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Applications of transition metal complexes containing aminophosphine ligand to transfer hydrogenation of ketones.

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Full Paper
Received: 13 March 2010
Revised: 22 September 2010
Accepted: 25 October 2010
Published online in Wiley Online Library: 21 January 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1753
Applications of transition metal complexes
containing aminophosphine ligand to transfer
hydrogenation of ketones
Murat Aydemir∗ , Akın Baysal and Yılmaz Turgut
Hydrogen transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational
simplicity. Reaction of [Ph2 PNHCH2 -C4 H3 S] with [Ru(η6 -benzene)(µ-Cl)Cl]2 , [Rh(µ-Cl)(cod)]2 and [Ir(η5 -C5 Me5 )(µ-Cl)Cl]2 gave
a range of new monodendate complexes [Ru(Ph2 PNHCH2 -C4 H3 S)(η6 -benzene)Cl2 ], 1, [Rh(Ph2 PNHCH2 -C4 H3 S)(cod)Cl], 2, and
[Ir(Ph2 PNHCH2 -C4 H3 S)(η5 -C5 Me5 )Cl2 ], 3, respectively. All new complexes were fully characterized by analytical and spectroscopic
methods. 1 H– 31 P NMR, 1 H– 13 C HETCOR or 1 H– 1 H COSY correlation experiments were used to confirm the spectral assignments.
1–3 are suitable catalyst precursors for the transfer hydrogenation of acetophenone derivatives. Notably [Ru(Ph2 PNHCH2 C4 H3 S)(η6 -benzene)Cl2 ], 1, acts as an excellent catalyst, giving the corresponding alcohols in 98–99% yields in 30 min at 82 ◦ C
(TOF ≤200 h−1 ) for the transfer hydrogenation reaction in comparison to analogous rhodium or iridium complexes. This transfer
c 2011 John Wiley & Sons, Ltd.
hydrogenation is characterized by low reversibility under these conditions. Copyright Supporting information may be found in the online version of this article.
Keywords: aminophosphine; transition metals; transfer hydrogenation; catalysis
Introduction
270
Tertiary phosphines have long been used in the synthesis of
transition metal complexes with catalytic properties.[1] Small
variations in these ligands can cause significant changes in their
coordination behavior and the structural features of the resulting
complexes.[2,3] The use of tertiary phosphines is widespread
in organometallic chemistry and in homogeneous catalysis as
these ligands can be used to fine tune the metal reactivity
and selectivity. The complexes incorporating this ligand type
are of considerable interest because of their potential use in
many processes such as reductive elimination and oxidative
addition for making and breaking C–H bonds,[4,5] formation
and cleavage of N–H and O–H bonds.[6] Also their extensive
use in classical catalytic processes such as allylic alkylation,[7]
Heck,[8,9] Suzuki,[10 – 12] hydrogenation,[13,14] isomerization and
decarbonylation[15,16] cannot be ignored.
Catalytic transfer hydrogenation with the aid of a stable
hydrogen donor is a useful alternative method for catalytic
hydrogenation by molecular hydrogen.[17] Transfer hydrogenation
of ketones by iso-PrOH is convenient in large-scale synthesis since
there is no need to employ a high hydrogen pressure or to
use hazardous reducing agents.[18] Today, the catalytic transfer
hydrogenation[19] of prochiral ketones is one of the most attractive
methods for synthesizing optically active secondary alcohols,
which form an important class of intermediates for fine chemicals
and pharmaceuticals.[20,21] There are several metal sources
available that have to mediate the hydride transfer from the donor
to the substrate. Even if main-group metals like aluminum have
historically been used in the transfer hydrogenation reactions,[22,23]
today’s catalysts of choice are transition metal complexes
predominantly of ruthenium, rhodium[24] or iridium.[25 – 27]
As part of our research program we report here the
synthesis and full characterization of three aminophos-
Appl. Organometal. Chem. 2011, 25, 270–275
phine complexes [Ru(Ph2 PNHCH2 -C4 H3 S)(η6 -benzene)Cl2 ], 1,
[Rh(Ph2 PNHCH2 -C4 H3 S)(cod)Cl], 2, and [Ir(Ph2 PNHCH2 -C4 H3 S)(η5 C5 Me5 )Cl2 ], 3, and their catalytic evaluation in the transfer
hydrogenation of acetophenone derivatives.
