Effective immobilisation of a metathesis catalyst bearing an ammonium-tagged NHC ligand on various solid supports

Article abstract of DOI:10.3762/bjoc.12.2

An ammonium-tagged ruthenium complex, 8, was deposited on several widely available commercial solid materials such as silica gel, alumina, cotton, filter paper, iron powder or palladium on carbon. The resulting catalysts were tested in toluene or ethyl acetate, and found to afford metathesis products in high yield and with extremely low ruthenium contamination. Depending on the support used, immobilised catalyst 8 shows also additional traits, such as the possibility of being magnetically separated or the use for metathesis and subsequent reduction of the obtained double bond in one pot.

Full text of DOI:10.3762/bjoc.12.2

Effective immobilisation of a metathesis catalyst bearing an
ammonium-tagged NHC ligand on various solid supports
Krzysztof Skowerski*1, Jacek Białecki1, Stefan J. Czarnocki1, Karolina Żukowska2
and Karol Grela*3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 5–15.
1
Apeiron Synthesis, Duńska 9, 54-427 Wrocław, Poland, 2Institute of
Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,
1-224 Warsaw, Poland and 3Biological and Chemical Research
Centre, Faculty of Chemistry, University of Warsaw, Żwirki i Wigury
01, 02-089 Warsaw, Poland
doi:10.3762/bjoc.12.2
0
Received: 03 November 2015
Accepted: 17 December 2015
Published: 05 January 2016
1
Email:
This article is part of the Thematic Series "N-Heterocyclic carbenes".
Guest Editor: S. P. Nolan
Krzysztof Skowerski* - krzysztof.skowerski@apeiron-synthesis.com;
Karol Grela* - karol.grela@gmail.com
*
Corresponding author
© 2016 Skowerski et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
catalysis; immobilisation; N-heterocyclic carbenes; olefin metathesis;
ruthenium
Abstract
An ammonium-tagged ruthenium complex, 8, was deposited on several widely available commercial solid materials such as silica
gel, alumina, cotton, filter paper, iron powder or palladium on carbon. The resulting catalysts were tested in toluene or ethyl acetate,
and found to afford metathesis products in high yield and with extremely low ruthenium contamination. Depending on the support
used, immobilised catalyst 8 shows also additional traits, such as the possibility of being magnetically separated or the use for
metathesis and subsequent reduction of the obtained double bond in one pot.
Introduction
Over the past decade olefin metathesis has undergone a grand plexes was also extensively tested, as their applications in a
development. The design of stable and active ruthenium-based solid form can be beneficial [6]. The efficient removal of ruthe-
metathesis catalysts has been the cardinal factor to distribute nium from metathesis products, possibility of catalyst recovery
olefin metathesis in the synthesis of many important com- and reuse as well as their potential use in continuous processes
pounds [1-4]. Commercially available homogeneous com- are the main benefits of heterogeneous systems [7]. Unfortu-
plexes, including phosphine-containing Gru-II, Ind-II or phos- nately, their application is associated with some drawbacks.
phine-free Hov-II and Gre-II are usually employed in such These catalysts usually exhibit lower activity than their homo-
cases (Figure 1) [5]. However, heterogenisation of these com- geneous counterparts as reflected by a noticeably lower
5

Beilstein J. Org. Chem. 2016, 12, 5–15.
turnover frequency (TOF). Moreover, their synthesis, due to the explored by Grubbs et al. who obtained catalysts 2 and 3 cova-
need for sophisticated linkers and tags, is significantly more lently bonded to silica gel through the NHC ligand [27-29].
complicated.
This work revealed that, for complexes supported on silica gel,
the heterogenization via the NHC backbone is a much better ap-
Several protocols were developed for heterogenisation of ruthe- proach than the previously used ones (e.g., via phosphine or
nium catalysts and this topic has been thoroughly reviewed benzylidene ligands).
[
8-18]. The implementation of such concepts requires the pres-
ence of remotely functionalised ligands within the metal coordi- An early example of a non-covalent attachment is complex 4,
nation sphere. A very efficient covalent immobilisation through an activated catalyst deposited on glass polymer Raschig rings,
anionic ligands was reported by Buchmeiser et al., who synthe- which was tested in various metathesis reactions carried out in
sised a series of monolith-supported catalysts (such as 1) which batch and circulating flow reactor, as well as in an industrial
gave metathesis products with extremely low residual ruthe- setup [30,31]. The concept was explored further, and a pyri-
nium [19-24]. In other contributions originating from the same dinium-tagged complex deposited on modified silica gel was
group, heterogeneous catalysts covalently connected to a mono- recently obtained by Kirschning, Mauduit et al., exhibiting
lithic support via NHC ligands were presented [25,26]. These much better activity and efficiency than 4 [32]. Specially func-
initiators were suitable for continuous metathesis processes and tionalised Hoveyda-type catalysts bearing polar ammonium
provided products with low residual ruthenium; however, they groups were non-covalently immobilised on silica-gel [33-36].
were less active than complex 1. A very similar idea was However, it is worth highlighting that alkylidene and pyridine
Figure 1: Selected classical and heterogeneous ruthenium complexes.
6

