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

Article abstract of DOI:10.1002/aoc.1753

Hydrogen transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational simplicity. Reaction of [Ph₂PNHCH₂-C₄H₃S] with [Ru(ν⁶-benzene)(μ-Cl)Cl]₂, [Rh(μ-Cl)(cod)] ₂ and [Ir(ν⁵-C₅Me₅)(μ-Cl)Cl] ₂ gave a range of new monodendate complexes [Ru(Ph ₂PNHCH₂-C₄H₃S)(ν⁶- benzene)Cl₂], 1, [Rh(Ph₂PNHCH₂-C ₄H₃S)(cod)Cl], 2, and [Ir(Ph₂PNHCH ₂-C₄H₃S)(ν⁵-C₅Me ₅)Cl₂], 3, respectively. All new complexes were fully characterized by analytical and spectroscopic methods. ¹H- ³¹P NMR, ¹H-¹³C HETCOR or ¹H- ¹H 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(Ph₂PNHCH₂-C ₄H₃S)(ν⁶-benzene)Cl₂], 1, acts as an excellent catalyst, giving the corresponding alcohols in 98-99% yields in 30 min at 82 °C (TOF a200 h;bsupesup) for the transfer hydrogenation reaction in comparison to analogous rhodium or iridium complexes. This transfer hydrogenation is characterized by low reversibility under these conditions.

Full text of DOI:10.1002/aoc.1753

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 [Ph2PNHCH2-C4H3S] with [Ru(η6-benzene)(µ-Cl)Cl]2, [Rh(µ-Cl)(cod)]2 and [Ir(η5-C5Me5)(µ-Cl)Cl]2 gave
a range of new monodendate complexes [Ru(Ph2PNHCH2-C4H3S)(η6-benzene)Cl2], 1, [Rh(Ph2PNHCH2-C4H3S)(cod)Cl], 2, and
[Ir(Ph2PNHCH2-C4H3S)(η5-C5Me5)Cl2],3,respectively.Allnewcomplexeswerefullycharacterizedbyanalyticalandspectroscopic
methods. 1H–31P NMR, 1H–13C HETCOR or 1H–1H 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(Ph2PNHCH2-
C4H3S)(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)forthetransferhydrogenation reaction in comparison to analogousrhodium oriridium complexes. Thistransfer
c
hydrogenation is characterized by low reversibility under these conditions. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: aminophosphine; transition metals; transfer hydrogenation; catalysis
Introduction
phine complexes [Ru(Ph2PNHCH2-C4H3S)(η6-benzene)Cl2], 1,
[Rh(Ph2PNHCH2-C4H3S)(cod)Cl], 2, and [Ir(Ph2PNHCH2-C4H3S)(η5-
C5Me5)Cl2], 3, and their catalytic evaluation in the transfer
hydrogenation of acetophenone derivatives.
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
hydrogenationbymolecularhydrogen.[17] Transferhydrogenation
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
historicallybeenusedinthetransferhydrogenationreactions,[22,23]
today’s catalysts of choice are transition metal complexes
predominantly of ruthenium, rhodium[24] or iridium.[25–27]
Experimental
Materials and Methods
Unless otherwise stated, all reactions were carried out un-
der an atmosphere of argon using conventional Schlenk
glassware. Solvents were dried using established procedures
and distilled under argon immediately prior to use. Ana-
lytical grade and deuterated solvents were purchased from
Merck. PPh2Cl and thiophene-2-methylamine are purchased
from Fluka and were used as received. The starting materi-
als thiophene-2-(N-diphenylphosphino)methylamine (to be pub-
[29]
lished elsewere), [Ru(η6-benzene)(µ-Cl)Cl]2,[28] Rh(µ-Cl)(cod)]2
[30]
and [Ir(η5-C5Me5)(µ-Cl)Cl]2
were prepared according to litera-
ture procedures. The IR spectra were recorded on a Mattson 1000
ATI Unicam FT-IR spectrometer as KBr pellets. 1H (400.1 MHz), 13
C
NMR (100.6 MHz) and 31P–{ H} NMR spectra (162.0 MHz) were
recorded on a Bruker Avance 400 spectrometer, with δ referenced
to external TMS and 85% H3PO4 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.
1
∗
Correspondence to: Murat Aydemir, Dicle University, Department of Chemistry,
TR-21280 Diyarbakır, Turkey. E-mail: aydemir@dicle.edu.tr
As part of our research program we report here the
synthesis and full characterization of three aminophos-
Dicle University, Department of Chemistry, TR-21280 Diyarbakır, Turkey
c
Appl. Organometal. Chem. 2011, 25, 270–275
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Transition metal complexes containing aminophosphine ligand
GC Analyses
Synthesis of [Ir(Ph2PNHCH2-C4H3S)(η5-C5Me5)Cl2], (3)
A mixture of [Ir(η5-C5Me5)(µ-Cl)Cl]2 (0.203 g, 0.244 mmol) and
[Ph2PNHCH2-C4H3S] (0.145 g, 0.488 mmol) in 20 ml of tetrahydro-
furan 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).
Yield312 mg, 92%, m.p. = 204–206◦C. 1HNMR(400.1 MHz, CDCl3)
δ = 7.95–7.90 (m, 4H, o-protons of phenyls), 7.63–7.42 (m, 6H, m-
and p- protons of phenyls), 7.11 (dd, 1H, 3J = 4.0 and 4J = 1.9 Hz,
H-5), 6.85–6.83 (m, 2H, H-4 and H-3), 3.94 (dd, 2H, 3J = 5.8
and 6.4 Hz, -CH2-), 3.12 (m, 1H, NH), 1.42 (d, 15H, 4J = 2.0 Hz,
CH3 of Cp∗ (C5Me5); 13C NMR (100.6 MHz, CDCl3): δ = 8.25
(C5Me5), 42.92 (d, 2J = 9.0 Hz, -CH2-), 92.42 (C5Me5), 124.13
(C-5), 124.62 (C-4), 126.65 (C-3), 128.96 (d, 3J = 10.1 Hz, m-carbons
of phenyls), 130.89 (d, 4J = 3.0 Hz, p-carbons of phenyls), 131.44
GC analyses were performed on a HP 6890N gas chromatograph
equipped with a capillary column (5% biphenyl, 95% dimethyl-
siloxane; 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.
Syntheses of Transition Metal Complexes
Synthesis of [Ru(Ph2PNHCH2-C4H3S)(η6-benzene)Cl2], (1)
A mixture of [Ru(η6-benzene)(µ-Cl)Cl]2 (0.122 g, 0.244 mmol) and
[Ph2PNHCH2-C4H3S] (0.145 g, 0.488 mmol) in 20 ml of tetrahydro-
furan 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 microc◦rystalline powder (Scheme 1).
Yield245 mg, 92%, m.p. = 186–188 C. 1HNMR(400.1 MHz, CDCl3)
δ = 7.96 (dd, 4H, 3J = 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,
3J = 4.8 Hz, H-5), 6.78–6.72 (m, 2H, H-4 and H-3), 5.41 (d, 6H,
3J = 4.8 Hz, aromatic protons of benzene), 3.88 (dd, 2H, 3J = 6.2
and 6.4 Hz, -CH2-), 3.56 (m, 1H, NH); 13C NMR (100.6 MHz, CDCl3):
δ = 42.27 (-CH2-), 88.78 (aromatic carbon of benzene), 124.40 (C-
5), 124.95 (C-3), 126.62 (C-4), 128.46 (d, 3J = 10.4 Hz, m-carbons of
phenyls), 131.12 (s, p-carbons of phenyls), 132.68 (d, 2J = 10.3 Hz,
o-carbonsofphenyls), 134.0(d, 1J = 53.9 Hz, i-carbonsofphenyls),
143.05 (C-2); assignment was based on the 1H–13C HETCOR and
1
2
(d, J = 61.4 Hz, i-carbons of phenyls), 133.43 (d, J = 12.2 Hz,
o-carbons of phenyls), 143.42 (d, 3J = 8.0 Hz, C-2); assignment was
1
based on the 1H–13C HETCOR and 1H–1H COSY spectra; 31P–{ H}
NMR(162 MHz, CDCl3): δ = 34.1 (s);IR, (KBr): υ = 3344(N–H), 1035
(P–N), 1438 (P–Ph) cm−1. C27H31NSPIrCl2 (695.7 g/mol): calcd C
46.61, H 4.49, N 2.01; found C 46.48, H 4.41, N 1.93.
General procedure for the transfer hydrogenation of ketones
Typical procedure for the catalytic hydrogen transfer reaction: a
solution of complexes [Ru(Ph2PNHCH2-C4H3S)(η6-benzene)Cl2], 1,
[Rh(Ph2PNHCH2-C4H3S)(cod)Cl], 2, or [Ir(Ph2PNHCH2-C4H3S)(η5-
C5Me5)Cl2], 3 (0.005 mmol), KOH (0.025 mmol) and the corre-
sponding 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 ace-
tone and analyzed immediately by GC. Conversions obtained are
related to the residual unreacted ketone.
1
1H–1H COSY spectra; 31P–{ H} NMR (162 MHz, CDCl3): δ = 61.7
(s); IR, (KBr): υ = 1066 (P–N), 1438 (P–Ph), 3344 (N–H) cm−1
;
C23H22NPSRuCl2 (547.5 g/mol): calcd C 50.46, H 4.05, N 2.56; found
C 50.32, H 3.97, N 2.46.
Results and Discussion
Synthesis of [Rh(Ph2PNHCH2-C4H3S)(cod)Cl], (2)
Synthesis and Characterization of the Complexes
A
mixture of [Rh(µ-Cl)(cod)]2 (0.123 g, 0.244 mmol) and
The ligand thiophene-2-(N-diphenylphosphino)methylamine
[Ph2PNHCH2-C4H3S] (0.145 g, 0.488 mmol) in 20 ml of tetrahy-
drofuran 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.). 1H NMR (400.1 MHz,
CDCl3) δ = 7.99 (dd, 4H, 3J = 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, 3J = 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, 3J = 5.2
and 6.0 Hz, -CH2-), 2.42 (br, 4H, CH2 of cod), 2.06 (br, 4H, CH2 of
cod); 13C 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, 3J = 9.7 Hz, m-carbons of phenyls), 130.7 (s, p-carbons
[Ph2PNHCH2-C4H3S] was synthesized (rhe details of this prepara-
tionwillbedescribedelsewhere:M.Aydemir,A.Baysal,S.Ozkar,L.T.
¨
Yıldırım, to be submitted for publication) successfully from a com-
mercially available starting material thiophene-2-methylamine
and an equivalent of PPh2Cl in the presence of triethylamine
by aminolysis at 0 ◦C. The 31P NMR spectrum of [Ph2PNHCH2-
C4H3S] 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 decom-
poses rapidly on exposure to air or moisture. When the re-
actions were monitored by 31P NMR spectroscopy, the for-
mation of P(O)Ph2PPh2 was observed, as indicated by signals
1
at δ 34.60 (d) ppm and δ −24.30 (d) ppm, { J(PP) 226 Hz}.
Solution of thiophene-2-(N-diphenylphosphino)methylamine in
CDCl3, prepared under anaerobic conditions, is also un-
stable and decomposes gradually to give oxide and
bis(diphenylphosphino)monoxide [P(O)Ph2PPh2] derivatives. The
reaction between [Ru(η6-benzene)(µ-Cl)Cl]2 with two equivalents
of [Ph2PNHCH2-C4H3S] in tetrahydrofuran led to a stable and
2
of phenyls), 133.4 (d, J = 12.2 Hz, o-carbons of phenyls), 132.3
1
(d, J = 46.3 Hz, i-carbons of phenyls), 143.04 (C-2); assignment
was based on the 1H–13C HETCOR and 1H–1H COSY spectra;
1
31P–{ H} NMR (162 MHz, CDCl3): δ = 60.6 (d, 1J (103Rh–31P) =
158.8 Hz); IR, (KBr): υ = 3454 (N–H), 1093 (P–N), 1432 (P–Ph)
cm−1. C25H28NSPRhCl (543.9 g/mol): calcd C 55.21, H 5.19, N 2.58; neutral P-coordinated ruthenium(II) complex [Ru(Ph2PNHCH2-
found C 55.09, H 5.11, N 2.48.
C4H3S)(η6-benzene)Cl2], 1 in 92% yield (Scheme 1). This ligand
c
Appl. Organometal. Chem. 2011, 25, 270–275
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Aydemir, A. Baysal and Y. Turgut
Scheme 1. Synthesisofthe[Ph2PNHCH2-C4H3S]andits[Ru(Ph2PNHCH2-C4H3S)(η6-benzene)Cl2],1,[Rh(Ph2PNHCH2-C4H3S)(cod)Cl],2and[Ir(Ph2PNHCH2-
C4H3S)(η5-C5Me5)Cl2], 3 complexes. (i) 1 equiv. Ph2PCl, 1 equiv. Et3N, 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-C5Me5)(µ-Cl)Cl]2, thf.
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
CH2Cl2 and slightly soluble in hexane and it can be crystallized
from CH2Cl2 –hexane solution. The chemical purity of the complex
(see Supporting Information, Fig. S1). IR spectroscopy and
elemental analysis of product 2 are consistent with the suggested
molecularformula. 1Hand13CNMRspectraofcompound2display
all the signals of coordinated ligands. The 1H 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, 3J = 5.2 and 6.0 Hz]. In
1
1 was confirmed by a single 31P–{ H} NMR signal at 61.7 ppm
1
(Supporting Information, Fig. SI.1). Analysis of complex 1 by H
NMR exhibits signals corresponding to the aromatic rings for 1 at
7.96–7.46 ppm. The 1H NMR spectrum of complex 1 displays the
(CH2) resonance as a doublet of doublet signal at 3.88 ppm (2H)
and the C6H6 protons as a doublet at 5.41 ppm (d, 3J = 4.8 Hz). In
the 13C NMR spectrum of 1, the CH2 carbon signal was observed
at 42.27 ppm and the C6H6 carbon resonance was occurred at
1
the 13C–{ H} NMR spectrum of compound 2, J(31P–13C) coupling
constants of the carbons of the phenyl rings were observed,
which are consistent with the literature values[38] (for details see
Experimental section).
1
In the 31P–{ H} NMR, a singlet at 34.1 ppm was assigned
to compound [Ir(Ph2PNHCH2-C4H3S)(η5-C5Me5)Cl2], 3, in line
with the values previously observed for similar compounds.[39,40]
Analysis by 1H 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
1
88.78 (s) ppm. Furthermore in the 13C–{ H} NMR spectrum of
1, J(31P–13C) 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 spec-
troscopy and microanalysis, and found to be in good agreement
with the theoretical values.
We also examined some simple coordination chemistry of
[Ph2PNHCH2-C4H3S] with [Rh(µ-Cl)(cod)]2 and [Ir(η5-C5Me5)(µ-
Cl)Cl]2 precursors. Reactions of [Ph2PNHCH2-C4H3S] with Rh(µ-
Cl)(cod)]2 and [Ir(η5-C5Me5)(µ-Cl)Cl]2 in tetrahydrofuran in a
ratio of 1 : 1/2 at room temperature for 2 h gave micro-
crystalline 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 [Ph2PNHCH2-C4H3S] gave the
mononuclear compound [Rh(Ph2PNHCH2-C4H3S)(cod)Cl], 2, in
good yield. The coordination of the ligand through the P donor
1
NH groups. Furthermore in the 13C–{ H} NMR spectrum of 3,
J(31P–13C) 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 Deriva-
tives
Recently, we have reported that the novel half-sandwich com-
plexes, 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
1
was confirmed by the 31P–{ 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]
c
wileyonlinelibrary.com/journal/aoc
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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 acetophenonewith iso-PrOH cat-
alyzedby[Ru(Ph2PNHCH2-C4H3S)(η6-benzene)Cl2],1,[Rh(Ph2PNHCH2-
C4H3S)(cod)Cl], 2 and [Ir(Ph2PNHCH2-C4H3S)(η5-C5Me5)Cl2], 3
Table 2. Transfer hydrogenation results for substituted acetophe-
nones with the catalyst systems prepared from [Ph2PNHCH2-C4H3S]
and [Ru(η6-benzene)(µ-Cl)Cl]2, [Rh(µ-Cl)(cod)]2 and [Ir(η5-C5Me5)(µ-
a
Cl)Cl]2
Entry Catalyst S:C:KOH
Time
1 h
Conversion (%)i TOF(h−1 k
)
1
1a
2a
3a
1b
2b
3b
1c
2c
3c
1d
2d
3d
1e
2e
3e
1f
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1
<1
<1
<1
<5
<5
<5
98
98
97
98
99
98
98
98
97
97
98
98
99
99
98
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
12
2
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
Entry
R
Time
Conversion (%)b
TOF (h−1 c
)
4
5
100 : 1
Cat: Ru(II) complex, 1
6
100 : 1
1
2
3
4
5
4-F
30 min
30 min
30 min
30 min
30 min
99
98
98
97
95
198
196
196
194
190
7
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
4-Cl
8
<3
<3
16
4-Br
9
2-MeO
4-MeO
10
11
12
13
14
15
16
17
18
19
20
21
<5
<5
49
Cat: Rh(I) complex, 2
6
4-F
2 h
2 h
2 h
2 h
2 h
99
98
96
95
93
50
49
48
48
47
7
4-Cl
12
8
4-Br
12
9
2-MeO
4-MeO
1000 : 1 : 5 3.5 h
1000 : 1 : 5 15 h
1000 : 1 : 5 15 h
28
2f
<7
<7
198
50
10
3f
Cat: Ir(III) complex, 3
1g
2g
3g
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
30 min
11
12
13
14
15
4-F
2 h
2 h
2 h
2 h
2 h
98
97
96
94
92
49
49
48
47
46
4-Cl
2 h
2 h
4-Br
49
2-MeO
4-MeO
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 H2O.
d Refluxing the reaction in air. e Refluxing in iso-PrOH; ace-
tophenone–Ru–KOH, 500 : 1 : 5. f Refluxing in iso-PrOH; acetophe-
none–Ru–KOH, 1000 : 1 : 5. g Refluxing in iso-PrOH; acetophe-
none–Ru–KOH, 100 : 1 : 5. i Determined by GC (three independent
catalytic experiments). k Referred at the reaction time indicated in
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
.
column; TOF = [mol product/mol Ru(II)Cat.] × h−1
.
presence of KOH are necessary to observe appreciable conver-
sions. The base facilitates the formation of alkoxide by abstracting
proton of the alcohol 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 work-
ers on the studies of ruthenium catalyzed transfer hydrogenation
reaction by metal hydride intermediates.[47,48] In addition, per-
forming the reaction in the presence of air or with the addition
of water slowed the reaction but did not affect conversion of the
product.
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 six-
membered 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.
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-to-
catalyst 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
iso-PrOHistheconventionalhydrogensourcehavingfavourable
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 1-
phenylethanol was observed (Table 1, entries 1–3). As can be
inferred from Table 1 (entries 4–6), the catalysts as well as the
c
Appl. Organometal. Chem. 2011, 25, 270–275
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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M. Aydemir, A. Baysal and Y. Turgut
Table 3. Transfer hydrogenation results for substituted alkyl phenyl ketones with the catalyst systems prepared from [Ph2PNHCH2-C4H3S] and
a
[Ru(η6-benzene)(µ-Cl)Cl]2, [Rh(µ-Cl)(cod)]2 and [Ir(η5-C5Me5)(µ-Cl)Cl]2
Entry
Catalyst
Time (h)
Substrate
Product
Conversion (%)b
TOF (h−1 c
)
1
2
1
2
1.5
5
97
98
65
20
3
4
5
3
1
2
5
2.5
8
97
95
94
19
38
12
6
7
8
3
1
2
8
4
96
96
95
12
24
11
<10
9
3
1
2
11
6
94
91
90
<10
15
10
11
16
<6
12
3
16
92
<6
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
.
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
thoseofthecod–Rh(I),2andCp∗ –Ir(III),3complexes.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
thanthecorrespondingprecursor:[Ru(η6-benzene)(µ-Cl)Cl]2 (37%
maximum yield in 24 h).[49]
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]
In order to investigate the evolution of the catalysts 1–3,
1
31P–{ 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,
Ph2P(O)H.[50–52]
Conclusions
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).
Asalreadystated, electronicproperties(thenatureandposition)
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
In summary, we have synthesized a series of selected transition-
metal [Ru(II), Rh(I), Ir(III)] complexes based thiophene-2-(N-
diphenylphosphino)methylaminemonodendateligand.Wefound
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
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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.
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Acknowledgment
¨
Partial support from Dicle University (project number: DUAPK
05-FF-27) is gratefully acknowledged.
¨
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