Stereospecific carbene polymerization with oxygenated Rh(diene) species

Article abstract of DOI:10.1002/anie.201200069

Breath-taking activation: Stereoregular carbene polymerization proceeds via cationic [(allyl)Rh^III-polymeryl]⁺ species. These are most efficiently generated by oxygenation of the [(diene)Rh^I] precatalysts, which involves a

Full text of DOI:10.1002/anie.201200069

Angewandte
Chemie
DOI: 10.1002/anie.201200069
Carbene Polymerization
Stereospecific Carbene Polymerization with Oxygenated Rh(diene)
Species**
´
´
Annemarie J. C. Walters, Oliver Troeppner, Ivana Ivanovic-Burmazovic, Cristina Tejel,
M. Pilar del Rꢀo, Joost N. H. Reek, and Bas de Bruin*
Polymers bearing polar functionalities are important materi-
als, since they exhibit beneficial properties with respect to
adhesion, paint/printability, and other surface properties.[1]
The commercial synthesis of these materials is mainly based
on radical processes, for which stereocontrol is difficult to
achieve.[2] Stereocontrolled polymerization generally requires
(transition-)metal catalysis. However, to the best of our
knowledge there are no catalysts known that can polymerize
1,2-difunctionalized olefins such as fumarates or maleates in
a stereocontrolled manner. Therefore, the synthesis of high
molecular weight and stereoregular densely functionalized
sp3-carbon chain (co)polymers containing a polar substituent
at every carbon atom of the polymer backbone is currently
restricted to the Rh-mediated carbene polymerization tech-
niques recently developed in our group (C1 polymerization;
Scheme 1).[3] These (co)polymers reveal interesting and
unexpected material properties, such as thermotropic and
lyotropic liquid crystallinity, gel formation, a broad thermal
stability range, and a high storage modulus up to high
temperatures.[3g] Such polymers are not accessible from (1,2-
difunctionalized) olefins,[4] and application of these new
materials[5,6] therefore relies on increasing the efficiency of
the carbene polymerization reaction.[3a]
Scheme 1. Rh-catalyzed carbene polymerization.
2 (Scheme 1) leads to markedly improved polymer yields and
a higher amount of active Rh-species,[7] but the underlying
reason was poorly understood.
As is commonly encountered in transition-metal catalysis,
the catalyst precursor is (partly) transformed in situ into the
active species and a detailed understanding of the reaction,
which is needed for further exploration of the potential of this
promising unique polymerization reaction, requires identifi-
cation of the active species. Faced with seemingly contrasting
observations in our initial studies,[8] we now report the nature
of the active polymer-forming growing-chain rhodium species
during carbene polymerization, which are formed after
oxidation of the catalyst precursor with O2.
Complex 2 reacts sluggishly with air or O2, and hence it
takes a long time to convert the solid starting material to the
fully oxidized solid (3).[7,9,10] NMR spectra of this material are
complicated, which is indicative of the presence of multiple
species with overlapping signals, and are therefore not very
informative (Figure S12 in the Supporting Information). They
are, however, suggestive of the presence of (different geo-
metrical isomers of) monooxygenated [Rh(Me2cod)] species
(3) (Scheme 2; Me2cod = 1,5-dimethylcycloocta-1,5-diene).[11]
This hypothesis was confirmed by ESI-MS; the clear and
clean mass spectra revealed only monooxygenated species
[3H]+. The high-resolution mass spectrometric measurements
and isotopic distribution of [3H]+ fit exactly to the mass of the
starting material plus one oxygen atom, and a proton
functions as the charge carrier: [2 + O + H]+ = [3H]+ (Fig-
The reaction mechanism of this intriguing carbene
polymerization reaction has been the subject of intensive
investigations in our group, but the exact nature of the
polymer forming Rh-species remained elusive thus far.
