Ruthenium(II)-catalyzed transfer hydrogenation of aldehydes with new water-soluble monotosylated ethylenediamines as ligands

Article abstract of DOI:10.1007/s11164-011-0345-4

New water-soluble monotosylated ethylenediamines containing quaternary ammonium groups were conveniently synthesized from ethylenediamine. The ruthenium catalysts prepared in situ from ruthenium complex [RuCl ₂(p-cymene)]₂ and water-soluble monotosylated ethylenediamine ligands were used in transfer hydrogenation of aldehydes, and excellent conversions and chemoselectivities were achieved with sodium formate as reductant in neat water. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0345-4

Res Chem Intermed (2012) 38:261–268
DOI 10.1007/s11164-011-0345-4
Ruthenium(II)-catalyzed transfer hydrogenation
of aldehydes with new water-soluble monotosylated
ethylenediamines as ligands
•
•
•
Hengrui Huo Zhongqiang Zhou Aiqing Zhang
Lamei Wu
Received: 23 April 2011 / Accepted: 20 June 2011 / Published online: 6 July 2011
Ó Springer Science+Business Media B.V. 2011
Abstract New water-soluble monotosylated ethylenediamines containing quater-
nary ammonium groups were conveniently synthesized from ethylenediamine. The
ruthenium catalysts prepared in situ from ruthenium complex [RuCl2(p-cymene)]2
and water-soluble monotosylated ethylenediamine ligands were used in transfer
hydrogenation of aldehydes, and excellent conversions and chemoselectivities were
achieved with sodium formate as reductant in neat water.
Keywords Ruthenium ꢀ Monotosylated ethylenediamine ꢀ
Transfer hydrogenation ꢀ Water ꢀ Aldehyde
Introduction
Reduction of aldehydes to the corresponding alcohols is a synthetically important
transformation, both in the pharmaceutical and chemical industries. Among the
methods used to carry out this reaction, catalytic transfer hydrogenation (CTH) with
the aid of a stable hydrogen source has emerged as the most viable. Water is a
desirable solvent for catalysis with respect to environmental concerns, safety, and
cost [1, 2]. Aqueous-phase catalytic transfer hydrogenation has attracted attention
for the relatively benign nature of the reagents and mild reaction conditions
employed [3–9]. An interesting approach to aqueous catalysis is the use of water-
soluble catalysts, which are assembled through the use of water-soluble ligands
[10]. The first example of transfer hydrogenation of aldehydes catalyzed by a water-
soluble ruthenium-phosphine catalyst in neat water was described by Joo and
H. Huo ꢀ Z. Zhou (&) ꢀ A. Zhang ꢀ L. Wu
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and
Ministry of Education, College of Chemistry and Materials Science, South-Central University
for Nationalities, Wuhan 430074, China
e-mail: zhou-zq@hotmail.com
123

262
H. Huo et al.
coworkers [11, 12]. In this respect, the development of water-tolerant catalysts and
water-soluble ligands has become an area of intense research [13–28]. Given the
interest in conducting transfer hydrogenation of aldehydes in water, the search for
new catalysts, which are active and stable in aqueous medium, is a significant
challenge. In continuing our efforts for developing water-soluble ligands [29],
herein we describe synthesis of new water-soluble monotosylated ethylenediamines
and their application in ruthenium(II)-catalyzed transfer hydrogenation of aldehydes
in neat water using sodium formate as the hydrogen source.
Experimental
Materials and instruments
Melting points were determined on a melting point apparatus and were uncorrected.
1
IR spectra were recorded on a Nicolet Nexus 470 FTIR spectrometer. H NMR
spectra were recorded on a Bruker Avance III 400 with TMS as internal standard.
The conversions were measured by GC–MS on an Agilent 5973 N (HP-5 ms
capillary column). Reactions were monitored by thin layer chromatography using
precoated silica plates (silica gel for thin layer chromatography, GF254, Qingdao
Haiyang Chemical Co., Ltd). The chemicals used in this work were purchased from
the Alfa Aesar, Acros and Sinopharm Chemcial Reagent Co., Ltd.
