Chirality-Amplifying, Dynamic Induction of Single-Handed Helix by Chiral Guests to Macromolecular Chiral Catalysts Bearing Boronyl Pendants as Receptor Sites

Article abstract of DOI:10.1021/jacs.8b00529

Helical chirality of poly(quinoxaline-2,3-diyl)s bearing a boronyl pendant at the 5-position of the quinoxaline ring was induced by condensation with chiral guests such as a diol, diamine, and amino alcohol. Reversible induction of a single-handed helical

Full text of DOI:10.1021/jacs.8b00529

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Communication
Chirality-Amplifying, Dynamic Induction of Single-
Handed Helix by Chiral Guests to Macromolecular Chiral
Catalysts Bearing Boronyl Pendants as Receptor Sites
Takeshi Yamamoto, Ryo Murakami, Satoko Komatsu, and Michinori Suginome
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00529 • Publication Date (Web): 02 Mar 2018
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Chirality-Amplifying, Dynamic Induction of Single-Handed Helix by
Chiral Guests to Macromolecular Chiral Catalysts Bearing Boronyl
Pendants as Receptor Sites
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Takeshi Yamamoto, Ryo Murakami, Satoko Komatsu, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
Supporting Information Placeholder
advantages, helix induction by chiral guests has never yet been
combined with application to asymmetric catalysis.
ABSTRACT: Helical chirality of poly(quinoxaline-2,3-diyl)s
bearing a boronyl pendant at the 5-position of the quinoxaline ring
was induced by condensation with chiral guests such as a diol, dia-
mine, and amino alcohol. Reversible induction of a single-handed
helical structure was achieved by using less than an equimolar
amount of chiral amino alcohols to the boronyl pendants. Majority-
rule-effect-based chiral amplification on the polyquinoxaline main
chain was demonstrated with chiral amino alcohols with low enan-
tiomeric excess (ee). The helical macromolecular scaffold whose
helicity was thus induced was utilized in palladium-catalyzed
asymmetric silaboration of meso-methylenecyclopropane (up to
92% ee) by introducing (diarylphosphino)phenyl pendants at their
side chains.
In this paper, we demonstrate the induction of a single-handed
helical structure to helical poly(quinoxaline-2,3-diyl)s4q,r (PQX
hereafter) bearing boronyl pendants (PQXboh) using several chiral
guest molecules.6,7 The single-handed PQX was used as a chiral
catalyst in asymmetric catalytic reactions with high
enantioselectivity. We also demonstrate the use of a chiral guest
with low enantiopurity to obtain high enantioselectivities through
chiral amplification.
A binary, “achiral” random copolymer PQXboh bearing boronyl
pendants along with propoxymethyl side chains was chosen as a
scaffold and synthesized by living polymerization. PQXboh is
reported to be a versatile synthetic intermediate, to which various
pyridine-based pendants such as 4-aminopyrid-3-yl and 2,2-
bipyrid-6-yl are easily introduced by postpolymerization cross-
coupling.2p,q PQXboh(190/10*) containing 10 boronyl pendants
(on average) was dissolved in toluene in the presence of molecular
sieves 4A with various chiral diol, diamine, and amino alcohols
(0.01 M, 200 equiv to boronyl group), separately (Figure 1a). After
stirring for 15–24 h at room temperature, circular dichroism (CD)
spectra were measured to determine the degree of helix induction
without removing the chiral guest. The screw-sense excess (se) of
each sample was determined by comparison of the g value (Kuhn
dissymmetry factors, /) at 371.5 nm with the expected g value
(gmax) for purely single-handed PQX (vide infra) (Figure 1b).8 The
chiral diol (S,S)-1 induced M-helical structure (g = –1.87  10–3),
which was assumed to be ca. 90% se, while the diamine derivative
(S,S)-2 induced M-helical structure with moderate se. We found
that the corresponding amino alcohol (S,S)-3 induced almost pure
M-helical conformation efficiently (g = –2.10  10–3). Similarly, its
diastereomer, (S,R)-4, afforded single-handed P-helical
conformation efficiently (g = 2.03  10–3). The two diastereomers
(S,S)-3 and (S,R)-4 gave a pair of mirror-image CD spectra of P-
and M-helices (Figure 1c). Aminoindanol (S,R)-5 also induced the
M-helix efficiently. We further tested amino alcohols derived from
amino acids. Phenylglycinol (R)-6 showed efficient induction of
the P-helical structure (g = 2.16  10–3), while valinol (R)-7
showed moderate but clear induction of the P-helix. Use of alaninol
(S)-8 with opposite absolute configuration resulted in formation of
the M-helical structure with much less screw-sense induction.
