Multiphase enantioselective Kharasch-Sosnovsky allylic oxidation based on neoteric solvents and copper complexes of ditopic ligands

Article abstract of DOI:10.1039/c2dt30352c

Recoverable multiphase enantioselective catalytic systems for the Kharasch-Sosnovsky oxidation of cycloalkenes with tert-butyl peroxybenzoate are described, based on the use of [BMIM][PF₆] and a new solvent derived from glycerol as the catalyst

Full text of DOI:10.1039/c2dt30352c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 8285
www.rsc.org/dalton
PAPER
Multiphase enantioselective Kharasch–Sosnovsky allylic oxidation based on
neoteric solvents and copper complexes of ditopic ligands†‡
Luis Aldea, José I. García* and José A. Mayoral
Received 15th February 2012, Accepted 1st May 2012
DOI: 10.1039/c2dt30352c
Recoverable multiphase enantioselective catalytic systems for the Kharasch–Sosnovsky oxidation of
cycloalkenes with tert-butyl peroxybenzoate are described, based on the use of [BMIM][PF ] and a new
6
solvent derived from glycerol as the catalyst reservoir phases, and chiral copper complexes with different
ligands from the bis(oxazoline) family. The best results are obtained with the glycerol-derived solvent and
a recently described azabisoxazoline-based ditopic ligand, allowing up to four uses of the catalytic phase
with good results.
Introduction
hand, when the catalyst was precipitated after the reaction
with hexane, the recovered catalyst was still active but enantio-
selectivity decreased to 37% ee. Le Bras and Muzart described
the use of Cu (L-prolinate) complexes in an aqueous biphasic
system with enantioselectivities decreasing from 32% ee in the
1
,2
The Kharasch–Sosnovsky allylic oxidation of alkenes with tert-
butyl peroxybenzoate promoted by Cu(I) represents a cleaner cata-
lytic alternative to the stoichiometric oxidation using selenium
reagents. Although this reaction is known from the late fifties, the
enantioselective version was first described at mid-nineties.
In this regard enantioselectivities ranging from moderate to
good (usually over 60% ee) have been reached by using widely
20
first cycle to 22% ee in the fourth use.
3
–13
The best results described to date in the preparation of a re-
cyclable enantioselective catalyst for the Kharasch–Sosnovsky
reaction are based, however, in a different strategy, as is the
release and capture mechanism through the formation of insoluble
1
0–14
available ligands of the bis(oxazoline) family.
The whole
21,22
mechanism of the enantioselective reaction of cyclohexene with
tert-butyl perbenzoate, catalyzed by a bisoxazoline–copper
complex, with particular interest on the enantioselection mechan-
coordination polymers.
In this work, we described the use of
a chiral ditopic diazabis(oxazoline) (DAX) ligand, able to form
insoluble polymers with copper salts, for the allylic oxidation of
cycloalkenes with tert-butyl peroxybenzoate. Using acetonitrile as
the solvent results in good yields and enantioselectivities over
1
5
ism, has been recently studied.
The development of recoverable catalysts for this interesting
reaction has shown to be a difficult task and only a few examples
have been described in the literature to date, in all cases based
on non-covalent approaches. The first attempt was carried out
by Fache and Piva who described the use of proline modified
2
3
6
5% ee were reached after at least five recycles.
The catalyst was precipitated in hexane for recovery, following
the methodology also described by our group for the reaction of
1
6
24,25
cyclopropanation of alkenes.
During the last years the use of multiphase homogeneous cata-
lysis has appeared as an interesting approach regarding easy
catalyst recovery and reuse. In this regard the use of ionic liquids
as a reaction medium in reactions promoted by transition metal
17
with a fluorous ponytail and Cu O as the catalyst precursor.
2
26
The reaction was carried out in hexafluoroisopropanol leading to
only 20% ee in the first use, and decreasing to 13% ee upon
recovery. Better results were obtained by Bayardon and Sinou by
using bis(oxazolines) modified by fluorous ponytails in the
27,28
29
complexes
and, in particular, in enantioselective reactions
18,19
biphasic FC72/CH CN system.
