATH) reactions
[1e6]. Among the various transition metals, Rh [7e9], Ir [10,11], Os
[12e14] and Ru [15e21] have attracted the most attention. Noyori
and co-workers, in particular, have pioneered the use of Ru-NH
catalyst systems for accelerating reaction rate and efficiency [22].
Since this seminal contribution, considerable effort has been
dedicated to the design of more efficient catalysts which contain
NH functionality [23,24]. Due to their phenomenal rigidity in co-
ordination chemistry, N-heterocyclic framework compounds have
been frequently applied as ligands. The Yu group has previously
reported the highly stereoselective ATH reaction of ketones using
pyridyl NNN-type coordinated ruthenium catalysts 1 [25]. Similarly,
containing amino pyridyl (ampy) 2 donor groups by Baratta and co-
workers [13,26,27]. Furthermore, our own research group has
previously developed quinazoline based ruthenium complexes 6a
for transfer hydrogenation of acetophenone derivatives, obtaining
excellent conversions (up to 99%) and high TOF values (up to
118800 hꢀ1) [28]. More recently, we have also reported ATH reac-
tion of ketones using ruthenium complexes 6b-d based on the
chiral quinazolines 3b-d which were synthesized from optically
pure amino acids, which resulted in very good enantioselectivities
and conversions (Scheme 1) [29]. In the light of these results, we
reported the preparation of new ruthenium catalytic system
bearing another chiral quinazoline ligand 3e derived from L-
phenylalanine and tested as a catalyst in ATH reaction of ketones.
2. Results and discussion
As the chiral center
a to the 2 position of quinazoline ring can be
modified [30e35] quickly by employing readily available optically
pure amino acids under basic conditions, quinazoline structure 3b-
e was chosen as an ideal modular chiral ligand scaffold. We syn-
thesized chiral chloroquinazoline 4e according to our reported
* Corresponding author.
E-mail address: semistan@ksu.edu.tr (S. Karabuga).
http://dx.doi.org/10.1016/j.jorganchem.2016.07.001
0022-328X/© 2016 Elsevier B.V. All rights reserved.

190
A. Agac et al. / Journal of Organometallic Chemistry 819 (2016) 189e193
Scheme 1. N-Heterocyclic chiral ligands for Ruthenium (II) complexes.
procedure with very good yield by choosing
L
-phenylalanine as a
could be obtained in good conversion in under 2 min, prolonged
reaction times caused the ee to diminish. These results suggested
that racemication might be responsible. Reducing the reaction
temperature to 50 ꢁC increased enantioselectivity (85% ee), how-
ever longer reaction times were required for good conversion. The
similar results were obtained when the reaction was conducted at
ambient temperature (92% conv., 85% ee). On the other hand
precursor [33]. The target optically pure amino methyl quinazoline
type ligand (amq) 3e was then achieved in a two-step reaction
sequence involving first cross-coupling reaction of phenyl boronic
acid and subsequent deprotection of the Boc group with TFA
(Scheme 2).
Finally, the new quinazoline-based ruthenium complex 6e was
prepared by treating ligand 3e with RuCl2(PPh3)dppb in the pres-
ence of Et3N and iPrOH as a solvent at reflux for 3 h (Scheme 3). As
described in our previous report [28], the amino quinazoline based
ruthenium (II) complex can generate two isomeric forms 6aı-6aıı
coordinating metal with ligand due to the position of nitrogen
atoms on the quinazoline ring. It was found that the major complex
6aı was energetically more stable than the minor complex 6aıı in
accordance with the density functional theory (DFT) calculations.
Furthermore, we obtained complex 6bı as a single isomer for X-ray
diffraction analysis. These results indicated that the possibility of
complex formation 6aı-dı which coordinated with N atom at 1-
position of quinazoline ring was higher than complex 6aıı-dıı at 3
position of quinazoline ring [29]. Similarly, in this study, the new
ruthenium complex 6e was acquired as a mixture of two isomers
6eı-6eıı in a ratio of 6.5:1 characterized by 1H and 31P NMR analyses.