N-Boc-N0-(4-Bromomethylphenylsulfonyl)-ethylenediamine (3)
Triethylamine (2.40 mL, 16 mmol) and N-Boc-ethylenediamine (2.40 g, 15 mmol)
were dissolved in CH2Cl2 (40 mL). The solution was stirred at 0 °C and
4-(bromomethyl)benzenesulfonyl chloride (4.32 g, 16 mmol) in CH2Cl2 (20 mL)
was added dropwise. The mixture was allowed to warm to room temperature and
stirred for an additional 22 h at room temperature. The mixture was washed
successively with 0.5 M citric acid, saturated sodium bicarbonate, and brine and
dried over anhydrous Na2SO4. After concentration, the crude product was purified by
recrystallization from benzene to afford 3 as a white powder (3.85 g, 65.3% yield).
M.p. 130–131 °C. IR (KBr), m: 3,373, 3,285, 2,980, 1,688, 1,530, 1,447, 1,327, 1,275,
1
1,157, 1,089, 894, 783, 692, 568 cm-1. H NMR (CDCl3, 400 MHz): d 7.84 (d,
J = 8.2, 2H), 7.54 (d, J = 8.0, 2H), 5.52 (s, 1H), 4.94 (s, 1H), 4.63 (s, 1H), 4.50 (s,
1H), 3.24 (s, 2H), 3.08 (s, 2H), 1.43 (s, 9H). 13C NMR (CDCl3, 100 MHz): d 156.65,
142.17, 139.76, 129.20, 127.48, 80.01, 43.80, 40.28, 31.52, 28.34.
Preparation of water-soluble monotosylated ethylenediamines (5)
To a solution of 3 (393 mg, 1.0 mmol) in CH3CN was added triethylamine (303 mg,
3.0 mmol). The mixture was stirred under reflux for 8 h. After cooling to room
temperature, the reaction mixture was concentrated. The residue was purified by
column chromatography on silica gel (petroleum ether/AcOEt = 1:1, methanol as
eluents) to give the desired product 4a as a colorless solid (481 mg, 97.3%). 5 M HCl
123

Ruthenium(II)-catalyzed transfer hydrogenation of aldehydes
263
methanol solution (18.5 mL) was added to 4a (481 mg,0.973 mmol) at 0 °C. After
stirring for 2 h at 0 °C, the mixture was allowed to warm to room temperature and
stirred for an additional 6 h at room temperature. The solution was concentrated
under reduced pressure to give 5a as a pale yellow solid (375 mg, 100%). M.p.
98–99 °C. IR (KBr), m: 3,438, 2,988, 1,477, 1,328, 1,159, 1,095, 1,027, 784, 692,
595 cm-1. 1H NMR (D2O, 400 MHz): d 7.83 (d, J = 7.2, 2H), 7.62 (d, J = 7.2, 2H),
4.38 (s, 2H), 3.12–3.10 (6H), 3.05 (s, 2H), 3.01 (s, 2H), 1.25 (s, 9H). 13C NMR (D2O,
100 MHz): d 139.85, 133.80, 132.78, 127.81, 59.03, 52.81, 40.00, 39.28, 7.16.
Compound 5b was similarly prepared in 75.6% yield (two steps). M.p.
128–130 °C. IR (KBr), m: 3,382, 2,963, 1,470, 1,331, 1,160, 1,095, 1,033, 865,
1
737, 606, 570 cm-1. H NMR (D2O, 400 MHz): d 7.84 (d, J = 6.8, 2H), 7.59 (d,
J = 6.8, 2H), 4.42 (s, 2H), 3.04–3.01 (10H), 1.66 (s, 6H), 1.22–1.21 (6H), 0.81 (s,
9H). 13C NMR (D2O, 100 MHz): d 139.74, 133.60, 132.74, 127.47, 60.33, 58.07,
39.84, 39.11, 23.35, 19.00, 12.80.