Recently, we discovered that partial oxidation of precatalyst
[*] A. J. C. Walters, Dr. M. P. del Rꢀo, Prof. Dr. J. N. H. Reek,
Dr. B. de Bruin
Homogeneous and Supramolecular Catalysis Group, Van‘tHoff
Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
E-mail: b.debruin@uva.nl
´
´
Dipl.-Chem. O. Troeppner, Prof. Dr. I. Ivanovic-Burmazovic
Lehrstuhl fꢁr Bioanorganische Chemie, Department Chemie und
Pharmazie, Universitꢂt Erlangen-Nꢁrnberg (Germany)
Dr. C. Tejel
C. Tejel, Departamento de Quꢀmica Inorgꢃnica, Instituto de Sꢀntesis
Quꢀmica y Catꢃlisis Homogꢄnea (ISQCH) CSIC,
Universidad de Zaragoza (Spain)
[**] Financial support from the European Research Council (ERC, grant
agreement 202886-CatCIR), the Netherlands Organization for
Scientific Research (NWO-CW), the Dutch Polymer Institute (DPI
projects no. 646/647), the University of Amsterdam, the DFG (SFB-
583 and Major Research Instrumentation Program) and MICINN/
FEDER (Projects CTQ2008-03860 and CTQ2011-22516) is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200069.
Scheme 2. Likely structures of 3 formed by oxidation of 2.
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ure S1 in the Supporting Information). On the basis of our
previous work in the field of RhI(olefin) oxygenation
reactions,[11] the most likely monooxygenated species in this
mixture (NMR) are (different isomers of) the 2-metalla-
oxetane[12] species 3a, its ring-closed analogue 3b, and the
allyl-b-alkyl-hydroxide species 3c (Scheme 2).
Interestingly, the fully oxidized samples (3), in which no
starting material (2) is present anymore according to NMR
spectroscopy,[9] give excellent yields in the carbene polymer-
ization reaction. In fact, even higher polymer yields are
obtained with fully oxidized samples (3) than previously with
partly oxidized 2. Furthermore, the percentage of active
species is higher for the oxygenated species 3 (Table 1).[7]
Table 1: Carbene polymerization with precatalysts 1–6.[a]
Catalyst
Polymer
Oligomer
yield [%][c]
MW
MW/Mn
IE
yield [%][b]
[kDa][d]
[%][e]
1[3d,g]
2
50
45
85
25
66
80
30
30
15
38
19
12
150
590
311
117
357
111
3.6
4.2
2.4
3.4
4.5
2.4
4
1.6
2.4
3.2
3.6
7.4
3[9]
4
Scheme 3. Reaction mechanism showing series I–IV (R’=H, Me,
R=Et, Bn).
5
6
[a] Conditions: 0.04 mmol catalyst; 2 mmol EDA, 5 mL chloroform,
room temperature, 14 h. [b] Isolated by precipitation and washing with
methanol. [c] Low molecular weight (Mw <2 kDa) material.[d] Size-
exclusion chromatography (SEC) (polystyrene standards). [e] Initiation
efficiency: Number of polymer chains per Rh center in% (mol/
molꢅ100%).
containing a methoxy chain end (series IV, Scheme 3). These
observations clearly show that the polymer initiates at
a hydroxide fragment generated from the oxidized precatalyst
3. The chains terminate by protonation, either involving H2O
or an alcohol.[3k] In the presence of MeOH, the alcohol clearly
functions as a chain-transfer agent,[3k] thus generating H-
(CHCOOR)n-OMe terminated polymer chains and allylic
[(Me2C8H9)RhIII-OMe]+ species from which a new chain
starts growing (Scheme 3). Representative MS spectra show-
ing series I–IV are shown in Figures S2–S5 (Supporting
Information).
Next, we monitored the polymerization reaction by ESI-
MS using complex (2 and) 3 as the catalyst. These experi-
ments revealed the exact nature of the active propagating Rh-
species and allowed us to draw several additional valuable
conclusions about the carbene polymerization process. The
assignment of all detected species is described below and is
based on high-resolution mass determinations (deviations
between measured and simulated m/z isotopic pattern masses
are consistently smaller than 0.002 Da).
Addition of ethyl diazoacetate (EDA) to a solution of
precatalyst 2 in CH2Cl2 reveals only [2H]+, and these peaks do
not visibly decrease over a period of more than an hour.
