P-chiral phosphine-sulfonate/palladium-catalyzed asymmetric copolymerization of vinyl acetate with carbon monoxide

Article abstract of DOI:10.1021/ja3044344

Utilization of palladium catalysts bearing a P-chiral phosphine-sulfonate ligand enabled asymmetric copolymerization of vinyl acetate with carbon monoxide. The obtained γ-polyketones have head-to-tail and isotactic polymer structures. The origin of the regio- and stereoregularities was elucidated by stoichiometric reactions of acylpalladium complexes with vinyl acetate. The present report for the first time demonstrates successful asymmetric coordination-insertion (co)polymerization of vinyl acetate.

Full text of DOI:10.1021/ja3044344

Communication
pubs.acs.org/JACS
P‑Chiral Phosphine−Sulfonate/Palladium-Catalyzed Asymmetric
Copolymerization of Vinyl Acetate with Carbon Monoxide
Akifumi Nakamura, Takeharu Kageyama, Hiroki Goto, Brad P. Carrow, Shingo Ito, 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
was achieved using nonpolar olefins such as propylene and
vinylarenes.1,4,5 Until now, however, there have been no reports
ABSTRACT: Utilization of palladium catalysts bearing a
P-chiral phosphine−sulfonate ligand enabled asymmetric
copolymerization of vinyl acetate with carbon monoxide.
The obtained γ-polyketones have head-to-tail and isotactic
polymer structures. The origin of the regio- and
stereoregularities was elucidated by stoichiometric reac-
tions of acylpalladium complexes with vinyl acetate. The
present report for the first time demonstrates successful
asymmetric coordination−insertion (co)polymerization of
vinyl acetate.
of the regulation of tacticity and enantioselectivity by
coordination−insertion polymerization of fundamental polar
vinyl monomers such as vinyl acetate and methyl acrylate with
carbon monoxide.6,7 The introduction of polar functional
groups into polyketones in a highly controlled manner has been
a significant challenge, since it would drastically alter the
polymer properties and expand the range of applications. In
2007, we reported the first example of copolymerization of
vinyl acetate and carbon monoxide using palladium/phos-
phine−sulfonate catalysts, but the resulting copolymers showed
low regio- and stereoregularities.8,9 Here we report the
synthesis of the first optically active P-chiral phosphine−
sulfonate ligands10 and their application to palladium-catalyzed
asymmetric copolymerization of vinyl acetate with carbon
monoxide.
A series of P-chiral ligand precursors [1b]H−[1d]H were
prepared according to literature procedures.10a,c,11 The
enantiomerically pure ligands 1b and 1d were prepared by
HPLC separation of the corresponding racemates with a chiral
stationary phase (CHIRALPAK IC). Copolymerization of vinyl
acetate with carbon monoxide was performed in the presence
of a mixture of Pd2(dba)3·CHCl3 and [1]H (Table 1). The use
of achiral ligand 1a resulted in the formation of polyketone A
with 71% head-to-tail selectivity (entry 1). The regioselectivity
was estimated by the integration ratio of the ketone carbonyl
region in 13C NMR spectra (Figure 2), assuming that the broad
signal at 201.5 ppm corresponds to either head-to-head or tail-
to-tail structure and that multiple signals from 202−204 ppm
corresponds to all the other regioisomeric structures.12,13 In
entries 2−4, three types of racemic P-chiral ligands 1b−1d were
employed for the copolymerization. In entries 2 and 3, the
copolymerization produced polyspiroketal B as a major product
instead of polyketone A.3 In contrast, higher chemo-, regio- and
stereoselectivities were obtained when using ligand 1d, bearing
phenyl and 2′,6′-dimethoxy(1,1′-biphenyl)-2-yl groups on the
phosphorus atom (entry 4), and the ratio of head-to-tail
structure was improved to 84%. Furthermore, the resonance at
203.5 ppm became predominant, which suggests higher regio-
and stereoregularities.
ynthesis and application of optically active polymers has
Sbeen an important subject in polymer science because their
chiral nature is essential for many well-defined functions and
properties, as exemplified by polypeptide and nucleic acid
chemistry.1,2 Among various artificial polymers, γ-polyketones
obtained by the alternating copolymerization of monosub-
stituted ethenes with carbon monoxide (CO) are endowed with
various structural regularities (Figure 1): head-to-tail regior-
egularity, syndiotactic or isotactic stereoregularity (tacticity),
and in the isotactic structure, enantioselectivity to control the
absolute stereochemistry.3 During the last few decades,
asymmetric monosubstituted ethenes/CO copolymerization
Encouraged by the result in entry 4, asymmetric copoly-
merization was examined with enantiomerically pure ligand (S)-
(−)-1d (entry 5). Unexpectedly, the regio- and stereo-
Figure 1. Possible structural regularities in monosubstituted ethene/
carbon monoxide copolymers.