Experimental
Materials and Methods
Unless otherwise stated, all reactions were carried out under an atmosphere of argon using conventional Schlenk
glassware. Solvents were dried using established procedures
and distilled under argon immediately prior to use. Analytical grade and deuterated solvents were purchased from
Merck. PPh2 Cl and thiophene-2-methylamine are purchased
from Fluka and were used as received. The starting materials thiophene-2-(N-diphenylphosphino)methylamine (to be published elsewere), [Ru(η6 -benzene)(µ-Cl)Cl]2 ,[28] Rh(µ-Cl)(cod)]2 [29]
and [Ir(η5 -C5 Me5 )(µ-Cl)Cl]2 [30] were prepared according to literature procedures. The IR spectra were recorded on a Mattson 1000
ATI Unicam FT-IR spectrometer as KBr pellets. 1 H (400.1 MHz), 13 C
NMR (100.6 MHz) and 31 P–{1 H} NMR spectra (162.0 MHz) were
recorded on a Bruker Avance 400 spectrometer, with δ referenced
to external TMS and 85% H3 PO4 respectively. Elemental analysis
was carried out on a Fisons EA 1108 CHNS-O instrument. Melting
points were recorded by Gallenkamp Model apparatus with open
capillaries.
∗
Correspondence to: Murat Aydemir, Dicle University, Department of Chemistry,
TR-21280 Diyarbakır, Turkey. E-mail: aydemir@dicle.edu.tr
Dicle University, Department of Chemistry, TR-21280 Diyarbakır, Turkey
c 2011 John Wiley & Sons, Ltd.
Copyright Transition metal complexes containing aminophosphine ligand
GC Analyses
Synthesis of [Ir(Ph2 PNHCH2 -C4 H3 S)(η5 -C5 Me5 )Cl2 ], (3)
GC analyses were performed on a HP 6890N gas chromatograph
equipped with a capillary column (5% biphenyl, 95% dimethylsiloxane; 30 m × 0.32 mm × 0.25 µm). The GC parameters were as
follows for transfer hydrogenation of ketones: initial temperature,
110 ◦ C; initial time, 1 min; solvent delay, 4.48 min; temperature
ramp 80 ◦ C/min; final temperature, 200 ◦ C; final time, 21.13 min;
injector port temperature, 200 ◦ C; detector temperature, 200 ◦ C,
injection volume, 2.0 µl.
A mixture of [Ir(η5 -C5 Me5 )(µ-Cl)Cl]2 (0.203 g, 0.244 mmol) and
[Ph2 PNHCH2 -C4 H3 S] (0.145 g, 0.488 mmol) in 20 ml of tetrahydrofuran was stirred at room temperature for 2 h. The volume of
the solvent was then reduced to 0.5 ml before addition of diethyl
ether (10 ml). The precipitated product was filtered off and dried
in vacuo yielding 3 as an orange microcrystalline solid (Scheme 1).
