Synthesis and characterization of new ruthenium N-heterocyclic carbene Hoveyda II-type complexes. Study of reactivity in ring closing metathesis reactions

Article abstract of DOI:10.1039/c2dt31540h

This manuscript describes the synthesis and structural study of new second generation Hoveyda-Grubbs catalysts: 1,3-dimesityl-acenaphthylenyl-4,5- imidazolin-2-ylidene (BIAN-NHC) ruthenium isopropoxybenzylidene dichloride 3 and 1,3-bis(2,6-dimethylphenyl)

Full text of DOI:10.1039/c2dt31540h

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COMMUNICATION
Synthesis and characterization of new ruthenium N-heterocyclic carbene
Hoveyda II-type complexes. Study of reactivity in ring closing metathesis
reactions†
Estíbaliz Merino,*a,b Evelyne Poli,b Urbano Díaz*b and Daniel Brunelb
Received 12th July 2012, Accepted 27th July 2012
DOI: 10.1039/c2dt31540h
stability and recycling of the second generation Grubbs cata-
lyst13,14 due to the discovery of the phosphine-free catalyst 1 by
using the chelating isopropoxybenzylidene ligand. This catalyst
had the highest general reactivity towards electron-deficient
olefins15 and the best air-stability.13 The phosphine-free catalyst
2 bearing 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes)
in place of 1,3-bis-(2,4,6-trimethylphenyl)-imidazolidin-2-
ylidene (SIMes) as the NHC ligand has also been synthesized
(Fig. 1).16,17
This manuscript describes the synthesis and structural study of
new second generation Hoveyda–Grubbs catalysts: 1,3-dimesityl-
acenaphthylenyl-4,5-imidazolin-2-ylidene
(BIAN-NHC)
ruthenium isopropoxybenzylidene dichloride 3 and 1,3-bis-
(2,6-dimethylphenyl)-2,3-dihydro-1H-imidazole Cl2Ru(vCH-
o-O-i-PrC6H4) 4. The electrochemical and catalytic behavior
of these new complexes was compared with the conventional
NHC carbene Hoveyda II IMes-type complexes 1 and 2 for
ring closing metathesis reactions.
In the last decade, many investigations have been reported
with a view to fine-tuning the reactivity and specificity of this
catalyst type for expanding its application profile. Efforts have
been directed towards the modification of steric and electronic
properties of the ligands around the Ru center. In this regard,
Blechert et al. have shown that the replacement of the isopropoxy-
styrene ligand by biphenyl- or binol-based styrene in 1 resulted
in a significant improvement of the catalyst activity attributed to
their steric hindrance.18 Furthermore, Grela et al. investigated the
electronic effects induced by modification at the 5-position of
the styrenyl ligand. A decrease in the electron density at the
oxygen atom of the 2-isopropoxy group by the introduction of a
strong electron-withdrawing group led to a dramatic enhance-
ment of the catalyst activity for ring-closing metathesis (RCM)
as well as for more demanding cross metathesis (CM) reac-
tions.19 Using different substituents at the styrene nucleus at the
ortho, meta and para positions, Blechert et al. have suggested
the effect of the Ru–O-ligation of the precatalyst for the concen-
tration control of the 14-electron propagating species. Also
described was the limitation of its decomposition reaction which
was in competition with the efficient olefin binding,20 according
to the suggested reaction pathway including the release-return
steps.13a,21,22 On the other hand, the role of π-electron density
Since the isolation of bis-(1,3-diphenyl-imidazolin-2-ylidene)
mercury perchlorate by Wanzlick and Schonherr1 and (1,3-
dimethyl-imidazolin-2-ylidene)-pentacarbonyl chromium as the
first transition-metal N-heterocyclic carbene (NHC) complex by
Öfele,2 NHCs have become ubiquitous ligands in coordination
chemistry.3,4 Arduengo described the first synthesis of stable
crystalline NHCs, with a 1,3-diadamantyl-imidazolin-2-ylidene5
or 1,3-dimesityl-imidazolidin-2-ylidene6 skeleton, i.e. possessing
sterically demanding groups at each nitrogen atom. The most
important feature of these ligands is their strong σ-donor ability7
leading to enhanced complex stabilization through the strongest
metal–carbene bond.8 This characteristic was particularly impor-
tant in the olefin metathesis area through the replacement of one
of the two phosphine donors from the neutral 16-electron ruthe-
nium vinylidene or benzylidene dichloride-based catalyst
