Catalysis with gold complexes immobilised on carbon nanotubes by π-π Stacking interactions: Heterogeneous catalysis versus the boomerang effect

Article abstract of DOI:10.1002/chem.201300998

A new pyrene-tagged gold(I) complex has been synthesised and tested as a homogeneous catalyst. First, a simple 1,6-enyne was chosen as a model substrate for cyclisation by using different solvents to optimise the reaction conditions. The non-covalent immobilisation of our pyrene-tagged gold complex onto multi-walled carbon nanotubes through π-π stacking interactions was then explored to obtain a supported homogeneous catalyst. The heterogenised catalyst and its homogeneous counterpart exhibited similar activity in a range of enyne cyclisation reactions. Bearing in mind that π-π interactions are affected by temperature and solvent polarity, the reuse and robustness of the supported homogeneous catalyst was tested to explore the scope and limitations of the recyclability of this catalyst. Under the optimised conditions, recyclability was observed by using the concept of the boomerang effect. Copyright

Full text of DOI:10.1002/chem.201300998

FULL PAPER
Supported Catalysts
C. Vriamont, M. Devillers, O. Riant,*
S. Hermans* ................... &&&& — &&&&
Catalysis with Gold Complexes Immo-
bilised on Carbon Nanotubes by p–p
Stacking Interactions: Heterogeneous
Catalysis versus the Boomerang Effect
Recycling the boomerang: A pyrene-
tagged gold complex has been synthes-
ised and immobilised onto multi-
walled carbon nanotubes (MWNTs)
through p–p stacking interactions. The
catalyst displays good catalytic activity
in a number of enyne cyclisation reac-
tions (see figure). The recyclability by
heterogeneous catalysis and the boo-
merang effect was evaluated. Under
the optimised conditions, recycling is
possible when the boomerang system
is used and provides the first example
of a supported gold catalyst on carbon
nanotubes.
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

DOI: 10.1002/chem.201300998
Catalysis with Gold Complexes Immobilised on Carbon Nanotubes by p–p
Stacking Interactions: Heterogeneous Catalysis versus the Boomerang Effect
Charles Vriamont, Michel Devillers, Olivier Riant,* and Sophie Hermans*[a]
Abstract: A new pyrene-tagged gold(I)
complex has been synthesised and
tested as a homogeneous catalyst. First,
a simple 1,6-enyne was chosen as a
model substrate for cyclisation by using
different solvents to optimise the reac-
tion conditions. The non-covalent im-
mobilisation of our pyrene-tagged gold
complex onto multi-walled carbon
nanotubes through p–p stacking inter-
actions was then explored to obtain a
supported homogeneous catalyst. The
heterogenised catalyst and its homoge-
neous counterpart exhibited similar ac-
tivity in a range of enyne cyclisation re-
actions. Bearing in mind that p–p inter-
actions are affected by temperature
and solvent polarity, the reuse and ro-
bustness of the supported homogene-
ous catalyst was tested to explore the
scope and limitations of the recyclabili-
ty of this catalyst. Under the optimised
conditions, recyclability was observed
by using the concept of the boomerang
effect.
Keywords: gold · nanotubes · pi in-
teractions · recycling · supported
catalysts
Introduction
most referenced is the covalent approach, which comprises
reductive alkylation, cycloadditions, carbene or nitrene addi-
tion, reactions involving radicals or the well-known func-
tionalisation of carboxylic acid groups obtained by the oxi-
dative treatment of CNTs.[8–11] These covalent functionalisa-
tion strategies modify the carbon hybridisation from sp2 to
sp3 and therefore lead to a partial or total loss of the intrin-
sic properties of the pristine carbon nanotubes, which is in
contrast to non-covalent functionalisation. The non-covalent
modification of carbon nanotubes is of growing interest and
different types of molecular structures can be immobilised
onto CNTs, such as aromatic small molecules (pyrene, por-
phyrin and their derivatives), biomacromolecules (proteins,
enzymes, DNA, (poly)saccharides, etc) and polymers. This
strategy has been applied to various areas of interest, such
as nanoelectronics, nanomaterials science, drug delivery and
catalysis.[12–15] We have chosen carbon nanotubes as a sup-
port for homogeneous catalysts for their potential to interact
strongly with polyaromatics through non-covalent p–p inter-
actions. The selected method has some non-negligible ad-
vantages compared with covalent functionalisation. It is sim-
pler because the initial functionalisation of CNTs before an-
choring of the catalyst is not required. Moreover, the use of
a non-polar solvent provides a simple and convenient way
of recovering both the intact catalyst and the nanotubes un-
altered. The preliminary choice of a pyrene-tagged group is
of critical importance for the immobilisation of catalysts.
Some studies relating to p–p stacking have been carried out
and indicated that stacked structures become increasingly
favourable with increasing arene size. Systems such as
pyrene or coronene show p–p stacking with the hydrogen
atoms located roughly over ring centres.[16,17] Pyrene seems
to be the smallest molecular unit able to interact non-cova-
lently with carbon nanotubes. Compared with coronene or
larger arenes able to perform the same interactions, we se-
Supported homogeneous catalysis, that is, the immobilisa-
tion of a homogeneous catalyst onto a solid support, allies
advantages of both homogeneous and heterogeneous cataly-
sis. A highly active and well-characterised catalyst combined
with the possibility of separating it from products, allowing
their possible use in continuous flow processes, is very at-
tractive. This “heterogenisation” can be conducted by differ-
ent routes 1) the formation of covalent bonds between the
support and the complex, 2) adsorption or ion pairing with
the support and 3) encapsulation. Many of these strategies
have already been implemented for the immobilisation of
various metal complexes on to different materials such as
MCM, zeolites and polymers.[1–6] Surprisingly, the expected
emergence of these molecular catalysts on carbon nanotubes
(CNTs) has not yet appeared despite the rich chemistry as-
sociated with the functionalisation of these tubes.[7]
CNTs possess remarkable properties such as high chemi-
cal and thermal stability and conductivity, resistance to acid
and basic media, and the possibility of being functionalised
by a large—and not yet fully tested—variety of chemical re-
agents. Their functionalisation can be divided into three
families: defect-site functionalisation, covalent sidewall
functionalisation and non-covalent functionalisation. The
[a] C. Vriamont, Prof. M. Devillers, Prof. O. Riant, Prof. S. Hermans
Institute of Condensed Matter and Nanosciences,
Molecules, Solids and Reactivity (IMCN/MOST)
Universitꢁ catholique de Louvain
Place Louis Pasteur 1, 1348 Louvain-la-Neuve (Belgium)
Fax : (+32)10-472330
E-mail: olivier.riant@uclouvain.be
sophie.hermans@uclouvain.be
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300998.
