Novel cyclohexyl-based aminophosphine ligands and use of their Ru(II) complexes in transfer hydrogenation of ketones

Article abstract of DOI:10.1002/aoc.3096

Two new aminophosphines - furfuryl-(N-dicyclohexylphosphino)amine, [Cy ₂PNHCH₂-C₄H₃O] (1) and thiophene-(N-dicyclohexylphosphino)amine, [Cy₂PNHCH ₂-C₄H₃S] (2) - were prepared by the reaction of chlorodicyclohexylphosphine with furfurylamine and thiophene-2-methylamine. Reaction of the aminophosphines with [Ru(η⁶-p-cymene)(μ-Cl)Cl] ₂ or [Ru(η⁶-benzene)(μ-Cl)Cl]₂ gave corresponding complexes [Ru(Cy₂PNHCH₂-C₄H ₃O)(η⁶-p-cymene)Cl₂] (1a), [Ru(Cy ₂PNHCH₂-C₄H₃O)(η⁶- benzene)Cl₂] (1b), [Ru(Cy₂PNHCH₂-C ₄H₃S)(η⁶-p-cymene)Cl₂] (2a) and [Ru(Cy₂PNHCH₂-C₄H₃S) (η⁶-benzene)Cl₂] (2b), respectively, which are suitable catalyst precursors for the transfer hydrogenation of ketones. In particular, [Ru(Cy₂PNHCH₂-C₄H ₃S)(η⁶-benzene)Cl₂] acts as a good catalyst, giving the corresponding alcohols in 98-99% yield in 30 min at 82 °C (up to time of flight ≤ 588 h^-1).

Full text of DOI:10.1002/aoc.3096

Full Paper
Received: 25 September 2013
Revised: 12 October 2013
Accepted: 20 October 2013
Published online in Wiley Online Library: 15 January 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3096
Novel cyclohexyl-based aminophosphine
ligands and use of their Ru(II) complexes
in transfer hydrogenation of ketones
Cezmi Kayana*, Nermin Meriça, Murat Aydemira, Yusuf Selim Ocakb,
Akın Baysala and Hamdi Temelc,d
Two new aminophosphines – furfuryl-(N-dicyclohexylphosphino)amine, [Cy2PNHCH2–C4H3O] (1) and thiophene-(N-dicyclohexy-
lphosphino)amine, [Cy2PNHCH2–C4H3S] (2) – were prepared by the reaction of chlorodicyclohexylphosphine with furfurylamine
and thiophene-2-methylamine. Reaction of the aminophosphines with [Ru(η6-p-cymene)(μ-Cl)Cl]2 or [Ru(η6-benzene)(μ-Cl)Cl]2
gave corresponding complexes [Ru(Cy2PNHCH2–C4H3O)(η6-p-cymene)Cl2] (1a), [Ru(Cy2PNHCH2–C4H3O)(η6-benzene)Cl2]
(1b), [Ru(Cy2PNHCH2–C4H3S)(η6-p-cymene)Cl2] (2a) and [Ru(Cy2PNHCH2–C4H3S)(η6-benzene)Cl2] (2b), respectively,
which are suitable catalyst precursors for the transfer hydrogenation of ketones. In particular, [Ru(Cy2PNHCH2–C4H3S)(η6-
benzene)Cl2] acts as a good catalyst, giving the corresponding alcohols in 98–99% yield in 30min at 82°C (up to time of
flight≤ 588 hꢀ1). Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: aminophosphine; ruthenium(II); transfer hydrogenation; homogeneous catalysis
transformations.[21] Complexes including this type of ligand are of
considerable interest because of their potential use in many pro-
Introduction
Transition-metal catalyzed processes for transfer hydrogenation
of a wide variety of functional groups by different hydrogen do-
nors are interesting alternatives to molecular hydrogenation.[1–3]
The hydrogenation of ketones, which is one of the most exciting
and powerful methods of synthesizing alcohols, has been receiv-
ing increased attention and has led to extraordinary success.[4,5]
In particular, the catalytic transfer hydrogenation of ketones[6] is
one of the most attractive methods for synthesizing secondary
alcohols, which form an important class of intermediates for
fine chemicals and pharmaceuticals.[7,8] The hydrogen transfer
from 2-propanol to ketones is an elegant and economic method
owing to its operational simplicity, since no hydrogen gas is
required, enabling the avoidance of high-pressure equipment, as
well as many favorable properties of the reductant;[9,10] for
example, it is stable, easy to handle, non-toxic and inexpensive.[11]
Moreover, when 2-propanol is used as a hydrogen donor, the only
side product is acetone, which is easily removed by distillation
during workup.[12,13]
cesses such as reductive elimination and oxidative addition for mak-
ing and breaking C–H bonds,[22,23] formation and cleavage of N–H
and O–H bonds.[24] Furthermore, their extensive use in classical cat-
alytic processes such as allylic alkylation,[25] Mizoroki–Heck,[26,27]
Suzuki–Miyaura,[28,29]
hydrogenation,[30,31]
isomerization,
decarbonylation, etc.[32,33] cannot be underrated.
