Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s Bearing Pyridine N-Oxide Groups and Their Use as Asymmetric Organocatalysts

Article abstract of DOI:10.1002/pola.29501

Novel poly(biphenylylacetylene) derivatives carrying different types of pyridine N-oxide units with a bulky or less-bulky substituent at a different position as the functional pendant groups (poly-2a and poly-2b) were synthesized by the rhodium-catalyzed

Full text of DOI:10.1002/pola.29501

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Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s
Bearing Pyridine N-Oxide Groups and Their Use as Asymmetric
Organocatalysts
1,2
Mitsuka Ando,1 Ryoma Ishidate,2 Tomoyuki Ikai ,1 Katsuhiro Maeda
,
3,4 Eiji Yashima
1Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya
464-8603, Japan
2Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya
464-8603, Japan
3Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
4Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
Correspondence to: E. Yashima (E-mail: yashima@chembio.nagoya-u.ac.jp)
Received 31 July 2019; Revised 31 August 2019; accepted 5 September 2019
DOI: 10.1002/pola.29501
ABSTRACT: Novel poly(biphenylylacetylene) derivatives carrying
different types of pyridine N-oxide units with a bulky or less-bulky
substituent at a different position as the functional pendant
groups (poly-2a and poly-2b) were synthesized by the rhodium-
catalyzed polymerization of the corresponding monomers. The
influence of the steric environment around the catalytically active
pyridine N-oxide sites on the helicity induction and its static mem-
ory as well as the asymmetric catalytic activities of the resulting
helical polymers with a macromolecular helicity memory was
investigated. The polyacetylenes formed an excess one-handed
helical conformation upon noncovalent interactions with optically
active alcohols and the induced macromolecular helicities of the
polyacetylenes were efficiently memorized after the removal of
the chiral inducers. Poly-2b with the macromolecular helicity
memory showed an enantioselectivity for the catalytic asymmetric
allylation of benzaldehydes, producing optically active allyl alco-
hols, although their enantioselectivities were low. On the other
hand, poly-2a exhibited a negligible catalytic activity probably due
to the bulky substituent at the o-position of the pyridine N-oxide
residues, while poly-2a underwent a unique helix-inversion with
the increasing concentration of chiral alcohols and the opposite
helicity of poly-2a was further successfully memorized. © 2019
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019
KEYWORDS: asymmetric polymer catalyst; biphenyl; helicity
induction; memory; organocatalyst; polyacetylene
these cases, the use of optically active monomers is essential to
control their helical handedness except for poly(methacrylate)s
with a bulky pendant prepared by helix-sense-selective polymer-
ization using chiral catalysts or initiators.2,5,45 Otherwise, racemic
helices with no optical activity will be produced, thus showing
no enantioselective catalytic activity.
INTRODUCTION Inspired by the sophisticated functions of bio-
logical helical polymers and their assemblies, a rich variety of
artificial helical polymers has been prepared over the past
decades1–8 in order to develop advanced chiral materials for the
resolution of enantiomers by chromatography,9–17 sensing of chi-
ral molecules,18–21 and circularly polarized luminescence.22–27
The application of optically active helical polymers for asymmet-
ric catalysis has also attracted growing interest, because a one-
handed helical chirality can significantly contribute to the
enantioselectivity due to a one-handed helical arrangement of
the catalytically active, chiral/achiral units along the one-handed
helical polymer backbone.8,28–30 Up to now, a variety of helical
polymer-based asymmetric catalysts have been developed
mainly by introducing catalytically active, chiral/achiral units as
components in or along artificial helical polymers that involve
polyacetylenes,31–35 poly(methacrylate)s,36–38 polyisocyanates,37
polyisocyanides,39–41 and poly(quinoxaline-2,3-diyl)s.29,30,42–44 In
We previously reported a conceptually new and versatile method,
namely “helicity induction and memory” principle,8,18,19,46–48 to
develop helical polymer-based asymmetric catalysts. A preferred-
handed helix induced in an optically inactive polyisocyanide via
noncovalent chiral interactions with optically active guests is
retained (“memorized”) after complete removal of the optically
active guests. Further modification of the pendant groups with
catalytically active, but achiral units, produces functional helical
polyisocyanides while maintaining the helicity memory, which
catalyzes the asymmetric direct aldol reaction, although its
Additional supporting information may be found in the online version of this article.
© 2019 Wiley Periodicals, Inc.
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catalytically active, achiral residues at the pendant groups of
the PBAs.
To this end, we designed and synthesized novel PBAs (poly-2a
and poly-2b) bearing pyridine N-oxide groups with a bulky
(poly-2a) or a less-bulky substituent (poly-2b) at a different
position as catalytically active units at the 40-position of the
biphenyl pendants and investigated their helicity induction and
memory effects along with their asymmetric catalytic activities
for the allylation of benzaldehydes with allyltrichlorosilane
(Fig. 1).51
CHART
alcohols ((R)-1 and (S)-1).
1 Structures of PBA (poly-A) and optically active
EXPERIMENTAL
The monomers (2a and 2b) were synthesized according to
Scheme 1(a,b).