&
2
&
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Catalysis with Gold Complexes on Carbon Nanotubes
FULL PAPER
lected pyrene as the smallest unit in the hope of optimising
the catalyst immobilisation on CNTs per unit surface area.
Homogeneous gold catalysis has been blooming during
the past ten years since this metal has had a resurgence.[18–23]
Gold has been identified as a captivating metal able to play
in a wide range of chemical transformations,[24–30] such as
enyne cycloisomerisation,[31–41] hydroamination,[40,42,43] hydro-
alkoxylation,[44] alkyne hydration,[45] benzannulation,[46]
furan/yne cyclisation[47] or hydrophosphoryloxylation.[48]
However, despite their excellent reactivity allowing low cat-
alyst loading, gold complexes are quite expensive and there-
fore their potential reuse is of critical importance.
which was finally coupled with 1-pyrenebutyric acid to give
the bifunctional ligand 4 in an overall yield of 55% for the
three steps. This air-stable ligand 4 could thus be easily pre-
pared on a multi-gram scale and was fully characterised
1
(mass spectrometry, H, 13C and 31P NMR, see the Support-
ing Information).
Ligand exchange between ligand 4 and the commercially
available chloro(dimethylsulfide)gold(I) complex allowed us
to obtain 5 (Scheme 2). The desired pyrene-tagged gold cat-
To the best of our knowledge only a few publications
have described the covalent anchoring of molecular catalysts
on to CNTs[49–58] and even fewer with a non-covalently im-
mobilised molecular catalyst through p–p interactions.[59–62]
Despite some elegant applications in catalysis, little reliable
quantitative data is available regarding the immobilisation
capacities of CNTs for homogeneous catalysts. Liu et al. re-
ported the immobilisation of pyrene-tagged ruthenium car-
bene complexes for ring-closing metathesis, but their quanti-
fication was based only on UV titration of the non-immobi-
lised complexes.[59] In a closely related study, Reiser and co-
workers reported an elegant method for immobilising palla-
dium complexes on to carbon-coated magnetic cobalt nano-
particles.[63]
We report herein the synthesis of a pyrene-tagged gold
complex as well as a study of its non-covalent immobilisa-
tion on to carbon nanotubes through p–p stacking. The im-
mobilised complex was fully characterised and studied in
some selected benchmark reactions to assess and quantify
its potential recyclability.
Scheme 2. Synthesis of the pyrene-tagged gold complex 6.
alyst 6 was finally obtained by treating complex 5 with
1 equivalent of AgNTf2 in an overall yield of 40% starting
from 1. This complex was extensively characterised by ther-
mal gravimetric analysis (TGA), HRMS and 19F, 31P, 13C and
1H NMR spectroscopy (see the Supporting Information).
The bis(trifluoromethanesulfonyl)imidate moiety was
chosen as the weakly coordinating counter anion because it
is a valuable candidate for the synthesis of stable gold(I)
catalysts. Such complexes are known to be stable, non-hy-
groscopic and highly active as catalysts for a wide range of
reactions such as enyne cycloisomerisations.[37]
Results and Discussion
Synthesis of the pyrene-tagged gold complex: The desired
bifunctional pyrene ligand was obtained according to the
synthetic route outlined in Scheme 1. A halogen/lithium ex-
change reaction between 4-bromobenzonitrile (1) and
nBuLi followed by trapping with chlorodiphenylphosphine
gave the corresponding phosphine 2. LiAlH4 reduction of
the nitrile group yielded the corresponding amine 3,[64]
Incorporation and immobilisation of the gold complex 6
onto multi-walled carbon nanotubes (MWNTs): The pyrene
group is important, as mentioned above, for non-covalently
immobilising a homogeneous catalyst onto a CNT. Due to
the lipophilicity of both pyrene and MWNTs, the use of a
polar solvent would seem to be required to maintain the p–
p interactions during the immobilisation step.[16] Complex 6
is soluble in most common polar solvents. Acetone was
chosen to study the incorporation of 6 onto carbon nano-
tubes with time (Scheme 3). The MWNTs were dispersed in
the solvent in an ultrasound bath before the addition of 6
and magnetic stirring for various periods of time (from a
few minutes to a few hours). Finally, the mixtures were sub-
Scheme 1. Synthesis of the bifunctional ligand 4.
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

O. Riant, S. Hermans et al.
mine whether the use of a
polar solvent was necessary
(Table 2). The manipulations
were carried out a number of
times with different quantities
of reagent (from 15 to 100 mg)
to verify the reproducibility. In
all cases, the ratios between
the different elements (F, N, P,
and S) and gold determined by
XPS were similar and close to
the calculated values. The XPS
ratio of gold/carbon fluctuates
between 0.0027 and 0.0037,
which indicates that the meth-
Scheme 3. Incorporation of the pyrene-tagged gold complex 6 onto MWNTs.
odology is reproducible. How-
ever, this ratio seems lower
jected to centrifugation to separate the functionalised
MWNTs (f-MWNTs) 7 from the liquid phase. The solutions
were analysed by inductively coupled plasma (ICP)
(Table 1), which showed that the majority of introduced 6
was indeed incorporated on to the MWNTs irrespective of
the time. The highest degree of incorporation of 6 was ob-
when acetonitrile was used. Complex 6 was analysed by
31P NMR spectroscopy over time to verify its stability in
these three solvents (see the Supporting Information). The
31P NMR spectra show the appearance of a second minor
peak in acetone and acetonitrile that is not present in di-
chloromethane; this second peak does not correspond to
either an oxidised species or the free ligand and remains un-
assigned, however, it must be due to a new complex formed
by rearrangement. For these reasons, dichloromethane was
chosen as the best compromise between quantity, quality of
incorporation and stability of the complex. The sample pre-
pared in dichloromethane was characterised by nitrogen
physisorption and the data compared with the starting
MWNTs (see Table S3 and Figure S13 in the Supporting In-
formation). As expected, the specific surface area (SBET and
SBJH) and pore volume were found to decrease after the im-
mobilisation of 6. Indeed, the layer of immobilised com-
pounds on the CNTs fills the voids that existed previously
between entangled nanotubes. The average pore size, how-
ever, was found to increase, probably due to blocking of the
smallest micropores (defects and opening of the internal
cavity) by adsorbed gold complex molecules.
