Rhodium-catalyzed polymerization of 3,3-diarylcyclopropenes involving a 1,4-rhodium migration

Article abstract of DOI:10.1021/ja5032002

A new mode of metal-catalyzed polymerization reaction has been developed by exploiting the ability of 1,4-rhodium migration of an organorhodium(I) species. Specifically, it has been demonstrated that 3,3-diarylcyclopropenes undergo polymerization through

Full text of DOI:10.1021/ja5032002

Communication
pubs.acs.org/JACS
Rhodium-Catalyzed Polymerization of 3,3-Diarylcyclopropenes
Involving a 1,4-Rhodium Migration
Ryo Shintani,* Ryo Iino, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
S
* Supporting Information
Scheme 1. Schematic Representation of Rhodium-Catalyzed
Polymerization of 3,3-Diphenylcyclopropene via 1,4-
Rhodium Migration to Yield a Poly(cyclopropylene-o-
phenylene)
ABSTRACT: A new mode of metal-catalyzed polymerization
reaction has been developed by exploiting the ability of
1,4-rhodium migration of an organorhodium(I) species.
Specifically, it has been demonstrated that 3,3-diarylcyclopro-
penes undergo polymerization through an insertion−1,4-
rhodium migration sequence by using an arylrhodium(I)
initiator/catalyst to give poly(cyclopropylene-o-phenylene)s,
which are difficult to synthesize by conventional polymer-
ization methods.
hodium-catalyzed carbon−carbon bond forming reac-
Rtions through insertion of a carbon−carbon or carbon−
heteroatom unsaturated bond into a carbon−rhodium bond of
organorhodium(I) complexes constitute powerful synthetic
tools in modern organic chemistry,1 and this mode of carbon−
carbon bond formation has been successfully applied to the
polymerization of arylacetylenes with high catalytic activity.2 In
addition to this simple insertion (carborhodation) process,
rhodium(I) complexes are also known to undergo intra-
molecular 1,4-rhodium migration from an alkyl-3 or alkenylrho-
3,3-diarylcyclopropenes as monomers in the presence of a
dium4 species to an arylrhodium species. This process can
rhodium catalyst (Scheme 1). Thus, a catalytic amount of
quickly add molecular complexity to organic compounds
through a C−H bond functionalization and has been effectively
utilized in various small molecule syntheses since the first
report by Miura et al. in 2000.3a,5 Unfortunately, however, this
attractive feature of rhodium(I) complexes has never been
exploited in the polymerization chemistry despite its potential
utility for synthesizing a variety of new types of polymers that
are currently inaccessible. Here, we describe the first polymer-
ization reaction involving a 1,4-rhodium migration in the
context of a rhodium-catalyzed polymerization of 3,3-diary-
lcyclopropenes through an insertion−1,4-rhodium migration
sequence to give a new class of polymers, poly(cyclopropylene-
o-phenylene)s, with high efficiency.
Cyclopropenes are highly reactive cyclic alkenes due to their
ring strains,6 but aside from spontaneous polymerization of
unsubstituted cyclopropene,7 only a limited number of reports
have been made on the use of cyclopropenes as monomers
for polymerization reactions in a controlled manner, such as a
palladium-catalyzed insertion-polymerization of 3,3-dialkylcy-
clopropenes8 and a molybdenum- or ruthenium-catalyzed ring-
opening metathesis polymerization of 3,3-disubstituted cyclo-
propenes.9,10 Under these circumstances, we envisioned that a
new type of polymerization process composed of arylrhoda-
tion and 1,4-rhodium migration could be realized by the use of
arylrhodium(I) complex undergoes alkene insertion of a 3,3-
diarylcyclopropene to give cyclopropylrhodium(I) intermediate
A. Instead of the direct insertion of another molecule of 3,3-
diarylcyclopropene, intramolecular 1,4-rhodium migration
would lead to arylrhodium(I) species B.11 This then repeats
the alkene insertion (arylrhodation)−1,4-rhodium migration
sequence to give poly(cyclopropylene-o-phenylene) E as the
final product.
