Tunable ferrocenyl-phosphinite ligands for the ruthenium(II)-catalyzed asymmetric transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.molcata.2013.08.005

We report here new examples of enantiomerically pure monodendate phosphinite ligands containing both a ferrocene moiety and NH bridging moiety adjacent to the stereocenter, as well as their ruthenium(II) dichloro complexes. The phosphinites based on ferrocenyl moiety possessing stereogenic center have been screened as ligands for ruthenium(II)-catalyzed transfer hydrogenation of aromatic ketones to give corresponding secondary alcohols using iso-PrOH as the hydrogen source in the presence of NaOH. Up to 99% conversion with 97% ee was obtained in the transfer hydrogenation of acetophenone derivatives. Furthermore, the catalytic properties of these catalysts based on ferrocenyl-phosphinite backbone are also discussed briefly. The structures of these ligand and their corresponding complexes have been elucidated by a combination of multinuclear NMR spectroscopy, IR spectroscopy and elemental analysis.

Full text of DOI:10.1016/j.molcata.2013.08.005

Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Tunable ferrocenyl-phosphinite ligands for the
ruthenium(II)-catalyzed asymmetric transfer hydrogenation
of ketones
Ug˘ur Is¸ ıka, Murat Aydemira,∗, Nermin Meric¸ a, Feyyaz Durapa, Cezmi Kayana,
Hamdi Temelb, Akın Baysala
a Department of Chemistry, Faculty of Science, University of Dicle, 21280 Diyarbakir, Turkey
b Department of Chemistry, Faculty of Education, University of Dicle, 21280 Diyarbakir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2013
Received in revised form 26 July 2013
Accepted 7 August 2013
Available online 2 September 2013
We report here new examples of enantiomerically pure monodendate phosphinite ligands containing
both a ferrocene moiety and NH bridging moiety adjacent to the stereocenter, as well as their ruthe-
nium(II) dichloro complexes. The phosphinites based on ferrocenyl moiety possessing stereogenic center
have been screened as ligands for ruthenium(II)-catalyzed transfer hydrogenation of aromatic ketones to
give corresponding secondary alcohols using iso-PrOH as the hydrogen source in the presence of NaOH.
Up to 99% conversion with 97% ee was obtained in the transfer hydrogenation of acetophenone deriva-
tives. Furthermore, the catalytic properties of these catalysts based on ferrocenyl-phosphinite backbone
are also discussed briefly. The structures of these ligand and their corresponding complexes have been
elucidated by a combination of multinuclear NMR spectroscopy, IR spectroscopy and elemental analysis.
Keywords:
Monodendate ferrocenyl-phosphinite
Homogeneous catalysis
Asymmetric transfer hydrogenation
Aromatic ketones
© 2013 Elsevier B.V. All rights reserved.
Ruthenium(II)
1. Introduction
Despite the encouraging performance of many ferrocene based
bidendate phosphorus-chelate ligands [17–23], the past few years
Asymmetric catalysis is a powerful technology for generat-
ing single enantiomer materials [1,2]. An increasing number of
chiral compounds and enantiomerically pure drugs are prepared
through transition metal-catalyzed asymmetric reactions [3–5].
Since the reactivity and stereoselectivity of an asymmetric trans-
formation are highly dependent on the structure of the chiral
ligand coordinated to the transition metal, the design and synthe-
sis of efficient chiral ligands are important in this area and have
attracted a great deal of attention from both academia and indus-
try [6–8]. Therefore, among several applications such as polymers
[9] to bioorganometallic chemistry [10–12], the use of ferrocene-
based chiral ligands in asymmetric synthesis is the most prominent
[13,14]. The ferrocene moiety has been extensively explored as a
backbone of chiral phosphine ligands due to its easy modifiabil-
ity and highly electron donating property [15]. Furthermore, the
unique structure of ferrocenes allows one to design a variety of
chiral ferrocenyl phosphine ligands, which are useful tools in metal-
catalyzed asymmetric reactions [16].
have witnessed a renewed interest in the development of chiral
monodendate phosphorus ligands for use in asymmetric hydro-
genation reactions [24–28]. This resurgence in monodendate
ligands is due to the ready accessibility of a range of diverse ligand
structures, and often their lower cost compared to bidendate lig-
ands [29]. Following the studies from the groups of Pringle [30],
Reetz [31], Feringa [32] and more recently Chan [33] and Zhou [34]
a large number of chiral monodendate phosphonite, phosphite, and
phosphoramidite ligands have been found to induce good to excel-
lent enantioselectivities in asymmetric hydrogenation reactions,
comparable to or exceeding those obtained with bidendate ligands.
Additionally, the most important advantage of chiral phosphinite
ligands over the corresponding P-based ligands is the easiness of
preparation, which leads to a substantial interest to develop highly
effective chiral monodendate phosphinite ligands for asymmetric
catalysis [35–37].
Ferrocenyl-phosphines are effective ligands in transition-metal
complexes catalyzing asymmetric reactions, often with high enan-
tioselectivity [38–41]. Because of its great success in catalytic
asymmetric reactions [42], chiral ferrocenyl-phosphine ligands
have attracted considerable attention in recent years [43,44].
These ligands are very efficient for a wide range of reactions,
such as hydrogenation [45], hydrosilylation [46], allylic alkylation
∗
Corresponding author. Tel.: +90 412 248 8550; fax: +90 412 248 8300.
E-mail address: aydemir@dicle.edu.tr (M. Aydemir).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.08.005

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U. Is¸ık et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
[47], palladium catalyzed cross-coupling reactions [48–50] and
cyclopropanation [51]. Although ferrocenyl-phosphine ligands
have found widespread applications in transition metal catalyzed
asymmetric transfer hydrogenations [52–54], the analogous phos-
phinites provide different chemical, electronic and structural
properties compared to phosphines. The metal–phosphorus bond
is often stronger for phosphinites compared to the related phos-
phine due to the presence of electron-withdrawing P-OR group.