ICP analysis was performed on some samples to deter-
mine the bulk amount of gold on the CNT supports (Table 3
and Table S2 in the Supporting Information for all values).
The results showed between 0.12 and 0.22 mmol of 7 per g
of the MWNTs, which clearly demonstrates that a non-negli-
gible amount of the catalyst 6 was incorporated onto the
MWNTs by a non-covalent pathway.
As already mentioned in the introduction, only four publi-
cations have reported the immobilisation of a molecular cat-
alyst on to MWNTs through p–p interactions.[59–62] Two of
them focused on the immobilisation of pyrene-functional-
ised electroactive transition-metal complexes on to elec-
trode-supported CNTs for electrocatalysed reactions. In the
two remaining references, UV titration with the pyrene fin-
gerprint was used to determine the quantity of immobilised
catalyst. The metal loading of these complexes on carbon
nanotubes was compared with the values we obtained for
our gold complex determined by ICP; the reported values
Table 1. ICP and XPS data after the incorporation of 6 onto MWNTs
after different periods of time.
Incorporation
time [min][a]
Au remaining
Au/C[c] F/Au N/Au P/Au S/Au
in solution [%][b]
20
60
160
210
26.4
12.5
16.5
26.4
22.4
26.4
–
3.4
4.6
3.3
3.4
3.5
4.5
–
7.04 3.42
7.06 2.39
7.51 2.10
9.67 3.39
6.82 1.84
8.84 2.17
0.88
0.93
0.80
1.00
0.84
0.75
1
2.22
1.81
2.43
2.85
2.09
2.33
2
300
1440
calculated ratio
6
2
[a] Experimental conditions. [b] Determined by ICP analysis of the fil-
trates. [c] Value ꢂ1000.
served after 60 min. The solids were analysed by X-ray pho-
toelectron spectroscopy (XPS). Based on the surface gold
atomic percentage, this analysis showed that complex 6 is
indeed incorporated on to the MWNTs to give 7 irrespective
of the incorporation time (see Table S1 in the Supporting In-
formation). The other elements in the gold complex (F, N, P,
S) were also analysed. The experimental ratios between
these elements and gold are close to the theoretical numbers
(Table 1), which indicates that the complex remains intact
after incorporation. The best values were found after 60 and
300 min. On the basis of these results, an incorporation time
of 60 min was chosen. Figure 1 shows both the XPS spectra
of pristine MWNTs and functionalised MWNTs 7 after
60 min. This latter spectrum shows the presence of the de-
sired elements (F, N, S, P, Au) and evidence that the oxida-
tion state of gold remains AuI after incorporation.
The incorporation procedure was performed in different
solvents such as acetonitrile, acetone and dichloromethane
to identify the best medium for incorporation and to deter-
&
4
&
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Catalysis with Gold Complexes on Carbon Nanotubes
FULL PAPER
Table 3. ICP values for the loading of 7 and some literature supported
homogeneous catalysts immobilised covalently or non-covalently onto
CNTs.
Entry
Metal
Immobilisation
methodology
Immobilisation
loading[a]
Ref.
1
2
3
4
5
6
7
8
Au
Rh
Ru
V
Mo
W
p–p interactions
p–p interactions
p–p interactions
covalent
covalent
covalent
0.12–0.22 (ICP)
0.17 (UV)
0.16 (UV)
0.08 (ICP)
0.25–0.35 (ICP)
0.03 (ICP)
this work
[61]
[59]
[53]
[54]
[56]
[57]
[58]
Mn
Mn
covalent
covalent
0.06 (ICP)
0.14 (ICP)
[a] Units: mmol catalyst per g of CNTs. Techniques used to determine
the metal loading are given in parentheses.
lie in the range of our results (Table 3). Note that, to the
best of our knowledge, the only other paper describing p–p
stacking between CNTs and a pyrene-tagged complex of
terbium (single molecular magnets) reported an ICP value
of 0.28 mmol of terbium complex per g of CNT.[65] In order
to compare 7 with a wider range of catalysts, a particular at-
tention was focused on molecular catalysts covalently
bonded to carbon nanotubes. The most outstanding exam-
ples determined by ICP are listed in Table 3; the loading of
7 between 0.12 and 0.22 mmolgÀ1 falls in the range of cova-
lently and non-covalently CNT supported homogeneous cat-
alysts reported in the literature.
Catalytic tests: Optimisation of homogeneous catalysis:
Complex 6 was first tested as a homogeneous catalyst to
prove its activity in well-mastered representative reactions
in the field of gold chemistry. The simple enyne 8 was ini-
tially chosen as a model substrate for cyclisation (Table 4),
and different solvents were used to optimise the reaction
conditions. Complex 6 was active in the cyclisation of enyne
8 and gave complete conversion with dichloromethane or
acetone as solvent. Interestingly, a change of solvent in-
duced a change in the outcome of the reaction. The use of
dichloromethane predominantly gave the 5-exo-dig product
9 and a minor amount of the 6-endo-dig product 10 in a
ratio of 88:12. When acetone was used, product 9 was again
predominant with a minor amount of 10, but the side-prod-
uct 11 was also formed, probably due to the inherent pres-
Table 4. Effect of solvents on the cycloisomerisation of enyne 8.
Figure 1. XPS spectra of pristine MWNTs (top) and f-MWNTs
(bottom). Inset: Gold narrow scan.
7
Entry
Solvent
Catalyst
6 [mol%]
t [h]
T [8C]
Conversion [%]
(products, ratio)
Table 2. Incorporation of 6 onto MWNTs in various solvents.
Entry
Solvent
Au/C[a]
F/Au
N/Au
P/Au
S/Au
1
2
3
4
5
CH2Cl2
acetone
CH3CN
DMF
2
2
2
2
2
0.5
1
24
24
24
RT
35
35
35
RT
100 (9+10, 88:12)
1
2
3
acetone[b]
CH2Cl2
CH3CN
3.65
3.35
2.72
6.92
7.70
6.17
2.04
2.39
1.83
0.96
0.95
1.12
2.11
2.54
2.23
100 (9+10+11)[a]
0
0
0
MeOH
[a] Value ꢂ1000. [b] Average of eight experiments (all values are report-
ed in the Supporting Information).