Yield 312 mg, 92%, m.p. = 204–206 ◦ C. 1 H NMR (400.1 MHz, CDCl3 )
δ = 7.95–7.90 (m, 4H, o-protons of phenyls), 7.63–7.42 (m, 6H, mand p- protons of phenyls), 7.11 (dd, 1H, 3 J = 4.0 and 4 J = 1.9 Hz,
H-5), 6.85–6.83 (m, 2H, H-4 and H-3), 3.94 (dd, 2H, 3 J = 5.8
and 6.4 Hz, -CH2 -), 3.12 (m, 1H, NH), 1.42 (d, 15H, 4 J = 2.0 Hz,
CH3 of Cp∗ (C5 Me5 ); 13 C NMR (100.6 MHz, CDCl3 ): δ = 8.25
(C5 Me5 ), 42.92 (d, 2 J = 9.0 Hz, -CH2 -), 92.42 (C5 Me5 ), 124.13
(C-5), 124.62 (C-4), 126.65 (C-3), 128.96 (d, 3 J = 10.1 Hz, m-carbons
of phenyls), 130.89 (d, 4 J = 3.0 Hz, p-carbons of phenyls), 131.44
(d, 1 J = 61.4 Hz, i-carbons of phenyls), 133.43 (d, 2 J = 12.2 Hz,
o-carbons of phenyls), 143.42 (d, 3 J = 8.0 Hz, C-2); assignment was
based on the 1 H– 13 C HETCOR and 1 H– 1 H COSY spectra; 31 P–{1 H}
NMR (162 MHz, CDCl3 ): δ = 34.1 (s); IR, (KBr): υ = 3344 (N–H), 1035
(P–N), 1438 (P–Ph) cm−1 . C27 H31 NSPIrCl2 (695.7 g/mol): calcd C
46.61, H 4.49, N 2.01; found C 46.48, H 4.41, N 1.93.
Syntheses of Transition Metal Complexes
Synthesis of [Ru(Ph2 PNHCH2 -C4 H3 S)(η6 -benzene)Cl2 ], (1)
A mixture of [Ru(η6 -benzene)(µ-Cl)Cl]2 (0.122 g, 0.244 mmol) and
[Ph2 PNHCH2 -C4 H3 S] (0.145 g, 0.488 mmol) in 20 ml of tetrahydrofuran was stirred at room temperature for 2 h. The volume of
the solvent was then reduced to 0.5 ml before addition of diethyl
ether (10 ml). The precipitated product was filtered off and dried
in vacuo yielding 1 as a red microcrystalline powder (Scheme 1).
Yield 245 mg, 92%, m.p. = 186–188 ◦ C. 1 H NMR (400.1 MHz, CDCl3 )
δ = 7.96 (dd, 4H, 3 J = 6.8 and 8.0 Hz, o-protons of phenyls),
7.52–7.46 (m, 6H, m- and p- protons of phenyls), 7.09 (d, 1H,
3 J = 4.8 Hz, H-5), 6.78–6.72 (m, 2H, H-4 and H-3), 5.41 (d, 6H,
3
J = 4.8 Hz, aromatic protons of benzene), 3.88 (dd, 2H, 3 J = 6.2
and 6.4 Hz, -CH2 -), 3.56 (m, 1H, NH); 13 C NMR (100.6 MHz, CDCl3 ):
δ = 42.27 (-CH2 -), 88.78 (aromatic carbon of benzene), 124.40 (C5), 124.95 (C-3), 126.62 (C-4), 128.46 (d, 3 J = 10.4 Hz, m-carbons of
phenyls), 131.12 (s, p-carbons of phenyls), 132.68 (d, 2 J = 10.3 Hz,
o-carbons of phenyls), 134.0 (d, 1 J = 53.9 Hz, i-carbons of phenyls),
143.05 (C-2); assignment was based on the 1 H– 13 C HETCOR and
1
H– 1 H COSY spectra; 31 P–{1 H} NMR (162 MHz, CDCl3 ): δ = 61.7
(s); IR, (KBr): υ = 1066 (P–N), 1438 (P–Ph), 3344 (N–H) cm−1 ;
C23 H22 NPSRuCl2 (547.5 g/mol): calcd C 50.46, H 4.05, N 2.56; found
C 50.32, H 3.97, N 2.46.
General procedure for the transfer hydrogenation of ketones
Typical procedure for the catalytic hydrogen transfer reaction: a
solution of complexes [Ru(Ph2 PNHCH2 -C4 H3 S)(η6 -benzene)Cl2 ], 1,
[Rh(Ph2 PNHCH2 -C4 H3 S)(cod)Cl], 2, or [Ir(Ph2 PNHCH2 -C4 H3 S)(η5 C5 Me5 )Cl2 ], 3 (0.005 mmol), KOH (0.025 mmol) and the corresponding ketone (0.5 mmol) in degassed iso-PrOH (5 ml) were
refluxed for 30 min for 1, and 2 h for 2 and 3. After this period a
sample of the reaction mixture was taken off, diluted with acetone and analyzed immediately by GC. Conversions obtained are
related to the residual unreacted ketone.