Beilstein J. Org. Chem. 2016, 12, 5–15.
ligands dissociate during the catalytic cycle and as a result the powerful procedure gives access to a modified complex with
active species containing heavy metal are leaching to the solu- multitude of possible applications [43-47]. We have found that
tion.
the introduction of an ammonium chloride tag into the NHC
ligand results in catalysts with interesting properties such as
A different valuable concept is the covalent or electrostatic solubility in neat water as well as extremely high affinity to
immobilisation of ruthenium initiators on magnetically active silica gel. The latter property was of particular interest to us.
nanoparticles. Complexes 5 [37] and 6 [38] presented in Having in hand a complex with such properties we became
Figure 1 can serve as an examples, but others were also interested in its ability to bind non-covalently to various easily
prepared [39]. These compounds are carefully designed to facil- available supports.
itate straightforward removal of the metal-containing species
after the reaction is completed. Although this idea is simple, the What is more important, the resulting material should inherit the
preparation of such sophisticated nanoparticle-supported cata- key characteristic of the support, thus allowing for easy and
lysts is often a complicated multistep procedure.
efficient removal of residual ruthenium from metathesis prod-
ucts, or being utilised in different ways. This heterogenisation
Definitely, the simplest and quickest immobilisation strategy is strategy seemed to us a much more straightforward and
to support an unmodified, commercially available homoge- universal approach than methods requiring a sophisticated
neous catalyst on silica gel. This plan was tested by Jacobs et al. design of the catalyst, linker or support. However, the reported
[
40] and more recently Limbach and co-workers [41], who physicosorbed systems [40-42] consisting of supported
proved that even the classical Hov-II catalyst can be success- commercial catalysts can only be used in non-polar solvents
fully immobilised on silica gel via physisorption. Recently, two [33-36,43-47]. Taking into account that many advanced poly-
other commercial Hoveyda-type homogeneous catalysts have functionalised substrates can be insoluble in pentane, this could
been immobilised on silica using the same physisorption ap- be considered as a drawback. Therefore, we decided to further
proach [42]. This strategy, although easy and economical, has a explore the potential of NHC-ammonium tagged catalysts,
drawback of being limited in terms of the solvents in which the aiming to develop a more versatile process.
system stays heterogeneous. Since these commercial Hoveyda
complexes have good solubility in methylene chloride (CH2Cl2) In this present report, we disclose a new heterogeneous catalytic
and toluene, olefin metathesis reactions with these systems have system, compatible with more polar solvents and substrates. It
to be conducted in pentane or hexane.
is important to note that in this study we focused on estab-
lishing the scope of this heterogenisation method, i.e., by testing
Recently we have reported on the synthesis and catalytic a wide number of potential supports, such as silica, alumina,
activity of a series of olefin metathesis catalysts bearing a charcoal, iron powder, as well as biocompatible wool and paper,
quaternary ammonium group attached to the NHC ligand and by examining different catalyst removal strategies, rather
(
Figure 2) [43-47]. These, now commercially available [48], than tackling recyclability issues. The latter was studied in a
complexes were synthesised by on-site quarternisation of cata- separate project conducted in our laboratories and recently
lysts containing a tertiary amine functional group with the use published [49]. In that parallel study the NHC ammonium-
of either methyl chloride or methyl iodide. This simple yet tagged catalyst was heterogenised and subjected to various re-
Figure 2: Applications of NHC ammonium-tagged catalysts.
7