In spite of several attempts to provide recrystallization, major
complex 6eı could not be obtained as a single isomer. Consequently,
in later attempts to investigate its catalytic activity, complex 6e was
used as a mixture in ATH reactions.
i
t
complexes 6b-d which are bearing Me, Pr and Bu groups on the
chiral center, exhibited very low reactivity at room temperature
(less than 10% conv.). These results showed that the benzyl group
on the chiral center influenced the catalytic efficiency positively.
With suitable conditions in hand (1/500 substrate-complex ra-
i
tio, 0.4 mL 1 M NaOiPr, in 10 mL PrOH at room temperature), a
series of aromatic and aliphatic ketones were explored in ATH re-
action and the results are shown in Table 1. In general, high con-
versions and enantioselectivities ranging between 82 and 94% were
observed in reduction of acetophenone derivatives (entries 1e12)
again indicative of a potential
p-stacking interaction between the
catalyst and substrate. Acetophenones bearing methyl and bromine
in all positions of the phenyl ring showed almost the same re-
activities (entries 2e4 and 8e10). While obtaining 94% ee for o-
methoxy substituted acetophenone, m- and p-substituted ones
gave moderate ee values (entries 5e7). Chloroacetophenones fur-
nished relatively better enantioinduction when compared with
bromine counterparts (entries 11 and 12). Also, the modest ees and
conversions were gained from reduction of a- and b-acetyl naph-
i
Initial tests were carried out in refluxing PrOH with acetophe-
thalenes (entries 13 and 14). But, when methyl group was displaced
with ethyl or isopropyl in acetophenone, the enantioselectivity
diminished significantly (entries 15 and 16). The similar ees were
detected as a result of reduction of the tetralone at reflux temper-
ature (entry 17). As a result of using heteroaryl ketones as sub-
strates, 2-acetylpyridine gave the desired product in excellent ee
and conversion (entry 18). On the contrary, 2-acetylthiophene and
2-acetylfuran were successively converted to the related alcohols
with acceptable results (entries 19 and 20). But, the catalyst showed
low reactivity in the reduction of aliphatic ketones at room tem-
perature (entries 21 and 22). However, when the reactions were
conducted under reflux conditions, very good conversions were
detected with lower ee (entries 22 and 23). Similarly, ATH reaction
of diaromatic ketones did not give a significant conversions at room
temperature, but also great conversions were succeeded with poor
ee at reflux as well (entries 24e26).
none was chosen as a substrate. In our recently published report
[29], we disclosed two different methods for these reactions
differing in the reagent addition sequence and initial reaction
temperature. According to the first method, the complex and base
were added to the solution of acetophenone, heated to reflux
temperature in iPrOH. Although this method resulted in good
conversions, no significant enantiomeric excess could be observed.
In contrast, when all of the reagents were added simultaneously at
room temperature and then rapidly heated to reflux, good enan-
tioselection was achieved although with diminished rates of
conversion.
In order to determine efficient reaction parameters such as base,
temperature, and substrate-catalyst ratio, a series of reactions were
conducted according to the second method. The best enantiomeric
excess and conversion were acquired via use of 1/500 substrate-
complex ratio in the presence of NaOiPr as a base. While 79% ee
These results indicated that acetophenone derivatives showed
Scheme 2. Synthesis of amino methyl quinazoline 3e.

A. Agac et al. / Journal of Organometallic Chemistry 819 (2016) 189e193
191
Scheme 3. Preparation of Ruthenium (II) complexes 6a-e.
Table 1
Asymmetric transfer hydrogenation of ketones catalyzed by complex 6e.
