The doping effect of fluorinated aromatic solvents on the rate of ruthenium-catalysed olefin metathesis

Article abstract of DOI:10.1002/chem.201100160

A study concerning the effect of using a fluorinated aromatic solvent as the medium for olefin metathesis reactions catalysed by ruthenium complexes bearing N-heterocyclic carbene ligands is presented. The use of fluorinated aromatic hydrocarbons (FAH) as

Full text of DOI:10.1002/chem.201100160

FULL PAPER
DOI: 10.1002/chem.201100160
The Doping Effect of Fluorinated Aromatic Solvents on the Rate of
Ruthenium-Catalysed Olefin Metathesis
Cezary Samojłowicz,[a] Michał Bieniek,[a, b] Aleksandra Pazio,[b] Anna Makal,[b]
Krzysztof Wozniak,[b] Albert Poater,[c, d] Luigi Cavallo,[d] Jacek Wꢀjcik,[e]
´
Konrad Zdanowski,[e, f] and Karol Grela*[a]
Dedicated to Professor Christian Bruneau on the occasion of his 60th birthday
Abstract:
A
study concerning the
polyfunctional molecules, including
natural and biologically active com-
pounds. Interactions between the FAH
and the second-generation ruthenium
catalysts, which apparently improve the
efficiency of the olefin metathesis
transformation, have been studied by
X-ray structure analysis and computa-
tions, as well as by carrying out a
number of metathesis experiments. The
optimisation of reaction conditions by
using an FAH can be regarded as a
complementary approach for the
design of new improved ruthenium cat-
alysts. Fluorinated aromatic solvents
are an attractive alternative medium
for promoting challenging olefin meta-
thesis reactions.
effect of using a fluorinated aromatic
solvent as the medium for olefin meta-
thesis reactions catalysed by ruthenium
complexes bearing N-heterocyclic car-
bene ligands is presented. The use of
fluorinated aromatic hydrocarbons
(FAH) as solvents for olefin metathesis
reactions catalysed by standard com-
mercially available ruthenium pre-cata-
lysts allows substantially higher yields
of the desired products to be obtained,
especially in the case of demanding
Keywords: doping effects · fluori-
nated aromatic hydrocarbons
metathesis N-heterocyclic car-
benes · ruthenium
·
·
Introduction
science.[1] Ruthenium-based pre-catalysts Gru, Ind, and Hov
(Figure 1)[2] display a high functional group tolerance and
satisfactory to excellent air and thermodynamic stabilities.
Unfortunately, relatively high loadings of these ruthenium
pre-catalysts are often required, sometimes leading to sub-
optimal use of this powerful methodology.[3,4] In the context
of greener chemistry, it is desirable to use these costly and
potentially toxic complexes more efficiently in order to pro-
Olefin metathesis has emerged as a versatile and powerful
tool for target-oriented organic synthesis as well as material
[a] C. Samojłowicz, Dr. M. Bieniek, Prof. Dr. K. Grela
Institute of Organic Chemistry, Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
Fax : (+48)22-632-66-81
http://www.karolgrela.eu/
E-mail: klgrela@gmail.com
´
[b] Dr. M. Bieniek, A. Pazio, Dr. A. Makal, Prof. Dr. K. Wozniak
Department of Chemistry, Warsaw University
Pasteura 1, 02-093 Warsaw (Poland)
[c] Dr. A. Poater
Catalan Institute for Water Research (ICRA)
Scientific and Technological Park of the University of Girona
H2O Building, Emili Grahit 101, 17003 Girona (Spain)
[d] Dr. A. Poater, Prof. Dr. L. Cavallo
Dipartmento di Chimica, Universitꢀ di Salerno
Via Salvador Allende, 84081 Baronissi (SA) (Italy)
[e] Dr. J. Wꢁjcik, Dr. K. Zdanowski
Institute of Biochemistry and Biophysics
Polish Academy of Sciences
´
Pawinskiego 5a, 02-106 Warsaw (Poland)
[f] Dr. K. Zdanowski
Institute of Chemistry
University of Natural Sciences and Humanities
3 Maja 54, 08-110 Siedlce (Poland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100160.
Figure 1. Selected ruthenium-based pre-catalysts for olefin metathesis.
Chem. Eur. J. 2011, 17, 12981 – 12993
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12981

tect the environment and to reduce the costs of industrial
processes.[4] Therefore, further research aimed at the devel-
opment of new catalysts of improved stability and/or activity
is of a vital importance.[4,5] Alongside evolutionary improve-
ment of the catalyst structure,[1,5] research aimed at finding
new reaction conditions that allow more efficient use of
known catalysts can be considered as a complementary ap-
proach.[6]
In particular, we are interested in learning to what extent
the solvent used for a metathesis reaction can affect the
turnover number (TON) of a given catalyst.[7] Recently, we
reported an unexpectedly strong effect of fluorinated aro-
matic solvents on olefin metathesis reactions promoted by
typical ruthenium catalysts bearing N-heterocyclic carbene
(NHC) ligands.[8] Our initial observation[9] was also con-
firmed by Blechert et al.,[10a] Collins et al.,[10b] Nolan
et al.,[10c] and Dorta et al.[10d] These authors reported that the
fluorinated aromatic hydrocarbon (FAH) effect is of quite
general nature for second-generation ruthenium catalysts,
both Grubbs and Hoveyda type complexes, bearing NHCs
with mesityl, tolyl, naphthyl and other aromatic substituents.
In this article, we assess the practical benefits of using FAHs
for challenging olefin metathesis reactions. To provide more
data on this interesting effect, X-ray, analytical, synthetic,
31P NMR and computational studies have been performed.
Figure 2. Solvent effect on RCM of 1a. Conditions: 0.02 mol% of cata-
lyst, c1a =0.02m, 708C, 1 h.
Results and Discussion
During our recent studies on the comparative screening of
various second-generation ruthenium-based pre-catalysts at
low loadings (Figure 1), a strong temperature effect was
noted in all of the tested reactions.[7] In the next step, we at-
tempted to study the effect of different solvents on the rate
of olefin metathesis reactions at the same temperature.