RuCl2(vCHAr)(PCy3)2 as described by Grubbs et al.9 leading
to the second generation Grubbs catalyst RuCl2(vCHAr)NHC-
(PCy3) containing 1,3-mesityl-imidazol-2-ylidene or a saturated
analogous ligand.10 This new type of complex exhibited higher
stability and functional group tolerance than the first generation
complexes as has been independently reported by three different
research groups.8,11,12 Subsequent research further improved the
aInstituto de Química Orgánica General, CSIC, C/ Juan de la Cierva, 3,
28006 Madrid, Spain. E-mail: estibaliz.merino@iqog.csic.es
bInstituto de Tecnología Química, UPV-CSIC, Avenida de los Naranjos
s/n, 46022 Valencia, Spain. Fax: +34 96 3877809;
Tel: +34 96 3877800; E-mail: udiaz@itq.upv.es
†Electronic supplementary information (ESI) available: Experimental
procedures to synthesize complexes 2, 3 and 4. General procedure to
carry out the kinetics. Details of cyclic voltammetry. 1H NMR spectra of
1
the reaction mixtures. H NMR spectra of intermediates and complexes
of ruthenium. X-ray crystal data and structure refinements for complexes
3 and 4. CCDC 831083 and 831082. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt31540h
Fig. 1 Structures of catalysts 1 and 2.
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delocalization between the phenyl and the carbene in the pre-
catalyst has been investigated very recently through a DFT
mechanistic study by Pleixats et al.23 It was found that this de-
localization was almost completely lost in the transition state and
is the key to controlling the alkoxy-dissociation energy barrier.
The higher the π delocalization the higher the energy barrier is
and the lower the catalytic activity is. Interestingly, there is
another suggestion about an electron donation from the aromatic
π-face of the NHC aryl groups towards the metal donor proper-
ties of N-heterocyclic carbenes as reported by Plenio and
Süβner.17 This hypothesis was supported by the variation of the
redox potentials of Ru(II)/Ru(III) as a function of the para substi-
tuent in the saturated or unsaturated NHC aryl ligands for the
two series of Grubbs–Hoveyda type complexes. More recently,
the same group has shown that the differences in the reactivity of
these two types of Grubbs II NHC complexes do not originate
from different electron density at the Ru center by a through-
bond transfer from the 4-aromatic nucleus-substituents. This
difference is mainly due to steric effects resulting from the trans-
annular interaction of the mesityl flap and the RuvCHAr unit.
This π-stacking effect is considerable in the oxidized NHC metal
complex.24 Moreover, the interaction of the π-system of the
ligand NHC with rhodium has been investigated considering
also the effects on the NHC donor strength achieved by modifi-
cation at another position, i.e., at the 4,5-positions of the unsatu-
rated imidazolylidene ligands.25 Following this approach,
Glorius et al. have focused their attention on the quantification
of the electronic, steric and bond strength, which are important,
to design, select and use metal complexes with tailor-made
NHC-type ligands in catalysis.26 It is noteworthy that the com-
parison of the annulated five-membered normal NHCs was made
considering the unsaturated carbene. One of the most interesting
aspects of these metal complexes at the 4 and 5 positions with
π-active groups would be metal-to-carbene π-back bonding, with
an electron-rich metal such as Pd(0) or Pt(0), which would
modify their properties.27 To the best of our knowledge, there is
only one example reported on the use of complex 2 for cata-
lysis,28 showing the inactivity of this complex in the cyclopoly-
merization of 1,6-heptadyines.
With this outlook, we report the preparation and structural
study of new annulated Hoveyda-type complex 3, (BIAN-NHC)
Cl2Ru(vCH-o-O-i-PrC6H4), and the complex 1,3-bis(2,6-
dimethylphenyl)-2,3-dihydro-1H-imidazole Cl2Ru(vCH-o-O-
i-PrC6H4) 4. The electrochemical behaviour and the catalytic
activity of the new complexes for ring closing metathesis reac-
tions were studied and compared to the previously reported com-
plexes 1 and 2. Complex 4 was synthesized as an analogue of
complex 2 in order to do the comparative study with complex 3,
as crystals of complex 2 could not be obtained for X-ray studies.
