Hydrogenation of acetophenone and its derivatives with 2-propanol using aminomethylphosphine-ruthenium catalysis

Article abstract of DOI:10.1080/10426500902754278

The reaction of 3′-aminopropyltriethoxysilane with the phosphonium salt ([Ph₂P(CH₂OH)₂]Cl) gives an aminomethylphosphine-type ligand ((CH₃CH₂O) ₃Si(CH2)3N(CH₂PPh₂)

Full text of DOI:10.1080/10426500902754278

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Phosphorus, Sulfur, and Silicon and the
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Hydrogenation of Acetophenone and
Its Derivatives with 2-Propanol Using
Aminomethylphosphine-Ruthenium
Catalysis
a
b
b
Mustafa Keleş , Tuğba Keleş , Osman Serindağ , Sedat Yaşar c &
İsmail Özdemir c
a Department of Chemistry , Osmaniye Korkut Ata University ,
Osmaniye, Turkey
b Department of Chemistry , University of Çukurova , Adana, Turkey
c Department of Chemistry , University of Inonu , Malatya, Turkey
Published online: 28 Dec 2009.
To cite this article: Mustafa Keleş , Tuğba Keleş , Osman Serindağ , Sedat Yaşar & İsmail
Özdemir (2009) Hydrogenation of Acetophenone and Its Derivatives with 2-Propanol Using
Aminomethylphosphine-Ruthenium Catalysis, Phosphorus, Sulfur, and Silicon and the Related
Elements, 185:1, 165-170
To link to this article: http://dx.doi.org/10.1080/10426500902754278
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Phosphorus, Sulfur, and Silicon, 185:165–170, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902754278
HYDROGENATION OF ACETOPHENONE AND ITS
DERIVATIVES WITH 2-PROPANOL USING
AMINOMETHYLPHOSPHINE-RUTHENIUM CATALYSIS
Mustafa Keles¸,1 Tug˘ba Keles¸,2 Osman Serindag˘,2 Sedat Yas¸ar,3
3
˙
¨
and Ismail Ozdemir
1Department of Chemistry, Osmaniye Korkut Ata University, Osmaniye, Turkey
2Department of Chemistry, University of C¸ ukurova, Adana, Turkey
3Department of Chemistry, University of Inonu, Malatya, Turkey
The reaction of 3ꢀ-aminopropyltriethoxysilane with the phosphonium salt ([Ph2P(CH2OH)2]
Cl) gives an aminomethylphosphine-type ligand ((CH3CH2O)3Si(CH2)3N(CH2PPh2)2). Re-
action of the ligand ((CH3CH2O)3Si(CH2)3N(CH2PPh2)2) with silica gives silica supported
aminomethylphosphine ligand (SiO2)-O-Si(CH2)3N(CH2PPh2)2). The ligands were refluxed
with [RuCl2(p-cymene)]2 in toluene to give [RuCl2((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)]
and silica-supported SiO2)-O-[RuCl2((Si(CH2)3N(CH2PPh2)2)2] complex, respectively. The
catalytic studies show that the complexes [RuCl2((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)]
and SiO2)-O-[RuCl2((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)2] are very active catalysts for
the transfer hydrogenation of acetophenone by 2-propanol in basic media. The best
yield was observed in hydrogenation of m-methoxy acetophenone (95%) with catalyst
[RuCl2((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)].
Keywords Acetophenone; aminomethylphosphine; hydrogenation; silica
INTRODUCTION
In a homogeneous system, the use of transition metal complexes for catalysis has
many advantages in organic synthesis.1 The problems of homogeneous systems are sepa-
ration and purification of heavy metals from the product. These problems of homogeneous
catalysis systems restrict their application in organic synthesis.1,2 Recently immobilizations
of catalysts to solid supports are able to combine the advantages of homogenous and het-
erogeneous catalysis to overcome these problems.3 Solid-supported catalysts are readily
separated from the product, can be easily removed from the reaction mixture by filtration,
and can be reused in subsequent reactions.4
Solid-supported catalysts have been studied by many researchers to compare transi-
tion metal complexes in homogeneous systems and heterogeneous systems due to higher
catalytic activity under the same reaction parameters.1,5 Phosphine functionalized Ru(II)
complexes have been used as catalysts to generate highly enantioselective products for
Received 18 December 2008; accepted 14 January 2009.
We would like to thank Research Fund of C¸ ukurova University for supporting this project.
Address correspondence to Mustafa Keles¸, Department of Chemistry, Osmaniye Korkut Ata University,
80000 Osmaniye, Turkey. E-mail: myskeles@hotmail.com
165

