The application of tunable tridendate P-based ligands for the Ru(II)-catalysed transfer hydrogenation of various ketones

Article abstract of DOI:10.1002/aoc.3202

Two novel versatile tridendate aminophosphine-phosphinite and phosphinite ligands were prepared and their trinuclear neutral ruthenium(II) dichloro complexes were found to be effective catalysts for the transfer hydrogenation of various ketones in excellent conversions up to 99% in the presence of 2-propanol/NaOH in 0.1M isopropanol solution. Particularly, [Ru3 (PPh₂OC₂H₄)₂N-PPh₂ (η⁶-p-cymene)₃Cl₆] acts as an excellent catalyst giving the corresponding alcohols in excellent conversion up to 99% (turnover frequency ≤ 1176 h^-1). A comparison of the catalytic properties of the complexes is also discussed briefly. Furthermore, the structures of these ligands and their corresponding complexes have also been clarified using a combination of multinuclear NMR spectroscopy, infrared spectroscopy and elemental analysis. ¹H-¹³C HETCOR or ¹H-¹H COSY correlation experiments were used to confirm the spectral assignments.

Full text of DOI:10.1002/aoc.3202

Full Paper
Received: 4 July 2014
Revised: 24 July 2014
Accepted: 25 July 2014
Published online in Wiley Online Library: 16 September 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3202
The application of tunable tridendate P-based
ligands for the Ru(II)-catalysed transfer
hydrogenation of various ketones
a,b
a,b
a,b
b
Nermin Meriç *, Feyyaz Durap , Murat Aydemir and Akın Baysal
Two novel versatile tridendate aminophosphine–phosphinite and phosphinite ligands were prepared and their trinuclear neutral
ruthenium(II) dichloro complexes were found to be effective catalysts for the transfer hydrogenation of various ketones in
excellent conversions up to 99% in the presence of 2-propanol/NaOH in 0.1M isopropanol solution. Particularly, [Ru
3
6
(
PPh OC H ) N–PPh (η -p-cymene) Cl ] acts as an excellent catalyst giving the corresponding alcohols in excellent conversion
2
2
4 2
2
3
6
ꢀ
1
up to 99% (turnover frequency ≤ 1176 h ). A comparison of the catalytic properties of the complexes is also discussed briefly.
Furthermore, the structures of these ligands and their corresponding complexes have also been clarified using a combination
1
13
1
1
of multinuclear NMR spectroscopy, infrared spectroscopy and elemental analysis. H– C HETCOR or H– H COSY correlation
experiments were used to confirm the spectral assignments. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: AMPP; phosphinite; ruthenium; catalysis; transfer hydrogenation; ketone
groups. In addition, the empty σ*-orbital of the phosphinite P(OR)
Introduction
R is stabilized, making the phosphinite a better acceptor, and
2
[25]
Catalytic transfer hydrogenation with the support of a stable hydro-
gen donor is a useful alternative method for catalytic hydrogena-
tion by molecular hydrogen for the reduction of ketones.
another advantage is the easy preparation of phosphinites.
We have recently reported many phosphinite ligands, and have
employed them successfully as ligands in transition metal-
[
1–3]
In
[
26,27]
transfer hydrogenation, organic compounds such as primary and
promoted transfer hydrogenation of ketones.
is well known that many modified phosphine ligands
variety of chiral AMPP ligands have important applications in
Furthermore, it
[4]
[5]
[28–30]
secondary alcohols or formic acid and its salts have been
employed as the hydrogen source. The use of a hydrogen donor
has a lot of advantages over the use of molecular hydrogen since
it avoids the risks and constraints associated with hydrogen gas
as well as the necessity for pressure vessels and other equipment.
It is well known that a variety of transition metal complexes are
strikingly active for the hydrogenation of ketones. Particularly,
over the last three decades, most effort on hydrogenation has been
focused on the use of ruthenium catalysts. One reason is that ruthe-
and a
[
31–33]
organometallic chemistry and catalysis.
Taking into conside-
ration the mentioned factors and as a part of our interest in design-
ing new ligand systems with various spacers to control the
electronic attributes at phosphorus centres and to explore their
coordination chemistry, we report here the synthesis of novel
tridendate phosphinite and AMPP ligands and their catalytic evalu-
ation in the transfer hydrogenation of various ketones.
