Pd-catalyzed ring-opening copolymerization of 2-aryl-1-methylenecyclopropanes with CO to afford polyketones via alternating insertion of the two monomers and C - C bond activation of the three-membered ring

Article abstract of DOI:10.1021/ja017460s

Pd-bpy complexes catalyze the ring-opening copolymerization of 2-aryl-1-methylenecyclopropanes with CO, affording the new polyketones composed of ring-opened structural units only. Kinetic and isotope-labeling studies revealed the mechanism of the reactio

Full text of DOI:10.1021/ja017460s

Published on Web 01/08/2002
Pd-Catalyzed Ring-Opening Copolymerization of
2-Aryl-1-methylenecyclopropanes with CO to Afford Polyketones via
Alternating Insertion of the Two Monomers and C-C Bond Activation of
the Three-Membered Ring
Sunwook Kim, Daisuke Takeuchi, and Kohtaro Osakada*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama, 226-8503, Japan
Received November 2, 2001; Revised Manuscript Received December 7, 2001
Palladium-complex-catalyzed copolymerization of alkene and CO
forms various polyketones via alternating copolymerization of the
two monomers.1,2 The reactions of strained cyclic olefins with CO,
which involve ring opening of the monomer, are thermodynamically
favorable and are expected to produce the polymers with novel
structures. Only a few reports of such polymerization, however,
have appeared in the literature.3 Copolymerization of methylene-
cyclopropane with CO catalyzed by a Pd-phosphine complex was
recently shown to give the polyketone that contains a structural
unit formed via ring-opening copolymerization as a minor com-
ponent.4 2-Aryl-1-methylenecyclopropanes were found to undergo
homopolymerization catalyzed by Pd-diimine complexes and form
the structurally regulated hydrocarbon polymers.5 In this contribu-
tion, we report ring-opening copolymerization of the cyclic
monomer with CO, affording the polyketones exclusively composed
of ring-opened structural units.
Figure 1. 13C{1H} NMR spectra of (i) Ia and (ii) Ia-13C in CDCl3 at 25
°C. The copolymerization of 2-phenyl-1-methylenecyclopropane (1a, 1a-
13C) with CO was carried out in the presence of PdCl(Me)(bpy) ([Pd] )
25 mM, [1a]/[Pd] ) 100) at 1 atm in CH3CN at room temperature. The
peaks with asterisks are due to the solvent.
of the polymer between the copolymerization and homopolymer-
ization. The copolymerization of 2-phenyl-1-methylenecyclopro-
pane-3-13C (1a-13C) with CO produces Ia-13C whose 13C{1H} NMR
spectrum is shown in Figure 1(ii). The signals at δ 35.1-36.2 and
2-Phenyl-1-methylenecyclopropane (1a) reacts with CO (1 atm)
in the presence of the catalyst, prepared in situ from PdCl(Me)-
(bpy) and NaBARF (BARF ) [B{C6H3(CF3)2-3,5}4]-), to produce
polyketone Ia (Mn ) 19 000, Mw/Mn ) 1.44 by GPC, polystyrene
standards) in 84% yield at room temperature (eq 1). NMR
spectroscopy including DEPT and 13C-1H COSY technique
revealed the ring-opened structure of all the monomer units in the
product. The polyketone contains the isomeric structural units A
and B, both via ring-opening copolymerization, in 62:38 ratio.
Figure 1(i) depicts the 13C{1H} NMR spectrum of Ia, showing the
dCH2 carbon signals at δ 123.5 and 124.9. The signals at δ 41.6-
43.1 and 40.6-41.6 are assigned to the CH2 and CHPh carbons of
structural unit A in eq 1, respectively, on the basis of comparison
of the positions with those of model compounds.6 The peaks at δ
35.2-36.1 and 50.2-51.8 correspond to the structural unit B.
The structural unit of Ia contains a vinylidene group, which is
similar to that of the homopolymer obtained by the Pd-diimine
complex-catalyzed polymerization of the monomer at elevated
temperature.5 Isotope-labeled experiments, however, indicated a
different insertion mode of the monomer and different end structure
41.0-43.3 correspond to the 13C-enriched CH2 carbons in the
polymer chain. In contrast, the homopolymerization of the same-
labeled monomer introduces the enriched 13C only at the vinylidene
carbon of the polymer.
The above-mentioned labeled position of polyketone Ia-13C
indicates cleavage of a proximal C-C bond during the polymer-
ization, as shown in Scheme 1. Insertion of CO into the Pd-alkyl
bond of the growing polymer gives the acylpalladium intermediate.
1,2-Insertion of the methylenecyclopropane into the Pd-acyl bond
forms a â-cyclopropylidenealkyl palladium intermediate that un-
dergoes rapid â-alkyl elimination,7 resulting in the formation of a
Pd complex having an R- or â-phenyl alkyl ligand. Repetition of
the above procedures accounts for the alternating copolymerization
to produce the polyketone. The activation of C-CH2 and C-CHPh
bonds, promoted by Pd, forms the two repeating units A and B,
respectively. The existence of units A and B in the polymer chain
is consistent with 1,2-insertion of 2-phenyl-1-methylenecyclopro-
pane. Copolymerization of R-olefins with CO also proceeds mostly
via 1,2-insertion of the R-olefins into the Pd-acyl bond.1,2c It
contrasts with the mechanism of the homopolymerization of 2-aryl-
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762 VOL. 124, NO. 5, 2002 J. AM. CHEM. SOC.
10.1021/ja017460s CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
also promoted the copolymerization to give the copolymer with a
high molecular weight (Mn ) 450 000) in a lower yield (run 5). It
is attributed to a much slower insertion of CO into the Pd-π-allyl
bond than that into the Pd-Me bond.11 2-Phenyl-1-methylene-
cyclopropanes having Me or F substituents on the phenyl ring also
undergo copolymerization to give the corresponding polyketones
(runs 6, 7).
In summary, we found the Pd-complex-catalyzed ring-opening
copolymerization of 2-aryl-1-methylenecyclopropanes with CO to
afford the new polymers. The polymer growth, including alternating
insertion of the monomers and C-C bond activation of the three-
membered ring of the polymer, is well-regulated by the Pd complex.
Table 1. Copolymerization of 2-Aryl-1-methylenecyclopropanes
Acknowledgment. The present work was partly supported by
Sumitomo Foundation.
with CO Catalyzed by Pd Complexesa
time
(h)
yield
(%)
Supporting Information Available: Experimental procedures for
polymerization and IR and NMR data of the products (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
runst
catalyst
M (M /M )b
n w n
1c
2c
3c
4
PdCl(Me)(bpy) + NaBARF
PdCl(Me)(bpy) + NaBF4
PdCl(Me)(bpy) + AgOTf
PdCl(Me)(bpy)
[(π-PhCHCHCH2)Pd(bpy)]BF4
PdCl(Me)(bpy) + AgOTf
PdCl(Me)(bpy) + AgOTf
3
3
3
66
15
3
84
85
92
60
37
87
77
19000 (1.44)
20000 (2.17)
23000 (3.19)
17000 (1.72)
450000 (2.40)
28000 (4.57)
34000 (3.65)
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