[a] No ratio is indicated because other side-products were also observed.
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

O. Riant, S. Hermans et al.
ence of water in acetone (see the Supporting Information).
Furthermore, when acetone was used, a longer reaction time
and higher temperature were needed to reach quantitative
conversion. Finally, when other polar solvents were used
(CH3CN, DMF, MeOH), no conversion was observed.
On the basis of these results, dichloromethane was select-
ed as the solvent for a small collection of benchmark sub-
strates, that is, enynes 8 and 12–15 and the propargylic car-
bonates 16 and 17 (Scheme 4).[34,66–68] The cyclisations of
substrates 8 and 12–16 proceeded readily at room tempera-
ture in CH2Cl2 with a 2 mol% catalytic loading of 6 and
were complete in 30 min. A longer time, that is, 7 h, was re-
quired for the cyclisation of 17. The rearrangement was ster-
eospecific for enyne 12 with a Z configuration of the alkene,
giving the Z-diene 18 quantitatively.[34] The enyne 13 was
quantitatively cyclised to give a mixture of two products by
an exohedral (19) and endohedral (20) skeletal rearrange-
ment in a ratio of 57:43, respectively. The N-toluene-4-sul-
fonyl derivatives 14 and 15 both underwent exohedral rear-
rangement and were converted into 21 and 22, respectively.
In both cases, the conversion was equal to or higher than
90%. A longer reaction time did not increase the conver-
sion rate. Finally, the propargylic carbonates 16 and 17 were
quantitatively transformed into the known methylene carbo-
nates 23 and 24.[71]
Supported homogeneous catalysis: Once the reactivity of 6
as catalyst in the model reactions had been demonstrated,
catalyst 7 was tested to compare its activity and selectivity
with its homogeneous counterpart 6. The results obtained
are presented in Table 5. The supported catalyst displayed
Table 5. Comparison of 6 and 7 as catalysts in the homogeneous and het-
erogeneous catalytic reactions.
Entry
Substrate
Catalyst
Solvent
Products
(yield[a] [%], ratio[b]
)
U
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
8
8
8
6
7
6
7
6
7
6
7
6
7
6
7
6
7
6
7
acetone
acetone
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
9+10+11
9+10+11
9+10 (100, 88:12)
9+10 (100, 88:12)
18 (100)
8
12
12
13
13
14
14
15
15
16
16
17
17
18 (100)
19+20 (100, 57:43)
19+20 (100, 57:43)
21 (93)
21 (93)
22 (90)
22 (90)
23 (100)
23 (100)
24 (100)
24 (100)
[a] Isolated yield. [b] Measured by 1H NMR analysis of the crude reac-
tion mixture. Cyclisation of molecules with gold complex 6 and supported
catalyst 7. Reagents and conditions: CH2Cl2 (acetone for molecule 8),
2 mol% 6 or 7, room temperature (358C when acetone was used), 30 min
(8, 12–16) or 7 h (17).
good catalytic activity and gave the same conversion as its
homogeneous counterpart irrespective of the solvent. Fur-
thermore, the same selectivity was observed towards the
skeletal rearrangement in the cyclisation of enynes 8 and 13.
These results suggest that the support does not play a role
during the catalysis. The absence of steric or electronic ef-
fects is probably due to the long distance between the active
site for catalysis, which is the AuI atom, and the pyrene
moiety in interaction with nanotubes.
Scheme 4. Cyclisation of enynes and propargylic carbonates with gold
complex 6. Reagents and conditions: CH2Cl2 (acetone for molecule 8),
2 mol% 6, room temperature (358C when acetone was used), [a] 30 min;
[b] 7 h.
Recyclability of the supported homogeneous gold catalyst:
In view of the excellent results obtained with the heteroge-
nised catalyst 7, the ultimate goal was to explore its recycla-
&
6
&
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Catalysis with Gold Complexes on Carbon Nanotubes
FULL PAPER
bility in the two media used during this work, acetone and
dichloromethane. In both cases the supported homogeneous
catalyst 7 (2 mol% based on ICP values) was first subjected
to ultrasound before addition of substrate 8 in acetone or
substrates 8 and 14 in dichloromethane. After 30 min, the
cyclisation products were separated from the f-MWNTs 7 by
centrifugation. This was carried out at room temperature in
acetone, but in dichloromethane the solution was first
cooled to À408C before centrifugation. The recovered cata-
lysts were then ready for a new catalytic run, and the solu-
1
tion containing the products was analysed by H NMR spec-
troscopy. The recyclability in acetone was tested a couple of
times and gave unambiguous results: the second run did not
give any cyclisation products with the model substrate 8,
which indicates that deactivation had occurred. The recycla-
bility of the catalyst in the reactions of enynes 8 and 14 in
dichloromethane gave good results (Schemes 5 and 6).
Scheme 6. Recycling of catalyst 7 (2 mol%) in the cyclisation of enyne
14.
the polarity of the solvent and by the temperature. These
two approaches are conceivable and are presented in
Scheme 7. The first pathway (right) shows the pyrene-
tagged gold complex strongly immobilised onto the CNTs
during catalysis and easily separable from the products once
the reaction is finished, that is, acting as a heterogeneous
catalyst due to the polar nature of the solvent. The second
pathway (left), in a non-polar solvent, involves a boomerang
effect: the gold complex acts as a homogeneous catalyst
before being re-immobilised onto the CNTs following a low-
ering of the temperature (from room temperature to
À408C). These two pathways were tested with varying de-
grees of success based on the results of the recyclability
study described above. To test the concept of heterogeneous
catalysis, acetone was used as the polar medium. Indeed, the
Scheme 5. Recycling of catalyst 7 (2 mol%) in the cyclisation of enyne 8.
Indeed, four successive catalytic runs were accomplished
with quantitative cyclisation of enyne 8 into the correspond-
ing exo and endo products 9 and 10 (ratio 88:12) without
any loss of catalytic activity. However, loss of activity was
observed during runs 5 and 6, with conversions of 67 and
37%, respectively (Scheme 5). The ratio between the two
cyclised products remained unchanged during each run. The
cyclisation of enyne 14 into the corresponding product 21
was also tested. A slow decrease in the conversion was ob-
served during the first four runs before an abrupt fall during
the fifth run. Conversions of 93, 83, 63, 56 and finally 6%
for the final run were observed (Scheme 6).