For instance, in the case of the ring-closing metathesis
(RCM) reactions shown in Scheme 1, the replacement of an
aliphatic chlorinated solvent by toluene has a strong effect
on catalyst activity, allowing, in the best case, a doubling of
yield without increasing the catalyst loading or extending
the duration of the reaction (Figures 2 and 3).[7] The forma-
tion of tetrasubstituted double bonds is one of the most
challenging transformations for Ru-based olefin metathesis
catalysts.[1,10] This reaction typically requires application of
catalysts at high loadings and even then does not lead to
quantitative yields.[11] Therefore, as the first model reaction
in the present study, we examined the RCM of diene 1c
(Scheme 2) promoted by a representative set of catalysts.[1f]
The reactions were conducted in 1,2-dichloroethane and tol-
Figure 3. Solvent effect on RCM of 1b. Conditions: 0.02 mol% of cata-
lyst, c1b =0.02m, 708C, 1 h.
Scheme 2. Two models representing challenging RCM reactions.
uene under otherwise identical conditions (catalyst loading,
time and temperature).
It is clear from the obtained results that replacing 1,2-di-
chloroethane by toluene has a strong effect on the turnover
numbers, leading to a tenfold increase in yield in the best
case (Hov-II, Figure 4) at the same temperature. We have
become interested in finding an explanation for such a pro-
nounced improvement of activity. It was reported by Fꢃrst-
ner and Nolan that some RCM reactions promoted by the
second-generation
catalyst
[PCy3ACTHUGNTERNNU(G IMes)Cl2Ru=CHPh]
(IMes=1,3-bis(2,4,6-trimethylphenyl)-2-imidazolylidene) in
toluene at 808C were not only faster than those in dichloro-
Scheme 1. Model RCM reactions.
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Ru-Catalysed Olefin Metathesis
FULL PAPER
Figure 4. Solvent effects in RCM of 1c. Conditions: 0.5 mol% of catalyst,
Figure 5. Observed solvent influence on RCM of 1d. Conditions:
2 mol% of catalyst, c1d =0.02m, 708C, 3 h.
c
1c =0.02m, 708C, 3 h.
methane at 408C, but also faster than the reactions conduct-
ed in 1,2-dichloroethane at 808C.[12a] Recently, Ledoux et al.
noted that in ring-opening metathesis polymerization
(ROMP) of cyclooctadiene, the Grubbs pre-catalyst Gru-II
was unambiguously more active in C6D6 than in CDCl3 at
the same temperature.[12b] A plausible explanation for this
effect, based on p–p interactions of the N-mesityl group
with the aromatic solvent molecules,[12b] has been proposed.
Encouraged by these reports, we decided to test fluorinat-
ed aromatic hydrocarbons as solvents, since these are known
to form strong face-to-face p–p complexes with phenyl
rings.[12,13] We set out to focus only on challenging olefin
metathesis reactions, specifically RCM and enyne reactions
(Est-II, Hov-II) show that this picture is more complicated
(Figure 5). Even if not yet fully explained, this observation
is very important from a practical point of view because in
the most pronounced case of Hov-II it was possible to in-
crease the reaction yield 18-fold merely by changing the sol-
vent from 1,2-dichloroethane to perfluorotoluene (Figure 5).
The challenging enyne 1e was chosen as another excellent
forum for evaluation of the scale of this intriguing solvent
effect (Scheme 3).[10,12] Again, the reactions conducted in
ꢀ
in which a tetrasubstituted C C double bond is formed. The
ꢀ
formation of tetrasubstituted C C double bonds typically
requires high catalyst loadings and even then does not lead
to quantitative yields.[11] Dimethylallyl malonate (1d) can be
considered as a very challenging substrate for ruthenium
catalysts, with Gru-II and Hov-II giving product yields of
only 17 and 6%, respectively (CH2Cl2, 308C, 96 h).[11,12a]
Therefore, this transformation seemed to be the best model
to test the extended palette of aromatic solvents, including
toluene, trifluoromethylbenzene, perfluorobenzene, and per-
fluorotoluene. We decided to perform a comparative study
at 708C, which was found to be optimal during our previous
investigation.[7] The obtained results (Figure 5) were quite
striking. First, we noted that the fluorinated aromatic sol-
vents led in general to much higher TONs as compared to
chlorinated aliphatic solvents and toluene. Indeed, the ring
closure of 1d was accomplished in 94% yield within 3 h
using only 2 mol% of catalyst Ind-II in perfluorotoluene at
708C. Under the same conditions but using toluene as the
solvent, only 27% of tetrasubstituted product 2d was fur-
nished.
Scheme 3. Model enyne reaction.
fluorinated aromatic solvents led to much higher TONs as
compared to those conducted in 1,2-dichloroethane and tol-
uene (for example, a sixfold increase in yield in the case of
Gru-II, or even a 13-fold increase in the case of Ind-II)
under otherwise identical conditions (catalyst loading, time,
and temperature; see Figure 6).
Being aware of the increasing importance of cross-meta-
thesis (CM) in the synthesis of natural and biologically
active products, we included this transformation in our
study.[14] To estimate the potential of our newly developed
conditions, we focused only on the most demanding cases,
such as the CM of geminally disubstituted or electron-defi-
cient alkenes (Scheme 4).[14,15] The results presented in
Table 1 show that in 1,2-dichloroethane none of the second-
generation catalysts was sufficiently potent to induce high
TONs in the CM between 1 f and (Z)-1,4-diacetoxy-2-
butene (Scheme 4a). In 1,2-dichloroethane, the highest yield
(27%) was obtained with Gru-II. Importantly, applying per-
fluorotoluene as the solvent led to an increased yield of
50%.
For some phosphine-containing Ru catalysts (Gru-II, Ind-
II, and Ind-II’), we observed that TON increases directly
with the number of fluorine atoms in the solvent molecule,
reaching the maximum in the case of perfluorotoluene.
However, the results obtained with Hoveyda type catalysts
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12983

K. Grela et al.
CM reactions by promoting the reactivity of trisubstituted
electron-poor substrates (Scheme 4c).[14] It has been report-
ed that such an effect can be achieved through capturing
phosphine from the Grubbs type complexes.[6b] Methacrylo-
nitrile (3b) is one of the most reluctant substrates to under-
go CM, especially for Gru-II type initiators.[5b,d,15b,c] For ex-
ample, in the reaction between 1h and 3b in toluene
(Scheme 4c), we observed that Gru-II, Ind-II, and Ind-II’
gave much lower yields (13, 12, and 16%, respectively) as
compared with Est-II and Hov-II (56 and 58%, respectively;
conditions: 5 mol% of catalyst, 708C, 3 h). Importantly, the
replacement of toluene with perfluorotoluene led to an up
to fourfold increase in yield (in the case of Ind-II)
(Table 3).[16] It should be noted that in the case of CM with
3b, the enhancing effect of the FAHs was only pronounced
in the case of phosphine-containing complexes.