First, the known 1,3-bis-(2,4,6-trimethylphenyl)imidazol-
2-ylidene (IMes) complex 2 was prepared according to the pre-
viously described method17 starting from the commercially
available 1,3-dimesityl-imidazolium chloride, by treatment
with the 2nd generation Hoveyda complex in the presence of
potassium 1,1-dimethyl-propylate and cuprous chloride in
hexane. The resulting complex 2 was purified by flash column
chromatography to be used as a reference metathesis catalyst.
As the crystallization of this compound failed, the unknown
parent ruthenium dichloride complex 4 bearing 1,3-bis-(2,6-
Scheme 1 Synthetic route of complex 3.
dimethylphenyl)imidazol-2-ylidene as the ligand was similarly
prepared and successfully crystallized and characterized. The
electrochemical and catalytic properties of this complex were
studied and compared with the other complexes.
The new complex 3, (BIAN-NHC) Cl2Ru(vCH-o-O-
i-PrC6H4), has been obtained according to the synthetic route
presented in Scheme 1. It starts from acenaphthylene-1,2-dione
via the known bis-(mesityl)-acenaphthenequinonedimine (Mes-
BIAN).29
The preparation of 1,3-dimesityl-acenaphthylenyl-4,5-imida-
zolium-2-ylidene30 from Mes-BIAN using p-formaldehyde in
the presence of hydrochloric acid or chlorotrimethylsilane failed.
In contrast, the reaction of the bis-imine with chloroethyl
methyl ether successfully yielded the Mes-BIAN imidazolium
chloride as has been similarly reported by Cowley et al.30a The
treatment of the Mes-BIAN imidazolium chloride according to
the two same procedures that were used to obtain IMes complex
2 led to complex 3.
The choice of an appropriate solvent and the corresponding
base for the synthesis of NHCs may be restricted due to the in-
solubility of the imidazolium salts and/or the decomposition of
the NHCs once it has been generated. Here, the presence of iso-
propoxystyryl ruthenium tricyclohexylphosphane dichloride
during the deprotonation of the imidazolium derivative using
potassium 1,1-dimethyl-propylate in the presence of cuprous
chloride gave directly the anticipated Hoveyda II′ IMes-type
complex 3.31 This compound directly precipitated from a
methanol/dichloromethane (10 : 1) solution preventing column
chromatography purification. Complex 3 was characterized by
1H and 13C NMR, HRMS (ESI†). A summary of the most
1
relevant data from H (black) and 13C-NMR (red) is given in
Fig. 2. There are no large differences in the chemical shifts of
the H-NMR spectra of complexes 2 and 3. Slight differences
are observed in the aromatic protons and methyl substituents of
1
10914 | Dalton Trans., 2012, 41, 10913–10918
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Fig. 2 Data of 1H (black) and 13C-NMR (red) of complexes 2 and 3.
the phenyl groups on the imidazole ring. The corresponding
protons to methyl groups are at 2.27 ppm in complex 2 and
2.37 ppm in complex 3. The hydrogens of phenyl groups appear
at 7.12 ppm (complex 2) and 7.21 ppm (complex 3). In light of
these results, we could suggest that the BIAN ligand does not
have an influence on the electron density of the aromatic rings.
The chemical shift of the proton attached to the Csp2 atom
bonded directly to the ruthenium atom in both complexes
(16.85 ppm for 2 and 16.71 ppm for 3) and the proton attached
to the carbon atom bonded directly to the oxygen atom
(4.93 ppm for 2 and 4.98 ppm for 3) is similar. With respect to
13C-NMR, the highest difference observed is in the carbene
atom. In complex 2, this carbon appears at 175.9 ppm and at
183.9 ppm in the case of complex 3. When the imidazole ring is
saturated, the chemical shift associated with the carbene atom
appears at 211.1 ppm (complex 1).13a
Fig. 3 ORTEP diagram of the molecular structure of complex 1,3-
dimesityl-acenaphthylenyl-4,5-imidazolin-2-ylidene ruthenium isopropoxy-
benzylidene dichloride 3 and selected bond lengths and bond angles.
The structures of complexes 3 and 4 were completely
confirmed by single crystal X-ray diffraction data.32 Fig. 3 and 4
summarize a selection of pertinent parameters for complexes 3
and 4. Further details are available as ESI.† Complexes 3 and 4
crystallize in different crystal settings and space groups.