Compound 5c was similarly prepared in 72.7% yield (two steps). M.p.
111–112 °C. IR (KBr), m: 3,398, 2,927, 1,475, 1,329, 1,161, 1,094, 1,030, 730,
1
687, 601, 570 cm-1. H NMR (D2O, 400 MHz): d 7.94 (d, J = 7.2, 2H), 7.74 (d,
J = 7.2, 2H), 4.53 (s, 2H), 3.24–2.99 (m, 12H), 1.80 (s, 2H), 1.27 (s, 4H), 1.18 (s,
6H), 0.76 (s, 3H). 13C NMR (D2O, 100 MHz): d 139.91, 134.13, 132.72, 127.52,
66.43, 64.71, 57.90, 42.58, 39.92, 39.19, 30.97, 28.14, 25.44, 23.87, 21.98, 13.44.
Compound 5d was similarly prepared in 86.9% yield (two steps). M.p.
124–125 °C. IR (KBr), m: 3,424, 3,053, 1,631, 1,488, 1,327, 1,157, 1,094, 1,028,
1
768, 686, 608, 562 cm-1. H NMR (D2O, 400 MHz): d 8.87 (s, 2H), 8.53 (s, 1H),
8.04 (s, 2H), 7.87 (d, J = 4.4, 2H), 7.60 (s, 2H), 5.88 (s, 2H), 3.11–3.07 (4H). 13C
NMR (D2O, 100 MHz): d 146.48, 144.67, 138.90, 138.35, 129.92, 128.65, 127.85,
63.57, 39.89, 39.15.
Compound 5e was similarly prepared in 88.2% yield (two steps). M.p. 87–88 °C. IR
(KBr), m: 3,422, 3,094, 1,579, 1,448, 1,328, 1,159, 1,093, 1,026, 759, 683, 575 cm-1
.
1H NMR (D2O, 400 MHz): d 8.76 (s, 1H), 7.85 (s, 2H), 7.52 (s, 2H), 7.41 (s, 2H), 5.45
(s, 2H), 3.83 (s, 3H), 3.11 (s, 2H), 3.08 (s, 2H). 13C NMR (D2O, 100 MHz): d 139.57,
138.38, 136.56, 129.47, 127.77, 124.24, 122.56, 52.10, 40.00, 39.28, 36.00.
General procedure for transfer hydrogenation
[RuCl2(p-cymene)]2 (3.1 mg, 0.005 mmol) and 5 (0.012 mmol) were dissolved in
degassed water (10 mL). The resulting solution was stirred at 80 °C for 1 h under an
argon atmosphere. HCO2Naꢀ2H2O (5.2 g, 50.0 mmol) and benzaldehyde
(10.0 mmol) were then introduced. The mixture was stirred at 80 °C for a certain
period of time. After cooling to room temperature, the organic compounds were
extracted with ethyl ether (5 9 6 mL) by using a syringe. The conversions were
determined by GC–MS analysis.
Recycle experiment for the transfer hydrogenation of benzaldehyde in water
[RuCl2(p-cymene)]2 (3.1 mg, 0.005 mmol) and 5e (0.012 mmol) were dissolved in
degassed water (2 mL). The resulting solution was stirred at 80 °C for 1 h under an
123

264
H. Huo et al.
argon atmosphere. HCO2Naꢀ2H2O (1.04 g, 10.0 mmol) and benzaldehyde
(2.0 mmol) were then introduced. The mixture was stirred at 80 °C for a certain
period of time. After cooling to room temperature, the organic compounds were
extracted with ethyl ether (5 9 6 mL) by using a syringe. The conversions were
determined by GC–MS analysis. After the organic compounds were extracted, the
residual aqueous phase containing the catalyst was reused by adding formic acid
(0.078 mL, 2.0 mmol) to regenerate sodium formate and then benzaldehyde
(2.0 mmol) was added into the aqueous solution for a new reaction cycle, and the
second cycle of the reaction was started under the same conditions.