These results clearly indicate that the stereochemistry of a N-bound
stereogenic carbon center serves as a determinant of the helical
chirality of PQXboh. Amino alcohol (R)-9, which lacks a N-bound
Because of their unique chiral structures, helical synthetic
macromolecules that contain either P- or M-helical conformation
in excess are finding various applications, including chiral
separation, chirality sensing, circularly polarized light (CPL)
emission, and CPL reflection.1 Particular interest is being focused
on the use of purely single-handed helical macromolecules as chiral
catalysts in asymmetric catalysis,2 although the requirement for the
induction of the “pure” helical sense still hampers this development.
In all these applications, the dynamic nature of helical structures
along with their rod-like molecular shape makes them highly
characteristic, in comparison with chiral small molecules, thus
enabling switchable chiral functions, where the change of external
conditions alters the enantiodiscrimination or CPL handedness.3
One major strategy for induction of the nonracemic helical sense
involves introduction of chiral side chains through inconvertible,
strong covalent bonds into all planar or quasiplanar repeating
units.4 Such inconvertible chiral groups in the macromolecular
scaffolds make the whole structure robust, but it does make their
synthesis laborious.
In contrast, there is another class of induction where chiral guests
serve as a source of helical chirality.5 This strategy allows the use
of macromolecules devoid of chiral groups on their backbones, to
which chirality is induced by the addition of chiral guests. Most
typically, Yashima and coworkers reported that achiral
polyacetylenes bearing carboxyl or boronyl groups at the pendant
groups adopt nonracemic helical structures upon addition of chiral
guests to their solutions.5b–d This type of helix induction by a chiral
guest has the advantages of less-laborious synthesis and wider
variation of the choice of chiral sources. However, despite these
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stereogenic carbon center, resulted in the formation of a M-helix
with moderate se. N-Methylated amino alcohol (S,S)-10 exhibited
the induction of screw-sense opposite to that of nonmethylated
(S,S)-3 with much lower se.
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Figure 2. (a) Relationships between DP (n + m) and dissymmetry
factor g of PQXboh(n/m*) (n/m = 19/1) at 371.5 nm in toluene at
20 C: (S,S)-3 (●), (S,R)-4 (▼), (S,R)-5 (▲), and (R)-6 (♦). In the
table insert, gmax (g values for pure helix) and Gh (helix
stabilization energy per unit) calculated from curve fittings are
shown. (b) Helical chirality induction of PQXboh(380/20*) with
different equivalents (equiv) of (S,S)-3.
By reprecipitation from acetonitrile, PQXboh(190/10*)/(S,S)-3,
in which the boronyl group was converted to an oxazaborolidine
group, was isolated (Scheme 1). The isolated PQX showed pure
left-handed helical structure on measurement of its CD spectrum in
the absence of the excess chiral guest (toluene, 20 C).
PQXboh(190/10*)/(S,S)-3 was made CD silent within 3 h upon
hydrolysis (1 M H2O in tetrahydrofuran at 20 C). Direct
replacement of the chiral amino alcohol on PQXboh was achieved
by mixing PQXboh(190/10*)/(S,S)-3 with 200 equiv of (R,R)-3,
resulting in complete reversal of the screw-sense (Scheme 1).
Scheme 1. Reversible Helical Chirality Induction of
PQXboh with Chiral Amino Alcohol
Figure 1. (a) Helical chirality induction of PQXboh(190/10*) with
chiral guests. (b) Induced se of PQXboh(190/10*) with chiral
guests in toluene at 20 C. (c) CD spectra of PQXboh(190/10*)
with chiral guests (S,S)-3 and (S,R)-4 in toluene at 20 C.
To determine the effect of chiral guests in detail, the helix
stabilization energy (Gh) per chiral guest was estimated by
changing the degree of polymerization (DP) of PQXboh(n/m*) ([n
 m] = 60–400), while the ratio of boronyl units (n/m = 19/1) was
maintained. The g values were plotted against the DP of
PQXboh(n/m*) (Figure 2a). Hyperbolic tangent curves fit into the
obtained positive nonlinear plots.4e The Gh of (S,S)-3 in toluene
was found to be highest; it was estimated to be –1.54 kJ mol–1. It
should be remarked here that this value is 1.5 times higher than the
highest Gh for PQX bearing covalently attached 2-alkoxymethyl
groups at the 6- and 7-positions of the quinoxaline rings (–1.01
kJ/mol for 2-octyloxymethyl).4r We found that the amount of chiral
guest could be reduced to 1.0 equiv to boronyl groups for the
induction of pure helical sense of PQXboh(380/20*) (Figure 2b).