166
M. KELES¸ ET AL.
many asymmetric organic process such as the reduction of prochiral ketones to synthesize
biologically active compounds.6,7
Catalytic transfer hydrogenation with the aid of a stable hydrogen donor is a useful
alternative method for catalytic hydrogenation by molecular hydrogen.8 In transfer hydro-
genation, organic molecules such as primary and secondary alcohols9 or formic acid and its
salts10 have been employed as the hydrogen source. The use of a hydrogen donor has some
advantages over the use of molecular hydrogen, since it avoids the risks and the constraints
associated with hydrogen gas as well as the necessity for pressure vessels and other equip-
ment. Although metal catalyzed transfer hydrogenation using a stable H-donor has been
found to be reliable, the current emphasis on cleaner methods for chemical transformations
requires high selectivity, low cost, and minimum waste production.
Transition-metal catalyzed transfer hydrogenation using 2-propanol as a hydrogen
source has become an efficient method in organic synthesis as illustrated by several useful
applications reported in recent years.11 The features for this important process are relatively
mild conditions and environmentally friendly processes. The most commonly used catalysts
for this reaction are ruthenium (II) complexes, but some rhodium and iridium derivatives
have also been used.12
In this article, we report the synthesis of silica supported and unsupported NꢁN-
bis(diphenylphosphinomethyl)aminopropyltriethoxysilane ligand and its Ru(II) complex.
These complexes applied the reduction of derivative ketones as catalyst to compare catalytic
activity of homogenized and heterogenized catalysts.
RESULTS AND DISCUSSION
Synthesis and Characterization
Tertiary diphosphine ((CH3CH2O)3Si(CH2)3N(CH2PPh2)2) (1) was synthesized us-
ing a Mannich reaction by treating ([Ph2P(CH2OH)2]Cl) with the appropriate primary
amine. The silica-supported (SiO2)-O-Si(CH2)3N(CH2PPh2)2 ligand (3) was synthe-
sized by modification of the method in the literature.13 The Ru((II) complexes of the
bis(diphenylphosphinomethyl)amino ligands were prepared using [RuCl2(p-cymene)]2
complex as shown in Scheme 1.
The 13C NMR spectrum of ligand 3 showed that signals appeared at 7.8 ( SiCH2),
23.7 (SiCH2CH2),and 60.9 (CH2CH2N) ppm were assigned to SiCH2CH2CH2N group.
The carbon atoms of OCH2CH3 signals were occurred at 58.1 and 19.0 ppm, respectively.
The signal at 62.9 ppm was assigned to P CH2 N carbon atom. All these spectra were
consistent as reported in the literature.14,15
Complex 2 was obtained by reacting 2 equivalents of 1 with 1 equivalent of [Ru(p-
cymene)Cl2]2 under reflux for 6 h in toluene. The product was purified with acetone. The
31P NMR signal of 1 was observed at −28.2 ppm and shifted to 27.1 ppm when it reacted
with [Ru(p-cymene)Cl2]2. Silica-supported complex 4 was synthesized by reacting ligand
3 with [Ru(p-cymene)Cl2]2 in toluene and refluxing for 12 h. The product was purified with
toluene and acetone. 31P NMR signal of 3 shifted from −27.0 to 26.3 when reacted with
[RuCl2(p-cymene)]2.
It was found that 31P NMR signals of two complexes showed more shielded signals
compared with the uncoordinated aminomethylphosphine ligands. The coordination shift
values of the complexes (ꢀ), as varied depending on the metal centers and the chemical
structures of the ligands, showed that the ligands were bound to Ru(II) via a P-P bidentate to

HYDROGENATION OF ACETOPHENONE
167
Scheme 1
give chelated complexes rather than Ru-N interaction.16,17 Based on the elemental analysis
of complex 2, the ligand to metal ratio was found to be 2:1 having chlorides placed in the
coordination sphere (Scheme 1).
Hydrogenation of Acetophenone and Its Derivatives
The catalytic activity of the Ru(II) complexes was tested on the hydrogenation of
aromatic ketones giving secondary alcohols (Scheme 2). Complexes 2 or 4 (0.01 mmol),
2-propanol (10 mmol), t-BuOK (5 mmol%), and the ketones (1 mmol) each were introduced
into a Schlenk tube under an argon atmosphere. The resulting solution was heated at 80◦C
for 12 h. The hydrogenation results of acetophenone and its derivatives with the catalysts 2 or
4 are listed in Table I. The yield was affected by the reaction conditions as well as properties
of the substituents of the ketones. With catalyst 2, the ketones were hydrogenated in very
high yield. The m-methoxy acetophenone was reduced in a 95% yield, and o-methoxy and
p-methoxy acetophenone were reduced to secondary alcohols with 90% and 85% yield,
Scheme 2

168
M. KELES¸ ET AL.
Table I Hydrogenation of acetophenone and its derivatives with Ru(II) complexes with 2-propanol
Entry
1
Substrate
Product
Catalyst Yield (%)a,b TOFc
2
86
717
2
3
4
2
82
90
683
750
4
5
4
2
92
95
767
791
6
7
4
2
93
85
775
708
8
9
4
2
83
75
692
625
10
11
4
2
73
72
608
600
12
13
4
2
63
72
525
600
14
4
65
542
aCatalyst (0.01 mmol), substrate (1 mmol), iPrOH (10 mL), KOBut (5 mmol %), 80◦C, 12 h.
bPurity of compounds is checked by NMR and GC; yields are based on methyl aryl ketone.
cTOF: (mol substrate/mol cat.)x percent/h.
respectively (Table I). The ketones having electron-withdrawing groups such as 2-fluoro,
2-chloro, and 2-bromo acetophenone reduced to secondary alcohols with 75%, 72%, and
72% yields, respectively.
All the ketones shown above were also investigated with the silica-supported Ru(II)
complex 4. By reducing the acetophenone and its derivatives to secondary alcohols, catalyst
4 exhibited small differences compared to the catalyst 2. With catalyst 4, m-methoxy
acetophenone was reduced in high yield (93%) compared with o-methoxy acetophenone
(92%) and p-methoxy acetophenone (85%) as catalyst 2. All these results, as described
in Table I, showed that ketones having electron-donating or electron-withdrawing groups