Under these conditions 57% ee
3
has increased. Studies on oxidation catalysis in ionic liquids
30
was reached in the first run but results with the recovered catalyst
depended on the method used to recover it, so direct reuse of the
fluorous phase led to an almost inactive catalyst. On the other
have been recently reviewed and, to the best of our knowledge,
the only precedent in allylic oxidation is the use of a non-chiral
31
Cu (salen), with an ionic pyridinium tag in the backbone of the
ligand, in the allylic oxidation of cyclohexene with oxygen.
There are also several examples describing the use of catalysts
Instituto de Síntesis Química y Catálisis Homogénea, CSIC-Univ. de
Zaragoza, Calle Pedro Cerbuna, 12. E-50009 Zaragoza, Spain.
E-mail: ji.garcia@csic.es; Fax: +34 976762077; Tel: +34 976762271
32–36
bearing ligands of the bis(oxazoline) family in ionic liquids,
36
but only one of them describe an oxidation reaction, namely
the epoxidation of alkenes using dioxomolybdenum(VI) com-
plexes containing chiral oxazolines.
In the present paper we describe the use of bis(oxazoline)–
copper complexes to promote the Kharasch–Sosnovsky allylic
oxidation in neoteric solvents.
†
Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
‡
Electronic supplementary information (ESI) available: Typical HPLC
chromatograms of the reaction products, and chromatography conditions.
Numerical data used to prepare Fig. 2. See DOI: 10.1039/c2dt30352c
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8285–8289 | 8285

View Article Online
Table 1 Results obtained from the allylic oxidation of cyclohexene in
Results and discussion
[
BMIM][PF
6
], using different chiral ligands and copper salts
CuPF CuOTf Cu(OTf)
Run Yield (%) % ee Yield (%) % ee Yield (%) % ee
Catalysis in [BMIM][PF6]
6
2
Given that no previous studies had been carried out for this reac-
tion using ionic liquids as solvents, first of all we tested a series
of conventional chiral ligands of the bisoxazoline family, whose
behaviour in common organic solvents was already known.
Thus, bisoxazoline (Box), azabisoxazoline (Azabox) and pyri-
dine bisoxazoline (Pybox) bearing iso-propyl groups in the oxa-
zoline ring (Fig. 1) were compared to the oxidation of
a
b
Ligand
i
PrBox
1
2
3
1
2
3
1
2
3
1
2
3
1
63
70
42
49
63
59
94
92
78
69
61
72
65
41
33
19
53
23
17
13
3
72
47
7
32 73
37 42
27 36
17 45
21 43
25 60
57 60
19 62
11 49
23 44
31 60
21 46
50
45
34
37
23
15
43
9
i
PrAzaBox
43
47
58
72
62
49
41
45
48
—
i
PrPyBox
cyclohexene (Scheme 1) carried out in [BMIM][PF ] and using
6
i
different copper salts. We also included the ditopic PrDAX and
3
4
i
i
Pr-click-DAX ligands in the study (in these cases, a 2 : 1
PrDAX
49
55
17
53
55
31
11
—
3
7
copper–ligand ratio was used in the experiments).
After completion of the reaction, the products were extracted
with n-hexane and the results, determined by gas chromato-
graphy and HPLC as previously described, are gathered in
Table 1. As can be seen the best conversions and enantioselectiv-
i
Pr-click-
—
—
DAX
1
6
Experimental conditions: 5 mmol of cyclohexene, 0.5 mmol of tert-
butyl perbenzoate and 0.025 mmol of Cu complex in 2 ml of ionic
liquid are stirred at room temperature for 36 h. Products are extracted
1
2
ities are obtained using the Pybox ligand and CuOTf· C H
6
6
a
(henceforth CuOTf); although higher yields are obtained with
from the ionic liquid phase with n-hexane. Determined by GC.
b
Determined by HPLC.
CuPF ·4CH CN (henceforth CuPF ) the enantioselectivity is
6
3
6
very low. In general both yield and % ee decrease upon recovery.