In addition, the empty ␴*-orbital of the phosphinite P(OR)R2 is
stabilized, making the phosphinite a better acceptor [55]. Consid-
ering the advantage of phosphinites, in recent years our research
group has reported the synthesis [56–58], characterization and
coordination properties of this kind of ligands [59–61]. With an
aim to design the efficient phosphinite ligands for asymmetric
transfer hydrogenation (ATH) of ketones, herein, we describe the
synthesis and characterization of ferrocenyl-phosphinite ligands
and neutral ruthenium(II)-(benzene) complexes. As far as we know,
there are not many reports on asymmetric transfer hydrogena-
tion of ketones by using chiral ferrocenyl phosphinites as ligands.
These ferrocenyl-phosphinites have been employed successfully
as ligands in ruthenium(II)-promoted asymmetric transfer hydro-
genation of aromatic ketones.
2.2. General procedures for synthesis of ferrocene based
Ru(II)-phosphinites complexes (17–24)
[Ru(␩6-benzene)(␮-Cl)Cl]2 (0.15 mmol) and ferrocene based
phosphinite (0.30 mmol) were dissolved in 20 mL of toluene and
stirred for 1 h at room temperature. The volume was concentrated
to ca. 1–2 mL under reduced pressure and addition of petroleum
ether (20 mL) gave the desired complex a tile red solid which was
collected by filtration and dried in vacuum.
2.2.1. [(2R)-2-(ferrocenylmethylamino)-2-phenylethyl
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))], (17)
Yield: 200 mg, 87%; mp: 133–135 ◦C; [˛]D20 −38.6 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 3.28 (d, 1 H, J = 12.8 Hz,
CH2NH (a)), 3.47 (d, 1H, J = 12.8 Hz, CH2NH (b)), 3.80–3.89 (m,
2H, CH2OP + 1H CHNH), 4.09–4.12 (m, 2H, C5H4 + 5H C5H5), 4.20
(s, 2H, C5H4), 5.36 (d, 6H, J = 4.4 Hz, aromatic protons of benzene),
7.29–7.39 (m, 6H, m- and p-C6H5P + 5H, (C6H5), 7.83–7.85 (m, 4H,
o-C6H5P); 13C NMR (100.6 MHz, CDCl3, ppm) ı: 46.31 (CH2NH),
62.22 (CHNH), 67.68, 67.92, 68.20, 68.42 (C5H4), 68.58 (C5H5), 71.41
(CH2OP), 87.05 (i-C5H4), 89.28 (d, J = 3.2 Hz, aromatic carbons of
benzene), 127.73, 127.91, 127.98, (C6H5), 128.01 (d, J = 11.0 Hz,
m-C6H5P), 130.95 (s, p-C6H5P), 132.49 (d, J = 10.6 Hz, o-C6H5P),
136.33 (d, J = 50.3 Hz, i-C6H5P), 138.20 (i-C6H5); 31P–{ H} NMR
1
2. Materials and methods
(162.0 MHz, CDCl3, ppm) ı: 111.78 (s, OP(Ph)2); IR (KBr pellet in
cm−1) ꢁ: (N H): 3281, (C-Cp): 3086, (C C-Cp): 1455, (O P): 1020;
Anal. Calc. for [C37H36NOPFeRuCl2] (769.49 g/mol): C 57.75, N 1.82,
H 4.72; found: C 57.62, N 1.78, H 4.70%.
Unless otherwise mentioned, all reactions were carried out
under an atmosphere of argon using conventional Schlenk glass-
ware, solvents were dried using established procedures and
distilled under argon immediately prior to use. Analytical grade
and deuterated solvents were purchased from Merck. The starting
materials d-, l-phenylglycine, d-, l-phenylalanine, d-, l-valine, l-
leucine, l-isoleucine, PPh2Cl and Et3N are purchased from Fluka and
were used as received. Ferrocenecarboxaldehyde, [62] and [Ru(␩6-
benzene)(␮-Cl)Cl]2, [63] were prepared according to literature
2.2.2. [(2S)-2-(ferrocenylmethylamino)-2-phenylethyl
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))], (18)
Yield: 197 mg, 85%; mp: 133–135 ◦C; [˛]D20 +38.6 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 3.22 (br, 1H, CH2NH (a)), 3.39
(br, 1H, CH2NH (b)), 3.80 (br, 2H, CH2OP + 1H, CHNH), 4.00–4.14
(m, 4H, C5H4 + 5H, C5H5), 5.36 (d, 6H, 3J = 4.4 Hz, aromatic protons
of benzene), 7.26 (br, 6H, m- and p-C6H5P + 5H, C6H5), 7.75 (br, 4H,
o-C6H5P); 13C NMR (100.6 MHz, CDCl3, ppm) ı: 46.26 (CH2NH),
62.18 (d, J = 7.1 Hz, CHNH), 65.86, 67.73, 67.99, 68.43 (C5H4), 68.69
(C5H5), 71.25 (CH2OP), 86.90 (i-C5H4), 89.28 (d, J = 3.2 Hz, aromatic
carbons of benzene), 127.98, 128.53, 130.89, 131.05, 132.46, 132.71,
136.03, 136.53 (o-, m-, p- and i-C6H5P and o-, m-, p- and i-C6H5);
1
procedures. 1H (at 400.1 MHz), 13C (at 100.6 MHz) and 31P–{ H}
NMR (at 162.0 MHz) spectra were recorded on a Bruker Avance 400
spectrometer, with TMS (tetramethylsilane) as an internal refer-
ence for 1H NMR and 13C NMR or 85% H3PO4 as external reference
1
for 31P–{ H} NMR. The IR spectra were recorded on a Mattson 1000
ATI UNICAM FT-IR spectrometer as KBr pellets. Specific rotations
were taken on a Perkin-Elmer 341 model polarimeter. 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. GC analyses were performed on a Shi-
madzu GC 2010 Plus Gas Chromatograph equipped with cyclodex-B
(Agilent) capillary column (30 m × 0.32 mm I.D. × 0.25 ␮m film
thickness). Racemic samples of alcohols were obtained by reduc-
tion of the corresponding ketones with NaBH4 and used as the
authentic samples for ee determination. The GC parameters for
asymmetric transfer hydrogenation of ketones were as follows; ini-
tial temperature, 50 ◦C; initial time 1.1 min; solvent delay, 4.48 min;
temperature ramp 1.3 ◦C/min; final temperature, 150 ◦C; initial
time 2.2 min; temperature ramp 2.15 ◦C/min; final temperature,
250 ◦C; initial time 3.3 min; final time, 44.33 min; injector port tem-
perature, 200 ◦C; detector temperature, 200 ◦C, injection volume,
2.0 ␮L.