Entry
Ketone
Time
Conv. %a
ee %b
Config.c
Entry
Ketone
Time
Conv. %a
ee %b
Config.c
1
2
3
4
5
6
7
8
9
1 h
92
70
90
85
95
93
65
95
96
85
87
86
85
94
84
82
85
85
R [36]
R [36]
R [36]
R [36]
R [36]
R [36]
R [36]
R [40]
R [41]
14
15
16
17
18
19
20
21
22
2 h
95
97
64
80
96
78
81
70
37
96
96
93
95
99
83b
65
63
54
97
70
70
20
27
3
R [36]
R [36]
R [36]
R [37]
R [38]
R [39]
R [38]
nd
24 h
1.5 h
1.5 h
3 h
1 h
1 h
0.2 h
5 hd
2 hd
2 h
1 h
0.5 h
4 h
1 h
1 h
0.5 hd
nd
10
11
12
13
1 h
0.5 h
1 h
97
83
96
85
85
89
89
77
R [41]
R [36]
R [36]
R [36]
23
24
25
26
6 hd
6
R [36]
R [42]
R [43]
nd
0.15 hd,e
0.15 hd,e
0.15 hd,e
14
15
5
3 h
a
Determined by GC.
Determined by GC or HPLC (see supplementary content).
b
c
The absolute configurations of adducts were assigned in accordance with literature.
Under reflux temperature.
d
e
1.2 mL NaOiPr was used.
considerably higher catalytic activity when compared to the dia-
romatic and dialiphatic ketones. We assumed that the increase in
the enantioselectivity of catalyst stemmed from the interaction
between phenyl ring of benzyl group on the complex 6e and aryl
group on the ketones. Probably, the lack of aromatic structure on
the aliphatic ketones did not allow for the interaction with benzyl
group on complex 6e. Nevertheless, the competitive reaction pro-
ceeded due to the presence of two aromatic structures on diaryl
ketone. Unfortunately, a good selectivity could not be obtained
from these substrates further suggesting that arene coordination is
important for stereoinduction as differentiation between two
nearly identical aryl substituents gives minimal enantioselectivity.
In the light of these results, a possible transition state structure was
proposed (Fig. 1).
3. Conclusion
In conclusion, ruthenium (II) complex 6e was obtained from the
reaction [RuCl2(PPh3)dppb] with optically pure amino methyl 4-
phenyl quinazoline 3e synthesized in a few steps starting from L-
phenylalanine, and the catalytic performance of the complex 6e
was tested in ATH reaction of the ketones. In general, the alkyl-
(hetero)aryl ketones were converted to the related secondary al-
cohols with good enantioselectivities and conversions. Unfortu-
nately, poor selectivities were obtained for the reduction of
dialiphatic and diaromatic ketones. In this study, we investigated
the efficiency of different groups on the chiral center by comparing
the selectivity with our previous report. Interestingly, while pre-
vious reactions gave less than 10% conversion after 24 h at room
temperature in the presence of 6b-d catalysts, considerable reac-
tivity was accomplished when the new catalyst 6e containing
benzyl group in the chiral center was employed. While Ru-NH
moiety is known to have a positive effect on the catalytic activity
[22,44-46], we believe that the quinazoline ring makes an addi-
tional contribution to the reactivity as well. Moving forward, these
ligands are exciting because the chiral environment of quinazoline
structure can be easily modified by employing the range of
commercially available optically pure a-amino acids [30e32]. To
explore these effects in the ATH reaction, further modifications are
ongoing modifying the substituents on the 4-position of quinazo-
line with more electron-donating and electron-withdrawing
groups.
Fig. 1. Proposed mechanism and transition state geometry for ATH reaction of ketones.

192
A. Agac et al. / Journal of Organometallic Chemistry 819 (2016) 189e193
4. Experimental
found 326.1657.
4.1. General remarks
4.1.3. Preparation of ruthenium complex 6e
i
Ru(PPh3)dppbCl2 (1.1 g, 1.27 mmol) was suspended in PrOH
(6 mL) and (S)-2-phenyl-1-(4-phenylquinazolin-2-yl)ethan-1-
amine 3e (0.5 g, 1.5 mmol) and Et3N (1.3 g, 12.9 mmol) were added.