In general, no clear relationship can be found between the use
38
of a particular ligand and copper salt. Yields tend to be some-
what better and more stable upon recovery with CuPF , followed
6
by Cu(OTf) (the latter requires the use of phenylhydrazine as
2
an external reducing agent). However, enantioselectivities are in
general low, hardly above 50% ee, and tend to decrease quickly
upon recovery. This behaviour might to be due to the partial loss
of ligand in the hexane extractions after the reaction, a point that
i
we will discuss later on. The use of the ditopic PrDAX or
i
Pr-click-DAX ligands does not provide better results than the
conventional monotopic ligands in this solvent.
Although the results obtained with ionic liquids are superior
to those previously described with fluorous solvents and ligands
11,12
modified with fluorous tags,
they are far from being useful
from a practical point of view, so we decided to test another type
of neoteric solvent.
Catalysis in a glycerol-derived solvent
We have recently described a new family of green solvents with
tuneable properties derived from glycerol, consisting of 1,3-di-
alkoxy-2-propanols and 1,2,3-trialkoxypropanes. Some of these
Fig. 1 Chiral ligands used in this work. In all cases, the stereogenic
carbon atoms have S configuration.
39
solvents have shown to be very efficient in enzyme catalysed
40–42
43,44
reactions,
as well as in some oxidation reactions.
Inter-
estingly, the glycerol-derived solvents bearing short fluorinated
alkyl chains, like trifluoroethyl, display some interesting combi-
nations of physico-chemical properties, such as being simul-
taneously hydrophobic and lipophobic, highly polar and
hydrogen bond donating, and possessing very low vapour
39
pressure. These properties seem closer to those of ionic liquids
rather than to those of conventional organic solvents, so similar
applications can be envisaged for this new family of neoteric sol-
vents. In fact, our group has demonstrated that 1,3-bis(2,2,2-
trifluoroethoxy)-2-propanol (henceforth BTFEP, Scheme 2) can
be advantageously used instead of ionic liquids to prepare
Scheme 1 Enantioselective Kharasch–Sosnovsky reactions catalysed
by copper complexes of bisoxazoline-type chiral ligands. The S-enantio-
mer of the corresponding cyclohexenyl benzoate is the major product in
all cases.
8286 | Dalton Trans., 2012, 41, 8285–8289
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 3 Results obtained from the allylic oxidation of cyclohexene
i
and cycloheptene in BTFEP, using the Pr-click-DAX-CuPF
catalyst
6
complex as
a
b
Scheme 2 Structure of BTFEP.
Alkene
Run
Yield (%)
% ee
Cyclohexene
1
2
3
1
2
3
4
63
58
55
77
75
75
76
80
76
66
85
83
84
78
Table 2 Results obtained from the allylic oxidation of cyclohexene in
BTFEP, using different chiral ligands and copper salts
Cycloheptene
CuPF
6
CuOTf
Cu(OTf)
2
a
b
Ligand
Run Yield (%) % ee Yield (%) % ee Yield (%) % ee
Experimental conditions: 5 mmol of cyclohexene, 0.5 mmol of tert-
butyl perbenzoate and 0.025 mmol of Cu complex in 2 ml of BTFEP are
stirred at room temperature for 36 h. Products are extracted from the
BTFEP phase with n-hexane. Determined by GC. Determined by
HPLC.
i
PrBox
1
2
3
78
73
34
81
76
74
60
29
31
95
84
88
43
35
33
63
52
25
7
63
67
61
67
57
55
62
35
37
62
76
78
15
13
7
25
37
11
9
7
5
41
23
17
53
64
60
63
76
74
57
27
24
69
85
87
45
28
19
61
27
31
17
7
a
b
i
PrAzaBox 1
2
3
i
PrPyBox
PrDAX
1
2
3
1
2
3
9
enantioselectivity in the first use is the highest obtained in this
series of catalytic experiments (80% ee), and, although it
decreases after catalyst recovery, this decrease is not so marked
as in the rest of cases, keeping 65% ee in the third use.