1
31P–{ H} NMR (162.0 MHz, CDCl3, ppm) ı: 111.54 (s, OP(Ph)2); IR
(KBr pellet in cm−1) ꢁ: (N H): 3291, (C-Cp): 3087, (P-Ph): 1442,
(C C-Cp): 1441, (O P): 1016; Anal. Calc. for [C37H36NOPFeRuCl2]
(769.49 g/mol): C 57.75, N 1.82, H 4.72; found: C 57.69, N 1.79, H
4.69%.
2.2.3. [(2R)-2-(ferrocenylmethylamino)-3-phenylpropyl
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))], (19)
Yield: 195 mg, 83%; mp: 120–122 ◦C; [˛]D20 +41.5 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 2.27 (br, 1H, NH), 2.73 (br,
2H CH2C6H5), 2.94 (br, 1H, CHNH), 3.33 (br, 1H, CH2NH, (a)),
3.40 (br, 1H, CH2NH, (b)), 3.65 (br, 1H, CH2OP (a)), 3.74 (br,
1H, CH2OP (b)) 3.92–4.05 (br, 4H, C5H4 + 5H, C5H5), 5.42 (d, 6H,
3J = 4.5 Hz, aromatic protons of benzene), 7.05–7.32 (m, 6H, m-
and p-C6H5P + 5H, CH2C6H5) 7.80 (d, 4H, J= 6.8 Hz, o-C6H5P); 13C
NMR (100.6 MHz, CDCl3, ppm) ı: 38.35 (CH2Ph), 46.65 (CH2NH),
58.79 (CHNH), 67.83, 67.84 68.15, 68.46 (C5H4), 68.41 (C5H5),
70.36 (CH2OP), 87.22 (i-C5H4), 89.60 (d, J = 3.0 Hz, aromatic car-
bons of benzene), 126.51, 128.61, 129.28, (CH2C6H5), 127.97 (d,
J = 9.9 Hz, m-C6H5P), 130.99 (d, J = 18.0 Hz, p-C6H5P), 132.6 (d,
J = 10.3, o-C6H5P), 137.19 (i-CH2C6H5), 138.35 (d, J = 52.0 Hz, i-
2.1. General procedure for the transfer hydrogenation of ketones
Typical procedure for the catalytic hydrogen-transfer reaction:
a solution of the ruthenium complexes 17–24, (0.005 mmol), NaOH
(0.025 mmol) and the corresponding ketone (0.5 mmol) in degassed
iso-PrOH (5 mL) was refluxed until the reaction completed. Then,
a sample of the reaction mixture is taken off, diluted with acetone
and analyzed immediately by GC, conversions obtained are related
to the residual unreacted ketone.
1
C6H5P); 31P–{ H} NMR (162.0 MHz, CDCl3, ppm) ı: 112.46 (s,
OP(Ph)2); IR (KBr pellet in cm−1) ꢁ: (N H): 3268, (C-Cp): 3084,
(C C-Cp): 1465, (C C-Cp): 1448, (P-Ph): 1442, (O P): 1022; Anal.

U. Is¸ık et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
227
(b)), 4.14–4.25 (br, 4H, C5H4 + 5H, C5H5), 5.45 (d, 6H, 3J = 4.6 Hz,
aromatic protons of benzene), 7.43 (m, 6H, m- and p-C6H5P),
7.90–7.92 (m, 4H, m- and o-C6H5P); 13C NMR (100.6 MHz, CDCl3,
ppm) ı: 22.46, 22.94 (CH(CH3)2), 24.68 (CH(CH3)2), 40.94 (CH2CH),
46.25 (CH2NH), 55.28 (CHNH), 64.20 (CH2OP), 68.11, 68.52, 68.97,
68.99 (C5H5)), 69.13 (C5H5)), 87.32 (i-C5H4), 89.36 (d, J = 3.2 Hz,
aromatic carbons of benzene), 128.01 (d, J = 22.0 Hz, m-C6H5P),
131.03 (s, p-C6H5P), 133.16 (d, J = 25.9 Hz, o-C6H5P); 138.00 (d,
Calc. for [C38H38NOPFeRuCl2] (783.52 g/mol): C 58.25, N 1.79, H
4.89; found: C 57.10, N 1.52, H 4.59%.