In recent years, we have reported that the novel half-sandwich
complexes, based on ligands with a P–N backbone, are active
catalysts in the reduction of aromatic ketones.[34–36] By choosing
an appropriate ligand system it is possible to fine tune the
electronic and steric properties of the donor centers to bind to
metals in the desired oxidation state with a specific coordination
number so that a catalyst may be generated for a specific organic
transformation. In this context, aminophosphines play a major
role in catalysis because of their easy and high-yielding synthetic
methodology as well as their flexible reactivity toward transition
metals in their various oxidation states.[37–41] The above consi-
derations prompted us to launch a research program on the
The influence of ligand type on structure and reactivity of
transition metal complexes is an important topic of research in
coordination and organometallic chemistry.[14] Ligands contai-
ning direct phosphorus–nitrogen bonds are quite attractive since
structural modifications can be introduced via simple P–N bond
formation[15,16] and they have proved to be versatile ligands;
changing the substituents on both P- and N-centers leads to
alterations in the P–NH angle and conformation around the
P-centers.[17,18] Small modifications in these ligands can cause
striking changes in their coordination behavior and the structural
properties of the resulting complexes.[19,20] Synthesis of new
aminophosphines to stabilize transition metals in low valent
states is considered to be a most challenging task in view of
their potential use in a variety of metal-mediated organic
*
Correspondence to: Cezmi Kayan, Department of Chemistry, Dicle University,
TR-21280 Diyarbakır, Turkey. Email: cezmik@dicle.edu.tr
a Department of Chemistry, Faculty of Science, Dicle University, 21280
Diyarbakir, Turkey
b Department of Science, Faculty of Education, Dicle University, 21280 Diyarbakir,
Turkey
c
Department of Chemistry, Faculty of Education, Dicle University, 21280
Diyarbakir, Turkey
d Science and Technology Application and Research Center, Dicle University,
21280 Diyarbakir, Turkey
Appl. Organometal. Chem. 2014, 28, 127–133
Copyright © 2014 John Wiley & Sons, Ltd.

C. Kayan et al.
aminophosphine complexes. To the best of our knowledge, there
is no report on the exploitation of these complexes, which
include aminophosphines carrying a cyclohexyl moiety on the
phosphorus atom, in transfer hydrogenation reactions. Herein,
we describe the results attained by the transfer hydrogenation
of ketones using the novel Ru(II)–aminophosphine complexes
having a cyclohexyl moiety. This study was completed with a
synthesis and full characterization of aminophosphine ligands
and their neutral ruthenium complexes.
1 J = 11.1 Hz), 45.86 (d, CH2–, 2 J = 27.2 Hz), 105.56 (C-3), 110.09
(C-4), 141.27 (C-5), 156.23 (d, C-2, 3 J = 3.0 Hz), assignment was
based on the 1H-13C HETCOR, DEPT and 1H-1H COSY spectra;
31P-{1H} NMR (162.0 MHz, CDCl3): δ 61.61 (s, NH–P(Cy)2); IR, (KBr):
υ 801 (P–N), 3257 (N–H) cmꢀ1; C17H28NOP (293.39 g molꢀ1): calcd
C 69.60, H 9.62, N 4.77; found C 69.49, H 9.54, N 4.61%.
Thiophene-(N-dicyclohexylphosphino)amine, [Cy2PNHCH2–C4H3S] (2)
Chlorodicyclohexylphosphine (0.212 g, 0.88 mmol) was added
dropwise over a period of 15 min to a stirred solution of
thiophene-2-methylamine (0.100 g, 0.88 mmol) and triethylamine
(0.090 g, 0.88 mmol) in CH2Cl2 (40 ml) at 0 °C). The mixture was
stirred at room temperature for 1 h and the solvent was removed
under reduced pressure. After addition of dry THF, the white
precipitate (triethylammonium chloride) was filtered off under
argon and the solvent was removed under reduced pressure.
The residue was then washed with dried cold petroleum ether
(2 × 10 mL) and dried in vacuo to produce a clear, white, viscous,
oily compound 2. Yield 0.261 g, 95.6%. 1H NMR (400.1 Hz, CDCl3):
δ 7.18 (d, 1H, H-5, 3 J = 5.0 Hz), 6.90–6.94 (m, 2H, H-3 and H-4), 4.27
(dd, 2H, CH2–, 3 J(HP) = 6.4 and 3 J(HH) = 6.3 Hz), 2.98 (m, 1H, NH),
1.24–1.81 (m, 22H, protons of cyclohexyls); 13C NMR (100.6 MHz,
CDCl3): δ 26.01, 26.57, 27.00, 27.70, 29.43, 29.62 (CH2 of cyclohexyls),
36.58 (d, CH of cyclohexyls, 1 J = 10.1 Hz), 48.16 (d, CH2, 2 J = 4.0Hz),
123.46 (C-3), 123.67 (C-5), 126.36 (C-4), 147.40 (C-2), assignment was
based on the 1H-13C HETCOR, DEPT and 1H-1H COSY spectra;
31P-{1H} NMR (162.0 MHz, CDCl3): δ 61.69 (s, NH–P(Cy)2); IR, (KBr):
υ 798 (P–N), 3253 (N–H) cmꢀ1; C17H28NSP (309.45 g molꢀ1): calcd
C 65.98, H 9.12, N 4.53; found C 65.87, H 8.99, N 4.35%.
