Bottom-Up Synthesis of Acrylic and Styrylic RhII Carboxylate Polymer Beads: Solid-Supported Analogs of Rh2(OAc)4

Article abstract of DOI:10.1002/ejoc.201800953

We have developed a short and efficient bottom-up synthesis of acrylic and styrylic polymer beads containing dirhodium(II) tetracarboxylates. The solid supported dirhodium(II) tetracarboxylate catalysts were synthesized in as little as two steps overall from dirhodium tetratrifluoroacetate and commercially available carboxylic acids, making the bottom-up approach a viable alternative to the post-modification approach commonly used. The dirhodium(II) tetracarboxylate polymer beads have a convenient size (ca. 100 μm), are easy to handle, and can be considered solid-supported analogs of Rh₂(OAc)₄. Beads generated from dirhodium(II) tetracarboxylates with four polymerizable carboxylate ligands displayed the best catalytic performance and compared favorably to Rh₂(OAc)₄ in benchmarked cyclopropanation reactions. The results imply that the cumbersome synthesis of monomeric dirhodium(II) tetracarboxylates with mixed ligands systems can be avoided and that immobilized dirhodium(II)-catalysts with a higher degree of crosslinking is a viable option to catalysts linked in an anchor-like fashion. We demonstrate recovery and recycling, and a potential use of the beads as catalysts in a cyclopropanation reaction towards the insecticide chrysanthemic acid.

Full text of DOI:10.1002/ejoc.201800953

DOI: 10.1002/ejoc.201800953
Full Paper
Dirhodiumtetracarboxylate Beads
II
Bottom-Up Synthesis of Acrylic and Styrylic Rh Carboxylate
Polymer Beads: Solid-Supported Analogs of Rh (OAc)
2
4
[
a]
[a]
[a]
[a]
Vladimir Levchenko, Bård Sundsli, Sigurd Øien-Ødegaard, Mats Tilset,
[a]
[a]
Finn K. Hansen, and Tore Bonge-Hansen*
Abstract: We have developed a short and efficient bottom-up polymerizable carboxylate ligands displayed the best catalytic
synthesis of acrylic and styrylic polymer beads containing dir- performance and compared favorably to Rh (OAc) in bench-
2
4
hodium(II) tetracarboxylates. The solid supported dirhodium(II) marked cyclopropanation reactions. The results imply that the
tetracarboxylate catalysts were synthesized in as little as two cumbersome synthesis of monomeric dirhodium(II) tetracarbox-
steps overall from dirhodium tetratrifluoroacetate and commer- ylates with mixed ligands systems can be avoided and that im-
cially available carboxylic acids, making the bottom-up ap- mobilized dirhodium(II)-catalysts with a higher degree of cross-
proach a viable alternative to the post-modification approach linking is a viable option to catalysts linked in an anchor-like
commonly used. The dirhodium(II) tetracarboxylate polymer fashion. We demonstrate recovery and recycling, and a poten-
beads have a convenient size (ca. 100 μm), are easy to handle, tial use of the beads as catalysts in a cyclopropanation reaction
and can be considered solid-supported analogs of Rh (OAc) . towards the insecticide chrysanthemic acid.
2
4
Beads generated from dirhodium(II) tetracarboxylates with four
Introduction
able for making solid-supported catalysts: A post-modification
[
5]
approach and a bottom-up approach. In the post-modifica-
tions approach, the catalysts of choice are grafted onto a pre-
made resin after the macromolecular synthesis. The primary
function of the premade resin is to work as an inert scaffold
after anchoring of the catalyst. In the bottom-up approach, the
catalysts are introduced as a part of the macromolecular syn-
thesis. The immobilization of the catalysts is modular at the
monomeric rather at the polymeric level and usually consists of
some sort of copolymerization. The majority of reports on solid-
supported dirhodium(II) tetracarboxylates fall into the post-
The development of novel synthetic methodology based on
transient Rh-carbenes can be traced back to work published by
Teyssie and co-workers in the beginning of the 1970s.[ Two of
their key findings were the unusually high kinetic activity of
Rh (OAc) in reactions with ethyl diazoacetate (EDA) and the
1]
2
4
highly efficient insertion of the transient Rh-carbenes into O–H
bonds. Ever since then, the development and use of di-
rhodium(II) tetracarboxylates as catalysts for the generation of
rhodium-carbenes from diazo compounds have gained an
enormous momentum in organic chemistry. Due to the unique
properties of Rh-carbenes to undergo C-H insertion reactions,
they play a vital role in the field of C-H functionalization.[2] Tran-
sient Rh-carbenes are also versatile intermediates in other state-
of-the-art organic reactions as they can for example partake in
cyclopropanations, cyclopropenation, [4+3] cycloadditions, in-
modification category, and they have been immobilized in two
[6,7]
ways: 1. By covalent grafting onto silica,
polymeric hollow
[8]
[9]
fibers and Merrifield resins. 2. By encapsulation and axial
coordination to various pyridine-containing resins.