2.2.4. [(2S)-2-(ferrocenylmethyl-amino)-3-phenylpropyl
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))], (20)
Yield: 194 mg, 83%; mp: 120–122 ◦C; [˛]D20 −41.5 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 2.25 (br, 1H, NH), 2.70 (m,
2H, CH2C6H5), 2.97 (m, 1H, CHNH), 3.40 (m, 2H, CH2NH), 3.61 (m,
1H, CH2OP (a)), 3.71 (m, 1H, CH2OP (b)), 3.96 (m, 2H, C5H4 + 5H,
C5H5), 4.07 (m, 2H, C5H4), 5.42 (d, 6H, 3J = 4.5 Hz, aromatic protons
of benzene), 7.25 (m, 5H, CH2C6H5), 7.29 (m, 6H, m- and p-C6H5P),
7.82 (m, 4H, o-C6H5P); 13C NMR (100.6 MHz, CDCl3, ppm) ı: 38.38
(CH2Ph), 46.70 (CH2NH), 58.75 (CHNH), 64.42 (CH2OP), 67.74,
67.84, 68.12, 68.43 (C5H4), 68.43 (C5H5), 87.32 (i-C5H4), 89.60 (d,
J = 3.0 Hz, aromatic carbons of benzene), 127.82, 127.95, 128.71
(CH2C6H5), 130.92 (d, J = 10.0, m-C6H5P), 132.08 (d, J = 10.1 Hz, p-
C6H5P), 133.12 (d, J = 11.0, o-C6H5P), 137.40 (i-CH2C6H5), 138.92
1
J = 49.6 Hz, i-C6H5P); 31P–{ H} NMR (162.0 MHz, CDCl3, ppm) ı:
110.01 (s, OP(Ph)2); IR (KBr pellet in cm−1) ꢁ: (N H): 3305, (C-
Cp): 3091, (P-Ph): 1446, (C C-Cp): 1438, (O P): 1024; Anal. Calc.
for [C35H40NOPFeRuCl2] (749.50 g/mol): C 56.09, N 1.87, H 5.38;
found: C 55.98, N 1.83, H 5.34%.
2.2.8. [(2S-3S)-2-(ferrocenylmethylamino)-3-methylpentyl
diphenylphosphinito(dichloro(ꢀ6-p-benzene)ruthenium(II)] (24)
Yield: 198 mg, 88%; mp: 132–135 ◦C; [˛]D20 +31.9 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 0.78–0.83 (m, 3H, CH2CH3 + 3H,
CH(CH3)), 1.29 (m, 2H, CHCH2(CH3)), 1.52 (m, 1H, CHCH2(CH3)),
2.62 (m, 1H, CHNH), 3.63 (m, 2H, CH2NH), 3.73 (m, 1H, CH2OP
(a)), 3.96 (m, 1H, CH2OP (b)), 4.15 (m, 2H, C5H4 + 5H, C5H5), 4.28
(m, 2H, C5H4), 5.47 (d, 6H, J = 4.8 Hz, aromatic protons of benzene),
1
(d, J = 50.3 Hz, i-C6H5P); 31P–{ H} NMR (162.0 MHz, CDCl3, ppm)
ı: 112.20 (s, OP(Ph)2); IR (KBr pellet in cm−1) ꢁ: (N H): 3268, (C-
Cp): 3089, (C C-Cp): 1448, (P-Ph): 1442, (O P): 1023; Anal. Calc.
for [C38H38NOPFeRuCl2] (783.52 g/mol): C 58.25, N 1.79, H 4.89;
found: C 58.11; N 1.77, H 4.85%.
7.42–7.43 (m, 6H, m- and p-C6H5P), 7.83–7.92 (m, 4H, o-C6H5P); 13
C
2.2.5. [(2R)-2-(ferrocenylmethylamino)-3-methylbutyl
NMR (100.6 MHz, CDCl3, ppm) ı: 11.93 (CH2CH3), 14.70 (CHCH3),
25.79 (CHCH2CH3), 35.28 (CHCH2CH3), 46.78 (CH2NH), 60.90
(CHNH), 67.03, 68.12, 68.54, 68.57 ((C5H4), 69.07 (C5H5), 68.30
(CH2OP), 86.33 (i-C5H4), 89.33 (d, J = 3.0 Hz, aromatic carbons of
benzene), 127.89 (d, J = 11.0, m-C6H5P), 130.91 (s, p-C6H5P), 132.61
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))] (21)
Yield: 186 mg, 84%; mp: 130–133 ◦C; [˛]D20 −53.4 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 0.73–0.82 (m, 6H, CH(CH3)2),
1.79 (br, 1H, CH(CH3)2), 2.46 (br, 1H, CHNH), 3.48 (br, 2H, CH2NH),
3.65 (br, 1H, CH2OP, (a)), 3.83 (br, 1H, CH2OP, (b)), 4.02–4.14 (m, 4H,
C5H4 + 5H, C5H5), 5.44 (d, 6H, 3J = 4.6 Hz, aromatic protons of ben-
zene), 7.33 (m, 6H, m- and p-C6H5P), 7.78–7.84 (br, 4H, o-C6H5P);
13C NMR (100.6 MHz, CDCl3, ppm) ı: 18.60, 18.70 (CH(CH3)2), 28.69
(CH(CH3)2), 47.20 (CH2NH), 62.36 (CHNH), 69.12 (CH2OP), 67.97,
68.49, 68.71, 68.94 (C5H4), 69.12 (C5H5), 87.59 (i-C5H4), 89.45
(d, J = 3.2 Hz, aromatic carbons of benzene), 127.92 (d, J = 10.0 Hz,
m-C6H5P), 131.00 (d, J = 10.0 Hz, p-C6H5P), 132.75 (d, J = 10.1 Hz, o-
1
(d, J = 11.1, o-C6H5P), 136.60 (d, J = 48.2 Hz, i-C6H5P); 31P–{ H} NMR
(162.0 MHz, CDCl3, ppm) ı: 110.97 (s, OP(Ph)2); IR (KBr pellet in
cm−1) ꢁ: (N H): 3268, (C-Cp): 3100, (P-Ph): 1446, (C C-Cp): 1461,
(O P): 1027; Anal. Calc. for [C35H40NOPFeRuCl2] (749.50 g/mol): C
56.09, N 1.87, H 5.38; found: C 56.00, N 1.83, H 5.33%.
3. Results and discussion
1
C6H5P), 136.17 (J = 48.3 Hz, i-C6H5P); 31P–{ H} NMR (162.0 MHz,
CDCl3, ppm) ı: 111.13 (s, OP(Ph)2); IR (KBr pellet in cm−1) ꢁ:
(N H): 3330, (C-Cp): 3090, (C C-Cp): 1458, (P-Ph): 1441, (O P):
1030; Anal. Calc. for [C34H38NOPFeRuCl2] (735,48 g/mol): C 55.53,
N 1.90, H 5.21; found: C 55.24, N 1.86, H 5.17%.