Figure 6. Observed solvent influence on the cycloisomerisation of 1e.
Conditions: 5 mol% of catalyst, c1e =0.02m, 708C, 5 h.
Table 3. Yields [%] observed in the CM of 1h with 3b.[a]
solvent
Est-II
Hov-II
Gru-II
Ind-II
Ind-II’
toluene
trifluoromethylbenzene
perfluorobenzene
56
41
49
58
39
51
13
41
50
12
40
55
16
25
39
[a] c1h =0.02m, 5 mol% of catalyst, 708C, 3 h.
In our initial report on the doping effect of FAHs in
olefin metathesis, we studied a set of biologically active
compounds.[9a] It was found that many highly polar mole-
cules, such as a derivative of the antibacterial agent moxi-
floxacin, are insoluble in pure fluorinated aromatic solvent-
s.[9a] To solve this problem, we used FAH mixed with chlori-
nated aliphatic or aromatic co-solvents, and still observed
substantial improvements in yields.[9a] In the current study,
we decided to try an alternative strategy that allows us to
work in pure FAH media. To do so, we used short fluorinat-
ed tags (such as octafluorobutane), which can enhance the
solubility of polar substrates in neat fluorinated solvents
(Scheme 5).[17a] One such substrate is the antibacterial steroi-
dal compound, fusidic acid. In the CM reaction between an
fluorous tagged fusidic acid derivative (1i) and 3a catalysed
Scheme 4. Models representing challenging CM reactions.
Table 1. Yields [%] observed in the CM of 1 f with 3a.[a]
solvent
Est-II
Hov-II
Gru-II
Ind-II
Ind-II’
ClCH2CH2Cl
toluene
perfluorobenzene
perfluorotoluene
13
42
47
50
7
27
37
45
50
25
38
47
51
24
39
32
13
20
39
44
[a] c1f =0.02m, 5 mol% of catalyst, 708C, 3 h.
Table 2. Yields [%] observed in the CM of 1g with 3a.[a]
solvent
Est-II
Hov-II
Gru-II
Ind-II
Ind-II’
ClCH2CH2Cl
toluene
perfluorobenzene
24
46
52
12
29
42
31
40
50
27
40
n.d.
29
40
40
[a] c1g =0.02m, 5 mol% of catalyst, 708C, 3 h; n.d.=not determined.
A similar trend (Table 2) was observed for another chal-
lenging geminally disubstituted olefin, 1g (Scheme 4b).[5c] It
is known that while the Grubbs catalyst (Gru-II) is not very
efficient in CM reactions of some electron-deficient alkenes,
the Hoveyda type catalysts perform particularly well in such
cases.[14b,15b] Therefore, we became interested in ascertaining
whether fluorinated aromatic solvents could aid demanding
Scheme 5. CM of fusidic acid derivative 1i. ([a] Isolated yields after
column chromatography. Conversions calculated by H NMR are given in
parentheses; n.d.=not determined).
1
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Ru-Catalysed Olefin Metathesis
FULL PAPER
ꢀ
by 5 mol% of Gru-II, the trisubstituted C C double bond
of 1i is converted into an allyl acetate fragment (2i).[18] This
challenging transformation is significantly improved in octa-
fluorotoluene at 708C (reaching 92% conversion) as com-
pared to “classical” conditions using dichloromethane at
408C (only 28% conversion), under otherwise identical con-
ditions.
effect, the experiments described in the following subsec-
tions were conducted.
Determination of RCM profiles: Detailed RCM profiles for
the cyclisation of 1d promoted by a representative catalyst
(Ind-II) were measured in selected solvents (Figure 7). The
results revealed that the reaction performed in fluorinated
aromatic solvents proceeds with an overall increased rate as
compared to RCM carried out in 1,2-dichloroethane and tol-
uene, respectively. Indeed, the rate is enhanced in FAH, as
evidenced by results obtained in various solvents after 2 h
(RCM of 1d catalysed by 5 mol% of Ind-II): perfluoroto-
For further investigations, we selected an estradiol deriva-
tive 1j, which is insoluble in pure octafluorotoluene, and its
analogue 1k, decorated at the phenol moiety with a fluorous
tag, which significantly enhances solubility in FAH solvents
(Scheme 6). Comparative CM reactions of 1j and 1k with
challenging olefin metathesis partners, such as tert-butyl ac-
rylate (3c) and phenyl vinyl sulfone (3d), provided some in-
teresting results (Table 4). The CM reactions of 1j were per-
formed in a chlorinated aliphatic solvent (CH2Cl2 at 408C),
while the reactions of 1k were conducted in a fluorinated ar-
omatic solvent (octafluorotoluene at 708C). In all cases, we
observed improved efficiency for the reactions performed in
octafluorotoluene as compared to CH2Cl2. CM of tert-butyl
acrylate with estradiol derivatives 1j and 1k catalysed by
1 mol% of Gru-II led to products 2j (in CH2Cl2) and 2k (in
octafluorotoluene) in isolated yields of 32 and 93%, respec-
tively. On the other hand, the same CM partners with
phenyl vinyl sulfone and 5 mol% of Gru-II gave products
2m (in CH2Cl2) and 2n (in octafluorotoluene) in isolated
yields of 29 and 69%, respectively.[19]
^
*
luene ( ) 94%, 1,3-bis(trifluoromethyl)benzene ( ) 88%,
*
chloropentafluorobenzene ( ) 83%, pentafluorobenzene
~
&
(&) 68%, trifluoromethylbenzene ( ) 55%, toluene ( )
~
^
33%, 1,2-dichloroethane ( ) 22% and nitrobenzene ( )
4% (Figure 7).
The results that we obtained in the above model RCM,
enyne and CM experiments showed that fluorinated aromat-
ic solvents can modify the properties of commercially avail-
able catalysts in a significant manner, allowing the recovery
of much higher yields from difficult metathesis transforma-
tions compared to “classical” solvents used in olefin meta-
thesis. To gain further insight into this striking solvent
^
Figure 7. Benchmark RCM activity of Ind-II in different solvents ( =ni-
~
~
&
trobenzene, =1,2-dichloroethane, =toluene, =trifluoromethylben-
*
*
=1,3-
zene, & =pentafluorobenzene,
=chloropentafluorobenzene,
^
bis(trifluoromethyl)benzene, =octafluorotoluene). Conditions: 5 mol%
of catalyst, c1d =0.2m, 708C, 6 h.