[10–13]
The
only examples of beads containing dirhodium(II) catalysts using
a bottom-up approach have been reported by Hashimoto's
[
3]
sertion reactions (X-H, X = Si, O, N, S) and ylide formations.
[14,15]
group.
They introduced one polymerizable carboxylate li-
Characteristic traits in many of these transformations are excep-
tionally high turnover-numbers and -frequencies in addition to
high levels of chemo-, regio-, diastereo- and enantioselectivity.
Rhodium is an expensive and precious metal, and consequently
solid-supported dirhodium(II) catalysts to facilitate catalyst re-
covery and recycling have received increasing attention over
gand into well-known chiral catalyst and performed suspension
polymerizations to make heterogeneous polymer beads, which
performed very well in asymmetric benchmark reactions.
The post-modification approach using encapsulation and ax-
ial coordination to pyridine-containing resins is highly attractive
due to its simplicity and generality, but unfortunately leaching
is an inevitable drawback. Most other reports towards heteroge-
neous catalysts with covalently immobilized dirhodium(II) tetra-
carboxylates have some features in common. They are almost
all chiral catalysts and they are linked to the inert scaffold in an
[
4]
the last two decades. There are two general strategies avail-
[
a] Department of Chemistry, University of Oslo,
P.O. Box 1033, Blindern NO-0315 Oslo, Norway
E-mail: tore.bonge-hansen@kjemi.uio.no
https://www.mn.uio.no/kjemi/personer/vit/torehans/index.html
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201800953.
“
anchor-like” fashion where only one of the four carboxylate
ligands is connected to the heterogeneous network. Both the
post modification and the bottom-up strategy usually require
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Scheme 1. Immobilization of a dirhodium(II) tetracarboxylate using a post modification approach.[17]
some sort of modification of the original chiral catalyst and/or rhodium(II)-catalysts, to make dirhodium(II) tetracarboxylate
the premade resin in order for the immobilization to take beads with catalytic activity comparable to Rh (OAc)4 (1).
2
place.
Rather than using only one of the four carboxylate ligands as a
Even though many new chiral dirhodium(II) catalysts have link to the solid support (standard anchor-like fashion), we
been synthesized over the last 40 years, the generic achiral cat- wanted to explore the effect of using two and four attachment
alyst of choice is still Rh (OAc) . The development of solid- points to the solid support. This increased number of attach-
2
4
supported, achiral analogs of Rh (OAc) has received very little ment points could possibly increase the stability, and affect
2
4
attention compared to the chiral catalysts. There are several turnover-numbers and the lifetime of the catalysts. We also
examples of dirhodium(II) tetracarboxylate catalysts where re- wanted to explore the effect of an acrylic-based polymer net-
[
16]
covery and recycling is facilitated by liquid–liquid extraction.
work (relative to the standard styrenic network commonly
However, the only examples of solid-supported achiral di- used), since the presence of esters are compatible with the mild
rhodium(II) tetracarboxylates in the shape of beads, are a Merri- reaction conditions that apply to many Rh-carbene reactions.
[
9]
field resin and benzylamine functionalized polystyrene(PS)-
[
17]
supported catalyst (Scheme 1).
The approach falls into the post-modification category and
Results and Discussion
has the advantage of being simple and straightforward. Major At the outset of the project we needed fast and easy access
drawbacks of the post modification approach are the limited to dirhodium(II) tetracarboxylate monomers with the desired
availability of polymer supports and the lack of flexibility con- characteristics. We elected commercially available dirhodium(II)
cerning their properties such as porosity, swelling, and degree tetratrifluoroacetate [Rh (TFA) , 2] as a starting material for two
2
4
of cross-linking.
reasons. Stepwise replacements of the trifluoroacetate (TFA) li-
There are often significant differences in catalytic perform- gands allow for a controlled ligand exchange with respect to
ance when comparing immobilized, heterogeneous catalysts the number of TFA-ligands and the cis/trans relationship of the
[
21]
prepared by a bottom-up approach relative to the post-modifi- ligands.
cation approach.