11
75
63
37
5
i
77
37
31
Experimental conditions: 5 mmol of cyclohexene, 0.5 mmol of tert-
butyl perbenzoate and 0.025 mmol of Cu complex in 2 ml of BTFEP are
stirred at room temperature for 36 h. Products are extracted from the
BTFEP phase with n-hexane. Determined by GC. Determined by
HPLC.
We then used the same catalytic system with another analo-
gous substrate, as in cycloheptene. The results obtained are even
better, with yields around 75% in four uses, and enantioselectiv-
ities ranging around 85% ee (>92 : 8 enantiomeric ratio) in the
three first uses. Only in the fourth one a slight decrease is
observed, but still keeping a fairly good 78% ee.
a
b
In order to gain some information about the deactivation
mechanism of these biphasic catalytic systems, some recovery
experiments were conducted as described below.
recoverable catalytic phases for an enantioselective conjugate
reduction.
45
We decided to test the same strategy to prepare an immobi-
lized catalytic phase for the Kharasch–Sosnovsky allylic oxi-
dation, i.e. dissolving the catalytic complex in BTFEP, and
extracting the reaction products with n-hexane after the com-
pletion of the reaction, given the immiscibility of both phases.
The results obtained in the recovery experiments are gathered in
Table 2.
The first run was conducted in the same conditions used
throughout the study. Recovery and reuse was also carried out
using the same procedures, until the third run. Then, the catalytic
phase was divided into two aliquots. One of them was directly
used in the fourth run, and the other was supplemented with
additional 0.002 mmol (15% with regard to the initial amount)
i
of Pr-click-DAX ligand. As can be seen, yields remain between
As a whole, the results obtained with BTFEP are not far
6
5–70% throughout the series of experiments. As in the prece-
different from those previously observed for [BMIM][PF ],
6
i
ding cases, enantioselectivity decreases upon recovery from an
initial 83% ee to 45% ee in the fourth run. However, when a
small amount of supplementary ligand is added, the enantio-
selectivity increases up to ca. 70% ee, and it is maintained for
two additional runs (Fig. 2). These results point to a loss of
chiral ligand from the catalytic phase as the most probable
reason for the enantioselectivity decrease observed upon recov-
ery. Consequently, more recoverable catalytic phases can be
envisaged by using stronger ligands, which remain coordinated
to copper, as well as ligands less soluble in hexane, the extrac-
tion solvent.
except in the case of the ditopic ligand PrDAX. With this
ligand, yields are fairly good upon recovery both with CuPF6
and Cu(OTf) . More interestingly, the enantioselectivities
2
obtained in the first use with both copper salts are around 75%
ee, better than those obtained in the ionic liquid. With CuPF6,
the enantioselectivity is still over 60% ee in the second use of
the catalytic phase. However, in the third run, the enantioselec-
tivity dramatically falls in all cases, although that obtained with
i
the PrDAX/CuPF combination is still the best one (37% ee).
6
At this point we decided to test another ditopic ligand of the
Azabisoxazoline family, namely the so-called click-DAX,
2
5
recently described by our group, based on a previous synthetic
4
6,47
strategy published by Reiser and co-workers.
The click-
Experimental
DAX ligand should be less soluble in n-hexane, thus improving
the recoverability of the catalytic systems. The results obtained
All the reagents, and solvents were commercially available,
except 1,3-bis(2,2,2-trifluoroethoxy)-2-propanol, which was pre-
pared from epichlorohydrin and 2,2,2-trifluoroethanol following
i
with the complexes of Pr-click-DAX and CuPF in BTFEP are
6
gathered in Table 3. As can be seen, the results obtained with the
click-DAX ligands are superior to those described in the pre-
vious experiments. In the reactions of cyclohexene, yields are
moderate, but rather stable upon recovery. More important, the
39
the previously described synthetic procedure. Copper(I) salts
1
used were CuOTf· C H and CuPF ·4CH CN. Chiral ligands
6
6
6
3
2
26,48,49
were synthesized according to literature procedures.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8285–8289 | 8287

View Article Online
To reuse the catalytic phase, the rest of the hexane was first
evaporated under vacuum, and new portions of cyclohexene and
perbenzoate were added, and the reaction conducted in the same
way. After the third use, the reaction was divided into two ali-
quots of 0.5 ml each. One of them was used in the preceding
way. To the second aliquot, 1.7 mg (0.002 mmol) of fresh
i
Pr-click-DAX ligand were added, and then a normal reaction
was carried out. This aliquot was reused two more times, with
the results shown in Fig. 2.