In general, a correlation between TONs in RCM of 1d
catalysed by Ind-II and the number of fluorine atoms in the
solvent molecules was observed. Moreover, solvents such as
nitrobenzene, containing strong Lewis base substituents,
possibly “arrest” 14-electron Ru active species, leading to
low activity.[20] Interestingly, 1,3,5-tris(trifluoromethyl)ben-
zene was not a suitable solvent for the model RCM reac-
tion, due to insolubility of the ruthenium pre-catalyst (Ind-
II) in the reaction medium.
Correlation between rate enhancement and volume fraction
of FAH: The overall reaction rate enhancement was corre-
lated with the volume fraction of the fluorinated aromatic
solvent present in the reaction medium (Table 5). To do so,
the same RCM model reactions of 1d were independently
conducted in toluene and in 1,2-dichloroethane containing
variable amounts of perfluorotoluene. It was found that the
activating effect is directly linked to the volume fraction of
the fluorinated aromatic solvent present in the reaction
medium. This challenging transformation was most efficient-
ly conducted in pure perfluorotoluene. These results are in
Scheme 6. CM of estradiol derivatives (1j and 1k).
Table 4. CM of estradiol derivatives with challenging partners.
CM partner
Gru-II
Solvent
Product
ACHTUNGTRENNUNG
(Yield [%])[a]
G
1
2
3
4
1j+tert-butyl acrylate (3c)
1k+tert-butyl acrylate (3c)
1j+phenylvinylsulfone (3d)
1k+phenylvinylsulfone (3d)
1
1
5
5
CH2Cl2 (40) 2j (32)
C6F5CF3 (70) 2k (93)
CH2Cl2 (40) 2m (29)
C6F5CF3 (70) 2n (69)
[a] Isolated yields after column chromatography.
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12985

K. Grela et al.
Table 5. Observed conversions in the RCM of 1d with Ind-II in per-
At this point, it becomes clearly apparent that the ob-
served rate enhancement effect is only realised when
second-generation ruthenium complexes, bearing an NHC
ligand with aromatic substituents, are combined with aro-
matic fluorine-containing solvents. This conjecture is also
supported by the recent report by da Costa and Gladysz,[22]
who tested some fluorous second-generation Grubbs cata-
lysts in aliphatic fluorous media,[17] such as perfluorocyclo-
fluorotoluene/solvent mixtures of various proportions.[a]
Fraction of
0
0.25
0.5
0.75
1
perfluorotoluene (v/v)
1,2-dichloroethane
toluene
28
45
35
65
50
80
75
90
>99
>99
[a] c1d =0.2m, 5 mol% of catalyst, 708C, 6 h.
hexane
(C6F12)
and
perfluoromethylcyclohexane
agreement with those reported by Nolan, who observed
only a slight increase of the reaction ratio in model RCM
using a mixture of CH2Cl2/C6F6 (9:1) as compared to the re-
action performed in pure CH2Cl2.[10c]
(CF3C6F11).[22b] For comparison purposes, the authors also
measured the reactivities of the standard Grubbs second-
generation pre-catalyst Gru-II in the same aliphatic fluori-
nated hydrocarbons. They concluded that in the case of
Gru-II no appreciable solvent effect was observed for these
media.[22]
The effects of different additives in the reaction mixtures:
First, we used 25 mol% of hexachlorobenzene (m.p. 2288C)
dissolved in 1,2-dichloroethane, which did not increase the
reaction rate of RCM of 1d catalysed by 5 mol% of Ind-II.
We also used CuCl, which is known to be an effective phos-
phine scavenger,[6b] but no improvements in the reaction
rate were observed compared to the same reaction per-
formed in the presence of CuCl in pure 1,2-dichloroethane.
Finally, when 25 mol% of trinitrobenzene (a strong p ac-
ceptor)[23a] was used as an additive in 1,2-dichloroethane,
only the starting material 1d was recovered.
Correlation between physico-chemical properties of solvents
and observed conversions in RCM: Initiation rates of Gru-I/
II[24] as well as the neat results of model RCM reactions[25a,b]
have already been correlated with the dielectric constants of
the solvents. On the other hand, viscosity plays a significant
role in reactions performed in ionic liquids.[25c] Nolan et al.
suggested that the FAH effect in olefin metathesis could be
related to the physical properties of the reaction medium.[10c]
In the present work, we have attempted to correlate the ob-
served effect (see Figure 7) with selected physicochemical
properties of the reaction medium, such as dielectric con-
stant and viscosity.[24,25] For instance, dielectric constant
values decrease in the following order: 1,2-dichloroethane>
trifluoromethylbenzene>1,3-bis(trifluoromethyl)benzene>
pentafluorobenzene>other nonpolar FAH (see Table 6).
On the other hand, viscosity is similar for all FAHs at 258C,
with them being slightly more viscous than “classical” sol-
vents. As the data collected in Table 6 show, it is difficult to
find a strong correlation between either viscosity or dielec-
tric constant and the results of challenging olefin metathesis
reactions of 1d. Recently, solvent effects were probed in an
RCM reaction using the multiple parameter fitting ap-
proach;[25e] however, we have not yet tried this method.
Evaluation of first-generation ruthenium complexes: The
NHC-deprived, first-generation ruthenium complexes were
tested in selected aromatic and aliphatic solvents (Figure 8).