Heterogeneous catalysis versus Boomerang effect: Liu[59]
and Reiser[63] and their co-workers showed that p–p stacking
interactions between pyrene and nanotubes are affected by
Scheme 7. Mechanisms for the boomerang effect (left) and supported ho-
mogeneous catalysis (right). R=reagents, P=Products.
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

O. Riant, S. Hermans et al.
cyclisation of enyne 8 with catalyst 7 gave quantitative con-
version in the first run in acetone (Table 5, entry 2). Never-
theless, poor recyclability was achieved by this methodology.
This was suspected to be related to the appearance of the
second peak in the 31P NMR spectra of complex 6 in this
solvent after incorporation onto the MWNTs (see Fig-
ure S12 in the Supporting Information). Simultaneously, a
colour change that increases over time was observed. How-
ever, XPS analysis of the used catalysts revealed the pres-
ence of gold still to be in the oxidation state I and all heter-
oatoms in the desired ratios. Thus, the deactivation can be
attributed to internal complex rearrangement.
In accordance with Scheme 7, recyclability by the boomer-
ang effect was also evaluated and showed that the catalyst
can be recycled in dichloromethane (Schemes 5 and 6).
However, a loss of activity was observed along the runs. A
partial loss of the pyrene-tagged gold complex from the
CNTs at the end of each run was postulated. To gain more
information, each solution was analysed by ICP during suc-
cessive runs of the transformation of enyne 14 into the cy-
clised molecule 21. The results are given in Table 6. After
Figure 2. XPS of AuI (in 7) before catalysis (top), after two runs (middle)
and after four runs (bottom).
cordance with Scheme 5: 2 mol% catalyst 7, 30 min, RT)
was tested without cooling to À408C before separation of
the CNTs. The second catalytic run did not give any cyclisa-
tion product, which supports the dependency of p–p stack-
ing on temperature. ICP analysis was realised after the first
catalytic run and showed gold leaching of 36%. This is
much greater than after the first run with workup at À408C
(16%). It is also even more than the sum of all the leaching
during five runs (sum of gold lost is 29%, Table 6). There-
fore the fall in activity is understandable but its complete
cancellation remains unexplained.
The initial goal of this work was reached to some extent
as conditions have been found for obtaining a supported ho-
mogeneous “boomerang” catalyst that is effective over suc-
cessive runs. As a result of p–p interactions, the recovery of
MWNTs is easily achieved by a simple change in solvent
and reload of these “pristine MWNTs” with fresh pyrene-
tagged gold catalyst.
Table 6. f-MWNTs 7 analysed by ICP after each run.[a]
Run
Au leaching [ppm][b]
Gold lost [%][c]
1
2
3
4
5.50
1.23
0.73
0.93
1.22
9.61
16.4
3.7
2.2
2.8
3.6
5
Total 1–5
28.7
[a] Quantity of gold in the f-MWNTs
7
before catalysis: 33.5 ppm.
[b] Leaching of gold in ppm in each catalytic cycle. [c] Percentage of gold
lost in each catalytic cycle (Au leaching (ppm) divided by the initial
quantity, 33.5 ppm).
the first run, substantial leaching was observed (i.e., 16%).
This might be due to the presence of immobilised complexes
in multiple layers with stacked pyrene units. During the first
run, the molecules not in direct contact with CNT might be
lost more easily and re-immobilised with more difficulty.
The leaching is thereafter quite constant and fluctuates be-
tween 2 and 4%. This slow descent might explain some of
the loss of catalytic activity over successive runs, although
the leaching seems still rather too low to fully explain the
high loss of activity observed after the second run. XPS
analyses were also carried out after each catalytic run to
check the quantity of the different elements and the oxida-
tion state of the gold (Figure 2). The measurements showed
that the oxidation state of gold always remained the same,
namely AuI. Thus, the deactivation is not a result of the oxi-
dation or reduction to AuIII or Au0, respectively. Secondly,
the leaching was also confirmed by XPS measurements: the
atomic ratio Au/C decreased after each catalytic test (see
Table S4 in the Supporting Information).
Conclusion
A pyrene-tagged gold complex has been designed for immo-
bilisation onto MWNTs through p–p interactions to give a
supported homogeneous “boomerang” catalyst. This catalyst
remained intact on the CNT surface after immobilisation
and remarkably its activity and selectivity in cyclisation was
not affected in comparison with its homogeneous counter-
part. This immobilisation through pyrene allowed a boomer-
ang effect to take place during catalysis and this effect was
found to be strongly dependent on the temperature. We
have shown that recycling is possible at low temperatures.
To the best of our knowledge, this is the first example of a
supported gold catalyst on carbon nanotubes, either by co-
valent or non-covalent functionalisation, and the approach
could open the door to further applications.
To validate the crucial importance of temperature in
these experiments, the cyclisation and recycling of enyne 14
under the same conditions as described previously (in ac-
&
8
&
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Catalysis with Gold Complexes on Carbon Nanotubes
FULL PAPER
General procedure for cyclisation of enynes 8 and 12–16 catalysed by the
AuI complex 6: The enyne (9.63ꢂ10À2 mmol) in CH2Cl2 (3 mL) was
added to gold catalyst 6 (2 mg, 1.925ꢂ10À3 mmol, 2 mol%) in CH2Cl2
(2 mL) and the mixture was stirred at room temperature for the duration
indicated in Scheme 4. The resulting mixture was filtered through SiO2
and the solvent evaporated to give the corresponding product (9–11, 18–
24; see the Supporting Information). These compounds have been report-
ed in the literature previously: 9,[69] 10,[66] 11,[70] 18,[34] 19,[34] 20,[34] 21,[34]
22,[34] 23,[71] 24.[71]
Experimental Section
General methods: Unless otherwise stated, all the manipulations were
carried out under an atmosphere of argon by using standard Schlenk
techniques and with anhydrous solvents. Hexane, diethyl ether and tetra-
hydrofuran were distilled from sodium benzophenone under argon. Di-
chloromethane and acetonitrile were distilled from CaH2. Anhydrous
acetone was used as received (Fisher chemical). MWNTs were obtained
from Nanocyl (Thin MWCNT, 95+% C purity). NMR spectra were re-
corded on Bruker spectrometers (more details can be found in the Sup-
porting Information), HRMS were recorded on a Q-Extractive orbitrap
spectrometer from ThermoFisher and XPS analyses were carried out
with a SSI-X-probe (SSX 100/206) photoelectron spectrometer from Sur-
face Science Instruments (more details can be found in the Supporting
Information). ICP analyses were realised by MEDAC Ltd. (U.K.). The
following compounds were obtained according to literature procedures:
8,[66] 12,[34] 13,[66] 14,[67] 15,[34]16[68] and 17.[68]
General procedure for the cyclisation of enynes 8 and 12–17 catalysed by
the AuI complex 6: f-MWNTs 7 (3.55ꢂ10À3 mmol, 2 mol% of supported
gold complex based on ICP values, typically 20 mg) were submitted to ul-
trasound in CH2Cl2 (4 mL) for 10 min. The enyne (0.178 mmol) in
CH2Cl2 (3 mL) was then added and the mixture stirred for 30 min (8, 12–
16) or 7 h (17). Agitation was then stopped and the solution was cooled
to À408C (bath CH3CN/N2) for 30 min. The reaction products were sepa-
rated from system 7 by centrifugation (6500 rpm, 5 min). The products
1
were filtered through Celite and analysed by H NMR spectroscopy.