[
6]
[
7,8]
nium catalysts have admirable performances.
Another is that
ruthenium has a cost advantage over other hydrogenation metals
such as rhodium and iridium.
[
9]
Experimental
The chemistry of aminophosphine–phosphinite (AMPP), amino-
Materials and Methods
phosphines, phosphines and phosphinites has also been much
[
10–13]
explored in recent years.
attractive as potential ligands since various structural modifications
These compounds are extremely
Unless otherwise stated, all reactions were carried out under an
atmosphere of argon using conventional Schlenk glassware. Sol-
vents were dried using established procedures and distilled under
argon immediately prior to use. Analytical-grade and deuterated
[
14,15]
are accessible via simple P–N, P–C and P–O bond formation.
Many modified phosphine ligands and a variety of AMPP ligands
have important applications in organometallic chemistry and
catalysis, giving selective catalysts for hydroformylation,
solvents were purchased from Merck. PPh Cl, diethanolamine and
2
[
16–18]
hydrosilylation, transfer hydrogenation, etc.
While much effort
has been devoted to the synthesis of aminophosphines and phos-
phines, similar studies of the analogous phosphinites are less
*
Correspondence to: Nermin Meriç, Science and Technology Application and
Research Center (DUBTAM), University of Dicle, 21280 Diyarbakir, Turkey. E-mail:
nmeric@dicle.edu.tr
[19,20]
extensive,
although phosphinites have different chemical,
electronic and structural advantages compared to phosphines and
[21–23]
a Department of Chemistry, Faculty of Science, University of Dicle, 21280 Diyarbakir,
Turkey
aminophosphines.
Phosphinites are versatile ligands which al-
[24]
low effective catalytic transformations.
The metal–phosphorus
bond is often stronger for phosphinites compared to the related
b Science and Technology Application and Research Center (DUBTAM), University of
phosphines due to the presence of electron-withdrawing P–OR
Dicle, 21280 Diyarbakir, Turkey
Appl. Organometal. Chem. 2014, 28, 803–808
Copyright © 2014 John Wiley & Sons, Ltd.

N. Meriç et al.
triethanolamine are purchased from Fluka and were used as
precipitate (triethylammonium chloride) was filtered under argon,
the solvent was removed and the remaining part was dried in
6
received. The starting material [Ru(η -p-cymene)(μ-Cl)Cl]
prepared according to literature procedures.
2
was
FT-IR spectra
were recorded with a Mattson 1000 ATI UNICAM FT-IR spectrome-
[
34,35]
vacuum to produce a yellow viscous oil of compound 2 (yield:
1
0.43 g, 93.3%). H NMR (CDCl
3
, δ, ppm): 7.46–7.49 (m, 12H,
1
13
31
1
ter. H NMR (400.1MHz), C NMR (100.6 MHz) and P-{ H} NMR
162.0MHz) spectra were recorded using a Bruker AV400 spectro-
o-protons of phenyls), 7.27–7.39 (m, 18H, m- and p-protons of
3
(
phenyls), 3.89 (t, 6H, JH-H = 6.9 Hz, NCH
2
CH
2
OPPh
2
), 2.89 (t, 6H,
, δ, ppm):
3
13
1
meter, with δ referenced to external tetramethylsilane and 85%
J
H-H = 6.9 Hz, NCH
2
CH
2
OPPh
2
). C-{ H} NMR (CDCl
3
1
H PO , respectively. Elemental analysis was carried out with a
142.1 (d, J31P-13C = 18.3 Hz, i-carbons of phenyls), 130.4 (d,
J31P-13C = 22.1 Hz, o-carbons of phenyls), 129.2 (s, p-carbons of
3
4
2
Fisons EA 1108 CHNS-O instrument. Melting points were recorded
using Gallenkamp Model apparatus with open capillaries.
