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Chemistry Letters Vol.36, No.8 (2007)
Synthesis and Helical Structure of Oligo(quinoline-2,3-diyl)s
Michinori Suginome,ꢀ Hiroyoshi Noguchi, and Masahiro Murakami
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
(Received June 4, 2007; CL-070595; E-mail: suginome@sbchem.kyoto-u.ac.jp)
Oligo(quinoline-2,3-diyl)s having a terminal bromo group
were synthesized by the Suzuki–Miyaura coupling of 5,8-disub-
stituted 2-bromoquinolin-3-ylboronic acid derivatives, and their
helical structures were elucidated by a single-crystal X-ray
analysis of the hexamer, UV measurements, and induction of a
non-racemic helical sense by introduction of a chiral end group.
by MS (Entries 2 and 3). In the reaction mixture after 10 h
(Entry 2), a considerable amount (18%, not included in the total
oligomer yield) of 2-bromo-5,8-dimethylquinoline 2 (n ¼ 1),
which may be derived by hydrolysis of 1, was obtained. A longer
reaction time resulted in the shift of the distribution toward
higher oligomers (n ¼ 8{12; 23%), giving a higher total yield
(78%) of oligomers (Entry 3). In Entries 4 and 5, the correspond-
ing pinacol boronate 1b was used to minimize hydrolysis of
the monomer. The boronate 1b showed a significantly lower
reactivity than 1a. Only 19% of oligomers was obtained, along
with a considerable amount of hydrolyisis product, in aqueous
dioxane with KF as an additive (Entry 4). However, the use of
anhydrous DMF as the solvent with use of NaHCO3 as the base
dramatically improved the yield of oligomers (94%) (Entry 5).
The reaction produced higher oligomers (n ¼ 8{12) in 10% total
yield in addition to trimer to heptamer (18, 13, 20, 19, and 14%
yield, respectively). Almost no hydrolysis product was detected
in the reaction mixture. It should be noted that extension of the
reaction time did not change the oligomer distribution.
On the basis of MS and NMR analysis, the oligomers were
found to lack the terminal boryl groups, but retained the bromo
group at the 2 positions of the oligomers.9 The oligomers higher
than trimer (n ꢁ 4) showed broadened 1H NMR signals at room
temperature (400 MHz). The pentamer showed a complicated
NMR spectrum at room temperature in benzene-d6, while sharp
signals were observed at 55 ꢂC. As for the hexamer, a well-
resolved NMR spectrum could not be obtained even at 80 ꢂC.
The oligomers (n ¼ 3{7) isolated as colorless solids showed
almost constant ꢀmax (273 nm) and a shoulder (ca. 340 nm) in
the UV spectra (Figure 3), suggesting that there is no significant
conjugation along the oligoquinoline skeleton and that the
oligomers do not adopt a planar structure.11
Helical organic polymers and oligomers attract much atten-
tion in organic chemistry, because they are expected to exhibit
unique properties arising from their helical structures.1 In partic-
ular, much effort has been devoted to selective synthesis of
helical polymers that are capable of retaining non-racemic heli-
cal structures.1,2 For further development in this field, it seems to
be very important to explore new polymer scaffolds constituting
stable helical structures.
We have been interested in the synthesis of aromatic poly-
mers and oligomers in which naphthalene-like monomer units
are linked together at their 2- and 3-positions.3,4 Such polymers
are not capable of adopting a planar structure because of steric
repulsion between the rings; instead they adopt helical structures
to minimize the steric repulsion in certain cases (Figure 1).
It has also been demonstrated that the helical structure can be
controlled by introduction of appropriate chiral groups at the
terminus of the polymer and oligomers.2c,3d Herein, we report
the synthesis of new quinoline oligomers,5 in which quinoline
units are linked at their 2- and 3-positions. The helical structure
of the oligoquinolines was established by a single-crystal X-ray
analysis. Moreover, the non-racemic helical sense was success-
fully induced in the oligoquinolines by introduction of optically
active amide groups at the termini of the oligomers.
2-Bromoquinolin-3-ylboronic acid derivatives 16 were sub-
jected to the Suzuki–Miyaura coupling7,8 under various reaction
conditions (Figure 2). A reaction of the boronic acid 1a in the
presence of a Pd(PPh3)4 catalyst produced trimer 2 (n ¼ 3) as
the major product (34%) along with the higher oligomers up to
the pentamer in 51% total yield (Entry 1).9 We then switched
the catalyst system to Pd[P(t-Bu)3]2 to obtain higher oligo-
mers.10 In the presence of KF as an additive, boronic acid 1a
produced higher oligomers up to the 12mer, which were detected
The oligo(quinoline-2,3-diyl)s were dissolved in acidic
aqueous solution. Although 2 M hydrochloric acid hardly dis-
solved the hexamer at room temperature, 6 M hydrochloric acid
5
3
n
2
8
R
5′′
X
R'
5′
8′′
Y
5
R
X = Y = N: quinoxaline-2, 3-diyl
X = Y = CH: naphthalene-2, 3-diyl
X = N, Y = CH: quinoline-2, 3-diyl
8′
8
planar structure
helical structure
(not possible)
Figure 1. A concept of helix formation by linking 5,8-disubstituted
naphthalene-2,3-diyl-like monomers.
Figure 2. Synthesis of oligo(quinoline-2,3-diyl)s by Suzuki–Miyaura
coupling of 1a or 1b.
Copyright Ó 2007 The Chemical Society of Japan
Suginome, Michinori