Conclusions
The combination of a neoteric solvent derived from glycerol and
the use of 1 : 2 complexes of a ditopic azabisoxazoline ligand
and CuPF allows good results to be obtained in the Kharasch–
6
Sosnovsky allylic oxidation of cycloalkenes. This solvent has
shown to be superior to [BMIM][PF ] regarding enantioselectiv-
6
Fig. 2 Recovery experiments from the reaction of cyclohexene with
ligand addition in the fourth run. Lighter shades refer to initial con-
ditions and darker shades to the experiments carried out after ligand
addition.
ity and, above all, catalyst recovery, allowing up to four uses
with good yields and enantioselectivities. This is far better than
previous results obtained with multiphasic fluorous or aqueous
systems, and opens the door to a new family of multiphasic
enantioselective catalytic systems for this reaction.
General procedure for catalytic experiments
A solution of the ligand (0.027 mmol for monotopic ligands or
Acknowledgements
0
(
.013 mmol for ditopic) and the corresponding copper salt
0.025 mmol) in 2 ml of anhydrous dichloromethane was stirred
at room temperature for 15 min under argon atmosphere, then
ml of the neoteric solvent was added and dichloromethane was
eliminated under vacuum. In the case of Cu(I) salts, cycloalkene
5 mmol) and tert-butyl peroxybenzoate (0.5 mmol) are directly
This work was made possible by the generous financial support
of the Ministerio de Economía y Competitividad (project
CTQ2011-28124).
1
(
Notes and references
added to the solution. In the case of using Cu(OTf) , phenylhy-
2
drazine (3 μl, 0.030 mmol) is first added to reduce Cu(II) to Cu(I)
and after 10 min the reagents are also added. Reactions were
stirred at room temperature and monitored by gas chromato-
graphy of aliquots extracted with n-hexane. After 36 h the reac-
tion mixture was extracted with n-hexane (3 × 1 ml) and the
final results were determined by gas chromatography and HPLC
1 M. S. Kharasch and G. Sosnovsky, J. Am. Chem. Soc., 1958, 80, 756–
7
56.
M. S. Kharasch, G. Sosnovsky and N. C. Yang, J. Am. Chem. Soc., 1959,
1, 5819–5824.
T. Punniyamurthy and L. Rout, Coord. Chem. Rev., 2008, 252, 134–154.
2
8
3
4 M. B. Andrus and J. C. Lashley, Tetrahedron, 2002, 58, 845–866.
5
J. Eames and M. Watkinson, Angew. Chem., Int. Ed., 2001, 40, 3567–
1
6
3571.
as previously described. The catalytic phase was further treated
under vacuum, to eliminate the possible residual amounts of n-
hexane and cycloalkene, and reused under the same conditions.
Typical chromatograms of these reactions and additional infor-
mation of experiments carried out with CuCl are available in the
ESI.‡
6
J.-M. Brunel, O. Legrand and G. Buono, C. R. Acad. Sci., Ser. II, 1999,
19–23.
7 H. Kwong, H. Yeung, C. Yeung, W. Lee, C. Lee and W. Wong, Coord.
Chem. Rev., 2007, 251, 2188–2222.
8
D. R. Boyd, N. D. Sharma, L. Sbircea, D. Murphy, T. Belhocine, J.
F. Malone, S. L. James, C. C. R. Allen and J. T. G. Hamilton, Chem.
Commun., 2008, 5535.