Beilstein J. Org. Chem. 2016, 12, 5–15.
cyclability tests, proving high potential (up to 23 recycles) and Having in hand the catalyst supported on activated carbon
really high total turnover numbers (up to 35,000).
(8-C*) we were eager to test its catalytic properties. A model
ring-closing metathesis (RCM) reaction leading to product 10
Results and Discussion
(
Scheme 2) was used to check the influence of temperature and
Synthesis of tagged catalyst 8
concentration on the activity of 8 on the solid support.
Complex 7 was synthesised according to the previously
reported procedure [43]. The obtained material was subse-
quently subjected to the on-site quaternisation reaction with
methyl chloride (Scheme 1). Catalyst 8 was isolated in excel-
lent yield after simple filtration through a short pad of neutral
aluminium oxide, using a mixture of EtOAc/MeOH 92:8 v/v as
eluent. We realised that one of the important properties of this
complex is its insolubility in toluene, a solvent of known indus-
trial potential [50].
Scheme 2: Model RCM reaction.
We observed that the use of higher temperature and concentra-
tion results in faster substrate consumption what is quite intu-
itive (Figure 3). We were pleased to see almost full conversion
of 9 after only 20 minutes of reaction carried out at 80 °C at
Preparation of high-surface 8 by precipitation
and by immobilisation on charcoal, silica,
alumina, cotton and paper and the applica-
tion of such materials in catalysis
Microcrystalline 8 showed only marginal activity in olefin
metathesis when applied as a suspension in toluene. This was
ascribed to the low surface area of this material. Therefore, we
decided to deposit 8 on a solid support characterised by a high
surface. The initially chosen support for the catalyst deposition
was activated carbon (charcoal, C*), which is known to have a
high surface area [51]. Addition of C* to a CH2Cl2 solution of 8
and subsequent removal of the solvent in vacuo resulted in a
complete deposition. Other solvents for the deposition process
were tested as well, with the AcOEt/MeOH 95:5 v/v system
being a greener alternative to CH2Cl2. The thus obtained ma-
terial was dried and used directly in metathesis reactions. In
both of these solvent systems, complete (100%) deposition was
achieved, according to gravimetric analysis of the obtained
loose powder. In addition, visual inspection showed no remains
of unsupported complex 8 deposited on the flask walls, etc.
Figure 3: Influence of temperature and concentration on RCM of 9.
Conditions: 1 mol % of 8-C* (5 wt % on C*), in toluene.
Scheme 1: Synthesis of ammonium-tagged complex 8.
8

Beilstein J. Org. Chem. 2016, 12, 5–15.
0
.2 M concentration. It is worth noting that such a fast reaction and on filter paper (8-cotton and 8-paper) was only slightly
is rather unusual in the case of heterogeneous olefin metathesis lower than that one of unsupported catalyst 8-powder. These
[
19-24,52].
non-expensive supports, however, are very attractive due to the
exceptional ease of handling and product purification. Lower
Encouraged by this initial success, we deposited 8 on several efficiency was observed in the case of catalyst supported on
other widely available solid supports, commonly utilised in neutral aluminium oxide (8-Al2O3) which provided product 10
everyday laboratory and industrial practice. Those included in diminished yield. On the other hand, the catalyst supported
silica gel (flash chromatography grade), neutral aluminium on unmodified silica gel (8-SiO2) showed the fastest initiation
oxide, cotton-viscose wool (cosmetic) and even filter paper. In rate among all tested materials, giving over 99% of conversion
each case 5 wt % of catalyst was deposited on the selected of 9 after only 10 minutes.
support (see Supporting Information File 1 for details). During
this part of the study we also discovered that the protocol We observed that simple decantation of the crude reaction mix-
involving fast precipitation of 8 from diluted CH2Cl2 solution ture or – in case of unsupported catalyst 8-powder, Figure 6 –
with c-hexane, can provide solid catalyst 8 as a fine powder ex- its filtration through a piece of cotton provides colourless
hibiting activity in RCM comparable with that observed for mixtures. This suggests that under these conditions there are no
immobilised 8-C*. The morphology of the obtained materials is active ruthenium species dissolved in toluene.
presented in Figure 4.
Figure 4: Presentation of various Ru-based catalysts. From the left:
20 mg of Gre-II powder, 20 mg of 8 as fine powder, 20 mg of 8
supported on 380 mg of silica gel, 20 mg of 8 supported on 380 mg of
activated carbon, 20 mg of 8 supported on 380 mg of cotton-viscose
wool, 20 mg of 8 supported on 380 mg of filter paper (6 × 8 cm).
Next, we aimed to test the activity of the obtained hetero-
genised 8 in the model RCM of 9. The results are presented in
Figure 5. The efficiency of 8 supported on cotton-viscose wool
Figure 6: Filtration of the reaction mixture after RCM of 9 catalysed by
mol % of 8-powder.
1
To check this hypothesis, split tests were carried out during the
RCM of 9 catalysed by 1 mol % of 8-powder, 8-C*, and
8
-SiO2. The split test procedure is commonly used to prove the
heterogeneity of a process in question [53]. After 3 minutes
of each investigated transformation, a part of the reaction mix-
ture was filtered via a very small piece of cotton to a new pre-
heated flask (the cotton plug was washed with hot toluene). The
filtered mixtures were immediately analysed to calculate the
conversion at the split time. After the following 30 minutes the
conversion was determined in both filtered and non-filtered
reaction mixtures, again. The appropriate data is presented in
Figure 7.
Figure 5: Influence of the support type on the metathesis outcome.
Conditions: 1 mol % 8, toluene 80 °C; [9] = 0.2 M.
9