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 Sigma-Aldrich. The
starting materials PCy2Cl, NH2CH2–C4H3O, NH2CH2–C4H3S,
[Ru(η6-p-cymene)(μ-Cl)Cl]2 and [Ru(η6-benzene)(μ-Cl)Cl]2 were
purchased from Fluka and used as received. IR spectra were
recorded on a Mattson 1000 ATI UNICAM FT-IR spectrometer as
KBr pellets. 1H (400.1 MHz), 13C NMR (100.6 MHz) and 31P-{1H}
NMR spectra (162.0 MHz) were recorded on a Bruker AV400
spectrometer, using tetramethylsilane and 85% H3PO4 as internal
and external references, respectively. CDCl3 was used as both
solvent and internal lock. Elemental analysis was carried out on
a Fisons EA 1108 CHNS-O instrument. Melting points were
recorded by a Gallenkamp model apparatus with open capillaries.
Synthesis of the Transition Metal Complexes
[Ru(Cy2PNHCH2–C4H3O)(η6-p-cymene)Cl2] (1a)
GC Analyses
GC analyses were performed on a Shimadzu 2010 Plus gas
chromatograph equipped with a capillary column (5% biphenyl
and 95% dimethylsiloxane) (30 m × 0.32 mm× 0.25 μm). The GC
parameters for transfer hydrogenation of ketones were as follows:
initial temperature, 110°C; initial time, 1 min; solvent delay,
4.48 min; temperature ramp 80 °C minꢀ1; final temperature, 200°C;
final time, 21.13 min; injector port temperature, 200 °C; detector
temperature, 200 °C; injection volume, 2.0μl.
To a solution of [Ru(η6-p-cymene)(μ-Cl)Cl]2 (0.291 g, 0.48 mmol)
in THF,
a solution (THF, 30 ml) of [Cy2PNHCH2–C4H3O],
1 (0.279 g, 0.95 mmol) was added and the resulting reaction
mixture was allowed to proceed with stirring at room tem-
perature for 1.5 h. After this time, the solution was filtered and
the solvent evaporated under vacuum. The solid residue thus
obtained was washed with petroleum ether (3 × 10 ml) and then
dried under vacuum. Following recrystallization from petroleum
ether/CH2Cl2, a red crystalline powder was obtained. Yield
0.528 g, 92.6%; m.p. 144–146 °C. 1H NMR (400.1 MHz, CDCl3):
δ 7.28 (br, 1H, H-5), 6.30 (m, 1H, H-4), 6.27 (d, 1H, H-3,
3 J = 2.7 Hz), 5.58 (s, 4H, aromatic protons of p-cymene),
4.06 (dd, 2H, CH2–, 3 J(HP) = 6.5 and 3 J(HH) = 6.4 Hz), 3.77
((dt, –NH–, 1H, 3 J(HH) = 6.4 and 2 J(HP) = 12.8 Hz), 2.87 (m, 1H,
CH– of p-cymene), 2.14 (s, 3H, CH3–Ph of p-cymene), 1.59–2.34
(m, 22H, protons of cyclohexyls) 1.31 (d, 6H, 3 J = 6.9 Hz, (CH3)2CHPh
of p-cymene); 13C NMR (100.6MHz, CDCl3): δ 18.22 (CH3Ph of
p-cymene), 22.41 ((CH3)2CHPh of p-cymene), 25.60, 26.55, 27.18,
27.50, 28.24, 28.60 (CH2 of cyclohexyls), 30.81 (–CH– of p-cymene),
40.37 (CH of cyclohexyls), 40.68 (d, 2 J=7.0Hz, CH2–), 83.56
(d, 2 J= 6.0 Hz, aromatic carbons of p-cymene), 88.50 (d, 2 J= 4.0Hz,
aromatic carbons of p-cymene), 94.65, 108.58 (quaternary carbons
of p-cymene), 106.59 (C-4), 110.32 (C-3), 141.22 (C-5), 154.19
(d, 3 J = 5.0Hz, C-2); assignment was based on the 1H-13C HETCOR,
Synthesis of the Ligands
Furfuryl-(N-dicyclohexylphosphino)amine, [Cy2PNHCH2–C4H3O] (1)
Chlorodicyclohexylphosphine (0.245 g, 1.02 mmol) was added
dropwise over a period of 15 min to a stirred solution of
furfurylamine (0.100 g, 99%, 1.02 mmol) and triethylamine
(0.104 g, 1.02 mmol) in CH2Cl2 (40 ml) at 0 °C. The mixture was
stirred at room temperature for 1 h and the solvent was removed
under reduced pressure. After addition of dry THF, the white
precipitate (triethylammonium chloride) was filtered off under
argon and the solvent was removed under reduced pressure.