Results and discussion
The route for the synthesis of water-soluble monotosylated ethylenediamines
containing quaternary ammonium groups is illustrated in Scheme 1. According to
the literature procedures [30], ethylenediamine was monoprotected by vigorous
stirring with di-tert-butyl dicarbonate. Sulfonylation of the N-Boc-ethylenediamine
with 4-(bromomethyl)benzenesulfonyl chloride in the presence of triethylamine
afforded 3 in 65.3% yield. Introduction of the triethylamine to the 3 was achieved in
refluxing acetonitrile to give the quaternary salt 4a. Then the 4a was deprotected by
using 5 M HCl in methanol solution to provide a novel ionic ligand 5a in
quantitative yield. The ligands 5b, 5c, 5d and 5e were similarly prepared using tri-n-
butylamine, N,N-dimethyloctylamine, pyridine and 1-methylimidazole in place of
triethylamine, respectively. All the ligands obtained are highly soluble in water, but
insoluble in less-polar solvents, such as diethyl ether, ethyl acetate, and hexane.
O
a
c
b
H2N
NH2
BocHN
HN
S
CH2Br
BocHN
NH2
O
1
2
3
O
S
O
S
d
H3N
HN
CH2NR1R2R3
2Cl
BocHN
HN
CH2NR1R2R3
Br
O
O
5a NR1R2R3=N(C2H5)3
4a NR1R2R3=N(C2H5)3
5b NR1R2R3=N(CH2CH2CH2CH3)3
5c NR1R2R3=CH3(CH2)6CH2N(CH3)2
4b NR1R2R3=N(CH2CH2CH2CH3)3
4c NR1R2R3=CH3(CH2)6CH2N(CH3)2
4d NR1R2R3=
5d NR1R2R3=
N
N
N
N
4e NR1R2R3=
5e NR1R2R3=
N
N
Scheme 1 Synthesis of monotosylated ethylenediamines 5. Reagents and conditions: (a) (Boc)2O,
CH2Cl2, rt; (b) 4-(bromomethyl)benzenesulfonyl chloride, triethylamine, CH2Cl2; (c) tertiary amine,
CH3CN, reflux; (d) HCl(g), MeOH, 0 °C
123

Ruthenium(II)-catalyzed transfer hydrogenation of aldehydes
265
These properties, together with the straightforward synthesis, suffice for practical
applications in aqueous catalysis.
Firstly, benzaldehyde was tested as model substrate in the ruthenium(II)-
catalyzed transfer hydrogenation reaction using 5 as ligands. The results are listed in
Table 1. The catalysts were generated in situ by reacting monotosylated ethylen-
ediamines with [RuCl2(p-cymene)]2 in neat water for 1 h. Following the addition of
sodium formate and benzaldehyde with a HCOONa/substrate/catalyst (HCOONa/S/
C) ratio of 5000:1000:1, benzaldehyde was converted into benzyl alcohol in 100%
conversion in 5 h. The reaction was found to be considerably influenced by the
ligand used. Catalysts Ru-5a, Ru-5b and Ru-5e showed better catalytic performance
than catalysts Ru-5c and Ru-5d.
We subsequently explored the scope of the transfer hydrogenation of aldehydes
using the catalyst Ru-5e, and the results are shown in Table 2. As can be seen from
Table 2, various aldehydes tested, including electron-deficient and electron-rich
variants, were reduced with excellent conversions using sodium formate as
hydrogen source and neat water as the solvent in several hours. It is important to
point out that only the carbonyl group is reduced; other reducible groups, such as
C=C double bonds and C-halogen bonds, are not affected. Chemoselective
reduction of a,b-unsaturated aldehydes is one of the active research areas of great
practical importance as the products a,b-unsaturated alcohols, generally, are
important intermediates in the production of pharmaceuticals, agrochemicals, and
fragrances. The present catalyst system shows a remarkable selectivity towards the
transfer hydrogenation of the conjugated C=O bond in the transfer hydrogenation of
a,b-unsaturated aldehydes (Table 2, entries 11–13). This feature makes it very
appropriate for the production of a,b-unsaturated alcohols.