Beilstein J. Org. Chem. 2016, 12, 5–15.
strong enough to allow heterogeneous catalysis even in the
solvent in which 8 is partially soluble and therefore cannot be
used unsupported. It should be noted that untagged catalysts,
like Hov-II immobilised on solid supports were reported to
work only in non-polar solvents such as hexane or pentane, in
which the catalyst is completely insoluble [40-42]. According to
this observation, Hov-II immobilised on charcoal (using the
procedure described above) and applied for RCM of 9 in AcOEt
lead to a non-heterogenous reaction and severe leaching
(
Table 1).
To establish the scope and limitations of the studied heteroge-
neous system, we selected the most active supported complex
(
(
8-SiO2) and a set of olefin metathesis reactions in toluene
Scheme 3). The outcome of this study along with the ruthe-
nium content in crude metathesis products is summarised in
Table 2.
Figure 7: Split test during RCM of 9 (1 mol % cat, toluene 80 °C,
9] = 0.2 M). The reaction mixtures were filtered through a piece of
cotton under argon to a flask placed in an oil bath pre-heated to 80 °C.
[
The products of RCM and ene–yne metathesis were obtained
In all the examined cases there was no increase of conversion in with good to excellent yields with the use of 0.1–0.5 mol % of
the filtered reaction mixtures. At the same time, the reactions catalyst. Functionalised olefins 19 and 21 were isolated with
proceeded further in the fractions that were not filtered. This excellent yield after cross metathesis (CM) reactions run with
proves that the active species are not present in solution, thus only 1 mol % of catalyst. Importantly, the residual ruthenium
suggesting that these reactions are truly heterogeneous.
content in the crude products was very low (determined by ICP-
MS method). Reaction work-up was performed solely by filtra-
We suspect that immobilisation of polar 8 is strengthened by tion of the reaction mixture through a piece of cotton and evap-
physicochemical interactions of the polar ammonium tag with oration of the volatiles, to yield the crude products. The
the surface of the support. To shed more light on this, we simplicity and effectiveness of this approach can be utilised in
compared the behaviour of 8-powder and 8-C* in the RCM many applications, e.g., in pharmaceutical R&D and produc-
reaction of diene 9 conducted in ethyl acetate at 50 °C. Com- tion [31,36]. The isolation of highly polar 21 in very good yield
plex 8 is partially soluble in this solvent. The split tests, made and with low residual ruthenium should be especially high-
during RCM reactions, revealed a great difference between the lighted in this context [54].
immobilised and unsupported 8 (Table 1). In the case of
8
-powder, the reaction was (at least in a part) homogeneous, Encouraged by the above results, in the final part of this work
and the Ru content present in solution was noticeable (257 ppm, we decided to utilise the observed great affinity of 8 to various
Table 1). On the other hand, the immobilised catalyst (8-C*) supports to briefly test some more unconventional heterogenisa-
worked in the same solvent in truly heterogeneous fashion, with tion strategies. It should be noted that two experiments de-
only minimal leaching (3.2 ppm). This seems to suggest that the scribed below shall be treated only as a preliminary endeavour
interactions between tagged catalyst 8 and activated carbon are aimed at getting a broader perspective.
Table 1: Results of olefin metathesis conducted in AcOEt.a
RCM of 9 in AcOEt
Promoted by 8-powder
Promoted by 8-C*
Promoted by Hov-II-C*
Conv. [%]
Residual Ru
ppm]
Conv. [%]
Residual Ru
Conv. [%]
Residual Ru
[ppm]
[
[ppm]
At split
70
91
n.d.
n.d.
257
57
57
88
n.d.
n.d.
3.2
80
n.d.
n.d.
Filtered (30 min)
Suspension (30 min)
>99
>99
>99
1760
an.d.: not determined.
10