The residue was then washed with dried cold petroleum ether
(2 × 10 ml) and dried in vacuo to produce a clear, white, viscous,
oily compound 1. Yield 0.279 g, 93.3%. 1H NMR (400.1 Hz, CDCl3):
δ 7.33 (d, 1H, H-5, 3 J = 1.6 Hz), 6.29 (dd, 1H, H-4, 3 J = 1.6 and
3.0 Hz), 6.13 (d, 1H, H-3, 3 J = 3.0 Hz), 4.04 (dd, 2H, CH2-, 3 J(HP) =
7.7 and 3 J(HH) = 7.6 Hz), 2.42 (dt, –NH–, 1H, 3 J(HH) = 7.6 and
2 J(HP) = 13.0 Hz), 1.16–1.77 (m, 22 H, protons of cyclohexyls);
13C NMR (100.6 MHz, CDCl3): δ 26.55, 27.03, 27.16, 27.28, 29.12,
29.30 (CH2– of cyclohexyls), 36.40 (d, CH– of cyclohexyls,
1
DEPT and H-1H COSY spectra; 31P-{1H} NMR (162.0 MHz, CDCl3):
δ 71.76 (s, NH–P(Cy)2); IR, (KBr): υ 851 (P–N), 3343 (N–H) cmꢀ1
;
C27H42NPORuCl2 (599.6 g molꢀ1): calcd C 54.09, H 7.06, N 2.50;
found C 53.96, H 6.95, N 2.41%.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 127–133

Novel cyclohexyl-based aminophosphine ligands
[Ru(Cy2PNHCH2-C4H3O)(η6-benzene)Cl2] (1b)
1
1
assignment was based on the H-13C HETCOR, DEPT and H-1H
COSY spectra; 31P-{1H} NMR (162.0 MHz, CDCl3): δ 72.47 (s, NH–P
(Cy)2); IR, (KBr): υ 795 (P–N), 3339 (N–H) cmꢀ1; C27H42NPSRuCl2
(615.7 g molꢀ1): calcd C 52.68, H 6.88, N 2.28; found C 52.31, H
6.52, N 2.13%.
A mixture of [Ru(η6-benzene)(μ-Cl)Cl]2 (0.238 g, 0.48 mmol) and
[Cy2PNHCH2–C4H3O] 1 (0.279 g, 0.95 mmol) in 15 ml of THF was
stirred at room temperature overnight. The volume of the solvent
was then reduced to 0.5 ml before addition of petroleum ether
(10 ml). The precipitated product was filtered and dried in vacuo,
yielding 1b as a red microcrystalline powder. Yield 0.470g, 90.9
%; m.p. 57–59°C. 1H NMR (400.1MHz, CDCl3): δ 7.29 (d, 1H,
3 J = 2.8Hz, H-5) 6.26 (m, 2H, H-3, H-4), 5.71 (s, 6H, aromatic protons
of benzene), 4.05 (dd, 2H, CH2–, 3 J(HP) = 6.5 and 3 J(HH)= 6.4Hz),
2.92 ((dt, –NH–, 1H, 3 J(HH)= 6.4 and 2 J(HP) = 13.3 Hz), 1.23–2.35
(m, 22H, protons of cyclohexyls); 13C NMR (100.6 MHz, CDCl3):
δ 26.42, 26.83, 27.33, 27.45, 28.72, 28.94 (CH2 of cyclohexyls),
40.56 (d, 2 J = 9.1 Hz, –CH2–), 41.09 (d, 1 J = 28.2 Hz, CH of
cyclohexyls), 87.14 (d, 2 J = 3.0Hz, aromatic carbon of benzene),
106.69, 110.36 (C-3 and C-4), 141.43 (C-5), 153.96 (C-2); assignment
[Ru(Cy2PNHCH2–C4H3S)(η6-benzene)Cl2] (2b)
A mixture of [Ru(η6-benzene)(μ-Cl)Cl]2 (0.211 g, 0.42 mmol) and
[Cy2PNHCH2–C4H3S] (0.261 g, 0.84 mmol) in 15 ml of THF was
stirred at room temperature overnight. The volume of the solvent
was then reduced to 0.5 ml before addition of petroleum ether
(10 ml). The precipitated product was filtered and dried in vacuo,
yielding 2b as a red microcrystalline powder. Yield 0.421 g, 89.2
1
%; m.p. 185 °C (dec.). H NMR (400.1 MHz, CDCl3) δ 7.15 (d, 1H,
3 J = 4.5 Hz, H-5), 7.00 (dd, 1H, H-4, 3 J = 3.4 and 4.5 Hz), 6.91
(d, 1H, 3 J = 3.4 Hz, H-3), 5.72 (s, 6H, aromatic protons of benzene),
4.26 (dd, 2H, CH2–, 3 J(HP) = 5.9 and 3 J(HH) = 5.8 Hz), 3.05 (br, 1H,
NH), 1.28–2.38 (m, 22H, protons of cyclohexyls); 13C NMR
(100.6 MHz, CDCl3): δ 26.42, 27.34, 27.42, 27.92, 28.23, 28.87
(CH2 of cyclohexyls), 41.15 (d, 1 J = 27.8 Hz, CH of cyclohexyls),
42.73 (d, 2 J = 10.0 Hz, CH2–), 87.12 (d, 2 J = 3.0 Hz, aromatic carbon
of benzene), 124.02 (C-5), 124.53 (C-4), 126.78 (C-3), 144.31 (C-2);
1
was based on the H-13C HETCOR, DEPT and 1H-1H COSY spectra;
31P-{1H} NMR (162 MHz, CDCl3): δ 75.05 (s, NH-P(Cy)2); IR, (KBr): υ
816 (P–N), 3325 (N–H) cm-1; C23H34NPORuCl2 (543.5 g molꢀ1): calcd
C 50.83, H 6.31, N 2.58; found C 50.66, H 6.20, N 2.23%.