The resulting mixture was then refluxed for 6 h at nitrogen atmo-
sphere. The precipitate was filtered and washed petroleum ether
(20 mL) and diethyl ether (2 ꢂ 20 mL). The ruthenium complex 6e
was obtained as an orange solid (0.78 g, 67%). mp: 224e226 ꢁC; IR
(KBr, cm-1) 3457, 3027, 2970, 1738, 1564, 1526, 1431, 1365, 696, 515,
Reagents and solvents were purchased from chemical suppliers
and purified to match the reported physical and spectroscopic data.
The solvents were carefully dried using standard methods. Melting
points were determined with an Electro thermal IA9100 apparatus.
IR spectra were obtained on KBr pellets with a Perkin Elmer
apparatus. 1H and 13C NMR spectra were recorded on Bruker
(300 MHz, 400 MHz and 600 MHz) spectrometers in CDCl3. The
conversions and enantioselectivity ratios were determined by GC
analysis with YL6500 Instrument and by HPLC analysis with Hitachi
7000 series. All column chromatography was performed on silica
gel (230e400mesh).
505.; 1H NMR (600 MHz, CDCl3)
d
10.50 (d, J ¼ 8.7 Hz, 1H), 8.41 (t,
J ¼ 8.6 Hz, 2H), 8.03 (t, J ¼ 7.6 Hz, 1H), 7.87e6.87 (m, 30H), 6.76 (d,
J ¼ 7.0 Hz, 2H), 6.45 (t, J ¼ 6.8 Hz, 2H), 4.21 (t, J ¼ 13.3 Hz, 1H),
4.11e3.82 (m, 3H), 3.69 (t, J ¼ 11.9 Hz, 1H), 2.90 (d, J ¼ 13.2 Hz, 1H),
2.76e2.25 (m, 3H), 2.14e1.78 (m, 2H); 31P NMR (121 MHz, CDCl3)
4.1.1. Synthesis of t-butyl (S)-(2-phenyl-1-(4-phenylquinazolin-2-
yl)ethyl)carbamate 5e
d
54.40 (d, J ¼ 39.2 Hz), 52.68 (d, J ¼ 39.2 Hz), 41.54 (d, J ¼ 39.0 Hz),
39.89 (d, J ¼ 39.0 Hz); HRMS (ESI) m/z calc. for C52H50ClN4P2Ru
[MꢀCl CH3CN] 929.2243, found 929.2248; Anal. calc. for
50H47Cl2N3P2Ru (%): C, 65.00; H, 5.13; Cl, 7.67; N, 4.55; found: C,
t-butyl
(S)-(1-(4-chloroquinazolin-2-yl)-2-phenylethyl)carba-
þ
mate 4e (5.1 g, 13,28 mmol), phenylboronic acid (2.73 g,
22.39 mmol), Pd(PPh3)4 (307 mg, 026 mmol) and Na2CO3 (29 mL,
1 M in water) were dissolved in dimethoxyethane (45 mL) and
ethanol (45 mL) and the resulting mixture was then refluxed for 4 h.
The solution was cooled to ambient temperature, after the addition
of water (100 mL), the organic material was extracted with ethyl
acetate (3 ꢂ 100 mL). The combined organic layers were dried over
Na2SO4, and concentrated under reduced pressure. The crude
product was purified by column chromatography (silica gel, hexane/
EtOAc, 6:1) to give t-butyl (S)-(2-phenyl-1-(4-phenylquinazolin-2-
C
64.29, H, 5.23, N, 4.35.
4.1.4. General procedure for the asymmetric transfer hydrogenation
reaction
Ruthenium complex (3.7 mg, 0.004 mmol), ketone (2 mmol) and
NaOiPr (0.4 mL, 0.1 M) were dissolved in degassed PrOH (10 mL)
i
and the mixture was stirred under nitrogen atmosphere at appro-
priate temperature. A small volume of sample was taken from re-
action mixture and diluted with diethyl ether (1:1), and rapidly
filtered using a short silica pad. The conversion and enantiomeric
excess were determined by GC using Agilent HP-Chiral 20B column
yl)ethyl)carbamate 5e as
a
colourless oil (4.64 g, 82%).
½aꢃ2D0 ꢀ 45 (c ¼ 1.6 CHCl3); ee: 99.9%; retention time 9.7 min, Chir-
alcel OD-H, 95:5 n-hexane-iPrOH, flow rate of 0.5 mL/min, 254 nm;
IR (KBr, cmꢀ1) 3423, 2925, 2856, 1710, 1610, 1557, 1492, 1385, 1248,
(30 m, 0.25 mm, 0.25
chiral columns.
mm) and by HPLC using Supelco AD-H, OD-H
1168, 1054, 1025, 774, 700; 1H NMR (600 MHz, CDCl3, ppm)
d 8.02
(1H, d, J ¼ 8.2 Hz), 7.96 (1H, d, J ¼ 8.0 Hz), 7.80 (1H, t, J ¼ 7.3 Hz), 7.63
(2H, dd, J ¼ 6.5, 2.8 Hz), 7.56e7.46 (4H, m), 7.07 (4H, bs), 6.92 (1H,
bs), 5.91 (1H, d, J ¼ 6.7 Hz), 5.38 (1H, d, J ¼ 6.7 CHCH2PhHz), 3.41
(1H, dd, J ¼ 13.4, 5.9 Hz, CH2Ph), 3.29 (1H, dd, J ¼ 13.4, 5.9 Hz,
Acknowledgment
This presented study was supported by The Scientific and
Technological Research Council of Turkey (TUBITAK) (project
number is 112T816). We thank David N. Primer (University of
Pennsylvania) for proof reading.
CH2Ph), 1.38 (9H, s); 13C NMR (151 MHz, DMSO, ppm))
d 168.4,165.9,
156.0, 151.1, 138.9, 137.1, 134.8, 130.6, 130.4, 129.7, 129.0, 128.6, 128.5,
128.3, 127.2, 126.6, 121.4, 78.4, 59.1, 28.6, 28.2; HRMS (ESI) m/z calc.
for C27H28N3O2 [MþH] 426,2182, found 426.2179.
Appendix A. Supplementary data
4.1.2. Synthesis of (S)-2-phenyl-1-(4-phenylquinazolin-2-yl)ethan-
1-amine 3e
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2016.07.001.
t-butyl (S)-(2-phenyl-1-(4-phenylquinazolin-2-yl)ethyl)carba-
mate 5e (3.98 g, 9.35 mmol) was dissolved in CH2Cl2 (100 mL) and
triflouroacetic acid (10.64 g, 93.35 mmol) was added slowly at 0 ꢁC.
After stirring 24 h at room temperature, the reaction was quenched
with saturated NaHCO3 (100 mL) and extracted with CH2Cl2
(2 ꢂ 100 mL). The combined organic layers were dried over Na2SO4,
and concentrated under reduced pressure. The crude product was
purified by column chromatography (silica gel, CH2Cl2/methanol,
12:1) to give (S)-2-phenyl-1-(4-phenylquinazolin-2-yl)ethan-1-
amine 3e as a colourless oil (2.99 g, 98%). ½aꢃ2D0 þ 20 (c ¼ 0.7,
EtOH); ee: 99%; retention time 6.3 min, Chiralcel AD-H, 90:10 n-
hexane-iPrOH, flow rate of 1 mL/min, 254 nm; IR (KBr, cmꢀ1) 3455,
3009, 2970, 1738, 1435, 1365; 1H NMR (600 MHz, CDCl3, ppm)
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