Similar experiments with the oxidized precatalyst 3, however,
led to an immediate decrease of the signals of [3H]+ and the
appearance of a clear repeating pattern of a [(Me2C8H9)Rh-
(CHCO2Et)n-OH]+ propagating species that builds in
DCHCOOEt carbene units (DM = 86 Da; see series I,
Scheme 3). The organometallic “(Me2C8H9)Rh” fragment
corresponds with the cationic [(allyl)RhIII-alkyl]+ species
depicted in Scheme 3, which is supported by additional
experiments (see below).
Similar results were obtained when using benzyl diazo-
acetate (BnDA) instead of EDA, thus confirming the above
assignments (Scheme 3, R = Bn; see Figures S6 and S7 in the
Supporting Information). The data therefore show that the
oxidized species 3 are capable of generating the active allyl-
Rh-alkyl species [(Me2C8H9)RhIII-P]+ (P = polymeryl). Appa-
rently, the hydroxide fragment of species 3c can be trans-
ferred to the metal to start the chain-propagation process
through carbene insertion into the thus formed RhIII OH
bond.
À
We were intrigued by the possibility that the active allyl
species [(Me2C8H9)RhIII-OH]+ (and hence related allylic
[(C8H11)RhIII-OH]+ species derived from cycloocta-1,5-
diene) could be generated from the 2-rhodaoxetane species
3a or the allyl-b-hydroxy–alkyl species 3c, and hence we
looked for similar activity in related compounds. Previously
reported [(C8H12O)Rh(N3-ligand)]+ species[11] turned out to
be nonreactive towards EDA, which can be expected for these
saturated 18-valence-electron RhIII species. Hence, we
decided to look at “unsaturated” analogues reported pre-
viously. The triazenide complexes 5 and 6 (Scheme 4)[13]
seemed good candidates to test our hypothesis that active
allylic [(C8H11)RhIII-OH]+ species can be generated from
Most interestingly, the MS spectra further show the
appearance of [H-(CHCOOEt)n-OH + Na]+ chains (series II,
Scheme 3). Hence, chain growth starts on a hydroxide frag-
ment. Furthermore, addition of MeOH results in the appear-
ance of a clear pattern corresponding to [(Me2C8H9)Rh-
(CHCOOEt)n-OMe]+ growing chains (series III, Scheme 3)
as well as terminated [H-(CHCOOEt)n-OMe + Na]+ chains
2
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a detailed understanding of the reaction mechanism. The
results are very similar to those obtained with 3 as the
precatalyst in EDA and BnDA polymerization reactions. In
absence of methanol, clear repeating patterns of
[(C8H11)RhIII-(CHCOOR)n-OH]+ growing chains were
detected (series I, R = Et, Scheme 3), as well as terminated
[H-(CHOOR)n-OH + Na]+ chains (series II, R = Et). Again,
in the presence of methanol patterns of [(C8H11)RhIII-
(CHCOOR)n-OMe]+ growing chains (series III, R = Et) and
patterns of terminated [H-(CHCOOR)n-OMe + Na]+ chains
containing a methoxy end group (series IV, R = Et) appeared
(Figures S8 and S9 in the Supporting Information). Similar
results were obtained using BnDA instead of EDA
(Scheme 3, R’ = H, R = Bn; Figure S10). Important to note
here is that these series I–IV (R’ = H) were detected using
Scheme 4. [(C8H12)Rh(N3Ph2)] (4), the 2-rhodaoxetane complex 5, and
its isomer allyl-b-alkyl-hydroxide compound 6.
precursors
containing
a
2-rhodaoxetane
fragment
(C8H12O)RhIII.
Complex 5 is formed in good yield and in good purity by
oxidation of the triazenide precursor [(C8H12)Rh(N3Ph2)] (4)
with O2.[13] Complexes 4–6 were evaluated as catalyst
precursors in the carbene polymerization of EDA (Table 1).
As expected, [(triazenide)RhI(cod)] complex 4 is active, but it
is less active than the non-oxidized [(R’2C8H10)RhI(prolinate)]
complexes (R’ = H: 1; R’ = Me: 2). However, oxidation of 4 to
5 markedly improves the polymerization over oligomeriza-
tion/dimerization selectivity, thus leading to substantially
improved polymer yield (66%). Conversion of 5 into 6
further increases the selectivity towards polymer formation
(80%), while at the same time the initiation efficiency
(percentage of active Rh-species) increases (see Table 1).