Results and Discussion
Synthesis and Characterization of the Complexes
A mixture of [Rh(µ-Cl)(cod)]2 (0.123 g, 0.244 mmol) and
[Ph2 PNHCH2 -C4 H3 S] (0.145 g, 0.488 mmol) in 20 ml of tetrahydrofuran was stirred at room temperature for 2 h. The volume of
the solvent was then reduced to 0.5 ml before addition of diethyl
ether (10 ml). The precipitated product was filtered off and dried
in vacuo, yielding 2 as a yellow microcrystalline solid (Scheme 1).
Yield 234 mg, 88%, m.p. = 214 ◦ C (dec.). 1 H NMR (400.1 MHz,
CDCl3 ) δ = 7.99 (dd, 4H, 3 J = 7.4 and 11.8 Hz, o-protons of
phenyls), 7.63–7.42 (m, 6H, m- and p- protons of phenyls), 7.18
(d, 1H, 3 J = 4.8 Hz, H-5), 6.91–6.82 (m, 2H, H-4 and H-3), 5.57
(br, 4H, CH of cod), 4.33 (m, 1H, NH), 3.98 (dd, 2H, 3 J = 5.2
and 6.0 Hz, -CH2 -), 2.42 (br, 4H, CH2 of cod), 2.06 (br, 4H, CH2 of
cod); 13 C NMR (100.6 MHz, CDCl3 ): δ = 28.59 (CH2 of cod), 42.38
(-CH2 -), 105.26 (CH of cod), 124.51 (C-5), 124.96 (C-3), 126.67 (C-4),
128.40 (d, 3 J = 9.7 Hz, m-carbons of phenyls), 130.7 (s, p-carbons
of phenyls), 133.4 (d, 2 J = 12.2 Hz, o-carbons of phenyls), 132.3
(d, 1 J = 46.3 Hz, i-carbons of phenyls), 143.04 (C-2); assignment
was based on the 1 H– 13 C HETCOR and 1 H– 1 H COSY spectra;
31 P–{1 H} NMR (162 MHz, CDCl ): δ = 60.6 (d, 1 J (103 Rh– 31 P) =
3
158.8 Hz); IR, (KBr): υ = 3454 (N–H), 1093 (P–N), 1432 (P–Ph)
cm−1 . C25 H28 NSPRhCl (543.9 g/mol): calcd C 55.21, H 5.19, N 2.58;
found C 55.09, H 5.11, N 2.48.
The ligand thiophene-2-(N-diphenylphosphino)methylamine
[Ph2 PNHCH2 -C4 H3 S] was synthesized (rhe details of this preparation will be described elsewhere: M. Aydemir, A. Baysal, S. Özkar, L. T.
Yıldırım, to be submitted for publication) successfully from a commercially available starting material thiophene-2-methylamine
and an equivalent of PPh2 Cl in the presence of triethylamine
by aminolysis at 0 ◦ C. The 31 P NMR spectrum of [Ph2 PNHCH2 C4 H3 S] showed single resonance at δ(P) 41.70 ppm, similar to
those found for closely related compounds.[31] Thiophene-2(N-diphenylphosphino)methylamine is not stable and decomposes rapidly on exposure to air or moisture. When the reactions were monitored by 31 P NMR spectroscopy, the formation of P(O)Ph2 PPh2 was observed, as indicated by signals
at δ 34.60 (d) ppm and δ −24.30 (d) ppm, {1 J(PP) 226 Hz}.