Synthesis of the bifunctional ligand 4: The first two steps, that is, com-
pounds 2 and 3, have been described previously.[64] 1-Pyrenebutyric acid
(1.54 g, 5.35 mmol), dimethylaminopyridine (DMAP; 160 mg, 1.34 mmol)
and N,N’-dicyclohexylcarbodiimide (DCC; 1.01 g, 4.91 mmol) were added
to a solution of 4-diphenylphosphanylbenzylamine (3; 1,3 g, 4.46 mmol)
in CH2Cl2 (40 mL). The resulting mixture was stirred at room tempera-
ture overnight. After extractive workup (1m HCl, H2O, 1 m NaOH, H2O),
the resulting organic phases were dried with MgSO4 and thereafter puri-
fied by chromatography (95:5 CH2Cl2/AcOEt) to obtain 4 (2.18 g, 87%)
as a pale-yellow solid. 1H NMR (300 MHz, CDCl3): d=8.28 (d, J=
9.3 Hz, 1H), 8.20–7.94 (m, 7H), 7.83 (d, J=7.8 Hz, 1H), 7.38–7.18 (m,
14H), 5.64 (s, 1H), 4.43 (d, J=5.7 Hz, 2H), 3.40 (t, J=7.1 Hz, 2H), 2.36–
2.17 ppm (m, 4H); 13C NMR (75 MHz, CDCl3): d=172.60, 138.91,
136.69, 135.62, 134.16, 133.90, 133.73, 133.48, 131.29, 130.77, 129.83,
128.80, 128.53, 128.44, 127.85, 127.75, 127.38, 127.33, 127.26, 126.65,
125.79, 124.86, 124.72, 123.25, 43.19, 35.69, 32.57, 27.30 ppm; 31P NMR
(121 MHz, CDCl3): d=À5.32 ppm; HRMS (ESI): m/z calcd for
C39H33O1N1P1: 562.22943 [M+1]+; found: 562.22876.
General procedure for the recycling of catalyst 7—cyclisation of enyne 8
and 14: The procedure was carried out in accordance with Scheme 5. f-
MWNTs 7 (3.55ꢂ10À3 mmol, 2 mol% of supported gold complex based
on ICP values) were submitted to ultrasound in CH2Cl2 (4 mL) for
10 min. Enyne 8 (42 mg, 0.178 mmol) or enyne 14 (49 mg, 0.178 mmol) in
CH2Cl2 (3 mL) was added and the mixture was stirred for 30 min. Agita-
tion was then stopped and the solution cooled to À408C (bath CH3CN/
N2) for 30 min. The mixture was submitted to centrifugation (6500 rpm,
5 min) and washed with cold CH2Cl2 (À408C, 5 mL, 6500 rpm, 5 min) to
separate the reaction products from the catalyst 7. The products were fil-
tered through Celite (except when leaching was analysed) and analysed
1
by H NMR spectroscopy. The f-MWNTs were vacuum-dried and used in
the subsequent catalytic cycles following the protocol described above.
Acknowledgements
We acknowledge the FRS-FNRS, FRIA, Fꢁdꢁration Wallonie-Bruxelles,
Loterie Nationale, the Belgian State (IAP Project No. P6/17) and the
Universitꢁ catholique de Louvain for funding. We are also grateful to
Nanocyl for supplying MWNTs, and to T. Haynes and J.-F. Statsijns for
technical support.
Synthesis of the gold complex 6: The bifunctional ligand 4 (112 mg,
0.2 mmol) was added to a solution of ClAuSMe2 (59 mg, 0.2 mmol) in
CH2Cl2 (10 mL). The resulting mixture was stirred at room temperature
for 20 min, the solvent was removed under reduced pressure and the resi-
due vacuum-dried to obtain the gold complex 5 as a pale-yellow solid.
Complex
5 was dissolved in CH2Cl2 (10 mL) and AgNTf2 (78 mg,
0.2 mmol) was added. The mixture was stirred at room temperature for
20 min, then filtered through a millipore membrane in a disposable plas-
tic syringe and the solution concentrated by removing most of the solvent
on a Schlenk line. Hexane was added (20 mL) to induce the precipitation
of 6 (176 mg, 85%) as a pale-yellow solid: 1H NMR (300 MHz, CDCl3):
d=8.32–7.75 (m, 9H), 7.61–7.36 (m, 14H), 5.81 (s, 1H), 4.46 (d, J=
5.8 Hz, 2H), 3.40 (t, J=7.2 Hz, 2H), 2.39–2.16 ppm (m, 4H); 13C NMR
(126 MHz, CDCl3): d=175.73, 142.70, 135.27, 134.33, 134.22, 133.99,
133.88, 132.80, 131.35, 130.78, 129.99, 129.72, 129.62, 128.83, 128.73,
128.64, 127.50, 127.40, 127.00, 126.86, 126.47, 126.02, 125.59, 125.09,
125.00, 124.88, 123.15, 121.00, 118.44, 43.36, 35.70, 32.63, 27.89 ppm;
[1] D. E. De Vos, M. Dams, B. F. Sels, P. A. Jacobs, Chem. Rev. 2002,
102, 3615–3640.
[2] C. E. Song, S. Lee, Chem. Rev. 2002, 102, 3495–3524.
[3] N. E. Leadbeater, M. Marco, Chem. Rev. 2002, 102, 3217–3274.