The ligand exchange reactions to replace TFA with
[
18]
The bottom-up approach is an unexplored the desired carboxylates take place under very mild conditions
method for the synthesis of beads containing achiral di- and are compatible with acrylates without decomposition. We
rhodium(II) tetracarboxylate catalysts. We have previous experi- synthesized four polymerizable dirhodium(II) carboxylate
ence with bottom-up immobilization of organocatalysts[19,20] monomers in a controlled manner from 2 and commercially
and we envisioned that our methods for making heterogene- available 4-vinyl benzoic acid (VBA) or mono-2-(methacryloyl-
ous organocatalysts could be applied to immobilization of di- oxy) ethyl succinate (MCES) as shown in Scheme 2.
Scheme 2. Synthesis of dirhodium(II) tetracarboxylate monomers 3–6 from Rh
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2 4
(TFA) .
2
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Careful ligand exchange reactions with VBA or MCES re- porting Information for full details). The dirhodium species con-
placed two of the four TFA-ligands in
2 and gave tained axially coordinated acetone molecules from the crystalli-
Rh (TFA) (VBA) (3) and Rh (TFA) (MCES) (4) in 57 % and 45 % zation solvent (Figure 1).
2
2
2
2
2
2
yield, respectively. All four TFA-ligands in 2 were replaced with
Table 1 contains bond lengths for the first coordination shell
either VBA or MCES after applying slightly more forcing condi- of rhodium for 3, 4, and 5, and for Rh (TFA) for comparison.
2
4
tions. We obtained Rh (VBA) (5) in 73 % yield and Rh (MCES)
(
determined by single-crystal X-ray diffraction analyses (see Sup- bond lengths are rather nonexceptional.
The geometrical features are as expected for such dinuclear
6) in 62 % yield. The molecular structures of 3, 4, and 5 were dirhodium paddlewheel structures, and the Rh–Rh and Rh–O
2
4
2
4
[
22]
Figure 1. ORTEP plots of dirhodium(II) tetracarboxylates 3–5 with 50 % ellipsoids. Each molecule contains two axially coordinated acetone molecules from the
crystallization solvent.
Table 1. Selected bond lengths [Å] in dirhodium tetracarboxylates 3–5.
Compd.
Rh–Rh
Rh–OTFA
Rh–ORCOO
Rh–OOCMe2
(
(
(
3) Rh
4) Rh
5) Rh
2
2
2
(TFA)
(TFA)
2
(VBA)
(MCES)
·2Me CO
2
·2Me
2
CO
CO
2.3976(2)
2.3987(3)
2.381(1)
2.041(1), 2.043(1)
2.035(1), 2.045(1)
n.a.
2.028(2), 2.031(2)
2.035(1), 2.036(1)
2.005(7)–2.055(8) (2.027 ave.)
n.a.
2.260(1)
2.2829(9)
2.281(8), 2.293(8)
2.240(15)
2
2
·2Me
2
(VBA)
·2Me
4
2
[
a]
Rh
2
(TFA)
4
2
CO
2.4060(27)
1.957(15)–2.061(13) (2.008 ave.)
[
a] Data obtained from ref.[23]
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With the four monomers 3–6 in hand, we synthesized four
Similarly, the two PS-systems were made from either 3 (two
dirhodium(II) carboxylate polymer systems, differing with re- polymerizable ligands) or 5 (four polymerizable ligands) with
spect to the nature of the polymer network [polyacrylate (PA) the monomers styrene and divinyl benzene. The dirhodium(II)
or polystyrene (PS)] and the number of attachment points (two tetracarboxylate beads were synthesized in a bottom-up fash-
or four) from the dirhodium(II) catalyst to the polymer network. ion using the suspension polymerization conditions shown in
The two PA-systems were made from either 4 (two polymeriza- Scheme 4.
ble ligands) or 6 (four polymerizable ligands) and the mono-
mers tert-butyl acrylate and pentaerythritol tetraacrylate using beads with mixed ligands systems: PA-Rh
the suspension polymerization conditions displayed in PS-Rh (TFA) (VBA) (8) in 92 and 94 % yield, respectively. The
Scheme 3.
beads with mono ligand systems, PA-Rh (MCES) (9) and
The suspension polymerization resulted in two types of
(TFA) (MCES) (7) and
2
2
2
2
2
2
2
4
Scheme 3. Synthesis of PA-beads 7 and 9 using bottom-up immobilization of monomers 4 and 6.
