[3+3]cycloalkyne dimers linked by an azo group: A stable cis-Azo compound forms polymeric aggregates by nonplanar π-π interactions

Article abstract of DOI:10.1021/ja034942q

We previously reported that the structure of the linker moiety controlled intramolecular and bimolecular aggregation of [3+3]cycloalkyne oligomers, which are cyclic acetylene derivatives containing helicenes. Here, novel [3+3]cycloalkyne dimers linked by

Full text of DOI:10.1021/ja034942q

Published on Web 07/15/2003
[3+3]Cycloalkyne Dimers Linked by an Azo Group: A Stable cis-Azo
Compound Forms Polymeric Aggregates by Nonplanar π-π Interactions
Yuto Saiki,† Hiroki Sugiura,† Keiichi Nakamura,† Masahiko Yamaguchi,*,† Tomonori Hoshi,‡ and
Jun-ichi Anzai‡
Departments of Organic Chemistry and Pharmaceutical Physicochemistry,
Graduate School of Pharmaceutical Sciences, Tohoku UniVersity, Aoba, Sendai 980-8578, Japan
Received March 1, 2003; E-mail: yama@mail.pharm.tohoku.ac.jp
Scheme 1
The π-π interactions are noncovalent bondings between π-elec-
tron systems with a face-to-face orientation. Construction of a self-
assembly system based on the π-π interactions, however, is not
facile, because the interactions are weak and result in a higher
degree of aggregation under forced conditions. Usually, external
factors such as templates,1 solvophobic effects,2 or hydrogen
bonding3 are required to promote and control such aggregation of
π-compounds. Recently, we have found that [3+3]cycloalkynes,
which are chiral macrocyclic alkynes containing three helicenes,
form a strong and selective bimolecular aggregate in organic sol-
vents.4 It was also observed that the aggregation of [3+3]cyclo-
alkyne oligomers could be controlled by changing the structure of
the linker moieties:5 Oligomers with flexible linkers form intra-
molecular aggregates, and those with rigid linkers form bimolecular
aggregates without forming a higher degree of aggregation. We
were then interested in the formation of polymeric aggregates using
this system, and we describe here our finding that a [3+3]cyclo-
alkyne dimer cis-(M,M,M)/(M,M,M)-1 possessing a cis-azo linker
does polymerize, while trans-(M,M,M)/(M,M,M)-1 forms a bi-
molecular aggregate without forming a higher degree of aggregation.
This is a self-assembly system employing only the nonplanar π-π
Figure 1. UV-vis spectra of (M,M,M)/(M,M,M)-1 (0.001 mM) in CHCl3
at 25 °C.
interactions of helicenes, which appear to be much stronger than
the planar π-π interactions. It was also observed that the trans-
and cis-(M,M,M)/(M,M,M)-1 do not isomerize when subjected to
heating or irradiation.
CD (CHCl3) spectra of trans-(M,M,M)/(M,M,M)-1 are concentra-
tion-independent below 0.01 mM and concentration-dependent
above 0.01 mM, providing isosbestic points at 280, 320, 345, and
395 nm (Figure 2a,c). 1H NMR (CDCl3) signals of trans-(M,M,M)/
(M,M,M)-1 shift upfield and broaden as the concentration increases
from 0.1 to 10 mM.6 Vapor pressure osmometry (VPO) in CHCl3
revealed bimolecular aggregate formation of the compound above
2 mM (Figure 3). The retention volume of GPC provided an
apparent molecular weight of 6400 in CHCl3 (2 mM); the MW of
trans-(M,M,M)/(M,M,M)-1 was 3033.6 These analyses indicate that
trans-(M,M,M)/(M,M,M)-1 is monomeric below 0.01 mM, and a
bimolecular aggregate is selectively formed above 2 mM without
Isomeric trans- and cis-(M,M,M)/(M,M,M)-1 were synthesized
forming a higher degree of aggregation.
as shown in Scheme 1. Cyclization of (M,M,M)-34 and 3,5-
CD (CHCl3) spectra of cis-(M,M,M)/(M,M,M)-1 are concentra-
diiodonitrobenzene 4 produced a nitro derivative (M,M,M)-5 in 50%
tion-independent between 0.001 and 0.5 mM (Figure 2b,d). 1H
yield, which was reduced by iron to form an amine (M,M,M)-6 in
NMR (CDCl3) spectra of cis-(M,M,M)/(M,M,M)-1 are concentra-
96% yield. Oxidative coupling with manganese(IV) oxide in toluene
tion-independent below 1 mM and broaden above 1 mM.6 VPO in
generated trans- and cis-(M,M,M)/(M,M,M)-1 in 47% and 17%
CHCl3 revealed trimolecular aggregate formation below 1 mM and
yields, respectively, which were readily separable by GPC.6 The
polymeric aggregate formation above 1 mM (Figure 3). The GPC
cis/trans-stereochemistry was determined by UV-vis spectroscopy
analysis gave a molecular weight of 8400 in CHCl3 (1 mM).6 It
(Figure 1): The trans-(M,M,M)/(M,M,M)-1 with a larger retention
has become clear that cis-(M,M,M)/(M,M,M)-1 polymerizes in
volume by GPC exhibits stronger absorption at 300-400 nm (π-
solution employing the nonplanar π-π interactions as the only
π* transition) and weaker absorption in the visible region (n-π*
driving force.
transition).7
It is also noticed that trans- and cis-(M,M,M)/(M,M,M)-1 are inert
toward heat and light. trans-(M,M,M)/(M,M,M)-1 in CHCl3 at 5
and 0.1 mM does not isomerize when subjected to irradiation at
† Department of Organic Chemistry.
‡ Department of Pharmaceutical Physicochemistry.
9
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J. AM. CHEM. SOC. 2003, 125, 9268-9269
10.1021/ja034942q CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 4. Aggregation behaviors of (M,M,M)/(M,M,M)-1.
exist in the same plane,11 they do not aggregate in a bimolecular
fashion. As a result, cis-(M,M,M)/(M,M,M)-1 gives trimolecular or
higher aggregates. It is in contrast to the bimolecular aggregate
formation of trans-(M,M,M)/(M,M,M)-1, the [3+3]cycloalkyne
moieties of which can exist in the same plane.5,12 The present
finding would provide a novel methodology for constructing a
controlled self-assembly system involving selective bimolecular
π-π interactions of helicene.
Figure 2. CD spectra of (a) trans-(M,M,M)/(M,M,M)-1, and (b) cis-
(M,M,M)/(M,M,M)-1 in CHCl3 at 25 °C. Plots of ∆ꢀ at 360 nm versus
concentration for (c) trans-(M,M,M)/(M,M,M)-1, and for (d) cis-(M,M,M)/
(M,M,M)-1.
Acknowledgment. The authors thank JSPS for providing
financial support.
Supporting Information Available: Experimental procedures for
the synthesis of 1, 2, 5, and 6, GPC chromatogram and 1H NMR spectra
of trans-(M,M,M)/(M,M,M)-1 and cis-(M,M,M)/(M,M,M)-1, isomeriza-
tion experiment procedures for 1 and 2 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 3. Degree of aggregation by VPO (CHCl3, 35 °C) for trans-
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A polymeric aggregate is currently obtained using cis-(M,M,M)/
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intramolecular aggregate or a bimolecular aggregate (Figure 4). The
rigid cis-azo structure possessing a 120° direction does not allow
the formation of the intramolecular aggregated structure. Because
two [3+3]cycloalkyne moieties of cis-(M,M,M)/(M,M,M)-1 cannot
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