3
phenyls), 128.3 (d, J3
= 7.0 Hz, m-carbons of phenyls), 68.4
1P-13C
(
NCH CH OPPh ), 56.0 (NCH CH OPPh ), assignment was based
2 2 2 2 2 2
1
13
31
1
on the H– C HETCOR spectrum. P-{ H} NMR (CDCl , δ, ppm):
1
Transfer Hydrogenation of Ketones
3
ꢀ
1
13.93 (s, O–PPh ). Selected FT-IR (cm ): 1026 (P–O), 1435
2
A typical procedure for the catalytic hydrogen transfer reaction was
(
P–Ph). Anal. Calcd for C H O P N (%): C, 71.89; H, 6.03; N,
42 42 3 3
6
as follows. A solution of complex [Ru (PPh OC H ) N–PPh (η -p-
3
2
2
4 2
2
2.00. Found (%): C, 71.51; H, 5.85; N, 1.77.
6
cymene) Cl ] (3) or [(RuPPh OC H ) N(η -p-cymene) Cl ] (4)
3
6
2
2
4 3
3
6
Synthesis of 3
(0.005 mmol), NaOH (0.025 mmol) and the corresponding ketone
(0.5 mmol) in degassed 2-propanol (5ml) were refluxed until the re-
6
To a solution of [Ru(η -p-cymene)(μ-Cl)Cl] (0.874 g, 1.43mmol) in
2
actions were completed. Then, a sample of the reaction mixture
was taken off, diluted with acetone and analysed immediately
using GC. The conversions were related to the residual unreacted
ketone. GC analyses were performed using a Shimadzu 2010 Plus
gas chromatograph equipped with a capillary column (5% biphe-
nyl, 95% dimethylsiloxane; 30 m ×0.32 mm × 0.25 μm). The GC
parameters for transfer hydrogenation of ketones were as follows:
initial temperature, 50°C; initial time, hold minimum 1 min; solvent
THF, a solution (THF, 30 ml) of 1 (0.627g, 0.95mmol) was added.
The resulting reaction was allowed to proceed with stirring at room
temperature for 2 h. Then, the solution was filtered and the solvent
evaporated under vacuum. The solid residue thus obtained was
washed with diethyl ether (3× 10 ml) and then dried under
vacuum. Following recrystallization from diethyl ether–CH
2 2
Cl , a
red crystalline powder was obtained. Yield 1.32 g, 88.1%; m.p.
1
1
68–170°C. H NMR (CDCl
3
, δ, ppm): 7.77–7.81 (m, 12H, o-protons
ꢀ
1
delay, 4.48min; temperature ramp, 15°Cmin ; final temperature,
70°C, hold minimum 5 min; final time, 20.67 min; injector port
temperature, 200°C; detector temperature, 200°C, injection volume,
.0 μl.
of O- and N-phenyls), 7.47 (br, 6H, m- and p-protons of N-phenyls),
2
7
1
3
.28–7.34 (m, 12H, m- and p-protons of O-phenyls), 5.23–5.30 (m,
2H, aromatic protons of p-cymene), 3.69 (br, 4H, NCH CH OPPh),
.32 (m, 4H, NCH CH OPPh), 2.55 (m, 3H, CH– of p-cymene), 1.73
–Ph of p-cymene), 1.02 (d, 18H, J= 6.3Hz, (CH
2
2
2
2
2
3
(
s, 9H, CH
of p-cymene). C NMR (CDCl
of phenyls), 132.76, 132.65 (o-carbons of phenyls), 130.77, 130.32
p-carbons of phenyls), 128.34, 127.93 (m-carbons of phenyls),
10.40, 95.93 (quaternary carbons of p-cymene), 90.41, 87.72
(aromatic carbons of p-cymene), 64.73 (NCH CH OPPh ), 48.05
3
3
)
2
CHPh
13
Synthesis of Ligands and Their Ruthenium(II) Complexes
3
, δ, ppm): 141.36, 141.54 (i-carbons
Synthesis of 2-[(diphenylphosphanyl)({2-[(diphenylphosphanyl)oxy]ethyl})amino]-
ethyldiphenylphosphinite (1)
(
1
Chlorodiphenylphosphine (0.66 g, 2.85 mmol) was added dropwise
over a period of 10 min to a solution of diethanolamine (0.10g,
2
2
2
(NCH CH OPPh ), 30.07 (CH– of p-cymene), 21.75 ((CH ) CHPh of
2
2
2
3 2
0
.95 mmol) and triethylamine (0.29 g, 2.85 mmol) in THF (30 ml)
3
p-cymene), 17.43 (CH Ph of p-cymene), assignment was based on
1
13
1
1
31
1
at room temperature with vigorous stirring. The mixture was
stirred at room temperature for 4 h, and then the white precipi-
tate (triethylammonium chloride) was filtered under argon, the
solvent was removed and the remaining part was dried vacuum
the H– C HETCOR, DEPT and H– H COSY spectra. P-{ H} NMR
(CDCl , δ, ppm): 114.4 (s, O–PPh ), 74.56 (s, N–PPh ). FT-IR (KBr,
3
2
2
ꢀ
1
cm ): 953 (P–N), 1030 (P–O), 1436 (P–Ph). Anal. Calcd for
ꢀ
1
C H NP O Ru Cl (1577.3 g mol ) (%): C, 53.31; H, 5.11; N, 0.89.