3.1. Synthesis and characterization of the amino alcohols,
phosphinites and their ruthenium(II) complexes
The synthesis of d-, l-phenylglycinol, d-, l-phenylalaninol, d-, l-
valinol, l-leucinol and l-isoleucinol were accomplished in one step
from d-, l-phenylglycine, d-, l-phenylalanine, d-, l-valine, l-leucine
or l-isoleucine, respectively, according to the procedures described
in the literature [64,65], using NaBH4-I2 in dry THF. The ferrocene
based compounds 1–8 were synthesized by the condensation reac-
tion between ferrocenecarboxaldehyde [66] and amino alcohols in
the presence of the base catalyst as shown in Scheme 1 (for details
see supporting information). The 1H NMR spectra of compounds
1–8 show characteristic features: the ␣- and ␤-cyclopentadienyl
(Cp) protons in the amino alcohols with relatively small non-
resolved multiplets appear approximately at ı 4.00–4.50 ppm The
magnetic non-equivalence of protons as well as carbon atoms of the
monosubstituted Cp ring was observed, which is caused by their
2.2.6. [(2S)-2-(ferrocenylmethylamino)-3-methylbutyl
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))] (22)
Yield: 192 mg, 87%; mp: 130–133 ◦C; [˛]D20 +53.4 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 0.73–0.80 (m, 6H, CH(CH3)2),
1.80 (br, 1H, CH(CH3)2), 2.45 (br, 1H, CHNH), 3.45 (br, 2H, CH2NH),
3.65 (br, 1H, CH2OP,(a)), 3.82 (br, 1H, CH2OP, (b)), 4.03–4.14 (br, 4H,
C5H4 + 5H, C5H5), 5.43 (d, 6H, 3J = 4.8 Hz, aromatic protons of ben-
zene), 7.33 (br, 6H, m- and p-C6H5P), 7.80–7.86 (m, 4H, o-C6H5P);
13C NMR (100.6 MHz, CDCl3, ppm) ı: 18.61, 18.63 (CH(CH3)2), 28.72
(CH(CH3)2), 47.22 (CH2NH), 69.13 (CH2OP), 62.44 (CHNH), 67.90,
68.50, 68.57, 68.90, (C5H4), 69.95 (C5H5)), 87.50 (i-C5H4), 89.27
(d, J = 3.1 Hz, aromatic carbons of benzene), 127.88, 130.90, 131.01,
1
1
136.72 (o-, m-, p- and i-C6H5P); 31P–{ H} NMR (162.0 MHz, CDCl3,
planar structure. The 13C–{ H} NMR spectra also exhibit the typi-
ppm) ı: 110.79 (s, OP(Ph)2); IR (KBr pellet in cm−1) ꢁ: (N H): 3275,
(C-Cp): 3091, (P-Ph): 1446, (O P): 1029, (C C-Cp): 1465; Anal. Calc.
for [C34H38NOPFeRuCl2] (735.48 g/mol): C 55.53, N 1.90, H 5.21;
found: C 55.40, N 1.87, H 5.19%.
cal signals for monosubstituted ferrocenes. The structures for these
ferrocene based chiral amino alcohols are consistent with the data
obtained from a combination of multinuclear NMR spectroscopy,
IR spectroscopy and elemental analysis.
The synthetic procedure for the preparation of the ferrocenyl-
phosphinite ligands is shown in Scheme 1. Ferrocenyl-phosphinite
ligands, 9–16 were synthesized by a hydrogen abstraction from
the described ferrocene based chiral amino alcohols 1–8, by a
base (Et3N) and the subsequent reaction with one equivalents
of Ph2PCl, in anhydrous toluene under inert argon atmosphere
(Scheme 1). The progress of this reaction was conveniently followed
2.2.7. [(2S)-2-(ferrocenylmethylamino)-4-methylpentyl
diphenylphosphinito(dichloro(ꢀ6-benzene)ruthenium(II))] (23)
Yield: 188 mg, 84%; mp: 133–136 ◦C; [˛]D20 +34.8 (c 1.2, MeOH)];
1H NMR (400.1 MHz, CDCl3, ppm) ı: 0.80 (br, 6H, CH(CH3)2), 1.32
(br, 2H, CH2CH), 1.47 (br, 1H, CH(CH3)2), 2.80 (m, 1H, CHNH), 3.57
(br, 2H, CH2NH), 3.70 (br, 1H, CH2OP, (a)) 4.06 (br, 1H, CH2OP,

228
U. Is¸ık et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
Scheme 1. Reagents and conditions (i) NaBH4-I2, THF; (ii) ferrocenecarboxaldehyde, K2CO3, CHCl3, NaBH4, 1.5 h, for 1–8; (iii) 1 equiv. Ph2PCl, 1 equiv. Et3N, toluene for 9–16
(iv) 1/2 equiv. [Ru(␩6-benzene)(␮-Cl)Cl]2, toluene for 17–24.
1
by 31P–{ H} NMR spectroscopy. The signal of the starting material
the values previously observed for similar compounds [78] with a
coordination shift of approximately ı 3.0 ppm (see SI) attributed
to formation of the corresponding ferrocenyl-phosphinite ligands.