9
V. D. M. Hoang, P. A. N. Reddy and T.-J. Kim, Organometallics, 2008,
2
7, 1026–1027.
0 A. Gokhale, A. B. E. Minidis and A. Pfaltz, Tetrahedron Lett., 1995, 36,
831–1834.
1 K. Kawasaki, S. Tsumura and T. Katsuki, Synlett, 1995, 1245–1246.
2 S. K. Ginotra and V. K. Singh, Tetrahedron, 2006, 62, 3573–3581.
13 M. B. Andrus and Z. Zhou, J. Am. Chem. Soc., 2002, 124, 8806–8807.
1
1
1
1
Recovery experiments with ligand re-addition
i
A solution of the Pr-click-DAX ligand (0.025 mmol) and
14 M. B. Andrus, A. B. Argade, X. Chen and M. G. Pamment, Tetrahedron
CuPF (0.05 mmol) in 1 ml of BTFEP was stirred at room temp-
6
Lett., 1995, 36, 2945–2948.
erature for 15 min under argon atmosphere, then 1 ml of the neo-
teric solvent was added and dichloromethane was eliminated
under vacuum. Cyclohexene (5 mmol) and tert-butyl peroxy-
benzoate (0.5 mmol) were directly added to the solution. The
reaction was stirred for 36 h at room temperature. After this time,
the excess of cyclohexene was eliminated by evaporation
through argon flow, and the reaction was extracted with n-hexane
15 J. A. Mayoral, S. Rodríguez-Rodríguez and L. Salvatella, Chem.–Eur. J.,
008, 14, 9274–9285.
6 J. M. Fraile, J. I. García and J. A. Mayoral, Chem. Rev., 2009, 109,
60–417.
17 F. Fache and O. Piva, Tetrahedron: Asymmetry, 2003, 14, 139–143.
18 J. Bayardon and D. Sinou, J. Org. Chem., 2004, 69, 3121–3128.
2
1
3
1
9 J. Bayardon and D. Sinou, Tetrahedron: Asymmetry, 2005, 16, 2965–
972.
0 J. Le Bras and J. Muzart, Tetrahedron: Asymmetry, 2003, 14, 1911–1915.
2
2
(3 × 1 ml).
21 Z. Wang, G. Chen and K. Ding, Chem. Rev., 2009, 109, 322–359.
8288 | Dalton Trans., 2012, 41, 8285–8289
This journal is © The Royal Society of Chemistry 2012

View Article Online
2
2
2
2
2
2 D. Farrusseng, S. Aguado and C. Pinel, Angew. Chem., Int. Ed., 2009,
of coordinating two different metal centres at two different positions of
the ligand. For a recent revision on ditopic ligands meeting this definition,
see: B. I. Kharisov, P. E. Martínez, V. M. Jiménez-Pérez,
O. V. Kharissova, B. Nájera-Martínez and N. Pérez, J. Coord. Chem.,
2010, 63, 1–25, and references cited therein.
4
8, 7502–7513.
3 L. Aldea, I. Delso, M. Hager, M. Glos, J. I. García, J. A. Mayoral and
O. Reiser, Tetrahedron, 2012, 68, 3417–3422.
4 J. I. García, B. Lopez-Sanchez and J. A. Mayoral, Org. Lett., 2008, 10,
4
995–4998.
38 Following the suggestion of one of the referees, we carried out some
additional experiments using CuCl. In most cases, activities, enantioselec-
tivities and recoverability of the systems were worse than those obtained
with the other copper salts. Full results of these experiments are available
in the ESI‡.
5 J. I. García, C. I. Herrerias, B. Lopez-Sanchez, J. A. Mayoral and
O. Reiser, Adv. Synth. Catal., 2011, 353, 2691–2700.
6 M. Lombardo, A. Quintavalla, M. Chiarucci and C. Trombini, Synlett,
2
010, 1746–1765.
2
2
7 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576.
8 P. Wasserscheid and P. Schulz, Transition Metal Catalysis in Ionic
39 J. I. García, H. García-Marín, J. A. Mayoral and P. Perez, Green Chem.,
2010, 12, 426–434.