Table 1 Transfer hydrogenation of benzaldehyde catalyzed by [RuCl2(p-cymene)]2/5
Entry
Ligand
Time (h)
Conversion (%)a
1b
2
5a
5a
5b
5c
5d
5e
5e
5e
5e
1.83
1
100
100
100
100
100
100
100
100
100
3
1
4
2.5
5
5
6c
7b
8
0.17
3.5
1
9d
1.5
Unless otherwise indicated, reactions were performed using 10 mmol of benzaldehyde, 5 equiv of
HCOONa, and a S/C ratio of 1,000 in 10 mL of water under argon atmosphere at 80 °C
a
Determined by GC–MS analysis on a HP-5 ms column
b
The reaction was performed at 65 °C
Reactions were performed using 2 mmol of benzaldehyde, 5 equiv of HCOONa, and a S/C ratio of 200
c
in 2 mL of water under argon atmosphere at 80 °C
d
Reactions were performed using 20 mmol of benzaldehyde, 5 equiv of HCOONa, and a S/C ratio of
2000 in 15 mL of water under argon atmosphere at 80 °C
123

266
H. Huo et al.
Table 2 Transfer hydrogenation of aldehydes catalyzed by [RuCl2(p-cymene)]2/5e
Entry
Aldehyde
Time (h)
Conversion (%)a
1
Benzaldehyde
1
100 (97)
100 (97)
100 (95)
100 (96)
100 (97)
100 (98)
81
2
p-Tolualdehyde
2
3
p-Anisaldehyde
1.25
1.33
1.67
1
4
2-Chlorobenzaldehyde
4-Fluorobenzaldehyde
3-Nitrobenzaldehyde
4-Dimethylaminobenzaldehyde
3-Methoxybenzaldehyde
4-Bromobenzaldehyde
2-Furaldehyde
5
6
7
4
8
0.75
1
100 (99)
100 (96)
100 (96)
99 (96)c
100 (96)c
100 (96)c
9
10
11b
12b
13b
0.5
0.5
3.5
0.5
Cinnamaldehyde
Citral
Crotonaldehyde
Unless otherwise indicated, reactions were performed using 10 mmol of aldehydes, 5 equiv of HCOONa,
and a S/C ratio of 1,000 in 10 mL of water under argon atmosphere at 80 °C
a
Determined by GC–MS analysis on a HP-5 ms column and numbers in the brackets refer to yields of
isolated products
b
Reactions were performed using 2 mmol of aldehydes, 5 equiv of HCOONa, and a S/C ratio of 200 in
2 mL of water under argon atmosphere at 80 °C
Product is the corresponding a,b-unsaturated alcohol
c
Joo and coworkers reported that unsaturated aldehydes can be reduced under very
mild conditions (30–80 °C) with good yields and excellent selectivities to the
corresponding unsaturated alcohols by hydrogen transfer from HCOONa/H2O with
a water-soluble ruthenium-phosphine catalyst. This is the first example of transfer
hydrogenation of aldehydes catalyzed by transition metal catalyst in neat water [11,
12]. Ajjou and coworkers reported that [Rh(COD)Cl]2/TPPTS (P(m-C6H4SO3Na)3)
system is very effective water-soluble catalyst for the hydrogen transfer reduction of
a variety of aliphatic and aromatic ketones and aldehydes. Treatment of
benzaldehyde (5 mmol) in the presence of catalytic amount of [Rh(COD)Cl]2
(0.01 mmol), TPPTS (0.3 mmol), and Na2CO3 (1 mmol) in a deoxygenated mixture
of distilled water (10 mL) and isopropanol (3 mL) at 80 °C selectively afforded
benzyl alcohol in 98% yield in 2 h [18]. Himeda et al. [24] reported the transfer
hydrogenation of benzaldehyde catalyzed by an iridium catalyst with 4,40-
dihydroxy-2,20-bipyridine (DHBP) in an aqueous formate solution, the highest
turnover frequency(TOF) of 8.1 9 104 h-1 were obtained at 80 °C. Xiao and
coworkers have demonstrated that Ir-Ts(en) and Ir-CF3Ts(en) are highly active and
chemoselective catalysts for the transfer hydrogenation of aldehydes in water. The
catalytic reduction can be carried out with S/C ratios as high as 5 9 104:1, the initial