Scheme 4. Synthesis of PS-beads 8 and 10 using bottom-up immobilization of monomers 3 and 5.
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PS-Rh (VBA) (10), were isolated in similarly high yields (89 % dichloromethane. The degree of swelling was measured to be
2
4
and 93 %, respectively). The glass-like polymer beads are easy 120 % for 11 and 100 % for 12.
to handle, and microscopy pictures of the beads (see SI) show
We also wanted to make dirhodium(II) tetracarboxylate
a perfect spherical shape and average diameter of 100 μm. beads by a post-modification approach for comparison of cata-
The beads were analyzed and characterized by microscopy, lytic performance. We grafted dirhodium(II) tetracarboxylate
SEM-EDX, TEM-analysis and surface area measurements (see monomer 6 onto commercially available (aminomethyl)polys-
SI for details). The BET absorption and SEM analysis of beads tyrene by means of a 1,4-addition reaction according to
7
–10 indicated absence of porosity or inner cavities in the dry Scheme 5. By using the dirhodium(II) monomer 6 as a catalyst/
state. Since the beads did not swell after soaking in organic monomer source, we obtained “post-modification beads”
solvents, there is probably no or a negligible degree of solvent- Rh (MCES-PS) 13, containing a similar (but not identical) di-
2
4
penetration in the polymer network. In order to increase the rhodium(II) tetracarboxylate catalyst to “bottom-up beads” 10.
porosity of the polymer network and to give access of solvent The catalytic performance of Rh (OAc) (1) and catalysts 2–
2
4
molecules and reactants to the inner part of the beads, we 13 were investigated using the standard cyclopropanation of
performed a second iteration of polymerization reactions with styrene with EDA as a benchmark reaction. The results are sum-
monomers 5 and 6. The reactions were executed according marized in Table 2. In general, the diasteromeric ratios of the
Scheme 3 and Scheme 4, but the degree of crosslinking was desired products cis- and trans-1-carbethoxy-2-phenyl-cyclo-
significantly reduced and a mixed PA-PS polymer network was propane (14) were almost independent of the catalyst structure.
generated from the monomers styrene and pentaerythritol The most surprising result when comparing the performance of
tetraacrylate. Standard suspension polymerization gave poly- the monomeric catalysts 1, 2, 5 and 6 (entry 1–6), was the high
mer beads PA-PS- Rh (MCES) (11) and PA-PS-Rh (VBA) (12), in yield of 14 in entry 6. Not only did monomer 6 perform simi-
2
4
2
4
5
1 and 50 % yield. The beads are spherical, hard and glass- larly to Rh (OAc) at a hundred times lower catalyst loading,
2 4
like in the dry state, but they become soft when swollen in but the undesired byproducts (ethyl maleate and fumarate, 15)
2 4
Scheme 5. Synthesis of Rh (MCES-PS) 13 by grafting of 6 onto (aminomethyl)-PS.
Table 2. Results from the reactions of styrene (5 equiv.) with EDA (1 equiv.) catalyzed by 1–13.