Remarkably, even the use of 0.5 equiv of chiral guest (S,S)-3
afforded 92% se.
Based on these results, a chiral-guest-responsive helical polymer
ligand was synthesized by incorporating coordinating groups as the
third component.2i–m,o The ternary PQXphos(360/20*/20) bearing
both boronyl and 2-[bis(3,5-dimethylphenyl)phosphino]phenyl
pendants was employed in the palladium-catalyzed asymmetric
silaborative C–C bond cleavage of meso-methylenecyclopropane
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(Table
1).2l,9
A
control
experiment
without
PQXphos(360/20*/20) but with (S,S)-3 gave only a trace amount
of product 13 (<1%, entry 1). In the absence of chiral guest,
PQXphos(360/20*/20) afforded racemic 13 (entry 2). However,
upon pretreatment of achiral PQXphos(360/20*/20) (2.4 mol% P
and 2.4 mol% B) with (S,S)-3 (1.2 mol%, 0.5 equiv to the boronyl
pendants) at 50 C for 24 h in toluene, the silaboration afforded
(R,R)-13 in 85% yield with 87% ee (entry 3). Use of larger amounts
of chiral guest in the pretreatment afforded the product with slightly
higher enantioselectivity (92% ee with 2.0 equiv chiral guest,
entries 4 and 5). Use of the enantiomeric chiral guest (R,R)-3
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resulted in the formation of an enantiomeric product with the same
ee (entry 6).
amplification was observed for phosphorous-containing, ternary
PQXphos(360/20*/20) (Figure 3b).
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These results suggest that the chirality of the chiral guest was
successfully transferred to the helical main chain of PQX and in
turn to the reaction center, as found in PQXphos bearing covalently
bonded chiral side chains. Note that ligand exchange on the
silylboron reagents was not observed during the reaction. It may
also be interesting to note that PQXphos(380/20) bearing no
boronyl pendant unexpectedly resulted in the formation of (S,S)-13
in 5% ee in the presence of (S,S)-3, which, however had preference
to form the opposite enantiomer (entry 7). As suggested by the
mechanism of chirality transfer, some other chiral guests (S,R)-4,
(S,R)-5, and (R)-6 (1.0 equiv) shown in Figure 2 can also be used
as the source of chirality in the silaboration reactions (entries 8–10).
Chirality-amplifying catalytic asymmetric synthesis was
demonstrated with the use of PQXphos(360/20*/20) and (S,S)-3
(Scheme 2). PQXphos(360/20*/20) was treated with 10 equiv of
(S,S)-3 with 33% ee at 80 C for 96 h, and then excess (S,S)-3 was
removed by precipitation with acetonitrile. The recovered (M)-
PQXphos(360/20*/20)/(S,S)-3 afforded (R,R)-13 with 87% ee.
This result demonstrates that PQX serves as an efficient chiral
amplifier in catalytic asymmetric synthesis.2o
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Table 1. Palladium-Catalyzed Asymmetric Silaborative C–
C Bond Cleavage of meso-Methylenecyclopropanea
Figure 3. Helical chirality induction of (a) PQXboh(380/20*)
and (b) PQXphos(360/20*/20) with 200 equiv of (S,S)-3 with
varying optical purity.
Scheme 2. Chiral Amplification on Polyquinoxaline Scaf-
fold toward Pd-Catalyzed Asymmetric Silaboration.
chiral guest
entry PQXphos
% yieldb % eec
(equiv to B atom)
1
–
(S,S)-3d
(360/20*/20) no addition
<1
88
N.D.e
2
0
3
(360/20*/20) (S,S)-3 (0.5 eq)
(360/20*/20) (S,S)-3 (1.0 eq)
(360/20*/20) (S,S)-3 (2.0 eq)
(360/20*/20) (R,R)-3 (1.0 eq)
85
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81
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93
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87 (R,R)
91 (R,R)
92 (R,R)
92 (S,S)
5 (S,S)
72 (S,S)
86 (R,R)
82 (S,S)
4
5
6
7
(380/20)
(S,S)-3 (1.0 eq)
8
(360/20*/20) (S,R)-4 (1.0 eq)
(360/20*/20) (S,R)-5 (1.0 eq)
(360/20*/20) (R)-6 (1.0 eq)
In conclusion, we demonstrated the efficient helical chirality
induction of PQXs by introducing boronyl pendants as chiral guest
receptor sites. Taking advantage of the long persistence length of
the helical PQX scaffold, a pure single-handed structure was
induced by condensation with a small amount of chiral amino
alcohol. Chiral amplification on the PQX scaffold was achieved by
using a chiral guest with low ee, forming a pure single-handed
helical structure. The induced helically chiral macromolecular
scaffold provided an efficient asymmetric reaction environment in
a palladium-catalyzed reaction. Separation of chirality induction
sites and catalytically active sites in the macromolecular scaffold
enables the rational design of chiral amplification systems.