In an initial investigation, we employed 3,3-diphenylcyclo-
propene (1a: 1.00 mmol) as the monomer and attempted a
polymerization reaction in the presence of [RhCl(cod)]2 (2
mol % Rh) as a catalyst without any initiator in THF (2.0 mL)
at 60 °C. Under these conditions, only 7% yield of the polymer
was obtained after precipitation from hexane/CH2Cl2 (Table 1,
entry 1). In contrast, by using Ph4BNa (3 mol %) as the
initiator, which is known to generate phenylrhodium(I) species
by the reaction with [RhCl(cod)]2,12 the polymerization
smoothly took place under otherwise the same conditions to
give poly-1a in 83% yield (entry 2). The number-averaged
molecular weight (Mn) was calculated to be 6400 g mol−1 with
PDI of 1.9 using size-exclusion chromatography against a
Received: March 31, 2014
Published: May 21, 2014
© 2014 American Chemical Society
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Table 1. Rhodium-Catalyzed Polymerization of 1a: Effect of Catalysts and Initiators
a
b
b
entry
Rh-catalyst
[RhCl(cod)]2
initiator
yield of poly-1a (%)
m/n
Mn (g mol−1
)
Mw/Mn
c
1
2
3
4
5
6
7
8
none
7
nd
4800
6400
2200
7000
5200
7900
2600
900
1.2
1.9
1.5
2.0
2.5
2.0
1.7
[RhCl(cod)]2
Ph4BNa
83
62
86
78
84
23
4
73/27
58/42
70/30
70/30
78/22
52/48
[RhCl(nbd)]2
Ph4BNa
[Rh(OH)(cod)]2
[Rh(OH)(cod)]2
[Rh(OH)(cod)]2
[Rh(OH)(cod)]2/PPh3
Ph4BNa
PhB(OH)2
3,5-Me2C6H3B(OH)2
3,5-Me2C6H3B(OH)2
3,5-Me2C6H3B(OH)2
d
c
[Rh(OH)(binap)]2
nd
1.1
a
b
c
d
Determined by 1H NMR. Determined using size-exclusion chromatography against a polystyrene standard. nd = not determined. Rh/PPh3 = 1/2.
polystyrene standard, and the degree of 1,4-rhodium migration
Table 2. Rhodium-Catalyzed Polymerization of 1a: Effect of
Concentrations
during the polymerization was deduced to be 73% (m/n =
1
73/27) by its H NMR spectrum (vide infra). The use of
13
[RhCl(nbd)]2 instead of [RhCl(cod)]2 gave poly-1a in a
lower yield, molecular weight, and the degree of 1,4-rhodium
migration (entry 3), whereas the use of [Rh(OH)(cod)]2 gave
almost the same result as that with [RhCl(cod)]2 (entry 4).
The polymerization was similarly well catalyzed by [Rh(OH)-
(cod)]2/PhB(OH)2, which is also known to generate a phenyl-
rhodium(I),14 to give poly-1a in 78% yield with m/n = 70/30
(entry 5). A somewhat better result was obtained by changing
the initiator from PhB(OH)2 to 3,5-Me2C6H3B(OH)2 (84%
yield, m/n = 78/22, Mn 7900 g mol−1; entry 6). In comparison,
the use of rhodium/phosphine catalysts instead of rhodium/
diene catalysts significantly lowered the polymerization efficiency
(entries 7 and 8).
[1a]0
yield of poly-1a
Mn
a
b
b
entry
(M)
(%)
m/n
(g mol−1
)
Mw/Mn
1
2
3
4
0.50
0.17
0.05
0.02
84
91
81
78/22
82/18
87/13
90/10
7900
9000
8700
8000
2.0
1.7
1.6
1.5
83
a
b
Determined by 1H NMR. Determined using size-exclusion
chromatography against a polystyrene standard.
In the present polymerization, the ideal reaction sequence is
a complete alternation of the insertion of cyclopropene 1a and
the successive 1,4-rhodium migration as illustrated in Scheme 1.
Under the conditions described in Table 1, however, the direct
insertion of 1a to a cyclopropylrhodium intermediate competes
with the desired 1,4-rhodium migration (e.g., m/n = 78/22 for
entry 5). As shown in Scheme 2, because the 1,4-rhodium
of 1,4-rhodium migration could be improved by lowering the
initial concentration of 1a as compiled in Table 2, and up to
90% was realized at 0.02 M concentration (1.00 mmol of 1a in
50 mL of THF; entry 4). It is remarkable that the poly-
merization smoothly proceeds at this low monomer concen-
tration to give poly-1a in 83% yield with Mn of 8000 g mol−1.