Entry
Catalyst
Cat load [mol-%]
Yield 14 [%][a]
cis/trans
Yield 15 [%][b]
1
2
3
4
5
6
7
8
9
Rh
Rh
Rh
Rh
Rh
Rh
PA-Rh
PS-Rh
PS-Rh
PA-Rh
PS-Rh
PA-PS- Rh
PA-PS- Rh
PA-PS- Rh
PA-PS-Rh
PA-PS-Rh
Rh
2
2
2
2
2
2
(OAc)
(OAc)
(OAc)
4
4
4
, 1
, 1
, 1
1
82
72
60[
40
30[
39:61
36:64
42:58
45:55
46:54
42:58
48:52
50:50
42:58
41:59
51:49
42:58
45:55
44:58
47:53
42:58
–
< 3
7
< 3
6
< 3
–
7
< 3
< 3
< 3
< 3
< 3
< 3
–
< 3
–
11
0.1
0.01
1
0.01
0.01
1
1
1
1
1
1
1
0.01
1
0.01
0.01
c]
(TFA)
4
, 2
, 5
, 6
(TFA) (MCES)
c]
(VBA)
(MCES)
4
[
c]
d]
e]
f]
4
81
2
2
2
, 7
, 8
, 8
46[
[
2
(TFA)
(TFA)
2
(VBA)
(VBA)
2
39
24[
63
81
81
2
2
2
10
11
12
13
14
15
16
17
2
(MCES)
4
, 9
2
(VBA) , 10
4
2
2
2
(MCES)
(MCES)
(MCES)
4
4
4
, 11
, 11
, 11
[
g]
74
[
c]
9
2
(VBA)
(VBA)
4
, 12
, 12
91
15[
–
c]
2
4
2
(MCES-PS)
4
, 13
[
a] Combined yield for the two diastereomers cis and trans 14. [b] Combined yield for diethyl fumarate and maleate measured by 1H-NMR spectroscopy. [c]
Dropwise addition of EDA over 1.5 h and 24 h stirring time. [d] 84 % conversion of EDA in 2 h. [e] Full conversion of EDA in 24 h. [f] 82 % conversion of EDA
in 2 h. [g] Addition of EDA in one portion.
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from the dimerization reactions were suppressed to the detec- propanation reaction between EDA and 2,5-dimethylhexa-2,4-
1
tion limit in the analysis of the crude H-NMR spectrum. Beads diene. We executed the reaction of ethyl chrysanthemate
7
and 8 with mixed ligand systems performed poorly (entry 7– similarly to the benchmark cyclopropanation of styrene with
9). Even though 8 decomposed EDA to 82 % conversion after EDA. The results are displayed in Table 4.
two hours, the yield of 14 was only 24 %. Beads 9 and 10,
where all the TFA-ligands have been displaced with VBA or
MCES, performed significantly better (entry 10–11). An interest-
ing observation is the reversal in performance when comparing
the monomeric catalysts 5 and 6 to the polymeric beads 10
and 9. At the monomeric stage, the acrylate-based catalyst 6
gave higher yield of 14 than styrene-based catalyst 5, but as
polymeric beads the PS-catalyst 10 outperformed PA-catalyst 9.
Beads 10 have catalytic activity similar to Rh (OAc) , and this is
Table 4. Results from the reactions of 2,5-dimethylhexa-2,4-diene with EDA
catalyzed by 1, 11, 12.
2
4
Yield [%][a]
Entry
Cat.
Cat. (mol-%)
rather surprising since BET and SEM analysis of 10 indicated no
or negligible solvent penetration. Catalysis most likely occur on
the surface of the beads with no or very low access to the
internal Rh-sites within the polymer network. All beads 7–12
1
2
3
1
12
11
1
1
1
80
76
58
[
a] Yield of ethyl chrysanthemate measured by analysis of crude 1H NMR
spectra.
were soaked in CH Cl prior to the cyclopropanation reaction
2
2
in order for the beads to swell and allow access to the interior
of the network. However, only beads 11 and 12 had a measure-
All three catalysts gave a diastereomeric mixture of chrysan-
able degree of swelling, and this increased degree of swelling themic acid ethyl esters. Catalyst 1 gave the desired products
had a significant impact on the yields of 14 (entries 12, 15). The in 80 % yield, marginally better than the 76 % yield obtained
PA-PS beads 11 were similar to Rh (OAc) in performance (entry from 12. Beads 11 gave 58 % yield, significantly lower than 1
2
4
1
2) while PA-PS beads 12 outperformed Rh (OAc) and gave and 12. These results imply that 12 can be used to synthesize
2
4
the desired product in 91 % yield (entry 15). At very low catalyst ethyl chrysanthemate in similar yields to Rh (OAc) , but with
2
4
loadings (0.01 mol-%), however, the immobilized catalysts per- two significant advantages. Catalyst 12 can be separated from
formed poorly and gave much lower yields compared to their the reaction mixture and recovered by simple filtration, and
monomeric precursors (entries 14, 16). The PS-beads 13 (entry reused 4–5 times without any significant drop in yields.