Liquids, Ionic Liquids in Synthesis,
Weinheim, 2nd edn, 2008.
2
Volume Set, Wiley-VCH,
40 M. J. Hernáiz, A. R. Alcantara, J. I. García and J. V. Sinisterra, Chem.–
Eur. J., 2010, 16, 9422–9437.
2
9 S.-G. Lee and Y. Y. Zhang, Enantioselective Catalysis in Ionic Liquids
and Supercritical CO , Handbook of Asymmetric Heterogeneous Cataly-
sis, Wiley-VCH, Weinheim, 2008.
41 M. Pérez-Sánchez, A. Cortés-Cabrera, H. García-Martín, J. Vicent
Sinisterra, J. I. García and M. J. Hernáiz, Tetrahedron, 2011, 67, 7708–
7712.
2
3
3
3
3
3
3
3
3
0 D. Betz, P. Altmann, M. Cokoja, W. A. Herrmann and F. E. Kühn, Coord.
Chem. Rev., 2011, 255, 1518–1540.
1 X. Yun, X. Hu, Z. Jin, J. Hu, C. Yan, J. Yao and H. Li, J. Mol. Catal. A:
Chem., 2010, 327, 25–31.
2 J. Fraile, J. Garcia, C. Herrerias, J. Mayoral, O. Reiser and M. Vaultier,
Tetrahedron Lett., 2004, 45, 6765–6768.
3 M. R. Castillo, L. Fousse, J. M. Fraile, J. I. García and J. A. Mayoral,
Chem.–Eur. J., 2007, 13, 287–291.
4 P. Goodrich, C. Hardacre, C. Paun, V. I. Pârvulescu and I. Podolean, Adv.
Synth. Catal., 2008, 350, 2473–2476.
5 M. R. Castillo, S. Castillón, C. Claver, J. M. Fraile, A. Gual, M. Martín,
J. A. Mayoral and E. Sola, Tetrahedron, 2011, 67, 5402–5408.
6 J. A. Brito, S. Ladeira, E. Teuma, B. Royo and M. Gómez, Appl. Catal.,
A, 2011, 398, 88–95.
42 M. Pérez-Sánchez, M. Sandoval, A. Cortés-Cabrera, H. García-Marín,
J. V. Sinisterra, J. I. García and M. J. Hernáiz, Green Chem., 2011, 13,
2810–2817.
43 H. García-Marín, J. C. van der Toorn, J. A. Mayoral, J. I. García and
I. W. C. E. Arends, Green Chem., 2009, 11, 1605–1609.
44 H. García-Marín, J. C. van der Toorn, J. A. Mayoral, J. I. García and
I. W. C. E. Arends, J. Mol. Catal. A: Chem., 2011, 334, 83–88.
45 L. Aldea, J. M. Fraile, H. García-Marín, J. I. García, C. I. Herrerias,
J. A. Mayoral and I. Perez, Green Chem., 2010, 12, 435–440.
46 A. Gissibl, C. Padié, M. Hager, F. Jaroschik, R. Rasappan, E. Cuevas-
Yañez, C.-O. Turrin, A.-M. Caminade, J.-P. Majoral and O. Reiser, Org.
Lett., 2007, 9, 2895–2898.
47 A. Schätz, M. Hager and O. Reiser, Adv. Funct. Mater., 2009, 19, 2109–
2115.
48 H. Werner, R. Vicha, A. Gissibl and O. Reiser, J. Org. Chem., 2003, 68,
10166–10168.
7 One of the referees raised the question on the adequacy of using the term
“
ditopic” to refer to our ligands. Admittedly, there are different uses of
this term in the literature, and there is no official definition given by the
IUPAC. As many other authors, we use ditopic to refer to the possibility
49 A. Cornejo, J. Fraile, J. Garcia, M. Gil, V. Martinez-Merino, J. Mayoral,
E. Pires and I. Villalba, Synlett, 2005, 2321–2324.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8285–8289 | 8289