The above data are in excellent agreement with the
markedly different polymerization kinetics of complex 4, 5,
and 6 under identical reaction conditions (Figures S16 and
S17 in the Supporting Information). The reaction with
complex 4 is very slow (full EDA conversion requires more
than 10 h). Complex 5 converts EDA substantially faster than
4 (full conversion in ca. 5 h), but much slower than 6 (full
conversion in only a few minutes). These kinetic data are best
explained by a progressively easy catalyst activation process
on going from 4 via 5 to 6 under the applied reaction
conditions, thus leading to a higher amount of active polymer-
forming Rh-species (in good agreement with the data in
shown in Table 1).
pure samples of 5 and 6 as precatalysts, and hence the RhIII
À
OH fragments (at which chain growth starts) must be formed
from these complexes. Therefore, 5 and 6 must liberate
a hydroxide fragment from their oxidized cod moiety to form
+
the observed active cationic allylic [(C8H11)RhIII OH]
À
species. A straightforward explanation is that 5 rearranges
to 6 under the catalytic conditions, after which the complex
loses the triazenide ligand and undergoes a b-hydroxy
elimination from the allyl-b-alkyl-hydroxide ligand to gen-
+
erate the active allylic [(C8H11)RhIII OH] species. DFT
À
calculations (b3lyp, def2-TZVP) confirm that this is an
energetically favorable and kinetically accessible pathway
(Scheme 5).
Hence, it is clear that the allyl-b-alkyl-hydroxide complex
6 more easily converts into the active polymer forming species
than 2-rhodaoxetane 5. Yet, still only a minor amount of
species 6 becomes active in the polymerization event
(ca. 7%), thus showing that complex 6 itself is still a pre-
catalyst that requires further activation under the applied
reaction conditions (i.e. loss of the triazenide ligand and
ligand rearrangement). The low efficiency of this process is
likely due to competing and unwanted side-reactions with
EDA during the incubation time of the reaction, possibly
related to sluggish displacement of the bidentate triazenide
N ligand from rhodium(III). However, complex 6 does give
easier access to the polymer forming growing-chain species
than non-oxidized [(cod)RhI] or [(Me2cod)RhI] complexes
such as 1, 2, and 4. In fact, complex 6 shows the highest
initiation activity of all Rh compounds studied so far. The
(oxygenated) Rh species 3, 5, and 6 produce polymers of the
same high syndiotacticity as reported previously.[14]
Scheme 5. DFT calculated pathway for activation of 2-rhodaIIIoxetane
precursors to form the active (allyl–cod)RhIII OH species.
À
Proton transfer from an allylic position to the 2-rhoda-
oxetane oxygen atom is exergonic by 18 kcalmolÀ1 and has
a barrier of 27 kcalmolÀ1 (simplified cationic model in the gas
phase; Scheme 5). This transformation is experimentally
observed in the rearrangement from 5 to 6 and in related
reported reactions.[11,13] More importantly, b-hydroxy elimi-
nation from the cationic allyl-b-alkyl-hydroxide rhodium(III)
species is exergonic by approximately 5 kcalmolÀ1 and has an
accessible transition state barrier of 28 kcalmolÀ1.[15] Clearly,
oxidation
of
[(R’2C8H10)RhI]-type
complexes
to
[(R’2C8H10O)RhIII]-type species, followed by rearrangement
to [(R’2C8H9)RhIII OH] allyl compounds (R’ = H, Me) is an
effective pathway to engage carbene polymerization activity.
Carbene polymerization is also observed for non-oxidized
rhodium(I) diene complexes under strict anaerobic condi-
+
À
The carbene polymerization reaction using species 5 and 6
was further probed by high resolution ESIMS to obtain
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tions. Hence, the active [(R’2C8H9)RhIII-Y]+ allyl species can
also be generated via a different pathway, directly from the
non-oxidized rhodium(I) diene precursors (albeit less effec-
tively). A plausible route for activation of the non-oxidized
[RhI(cod)] catalyst precursors is discussed in the Supporting
Information (Scheme S1).