[Ru(Cy2PNHCH2–C4H3S)(η6-p-cymene)Cl2] (2a)
1
1
assignment was based on the H-13C HETCOR, DEPT and H-1H
COSY spectra; 31P-{1H} NMR (162 MHz, CDCl3): δ 75.84 (s, NH–P
(Cy)2); IR, (KBr): υ 846 (P–N), 3334 (N–H) cmꢀ1; C23H34NPSRuCl2
(559.5 g molꢀ1): calcd C 49.37, H 6.12, N 2.50; found C 49.28, H
6.01, N 2.40%.
To a solution of [Ru(η6-p-cymene)(μ-Cl)Cl]2 (0.258 g, 0.42 mmol) in
THF, a solution (THF, 30 ml) of [Cy2PNHCH2–C4H3S], 2 (0.261 g,
0.84 mmol) was added and the resulting reaction mixture was
allowed to proceed with stirring at room temperature for 1.5 h.
After this time, the solution was filtered and the solvent evapo-
rated under vacuum. The solid residue thus obtained was washed
with petroleum ether (3 × 10 ml) and then dried under vacuum.
Following recrystallization from petroleum ether/CH2Cl2, a red
crystalline powder was obtained. Yield 0.491 g, 94.6%, m.p. 67–
General Procedure for the Transfer Hydrogenation
of Ketones
Typical procedure for the catalytic hydrogen transfer reaction: a
solution of complexes [Ru(Cy2PNHCH2–C4H3O)(η6-p-cymene)Cl2]
1
69 °C. H NMR (400.1 MHz, CDCl3): δ 7.14 (d, 1H, 3 J = 4.3 Hz, H-5),
7.02 (dd, 1H, H-4, 3 J = 4.3 and 4.1 Hz), 6.90 (d, 1H, 3 J = 4.1 Hz, H-
3), 5.59 (br, 4H, aromatic protons of p-cymene), 4.26 (dd, 2H,
CH2–, 3 J(HP) = 5.8 and 3 J(HH) = 5.7 Hz), 2.97 (m, 1H, NH), 2.88
(m, 1H, CH– of p-cymene), 2.14 (s, 3H, CH3–Ph of p-cymene),
1.36–1.92 (m, 22H, protons of cyclohexyls), 1.32 (d, 6H, (CH3)
2CHPh of p-cymene, 3 J = 8.0 Hz); 13C NMR (100.6 MHz, CDCl3):
δ 18.25 (CH3Ph of p-cymene), 22.44 ((CH3)2CHPh of p-cymene),
26.54, 27.20, 27.33, 27.49, 28.35, 28.76 (CH2 of cyclohexyls),
30.83 (CH– of p-cymene), 40.54 (d, 1 J = 26.2 Hz, CH of
cyclohexyls), 42.82 (d, 2 J = 10.1 Hz, CH2–), 88.45 (d, 2 J = 4.0 Hz,
aromatic carbons of p-cymene), 83.60 (d, 2 J = 6.0 Hz, aromatic
carbons of p-cymene), 94.72, 108.60 (quaternary carbons of
p-cymene), 123.86 (C-5), 124.47 (C-4), 126.70 (C-3), 144.50 (C-2);
(1a),
[Ru(Cy2PNHCH2–C4H3O)(η6-benzene)Cl2]
(1b),
[Ru
(Cy2PNHCH2–C4H3S)(η6-p-cymene)Cl2] (2a), [Ru(Cy2PNHCH2–
C4H3S)(η6-benzene)Cl2] (2b) (0.005 mmol), NaOH (0.025 mmol)
and the corresponding ketone (0.5 mmol) in degassed iso-PrOH
(5 ml) were refluxed for 1 h for (1a, 1b and 2a) and 30 min for
2b. Then, 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
Aminophosphine ligands Cy2PNHCH2–C4H3O
and Cy2PNHCH2–C4H3S, 1–2 were prepared
from the starting material furfurylamine and
Cl
Cl
NH-PCy2
Cl
Ru
ii
Ru
thiophene-2-methylamine, respectively, by
the aminolysis method (Scheme 1).[42,43] The
31P-{1H} NMR spectra of ligands 1–2 showed
single resonances at δ 61.61 and 61.69 ppm,
respectively, similar to those found for closely
related compounds. 1H NMR spectra of the
ligands show multiplets at δ 1.16–1.81 ppm,
which were assigned as protons of cyclohexyl
groups and triplets at ~ δ 4 ppm due to methy-
lene protons of ligands. In their 13C NMR
spectra, many singlets at δ 26.01–29.30 ppm
may be due to methylene carbons of
cyclohexyl groups and doublets at ~ δ
Cl
X
O
NH2
O
NH-PCy2
1a
i
1b
X
HN PCy2
Cl
Cl
NH-PCy2
Cl
Ru
iii
Ru
Cl
S
S
NH-PCy2
X: O, 1; S, 2
2b
2a
Scheme 1. Synthesis of ligands 1–2 and their Ru(II) complexes 1a, 1b, 2a and 2b. (i) 1 equiv.
