Received: September 30, 2014 | Accepted: October 7, 2014 | Web Released: October 15, 2014
CL-140909
Facile Preparation of Poly(quinoxaline-2,3-diyl)s via Aromatizing Polymerization
of 1,2-Diisocyanobenzenes Using Phosphine Complexes of Nickel(II) Salts
1
1
1,2
Yuuya Nagata, Yuan-Zhen Ke, and Michinori Suginome*
1
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto 606-8501
CREST, Japan Science and Technology Agency (JST), Katsura, Nishikyo-ku, Kyoto 615-8510
2
(
E-mail: suginome@sbchem.kyoto-u.ac.jp)
Polymerizations of 1,2-diisocyanobenzenes to afford poly-
quinoxaline-2,3-diyl)s were investigated in the presence of
Table 1. Polymerization of 1 in the presence of metal
acetylacetonates
(
various metal salts and their phosphine complexes, whereby
especially the stable and easily-accessible [NiCl2(PMe3)2] was
found to exhibit a high polymerization activity. The end groups
of the resulting polymers were determined by mass spectrometry
NC
NC
N
N
O
O
Initiator (1 mol%)
THF, RT
O
O
n
1
P1
+
measurements as P Me3 and H, indicative of a nucleophilic
attack of PMe3 towards an isocyano group as the initial step of
the polymerization. A chiral polymer ligand, prepared by such
a [NiCl2(PMe3)2]-mediated polymerization, demonstrated high
enantioselectivities in palladium-catalyzed asymmetric hydro-
silylation reactions, suggesting that the polymeric catalyst
adopted a single-handed helical backbone.
Proportion of the polymer region
(Mn > 2000, %)
Entry
Initiator
3 h
6 h
12 h
24 h
1
2
3
4
5
6
[Ni(acac)2]
[Co(acac)2]
[Pd(acac)2]
[Fe(acac)3]
[Cu(acac)2]
[Zn(acac)2]
43.0
16.1
9.7
51.5
28.5
12.9
3.2
61.5
32.1
13.4
2.9
77.5
34.3
13.2
2.4
3.1
2.7
0.3
0.3
0.1
In recent years, helical polymers have received much
5.2
3.1
1.3
5.1
1
attention, due to their attractive macromolecular functions
2
3
including chiral separation, asymmetric catalysis, and choles-
4
teric materials. Particular efforts have been devoted to the
Firstly, 1,2-diisocyano-3,6-dimethyl-4,5-bis(propoxymeth-
yl)benzene (1) was polymerized in the presence of various
metal acetylacetonates (1 mol %; Table 1). These complexes
were selected according to their performance in a study by Nolte
and Drenth, who investigated the polymerization of monoiso-
development of facile methods for the preparation of helical
polymers and their subsequent practical applications.
We have developed an aromatizing polymerization route for
5
1,2-diisocyanobenzenes affording poly(quinoxaline-2,3-diyl)s
9
with a rigid helical structure. Recently, we reported that a
single-handed screw-sense can be induced in these polymers by
introducing chiral side chains, and that the thus obtained helical
polymers exhibit almost absolute switches of their helical
cyanides. The polymerizations were monitored with a size-
exclusion chromatograph (SEC) equipped with a UV detector
(254 nm), calibrated with polystyrene standards. In the follow-
ing, the region of SEC traces, where Mn > 2000 will be referred
to as “polymer region,” whereas Mn < 2000 will be referred to
as “low-molecular-weight region” (including monomers, oligo-
mers, and initiators). Although the ratio of the integrated area
of the “polymer region” and the “low-molecular-weight region”
is not exactly equal to the molar ratio of the polymers and
smaller molecules, it is possible to use this ratio as a qualitative
measure for the polymerization activity of the metal complexes.
For example, polymerization of 1 in the presence of 1 mol %
[Ni(acac)2] afforded polymer P1, and although the reaction rate
was slower than that of the previous system using [o-
TolNiCl(PMe ) ], the proportion of the polymer region reached
6
chirality through solvent effects. We also reported that single-
handed helical poly(quinoxaline-2,3-diyl)s with diarylphosphino
7
pendants can be used as highly effective chiral ligands in
asymmetric catalysis. These chiral polymer ligands also showed
solvent-dependent switches of their helical chirality, which
enabled the highly selective formation of either product
enantiomer from a single chiral catalyst.8
These functional helical poly(quinoxaline-2,3-diyl)s are
usually prepared via living polymerizations of 1,2-diiso-
5a
cyanobenzenes catalyzed by organopalladium(II) or organo-
5
b,5d
nickel(II)
complexes. While these organometallic complexes
3
2
grant highly efficient living polymerization systems, affording
products with narrow polydispersity indexes (PDIs) and control
over the degree of polymerization, they are not commercially
available and their preparation is usually laborious, including
e.g. separation by silica gel chromatography under an inert gas
atmosphere. For further investigations on poly(quinoxaline-2,3-
diyl)s, it seemed both pertinent and timely to develop a more
convenient synthetic route. In this paper, we report the synthesis
of poly(quinoxaline-2,3-diyl)s from 1,2-diisocyanobenzenes in
the presence of simple and stable transition metal salts and their
phosphine complexes, respectively.
77.5% after 24 h. Similar polymers were obtained using
[Co(acac)2] and [Pd(acac)2], even though their polymerization
activity was low (Table 1, Entries 2 and 3). No polymerization
activity towards monomer 1 was observed for [Fe(acac)3],
[Cu(acac)2], or [Zn(acac)2] (Table 1, Entries 46). These
observations are in good agreement with results obtained from
the polymerizations of monoisocyanides in the presence of
various metal acetylacetonates. As [Ni(acac)2] exhibited the
best performance for the polymerization of 1,2-diisocyanoben-
zenes among all the tested candidates, we subsequently focused
our attention on various other nickel(II) complexes.
9
a
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