[4] A. Corma, H. Garcia, Top Catal. 2008, 48, 8–31.
[5] A. Choplin, F. Quignard, Coord. Chem. Rev. 1998, 178–180, 1679–
1702.
[6] M. H. Valkenberg, W. F. Hçlderich, Catal. Rev. Sci. Eng. 2002, 44,
321–374.
[7] A. Schaetz, M. Zeltner, W. J. Stark, ACS Catal. 2012, 2, 1267–1284.
[8] A. Hirsch, Angew. Chem. 2002, 114, 1933–1939; Angew. Chem. Int.
Ed. 2002, 41, 1853–1859.
[9] P. Singh, S. Campidelli, S. Giordani, D. Bonifazi, A. Bianco, M.
Prato, Chem. Soc. Rev. 2009, 38, 2214–2230.
[10] D. Tasis, N. Tagmataechis, A. Bianco, M. Prato, Chem. Rev. 2006,
106, 1105–1136.
[11] K. Balasubramanian, M. Burghard, Small 2005, 1, 180–192.
[12] Y. L. Zhao, J. F. Stoddart, Acc. Chem. Res. 2009, 42, 1161–1171.
[13] H. Huang, Q. Yuang, J. S. Sha, R. D. K. Misra, Adv. Drug Delivery
Rev. 2011, 63, 1332–1339.
[14] C. Schulz-Drost, V. Sgobba, C. Gerhards, S. Leubner, R. M. Krieck
Calderon, A. Ruland, D. M. Guldi, Angew. Chem. 2010, 122, 6569–
6574; Angew. Chem. Int. Ed. 2010, 49, 6425–6429.
[15] C. Vijayakumar, B. Balan, M.-J. Kim, M. Takeuchi, J. Phys. Chem. C
2011, 115, 4533–4539.
31P NMR (121 MHz, CDCl3): d=30.05 ppm; 19F NMR (282 MHz,
CDCl3): d=À75.44; HRMS (ESI): m/z calcd for C39H32O1N1197Au1P1
:
+
758.18815;
found
758.18809;
MS
(ESI)
m/z
calcd
for
C2O4N1F632S2À:279.91674; found 279.91830.
General procedure for the immobilisation of the gold complex 6 onto
MWNTs to obtain 7: Thin MWCNTs (95+% purity; 100 mg,
C
8.33 mmol) were submitted to ultrasound for 1 h in the selected solvent
(acetone, dichloromethane or acetonitrile; 30 mL). The pyrene-tagged
gold(I) complex 6 was then added (100 mg, 0.1 mmol) and the resulting
mixture was stirred at room temperature for various times (from 20 min
to 1 day). The resulting powder was submitted to centrifugation
(6500 rpm, 5 min) and washed twice with the selected solvent (6500 rpm,
5 min) to separate the non-immobilised gold complex 6 from the sup-
ported homogeneous catalyst system 7. The resulting supported homoge-
neous catalyst was vacuum-dried.
[16] C. Janiak, J. Chem. Soc. Dalton Trans. 2000, 3885–3896.
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
9
&
ÞÞ
These are not the final page numbers!

O. Riant, S. Hermans et al.
[17] D. A. Britzab, A. N. Khlobystov, Chem. Soc. Rev. 2006, 35, 637–659.
[18] Y. Ito, M. Sawamura, T. Hayashi, J. Am. Chem. Soc. 1986, 108,
6405–6406.
[19] Y. Ito, M. Sayamura, T. Hayashi, Tetrahedron Lett. 1988, 29, 239–
240.
[20] S. D. Pastor, A. Togni, J. Am. Chem. Soc. 1989, 111, 2333–2334.
[21] A. Togni, S. D. Pastor, J. Org. Chem. 1990, 55, 1649–1664.
[22] A. S. K. Hashmi, L. Schwarz, J.-H. Choi, T. M. Frost, Angew. Chem.
2000, 112, 2382–2385; Angew. Chem. Int. Ed. 2000, 39, 2285–2288.
[23] A. S. K. Hashmi, T. M. Frost, J. W. Bats, J. Am. Chem. Soc. 2000,
122, 11553–11554.
[48] P. Nun, J. D. Egbert, M.-J. Oliva-Madrid, S. P. Nolan, Chem. Eur. J.
2012, 18, 1064–1067.
[49] R. Giordano, P. Serp, P. Kalck, Y. Khin, J. Schreiber, C. Marhic, J.-L.
Duvail, Eur. J. Inorg. Chem. 2003, 610–617.
[50] S. Banerjee, S. S. Wong, J. Am. Chem. Soc. 2002, 124, 8940–8948.
[51] Y. Zhang, H.-B. Zhang, G.-D. Lin, P. Chen, Y.-Z. Yuan, K. R. Tsai,
Appl. Catal. A 1999, 187, 213–224.
[52] C. Baleiz¼o, B. Gigante, H. Garcia, A. Corma, J. Catal. 2004, 221,
77–84.
[53] C. Baleiz¼o, B. Gigante, H. Garcia, A. Corma, Tetrahedron 2004, 60,
10461–10468.
[54] M. Moghadam, S. Tangestaninejad, V. Mirkhani, I. Mohammadpoor-
Baltork, N. S. Mirbagheri, J. Organomet. Chem. 2010, 695, 2014–
2021.
[55] M. Moghadam, S. Tangestaninejad, V. Mirkhani, I. Mohammadpoor-
Baltork, N. S. Mirbagheri, J. Mol. Catal. A 2010, 329, 44–49.
[56] M. Nooraeipour, M. Moghadam, S. Tangestaninejad, V. Mirkhani, I.
Mohammadpoor-Baltork, N. Iravani, J. Coord. Chem. 2012, 65, 226–
238.
[57] M. Zakeri, M. Moghadam, I. Mohammadpoor-Baltork, S. Tangesta-
ninejad, V. Mirkhani, A. R. Khosropour, J. Coord. Chem. 2012, 65,
1144–1157.
[58] M. Moghadam, I. Mohammadpoor-Baltork, S. Tangestaninejad, V.
Mirkhani, H. Kargar, N. Zeini-Isfahani, Polyhedron 2009, 28, 3816–
3822.
[24] A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180–3211.
[25] Z. Li, C. Brouwer, C. He, Chem. Rev. 2008, 108, 3239–3265.
[26] N. Krause, C. Winter, Chem. Rev. 2011, 111, 1994–2009.