enantioselectivity was rather low, providing the first polymer-
based asymmetric catalysts based on the helicity memory.39 This
“helicity induction and memory” concept has recently been
applied to dynamic helical poly(quinoxaline-2,3-diyl)s to develop
powerful asymmetric catalysts by Nagata et al.49
Synthesis of 2-(p-Dodecyloxyphenyl)pyridine (5)
To a solution of 2-(p-dodecyloxyphenyl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (7.28 g, 18.7 mmol), 2-bromopyridine
(1.17 mL, 12.1 mmol), and potassium carbonate (4.58 g,
33.1 mmol) in a degassed 1,2-dimethoxyethane–water mixture
(3/1, v/v; 316 mL) was added tetrakis(triphenylphosphine)
palladium(0) (1.05 g, 0.909 mmol). The mixture was stirred at
80 ꢀC for 5 h. After evaporating the solvent, the residue was
diluted with an n-hexane–ethyl acetate mixture (10/1, v/v)
and the solution was washed with water and brine, and then
dried over MgSO4. The solvent was removed under reduced
pressure and the crude product was purified by silica gel
chromatography using n-hexane–ethyl acetate (5/1, v/v) as
the eluent to give the desired product as a yellow solid
(3.48 g, 86% yield). 1H NMR (500 MHz, CDCl3, rt): δ 8.64–8.66
(m, 1H, Ar H), 7.92–7.95 (m, 2H, Ar H), 7.66–7.73 (m, 2H,
Ar H), 7.15–7.18 (m, 1H, Ar H), 6.97–7.00 (m, 2H, Ar H),
4.01 (t, J = 6.6 Hz, 2H, OCH2CH2), 1.81 (quint, J = 7.1 Hz, 2H,
CH2CH2), 1.26–1.47 (m, 18H, CH2), 0.88 (t, J = 6.5 Hz, 3H,
CH2CH3). 13C NMR (126 MHz, CDCl3, rt): δ 160.07, 157.22,
149.53, 136.64, 131.79, 128.12, 121.34, 119.78, 114.69, 68.12,
31.93, 29.68, 29.65, 29.61, 29.59, 29.42, 29.36, 29.27, 26.06,
22.70, 14.13. HRMS (ESI+): m/z calcd for C23H34NO (M + H+),
340.2635; found 340.2695.
Recently, we also found unique helical polymers,
poly(biphenylylacetylene)s (PBAs), such as poly-A (Chart 1),
to which the “helicity induction and memory” principle can be
applied.13,50 In fact, poly-A formed a preferred-handed helical
conformation through noncovalent chiral interactions with the
optically active 1-phenylethanol ((R)-1 and (S)-1) both in
solution and the solid state, and the macromolecular helicity
induced in the PBA backbones could be memorized after com-
plete removal of the optically active alcohols. Taking advan-
tage of this helicity induction followed by memory of the
helicity, we successfully developed the first smart chiral sta-
tionary phase (CSP) capable of switching the elution order of
enantiomers. The observed reversible switching of the
enantioseparation ability of poly-A relies on the switching of
the macromolecular helicity by sequential column treatments
with optically active compounds, such as (R)- and (S)-1. The
successful application of poly-A to the switchable CSP implies
that helical polymer-based switchable asymmetric catalysts
could also be developed based on the unique helicity induc-
tion and memory effect observed for PBAs by introducing the
FIGURE 1 Structures of poly-2a and poly-2b and schematic illustration of the development of helical polymer-based asymmetric
catalysts based on the helicity induction and its memory strategy. [Color figure can be viewed at wileyonlinelibrary.com]
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SCHEME 1 Syntheses of 2a (a), 2b (b), and poly-2 (c).
Synthesis of 2-(p-Dodecyloxyphenyl)pyridine 1-Oxide (6)
5 (3.29 g, 9.96 mmol) was dissolved in anhydrous dic-
hloromethane (97 mL) and the solution was cooled to 0 ꢀC. To
this was added 3-chloroperoxybenzoic acid (mCPBA) (3.59 g,
20.8 mmol) dissolved in anhydrous dichloromethane (36 mL),
and the mixture was stirred at room temperature for 48 h.
After evaporating the solvent, the residue was diluted with
chloroform and the solution was washed with saturated
NaHCO3 aqueous solution and brine, and then dried over
MgSO4. The solvent was removed under reduced pressure and
the crude product was purified by silica gel chromatography
using chloroform–methanol (95/5, v/v) as the eluent to give
the desired product as a yellow solid (3.01 g, 87% yield). IR
(KBr, cm−1): 1246 (N O). 1H NMR (500 MHz, CDCl3, rt): δ
8.32 (d, J = 6.8 Hz, 1H, Ar H), 7.80–7.83 (m, 2H, Ar H), 7.42
(dd, J = 7.9, 2.1 Hz, 1H, Ar H), 7.27–7.29 (m, 1H, Ar H),
7.16–7.19 (m, 1H, Ar H), 6.97–7.00 (m, 2H, Ar H), 4.01 (t,
J = 6.6 Hz, 2H, OCH2CH2), 1.80 (quint, J = 7.1 Hz, 2H, CH2CH2),
1.27–1.49 (m, 18H, CH2), 0.88 (t, J = 7.0 Hz, 3H, CH2CH3). 13C
NMR (126 MHz, CDCl3, rt): δ 160.21, 140.57, 130.77, 126.95,
125.69, 124.58, 123.82, 114.26, 68.13, 31.93, 29.67, 29.64,
29.61, 29.59, 29.39, 29.36, 29.19, 26.02, 22.70, 14.13. HRMS
(ESI+): m/z calcd for C23H33NNaO2 (M + Na+), 378.2404;
found 378.2457.
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Synthesis of 2-Bromo-6-(p-dodecyloxyphenyl)pyridine
1-Oxide (7)
washed with water and brine, and then dried over MgSO4.
The solvent was removed under reduced pressure and the
residual compound was passed through a short pad of silica
gel using chloroform–methanol (97/3, v/v) as the eluent.
After concentrating in vacuo, the crude product was purified
by recycling preparative high-performance liquid chromatog-
raphy (HPLC) on JAIGEL-1H and JAIGEL-2H (60 cm × 2.0 cm
[i.d.]) to give the desired product as colorless oil (247 mg,
75% yield). IR (KBr, cm−1): 3284 ( CH), 2106 (C C), 1250
(N O). 1H NMR (500 MHz, CDCl3, rt): δ 7.87–7.90 (m, 2H,
Ar H), 7.35 (d, J = 1.5 Hz, 1H, Ar H), 7.17–7.26 (m, 5H,
Ar H), 7.06 (d, J = 2.4 Hz, 1H, Ar H), 7.01 (dd, J = 7.4, 2.6 Hz,
1H, Ar H), 6.97–6.99 (m, 2H, Ar H), 6.69 (dd, J = 8.2, 2.4 Hz,
1H, Ar H), 5.07 (s, 2H, OCH2O), 5.04 (s, 2H, OCH2O), 4.01 (t,
J = 6.6 Hz, 2H, OCH2CH2), 3.36 (s, 3H, OCH3), 3.33 (s, 3H,
OCH3), 3.08 (s, 1H, C C H), 1.80 (quint, J = 7.5 Hz, 2H,
CH2CH2), 1.27–1.49 (m, 18H, CH2), 0.88 (t, J = 7.1 Hz, 3H,
CH2CH3). 13C NMR (126 MHz, CDCl3, rt): δ 160.28, 156.75,
156.01, 155.27, 154.74, 150.33, 131.96, 131.55, 130.97,
129.36, 125. 65, 125.55, 124.54, 124.43, 122.33, 121.37,
118.91, 114.12, 113.17, 110.60, 106.12, 95.21, 95.16, 83.52,
68.12, 56.01, 55.99, 31.92, 29.67, 29.64 29.61, 29.58, 29.39,
29.36, 29.18, 26.01, 22.70, 14.14. HRMS (ESI+): m/z calcd for
6 (3.12 g, 8.44 mmol), lithium tert-butoxide (2.70 g, 33.8 mmol),
and carbon tetrabromide (6.99 g, 21.1 mmol) were dissolved in
anhydrous N,N-dimethylformamide (DMF)–m-xylene (1/1, v/v;
8 mL) and the mixture was stirred at 40 ꢀC for 1.5 h. After
quenching the reaction with water, the mixture was diluted with
ethyl acetate and the solution was washed with brine and then
dried over MgSO4. The solvent was removed under reduced
pressure and the crude product was purified by silica gel chro-
matography using n-hexane–ethyl acetate (1/1, v/v) as the elu-
ent to give the desired product as a light brown solid (1.20 g,
32% yield). IR (KBr, cm−1): 1245 (N O). 1H NMR (500 MHz,
CDCl3, rt): δ 7.79–7.82 (m, 2H, Ar H), 7.60 (dd, J = 7.9, 2.1 Hz,
1H, Ar H), 7.38 (dd, J = 7.9, 2.1 Hz, 1H, Ar H), 7.10 (t, J = 8.0 Hz
1H, Ar H), 6.95–6.98 (m, 2H, Ar H), 4.01 (t, J = 6.6 Hz, 2H,
OCH2CH2), 1.80 (quint, J = 7.1 Hz, 2H, CH2CH2), 1.27–1.49 (m,
18H, CH2), 0.88 (t, J = 7.1 Hz, 3H, CH2CH3). 13C NMR (126 MHz,
CDCl3, rt): δ 160.41, 150.75, 134.56, 130.89, 128.77, 125.47,
124.89, 124.65, 114.11, 68.15, 31.92, 29.67, 29.64, 29.61, 29.58,
29.39, 29.36, 29.19, 26.02, 22.70, 14.13. HRMS (ESI+): m/z calcd
for C23H32BrNNaO2 (M + Na+), 456.1509; found 456.1562.
Synthesis of 8
C
41H49NNaO7 (M + Na+), 690.3401; found 690.3435.
To
a
mixture of ((2,20-bis(methoxymethoxy)-40-hydroxy-
4-biphenylyl)ethynyl)triisopropylsilane
(BiPh-OH)
(0.32 g,
Synthesis of 9
0.69 mmol) and sodium hydride (60% dispersion in mineral oil;
49 mg, 1.0 mmol) in anhydrous DMF (3.0 mL) was added an
anhydrous tetrahydrofuran (THF) solution (12.5 mL) of
7 (0.30 g, 0.69 mmol). The mixture was stirred at 100 ꢀC for 4 h.
After evaporating the solvent, the residue was diluted with ethyl
acetate and the solution was washed with water and brine, and
then dried over MgSO4. The solvent was removed under reduced
pressure and the crude product was purified by silica gel chro-
matography using n-hexane–ethyl acetate (1/2, v/v) as the elu-
ent to give the desired product as a yellow oil (449 mg, 79%
yield). IR (neat, cm−1): 2150 (C C), 1250 (N O). 1H NMR
(500 MHz, CDCl3, rt): δ 7.87–7.90 (m, 2H, Ar H), 7.29 (d,
J = 1.4 Hz, 1H, Ar H), 7.15–7.25 (m, 5H, Ar H), 7.07 (d,
J = 2.4 Hz, 1H, Ar H), 6.96–7.00 (m, 3H, Ar H), 6.69 (dd, J = 8.4,
2.5 Hz, 1H, Ar H), 5.08 (s, 2H, OCH2O), 5.04 (s, 2H, OCH2O),
4.01 (t, J = 6.5 Hz, 2H, OCH2CH2), 3.36 (s, 3H, OCH3), 3.33 (s, 3H,
OCH3), 1.80 (quint, J = 7.5 Hz, 2H, CH2CH2), 1.27–1.49 (m, 18H,
CH2), 1.14 (s, 21H, TIPS), 0.88 (t, J = 7.1 Hz, 3H, CH2CH3). 13C
NMR (126 MHz, CDCl3, rt): δ 160.29, 156.89, 156.03, 155.16,
154.65, 150.34, 132.01, 131.44, 130.97, 128.94, 125.89, 125.54,
124.77, 124.47, 123.87, 121.28, 118.54, 114.13, 112.99, 110.70,
106.89, 106.17, 95.18, 95.15, 90.69, 68.14, 56.05, 55.99, 31.93,
29.67, 29.65, 29.61, 29.59, 29.39, 29.36, 29.20, 26.02, 22.70,
18.69, 14.13, 11.35. HRMS (ESI+): m/z calcd for C50H69NNaO7Si
(M + Na+), 846.4736; found 846.4730.
To a mixture of BiPh-OH (496 mg, 1.06 mmol), 2-bromo-
5-dodecyloxypyridine (365 mg, 1.06 mmol), copper(I) chlo-
ride (11.6 mg, 0.117 mmol), and potassium carbonate
(292 mg, 2.11 mmol) in anhydrous toluene (2.2 mL) was
added 1-butylimidazole (69 μL, 0.53 mmol). The mixture was
stirred at 120 ꢀC for 16 h. After evaporating the solvent, the
residue was diluted with ethyl acetate and the solution was
washed with water and brine, and then dried over MgSO4.
The solvent was removed under reduced pressure and the
crude product was purified by silica gel chromatography using
n-hexane–ethyl acetate (8/1, v/v) as the eluent to give the
desired product as a yellow solid (662 mg, 86% yield). IR
(KBr, cm−1): 2156 (C C). 1H NMR (500 MHz, CDCl3, rt): δ
7.91 (d, J = 3.3 Hz, 1H, Ar H), 7.27–7.30 (m, 2H, Ar H),
7.17–7.19 (m, 3H, Ar H), 6,99 (d, J = 2.5 Hz, 1H, Ar H), 6.91
(d, J = 8.8 Hz, 1H, Ar H), 6.75 (dd, J = 8.3, 2.1 Hz, 1H, Ar H),
5.08 (s, 2H, OCH2O), 5.04 (s, 2H, OCH2O), 3.97 (t, J = 6.6 Hz,
2H, OCH2CH2), 3.36 (s, 3H, OCH3), 3.33 (s, 3H, OCH3), 1.78
(quint, J = 7.1 Hz, 2H, CH2CH2), 1.27–1.48 (m, 18H, CH2), 0.88
(t, J = 7.0 Hz, 3H, CH2CH3). 13C NMR (126 MHz, CDCl3, rt): δ
157.06, 155.87, 155.74, 154.70, 152.12, 133.66, 131.72,
131.54, 129.36, 126.74, 125.89, 123.86, 123.66, 118.59,
112.87, 112.72, 107.61, 106.99, 95.21, 95.09, 90.53, 69.14,
56.03, 55.95, 31.92, 29.66, 29.64, 29.60, 29.57, 29.37, 29.35,
29.26, 25.96, 22.70, 18.17, 14.12, 11.35. HRMS (ESI+): m/z
calcd for
754.4483.
C
44H65NNaO6Si (M + Na+), 754.4473; found
Synthesis of 2a
To a solution of 8 (400 mg, 0.49 mmol) in anhydrous THF
(19 mL) was added tetra-n-butylammonium fluoride (TBAF)
(1.0 M in THF, 0.58 mL, 0.58 mmol) at 0 ꢀC and the mixture
was stirred at 0 ꢀC for 1 h. After evaporating the solvent, the
residue was diluted with ethyl acetate and the solution was
Synthesis of 10
9 (631 mg, 0.861 mmol) was dissolved in anhydrous dic-
hloromethane (8.9 mL) and the solution was cooled to 0 ꢀC.
To this was added mCPBA (297 mg, 1.72 mmol) dissolved in
4
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anhydrous dichloromethane (2.0 mL), and the mixture was
stirred at room temperature for 48 h. After evaporating the
solvent, the residue was diluted with chloroform and the solu-
tion was washed with saturated NaHCO3 aqueous solution
and brine, and then dried over MgSO4. The solvent was
removed under reduced pressure and the crude product was
purified by silica gel chromatography using chloroform–
methanol (94/6, v/v) as the eluent to give the desired product
as a yellow oil (384 mg, 59% yield). IR (neat, cm−1): 2150
(nbd)Cl]2) as a catalyst in a similar way as previously
reported.13,50
A typical procedure for the polymerization of 2a is described
as follows. The monomer 2a (100 mg, 0.15 mmol) was placed
in a dry ampule, which was then evacuated on a vacuum line
and flushed with dry nitrogen. After this evacuation-flush pro-
cedure was repeated three times, a three-way stopcock was
attached to the ampule, and freshly distilled THF (0.25 mL)
and triethylamine (Et3N) (63 μL, 0.45 mmol) were added
using a syringe. To this was added a solution of [Rh(nbd)Cl]2
(0.03 M) in distilled THF (50 μL) at 30 ꢀC. The concentrations
of the monomer and the rhodium catalyst were 0.5 and
0.005 M, respectively. After 3 h, THF (2 mL) was added to the
reaction mixture. The resulting polymer was then precipitated
into a large amount of methanol and collected by centrifuga-
tion. The obtained poly-2a was washed with methanol and
dried in vacuo (91 mg, 91% yield). In the same way, poly-2b
was prepared. The Mn and Mw/Mn of the polymers were
determined by size-exclusion chromatography (SEC) using
polystyrene standards in THF. The polymerization results are
summarized in Supporting Information Table S1.
1
(C C), 1244 (N O). H NMR (500 MHz, CDCl3, rt): δ 8.05 (d,
J = 2.5 Hz, 1H, Ar H), 7.28 (d, J = 1.5 Hz, 1H, Ar H),
7.14–7.20 (m, 3H, Ar H), 6.99–7.01 (m, 2H, Ar H), 6.89 (dd,
J = 9.1, 2.6 Hz, 1H, Ar H), 6.58 (dd, J = 8.4, 2.5 Hz, 1H, Ar H),
5.07 (s, 2H, OCH2O), 5.04 (s, 2H, OCH2O), 3.95 (t, J = 6.5 Hz,
2H, OCH2CH2), 3.35 (s, 3H, OCH3), 3.33 (s, 3H, OCH3), 1.79
(quint, J = 7.0 Hz, 2H, CH2CH2), 1.27–1.47 (m, 18H, CH2), 1.14
(s, 21H, TIPS), 0.88 (t, J = 7.0 Hz, 3H, CH2CH3). 13C NMR
(126 MHz, CDCl3, rt): δ 157.02, 155.68, 154.64, 153.29,
150.36, 131.94, 131.42, 128.93 128.04, 125.89, 124.52,
123.86, 118.56, 115.90, 115.81, 109.77, 106.89, 105.57, 95.18,
95.11, 90.69, 69.57, 56.05, 56.00, 31.92, 29.65, 29.63, 29.57,
29.52, 29.35, 29.28, 28.93, 25.85, 22.69, 18.17, 14.12, 11.34.
HRMS (ESI+): m/z calcd for
C
44H65NNaO7Si (M + Na+),
1
Spectroscopic data of poly-2a: IR (KBr, cm−1) 1251 (N O). H
770.4423; found 770.4428.
NMR (500 MHz, CDCl3, 50 ꢀC): δ 7.88–7.80 (br, 2H, Ar H),
7.20–7.00 (br, 2H, Ar H), 7.00–6.70 (br, 7H, Ar H), 6.50–6.35
(br, 2H, Ar H), 6.05–5.85 (br, 1H, C CH), 4.85–4.60 (br, 4H,
2OCH2O), 3.98–3.85 (br, 2H, OCH2CH2), 3.13–2.86 (br, 6H,
2OCH3), 1.80–1.72 (br, 2H, OCH2CH2), 1.46–1.20 (br, 18H, CH2),
0.87 (br, 3H, CH3). Calcd for C41H49NO7: C, 73.74; H, 7.40; N,
2.10. Found: C, 73.75; H, 7.25; N, 2.06.
Synthesis of 2b
To a solution of 10 (350 mg, 0.47 mmol) in anhydrous THF
(19 mL) was added TBAF (1.0 M in THF, 0.56 mL, 0.56 mmol)
at 0 ꢀC and the mixture was stirred at 0 ꢀC for 1 h. After evap-
orating the solvent, the residue was diluted with ethyl acetate
and the solution was washed with water and brine, and then
dried over MgSO4. The solvent was removed under reduced
pressure and the residual compound was passed through a
short pad of silica gel using chloroform–methanol (96/4, v/v)
as the eluent. After concentrating in vacuo, the crude product
was purified by recycling preparative HPLC on JAIGEL-1H and
JAIGEL-2H (60 cm × 2.0 cm [i.d.]) to give the desired product
as a white solid (149 mg, 54% yield). Mp: 73.2–74.0 ꢀC. IR
Spectroscopic data of poly-2b: IR (KBr, cm−1) 1241 (N O). 1H NMR
(500 MHz, CDCl3, 50ꢀC): δ 7.98–7.94 (br, 1H, Ar H), 6.92–6.76 (br,
6H, Ar H), 6.50–6.35 (br, 2H, Ar H), 6.05–5.85 (br, 1H, C CH),
4.85–4.60 (br, 4H, 2OCH2O), 3.98–3.85 (br, 2H, OCH2CH2),
3.13–2.86 (br, 6H, 2OCH3), 1.80–1.72 (br, 2H, OCH2CH2), 1.46–1.20
(br, 18H, CH2), 0.87 (br, 3H, CH3). Calcd for C35H45NO7: C, 71.04; H,
7.67; N, 2.37. Found: C, 71.03; H, 7.77; N, 2.34.
1
(KBr, cm−1): 3259 ( CH), 2103 (C C), 1240 (N O). H NMR
(500 MHz, CDCl3, rt): δ 8.05 (d, J = 2.7 Hz, 1H, Ar H), 7.34 (d,
J = 1.2 Hz, 1H, Ar H), 7.15–7.20 (m, 3H, Ar H), 7.00–7.02 (m,
2H, Ar H), 6.89 (dd, J = 9.2, 2.7 Hz, 1H, Ar H), 6.58 (dd,
J = 8.2, 2.4 Hz, 1H, Ar H), 5.06 (s, 2H, OCH2O), 5.04 (s, 2H,
OCH2O), 3.96 (t, J = 6.6 Hz, 2H, OCH2CH2), 3.35 (s, 3H, OCH3),
3.33 (s, 3H, OCH3), 3.08 (s, 1H, C C H), 1.82 (quint, J = 7.0 Hz,
2H, CH2CH2), 1.27–1.47 (m, 18H, CH2), 0.88 (t, J = 7.0 Hz, 3H,
CH2CH3).13C NMR (126 MHz, CDCl3, rt): δ 155.98, 155.77,
154.71, 153.32, 150.24, 131.88, 131.51, 129.34, 128.03,
125.64, 124.29, 122.32, 118.93, 116.02, 115.73, 109.67,
105.54, 95.19, 95.17, 83.50, 69.55, 56.00, 31.91, 29.64, 29.63,
29.57, 29.52, 29.35, 29.27, 28.92, 25.83, 22.69, 14.12. HRMS
(ESI+): m/z calcd for C35H45NNaO7 (M + Na+), 614.3088;
found 614.3103.
Typical Procedure for Asymmetric Allylation of
Benzaldehyde with Allyltrichlorosilane
hm(R)5-Poly-2b (3 μmol based on the pyridine N-oxide residue of
poly-2b) was dissolved in toluene (0.16 mL) under an argon
atmosphere. To this was added 3a (3.6 mg, 0.034 mmol) dis-
solved in toluene (20 μL), N-ethyldiisopropylamine (iPr2NEt)
(18 μL, 0.10 mmol), and allyltrichlorosilane (8.9 mg, 0.051 mmol)
dissolved in toluene (30 μL) at −10 ꢀC. The mixture was stirred
at −10ꢀC for 6 h. After the addition of aqueous NaOH (1.0 M,
0.20 mL) to quench the reaction, chloroform was added, and
the organic layer was washed with brine and dried over anhy-
drous MgSO4. After filtration, the solvent was evaporated to
dryness in vacuo. The conversion of 3a to the allyl alcohol 4a
was determined to be 21% by 1H NMR analysis using
1,1,2,2-tetrachloroethane as the internal standard. After the res-
idue was passed through a short pad of silica gel using chloro-
form as the eluent, the enantiomeric excess (ee) value of the
product 4a (5% ee, S-rich) was determined by chiral HPLC
Polymerization
The polymerizations of 2a and 2b were carried out according
to Scheme 1(c) in a dry glass ampule under a dry nitrogen
atmosphere using [(norbornadiene)rhodium(I) chloride]2 ([Rh
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analysis using
a
CHIRALCEL OD-H column (n-hexane–
the antipode (S)-1 was used instead of (R)-1 [Fig. 2(a)(ix)].
These results indicated that the poly-2a formed a preferred-
handed helical conformation in response to the chirality of
optically active guests. Based on the relationships between
the Cotton effect signs and the helical handedness of analo-
gous helical poly(phenylacetylene)s,55–58 the poly-2a back-
bone most likely had left- and right-handed helical
conformations in toluene containing (R)-1 and (S)-1 (5 vol%),
respectively. The CD spectral pattern of poly-2a was similar
to that of the previously reported poly-A with a one-handed
macromolecular helicity,13 indicating that the pyridine
N-oxide-based functional group introduced at the 40-position
of the biphenyl pendant caused a negligible structural alter-
ation in the helical polyacetylene backbone.
2-propanol (98/2, v/v); flow rate 0.8 mL/min; temperature
about 20 ꢀC: tr = 18.5 min (for R-isomer), tr = 21.0 min (for S-
isomer) (see Supporting Information Fig. S9).52 Allylation reac-
tions of 3b and 3c with allyltrichlorosilane were also per-
formed in a similar manner and the results are summarized in
Table 1.
Chromatographic condition for the ee determination of 4b:
CHIRALCEL OD-H column (n-hexane–2-propanol (98/2, v/v);
flow rate 0.8 mL/min; temperature about 20 ꢀC: tr = 26.0 min
(for R-isomer), tr = 31.7 min (for S-isomer).52
Chromatographic condition for the ee determination of 4c:
CHIRALCEL OB-H column (n-hexane–2-propanol (90/10, v/v);
flow rate 0.8 mL/min; temperature about 20 ꢀC: tr = 16.4 min
(for S-isomer), tr = 17.8 min (for R-isomer).52
We found that the main-chain absorption band of poly-2a in
(R)-1/toluene mixtures at 25 ꢀC was significantly red-shifted
as the volume fraction of (R)-1 increased, resulting in the
absorbance maximum (εmax) at 458 nm in (R)-1/toluene
(40/60 and 50/50, v/v) [Fig. 2(a)(vii, viii)] accompanied by a
color change from yellow to orange. Similar absorption spec-
tral changes were also observed when rac-1 was used instead
of (R)-1 (Supporting Information Fig. S4). Therefore, the
observed red-shift of the absorption spectra of poly-2a with the
color changes can be ascribed to a solvent-induced main-chain
conformational change from a tightly twisted helix to an
extended helix, leading to elongation of the π-conjugation length
of the poly-2a backbone,59 as observed in chloroform and dic-
hloromethane (Supporting Information Fig. S5).
RESULTS AND DISCUSSION
Synthesis
The synthetic route to prepare novel biphenylylacetylene mono-
mers (2a and 2b) containing pyridine N-oxide groups as a cata-
lytically active pendant is shown in Scheme 1(a,b), respectively.
The molecular design of 2a with a bulky p-alkoxyphenyl substitu-
ent at the o-position of the pyridine N-oxide group was based on
the previously reported pyridine N-oxide-based asymmetric cata-
lysts developed by Hayashi et al.53 The target monomer 2a was
synthesized from a previously reported 4-phenylphenol deriva-
tive (BiPh-OH)54 through etherification with 2-bromo-6-(p-dode-
cyloxyphenyl)pyridine 1-oxide followed by deprotection of a
triisopropylsilyl group. The analogous monomer 2b with a long
alkoxy substituent at the m-position of the pyridine N-oxide
group was also prepared from BiPh-OH in a similar manner. The
rhodium-catalyzed polymerization of the obtained 2a and 2b
was performed in a THF/Et3N mixture at 30 ꢀC according to a
previously reported procedure (Scheme 1(c) and Supporting
Information Table S1).13,50 Optically inactive PBAs, poly-2a and
poly-2b, carrying different pyridine N-oxide pendants were
obtained in good yields (≥82%) with the number-average molec-
ular weight (Mn) of 4.2 × 105 and 2.1 × 105, respectively, as esti-
mated by SEC. 1H NMR analysis revealed that the
stereoregularities of the obtained polymers were almost
completely cis-transoidal (Supporting Information Fig. S1).
The ICD signals almost completely disappeared in toluene con-
taining more than 30% (v/v) (R)-1, suggesting that poly-2a
with an elongated π-conjugated backbone may not adopt a
preferred-handed helical conformation [Fig. 2(a)(vi–viii)]. Inter-
estingly, when (R)-1/toluene solutions (30/70 and 40/60, v/v)
of poly-2a were cooled to −10 ꢀC, the ICD signals appeared
again along with a blue-shift of the absorption spectra and a
recovery of the solution color from orange to yellow accompa-
nied by inversion of the Cotton effect signs, while the absolute
ICD intensities were almost comparable with those at 25ꢀC in
(R)-1/toluene (5/95, v/v) [Fig. 2(b,c)]. Furthermore, both the
absorption and CD spectra of poly-2a in (R)-1/toluene (30/70
and 40/60, v/v) at −10ꢀC were almost identical to those in (S)-
1/toluene (5/95, v/v) at −10 ꢀC. These results clearly revealed
that both the right- and left-handed helices of poly-2a could be
produced and completely switched using a single enantiomer
by a change in temperature. Because poly-2a showed no
concentration-dependent CD spectral changes (0.1–1.0 mM) and
(Supporting Information Fig. S6), the formation of aggregates
through intermolecular interactions could be excluded for the
observed helix inversion. Interestingly, a one-handed helical con-
formation induced in the poly-2b backbone in (R)-1/toluene mix-
tures (1/99–40/60, v/v) at 25 as well as at −10 ꢀC [Fig. 3(a,b)]
was opposite (right-handed) to that of the poly-2a (left-handed)
induced in (R)-1/toluene mixtures (1/99–20/80, v/v) at 25 ꢀC.
Helicity Induction and Static Memory
The preferred-handed helicity induction ability of the optically
inactive poly-2a was first investigated using (R)-1 and (S)-1
as helix inducers in toluene (1/toluene, 5:95, v/v) at 25 ꢀC.
Poly-2a showed an induced circular dichroism (ICD) in the
absorption region of the polyacetylene backbone and its inten-
sity increased with time and became constant within 1 h
[Fig. 2(a) and Supporting Information Fig. S2]. Although the
saturated ICD intensity of poly-2a in (R)-1/toluene (5/95,
v/v) tended to decrease with the increasing temperature, an
excess handed helix was induced in poly-2a to some extent
even at 50 ꢀC (Supporting Information Fig. S3). As expected,
poly-2a exhibited a perfect mirror-image CD spectrum when
In addition,
a unique temperature-driven helix-inversion
observed in poly-2a did not take place for poly-2b in (R)-1/tolu-
ene (1/99–40/60, v/v) [Fig. 3(b)]. This may be ascribed to the
6
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FIGURE 2 (a,b) CD and absorption spectra of poly-2a in (R)-1/toluene (1/99–50/50, v/v) or (S)-1/toluene (5/95, v/v) at 25 (a) and −10ꢀC (b).
(c) Plots of ICD intensity (Δε2nd) of poly-2a measured at 25 (red circles) and −10ꢀC (blue circles) versus the volume fraction of (R)-1 in a
(R)-1/toluene mixture after standing at 25 and −10 ꢀC, respectively, until no further increase in the ICD intensity was observed. [poly-
2a] = 1.0 mM. [Color figure can be viewed at wileyonlinelibrary.com]
steric effect of the bulky (poly-2a) and less-bulky (poly-2b) sub-
stituents at different positions on the pyridine N-oxide groups.
helicity control was imperfect.60 To the best of our knowledge,
this is the first example of the preparation of both enantiomeric
left- and right-handed helicity-memorized polymers with an
almost perfect helical handedness using the chirality of a single
enantiomer.61
The right- and left-handed helical poly-2a and poly-2b induced
in toluene containing (R)-1 or (S)-1 (1/toluene = 5/95, v/v) at
−10ꢀC [Fig. 4(a)(i,iii) and (b)(i,ii), respectively] were then iso-
lated by precipitation into methanol to completely remove the
optically active alcohols (Supporting Information Fig. S7). The
CD spectral patterns and intensities of the isolated poly-2a [Fig. 4
(a)(iv,vi)] and poly-2b [Fig. 4(b)(iii,iv)] in toluene at −10ꢀC were
almost identical to those in the presence of the optically active
(R)-1 or (S)-1 (5%, v/v). These results indicated that the induced
right- and left-handed helices of poly-2a and poly-2b were
almost completely memorized showing the complete mirror
image CDs [Fig. 4(a,b)] as previously reported for poly-A.14 Tak-
ing advantage of the unique helix-inversion of poly-2a in toluene
with a different amounts of (R)-1 (5/95 and 35/65, v/v) at
−10 ꢀC followed by static memory of the induced helices, the
enantiomeric helices of poly-2a were also successfully memo-
rized using a single enantiomer as a helix inducer [Fig. 4(a)(iv,v)].
Maeda et al. reported that a pair of enantiomeric helical polymers
with a static helicity memory could be prepared from an achiral
PBA by using a single enantiomeric helix inducer with the assis-
tance of a temperature-driven helix inversion, although the
The stabilities of the static helicity memories of poly-2a and poly-
2b highly depended on the solvents and temperature (Supporting
Information Fig. S8). For example, the isolated poly-2a and poly-
2b from their (R)-1/toluene (5/95, v/v) solutions almost
maintained their helicity memories in toluene at −10 ꢀC for 24 h,
while the CD intensities gradually decreased with time in toluene
at 25 ꢀC. In dichloromethane, however, the CD instantly dis-
appeared at −10 ꢀC (Supporting Information Fig. S8c).
Enantioselective Allylation Catalyzed by Pyridine N-Oxide-
Appended Poly-2a and Poly-2b with a Static Helicity
Memory
The optically active helical poly-2a and poly-2b with a static
helicity memory were then utilized as organocatalysts for the
enantioselective allylation of benzaldehydes (3) with
allyltrichlorosilane in toluene at −10 ꢀC (Scheme 2) to suppress
the loss of helicity memory during the reactions (Supporting
Information Fig. S8). The results are summarized in Table 1.
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FIGURE 3 (a) CD and absorption spectra of poly-2b in (R)-1/toluene (1/99–40/60, v/v) at −10ꢀC. (b) Plots of ICD intensity (Δε2nd) of poly-
2b measured at 25 (red circles) and − 10 ꢀC (blue circles) versus the volume fraction of (R)-1 in a (R)-1/toluene mixture after standing
at 25 and −10 ꢀC, respectively, until no further increase in the ICD intensity was observed. [poly-2b] = 1.0 mM. [Color figure can be
viewed at wileyonlinelibrary.com]
SCHEME
2 Allylation reactions of benzaldehydes (3) with
allyltrichlorosilane catalyzed by achiral 2 and optically active
and inactive poly-2.
Toluene was used instead of dichloromethane, a typical solvent
for the asymmetric allylation reactions catalyzed by chiral pyri-
dine N-oxide-based organocatalysts,51–53,62–64 based on the sta-
bilities of the helicity-memorized poly-2a and poly-2b
(Supporting Information Fig. S8).
The as-prepared, optically inactive poly-2a and monomer 2a as
well as the helicity-memorized poly-2a (hm(R)5-poly-2a) showed
almost no catalytic activity in the allylation reaction of 3a with
allyltrichlorosilane (entries 1–3) mostly due to the steric and
electronic environments around the catalytically active pyridine
N-oxide residues of poly-2a and 2a. In contrast, the as-prepared
poly-2b, monomer 2b, and the helicity-memorized poly-2b
(hm(R)5-poly-2b) bearing a less-bulky substituent around the
catalytically active pyridine N-oxide sites catalyzed the allylation
reaction of 3a to produce the corresponding allyl alcohol (4a)
(entries 4–6). Importantly, hm(R)5-poly-2b could function as an
asymmetric organocatalyst during the allylation reaction of 3a
with allyltrichlorosilane to produce 4a with an optical activity,
indicating the indispensable role of the macromolecular helicity
memory of hm(R)5-poly-2b, although its enantioselectivity (5%
ee) was low (entry 6).
FIGURE 4 (a) CD and absorption spectra of poly-2a in the presence
of (R)-1 ((R)-1/toluene = 5/95 (i) and 35/65 (ii), v/v) or (S)-1 ((S)-1/
toluene = 5/95, v/v) (iii) after standing at −10 ꢀC until no further
increase in the ICD intensity was observed, and those of the
isolated poly-2a in toluene at −10 ꢀC recovered from (i) (iv; hm(R)5
-
poly-2a), (ii) (v; hm(R)35-poly-2a), and (iii) (vi; hm(S)5-poly-2a),
measured at −10 ꢀC. [poly-2a] = 1.0 mM. (b) CD and absorption
spectra of poly-2b in the presence of (R)-1 (i) or (S)-1 (ii) (1/
toluene = 5/95, v/v) after standing at −10 ꢀC until no further increase
in the ICD intensity was observed and those of the isolated poly-2b in
toluene at −10 ꢀC recovered from (i) (iii; hm(R)5-poly-2b) and (ii) (iv;
hm(S)5-poly-2b), measured at −10 ꢀC. [poly-2b] = 1.0 mM. [Color
figure can be viewed at wileyonlinelibrary.com]
The enantioselectivity and catalytic activity of hm(R)5-poly-2b
toward the asymmetric allylation reaction were further inves-
tigated using para-methoxy (3b) and -nitro (3c) substituted
benzaldehydes. Both the enantioselectivity and catalytic activ-
ity of hm(R)5-poly-2b were significantly decreased for 4b
8
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TABLE 1 Enantioselective Allylation of Benzaldehydes (3) with Allyltrichlorosilanea
Entry
Catalyst
Substrate
Product
Time (h)
Conv. (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
2a
3a
3a
3a
3a
3a
3a
3b
3c
3c
4a
4a
4a
4a
4a
4a
4b
4c
4c
24
24
24
6
<1
<1
<1
11
20
21
10
39
44
–
poly-2a
–
hm(R)5-poly-2a
2b
–
–
poly-2b
6
–
hm(R)5-poly-2b
hm(R)5-poly-2b
hm(R)5-poly-2b
hm(S)5-poly-2b
6
5 (S)
<1
9 (R)
8 (S)
6
6
6
a The reactions of 3 (0.15 M) with allyltrichlorosilane (1.5 equiv) were car-
c Determined by chiral HPLC (see Supporting Information Fig. S9). In
parentheses are shown the absolute configuration of the major enantio-
mer assigned by the retention times reported in the literature (Ref. 52).
i
ried out in the presence of a catalyst (10 mol %) and Pr2NEt (3.0 equiv.)
in toluene at −10 ꢀC.
b Determined by 1H NMR analysis.
(entry 7), but were only slightly improved for 4c (39% con-
version and 9% ee (R-rich)) (entry 8). The reason for this
inversion of the enantioselectivity of hm(R)5-poly-2b
depending on the substrates (3a and 3c) (entries 6 and 8 in
Table 1) is not clear at present, but may be due to the
electron-withdrawing nitro-substituent of 3c. A similar inver-
sion of the enantioselectivity depending on the substrates was
reported by Kotora et al.65 The fact that hm(S)5-poly-2b with
an opposite-handed helicity memory produced the product
with the opposite configuration (8% ee (S-rich)) while
maintaining its catalytic activity (entry 9) demonstrated that
optically active helical polymers bearing achiral, but catalyti-
cally active residues as the pendants whose optical activities
are solely derived from their static helicity memory indeed
work as asymmetric catalysts, leading to the development of
switchable asymmetric catalysts.
enantiomeric helical PBA derivatives with the static helicity
memory was found to catalyze the asymmetric allylation of
benzaldehydes to produce optically active enantiomeric allyl
alcohols. We believe that more powerful helical polymer-
based asymmetric catalysts with a more stable one-handed
helicity memory, showing a higher catalytic activity and
switchable enantioselectivity based on a switchable memory
of the helicity concept,13 can be developed through the ratio-
nal design of catalytically active pendants of helical PBAs with
a unique static helicity memory. Further studies toward these
goals are currently in progress in our laboratory and will be
reported in due course.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI (Grant-in-Aid for
Specially Promoted Research, No. 18H05209 (E.Y.).
Apparently, the observed enantioselectivity of the helicity-
memorized poly-2b in the allylation reactions evaluated in
this study was unexpectedly low, mostly like because the cata-
lytically active N-oxide units are located far from the helical
polyacetylene backbone as well as the lack of stability of the
helicity memory of the poly-2b during the allylation reactions
even in toluene at −10 ꢀC. The latter was evidenced by the
fact that the CD intensity of the hm(R)5-poly-2b significantly
decreased by approximately one-fourth after the allylation
reaction (Supporting Information Fig. S10), which was beyond
our expectation [Supporting Information Fig. S8(a,b)] proba-
bly caused by some sort of reagents and/or products.
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