Beilstein J. Org. Chem. 2016, 12, 5–15.
Scheme 3: Model metathesis reactions used in tests.
Table 2: Results of olefin metathesis conducted in toluene, 80 °C.
Entry
Product
Catalyst [mol %]
Time [min]
Conv. (yield) [%]a
Ru content [ppm]b
1
2
3
4
5
6
7
8
9
10
10
8-powder (1)
8-C* (1)
60
15
10
20
30
30
30
10
20
10
30
98 (94)
98 (98)
99 (93)
99 (93)
85 (83)
98 (95)
99 (92)
95 (92)e
98 (96)e,f
>95 (75)g
89 (85)
2.0
1.2
10
8-SiO2 (1)
8-SiO2 (0.5)
8-SiO2 (0.1)
8-SiO2 (0.2)
8-SiO2 (0.5)
8-SiO2 (1)
8-SiO2 (1)
8-SiO2 (1)
8-SiO2 (0.5)
1.4
10
2.4
12
0.30
b.d.l.c
19
14
16
19d
21d
25d
23
0.61
0.38
0.44
115
10
1
1
aYields of isolated pure products; bmeasured for products purified by filtration of reaction mixture through a piece of cotton; cb.d.l. – below detection
level; d4 equiv of 18 were used; eE/Z = 19:1; fdetermined by GC after esterification of the crude product; gyield of pure E-isomer.
Immobilisation of 8 on iron powder
would result in a magnetically removable system. In fact, com-
First, we attempted to prepare a model for a magnetically sepa- plex 8 can indeed be non-covalently immobilised on iron
rable catalyst [55]. This idea has been explored previously by powder, to form 8-Fe, although in this case the catalyst/support
attaching tagged Ru complexes to specially designed magneti- mass ratio had to be lowered to 0.01 in order to provide fully
cally active nanoparticles. We wondered whether supporting 8 Fe-supported complex 8. This method for the preparation of a
on commercially available iron powder (spherical, <10 μm), magnetically removable catalyst is obviously less complicated
11

Beilstein J. Org. Chem. 2016, 12, 5–15.
than the reported syntheses of catalysts on magnetic nanoparti- terial clung to the mixing element (Figure 9B). It was then
cles [37-39]. The main difference is that the latter materials are mechanically removed, providing the colourless product in very
much more technologically advanced and work rather as quasi- good yield (93%) and with low residual ruthenium content
homogeneous catalysts [56]. The resulting 8-Fe was tested in (25 ppm, Figure 9C). This level of contamination is higher than
the RCM of 9 and its overall efficiency was found to be almost in the case of 8-SiO2 or 8-C*, however, we think that
identical with that observed for the unsupported powdered cata- immobilisation of a metathesis catalyst on iron is still of
lyst (Figure 8).
interest. It brings some additional possibilities to be explored,
such as on-demand catalyst removal and insertion controlled by
a magnetic field.
If it is required, complex 8 can then easily be recovered from
the support by simply washing the heterogeneous material 8-Fe
with a polar solvent, such as water, alcohol or CH2Cl2
(
Figure 9D and Figure 9E) and possibly deposited on other
supports or used as a homogeneous catalyst. As it was noted
earlier, we are not considering high recyclability of immo-
bilised complex 8. However, the possibility of removing the
deactivated catalyst from the support and reloading the latter
with a fresh portion of the complex (or to replace a catalyst by
another one on a given support) is a potentially viable option,
worthy of being explored, especially in industrial setups [30].
Immobilisation of 8 on carbon containing
Figure 8: RCM of 9 catalysed by 8 and 8-Fe. Conditions: 1 mol %
catalyst, toluene 80 °C, [9] = 0.2 M.
10% Pd
It is described in the literature that ruthenium residues, present
in the reaction mixture after olefin metathesis, can serve as an in
Reactions with 8-Fe were performed in a standard reaction vial situ homogeneous catalyst for high pressure hydrogenation of
containing a magnetic stirring bar. During the reaction course the newly formed C–C double bonds [57-61]. On this basis, we
the catalyst remained fully suspended in the solution due to the assumed that the ruthenium complex impregnated on carbon
centrifugal force (Figure 9A), whereas once the solution was no 8-C* might serve as in situ heterogeneous catalyst for a tandem
longer stirred after reaction completion, all Fe-supported ma- olefin metathesis-hydrogenation transformation.
Figure 9: Removal of 8-Fe and subsequent recovery of 8. A: stirred reaction mixture containing 8-Fe, B: the same reaction mixture after stirring
ceased, C: catalysts 8-Fe attached to the magnetic rod and removed, D: catalyst 8-Fe being washed with CH2Cl2, E: CH2Cl2 solution of catalyst 8
removed from Fe powder.
12