Received: May 8, 2012
Published: July 23, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
a
Table 1. Copolymerization of Polar Vinyl Monomers with Carbon Monoxide
b
b
c
c
entry
ligand
1a
FG
OAc
yield (mg)
TOF (h−1
)
Mn (×103)
Mw/Mn
A or B
h-to-t (%)
1
2
3
4
5
6
7
320
150
500
230
170
106
101
14
65
14
44
25
20
10
12
1.6
1.5
1.5
1.2
1.8
1.2
1.1
A
71
(rac)-1b
(rac)-1c
(rac)-1d
(S)-1d
OAc
OAc
OAc
OAc
CO2Me
Ph
6.6
22
B
1:5 (A:B)
10
A
A
A
A
84
90
7.4
4.7
3.8
(S)-1d
>99
>99
(S)-1d
a
Conditions: Pd2(dba)3·CHCl3 (5 μmol), ligand 1 (12 μmol), and vinyl monomer (2.5 mL) were stirred under carbon monoxide pressure (6.0
b
c
MPa) for 20 h at 70 °C in a 50-mL autoclave. Determined by SEC analysis using polystyrene standards. Determined by NMR analysis.
Figure 2. Ketone regions in the 13C NMR spectra of the polyketones
obtained in entries 1, 4, and 5 in Table 1 (CDCl3).
regularities were improved compared with the results using
(rac)-1d (Figure 2).14 The solubility of the resulting polymer in
Figure 3. Asymmetric copolymerization of vinyl acetate with carbon
entry 5 was lower than that of entry 4 in CDCl3, indicating
different higher-order structures.15,16 Although the reason for
the different regio- and stereoregularities between entry 4 and 5
is still unclear, chain transfer between the two enantiomeric
palladium complexes may be operating during the polymer-
ization with rac-1d so that one chain contains segments formed
by each enantiomer of the catalyst.17
monoxide using optically active (S)-(−)-1d/Pd2(dba)3·CHCl3. Optical
rotation values of analogous model compounds are also shown for
comparison.19
uration of the asymmetric center in the polymer main chain was
inferred from the results of the reaction shown in Scheme 1
(vide infra).18 These data represent the first example of the
synthesis of an optically active γ-polyketone having ester
substituents directly attached to the main chain. Although the
absolute value of the specific optical rotation is not high, it does
The resulting vinyl acetate/carbon monoxide copolymer in
entry 5 exhibited moderate optical rotation ([Φ]D22 = −8.5, c =
0.32, CHCl3), which may correspond to regions of enantio-
enriched isotactic structure (Figure 3). The absolute config-
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Journal of the American Chemical Society
Communication
a
metal square plane as the 2′,6′-dimethoxy(1,1′-biphenyl)-2-yl
moiety of the phosphorus atom.
Scheme 1. Synthesis and Molecular Structure of 6d
The relative configuration in 6d and asymmetric copoly-
merization can be rationalized by the reaction sequence
outlined in Scheme 2. According to density functional theory
Scheme 2. Possible Explanation for the Enantioface
Selection in Asymmetric Copolymerization of Vinyl Acetate
a
with Carbon Monoxide
a
Hydrogen atoms and solvent molecules are omitted for clarity. Ar =
2′,6′-dimethoxy(1,1′-biphenyl)-2-yl.
not necessarily indicate low selectivity considering that reported
model compounds similar to the repeating unit of the
copolymer, such (S)-2, also exhibit moderate values (Figure
3).19a
Copolymerization reactions of other monosubstituted
ethenes with carbon monoxide were also investigated. The
use of methyl acrylate as a comonomer produced a regio-
controlled alternating copolymer, as confirmed by the
observation of a single resonance in the ketone carbonyl
region of the 13C NMR spectrum (entry 6).9 The copolymer
a
Ar = 2′,6′-dimethoxy(1,1′-biphenyl)-2-yl; poly = polymer chain.
26
exhibited an optical rotation value [Φ]D = +4.4 (c = 0.32,
CHCl3),20 which is consistent with a small optical rotation
value of analogous molecule (R)-319b as shown in Figure 3.
However, epimerization of the asymmetric center may have
occurred, since our previous studies revealed that the methine
proton in the methyl acrylate/CO copolymer was exchangeable
with deuterium by the treatment with MeOD.9b In contrast, the
styrene/CO copolymer obtained in entry 7 exhibited a large
calculation, vinyl acetate insertion proceeds via coordination of
vinyl acetate at the position cis to the phosphine moiety
followed by migration of the acetyl group.22 The relative
configuration in 6d can be explained by coordination of vinyl
acetate to the metal such that the acetoxy group is located on
the same side of the square plane as the smaller phenyl group
on the phosphorus atom to avoid steric repulsion with the
bulky 2′,6′-dimethoxy(1,1′-biphenyl)-2-yl group (TSI−II in
Scheme 2). If this stereoselective insertion of vinyl acetate
operates in the copolymerization reaction of entry 5 of Table 1,
R-configuration of the stereocenter would be generated using
(S)-1d, giving an isotactic-rich copolymer with R-configuration
in the main chain.