17) were tested only at very low catalyst loading (0.01 mol-
%
). The post-modification catalyst decomposed EDA to 22 %
conversion, but no cyclopropane 14 was detected in the crude
reaction mixture.
We elected the best catalysts, 11 and 12, for a study of recov-
ery and recycling. We used the same benchmark cyclopropan-
ation reaction and measured the yields of 14 over 7 cycles
Conclusions
The work presented here demonstrates four important aspects
with regard to synthesis of solid-supported dirhodium(II) tetra-
carboxylate catalysts: (1) It is feasible to make immobilized, het-
erogeneous dirhodium(II) tetracarboxylate catalysts efficiently,
using a bottom-up approach, in only two steps from commer-
cially available starting materials. Hence, the bottom-up ap-
proach can be a competitive alternative to the post-modifica-
tion approach, which is commonly used for immobilization of
high-value catalysts. (2) The heterogeneous dirhodium(II) tetra-
carboxylate bead catalysts can be as efficient catalysts as the
homogeneous, generic Rh (OAc) . (3) Beads made from dirho-
(
Table 3). Both catalysts showed a similar activity trend. The
yields were generally high and reproducible for the first four
cycles followed by a steady drop towards moderate yields after
seven cycles. Beads 12 appear to be more robust as the drop
in yield of 14 was slightly lower for reactions catalyzed by 12
compared to 11.
2
4
Table 3. Data from the reactions of styrene (5 equiv.) with EDA (1 equiv.)
dium(II) tetracarboxylate monomers with four polymerizable li-
gands are efficient catalysts despite a higher degree of cross-
linking. This has an important implication: The link from the
dirhodium(II) tetracarboxylate catalyst to the solid support does
not have to be an “anchor-like” fashion for the beads to func-
tion properly. Hence, the cumbersome synthesis of mixed li-
gand dirhodium(II) tetracarboxylates with only one polymeriza-
ble ligand can be avoided. (4) Beads made of PA work just as
well as PS beads with an inert scaffold. Since many acrylate
monomers are commercially available, PA-dirhodium(II) tetra-
catalyzed by 1 mol-% 11 or 12.
Catalyst Cycle[a]
Yield[b] [%]
1
2
3
4
5
6
7
12
91
96
92
88
85
62
78
[b]
trans/cis [%] 55:45 55:45 55:45 55:45 55:45 55:45 55:45
Yield[b] [%]
81 81 82 82 57 43 44
trans/cis [%] 58:42 58:42 58:42 58:42 58:42 58:42 58:42
a] The catalyst was recovered by filtration, washed and recycled. [b] Yield of
1
1
[b]
[
1
4 and cis/trans ratios were measured by analysis of crude 1H NMR spectra.
We picked catalysts 1, 11 and 12 and investigated their cata- carboxylate beads with excellent catalytic performance and a
lytic performance in cyclopropanation reactions towards a syn- large range of different chemical and physical properties should
thesis of chrysanthemic acid. Chrysanthemic acid is a commer- be readily available in only a few steps. By extending and fur-
cially available insecticide and it can be made by hydrolysis of ther optimizing the bottom-up strategy, we believe it should
ethyl chrysanthemate, which in turn can be made in a cyclo- be possible to prepare tailor-made immobilized dirhodium(II)
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catalysts that not only compete favorably with Rh (OAc) as 30 min, a solution of EDA (0.100 g, 0.876 mmol, 1 equiv.) in CH
Cl
2
2
4
2
(
12 mL) was added dropwise and reaction mixture was stirred at
catalyst in Rh-carbene reactions, but are also easily available at
a comparable price.
room temperature for 24 h. The solid catalyst was recovered by
filtration and mesitylene (105.3 mg, 0.876 mmol, 1 equiv.) was
added as internal standard. The yield of ethyl chrysanthemate was
1
calculated from analysis of the crude H NMR spectrum.
Experimental Section
CCDC 1847889 (for 5), 1847891 (for 4), and 1847890 (for 3) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre.