Interestingly, no improvement in the RCM of diethyl allyl-
methallyl malonate (1b) was observed when representative
first-generation pre-catalysts (Gru-I, Ind-I and Hov-I) were
used in the fluorinated aromatic solvents. These results are
in agreement with Fꢃrstnerꢄs observation[12a,21] that the posi-
tive influence of toluene on a reaction ratio is observed only
for ruthenium catalysts bearing N-aryl-substituted NHC li-
gands, whereas the activity of analogues with N-alkyl groups
is usually higher in chlorinated media.[11a,21]
Table 6. Dielectric constants and viscosities of various solvents, and con-
versions in the RCM of 1d with Ind-II observed in these media.[a]
Solvent
Dielectric
constant
[er][b]
Dynamic
viscosity
Conversion
in RCM of
1d [%]
AHCTUNGTRENNUNG
[mPas][b]
nitrobenzene
1,2-dichloroethane
toluene
35.60[c]
10.42[c]
2.38[c]
2.28[c]
9.22[c]
4.36
1.863[c]
0.779[c]
0.560[c]
0.604[c]
0.689
–
1.468
1.029
1.183
1.233
4
25
43
43
58
73
89
94
99
benzene
trifluoromethylbenzene
pentafluorobenzene
chloropentafluorobenzene
1,3-bis(trifluoromethyl)benzene
hexafluorobenzene
octafluorotoluene
2.27
7.49
2.03[c]
2.66
>99
Figure 8. Observed solvent effects in RCM of 1b. Conditions: 5 mol% of
catalyst, c1b =0.02m, 408C, 3 h.
[a] c1d =0.2m, 5 mol% of Ind-II catalyst, 708C, 6 h. [b] Measurements
were conducted at 258C. [c] Value taken from reference [25f].
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Ru-Catalysed Olefin Metathesis
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Comparison of commercially available non-degassed FAHs
with freshly distilled and degassed solvents in RCM experi-
ments: It is known that perfluorohydrocarbons dissolve rela-
tively large amounts of gases, especially oxygen.[17a] Being
aware of a negative influence of oxygen on the catalyst life-
time,[24] we performed a set of RCM reactions of 1d in com-
mercially available non-degassed toluene and hexafluoro-
benzene (the reactions were run in open vessels in air), and
compared the results with the outcome of reactions conduct-
ed in freshly distilled and degassed solvents (using Schlenk
techniques under argon atmosphere). Interestingly, we did
not observe a significant influence of air on the rate of
RCM performed in FAH (Table 7). Although difficult to ex-
plain, the results show some practical advantages of these
solvents.
Figure 9. Plots of the relative intensity of the 31P NMR signal of Gru-II as
*
*
a function of time in C2D4Cl2 at 30 ( ) and 508C ( ), and in a mixture of
&
C6F6/C2D4Cl2 (4:1, v/v) at 30 ( ) and 508C (&).
Table 7. Observed conversions in the RCM of 1d with Ind-II in commer-
cially available compared with distilled and degassed solvents.[a]
suggest that the facile dissociation/oxidation reaction of the
phosphine ligand in FAHs allows faster and irreversible for-
mation of catalytically active 14-electron species, which can
be subsequently stabilised by p-p stacking interactions be-
tween an electron-donating moiety (N-mesityl or benzyli-
dene) and electron-poor solvent molecules,[23,26] or even by
direct fluorine–ruthenium interactions.[27] In the case of
phosphine-free Hoveyda type systems, similar stabilisation
of the propagating species can be suggested. We postulate
that the interactions with fluorinated solvent molecules can
modify the through-space transfer of electron density be-
tween the Ru=CHR unit and the aromatic rings on the N-
substituents of the NHC ligand, thereby accelerating the
rate of the metathesis reaction,[26] such that high TONs can
be achieved even for very challenging substrates.
Solvent
Commercially available
solvent, conversion [%]
Distilled and degassed
solvent, conversion [%]
toluene
hexafluorobenzene
19
91
45
99
[a] c1d =0.2m, 5 mol% of Ind-II catalyst, 708C, 6 h.
31P NMR stability and decomposition studies of Gru-II in
solutions containing FAH: Intrigued by the above results,
we decided to study decomposition reaction rates of Gru-II
in “classical” chlorinated solvents and in the presence of
FAH. To do so, two sets of 31P NMR experiments were con-
ducted by recording spectra from solutions of Gru-II
(10 mm) in neat C2D4Cl2 and in a mixture C6F6/C2D4Cl2 (4:1,
v/v). The decay of the characteristic 31P NMR signal of Gru-
II (d=29 ppm) was recorded until its complete disappear-
ance. At the same time, a new signal appeared at 48 ppm,
which was unambiguously associated with the formation of
tricyclohexylphospine oxide, Cy3P=O. This signal was the
major feature in the 31P NMR spectrum, accompanied by
two signals at d=21 and 31 ppm (for copies of the spectra
and calculated thermodynamic data, see the Supporting In-
formation). We have not attempted to characterise other de-
composition products.
Three series of spectra were recorded from the respective
solutions at temperatures of 30, 40, and 508C. The measured
decomposition half-lives of Gru-II in C2D4Cl2 were 470 min
at 308C and 170 min at 508C. In the solution containing
C6F6, the catalyst decomposed much more rapidly, with half-
lives of 160 min at 308C and of only 60 min at 508C (see
Figure 9). A relatively narrow temperature window was
used in these experiments due to the pronounced tempera-
ture dependence of the process. Below room temperature,
the process becomes very slow, whereas above 508C it be-
comes too fast to be reasonably measured.
X-ray structural analysis of ruthenium complexes with FAH:
Fortunately,[10b] we were able to grow crystals of Hoveyda
pre-catalysts in FAH, which allowed us to study possible sol-
vent–catalyst interactions in the solid phase. The complex of
¯
Hov-II with perfluorobenzene crystallises in the P1 space
group of the triclinic system. There are two independent cat-
alyst molecules in the crystallographic asymmetric unit,
which differ with respect to the interactions with the per-
fluorobenzene solvent molecules.[29] Both molecules (denot-
ed hereinafter as M1 and M2 according to the ruthenium
atom numbering) occupy general positions (Figure 10). One
of the N-mesityl arms in M2 is disordered over two positions
with respective occupancies of 0.45 and 0.55. The asymmet-
ric unit also contains one-and-a-half perfluorobenzene mole-
cules. The solvent molecule containing atoms C101–C103
and F1–F3 (denoted hereinafter as S1) is located in a special
position with an inversion center at the middle of the mesi-
tyl ring and is perfectly ordered. The other solvent molecule
(denoted as S2) displays a complicated disorder. Three alter-
native positions are occupied by this molecule, with occupa-
tion factors of approximately 0.45, 0.35 and 0.2, respectively.
The former two orientations are coplanar and parallel to
one of the mesityl moieties in M2. The least occupied posi-
tion places the C125 atom of perfluorobenzene close to the
The observed faster decomposition of Gru-II in solutions
containing FAH may seem surprising in the light of the in-
variably better yields reported for reactions in these sol-
vents, as well as in the presence of oxygen (see Table 7). We
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K. Grela et al.