Cy2PCl, 1 equiv. Et3N, CH2Cl2; (ii, iii) 1 equiv. [Ru(η6-p-cymene)(μ-Cl)Cl]2 or [Ru(η6-benzene)
(μ-Cl)Cl]2, THF.
Appl. Organometal. Chem. 2014, 28, 127–133
Copyright © 2014 John Wiley & Sons, Ltd.
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C. Kayan et al.
36 ppm (1 J: 10–11 Hz) can result from methine carbons of
cyclohexyl groups. Unfortunately, aminophosphines 1–2
Table 1. Transfer hydrogenation of acetophenone with iso-PrOH cat-
alyzed by 1a, 1b, 2a and 2b
are unstable and decompose rapidly on exposure to air or mois-
ture. Because ligands 1–2 are not stable enough in solution, their
ruthenium(II) complexes were synthesized in situ.
Entry Catalyst
S:C:KOH
Time
Conversion TOF (hꢀ1
)
h
(%)g
1
1aa
1ba
2aa
2ba
1ab
1bb
2ab
2bb
1ac
1bc
2ac
2bc
1ad
1bd
2ad
2bd
1ae
1be
2ae
2be
1af
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1
1 h
1 h
99
99
96
96
<5
<5
<5
<5
<5
<5
<5
<5
95
96
97
96
95
94
96
97
95
95
93
96
98
98
The ability of dimers {[Ru(arene)(μ-Cl)Cl]2} to form mononu-
clear complexes of general formula [Ru(η6-arene)Cl2L] is well
known.[44] As expected, the reaction of 1–2 with [Ru(η6-p-
cymene)(μ-Cl)Cl]2 or [Ru(η6-benzene)(μ-Cl)Cl]2 gave correspond-
ing complexes [Ru(Cy2PNHCH2–C4H3O)(η6-p-cymene)Cl2] (1a),
[Ru(Cy2PNHCH2–C4H3O)(η6-benzene)Cl2] (1b), [Ru(Cy2PNHCH2–
C4H3S)(η6-p-cymene)Cl2] (2a), and [Ru(Cy2PNHCH2–C4H3S)
(η6-benzene)Cl2] (2b), respectively, in high yields as air-stable,
red microcrystalline powder (Scheme 1). The aminophosphine
ligands were expected to cleave the [Ru(p-cymene)Cl2]2 and
[Ru(benzene)Cl2]2 dimer to give the corresponding complexes
1a, 1b, 2a and 2b via monohapto coordination of the
aminophosphine group. The chemical purity of the complexes
was confirmed by single 31P-{1H} NMR signals at δ 71.76, 75.05,
72.47 and 75.84 ppm, respectively. These complexes are highly
soluble in CH2Cl2 and slightly soluble in petroleum ether, so they
were recrystallized from CH2Cl2/petroleum ether solution. In their
1H NMR spectra, the NH resonances of the complexes were
observed as triplet at δ 2.92–3.77 ppm (3 J 6.3–8.5 Hz) due to the
multiple coupling 3 J confirmed by H/D exchange experiment,
by which the signal of the NH simply disappears from the
spectrum or more commonly is replaced by a weak signal close
to δ 4.70 coming from suspended droplets of HDO. Other 1H
and 13C NMR data support the expected compounds. The struc-
tural compositions of complexes 1a, 1b, 2a and 2b were further
confirmed by IR spectroscopy and microanalysis, and found to be
in good agreement with the theoretical values.
2
3
1 h
—
4
30 min
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
1 h
—
—
—
—
—
—
—
—
—
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
100:1
100:1
100:1
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
95
1 h
96
97
192
24
24
24
49
32
32
31
64
1 h
30 min
4 h
4 h
4 h
2 h
3 h
1bf
2af
3 h
3 h
2bf
1.5 h
Reaction conditions:
aRefluxing in iso-PrOH; acetophenone:Ru:NaOH, 100:1:5.
bAt room temperature; acetophenone:Ru:NaOH, 100:1:5.
cRefluxing in iso-PrOH; acetophenone:Ru, 100:1, in the absence
of base.
Catalytic Transfer Hydrogenation of Ketones
dAdded 0.1 ml H2O.
eRefluxing the reaction in air.