Analysis by 1H NMR spectroscopy reveals that these complexes to
be diamagnetic, exhibiting signals corresponding to the aromatic
rings for 17–24 near at ı 8.0–7.0 ppm and the C6H6 protons as dou-
blets, approximately at ı 5.4 ppm (d, 3J = 4.8 Hz). In the 13C NMR
spectra of complexes, the C6H6 carbon resonance was occurred
approximately at 89.0 (d) ppm with J (31P–13C) coupling constants
of the carbons of the phenyl rings 3.0 Hz, which is consistent
with the literature values [79–81]. The most relevant signals in
PPh2Cl at ı 81.0 ppm disappeared and new singlet appeared down-
field due to the ferrocenyl-phosphinite ligands. The 31P–{ H} NMR
1
spectra of compounds, 9–16 show single resonances due to phos-
phinite at approximately ı 115 ppm [67–71] in line with the values
previously observed for similar compounds [72–75]. The assign-
ment of the 1H chemical shifts was derived from 2D HH-COSY
spectra and the appropriate assignment of the 13C chemical shifts
from DEPT and 2D HMQC spectra. Furthermore, IR spectra and C,
H, N elemental analyses are in agreement with the proposed struc-
tures for ferrocenyl-phosphinite ligands. Typical spectra of these
ligands are illustrated in supporting information.
1
13C–{ H} NMR spectra of the complexes are those corresponding
to mono- and unsubstituted ferrocene units [82–84] (for details
see experimental section). Elemental analyses of the complexes
are also consistent with the suggested molecular formulas. The
absorption bands corresponding to ferrocenyl-phosphinite ligands
in Ru(II) complexes in the IR spectra of complexes do not exhibit
significant differences with respect to those of free ligands (see SI
and experimental section). Although, single crystals of complexes
were obtained by slow diffusion of diethylether into a solution
of the compound in dichloromethane over several days, unfortu-
nately we were not able to protect them from rapid decomposition
in air.
The ability of dimers {[Ru(arene)(␮-Cl)Cl]2} to form mono-
nuclear complexes of general formula [Ru(␩6-arene)Cl2L] is
well-known [76]. The whole reactions resulting 17–24 are
depicted in Scheme 1. Treatment of [Ru(␩6-benzene)(␮-Cl)Cl]2
with ferrocenyl-phosphinite ligands 9–16 in ½:1 molar ratio in
toluene resulted the formation of mononuclear complexes 17–24
as orange–red crystalline solids. All the ruthenium complexes were
readily synthesized in good yields, which are air stable, red micro-
crystalline powders. The ferrocenyl-phosphinites were expected
to cleave the [Ru(benzene)Cl2]2 dimer to give the corresponding
complexes, 17–24 via monohapto coordination of the phosphi-
nite group. Furthermore, the reactions between Ru(II) precursor
and ferrocenyl-phosphinites are not affected by the molar ratio
of [Ru(␩6-benzene)(␮-Cl)Cl]2 as well as the steric and electronic
properties of the donor phosphorus atoms. The initial color change,
i.e., from clear orange to deep red was attributed to the dimer
cleavage most probably by the ferrocenyl-phosphinite ligand [77].
All the complexes were isolated as indicated by singlets in the
3.2. Asymmetric transfer hydrogenation of acetophenone
derivatives with isoPrOH
The use of organometallic complexes as catalysts for asymmet-
ric transfer hydrogenation from a suitable donor (usually isoPrOH
or formic acid) has been the subject of ongoing research for
some decades. Hydrogen gas presents considerable safety hazards
1
31P–{ H} NMR spectra at approximately ı 112 ppm, in line with

U. Is¸ık et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
229
especially for large scale reactions [85,86]. The use of a solvent that
can donate hydrogen overcomes these difficulties. isoPrOH is a pop-
ular reactive solvent for transfer hydrogenation reactions since it
is easy to handle and relatively non-toxic, environmentally benign,
and inexpensive. The volatile acetone by-product can also be eas-
ily removed to shift unfavorable equilibria. Efficient catalysts for
the asymmetric transfer hydrogenation of ketones include Evans’
samarium complexes with chiral amino alcohols ligand [87,88] and
Noyori and Ikariya’s ruthenium complexes containing arene and
N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine (TsDPEN) lig-
ands [89,90]. Additionally, chiral diphosphinite ligands derived
from the reaction of 1,1ꢀ-bi-2-naphthol (BINOL) with chlorodi-
arylphosphine were easily synthesized and modified and they were
widely used as chiral auxiliaries in rhodium, iridium and palladium
asymmetric catalytic reactions [91,92].
Ferrocenyl-phosphinite ligand families demonstrate outstand-
ing air stability and are designed to avoid any extreme conditions
showing the easiness of preparation. Thus, these kind ligands com-
bine a number of characteristics making uniquely attractive for
asymmetric catalysis [93]. The oxygen atom in phosphinite ligand
increases the distance between the chiral moiety and the PPh2
group. Consequently there is less control of stereoselectivity in
the catalyst-substrate interaction. Furthermore, the presence of
of base, such as KOH and NaOH, had little influence on the conver-
sion and enantioselectivity (Table 2, entries 1–4,b). Reduction of
acetophenone into (S)- or (R)-1-phenylethanol could be achieved
in high yield by increasing the temperature up to 82 ◦C (Table 2,
entries 1–8). Furthermore, optimization studies of the catalytic
reduction of acetophenone in isoPrOH showed that good activity
was obtained with a base/ligand ratio of 5:1 (Table 2, entries 9–16).
Although the conversions gradually decreased on increasing the
mole ratios of [acetophenone]/[Ru] from 100/1 to 500/1 or 1000/1,
except the time lengthened (lower TOF), the enantioselectivities
were still high (Table 2, entries 17–24). Namely, the ee’s remained
unchanged when the molar ratios of substrate to catalyst were
increased.