Beilstein J. Org. Chem. 2016, 12, 5–15.
To test this possibility we ran CM of 17 and 18 and when the manner. Depending on the nature of the support chosen, the
metathesis reaction was complete the resulting product was properties of the resulting catalyst are different, allowing for
subjected to reduction by applying hydrogen gas at atmos- various applications such as separation in magnetic field, or a
pheric pressure. Unfortunately, only traces of the desired com- one-pot metathesis–hydrogenation sequence. Practical advan-
pound 26 were observed after 2 h at 80 °C (Scheme 4). This tages of the immobilised NHC-tagged catalyst, such as possi-
failure did not discourage us from further attempts. Being inter- bility of being applied in more polar solvents (toluene, ethyl
ested in development of a catalytic system that would lead to acetate), wide substrate scope, good yields and very low
products of tandem metathesis–hydrogenation under mild residual ruthenium content obtained make it potentially interest-
conditions, we deposited 8 on commercially available palla- ing in target-oriented synthesis, applications in pharmaceutical
dium on carbon (10 wt % Pd/C) [62]. The resulting bimetallic industry and similar areas.
Ru–Pd heterogeneous catalyst 8-Pd/C exhibited the same
activity in metathesis as catalyst deposited on activated carbon. In this work we focused on studying the immobilisation tech-
Interestingly, the E/Z selectivity in reaction carried out with niques and determining the application profile of the resulted
8
-C* was significantly different from that observed in reaction system. In parallel work, the recyclability of an immobilised
promoted by 8-SiO2 (11.5:1 and 19:1, respectively).
Ru-catalyst bearing quaternary ammonium-tagged NHC ligand
was described.
Advantageously, clean and fast conversion of 19 into 26 was
observed when a balloon containing hydrogen gas was applied
at 80 °C. After optimisation of the conditions, the saturated
diester 26 was obtained in 91% isolated yield and was found to
contain only 5.7 ppm of residual ruthenium after simple filtra-
tion (Scheme 4). It should be emphasised that this result was
obtained without application of high pressure of hydrogen gas,
and did not require any specific equipment.
Supporting Information
Supporting Information File 1
Experimental procedures and analytical data of obtained
compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-2-S1.pdf]
Conclusion
In summary, we have reported on an olefin metathesis catalyst, Acknowledgements
bearing a quaternary ammonium-tagged NHC ligand. This cata- K.G. acknowledges the “TEAM” project operated within the
lyst can be non-covalently immobilised on various organic and Foundation for Polish Science Team program co-financed by
inorganic solid supports in a straightforward and universal the EU European Regional Development Fund, Operational
Scheme 4: Supported catalyst 8 in sequential cross metathesis and reduction.
13

Beilstein J. Org. Chem. 2016, 12, 5–15.
2
4.Vehlow, K.; Maechling, S.; Köhler, K.; Blechert, S.
Program Innovative Economy 2007-2013. K.Ż. thanks for the
Diamond Grant” research project financed from the govern-
J. Organomet. Chem. 2006, 691, 5267–5277.
doi:10.1016/j.jorganchem.2006.08.019
„
mental funds for science for 2012–2015.
25.Mayr, M.; Mayr, B.; Buchmeiser, M. R. Angew. Chem., Int. Ed. 2001,
40, 3839–3842.
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