7
0
80
3
2
3
6
1
to produce a viscous oil of compound 1 (yield: 0.59 g, 94.3%). H
Found (%): C, 53.25; H, 5.04; N, 0.86.
NMR (CDCl , δ, ppm): 7.29–7.50 (m, 30H, o-, m- and p-protons of
3
Synthesis of 4
phenyls), 3.96 (t, J = 6.5 Hz, 2H, NCH
2
CH
). C-{ H} NMR (CDCl
41.68 (i-carbons of phenyls), 132.12, 131.92 (o-carbons of
2
OPPh
2
), 3.40 (t, J = 6.5 Hz,
1
3
1
6
2
1
H, NCH
2
CH
2
OPPh
2
3
, δ, ppm): 141.87,
To a solution of [Ru(η -p-cymene)(μ-Cl)Cl] (0.603 g, 0.99mmol) in
2
THF, a solution (THF, 30 ml) of 2 (0.461g, 0.66mmol) was added.
The resulting reaction was allowed to proceed with stirring at room
temperature for 2 h. Then, the solution was filtered and the solvent
evaporated under vacuum. The solid residue thus obtained was
washed with diethyl ether (3× 10 ml) and then dried under
vacuum. Following recrystallization from diethyl ether–CH Cl , a
phenyls), 130.52, 130.31 (p-carbons of phenyls), 129.27, 128.33 (m-
carbons of phenyls), 68.0 (NCH CH OPPh ), 50.1 (NCH CH OPPh ),
assignment was based on the H– C HETCOR spectrum. P-{ H}
NMR (CDCl , δ, ppm): 111.10 (s, O–PPh ), 60.71 (s, N–PPh ). Selected
FT-IR (cm ): 804 (P–N), 1023 (ꢀP-O), 1434 (P–Ph). Anal. Calcd for
N (%): C, 72.94; H, 5.37; N, 2.13. Found (%): C, 72.83; H,
.46; N, 2.76.
2
1
2
13
2
2
2
31
2
1
3
1
2
2
ꢀ
2
2
C H O P
40 38 2 3
red crystalline powder was obtained. Yield 0.95 g, 89.5%; m.p.
132–134°C. H NMR (CDCl , δ, ppm): 7.81–7.85 (m, 12H, o-protons
1
5
3
of phenyls), 7.28–7.43 (m, 18H, m- and p-protons of phenyls),
Synthesis of 2-[bis({2-[(diphenylphosphanyl)oxy]ethyl})amino]ethyldiphenyl-
phosphinite (2)
5
.22–5.27 (m, 12H, aromatic protons of p-cymene), 3.78 (br, 6H,
NCH CH OPPh), 2.63 (br, 9H, NCH CH OPPh and –CH– of p-cymene),
1.83 (s, 9H, CH –Ph of p-cymene), 1.07 (d, 18H, J=6.1Hz, (CH
2
2
2
2
3
Chlorodiphenylphosphine (0.44 g, 1.98 mmol) was added dropwise
over a period of 10 min to a solution of triethanolamine (0.10 g
3
1
3
)
2
CHPh
3
1
of p-cymene). C NMR (CDCl
i-carbons of phenyls), 132.60 (d, J31P-13C = 11.0 Hz, o-carbons of
phenyls), 130.85 (s, p-carbons of phenyls), 128.05 (d, J31P-13C
3
, δ, ppm): 141.8 (d, J31P-13C = 16.2 Hz,
1
(
(
98%), 0.66 mmol) and triethylamine (0.20g, 1.98mmol) in THF
30 ml) at room temperature with vigorous stirring. The mixture
3
=
was stirred at room temperature for 1 h, and then the white
10.1Hz, m-carbons of phenyls), 111.42, 110.42 (quaternary carbons
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Copyright © 2014 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2014, 28, 803–808

P-based ligands for Ru(II)-catalysed transfer hydrogenation
of p-cymene), 90.54, 87.75 (aromatic carbons of p-cymene), 65.06
the reaction of the commercially available α-phellandrene (5-iso-
[46]
(
2
NCH
1.81 ((CH
assignment was based on the H– C HETCOR, DEPT and H– H
2
CH
2
OPPh
2
), 54.55 (NCH
2
CH
2
OPPh
2
), 30.10 (CH– of p-cymene),
propyl-2-methylcyclohexa-1,3-diene) with RuCl
3
.
As expected,
3 2
) CHPh of p-cymene), 17.58 (CH
3
Ph of p-cymene),
complexation reaction was straightforward, with coordination to
ruthenium being carried out at room temperature. Both of the
ruthenium complexes were readily synthesized in good yields.
1
13
1
1
3
1
1
COSY spectra. P-{ H} NMR (CDCl
FT-IR (KBr, cm ): 1030 (P–O), 1436 (P–Ph). Anal. Calcd for
3
, δ, ppm): 112.49 (s, O–PPh
2
).
ꢀ1
[47]
The initial colour change, i.e. from clear orange to deep red, is
ꢀ
1
C H84NP O Ru Cl
72 3 3 3 6
(1620.3g mol ) (%): C, 53.37; H, 5.23; N, 0.86.
attributed to the dimer cleavage, most probably by the AMPP or
Found (%): C, 53.26; H, 5.19; N, 0.81.
phosphinite ligands. The reaction of 1 or 2 with 3/2 equivalent of
[
Ru(p-cymene)Cl ] affords the corresponding 3 and 4, respectively,
2 2
31
1
as the main products. The P-{ H} NMR spectra are quite consistent
with the structure, namely the spectra of 3 and 4 show resonances
at 114.41 (s, O–PPh ), 74.56 ppm (s, N–PPh ) and 112.49 ppm (s, O–
Results and Discussion
2
2
Synthesis and Characterization of Ruthenium(II) Complexes
[
48,49]
13
PPh ), respectively (SI, Figs. 3a and 4a).
In the C NMR spectra,
2
1
The synthetic procedure for the preparation of the P-based ligands
is shown in Scheme 1. Ligands 1 and 2 were synthesized via hydro-
through-space P–C coupling is observed. Furthermore, the H
NMR spectral data of the complexes are consistent with the pro-
posed structures. The structural composition of the complexes is
also confirmed from FT-IR and elemental analyses (see experi-
mental section). Although single crystals of the complexes can
be obtained by slow diffusion of diethyl ether into a solution of
the compound in dichloromethane over several days, unfortu-
nately we were not able to keep the crystals from rapid decom-
position in air.
3
gen abstraction by a base (Et N) and the subsequent reaction with
three equivalents of Ph PCl in anhydrous THF under argon
2
atmosphere. The ammonium salt was separated by filtration and
the ligands were obtained by extracting the solvent in vacuo in
excellent yields. The progress of this reaction was conveniently
3
1
1
31
1
followed using P-{ H} NMR spectroscopy. The P-{ H} NMR spec-
tra of compounds 1 and 2 show single resonances due to AMPP at
1
11.10 ppm (s, O–PPh
at 113.93 ppm, respectively,
observed for similar compounds.
2
) and 60.71 ppm (s, N–PPh
2
) and phosphinite
[
36–39]
40–44]
in line with the values previously
Catalytic Transfer Hydrogenation of Ketones
[
31
1
Typical P-{ H} NMR spectra
We have reported that Ru(II)–arene complexes, based on ligands
with P–N, P–O and P–N–P backbones, are active catalysts in the
of these ligands are illustrated in the supporting information (SI,
spectra 1.1–1.4). The structures of these compounds are consistent
with the data obtained from a combination of multinuclear NMR
spectroscopy, FT-IR spectroscopy and elemental analysis (for
details, see the experimental section).
[
50]
reduction of various ketones.
The excellent catalytic perfor-
mance and the higher structural variability of phosphinite-based
[
51–53]
transition metal catalysts
prompted us to develop new Ru(II)
[54–59]
6
catalyst systems with well-shaped phosphinite ligands.
The
The reactions of [Ru(η -p-cymene)(μ-Cl)Cl]2 with AMPP and
most important advantage of phosphinite ligands over the corre-
sponding phosphine ligands is the ease of preparation, and by
taking into consideration this feature and the marked versatility of
phosphines, we also modified the tridendate phosphinite ligands
to analogous AMPP ligands. From a practical standpoint, it is of
substantial interest to develop highly efficient AMPP ligands for
phosphinite ligands are shown in Scheme 1. The ability of dimers
6
[
Ru(arene)(μ-Cl)Cl] to form complexes of general formula [Ru(η -ar-
2
[45]
ene)Cl L] is well known.
We examined simple coordination
2
chemistry of 1 and 2 with [Ru(p-cymene)Cl ] . The starting ruthe-
2
2
6
nium(II) complex, [Ru(η -p-cymene)(μ-Cl)Cl] , was synthesized from
2
Table 1. Transfer hydrogenation of acetophenone with isopropanol
catalysed by 3 and 4
Entry
Catalyst
Substrate/
cat./NaOH
Time
Conversion
TOF
(h )
a
ꢀ1 b
(%)
c
1
2
3
4
5
6
3
100:1:5
100:1:5
100:1
1 h
Trace
Trace
<5
—
—
c
4
1 h
d
3
2 h
—
d
4
100:1
2 h
<5
—
e
3
100:1:5
100:1:5
10 min
25 min
99
594
392
e
4
98
a
Determined by GC (three independent catalytic experiments).
b
Referred to the reaction time indicated in column; TOF = (mol
ꢀ
1
product/mol catalyst) × h
At room temperature.
.
Scheme 1. Synthesis of the 2-[(diphenylphosphanyl)({2-[(diphenylphos-
phanyl)oxy]ethyl})amino]ethyldiphenylphosphinite (1) and 2-[bis({2-[(diphenyl-
phosphanyl)oxy]ethyl})amino]ethyldiphenylphosphinite (2) ligands and their
c
d
Refluxing in isopropanol, in the absence of base.
6
6
e
[
Ru
3
(Ph
2
POC
2
H
4
)
2
NPPh
2
(η -p-cymene)
3
Cl
6
] (3) and [(RuPh
2
POC
2
H
4
)
3
N(η -p-
Refluxing in isopropanol.
cymene)
3
Cl ] (4) complexes.
6
Appl. Organometal. Chem. 2014, 28, 803–808
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

N. Meriç et al.
[
60]
6
transfer hydrogenation reactions. Therefore, the catalytic activity
of complexes 3 and 4 in the transfer hydrogenation of aromatic
ketones by isopropanol was investigated. In a typical experiment,
(Table 1, entries 1 and 2) and also the catalytic activity of [Ru(η -p-
2
cymene)(μ-Cl)Cl] under the applied experimental conditions is in-
significant. In addition, as can be inferred from Table 1 (entries 3
and 4), the precatalysts as well as the presence of NaOH are neces-
sary to observe appreciable conversions. The base facilitates the
formation of ruthenium alkoxide by abstracting a proton of the al-
cohol and subsequently alkoxide undergoes β-elimination to give
ruthenium hydride, which is an active species in this reaction. This
is the mechanism proposed by several research groups studying
ruthenium(II)-catalysed transfer hydrogenation reaction by metal
0
.005 mmol of the complex and 0.5mmol of acetophenone were
added to a solution of NaOH in isopropanol (0.005 mmol of NaOH
in 5 ml of isopropanol) and refluxed at 82°C, while the reaction
was monitored using GC.
The complexes were tested as precursors for the catalytic transfer
hydrogenation of acetophenone using isopropanol/NaOH as a re-
ducing system. The results are summarized in Table 1. At room tem-
perature no appreciable formation of 1-phenylethanol is observed
[
61–67]
hydride intermediates.
Specifically, the role of the base is to
generate a more nucleophilic alkoxide ion which would quickly
attack the ruthenium complex responsible for dehydrogenation
of isopropanol. As Table 1 shows, high conversions can be achieved
with these catalytic systems. The results obtained from optimi-
zation studies demonstrate clearly that excellent conversions are
achieved in the reduction of acetophenone to 1-phenylethanol
when 3 and 4 are used as the catalytic precursors, with a
substrate-to-catalyst molar ratio of 100:1 in isopropanol at 82°C
Table 2. Transfer hydrogenation results for substituted acetophenones
with catalyst systems 3 and 4
a
(Table 1, entries 5 and 6). It should also be pointed out that
complexes 3 and 4 are more active catalysts than the correspond-
b
ꢀ1 c
Entry
R
Time
Conversion (%)
TOF (h
)
6
ing precursor [Ru(η -p-cymene)(μ-Cl)Cl] (41% maximum yield in
2
[
68]
24h) with a 1/14 complex/NaOH ratio.
Furthermore, with a
Catalyst: 3
complex/NaOH ratio of 1/5, the complexes are very active leading
to a quantitative transformation of the acetophenone, with a good
1
4-F
5 min
99
98
97
96
94
1176
2
4-Cl
5 min
1176
582
230
376
ꢀ
1
turnover frequency (TOF) of less than 594h
.
3
4-Br
10 min
25 min
15 min
As seen from Table 1, the catalytic activities in the studied hydro-
gen transfer reactions are generally much higher for 3 than for 4.
For example, under identical conditions, transfer hydrogenation
of acetophenone derivatives with 3 leads to 99% conversions
within 10 min, whereas with 4, similar 98% conversions are
achieved only after 25 min (Table 1, entries 5 and 6). Complexes 3
and 4 were widely investigated with different substrates. The cata-
lytic reduction of acetophenone derivatives was tested with the
conditions optimized for acetophenone and the results are summa-
rized in Table 2, which illustrates conversions for the reduction
performed in 0.1M isopropanol solution containing 3 or 4 and
NaOH (ketone/catalyst/NaOH = 100:1:5). Electronic properties of
the substituents on the phenyl ring of the ketone cause the
changes in the reduction rate. An ortho- or para-substituted
acetophenone with an electron-donor substituent, i.e. 2-methoxy
4
2-MeO
4-MeO
5
Catalyst: 4
6
7
8
9
4-F
20 min
20 min
30 min
1.5 h
99
97
97
96
93
297
291
194
64
4-Cl
4-Br
2-MeO
4-MeO
10
1 h
93
a
Catalyst (0.005 mmol), substrate (0.5 mmol), isopropanol (5 ml), NaOH
0.025 mmol%), 82°C, concentration of acetophenone derivatives is
.1 M.
(
0
b
1
Purity of compounds checked by H NMR and GC (three independent
catalytic experiments), yields are based on methylaryl ketone.
c
ꢀ1
TOF = (mol product/mol catalyst) × h
.
Figure 1. Transfer hydrogenation results for substituted alkylphenyl ketones with catalyst system 3. Catalyst (0.005 mmol), substrate (0.5mmol), isopropanol
5ml), NaOH (0.025 mmol%), 82°C, concentration of alkylphenyl ketones is 0.1M.
(
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Appl. Organometal. Chem. 2014, 28, 803–808

P-based ligands for Ru(II)-catalysed transfer hydrogenation
Figure 2. Transfer hydrogenation of various simple ketones with isopropanol catalysed by 4. Refluxing in isopropanol; acetophenone/catalyst/
NaOH = 100:1:5.
or 4-methoxy, is reduced more slowly than acetophenone (Table 2,
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