TOF near 1.3 9 105 h-1 [20]. Recently, Li et al. [25] reported amphiphilic
polymer-based catalyst Ir-PTsEN is likely the most efficient one for the aqueous-
phase transfer hydrogenation of aldehydes, and the highest TOF is up to
123

Ruthenium(II)-catalyzed transfer hydrogenation of aldehydes
267
Table 3 Catalyst recycling in transfer hydrogenation of benzaldehyde in water
Run
Ligand
Time (h)
Conversion (%)a
1
2
3
4
5
5e
5e
5e
5e
5e
0.17
0.42
0.5
100
100
100
0.83
4
[99
[99
Reactions were performed using 2 mmol of benzaldehyde, 5 equiv of HCOONa, and a S/C ratio of 200 in
2 mL of water under argon atmosphere at 80 °C in the first run. Since the second run, 2 mmol HCOOH
was added to regenerate sodium formate in every recycling run
a
Determined by GC–MS analysis on a HP-5 ms column
3.0 9 105 h-1. Ru–Ts(en) is lower active catalysts for the aqueous-phase transfer
hydrogenation of aldehydes. Benzaldehyde was reduced with almost complete
conversion in 2 h using Ru–Ts(en) with a S/C ratio of 1,000:1 at 65 °C [20]. We
found that benzaldehyde was reduced with complete conversion in 1.83 h using
water-soluble Ru-monotosylated ethylenediamine with a S/C ratio of 1,000:1 at
65 °C (Table 1, entry 1). Ruthenium-arene complexes containing water-soluble
monotosylated ethylenediamines ligands can be used as catalysts for transfer
hydrogenation of aldehydes in neat water using sodium formate as hydrogen donor,
with TOF up to 2 9 103 h-1 obtained (Table 2, entry 10).
The reusability of the catalytic system was also examined using benzaldehyde as
a substrate. An attractive feature of the present catalytic system lies in the fact that
the catalyst can be readily removed from the product by addition of a low polarity
solvent. For each cycle, the catalyst aqueous solutions were carefully separated from
the organic phases, followed by the addition of 1.0 equiv. formic acid and the
substrate. The catalytic system could be reused at least five consecutive runs
(Table 3). Although a decrease in activity was observed from the second run, when
the catalyst was run for the fifth time, the reaction could still reach 99% conversion.
A longer reaction time was needed to complete the reaction, presumably due to the
decomposition of catalyst during the course of recovery. In the course of the
recycling experiments, the color of the aqueous phases containing the catalyst
changed from yellow to pale yellow, which indicate some decomposition of the
catalyst.
Conclusions
In conclusion, new water-soluble monotosylated ethylenediamine containing
quaternary ammonium groups have been successfully synthesized in good yields.
Water-soluble ruthenium catalysts prepared in situ from ruthenium complex
[RuCl2(p-cymene)]2 and monotosylated ethylenediamine ligands were used in
transfer hydrogenation of aldehydes, and high conversions and chemoselectivities
were achieved with sodium formate as reductant in neat water. The catalytic system
could be easily recovered and reused several times, although the activity of the
123

268
H. Huo et al.
catalyst showed a remarkable drop on the recycled application. Improvement of the
recyclability of the catalytic system and further investigations on the application of
water-soluble monotosylated ethylenediamine ligands in catalysis are still underway
in our laboratory.