The methyl hydrogen atoms of the mesityl group form nu-
merous short contacts with the S2 molecules both parallel
and perpendicular to the mesityl ring. The crystal packing
depends strongly on the mesityl–perfluorobenzene and per-
fluorobenzene–perfluorobenzene interactions.[23] The solvent
molecules are gathered on the (110) crystallographic plane.
The disordered S2 molecules form a continuous tape along
the [100] direction passing through 01= 0, while the S1 mol-
2
ecules are located directly along the [100] axis. Both the for-
mation of the characteristic layer and strong interactions
with the catalyst molecules account for the fact that the
evaporation of solvent causes slow crystal decomposition.[29]
The catalyst molecules themselves occupy space between
the perfluorobenzene layers. The molecules M1 interact
with M2 through the methyl groups at the para-position in
the mesityl moieties, which penetrate the space between the
mesityl rings of M2. The interacting mesityl and benzylidene
moieties surround a channel, which extends along the [100]
direction in the middle of each unit cell.
Figure 10. X-ray structure analysis of Hov-II with hexafluorobenzene in-
corporated in the crystal lattice. Ellipsoids represent 50% probability
level. Hydrogen atoms have been omitted for clarity.
crystallographic inversion center and the whole molecule in
the plane perpendicular to the more occupied positions. An
ORTEP representation of the catalyst molecule interacting
with perfluorobenzene is presented in Figure 10. The disor-
dered system is illustrated in Figure S1 in the Supporting In-
formation. The structures of both independent molecules of
the Hov-II catalyst do not differ from the crystal structures
of this catalyst crystallised with dichloromethane.[1e,30] The
coordination sphere of the ruthenium ion is particularly well
conserved. The most important geometrical parameters
characterizing the catalyst are reported in the Supporting In-
formation.
The interactions of perfluorobenzene with the Hoveyda
catalyst seem to be important for the crystal formation and
are quite different from what has been observed in the case
of catalyst crystallised from other solvents. The catalyst mol-
ecules M1, together with the solvent molecules S1, which
are located exactly between the C13–C21 N-mesityl arms of
M1 (Figure 10) and the same moiety related by the inver-
sion center, form a well localised stacked structure. The dis-
tance between the centroids of the perfluorobenzene mole-
cule and the mesityl moiety is 3.533(3) ꢅ, thus indicating
strong interaction between the rings. S1 is rotated with re-
spect to the mesityl ring by 23.6(6)8, so that its carbon
atoms are located above the bonds of the mesityl moiety.
The fluorine atom F2 is engaged in short contacts to methyl-
ene protons H3A (2.650 ꢅ), H57 (2.621 ꢅ), and H60A
(2.556 ꢅ) from the disordered part of M2 and F35 from the
disordered solvent, which results in a longer C-F distance
(1.340 ꢅ, compared with 1.330 ꢅ for F1). The C53–C61 me-
sityl moiety of M2, analogous to C13–C21 in M1, interacts
with perfluorobenzene S2 also mainly by ring stacking (see
Figure S1). In this case, S2 is not stacked between two mesi-
tyl moieties. However, another S2 molecule, related by an
inversion center, is located parallel to the first; therefore,
the fluorine atoms from one molecule lie above the carbon
atoms from the parallel molecule. The shortest distance be-
tween the mesityl moiety and S2 is 3.331 ꢅ, and the shortest
distance between the two parallel S2 molecules is 3.429 ꢅ.
The structure of Gru-II with perfluorobenzene: The crystals
¯
formed from Gru-II and C6F6 crystallise in the P1 space
group with one molecule of the catalyst and two molecules
of solvent (S1: C50–C55 ring and S2: C60–C65 ring) in the
independent part of the unit cell (Figure 11). The most im-
portant structural parameters for this structure are given in
the Supporting Information. There are several weak interac-
tions between the perfluorobenzene moieties and the
Grubbs pre-catalyst molecule, which may be important for
crystal formation and stability. Each catalyst molecule inter-
acts with two perfluorobenzene molecules through quite
complex interactions. First of all, the F atoms change the
sign of the molecular quadrupole moment. This means that
the negative charge in C6F6 is located in the plane of the
moiety, whereas the positive charge lies above and below
this plane. Consequently, the Cl atoms of the catalyst mole-
Figure 11. X-ray structure analysis of Gru-II with hexafluorobenzene in-
corporated in the crystal lattice. Ellipsoids represent 30% probability
level. Hydrogen atoms have been omitted for clarity.
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Ru-Catalysed Olefin Metathesis
FULL PAPER
cule point towards the carbon atoms of the S2 solvent mole-
cule (with Cl···C contacts of length about 3.3–3.4 ꢅ), as
might be expected for bond dipole···molecular quadrupole
interactions. In this way, catalyst–solvent–catalyst chains are
formed in the crystal lattice, which extend along the [100]
direction. At the same time, the C6F6 molecule S2 is orient-
ed perpendicularly to the plane of the other independent
solvent molecule S1 to allow close F···C contacts. This is
characteristic of molecular quadrupole···quadrupole interac-
tions. Numerous F atoms of both solvent molecules form
several weak H···F hydrogen bonds. Additionally, even short
F···F contacts are possible.
The S1 ring is located almost parallel to the C13–C21 me-
sityl group of the catalyst molecule, and the distance be-
tween their centroids is 3.870 ꢅ, which indicates some stack-
ing-type interactions. This corresponds to the Hoveyda sol-
vates, in which stacking is the main type of interaction be-
tween the catalyst and solvent molecules; however, the in-
teraction distances in the Hoveyda derivatives are shorter
(about 3.53–3.55 ꢅ for the two independent moieties in the
asymmetric unit). The distance between the centroids of the
other C4–C12 mesityl ring and the C22–C28 benzylidene
group bound to the Ru atom is
and recommended functionals for efficiently describing or-
ganometallic systems and weak interactions.[32] The geome-
tries that we examined are drawn in Scheme 7.