In conclusion, high-resolution ESI-MS measurements
allowed us to characterize the polymer-forming growing-
chain rhodium species that are active during Rh(diene)-
catalyzed stereoregular carbene polymerization reactions.
These turn out to be cationic [(allyl)RhIII-polymeryl]+ species.
Unexpectedly, these species are most efficiently generated
from oxygenated complexes [(“diene-O”)RhIII], which pro-
duce higher polymer yields and allow better initiation
efficiencies than their non-oxidized [(diene)RhI] precursors.
Rearrangement of 2-rhodaoxetanes to allyl-b-alkyl-hydroxide
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species followed by b-hydroxy elimination gives access to
+
[(allyl–diene)RhIII OH] species. To the best of our knowl-
À
edge, this is the first unambiguous example of a concerted b-
hydroxy syn-elimination reaction at a rhodium center,[16,17]
which adds further diversity to the remarkable palette of 2-
metallaoxetane reactivity.[12] The active [(allyl)RhIII-poly-
meryl]+ growing chains are formed by carbene insertion
À
into the Rh OH bond. These important new mechanistic
insights will greatly aid the development of Rh-mediated
stereoregular carbene polymerization reactions in the near
future, and hold the promise to make new polymeric materials
from carbene precursors in a much more effective, selective,
and better controllable manner. The data further imply that
catalyst oxidation can be crucial to convert catalyst precursors
into active species, and remind us of the fact that the cod
ligand (which is widely applied in [{M(m-Cl)(cod)}2] and
[(M(cod)2]BF4 precursors (M = Rh, Ir) for in situ catalyst
generation in a variety of catalytic reactions) should not be
inadvertently assumed to be labile or easily displaced, but can
actually be or become a potent ancillary ligand during
catalytic turnover.
[7] E. Jellema, A. L. Jongerius, A. J. C. Walters, J. M. M. Smits,
J. N. H. Reek, B. de Bruin, Organometallics 2010, 29, 2823 –
2826.
Received: January 4, 2012
Revised: March 9, 2012
Published online: && &&, &&&&
[8] For example, the non-oxidized precatalysts 1 and 2 (complex
1 does not even react with air) give clear access to carbene
polymerization under strict anaerobic and dry conditions,
whereas partial oxidation of 2 leads to higher activities (see
Keywords: diazo compounds · oxidation · polymerization ·
rhodium
.
Refs. [3] and [6]). All tested pre-formed RhI–allyl and RhIII
–
alkyl species have very poor (if any) activity (see Ref. [3i–k]).
[9] The oxidation process can be accelerated by crushing the
microcrystalline solid 2 before exposing it to a small amount of
water and pure O2. After several weeks, the less-soluble oxidized
material (3) was separated from starting material 2 by adding
CHCl3, followed by filtration and thorough washing of the solid
residue (3) with CHCl3 (see the Supporting Information).
[10] Reactions of 2 with air or with O2 in solution proceed differently
(NMR spectroscopic evidence). Such samples give incomplete
conversion in carbene polymerization.
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[14] No stereo-errors are detectable with 13C NMR spectroscopy; see
Ref. [3] for details.
[15] Note that these reactions are strongly exothermic.
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[17] Concerted b-hydroxy-elimination also explains the outcome of
the reaction of complex 6 with carbon monoxide: C. Tejel, M. P.
del Rꢀo, J. A. Lꢁpez, M. A. Ciriano, Chem. Eur. J. 2010, 16,
11261 – 11265.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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.
Angewandte
Communications
Communications
Carbene Polymerization
Breath-taking activation: Stereoregular
carbene polymerization proceeds via cat-
ionic [(allyl)RhIII–polymeryl]+ species.
These are most efficiently generated by
oxygenation of the [(diene)RhI] precata-
lysts, which involves an unusual rear-
rangement of 2-rhodaoxetane intermedi-
ates. This discovery gives detailed insight
in the reaction mechanism.
A. J. C. Walters, O. Troeppner,
´
´
I. Ivanovic-Burmazovic, C. Tejel,
M. P. del Rꢁo, J. N. H. Reek,
B. de Bruin*
&&&& — &&&&
Stereospecific Carbene Polymerization
with Oxygenated Rh(diene) Species
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!