HYDROGENATION OF ACETOPHENONE
169
influenced the catalytic reaction.18,19 Ketones that have methoxy group were reduced in high
yields in the presence of catalysts 2 or 4 compared with electron-withdrawing substituted
ketones (e.g., fluoro, chloro, or bromo). The results are consistent with the reported data.20
It is notable that solid-supported Ru(II) complex 4 exhibits small differences when
compared to unsupported Ru(II) complex 2 as catalyst. However based on the TOF, the
solid-supported complex 4 could have reuse potential to the same catalytic reaction without
losing its activity.
CONCLUSIONS
Two complexes of Ru(II) with bidentate tertiary bis(diphenylphosphinomethyl)amino
ligands have been synthesized and characterized using spectroscopic techniques. The 31P
NMR spectra of the complexes indicate that coordination of the aminomethylphosphine
ligand occurs via two phosphorus atoms. The complexes have been tested as catalysts for
hydrogenation of acetophenone. The results show that the homogeneous catalyst show
higher catalytic activity than silica supported catalyst.
EXPERIMENTAL
Apparatus
Elemental analysis was performed on a LECO CHNS 932. FT-IR spectra were
recorded in 4000–450 cm−1 range using a Perkin-Elmer RX1 FT-IR system as KBr pellets.
The 13C NMR and 31P NMR spectra were recorded at 25◦C in DMSO-d6 and CDCl3 using
a Varian Mercury 200 MHz NMR spectrometer. 31P NMR spectra were recorded with com-
plete proton decoupling and are reported in ppm using 85% H3PO4 as external standard.
Solid NMR spectra were recorded using a solid NMR Bruker superconducting FTNMR
spectrometer, model Avance TN 300 MHz WB.
General Procedures
All reactions were carried out under nitrogen atmosphere using conventional Schlenk
glassware. The solvents were dried using established procedures and then were immediately
distilled under nitrogen atmosphere prior to use.
(CH3CH2O)3Si(CH2)3N(CH2PPh2)2 (1). [Ph2P(CH2OH)2]Cl (0.6 g, 2.0 mmol)
was dissolved in degassed 2:1 H2O MeOH (20 mL) solvent system, and triethylamine (NEt3)
(0.5 mL, 1.7 mmol) was added to this solution and then 3ꢁ-aminopropyltriethoxysilane
(1.0 mmol) and acetone 5 mL were added and refluxed for 4 h. The product was extracted
with dichloromethane and crystallized at −20◦C over 24 h. The solvent was removed in
vacuum. Yield 0.50 g (78%). Calc. for (CH3CH2O)3Si(CH2)3N(CH2PPh2)2: C, 68.1; H,
7.3; N, 2.3%; Anal. Found: C, 67.9; H, 7.4; N, 2.2%. FT-IR (KBr, cm−1) 2974, 1640, 1433,
1164, 1078, 749–695. 31P NMR (CDCI3, 25◦C): δ −28.2 [PPh2] ppm.
[Ru((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)Cl2] (2). Ligand 1 (1.0 g, 1.6 mmol)
was added to a stirred solution of [RuCl2(p-cymene)] (0.2 g, 0.65 mmol) in toluene (5 mL).
The mixture was stirred under reflux for a further 6 h. Addition of diethylether (15 mL)
gave the red solid, which was then filtered and dried in vacuum. Yield 0.37 g (72%). Calc.
for [Ru((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)Cl2]: C, 53.2; H, 5.8; N, 1.7%; Anal. Found:
C, 53.1; H, 5.6; N, 1.2%. FT-IR (KBr, cm−1) 3051, 2924, 1651, 1435, 1096, 738–695. 31
P
NMR (CDCI3, 25◦C): δ 27.1 [Ru-PPh2] ppm.

170
M. KELES¸ ET AL.
Silica-Aupported (CH3CH2O)3Si(CH2)3N(CH2PPh2)2 (3). [Ph2P(CH2OH)2]Cl
(0.57 g, 2 mmol) was dissolved in degassed 2:1 H2O MeOH (20 mL) solvent system, and
triethylamine (NEt3) (0.5 mL, 1.75 mmol) was added to this solution. Silica-supported
3ꢁ-aminopropyltriethoxysilane (2g) and CH2Cl2 (10 mL) were added and refluxed for 6
h. The product was purified with acetone and dried in vacuum. Yield 2.3 g (75%). FT-IR
(KBr, cm−1) 3035, 2945, 1579, 1483, 1161, 1078, 797–690. 13C NMR (25◦C): 7.5–7.7,
18.6–18.7, 23.7, 55.4, 56.5, 58.1–59.8, 60.9–60.6, 128.3–133.7. 31P NMR (CDCI3, 25◦C):
δ −27.0 [PPh2] ppm.
Solid-Supported [Ru((CH3CH2O)3Si(CH2)3N(CH2PPh2)2)Cl2] (4). Ligand 3
(2.0 g) was added to a stirred solution of [RuCl2(p-cymene)] (1.2 g, 1.96 mmol) in toluene
(5 mL). The mixture was stirred and refluxed further 12 h. The product was purified with
CH2Cl2 (10 mL) and dried in vacuum. Yield 2.24 g (75%). FT-IR (KBr, cm−1) 3053, 2919,
1664, 1436, 1120, 1098, 743–695. 31P-NMR (CDCI3, 25◦C): δ 26.2 ppm.
Typical Procedure for the Transfer Hydrogenation of Ketones
The complexes (2–4) (0.01 mmol), 2-propanol (10 mL), t-BuOK (5 mmol%), and the
substrate (1 mmol) were introduced into a Schlenk tube under argon. The resulting solution
was heated at 80◦C for 12 h. The solvent was then removed under reduced pressure and
purified by flash chromatography (hexane:ethyl acetate, 10:1). Product distribution was
determined by 1H NMR spectroscopy and GC.
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