Acknowledgments Financial support of this work by the Natural Science Foundation of Hubei
Province (2007ABA291, 2008CDA067) is gratefully acknowledged.
References
1. A. Chanda, V.V. Fokin, Chem. Rev. 109, 725 (2009)
2. C.J. Li, L. Chen, Chem. Soc. Rev. 35, 68 (2006)
3. F. Joo, Acc. Chem. Res. 35, 738 (2002)
4. X.F. Wu, J.L. Xiao, Chem. Commun. 2449 (2007)
5. X.F. Wu, C. Wang, J.L. Xiao, Platinum Metals Rev. 54, 3 (2010)
6. J. Canivet, G. Labat, H. Stoeckli-Evans, G. Suss-Fink, Eur. J. Inorg. Chem. 4493 (2005)
7. P. Stepnicka, J. Ludvik, J. Canivet, G. Suss-Fink, Inorg. Chim. Acta 359, 2369 (2006)
8. J. Canivet, G. Suss-Fink, Green Chem. 9, 391 (2007)
9. J. Canivet, G. Suss-Fink, P. Stepnicka, Eur. J. Inorg. Chem. 4736 (2007)
10. K.H. Shaughnessy, Chem. Rev. 109, 643 (2009)
11. F. Joo, A. Benyei, J. Organomet. Chem. 363, C19 (1989)
12. A. Benyei, F. Joo, J. Mol. Catal. 58, 151 (1990)
13. R. Bar, Y. Sasson, J. Blum, J. Mol. Catal. 26, 327 (1984)
14. R. Bar, L.K. Bar, Y. Sasson, J. Blum, J. Mol. Catal. 33, 161 (1985)
15. S. Ogo, N. Makihara, Y. Watanabe, Organometallics 18, 5470 (1999)
16. N. Makihara, S. Ogo, Y. Watanabe, Organometallics 20, 497 (2001)
17. S. Ogo, N. Makihara, Y. Kaneko, Y. Watanabe, Organometallics 20, 4903 (2001)
18. A.N. Ajjou, J. Pinet, J. Mol. Catal. A: Chem. 214, 203 (2004)
19. S. Bolano, L. Gonsalvi, F. Zanobini, F. Vizza, V. Bertolasi, A. Romerosa, M. Peruzzini, J. Mol.
Catal. A: Chem. 224, 61 (2004)
20. X. Wu, J. Liu, X. Li, A. Zanotti-Gerosa, F. Hancock, D. Vinci, J. Ruan, J. Xiao, Angew. Chem. Int.
Ed. 45, 6718 (2006)
21. C.A. Mebi, R.P. Nair, B.J. Frost, Organometallics 26, 429 (2007)
22. Z.S. Liu, G.L. Rempel, J. Mol. Catal. A: Chem. 278, 228 (2007)
23. Z.S. Liu, G.L. Rempel, J. Appl. Polym. Sci. 108, 3262 (2008)
24. Y. Himeda, N. Onozawa-Komatsuzaki, S. Miyazawa, H. Sugihara, T. Hirose, K. Kasuga, Chem. Eur.
J. 14, 11076 (2008)
25. J. Li, Y. Zhang, D. Han, G. Jia, J. Gao, L. Zhong, C. Li, Green Chem. 10, 608 (2008)
26. L. He, J. Ni, L.-C. Wang, F.-J. Yu, Y. Cao, H.-Y. He, K.-N. Fan, Chem. Eur. J. 15, 11833–11836
(2009)
27. H. Cheng, R. Liu, J. Hao, Q. Wang, Y. Yu, S. Cai, F. Zhao, Appl. Organometal. Chem. 24, 763–766
(2010)
28. C. Leiva, H.C. Lo, R.H. Fish, J. Organomet. Chem. 695, 145 (2010)
29. Z. Zhou, L. Wu, Catal. Commun. 9, 2539 (2008)
30. T. Kofoed, H.F. Hansen, H. Ørum, T. Koch, J. Peptide Sci. 7, 402 (2001)
123