1
The H NMR spectrum of poly-1a thus obtained is shown in
Figure 1a. A broad peak from 8.5 to 4.7 ppm corresponds to
the proton signals on the aromatic rings and a broad peak from
4.3 to −0.3 ppm corresponds to the proton signals on the
cyclopropane rings.15 The area ratio of these two broad peaks is
calculated to be 3.13/1 (excluding the CHCl3 residual peak),
which can deduce the degree of 1,4-rhodium migration as 90%,
based on the theoretical area ratios of 3/1 for the ideal polymer
(9H for aromatic and 3H for aliphatic) and 5/1 for the
complete direct-insertion polymer (10H for aromatic and 2H
for aliphatic). It is also worth pointing out that the poly-1a
obtained in the present insertion−1,4-rhodium migration
polymerization shows high solubility (>100 mg/mL) at room
temperature in various common organic solvents such as
benzene, toluene, chlorobenzene, THF, DMF, CH2Cl2, and
CHCl3. This is in stark contrast to the poly(cyclopropene)s
obtained by simple addition polymerization under palladium
catalysis, most of which are known to show poor solubility.8,16
To further confirm the involvement of 1,4-rhodium migra-
tion in the present polymerization, we employed 1,2-bisdeuterio-
3,3-diphenylcyclopropene (1a-d2) as the monomer as shown in
eq 1, and poly(1a-d2) was obtained in 82% yield with Mn of
9000 g mol−1. According to the 1H NMR spectrum in Figure 1b,
Scheme 2. 1,4-Rhodium Migration vs Direct Insertion from
a Cyclopropylrhodium Intermediate
migration (A → B) is an intramolecular process whereas the
insertion of 1a (A → C′) is an intermolecular one, we decided
to control these two processes by changing the initial
concentration of 1a. As expected, we found that the degree
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Journal of the American Chemical Society
Communication
rhodium migration becomes somewhat lower (eq 2).17 In
contrast to these para-substituted 3,3-diarylcyclopropenes, 3,3-
bis(3,5-dimethylphenyl)cyclopropene (1d) is not suitable as a
monomer presumably due to the presence of meta-substituents
on the phenyl groups, which are known to retard the 1,4-
rhodium migration toward the ortho-position adjacent to them
(eq 3).3a,i,4c,f This also indirectly supports that 1,4-rhodium
migration is the key process for the present polymerization
reaction.
In summary, we have developed a new mode of metal-
catalyzed polymerization reaction by exploiting the ability of
1,4-rhodium migration of an organorhodium(I) species.
Specifically, it has been demonstrated that 3,3-diarylcyclopro-
penes undergo polymerization through an insertion−1,4-rhodium
migration sequence by using an arylrhodium(I) initiator/catalyst
generated in situ to give poly(cyclopropylene-o-phenylene)s,
which are inaccessible by conventional chain-growth polymer-
ization methods.18 Future studies will be directed toward fur-
ther improvement of this new type of polymerization reaction
including the development of a catalyst system for the control
of tacticity.
Figure 1. (a) 1H NMR spectrum of poly-1a obtained in Table 2, entry
4 (in dry CDCl3). (b) H NMR spectrum of poly(1a-d2) obtained in
eq 1 (in dry CDCl3).
1
the area ratio of the aromatic peak and the aliphatic peak is
9.96/1. If the polymerization takes place without 1,4-rhodium
migration, no peaks should appear in the aliphatic region, and
the polymerization with perfect 1,4-rhodium migration should
provide the area ratio of 9/1. This result therefore supports the
insertion−1,4-rhodium migration sequence for the present
polymerization, and the degree of 1,4-rhodium migration is
calculated to be 91%, which is consistent with the result
obtained using nondeuterated monomer 1a in Table 2, entry 4.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and compound characterization data
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
shintani@chembio.t.u-tokyo.ac.jp
nozaki@chembio.t.u-tokyo.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support has been provided in part by Funding Program for Next
Generation World-Leading Researchers (NEXT Program) of
JSPS, Japan.
We have also applied this polymerization reaction to other
3,3-diarylcyclopropenes. For example, monomers having
substituents at the para-positions of the aryl groups such as
1b (X = Me) and 1c (X = Cl) can also undergo polymerization
efficiently to give poly-1b and poly-1c in high yields with
higher Mn of 27000 g mol−1, although the degree of 1,4-
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