Recently, we reported that the novel half-sandwich complexes,
based on ligands with P–N and P–O backbone, are active cata-
lysts in the reduction of aromatic ketones.[45–47] The observed
activity of these complexes has encouraged us to examine other
analogous ligands and their ruthenium complexes. Furthermore,
it is well known that the complex with sp3-hybribized nitrogens
having N–H bonds exhibits a higher reaction rate. On the other
hand, the presence of the N–H group in the ligands may enable
stabilization of a six-membered cyclic transition state by forming
a hydrogen bond with the oxygen atom of ketones.[48] In this
context, complexes 1a, 1b, 2a and 2b were selected as catalysts,
iso-PrOH/NaOH as the reducing system, and acetophenone as a
model substrate (Scheme 2), and the results are listed in Table 1.
In a preliminary study, when the reaction temperature was
increased to 82 °C a smooth reduction of acetophenone into
1-phenylethanol occurred, with conversion ranging from 96%
to 99% after 30 min for 2b and 1 h for (1a, 1b and 2a) of reaction,
as seen in Table 1 (entries 1–4). From these results, it is worth
mentioning that the reactivities of complexes 1a–2b are
fRefluxing in iso-PrOH; acetophenone:Ru:NaOH, 500:1:5.
gDetermined by GC (three independent catalytic experiments).
hReferred at the reaction time indicated in column; TOF = (mol
product/mol Ru(II)cat.) × hꢀ1
.
different, with a complex:NaOH ratio of 1:5. Complex 2b is the
most active, converting acetophenone to 1-phenylethanol with
a time of flight (TOF) of 192 hꢀ1, implying that the η6-benzene–
ruthenium complex 2b is more effective than complexes
(1a, 1b and 2a). It should also be pointed out that complex 2b
is far more active than the corresponding precursor: [Ru(η6-
benzene)(μ-Cl)Cl]2 (37% maximum yield in 24 h).[49] At room
temperature no significant formation of 1-phenylethanol was
observed (Table 1, entries 5–8), which means the catalytic activity
of Ru(II) complexes at room temperature is unimportant. One can
infer from Table 1 (entries 9–12) that the catalysts as well as the
presence of base (NaOH) are necessary to observe noteworthy
conversions. It was proposed that base facilitates the formation
of alkoxide by abstracting a proton of the alcohol and then alkox-
ide undergoes β-elimination to give ruthenium hydride, which is
an active species in this reaction.[50,51] Furthermore, performing
the reaction with the addition of water did not slow the reaction
but however, reduced conversion of the product (Table 1, entries
13–16). Unfortunately, changing the reaction atmosphere from
HO
O
Cat. / NaOH
iso-PrOH
O
OH
+
Scheme 2. Hydrogen transfer from iso-PrOH to acetophenone.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 127–133

Novel cyclohexyl-based aminophosphine ligands
an inert gas to an ambient atmosphere slowed the reaction by
four times (Table 1, entries 17–20). Therefore, the coupling reac-
tions were carried out under an inert atmosphere of argon. As
presented in Table 1, increasing the substrate:catalyst ratio did
not cause significant changes in the conversions of the
products in most cases, despite prolonging the reaction time.
Remarkably, the transfer hydrogenation of acetophenone could
be achieved up to 96% conversion even when the substrate-to-
catalyst ratio reached 500:1 (Table 1).
hydrogenation.[53,54] We also performed further experiments to
investigate the influence of bulkiness of the alky groups on the
catalytic activity, and the results are given in Table 3 (entries 1–16).
A variety of simple alkyl phenyl ketones were transformed to the
corresponding secondary alcohols, and it was found that activity
was highly dependent on the steric bulkiness of the alkyl group.
The reactivity gradually decreased by increasing the bulkiness of
the alkyl groups;[55,56] i.e. when the size of an alkyl group was
increased, activity decreased significantly.[57,58]
The catalytic reduction of acetophenone derivatives was
tested with the conditions optimized for acetophenone, and
the results are summarized in Table 2. The fourth column in
Table 2 illustrates conversion of the reduction performed in
0.1 M iso-PrOH solution containing 1a–2b and NaOH (ketone:
Ru:NaOH = 100:1:5). As already stated, the electronic properties
(nature and position) of the substituents on the phenyl ring of
the ketone caused changes in the reduction rate. An ortho- or
para-substituted acetophenone with an electron-donor substit-
uent, i.e. 2-methoxy or 4-methoxy, is reduced more slowly than
acetophenone (Table 2, entries 4, 5, 9, 10, 14, 15, 19 and 20).[52]
In addition, the introduction of electron-withdrawing subst-
ituents, such as F, Cl and Br, to the para position of the aryl ring
of the ketone decreased the electron density of the C¼O bond
so that the activity was improved, giving rise to easier
Ruthenium-based catalysts have usually been applied in the
hydrogenation of simple ketones.[59–61] Encouraged by the high
catalytic activities obtained in these preliminary studies, we next
extended our investigations to involve transfer hydrogenation of
various simple ketones. We therefore studied the catalytic activity
of our Ru(II) complexes. Investigation of catalytic activity of these
complexes has shown that they are efficient catalysts, affording
almost quantitative transformation of the ketones in shorter
times, and 2b was more active than complexes (1a, 1b and 2a)
(Table 4). For instance, hydrogenation of cyclohexanone could
be achieved in 1.5 h and 45 min by (1a, 1b and 2a) and 2b, re-
spectively. Conversion of methyl isobutyl ketone occurred in 3 h
and 1.5 h by (1a, 1b and 2a) and 2b, respectively, while that of
diethyl ketone occurred in 5 h and 3 h by (1a, 1b and 2a) and
2b, respectively. As a result, the catalytic activities of (1a, 1b
and 2a) were generally lower than 2b in the studied hydrogen
transfer reactions.