Solution of thiophene-2-(N-diphenylphosphino)methylamine in
CDCl3 , prepared under anaerobic conditions, is also unstable and decomposes gradually to give oxide and
bis(diphenylphosphino)monoxide [P(O)Ph2 PPh2 ] derivatives. The
reaction between [Ru(η6 -benzene)(µ-Cl)Cl]2 with two equivalents
of [Ph2 PNHCH2 -C4 H3 S] in tetrahydrofuran led to a stable and
neutral P-coordinated ruthenium(II) complex [Ru(Ph2 PNHCH2 C4 H3 S)(η6 -benzene)Cl2 ], 1 in 92% yield (Scheme 1). This ligand
Appl. Organometal. Chem. 2011, 25, 270–275
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
271
Synthesis of [Rh(Ph2 PNHCH2 -C4 H3 S)(cod)Cl], (2)
M. Aydemir, A. Baysal and Y. Turgut
Scheme 1. Synthesis of the [Ph2 PNHCH2 -C4 H3 S] and its [Ru(Ph2 PNHCH2 -C4 H3 S)(η6 -benzene)Cl2 ], 1, [Rh(Ph2 PNHCH2 -C4 H3 S)(cod)Cl], 2 and [Ir(Ph2 PNHCH2 C4 H3 S)(η5 -C5 Me5 )Cl2 ], 3 complexes. (i) 1 equiv. Ph2 PCl, 1 equiv. Et3 N, thf; (ii) 1/2 equiv. [Ru(η6 -benzene)(µ-Cl)Cl]2, thf; (iii) 1/2 equiv. [Rh(µ-Cl)(cod)]2 , thf;
(iv) 1/2 equiv. [Ir(η5 -C5 Me5 )(µ-Cl)Cl]2 , thf.
272
was expected to cleave the [Ru(η6 -benzene)(µ-Cl)Cl]2 dimer to
give the corresponding complex via monohapto coordination of
the aminophosphine group. This complex is highly soluble in
CH2 Cl2 and slightly soluble in hexane and it can be crystallized
from CH2 Cl2 –hexane solution. The chemical purity of the complex
1 was confirmed by a single 31 P–{1 H} NMR signal at 61.7 ppm
(Supporting Information, Fig. SI.1). Analysis of complex 1 by 1 H
NMR exhibits signals corresponding to the aromatic rings for 1 at
7.96–7.46 ppm. The 1 H NMR spectrum of complex 1 displays the
(CH2 ) resonance as a doublet of doublet signal at 3.88 ppm (2H)
and the C6 H6 protons as a doublet at 5.41 ppm (d, 3 J = 4.8 Hz). In
the 13 C NMR spectrum of 1, the CH2 carbon signal was observed
at 42.27 ppm and the C6 H6 carbon resonance was occurred at
88.78 (s) ppm. Furthermore in the 13 C–{1 H} NMR spectrum of
1, J(31 P– 13 C) coupling constants of the carbons of the phenyl
rings were observed, which are consistent with the literature
values[32 – 34] (for details see Experimental section). The structural
composition of complex 1 was further confirmed by IR spectroscopy and microanalysis, and found to be in good agreement
with the theoretical values.
We also examined some simple coordination chemistry of
[Ph2 PNHCH2 -C4 H3 S] with [Rh(µ-Cl)(cod)]2 and [Ir(η5 -C5 Me5 )(µCl)Cl]2 precursors. Reactions of [Ph2 PNHCH2 -C4 H3 S] with Rh(µCl)(cod)]2 and [Ir(η5 -C5 Me5 )(µ-Cl)Cl]2 in tetrahydrofuran in a
ratio of 1 : 1/2 at room temperature for 2 h gave microcrystalline precipitate of neutral complexes 2 and 3, respectively.
Complexation reactions, with coordination to rhodium or iridium,
were carried out at room temperature easily. Bridge cleavage of
the dimer [Rh(µ-Cl)(cod)]2 with [Ph2 PNHCH2 -C4 H3 S] gave the
mononuclear compound [Rh(Ph2 PNHCH2 -C4 H3 S)(cod)Cl], 2, in
good yield. The coordination of the ligand through the P donor
was confirmed by the 31 P–{1 H} NMR spectroscopy. The spectrum
recorded in deuterated chloroform at room temperature shows a
doublet centered at δ(P) 60.6 ppm with a J(P−Rh) = 158.8 Hz[35 – 37]
wileyonlinelibrary.com/journal/aoc
(see Supporting Information, Fig. S1). IR spectroscopy and
elemental analysis of product 2 are consistent with the suggested
molecular formula. 1 H and 13 C NMR spectra of compound 2 display
all the signals of coordinated ligands. The 1 H NMR spectrum 2
displays the anticipated multiplets at δ 7.99–7.42 ppm for the
protons of phenyls, broad singlets at δ 5.57, 2.42 and 2.06 ppm for
cod protons, a broad multiplet at 4.33 ppm for NH and a doublet of
doublet at 3.98 ppm for -CH2 NH- [dd, 2H, 3 J = 5.2 and 6.0 Hz]. In
the 13 C–{1 H} NMR spectrum of compound 2, J(31 P– 13 C) coupling
constants of the carbons of the phenyl rings were observed,
which are consistent with the literature values[38] (for details see
Experimental section).