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a11 (0.15 mmol), 12 (0.10 mmol), Pd2(dba)3 (1.0 mol), and ligand
(2.4 mol) were heated with toluene (0.20 mL) at 50 C. bIsolated
c
yield. Determined by chiral SFC analysis after oxidation to the
corresponding -silyl ketones. d2.4 mol% to 12. eNot determined.
We then looked at the possibility and the degree of chiral
amplification10,11 using binary PQXboh(380/20*) (Figure 3a). In
the presence of 200 equiv of (S,S)-3 with varying optical purity,
helical chirality induction was carried out at 80 C. A positive
nonlinear relationship between the enantiopurity of the chiral guest
and screw-sense induction was observed in a CD spectrum,
measured in toluene at 20 C. The plot indicated that even a chiral
guest with 20% ee can induce >90% se. A similar chiral
ASSOCIATED CONTENT
Supporting Information
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(h) Takei, F.; Yanai, K.; Onitsuka, K.; Takahashi, S. Angew. Chem. Int. Ed.
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AUTHOR INFORMATION
Corresponding Author
suginome@sbchem.kyoto-u.ac.jp
ORCID
Takeshi Yamamoto: 0000-0002-5184-7493
Michinori Suginome: 0000-0003-3023-2219
Notes
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The authors declare no competing financial interests.
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ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI (Grant Number
JP15H05811 in "Precisely Designed Catalysts with Customized
Scaffolding" Area) and Japan Science and Technology Corporation
(CREST, "Establishment of Molecular Technology towards the
Creation of New Function" Area).
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(8) The screw-sense excess (se) of each sample (1, 2, and 7–10) was esti-
mated using an averaged gmax value ( 2.10) calculated from the values for
(S,S)-3, (S,R)-4, (S,R)-5, and (R)-6 (Figure 2a).
(9) Ohmura, T.; Taniguchi, H.; Kondo, Y.; Suginome, M. J. Am. Chem. Soc.
2007, 129, 3518–3519.
(10) Chiral amplification in macromolecules, see: (a) Green, M. M.; Park,
J. W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V.
Angew. Chem. Int. Ed. 1999, 38, 3138–3154. (b) Yashima, E.; Maeda, K.;
Nishimura, T. Chem. Eur. J. 2004, 10, 42–51. (c) Palmans, A. R. A.; Meijer,
E. W. Angew. Chem. Int. Ed. 2007, 46, 8948–8968.
(11) Chiral amplification in catalytic asymmetric synthesis, see: (a) Guil-
laneux D.; Zhao S.-H.; Samuel, O.; Rainford, D.; Kagan H. B. J. Am. Chem.
Soc. 1994, 116, 9430–9439. (b) Satyanarayana, T.; Abraham, S.; Kagan, H.
B. Angew. Chem. Int. Ed. 2009, 48, 456–494.
(3) (a) Shimomura, K.; Ikai, T.; Kanoh, S.; Yashima, E.; Maeda, K. Nat.
Chem. 2014, 6, 429–434. (b)Nagata, Y.; Uno, M.; Suginome, M. Angew.
Chem. Int. Ed. 2016, 55, 7126–7130. (c) Nishikawa, T.; Nagata, Y.;
Suginome, M. Acs Macro Lett. 2017, 431–435.
(4) Representative examples. Polyacetylenes: (a) Moore, J. S.; Gorman, C.
B.; Grubbs, R. H. J. Am. Chem. Soc. 1991, 113, 1704–1712. (b) Yashima,
E.; Huang, S.; Matsushima, T.; Okamoto, Y. Macromolecules 1995, 28,
4184–4193. Polyisocyanates: (c) Green, M. M.; Andreola, C.; Munoz, B.;
Reidy, M. P.; Zero, K. J. Am. Chem. Soc. 1988, 110, 4063–4065. (d) Green,
M. M.; Reidy, M. P.; Johnson, R. D.; Darling, G.; O′Leary, D. J.; Willson,
G. J. Am. Chem. Soc. 1989, 111, 6452–6454. (e) Lifson, S.; Andreola, C.;
Peterson, N. C. Green, M. M. J. Am. Chem. Soc. 1989, 111, 8850–8858. (f)
Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S.
Science, 1995, 268, 1860–1866. (g) Jha, S. K.; Cheon, K. S.; Green, M. M.;
Selinger, J. V. J. Am. Chem. Soc. 1999, 121, 1665–1673. Polyisocyanides:
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