optical rotation value [Φ]D = +463 (c = 0.053, CHCl3),20
24
which is one of the highest values reported for styrene/CO
copolymers.4,5 These results indicate that the introduction of P-
chirogenic centers into phosphine−sulfonate ligands is an
effective strategy for catalyst-controlled asymmetric polymer-
ization.11
In summary, we have developed asymmetric copolymeriza-
tion of vinyl acetate, methyl acrylate, or styrene with carbon
monoxide using palladium complexes ligated by P-chiral
phosphine−sulfonates to synthesize optically active polyke-
tones. The utilization of P-chiral phosphine−sulfonate ligands
enables the microstructure regulation of coordination−
insertion polymerization of polar vinyl monomers. Further
improvement of the selectivity in this copolymerization and
application of these chiral palladium/phosphine−sulfonate
catalysts to other reactions will be forthcoming.
The origin of regio- and stereoregularities in the copoly-
merization of vinyl acetate with carbon monoxide was
investigated by the stoichiometric reaction of palladium
complexes bearing chiral ligand (rac)-1d with vinyl acetate
(Scheme 1). First, acetylpalladium complex 5d bearing ligand
1d was synthesized by the reaction of phosphonium−sulfonate
[1d]H with PdMeCl(cod) in the presence of Hunig’s base
̈
followed by treatment with carbon monoxide. Next, reaction of
5d with an excess amount of vinyl acetate in the presence of
AgOTf to afforded 2,1-insertion complex 6d in 83% yield.
NMR and X-ray analyses revealed that the major isomer arises
from 2,1-insetion of vinyl acetate, and the ester carbonyl
coordinates to the palladium center preferentially over the
ketone carbonyl. The highly selective 2,1-insertion in this
reaction contrasts the corresponding reaction with ligand 1a
where both 2,1- and 1,2-insertion products were detected.21 In
complex 6d, the acetylmethyl group lies on the same side of the
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, supplementary experiments, NMR
spectra, and crystal data for (rac)-[1b]H, (S)-(−)-[1d]H, and
6d. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Journal of the American Chemical Society
Communication
F. Dalton Trans. 2011, 40, 8304−8313. (e) Piche, L.; Daigle, J.-C.;
Rehse, G.; Claverie, J. P. Chem.Eur. J. 2012, 18, 3277−3285.
(11) We have examined other achiral and chiral phosphine−sulfonate
ligands. See Supporting Information for details.
(12) See Supporting Information for the method to estimate head-to-
tail regioregularity.
AUTHOR INFORMATION
Corresponding Author
nozaki@chembio.t.u-tokyo.ac.jp
Notes
■
The authors declare no competing financial interest.
(13) For 13C NMR spectra in the region of the carbonyl group of
propylene/CO copolymers, see: (a) Barsacchi, M.; Batistini, A.;
Consiglio, G.; Suter, U. W. Macromolecules 1992, 25, 3604−3606.
(b) Jiang, Z.; Sen, A. J. Am. Chem. Soc. 1995, 117, 4455−4467.
(c) Gambs, C.; Chaloupka, S.; Consiglio, G.; Togni, A. Angew. Chem.,
Int. Ed. 2000, 39, 2486−2488. For styrene/CO copolymers, see:
(d) Bronco, S.; Consiglio, G. Macromol. Chem. Phys. 1996, 197, 355−
365.
ACKNOWLEDGMENTS
■
This work was supported by Funding Program for Next
Generation World-Leading Researchers, Green Innovation and
the Global COE Program “Chemistry Innovation through
Cooperation of Science and Engineering” from Japan Society
for the Promotion of Science (JSPS), Japan. A.N. is grateful to
the JSPS for a Research Fellowship for Young Scientists.
(14) In isotactic propylene/CO copolymers in ref 13, the half width
of the sharpest ketone signal in the 13C NMR spectra can be estimated
to be within 2−6 Hz. Since the width of the sharpest ketone signal of
the vinyl acetate/CO copolymer obtained in entry 5 was found to be 5
Hz, we concluded that the stereoregularity of the vinyl acetate/CO
copolymer was controlled to be isotactic.
(15) Differential scanning calorimetry (DSC) analysis revealed that
the copolymers obtained using (S)-1d showed glass transition
temperatures (Tgs) almost identical to those obtained using achiral
1a. See Supporting Information for details.
(16) The VAc/CO copolymer obtained using (S)-1d more easily
isomerizes to form polyspiroketal structure B than that obtained using
1a, also suggesting the presence of regio- and stereoregular structures.
(17) Rix, F. C.; Rachita, M. J.; Wagner, M. I.; Brookhart, M.; Milani,
B.; Barborak, J. C. Dalton Trans. 2009, 8977−8992.
(18) According to the proposed mechanism (discussed later), the
vinyl acetate/CO copolymer, which exhibited a negative optical
rotation value, would possess R-configuration in the main chain. This
is not consistent with the fact that model compound (R)-2 shows a
positive optical rotation value. This reverse rotation may be attributed
to the presence of higher order structures in the vinyl acetate/CO
polyketone.
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̈
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