Procedure for Monomer Synthesis Exemplified by the Synthesis
2
of Monomer 3: A dry bottom flask was charged with Rh (TFA)4
(30 mg, 0.046 mmol), 4-vinylbenzoic acid (61 mg, 0.41 mmol,
9
.0 equiv.), NaHCO3 (24 mg, 0.29 mmol, 6.4 equiv.) and dichloro-
Supporting information (see footnote on the first page of this
article): Full experimental procedures, pictures and spectroscopic
data are given in the supporting information file free of charge.
methane (5 mL). The mixture was stirred at the ambient tempera-
ture for 20 h. The organic phase was washed with a saturated NaH-
CO solution, dried (MgSO ), filtered and concentrated under vac-
3
4
uum to give a green residue. Column chromatography (SiO , 1:1
2
EtOAc:hex) gave 19 mg (57 % yield) of 3 as a green solid. The struc-
ture was determined by single-crystal X-ray diffraction after recrys-
tallization from acetone/hexane.
Acknowledgments
We thank Mr. Osamu Sekeguchi (Department of Chemistry) for
MS analysis and Michaela Salajkova at the EM lab (Department
Procedure for Bead Synthesis Exemplified by the Synthesis of
Beads 4:
Rh (TFA) (MCES) 4 (25 mg, 0.028 mmol) tert-butyl acrylate (0.6 g,
A
25 mL round-bottom flask was charged with of Biosciences) for SEM, TEM, and EDX analysis.
2
2
2
4
.68 mmol), pentaerythritol tetraacrylate (0.27 g, 0.77 mmol) and
Keywords: Heterogeneous catalysis · Rhodium ·
Cyclopropanation · Immobilization · Carbene ligands ·
Bottom-up synthesis
ethyl acetate (0.8 mL) containing a few drops of acetone (in order
to increase the solubility of 4). To this solution was added 0.5 %
aqueous polyvinyl alcohol (Mowiol 40–88, 3.5 mL) together with
2
mg potassium iodide (inhibits polymerization in the aqueous
phase). 2,2′-Azobisisobutyronitrile (AIBN, 8 mg, 0.049 mmol) was
dissolved in 0.25 mL ethyl acetate and carefully added to the result-
ing mixture under vigorous stirring. The suspension was polymer-
ized at 85 °C for 2.5 h at a constant rate of 1000 rpm. The suspen-
sion was cooled to room temperature and poured into beaker to-
gether with water/methanol (15/15 mL). The beads were allowed
to settle by gravity and the supernatant was decanted off. The poly-
mer beads were filtered, washed with water, dichloromethane and
dried at the room temperature to give 0.8 g (92 %) of glassy, light-
blue polymer beads 7. The aqueous filtrate and the water from the
washing process were extracted with dichloromethane. The organic
layers were combined with the dichloromethane phase form the
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3
805.
1
(
MgSO ), filtered and concentrated under vacuum. H-NMR analysis
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2
2
2
mer 4. The calculated loading of 4 in the beads is 0.035 mmol/g
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2
4
General Procedure for the Cyclopropanation of Styrene with
EDA Catalyzed by Beads 7–13: A 50 mL round-bottomed flask
equipped with a stirring bar was charged with beads of choice (7–
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1
3, 1 mol-% with respect to EDA) in CH Cl (3 mL) and left stirring
2 2
[
for 30 min for pre-swelling. A solution of ethyl diazoacetate (0.10 g,
0
.88 mmol, 1 equiv.) in CH Cl (12 mL) was added dropwise under
2
2
[
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8
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2
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1
General Procedure for the Synthesis of Ethyl Chrysanthemate:
The dirhodium(II) tetracarboxylate catalyst (0.00876 mmol, 1 mol-
%) and 2,5-dimethylhexa-2,4-diene (0.482 g, 4.38 mmol, 5 equiv.)
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were suspended in CH Cl (6 mL) under vigorous stirring. After
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2
2
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Full Paper
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Received: June 18, 2018
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Full Paper
Dirhodiumtetracarboxylate Beads
V. Levchenko, B. Sundsli,
S. Øien-Ødegaard, M. Tilset,
F. K. Hansen, T. Bonge-Hansen* .... 1–9
Bottom-Up Synthesis of Acrylic and
Styrylic Rh Carboxylate Polymer
Polymer beads containing dirhodium(II) Rh (OAc)4 and compare favorably in
2
II
tetracarboxylates can be made in only benchmark cyclopropanation reac-
two steps from commercially available tions of styrene with ethyl diazoacet-
starting materials. The beads can be ate.
Beads: Solid-Supported Analogs of
Rh (OAc)
2
4
considered immobilized analogs of
DOI: 10.1002/ejoc.201800953
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim