Synthesis of helical π-conjugated polymers bearing pyridine: N -oxide pendants and asymmetric allylation of aldehydes in the helical cavity

Article abstract of DOI:10.1039/c9ob01828j

Catalytically active chiral π-conjugated polymers (poly-1(NO)_r) bearing pyridine N-oxide pendants were synthesized by ternary copolymerization of a d-glucose-bound diethynyl compound with two types of thieno[3,4-b]thiophene comonomer, one of wh

Full text of DOI:10.1039/c9ob01828j

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Synthesis of helical π-conjugated polymers
bearing pyridine N-oxide pendants and
asymmetric allylation of aldehydes in the helical
cavity†
Cite this: DOI: 10.1039/c9ob01828j
Received 19th August 2019,
Accepted 7th September 2019
DOI: 10.1039/c9ob01828j
Tomoyuki Ikai
*
a,b and Takumu Yoshidaa
rsc.li/obc
Catalytically active chiral π-conjugated polymers (poly-1(NO)r)
Our group recently reported a series of chiral π-conjugated
bearing pyridine N-oxide pendants were synthesized by ternary polymers, poly(arylene ethynylene)s, containing an optically
copolymerization of a D-glucose-bound diethynyl compound with active D-glucose-linked biphenyl (GLB) unit in the main
two types of thieno[3,4-b]thiophene comonomer, one of which chain.26,33–35 Poly-1 containing a thieno[3,4-b]thiophene (TT)-
contained a pyridine N-oxide group. When the pyridine N-oxide type comonomer unit is a representative example of these poly-
content in the copolymer was 10 mol% (poly-1(NO)0.10), the mers (Fig. 1).19,27 We also demonstrated that poly-1 undergoes
polymer backbone formed
a one-handed helical structure in a reversible conformational transition between one-handed
acetonitrile. Pyridine N-oxide pendants arranged inside the helical helix and random-coil structures in response to the exterior
cavity of poly-1(NO)0.10 exhibited catalytic activity for the asym- solvent environment, and that poly-1 could be applied as a
metric allylation of benzaldehydes, producing the corresponding chiral stationary phase19 and circularly polarized luminescent
allyl alcohols with up to 43% ee.
material.27 Furthermore, when the poly-1 main chain was
folded into a helical conformation, the pendant groups on the
TT repeating units were shown to be arranged inside the
helical cavity (Fig. 1).
Taking inspiration from exquisitely designed biomacro-
molecules containing helical geometries, such as DNA and
proteins, various artificial helical polymers have been devel-
oped to date1–7 and applied to chiral functional materials in
various fields, such as chiral sensing,8–11 chromatographic
resolution12–20 and circularly polarized luminescence.21–27 The
application of helical polymers to asymmetric catalysts has
also attracted growing interest owing to the potential for
characteristic catalytic activity, which is achieved through the
one-handed helical arrangement of catalytically active units
along the polymer backbone.7 Thus far, various helical
polymer-based asymmetric catalysts have been developed by
embedding catalytically active pendants in artificial
helices.7,28–30 However, in most cases, catalytically active sites
were located at the periphery of helical structures, with helical
polymer-based asymmetric catalysis in the helical cavity
achieved only in a few cases.31,32
Considering the unique framework and chiral functions of
GLB-based polymers, we anticipated that a novel helical
polymer-based asymmetric catalyst, in which catalytically
active units are arranged inside the helical cavity, could be
developed through appropriate modification of the pendant
groups in GLB-based polymers. In this study, novel GLB-based
polymers (poly-1(NO)r in Scheme 1) bearing pyridine N-oxide
groups as catalytically active sites on the TT units were
designed and synthesized, and the helix-forming ability and
asymmetric catalytic activity in the allylation of benzaldehydes
were investigated (Fig. 2).
aGraduate School of Natural Science and Technology, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan
bDepartment of Molecular and Macromolecular Chemistry, Graduate School of
Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: ikai@chembio.nagoya-u.ac.jp
†Electronic supplementary information (ESI) available: Detailed experimental
procedures, characterization of molecules/polymers and additional spectro-
scopic data. See DOI: 10.1039/c9ob01828j
Fig. 1 Structure of poly-1.
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Fig. 3 CD and absorption spectra of poly-1(NO)0.10 in dichloro-
methane–acetonitrile (10 : 0–0 : 10, v/v) at 0 °C, [glucose unit] = 1.0 ×
10−4 M.
be almost identical to the feed ratios of these two TT comono-
mers by H NMR and elemental analyses. The resulting poly-
Scheme 1 Synthesis of GLB-based polymers bearing pyridine N-oxide
groups (poly-1(NO)r).
1
1(NO)0.10 showed good solubility in organic solvents, such as
THF, chloroform and dichloromethane, and was used in the
subsequent study accordingly.
Absorption and circular dichroism (CD) spectra of poly-
1(NO)0.10 recorded in dichloromethane/acetonitrile (10 : 0–0 : 10,
v/v) at 0 °C are shown in Fig. 3. In acetonitrile, poly-1(NO)0.10
exhibited an intense CD signal in the absorption region of the
π-conjugated backbone (400–520 nm), with the CD spectral
patterns and intensities closely resembling those previously
reported for poly-1 in the helically folded state (ESI Fig. S1†).19
This result indicated that poly-1(NO)0.10 could form a one-
handed helical conformation in acetonitrile. The spectral simi-
larity between poly-1(NO)0.10 and poly-1 in acetonitrile also
suggested that the pyridine N-oxide group introduced on the
TT unit did not cause a noticeable change in the helical
folding ability of GLB-based polymers. As observed for poly-1,
the TT-bound ester pendants, including pyridine N-oxide
units, were considered to be located inside the helical cavity.
In contrast, when the chiroptical properties of poly-1(NO)0.10
were measured in dichloromethane or dichloromethane/aceto-
nitrile mixtures, its CD intensities significantly decreased with
increasing dichloromethane content, accompanied by hyper-
chromic and hypsochromic shifts in the absorption spectra.
Given the spectral pattern of poly-1 with a random-coil confor-
mation (ESI Fig. S1†),19 the poly-1(NO)0.10 backbone did not
form a specific secondary structure in dichloromethane.
Poly-1(NO)0.10 was then applied to an organocatalyst for the
asymmetric addition of allyltrichlorosilane to benzaldehydes
Fig. 2 Schematic illustration of helical polymer-based asymmetric cat-
alysis occurring in the helical cavity.
A novel TT monomer bearing a catalytically active pyridine
N-oxide group (2) was prepared from a commercially available
phenol derivative and 4,6-diiodothieno[3,4-b]thiophene-2-car-
boxylic acid through a condensation reaction using activating
agents (ESI Scheme S1†). The Sonogashira–Hagihara coupling
copolymerization of GLB-1 and 2 was first conducted using a
palladium catalyst in a tetrahydrofuran (THF)–diisopropyl-
amine mixture (5 : 1, v/v) at 60 °C, similar to our previously
reported procedure.19 However, as the resulting polymer
showed very low solubility in common organic solvents, such
as THF, chloroform and dichloromethane, further characteriz-
ation could not be performed.
To ensure polymer solubility, the ternary copolymerization
of GLB-1, 2 and 3 was performed as described above to afford
poly-1(NO)r, where r is the molar fraction of 2 in the monomer
feed (Scheme 1). The ternary copolymerization performed with
[GLB-1]0/[2]0/[3]0
= 1.0 : 0.50 : 0.50 still proceeded hetero-
geneously, affording a certain amount of an insoluble precipi-
tate in the reaction system. When the feed mole fraction of 2
was decreased to 10% ([GLB-1]0/[2]0/[3]0 = 1.0 : 0.10 : 0.90), the
copolymerization proceeded homogeneously, affording opti-
cally active poly-1(NO)0.10 with a number-average molecular
mass (Mn) of 1.3 × 104 g mol−1, estimated by size-exclusion
chromatography (SEC) (ESI Table S1†). The compositional
Scheme 2 Allylation reactions of benzaldehydes (4) with allyltrichloro-
ratio of the 2 and 3 units in the copolymer was confirmed to silane catalyzed by poly-1(NO)0.10 and 2’.
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Table 1 Enantioselective allylation of benzaldehydes (4) with allyltrichlorosilanea
Entry
Catalyst
Temperature (°C)
Solvent
Substrate
Product
Yield (%)
eeb (%)
1
2
3
4
5
6
7
2′
0
0
−40
0
−40
−40
−40
Acetonitrile
Acetonitrile
Acetonitrile
Dichloromethane
Dichloromethane
Acetonitrile
4a
4a
4a
4a
4a
4b
4c
5a
5a
5a
5a
5a
5b
5c
25
25
13
13
8
20
15
—
Poly-1(NO)0.10
Poly-1(NO)0.10
Poly-1(NO)0.10
Poly-1(NO)0.10
Poly-1(NO)0.10
Poly-1(NO)0.10
26 (R)
43 (R)
5 (R)
6 (R)
39 (R)
31 (R)
Acetonitrile
a The reactions of 4 (0.5 M) with allyltrichlorosilane (1.2 equiv.) were carried out in the presence of a catalyst (1 mol%) and iPr2NEt (5.0 equiv.) in
acetonitrile or dichloromethane. Reaction time = 48 h. b Determined by chiral HPLC. In parentheses are shown the absolute configuration of the
major enantiomer assigned by the retention times and optical rotation signs reported in the literature [ref. 37].
(4) (Scheme 2).36 Table 1 summarizes the results of allylation TT-bound pyridine N-oxide pendants located inside the helical
reactions of benzaldehydes using the poly-1(NO)0.10-based cavity. Furthermore, poly-1(NO)0.10 was shown to function as a
organocatalyst. The corresponding result using the TT-based chiral organocatalyst in the asymmetric allylation of benzal-
model compound (2′; see Scheme 2) as catalyst is shown for dehydes, affording the corresponding allyl alcohols with up to
comparison. Poly-1(NO)0.10 catalyzed the allylation reaction of 43% ee. We anticipate that more powerful GLB-based asym-
4a in acetonitrile at 0 °C and gave the corresponding allyl metric organocatalysts with much higher catalytic activities and
alcohol (5a) in 25% yield after 48 h (entry 2), which was com- enantioselectivities can be developed through the delicate
parable to the result obtained using 2′ (25%, entry 1). This design of catalytically active pendant units. Related studies
indicated that the pyridine N-oxide pendants in poly-1(NO)0.10 towards these goals are currently underway in our laboratory.
caused little deterioration in the catalytic activity compared
with that of 2′, despite the catalytically active units in poly-
1(NO)0.10 being arranged in a closed environment inside the
helical cavity. Importantly, poly-1(NO)0.10 showed enantio-
Conflicts of interest
There are no conflicts to declare.
selectivity in the asymmetric allylation reaction of 4a in aceto-
nitrile at 0 °C, affording (R)-5a with 26% enantiomeric excess
(ee), as estimated by chiral HPLC (entry 2). When the reaction
temperature was decreased to −40 °C (entry 3), the enantio-
selectivity improved to 43% ee. In contrast, when the allylation
reactions were performed in dichloromethane at 0 and −40 °C
in the presence of poly-1(NO)0.10, the ee values of the product
clearly decreased compared with the corresponding results in
acetonitrile, while maintaining the absolute configuration of
the major enantiomer (R-rich) (entries 4 and 5). These results
indicated that the enantioselectivity obtained from the poly-
1(NO)0.10-based catalyst in acetonitrile was mainly attributed to
the special arrangement of the catalytically active pendants
inside the helical cavity. Poly-1(NO)0.10 also showed moderate
enantioselectivities in the allylation reactions using substituted
benzaldehydes 4b and 4c as substrates, affording (R)-5b with
39% ee and (R)-5c with 31% ee, respectively (entries 6 and 7).
Acknowledgements
This work was supported by the Japan Society for the
Promotion of Science (JSPS) KAKENHI Grant No. 17K05875
(Grant-in-Aid for Scientific Research (C)) and 18H05209
(Grant-in-Aid for Specially Promoted Research). This work was
also supported by the Shorai Foundation for Science and
Technology.
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