In the 31 P–{1 H} NMR, a singlet at 34.1 ppm was assigned
to compound [Ir(Ph2 PNHCH2 -C4 H3 S)(η5 -C5 Me5 )Cl2 ], 3, in line
with the values previously observed for similar compounds.[39,40]
Analysis by 1 H NMR reveals this compound to be diamagnetic,
exhibiting signals corresponding to the aromatic rings for 3 at
7.95–7.42 ppm. Another signal consisting of a doublet centered
at 1.42 ppm is due to the presence of the methyl protons
in the Cp∗ group; this information is complemented by the
presence of signals at 3.94 and 3.12 ppm due to the CH2 and
NH groups. Furthermore in the 13 C–{1 H} NMR spectrum of 3,
J(31 P– 13 C) coupling constants of the carbons of the phenyl
rings was observed. The structure of 3 was further confirmed
by IR spectroscopy and microanalysis, and found to be in good
agreement with the theoretical values.
Catalytic Transfer Hydrogenation of Acetophenone Derivatives
Recently, we have reported that the novel half-sandwich complexes, based on ligands with P–N and P–O backbone, are active
catalysts in the reduction of aromatic ketones.[41 – 43] The observed
activity of these complexes has encouraged us to investigate other
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 270–275
Transition metal complexes containing aminophosphine ligand
Scheme 2. Hydrogen transfer from iso-PrOH to acetophenone.
Table 1. Transfer hydrogenation of acetophenone with iso-PrOH catalyzed by [Ru(Ph2 PNHCH2 -C4 H3 S)(η6 -benzene)Cl2 ], 1, [Rh(Ph2 PNHCH2 C4 H3 S)(cod)Cl], 2 and [Ir(Ph2 PNHCH2 -C4 H3 S)(η5 -C5 Me5 )Cl2 ], 3
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Catalyst
S:C:KOH
Time
Conversion (%)i
TOF(h−1 )k
1a
2a
3a
1b
2b
3b
1c
2c
3c
1d
2d
3d
1e
2e
3e
1f
2f
3f
1g
2g
3g
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1
100 : 1
100 : 1
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
500 : 1 : 5
500 : 1 : 5
500 : 1 : 5
1000 : 1 : 5
1000 : 1 : 5
1000 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
1 h
1 h
1 h
1 h
1 h
1 h
8 h
36 h
36 h
6 h
20 h
20 h
2 h
8 h
8 h
3.5 h
15 h
15 h
30 min
2 h
2 h
<1
<1
<1
<5
<5
<5
98
98
97
98
99
98
98
98
97
97
98
98
99
99
98
....
....
....
....
....
....
12
<3
<3
16
<5
<5
49
12
12
28
<7
<7
198
50
49
Reaction conditions: a At room temperature; acetophenone–Ru–KOH,
100 : 1 : 5. b Refluxing in iso-PrOH; acetophenone–Ru, 100 : 1, in
the absence of base. c With addition of 0.1 ml of H2 O.
d Refluxing the reaction in air. e Refluxing in iso-PrOH; acetophenone–Ru–KOH, 500 : 1 : 5. f Refluxing in iso-PrOH; acetophenone–Ru–KOH, 1000 : 1 : 5. g Refluxing in iso-PrOH; acetophenone–Ru–KOH, 100 : 1 : 5. i Determined by GC (three independent
catalytic experiments). k Referred at the reaction time indicated in
column; TOF = [mol product/mol Ru(II)Cat.] × h−1 .