[27] A. Corma, A. Leyva-Pꢁrez, M. J. Sabater, Chem. Rev. 2011, 111,
1657–1712.
[28] E. Jimꢁnez-NfflÇez, A. M. Echavarren, Chem. Rev. 2008, 108, 3326–
3350.
[29] A. Arcadi, Chem. Rev. 2008, 108, 3266–3325.
[30] C. Aubert, L. Fensterbank, P. Garcia, M. Malacria, A. Simonneau,
Chem. Rev. 2011, 111, 1954–1993.
[31] L. Leseurre, C.-M. Chao, T. Seki, E. Genin, P. Y. Toullec, J.-P.
GenÞt, V. Michelet, Tetrahedron 2009, 65, 1911–1918.
[32] M. Raducan, C. Rodriguez-Escrich, X. C. Cambeiro, E. C. Escudo-
Adan, M. A. Pericàs, A. M. Ecchavarren, Chem. Commun. 2011, 47,
4893–4895.
[33] C. Nieto-Oberhuber, S. Lopez, M. Paz MuÇoz, E. Jimꢁnez-NuÇez,
E. BuÇuel, D. J. Cardenas, A. M. Echavarren, Chem. Eur. J. 2006,
12, 1694–1702.
[34] C. Nieto-Oberhuber, M. Paz MuÇoz, S. Lopez, E. Jimꢁnez-NuÇez,
C. Nevado, E. Herrero-Gomez, M. Raducan, A. M. Echavarren,
Chem. Eur. J. 2006, 12, 1677–1693.
[35] C. Nieto-Oberhuber, M. Paz MuÇoz, E. BuÇuel, C. Nevado, D. J.
Cardenas, A. M. Echavarren, Angew. Chem. 2004, 116, 2456–2460;
Angew. Chem. Int. Ed. 2004, 43, 2402–2406.
[59] G. Liu, B. Wu, J. Zhang, X. Wang, M. Shao, J. Wang, Inorg. Chem.
2009, 48, 2383–2390.
[60] F. Li, B. Zhang, X. Li, Y. Jiang, L. Chen, Y. Li, L. Sun, Angew.
Chem. 2011, 123, 12484–12487; Angew. Chem. Int. Ed. 2011, 50,
12276–12279.
[61] L. Xing, J.-H. Xie, Y.-S. Chen, L.-X. Wang, Q.-L. Zhou, Adv. Synth.
Catal. 2008, 350, 1013–1016.
[62] P. D. Tran, A. Le Goff, J. Heidkamp, B. Jousselme, N. Guillet, S. Pal-
acin, H. Dau, M. Fontecave, V. Artero, Angew. Chem. 2011, 123,
1407–1410; Angew. Chem. Int. Ed. 2011, 50, 1371–1374.
[63] A. Schätz, O. Reiser, W. J. Stark, Chem. Eur. J. 2010, 16, 8950–8967.
[64] M. Nishimura, M. Ueda, N. Miyaura, Tetrahedron 2002, 58, 5779–
5787.
[65] S. Kyatskaya, J. R. G. Mascaros, L. Bogani, F. Hennrich, M. Kappes,
W. Wernsdorfer, M. Ruben, J. Am. Chem. Soc. 2009, 131, 15143–
15151.
[36] C. Nieto-Oberhuber, S. Lopez, A. M. Echavarren, J. Am. Chem.
Soc. 2005, 127, 6178–6179.
[37] N. Mꢁzailles, L. Ricard, F. Gagosz, Org. Lett. 2005, 7, 4133–4136.
[38] E. Soriano, J. Marco-Contelles, Acc. Chem. Res. 2009, 42, 1026–
1036.
[39] C. Li, Y. Zeng, H. Zhang, J. Feng, Y. Zhang, J. Wang, Angew. Chem.
2010, 122, 6557–6561; Angew. Chem. Int. Ed. 2010, 49, 6413–6417.
[40] S. K. Hashmi, C. Hubbert, Angew. Chem. 2010, 122, 1026–1028;
Angew. Chem. Int. Ed. 2010, 49, 1010–1012.
[41] V. Loꢃpez-Carrillo, N. Huguet, A. Mosquera, A. M. Echavarren,
Chem. Eur. J. 2011, 17, 10972–10978.
[42] H. Duan, S. Sengupta, J. L. Petersen, N. G. Akhmedov, X. Shi, J.
Am. Chem. Soc. 2009, 131, 12100–12102.
[43] L. Leseurre, P. Y. Toullec, J.-P. GenÞt, V. Michelet, Org. Lett. 2007,
9, 4049–4052.
[44] N. Nishina, Y. Yamamoto, Tetrahedron Lett. 2008, 49, 4908–4911.
[45] A. Leyva, A. Corma, J. Org. Chem. 2009, 74, 2067–2074.
[46] a) A. S. K. Hashmi, W. Yang, F. Rominger, Angew. Chem. 2011, 123,
5882–5885; Angew. Chem. Int. Ed. 2011, 50, 5762–5765; b) A. S. K.
Hashmi, W. Yang, F. Rominger, Chem. Eur. J. 2012, 18, 6576–6580.
[47] A. S. K. Hashmi, Pure Appl. Chem. 2010, 82, 1517–1528.
[66] J. W. Faller, P. P. Fontaine, J. Organomet. Chem. 2006, 691, 1912–
1918.
[67] F. Monnier, C. Vovard-Le Bray, D. Castillo, V. Aubert, S. Dꢁrien,
P. H. Dixneuf, L. Toupet, A. Ienco, C. Mealli, J. Am. Chem. Soc.
2007, 129, 6037–6049.
[68] H. Yamamoto, M. Nishiyama, H. Imagawa, M. Nishizawa, Tetrahe-
dron Lett. 2006, 47, 8369–8373.
[69] S. Gaillard, J. Bosson, R. S. Ramon, P. Nun, A. M. Z. Slawin, S. P.
Nolan, Chem. Eur. J. 2010, 16, 13729–13740.
[70] M. Nishizawa, V. K. Yadav, M. Skwarczynski, H. Takao, H. Imaga-
wa, T. Sugihara, Org. Lett. 2003, 5, 1609–1612.
[71] A. K. Buzas, F. M. Istrate, F. Gagosz, Tetrahedron 2009, 65, 1889–
1901.
Received: March 15, 2013
Revised: May 21, 2013
Published online: && &&, 0000
&
10
&
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!