These ferrocenyl based monodendate phosphinite ligands
with amino (NH) moiety show much higher activity and enan-
tioselectivity. Similar tendency was reported in earlier studies
[112], indicating that the NH functional moiety in ligand plays
an important role in Ru(II)-ligand catalytic system. Higher
activity and enantioselectivity of amino containing phosphi-
nite ligand also may be due to the fact that NH moiety can
stabilize the catalytic transition state [117,115,118,114]. Fur-
thermore, it is noteworthy that the catalytic systems, 17–24
display the differences in reactivity. These results indicate
that the structure of the monodenate ferrocenyl-phosphinite
ligands is a crucial factor for acceleration the reaction. Com-
pared to the other complexes, [(2S)-2-(ferrocenylmethylamino)-
2-phenylethyl diphenylphosphinito(dichloro(␩6-benzene) ruthe-
nium(II))], 18 appears to provide a bit more effective chiral
environment around the ruthenium. In the context of the results, it
could be reasonably argued that the absolute configuration of the
product is governed by the carbon centered chirality.
the C
O P bond substantially increases the flexibility of the back-
bone and consequently decreases the enantioselectivity of the
catalyst. In spite of these drawbacks, the excellent catalytic per-
formance and the higher structural permutability of phosphinite
based transition metal catalysts [94–97], prompted us to develop
new catalyst systems with well-shaped monodendate phosphinite
ligands [98–102]. Encouraged by our recent success in the develop-
ment of new chiral and highly active ligands [103,104], we initiated
a study of the synthesis of a series of monodendate ferrocenyl-
phosphinite ligands, and investigated their efficiency in ruthenium
catalyzed asymmetric transfer hydrogenations.
In a preliminary study, complexes 17–24 were evaluated as
precursors for the catalytic asymmetric transfer hydrogenation of
acetophenone by isoPrOH. A comparison of complexes 17–24 as
precatalysts for the asymmetric hydrogenation of acetophenone by
isoPrOH is summarized in Table 1. Catalytic experiments were car-
ried out under inert (Ar) atmosphere using standard Schlenk-line
techniques. These systems catalyzed the reduction of acetophe-
none to corresponding alcohol ((S)-,(R)-1-phenylethanol) in the
presence of NaOH.
To an isoPrOH solution of Ru(II)-monodendate ferrocenyl-
phosphinite complex, an appropriate amount of acetophenone and
NaOH/isoPrOH solutions were added, at room temperature. The
solution was stirred, and then examined with capillary GC analy-
sis. At room temperature, transfer hydrogenation of acetophenone
occurred very slowly [105], with low conversion (up to 17%, 24 h)
and moderate to high enantioselectivity (up to 90% ee) in the reac-
tions (Table 1, entries 1–8). During this period, the color changed
from orange to deep red. As a result of the reversibility at room tem-
perature the prolonging the reaction time (72 h) led to a decreasing
of enantioselectivity, as indicated by the catalytic results collected
with catalysts, 17–20 (entries 9–12) [106,107].
As can be inferred from Table 1 (entries 13–20) the presence
of base is necessary to observe appreciable conversions. It is well-
known that the base facilitates the formation of ruthenium alkoxide
by abstracting proton of the alcohol and subsequently alkoxide
undergoes ␤-elimination to give ruthenium hydride, which is an
active species in this reaction. This is the mechanism proposed
by several research groups on the studies of ruthenium catalyzed
transfer hydrogenation reaction by metal hydride intermediates
[108–111]. Namely, role of the base is to generate a more nucleophi-
lic alkoxide ion which would rapidly attack the ruthenium complex
responsible for dehydrogenation of isoPrOH. In addition, the choice
As seen from Table 2, the catalytic activities in the studied
hydrogen transfer reactions were generally much higher for the
17–20 than those for the other complexes. So, these four complexes
were extensively investigated with acetophenone derivatives. The
catalytic reduction of acetophenone derivatives was tested with
the conditions optimized for acetophenone and the results are
summarized in Table 3 which illustrates conversions of the reduc-
tion performed in a 0.1 M of iso-PrOH solution containing 17–20
and NaOH (ketone:Cat.:NaOH = 100:1:5). The results demonstrate
that a range of acetophenone derivatives can be hydrogenated
with good enantioselectivities. It has also been shown that the
catalytic activities in the studied hydrogen transfer reactions were
generally much higher for the [(2S)-2-(ferrocenylmethylamino)-
2-phenylethyl
diphenylphosphinito(dichloro(␩6-benzene)
ruthenium(II))] 18 than for the complexes, 17, 19 and 20.
This different behavior in enantioselectivities can be explained
on the basis of aromatic moiety (phenyl or benzyl) near chiral
carbon center in the ligand backbone responsible for the opti-
mization molecular rigidity. These results have also shown that
enantiomeric purity of the product can be affected by electronic
factors, and steric factors of the substituents on the ligand.
The examination of the results indicates clearly that with each
of the tested complexes, the highest enantioselectivity was
found for transfer hydrogenation of o-methoxyacetophenone
(97% ee) when [(2S)-2-(ferrocenylmethylamino)-2-phenylethyl
diphenylphosphinito(dichloro(␩6-benzene) ruthenium(II))] 18
was used as the catalyst precursor. 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. The introduction of electron withdrawing substituents 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 [119,120] As expected, the lowest
enantioselectivity was observed in transfer hydrogenation of
p-methoxyacetophenone. From the results, the introduction of an

230
U. Is¸ık et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
Table 1
Transfer hydrogenation of acetophenone with iso-PrOH catalyzed by Ru-ferrocenyl monodendate phosphinite complexes (17–24).
H
O
HO
Cat.
*
isoPrOH
acetone
.
g
Entry
Complex
S/C/NaOH
Time (h)
Conversion (%)d
% eee
Configurationf
TOF (h−1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
17a
18a
19a
20a
21a
22a
23a
24a
17a
18b
19b
20b
17c
18c
19c
20c
21c
22c
23c
24c
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
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
24
24
24
24
24
24
24
24
72
72
72
72
1
1
1
1
1
1
1
1
13
17
11
12
12
13
11
10
48
49
30
36
<5
<5
<5
<5
<5
<5
<5
<5
85
90
79
76
72
76
60
65
81
88
75
72
–
–
–
–
–
–
–
–
R
S
R
S
R
S
S
S
R
S
R
S
–
–
–
–
–
–
–
–
<5
<5
<5
<5
<5
<5
<5
<5
Trace
Trace
Trace
Trace
–
–
–
–
–
–
–
–
Reaction conditions:
a
At room temperature; acetophenone/Ru/NaOH, 100:1:5.
b
c
d
e
f
At room temperature; acetophenone/Ru/NaOH, 100:1:5 (72 h).