Appl. Organometal. Chem. 2011, 25, 270–275
Entry
R
Cat: Ru(II) complex, 1
1
4-F
2
4-Cl
3
4-Br
4
2-MeO
5
4-MeO
Cat: Rh(I) complex, 2
6
4-F
7
4-Cl
8
4-Br
9
2-MeO
10
4-MeO
Cat: Ir(III) complex, 3
11
4-F
12
4-Cl
13
4-Br
14
2-MeO
15
4-MeO
Time
30
30
30
30
30
min
min
min
min
min
Conversion (%)b
TOF (h−1 )c
99
98
98
97
95
198
196
196
194
190
2
2
2
2
2
h
h
h
h
h
99
98
96
95
93
50
49
48
48
47
2
2
2
2
2
h
h
h
h
h
98
97
96
94
92
49
49
48
47
46
a
Catalyst (0.005 mmol), substrate (0.5 mmol), iso-PrOH (5 ml), KOH
(0.025 mmol %), 82 ◦ C, 30 min for 1 and 2 h for 2 and 3, respectively;
the concentration of acetophenone derivatives is 0.1 M. b Purity of
compounds is checked by NMR and GC (three independent catalytic
experiments); yields are based on methyl aryl ketone. c TOF = (mol
product/mol catalyst) × h−1 .
presence of KOH are necessary to observe appreciable conversions. The base facilitates the formation of alkoxide by abstracting
proton of the alcohol and subsequently alkoxide undergoes βelimination to give ruthenium hydride, which is an active species
in this reaction. This is the mechanism proposed by several workers on the studies of ruthenium catalyzed transfer hydrogenation
reaction by metal hydride intermediates.[47,48] In addition, performing the reaction in the presence of air or with the addition
of water slowed the reaction but did not affect conversion of the
product.
As shown in Table 1, increasing the substrate-to-catalyst ratio
do not lower the conversions of the product in most cases.
Remarkably, the transfer hydrogenation of acetophenone could
be achieved up to 98% conversion even when the substrate-tocatalyst ratio reached to 1000 : 1 (Table 1). When the reaction
temperature was increased to 82 ◦ C, smooth reduction of
acetophenone into 1-phenylethanol occured, with conversion
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
273
analogous ligands and other transition metal complexes of these
ligands.
Complex with sp3 -hybribized nitrogens containing N–H bonds
displays higher reaction rate. On the other hand, the presence
of N–H group in the ligands makes it possible to stabilize a sixmembered cyclic transition state by forming a hydrogen bond
with the oxygen atom of ketones.[44,45] In this context, complexes
1–3 were selected as catalysts, iso-PrOH–KOH as the reducing
system, and acetophenone as a model substrate (Scheme 2) and
the results are listed in Table 1.
iso-PrOH is the conventional hydrogen source having favourable
properties; it is stable, easy to handle (bp 82 ◦ C), non-toxic,
environmentally friendly, inexpensive and the acetone product
is readily removable.[46]
At room temperature no appreciable formation of 1phenylethanol was observed (Table 1, entries 1–3). As can be
inferred from Table 1 (entries 4–6), the catalysts as well as the
Table 2. Transfer hydrogenation results for substituted acetophenones with the catalyst systems prepared from [Ph2 PNHCH2 -C4 H3 S]
and [Ru(η6 -benzene)(µ-Cl)Cl]2 , [Rh(µ-Cl)(cod)]2 and [Ir(η5 -C5 Me5 )(µCl)Cl]2 a
M. Aydemir, A. Baysal and Y. Turgut
Table 3. Transfer hydrogenation results for substituted alkyl phenyl ketones with the catalyst systems prepared from [Ph2 PNHCH2 -C4 H3 S] and
[Ru(η6 -benzene)(µ-Cl)Cl]2 , [Rh(µ-Cl)(cod)]2 and [Ir(η5 -C5 Me5 )(µ-Cl)Cl]2 a
Conversion (%)b
TOF (h−1 )c
1.5
5
97
98
65
20
3
1
2
5
2.5
8
97
95
94
19
38
12
6
7
8
3
1
2
8
4
11
96
96
95
12
24
<10
9
10
11
3
1
2
11
6
16
94
91
90
<10
15
<6
12
3
16
92
<6
Entry
Catalyst
Time (h)
1
2
1
2
3
4
5
Substrate
Product
◦
a
Catalyst (0.005 mmol), substrate (0.5 mmol), iso-PrOH (5 ml), KOH (0.025 mmol%), 82 C, respectively; the concentration of alkyl phenyl ketones is
0.1 M. b Purity of compounds is checked by NMR and GC (three independent catalytic experiments); yields are based on methyl aryl ketone. c TOF =
(mol product/mol cat.) × h−1 .