Table 2. Transfer hydrogenation results for substituted acetophe-
nones with catalysts 1a, 1b, 2a and 2ba
TOF (hꢀ1
)
c
Conversion (%)b
Table 3. Transfer hydrogenation results for substituted alkyl phenyl
ketones with catalysts 1a, 1b, 2a and 2ba
Entry
R
Time
Cat. 1a
1
4-F
30 min
30 min
1 h
97
98
96
96
97
194
196
96
HO
R
O
R
2
4-Cl
4-Br
Cat
3
+ HO
+
O
4
2-MeO
4-MeO
2 h
48
5
1.5 h
65
Cat. 1b
c
Entry Catalyst Time
R
Conversion (%)b TOF (hꢀ1
)
6
4-F
30 min
30 min
1 h
95
96
95
96
97
190
192
95
7
4-Cl
1
1a
1b
2a
2b
1a
1b
2a
2b
1a
1b
2a
2b
1a
1b
2a
2b
2 h
2 h
CH2CH3
97
97
99
99
96
99
99
98
98
99
98
99
98
98
97
98
49
49
8
4-Br
2
9
2-MeO
4-MeO
2 h
48
3
2 h
50
10
1.5 h
65
4
1 h
99
Cat. 2a
11
5
4 h
Ch2CH2CH3
CH(CH3)2
C(CH3)2
24
4-F
30 min
30 min
1 h
97
97
96
98
98
194
194
96
6
4 h
25
12
4-Cl
7
4 h
25
13
4-Br
8
2 h
49
14
2-MeO
4-MeO
2 h
49
9
5 h
20
15
1.5 h
65
10
11
12
13
14
15
16
5 h
20
Cat. 2b
16
5 h
20
4-F
15 min
20 min
30 min
1 h
96
96
95
96
97
384
288
190
96
3 h
33
17
4-Cl
12 h
12 h
12 h
6 h
<10
<10
<10
16
18
4-Br
19
2-MeO
4-MeO
20
45 min
121
aCatalyst (0.005mmol), substrate (0.5mmol), iso-PrOH (5ml), NaOH
(0.025 mmol%), 82 °C; concentration of acetophenone
derivatives = 0.1M.
aCatalyst (0.005 mmol), substrate (0.5 mmol), iso-PrOH (5 ml), NaOH
(0.025 mmol%), 82 °C, respectively; concentration of alkyl phenyl
ketones was 0.1 M.
bPurity of compounds was checked by 1H NMR and GC (three
independent catalytic experiments); yields based on methyl
aryl ketone.
bPurity of compounds was checked by 1H NMR and GC (three
independent catalytic experiments); yields based on methyl
aryl ketone.
cTOF = (mol product/mol cat.) × hꢀ1
.
cTOF = (mol product/mol cat.) × hꢀ1
.
Appl. Organometal. Chem. 2014, 28, 127–133
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

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Appl. Organometal. Chem. 2010, 24, 17.
OH
O
OH
O
Cat
+
+
R1
R2
R1
R2
Entry Catalyst
R1, R2
Time
Conversion (%)b
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1
1a
1b
2a
2b
1a
1b
2a
2b
1a
1b
2a
2b
1a
1b
2a
2b
(CH2)5
(CH2)4
1.5 h
1.5 h
1.5 h
97
98
99
99
97
98
99
97
99
98
98
98
99
99
98
99
2
3
4
45 min
1.5 h
1.5 h
1.5 h
45 min
3 h
5
6
7
8
9
CH2CH(CH3)2, CH3
CH2CH3, CH2CH3
10
11
12
13
14
15
16
3 h
3 h
1.5 h
5 h
5 h
5 h
3 h
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bDetermined by GC (three independent catalytic experiments).
cPurity of compounds was checked by 1H NMR and GC (three
independent catalytic experiments); yields based on methyl
aryl ketone.
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Conclusions
We have synthesized two novel aminophosphine ligands and their
Ru(II) complexes. We have found that these complexes are efficient
homogeneous catalytic systems that can be readily implemented
and lead to secondary alcohols with good to excellent yields. It
was seen that [Ru(Cy2PNHCH2–C4H3S)(η6-benzene)Cl2] showed
stronger catalytic activity in the transfer hydrogenation reaction
than analogous complexes. Furthermore, the procedure is quite
simple and efficient towards various ketones and does not require
an induction period. Further work in this direction and in the
possible use of these kinds of complexes for catalytic purposes is
therefore in progress.
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