Calculations were started from geometry A in Scheme 7,
since it allows a direct comparison of the DFT-optimised
and X-ray structures. Geometrical analysis indicated that
the DFT-optimised structure (see Figure 12) is in excellent
agreement with the crystallographic structure, with RMSDs
(root-mean-square deviations) of only 0.055 ꢅ for distances
and 0.98 for angles.[33] The distance between the centre of
the C6F6 ring and the aromatic ring of the adjacent mesityl
ring is 3.52 ꢅ, in excellent agreement with the experimental
value of 3.59 ꢅ. This analysis validates the chosen computa-
tional approach for locating the geometry of these particular
systems. Replacing C6F6 with C6H6 results in a strong tilting
and shift of the C6H6 ring, leading to an offset stacked geo-
metry.[23a] The distance between the centres of the two aro-
matic rings increases to 4.51 ꢅ, with the C6H6 ring rotated
to point towards the nearby methylene group of the back-
bone of the NHC ligand. This results in the formation of a
weak C–H···aromatic interaction, as suggested by the short
distance of 2.81 ꢅ between the centre of the benzene and
3.799 ꢅ.
The crystal packing depends
on perfluorobenzene–Grubbs
complex
interactions
(Figure 11), and likewise in the
Hoveyda structure crystallised
with C6F6. Each molecule of the
Grubbs complex forms numer-
ous short contacts with seven
solvent moieties and only one
contact with another catalyst
molecule. Aromatic rings in the
lattice, except those of S2 en-
gaged in interactions with chlor-
ine atoms, are located almost
perpendicularly to the [100] di-
rection.
Computational results of ruthe-
nium complexes in FAHs: To
obtain insights into the energet-
ics of the interactions between
aromatic solvent molecules
(both C6F6 and C6H6 rings) and
the ruthenium pre-catalysts
considered above, we per-
formed
theory
density
(DFT)
functional
calculations.
Since one of the main interac-
tions could be a stacking be-
tween the aromatic rings of the
Ru complex and of the solvent,
we selected the M06L function-
Scheme 7. Structures examined, and interaction energies, in kcalmolꢀ1, between the Ru complexes and C6F6
and C6H6, in round and in square parentheses, respectively. For the sake of clarity, the F and H atoms of C6F6
al,[31] which is among the best and C6H6 have been omitted in the drawings.
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12989

K. Grela et al.
are essentially the same, which indicates that the interaction
of a solvent molecule with the mesityl rings is not selective.
To gain insight into the role of the N-substituents of the
NHC ligand, the SIMes (1,3-bis(2,4,6-trimethylphenyl)-2-
imidazolidinylidene) ligand of Hov-II was replaced by the
popular SIPr (1,3-bis(2,6-diisopropylphenyl)-2-imidazolidi-
nylidene) ligand, structure C in Scheme 7. Even in the pres-
ence of the bulkier ortho isopropyl substituents, the solvent
molecules can effectively approach the aromatic ring of the
N-substituents. Indeed, the distance between the centre of
the C6F6 ring and the nearby N-substituent of the SIPr-
based Ru complex, 3.85 ꢅ, compares well with the analo-
gous distance in the SIMes-based Ru complex. The interac-
tion energy between C6F6 and the SIPr-based Ru complex,
12.8 kcalmolꢀ1, is essentially the same, giving a further indi-
cation that the bulkier ortho iPr groups do not sterically de-
stabilise the interaction. In line with this trend, adding a
methyl group para to the N atoms in the SIPr-based Ru
complex, structure D in Scheme 7, stabilises the interaction
with C6F6 from 12.8 to 13.4 kcalmolꢀ1. In all of these cases,
the interaction with C6H6 is noticeably weaker. To assess the
role of the alkylidene unit, as well as the saturation of the
metal, we calculated the same kind of interactions in the
prototype second-generation Ru catalyst, and in the 14-elec-
tron species formed after phosphine dissociation, structures
E–H. The interaction energies reported in Scheme 7 for E–
H are rather similar to those calculated for A–D, indicating
that the nature of the alkylidene group or the metal satura-
tion have no role in determining the strength of the interac-
tion between the N-substituents and the solvent molecules.
Next, we examined the interactions of C6F6 and C6H6 with
the aromatic ring of the alkylidene moiety, structures I–K in
Scheme 7. Even with the iPrO group coordinated to the Ru
centre, structure I, a solvent molecule is able to engage
properly with the aromatic group of the alkylidene unit, and
the interaction is even slightly stronger. This interaction is
of course preserved after dissociation of the iPrO group,
which gives the opportunity to investigate the influence of a
solvent molecule on the activation step. In the absence of an
explicit solvent molecule coordinated to the Ru complex,
dissociation of the iPrO group has an energy requirement of
16.1 kcalmolꢀ1. Considering a solvent molecule coordinated
to the alkylidene group, dissociation incurs 13.7 and
13.0 kcalmolꢀ1 on going from I to J in the presence of C6F6
and C6H6, respectively, indicating that a solvent molecule
may slightly promote activation, since a better geometry of
interaction can be established between the solvent molecule
and the alkylidene aromatic group after dissociation of the
iPrO group. A rather strong interaction is also calculated for
that between a solvent molecule and the aromatic ring of
the phenylidene group; see structure K. In these cases as
well, C6F6 interacts with the Ru complex considerably more
strongly than C6H6.
Figure 12. DFT-optimised structures of the complexes between C6F6 (top)
or C6H6 (bottom) and the Hov-II catalyst. Distances in ꢅ.
the closest hydrogen atom of the methylene group. From an
energetic point of view, the interaction of C6F6 with the Ru
complex, 13.5 kcalmolꢀ1, is considerably stronger than that
involving C6H6, which amounts to just 5.8 kcalmolꢀ1; see
Scheme 7.