274
ranging from 98 to 99% after 30 min for 1 and 2 h for 2 and
3 of the reaction. From the results observed, it is noteworthy
that the complexes 1–3 display the differences in reactivities,
with a complex–KOH ratio of 1 : 5. Complex 1 is the most active,
quantitatively converting acetophenone with a moderate TOF
of 200 h−1 . From these preliminary results, it can be concluded
that the η6 -benzene–ruthenium complex, 1, is more effective than
those of the cod–Rh(I), 2 and Cp∗ –Ir(III), 3 complexes. Furthermore,
the decreasing the amount of base leads to the deactivation of the
catalyst. It should be pointed out that complex 1 is far more active
than the corresponding precursor: [Ru(η6 -benzene)(µ-Cl)Cl]2 (37%
maximum yield in 24 h).[49]
In order to investigate the evolution of the catalysts 1–3,
31 P–{1 H}-NMR spectra were recorded periodically after the
catalytic reaction. The singlet observed at 21.56 ppm at the end of
the third day (see Supporting Information, Fig. S2) in the spectrum
is corresponding to hydrolysis product diphenylphosphinous acid,
Ph2 P(O)H.[50 – 52]
The catalytic reduction of acetophenone derivatives was all
tested with the conditions optimized for acetophenone and the
results are summarized in Table 2. The fourth column in Table 2
illustrates conversions of the reduction performed in a 0.1 M of
iso-PrOH solution containing 1–3 and KOH (Ketone : Ru : KOH =
100 : 1 : 5).
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. An ortho- or para- substituted
acetophenone with an electron-donor substituent, i.e. 2-methoxy
wileyonlinelibrary.com/journal/aoc
or 4-methoxy is reduced more slowly than acetophenone (Table 2,
entries 4, 5, 9, 10, 14 and 15).[53] In addition, the introduction of
electron withdrawing substituents, such as F, Cl and Br 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 hydrogenation.[54,55] We also carried out
further experiments to investigate the effect of bulkiness of the
alky groups on the catalytic activity and the results were given
in Table 3 (entries 1–12). A variety of simple aryl alkyl ketones
were transformed to the corresponding secondary alcohols. It was
found that the activity is highly dependent on the steric bulk of
the alkyl group. The reactivity gradually decreased by increasing
the bulkiness of the alkyl groups.[56,57] That is to say, when the
size of the alkyl group increased, the activity was remarkable
decreased.[58,59]
Conclusions
In summary, we have synthesized a series of selected transitionmetal [Ru(II), Rh(I), Ir(III)] complexes based thiophene-2-(Ndiphenylphosphino)methylamine monodendate ligand. We found
that these complexes are efficient homogeneous catalytic systems
that can be readily implemented and lead to secondary alcohols
from good to excellent yields. Ru(II)–aminophosphine complex
showed stronger catalytic activity in the transfer hydrogenation
reaction than the analogous Rh(I) and Ir(III) complexes. The
modular construction of these catalysts and their flexibility toward
transfer hydrogenation make these systems promising, and future
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 270–275
Transition metal complexes containing aminophosphine ligand
investigations are aiming at the development of an asymmetric
version of this process.
Acknowledgment
Partial support from Dicle University (project number: DÜAPK
05-FF-27) is gratefully acknowledged.
Supporting information
Supporting information may be found in the online version of this
article.
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