Complex 3 forms orange crystals in the monoclinic P21/c space
Fig. 4 ORTEP diagram of the molecular structure of complex 1,3-bis-
(2,6-dimethylphenyl)-imidazolin-2-ylidene ruthenium isopropoxybenzy-
lidene dichloride 4 and selected bond lengths and bond angles.
ˉ
group, while green crystals of 4 belong to the triclinic P1 space
group. In both cases, the ruthenium centers are pentacoordinated.
The complex’s configuration at the metal atom is similar to that
described previously for other ruthenium complexes, a deformed
square pyramid with the carbene atom at the top and with all the
other atoms that are directly connected with the ruthenium atom
in the base of the pyramid. The geometry of the metal center and
the Ru–Cl, Ru–C bond lengths are almost identical in both com-
plexes. The length for the Ru–C2 bond is the same in both com-
plexes (1.98 Å). The difference in the value of the N1–C2–N3
bond angle is of 1°.
Table 1 Redox potentials of ruthenium complexes 1–4 studied by
cyclic voltammetry
Ruthenium complex
ΔE (mV)
(Ea − Ec) (mV)
1
2
3
4
889
804
768
824
104
97
92
87
The redox potentials of ruthenium complexes were established
by cyclic voltammetry (Table 1). The cyclic voltammetry of
compounds 1–4 consists of a reversible oxidation between
RuII/RuIII species. The electrochemical experiments were carried
out at room temperature in dichloromethane applying a scan rate
of 100 mV s−1. Cyclic voltammetry measurements indicate that
regard to complexes 1, 2 and 4 respectively. Also, additional ir-
reversible reduction is observed for complexes 2 and 4, with
E = 983 mV for complex 2, and E = 963 mV for complex 4.
We can suppose that in the case of complexes 2 and 4 new
species of ruthenium are formed.
Several cycles were applied on the samples and comparing the
results, we observe that complexes 2 and 3 present lower stability
in the oxidative process than the other two ruthenium complexes
(1 and 4).33 These former complexes also present an irreversible
oxidation with E = 1308 mVand 1334 mV, respectively.
It is noteworthy that when the number of cycles increases, the
intensity of the oxidative wave at 1308 mV for complex 2 and
1334 mV for complex 4 increases while the intensity of the
complex 3 undergoes a single reversible oxidation with E1/2
=
768 mV (Table 1). Also, reversible oxidation is observed in com-
plexes 1, 2 and 4 with E1/2 = 889, 804 and 824 mV, respectively.
The redox potential of complex 1 is slightly higher compared
with complex 2 as already mentioned by Plenio and Süβner17
and complex 4. The ruthenium complex 3 with a BIAN sub-
stituent is cathodically shifted by 121, 36 and 56 mV with
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reductive wave at initial E = 983 mV for complex 2 and 963 mV
for complex 4 strongly increases with a progressive potential dis-
placement. Changes in the CVs of complex 1 are not observed
and only small changes are observed in the case of complex 3
with respect to their highest stability.
of the imidazole ring and the saturated imidazole ring increase
the electrochemical stability of ruthenium complexes presenting
more cathodic redox potentials (Fig. 5).
Ring closing metathesis reactions have been widely used in
organic synthesis34 for instance for the synthesis of natural pro-
ducts.35 We have chosen this type of reaction to test the activity
of the prepared ruthenium catalysts. Diethyl diallylmalonate and
N-tosyl diallylamine were used as substrates to form disubsti-
tuted olefins. The course of the reaction was monitored by NMR
spectroscopy and the conversion of the starting material to the
product was measured over time (Fig. 6). All experiments were
duplicated and performed under a nitrogen atmosphere in closed
systems.33 The reactions were carried out with 1 mol% of the
catalyst in CD2Cl2 (0.1 M) at 30 °C. The newly synthesized
complexes 3 and 4 were tested in the RCM reaction of diethyl
diallylmalonate and compared with the previously described
complex 2 and the commercially available complex 1. Catalysts
2 and 3 show similar activity, 71% conversion after 10 min in
the case of complex 2 and 79% for complex 3. The activity of
The substitution with the aromatic system in the five mem-
bered ring of the NHC ligand (complex 3) results in a cathodic
shift of the redox potential. The para-substitution on the aro-
matic rings also has a slight effect on the stability of the ruthe-
nium complexes. In fact, the introduction of electron donating
substituents leads to a slight stabilization of the ruthenium com-
plexes, with complex 2 undergoing a cathodic shift of 20 mV
with respect to complex 4. For this, the redox potential of
complex 2 with more electron rich substituents in the para posi-
tion is more cathodic. Complex 1 presents the highest redox
potential value (889 mV) with an anodic shift of 85, 121 and
65 mV in comparison to complexes 2, 3 and 4, respectively.
With these data, we can propose that the introduction of electron
donating substituents in the aromatic rings and the 4,5-position
Fig. 5 CVs of ruthenium complexes 1–4 recorded in a CH2Cl2 solution of Bu4NPF6 (supporting electrolyte). Platinum (working electrode), platinum
wire (counter electrode), and an Ag/AgCl electrode (reference), at a scan rate of 100 mV s−1 from −1750 to 1750 mV, were used.
10916 | Dalton Trans., 2012, 41, 10913–10918
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Fig. 6 (a) Conversion of diethyl diallylmalonate to dimethyl 3-cyclopentene-1,1-dicarboxylate and (b) N,N′-diallyl-4-methylbenzenesulfonamide to
1-tosyl-2,5-dihydro-1H-pyrrole using complexes 1–4. TON (number of diethyl diallylmalonate moles consumed calculated by NMR versus moles of
the catalyst) and TOF (turnover frequency) values for catalysts 1–4.
complexes 1 and 4 was slightly inferior to the other two cata-
lysts, with 51% conversion at 10 min for complex 1 and 35%
conversion for complex 4 (Fig. 6a). In the RCM reaction of
N,N′-diallyl-4-methyl-benzenesulfonamide, the catalytic activity
is similar for the four catalysts and the conversion is complete
after 30 min (Fig. 6b). In the case of diethyl diallylmalonate,
when the electronic density on the aromatic rings of the imidazo-
lylidene ligand is decreased with 2,6-dimethyl phenyl groups
(complex 4) in front of 2,4,6-trimethyl phenyl groups (com-
plexes 2 and 3), the catalytic activity decreases significantly. The
activity of complex 1 is lower in comparison with the activity of
complexes 2 and 3 and slightly higher compared with the
activity of complex 4.
Complex 2 with the unsaturated imidazole ring and 2,4,6-tri-
methyl phenyl as the aromatic substituent and complex 3 with
the BIAN ligand present similar activity. These results suggest
that substitution of positions 4 and 5 of the imidazole ring does
not have a significant effect on the catalytic activity of the ruthe-
nium complex. The only structural difference between com-
plexes 2 and 4 is the methyl groups in the 4 position of the
aromatic rings, and therefore the activity of complex 4 is less
than that of complex 2 in the RCM of diethyl diallylmalonate.
In this sense, when the electronic density is decreased in the
aromatic ring of the imidazolylidene ligand the activity of the
catalyst is decreased in the RCM of diethyl diallylmalonate
according to “Grubbs rule”.4g In the case of the less demanding
reaction with N,N′-diallyl-4-methyl-benzenesulfonamide, the
four catalysts show the same activity in the RCM. The conver-
sion is not dependent on the structure of the Hoveyda II
ruthenium catalyst. The product obtained starting from N,N′-
diallyl-4-methyl-benzenesulfonamide is obtained with higher
conversion in less time compared with the product resulting
from the RCM with diethyl diallylmalonate. Then the substitu-
ents attached to the periphery of the ligand system do not have a
significant effect on the metal activity. In Fig. 6a, we observe
that the values of TONs for complexes 2 and 3 are similar (21.39
and 21.75, respectively) and higher than the TONs for com-
plexes 1 (6.73) and 4 (12.66). The same is observed when we
compare the TOF values. The TOF values of complexes 2 and 3
are 10.23 and 10.16 min−1 at 2 min, these values are higher than
the TOF values for complexes 1 (2.34 min−1) at 2.88 min and 4
(2.22 min−1) at 5.69 min. In Fig. 6b, we present the kinetics of
the RCM of N,N′-diallyl-4-methyl-benzenesulfonamide, the
TON values are higher compared with the values obtained in the
RCM of diethyl diallylmalonate. In this case, the TON values
are between 37.16 and 50.88 and the TOF values are between
20.38 and 14.35 min−1 at 2.1–2.81 min. Additionally, complexes
2 and 3 are air-stable, being controlled over 15 days, while
complex 4 decomposes (see the 1H-NMR spectrum in ESI†).
In summary, we have synthesized and characterized new
ruthenium complexes 3 and 4. We carried out a structural study
of these complexes by NMR and X-ray. We studied the catalytic
activities of complexes 3 and 4 which were compared with those
of complexes 1 and 2. The complexes show very good activity
for the RCM reaction. The catalytic activity of complex 3 with
the BIAN ligand is similar to the previously described catalyst
with unsaturated NHC ligand 2 and higher than the commer-
cially available complex 1 under the same experimental
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11 (a) A. K. Chatterjee and R. H. Grubbs, Org. Lett., 1999, 1, 1751;
(b) M. Scholl, T. M. Trnka, J. P. Morgan and R. H. Grubbs, Tetrahedron
Lett., 1999, 40, 2247.
12 (a) L. Ackermann, A. Fürstner, T. Weskamp, F. J. Kohl and W.
A. Herrmann, Tetrahedron Lett., 1999, 40, 4787; (b) L. Ackermann,
D. El Tom and A. Fürstner, Tetrahedron, 2000, 56, 2195; (c) A. Fürstner,
O. R. Thiel, L. Ackermann, H. J. Shanz and J. P. Nolan, J. Org. Chem.,
2000, 65, 2204.
13 (a) S. B. Garber, J. S. Kingsbury, B. L. Gray and A. H. Hoveyda, J. Am.
Chem. Soc., 2000, 122, 8168; (b) J. Cossy, S. BouzBouz and
A. H. Hoveyda, J. Organomet. Chem., 2001, 634, 216.
14 S. Gessler, S. Randl and S. Blechert, Tetrahedron Lett., 2000, 41, 9973.
15 S. Imhof, S. Randl and S. Blechert, Chem. Commun., 2001, 1692.
16 Without single-crystal X-ray structure analysis.
17 M. Süβner and H. Plenio, Chem. Commun., 2005, 5417.
18 (a) H. Wakamatsu and S. Blechert, Angew. Chem., Int. Ed., 2002, 41,
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2002, 41, 2403.
conditions. In the case of 4, the activity is slightly lower com-
pared with the other two complexes (2 and 3) in the RCM of
diethyl diallylmalonate. With these results we can conclude that
when the electronic density is decreased in the aromatic ring of
the imidazolylidene ligand, due to the para substituent absence,
the activity of the catalyst is decreased in the RCM of diethyl
diallylmalonate. The activity of the four catalysts is similar in the
RCM of N,N′-diallyl-4-methylbenzenesulfonamide as it is a less
demanding reaction. The redox potential of the Hoveyda II type
complex cannot be used as a unique parameter to predict their
activity in metathesis reactions as already mentioned.36 Complex
3 is an alternative to the ruthenium complexes described to date
for RCM processes.
We thank Dr Berta Gómez-Lor for allowing us to use the
cyclic voltammetry instrumentation, Dr J. A. Vidal-Moya for his
support in the kinetic experiments and Dr Marcelo Domine for
the HRMS measurements. We thank Avelino Corma, Félix
Sanchez and Marta Iglesias for helpful discussions. E. M. also
thanks Ministerio de Ciencia e Innovación for a contract of the
“Juan de la Cierva” program. Financial support by Consolider-
Ingenio 2010 (MULTICAT project) and MAT2011-29020-C02-
01 is additionally acknowledged.
19 K. Grela, S. Harutyunyan and A. Michrowska, Angew. Chem., Int. Ed.,
2002, 41, 4038.
20 (a) M. Zaja, S. J. Connon, A. M. Dunne, M. Rivard, N. Buschmann,
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M. Zaja and S. Blechert, Adv. Synth. Catal., 2005, 347, 1413.
21 J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus Jr. and A. H. Hoveyda,
J. Am. Chem. Soc., 1999, 121, 791.
22 T. Vorfalt, K. J. Wannowius, V. Thiel and H. Plenio, Chem.–Eur. J., 2010,
16, 12312.
23 X. Solans-Montfort, R. Pleixats and M. Sodupe, Chem.–Eur. J., 2010,
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24 S. Leuthäuβer, V. Schmidys, C. M. Thiele and H. Plenio, Chem.–Eur. J.,
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25 A. Bittermann, P. Härter, E. Herdtweck, S. D. Hoffmann and
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26 S. Urban, M. Tursky, R. Fröhlich and F. Glorius, Dalton Trans., 2009,
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