The formation of a nicely stacked structure in the case of
C6F6 and of a rather tilted structure in the case of C6H6 can
be explained in terms of the quadrupole moments of the
two molecules. The quadrupole moment of benzene (and
similarly of a mesityl ring) is large and negative, while the
electronegativity of fluorine makes the quadrupole moment
of C6F6 large and positive. The opposite quadrupole mo-
ments of C6F6 and of the mesityl ring maximise the electro-
static interaction energy and favour a stacked arrangement,
whereas the same-sign quadrupoles of C6H6 and of the mesi-
tyl ring electrostatically repel, favouring a T-shaped or offset
stacked geometry.[23a] We also investigated structures in
which the C6F6 and C6H6 rings are placed above the other
mesityl ring, that is, on the same side of the Ru=ylidene
bond, structure B in Scheme 7. In this case as well, the C6F6
ring remains well stacked over the mesityl ring, whereas the
C6H6 ring moves away to engage in an interaction with the
NHC bridge backbone. In short, both B structures are quite
similar to the corresponding A structure. The interaction en-
ergies between the solvent molecules and the Ru complex in
B, 12.9 and 5.8 kcalmolꢀ1 for C6F6 and C6H6, respectively,
Moving finally to the interaction of C6F6 or C6H6 with the
Ru centre in the 14-electron complexes derived from disso-
ciation of the iPrO group or of the phosphine, we first as-
sessed whether the solvent could interact with the metal
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Ru-Catalysed Olefin Metathesis
FULL PAPER
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
through direct coordination of one of the F or H atoms;
structure L in Scheme 7. This interaction is clearly disfav-
oured relative to solvent interaction with the alkylidene aro-
matic group in J, but is still attractive, since the energies of
solvent binding to the simple 14-electron species formed by
dissociation of the iPrO group from Hov-II amount to 9.7
and 7.3 kcalmolꢀ1 for C6F6 and C6H6, respectively. Coordi-
nation of a solvent molecule to the Ru centre through the
aromatic ring of C6F6, structure M, is even more favoured,
and it allows recovery of almost all of the energy lost in the
dissociation of the iPrO group. The rather strong interaction
between the p-system of the solvent and the Ru centre is
confirmed by the strong interaction with the Ru centre in
the phenylidene-based 14-electron structure N.
Static calculations: Static DFT calculations were performed at the GGA
level with the Gaussian 09 package using the M06L functional.[31] The
electronic configurations of the molecular systems were described with
the standard split-valence basis set with a polarisation function due to
Ahlrichs and co-workers for H, C, N, O, F, and Cl (SVP keyword in
Gaussian 09).[37] For Ru, we used the small-core, quasi-relativistic Stutt-
gart/Dresden effective core potential, with an associated (8s7p6d)/ACHTUNG-
TERNN(UG 6s5p3d) valence basis set contracted according to a (311111/22111/411)
scheme (standard SDD keywords in Gaussian 09).[38] The geometry opti-
misations were performed without symmetry constraints, and the located
stationary points were characterised by analytical frequency calculations.
Solvent effects, including contributions of non-electrostatic terms, have
been estimated by single-point calculations on the gas-phase-optimised
structures, based on the polarisable continuum solvation model PCM
using benzene and perfluorobenzene as solvents.[39]
Conclusion
The presented results demonstrate that interactions with ar-
omatic fluorinated solvent molecules can significantly influ-
ence the activity of NHC-ruthenium catalysts in olefin meta-
thesis. This effect is of high practical importance since it
offers a complementary method for activating already exist-
ing metathesis catalysts.[34] In addition, unlike freonꢆ and
some aromatic hydrocarbons, FAHs do not seem to pose
much environmental or biological risk.[35]
Acknowledgements
C.S. thanks the Foundation for Polish Science (“Ventures” Program) for
financial support. The project “Ventures/2008–1/5” is realised within the
“Ventures” program of the Foundation for Polish Science, co-financed
from the European Union Regional Development Fund. K.W. thanks the
Foundation for Polish Science for the award of a “Mistrz” professorship.
A.M. is also thankful for financial support in the form of a Polish Minis-
try of Science and Higher Education grant for Ph.D. students, no.
NN204030233. Special thanks are due to Dr. R. Kadyrov (Degussa-
Evonik) for initiation of the project, Prof. R. Hołyst (IChF PAN) for as-
sistance with viscosity measurements, and Dr. H. Szatyłowicz (Faculty of
Chemistry, Warsaw University of Technology) for assistance with the die-
lectric constant measurements. A.P. is grateful for the allocation of a
Ramꢁn y Cajal contract by the Spanish MICINN. We thank the HPC
team of Enea (www.enea.it) for the use of the ENEA-GRID and the
HPC facilities of CRESCO (www.cresco.enea.it) in Portici, Italy.
FAHs can improve the efficiency of the initiation step of
phosphine-containing Ru pre-catalysts, as was indicated by
much faster decomposition of the Grubbs catalysts Gru-II in
solutions containing FAH. One of the parameters that may
be responsible for the enhanced activity of the propagating
catalyst is the formation of p–p stacking interactions be-
tween the N-aromatic substituent and the aromatic fluori-
nated solvent molecules. A number of interactions, including
p–p stacking, have been observed between FAHs and
second-generation ruthenium catalysts in the solid state. In-
dependent calculations have indicated that an aromatic sol-
vent molecule can engage in a variety of possible interac-
tions with a ruthenium complex, stabilizing it and even pro-
tecting the 14-electron species by direct coordination to the
Ru centre. This contributes to a higher stability of the ruthe-
nium active species, which may be responsible for the re-
markable activation effect observed experimentally. Further,
the calculations indicated that C6F6 interacts remarkably
more strongly than C6H6, which is in line with the experi-
mental results. Finally, there is no reason to rule out the pos-
sibility that the interactions described above could work co-
operatively.[36]
[1] For selected reviews on olefin metathesis, see: a) Handbook of
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ized, and that under some conditions will catalyse the metathesis of
olefins, but that have not been proven to be essentially identical to
the active species for the metathesis reaction, are not well-defined
catalysts. They are catalyst precursors, or (pre)catalysts.” R. R.
Schrock, J. Mol. Catal. A Chem. 2004, 213, 21–30.
[3] For selected reviews on applications, see: a) I. Nakamura, Y. Yama-
moto, Chem. Rev. 2004, 104, 2127–2198; b) A. Deiters, S. F. Martin,
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Int. Ed. 2006, 45, 6086–6101; h) Metathesis in Natural Product Syn-
Experimental Section
Experimental details of the catalytic procedures, with complete character-
isation of all new compounds, including copies of the NMR spectra, X-
ray crystallographic tables, the data in crystallographic information file
(CIF) format, and all other information are included in the Supporting
Information. CCDC-759842 (Hov-II with perfluorobenzene) and 782769
(Gru-II with perfluorobenzene) contain the supplementary crystallo-
Chem. Eur. J. 2011, 17, 12981 – 12993
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12991

K. Grela et al.
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Chem. Eur. J. 2011, 17, 12981 – 12993

Ru-Catalysed Olefin Metathesis
FULL PAPER
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Received: January 17, 2011
Revised: June 25, 2011
Published online: September 29, 2011
(CVꢀEV)2/
A
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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12993