Refluxing in iso-PrOH; acetophenone/Ru, 100:1, in the absence of base.
Determined by GC (three independent catalytic experiments).
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column.
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values, (S) or (R) configuration was obtained in all experiments.
g
TOF = (mol product/mol Cat.) × h−1
.
Table 2
Transfer hydrogenation of acetophenone with isoPrOH catalyzed by Ru-ferrocenyl based monodendate phosphinite complexes (17–24).
g
Entry
Catalyst
S/C/NaOH
Time (h)
Conversion (%)d
% eee
Configurationf
TOF (h−1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
17a
18a
19a
20a
21a
22a
23a
24a
17c
17c
17c
17c
18c
18c
18c
18c
17
18
19
20
17
18
19
20
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:3
100:1:5
100:1:7
100:1:9
100:1:3
100:1:5
100:1:7
100:1:9
500:1:5
500:1:5
500:1:5
500:1:5
1000:1:5
1000:1:5
1000:1:5
100:01:5
1/3 (1/3)b
99 (98)
98 (97)
99 (98)
97 (97)
96
90 (87)
95 (92)
82 (78)
78 (76)
74
R
S
R
S
R
S
S
S
R
R
R
R
S
S
S
S
R
S
R
S
R
S
R
S
297 (294)
294 (291)
198 (294)
194 (291)
192
1/3 (1/3)b
1/2 (1/2)b
1/2 (1/2)b
1/2
1/2
1/2
1/2
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
3/2
3/2
2
99
97
95
95
99
94
92
93
98
94
90
98
99
99
99
98
72
73
71
84
90
86
80
90
95
88
84
91
95
80
79
88
198
194
190
285
297
282
276
279
294
282
270
65
66
50
50
33
2
3
3
5
98
99
98
94
81
79
33
20
20
5
Reaction conditions:
a
Refluxing in isoPrOH; acetophenone/Ru/NaOH, 100:1:5.
Refluxing in isoPrOH; acetophenone/Ru/KOH, 100:1:5.
Refluxing in iso-PrOH; acetophenone/Ru/NaOH, 500:1:5.
Determined by GC (three independent catalytic experiments).
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column.
b
c
d
e
f
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values, (S) or (R) configuration was obtained in all experiments.
g
TOF = (mol product/mol Cat.) × h−1
.

U. Is¸ık et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 225–233
231
Table 3
Asymmetric transfer hydrogenation results for substituted acetophenones catalyzed by Ru-ferrocenyl based monodendate phosphinite complexes (17–20).a
OH
O
O
OH
Cat / NaOH
*
R
R
+
+
.
d
Entry
1
Catalyst
Substrate
Product
Time (h)
Conv. (%)b
ee (%)c
86
TOF (h−1
388
)
Config.e
17
1/4
97
R
O
OH
*
2
3
4
18
19
20
1/4
1/3
1/3
98
99
97
91
84
79
392
297
291
S
R
S
F
F
5
17
1/3
98
84
294
R
O
OH
*
6
7
8
18
19
20
1/3
1/2
1/2
99
98
98
90
82
78
297
196
196
S
R
S
Cl
Br
Cl
Br
9
17
1/3
99
85
297
R
O
OH
*
10
11
12
18
19
20
1/3
1/2
1/2
99
98
98
89
82
75
297
196
196
S
R
S
13
17
1/4
98
87
392
R
O
OH
14
15
16
18
19
20
1/4
1/3
1/3
99
98
99
90
81
78
396
294
297
S
R
S
*
O2N
MeO
O2N
MeO
17
17
1/2
98
93
196
R
O
OH
*
18
19
20
18
19
20
1/2
1
1
99
99
98
97
83
80
198
99
98
S
R
S
21
17
1
98
78
98
R
O
OH
*
22
23
24
18
19
20
1
2
2
98
99
97
85
77
71
98
49
48
S
R
S
MeO
MeO
a
b
c
Catalyst (0.005 mmol), substrate (0.5 mmol), isoPrOH (5 mL), NaOH (0.025 mmol %), 82 ◦C, the concentration of acetophenone derivatives are 0.1 M.
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), yields are based on aryl ketone.
Determined by capillary GC analysis using a chiral cyclodex B (Agilent) capillary column (30 m × 0.32 mm I.D. × 0.25 ␮m film thickness).
d
e
TOF = (mol product/mol Cat.) × h−1
.
Determined by comparison of the retention times of the enantiomers on the GC traces with literature values.
electron-donating group such as methoxy group to the p-position
decelerates the reaction, but that to the o-position increases the
rate and improves the enantioselectivity. The introduction of
electron-withdrawing substituents, such as F or NO2, to the para
positions of the aryl ring of the ketone, resulted in improved
activity with good enantioselectivity (Table 3, entries 1–4 and
13–16).
demonstration that they are competent auxiliaries for catalysis
opens up a neglected vein in the rich chemistry of phosphorus lig-
ands. Studies further investigating the scope of these ligands are
currently in progress.
Acknowledgements
Partial support from Dicle University (Project number: DÜBAP
12-FF-140) is gratefully acknowledged.
4. Conclusions
In conclusion, we have demonstrated, for the first time, a new
class of readily available, air stable Ru(II)-ferrocenyl-phosphinite
catalysts, which are efficient for the asymmetric transfer hydro-
genation of aromatic ketones. High yield and moderate to excellent
enantioselectivities were obtained by using these complexes as cat-
alyst. The chirality of the carbon center in the ligand backbone is of
particular importance for asymmetric induction offering an obvi-
ous target for further optimization. The simplicity and efficiency
clearly make it an excellent choice of catalyst for the practical
preparation of highly valued alcohols via the catalytic asymmetric
transfer hydrogenation of ketones. Furthermore, the develop-
ment of a practical synthesis of ferrocenyl-phosphinites and
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2013.08.005.
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