First Dewar benzene approach to acetylenic oligophenylene macrocycles:
synthesis and structure of a molecular rectangle bearing two spindles
Masakazu Ohkita, Kohta Ando, Ken-ichi Yamamoto, Takanori Suzuki and Takashi Tsuji*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
E-mail: tsuji@sci.hokudai.ac.jp
Received (in Cambridge, UK) 2nd November 1999, Accepted 29th November 1999
A novel Dewar benzene approach to the construction of
oligophenylene macrocycles has been developed by in-
troducing Dewar benzene 1 as a building block and applied
to the synthesis of a molecular rectangle 8, whose structure
was characterized crystallographically.
In conclusion, we have successfully synthesized and charac-
terized a novel rectangular macrocycle 8 by introducing 1 as a
building block. The high efficiency of the macrocyclization of
4 into 5 demonstrates the synthetic utility of a Dewar structure
as a ‘masked’ p-phenylene unit. Further elaboration and
application of the present Dewar benzene strategy are now in
progress.
We thank Professor Tamotsu Inabe, Hokkaido University, for
the use of X-ray analytical facilities. This work was supported
in part by a Grant-in-Aid for Scientific Research (No.
09640620) from the Ministry of Education, Science, Sports and
Culture of Japan.
During the course of our study on small cyclophanes generated
from the corresponding Dewar derivatives,1 we were interested
in exploiting a Dewar benzene for the construction of
macrocyclic structures, especially for the synthesis of p-
phenylene-based rigid, and thus strained, macrocycles. Since
the bent shape of Dewar benzene is expected to provide a unique
template effect on the cyclization, novel macrocyclic systems
which are difficult to access from planar benzene derivatives
may well become available based on the Dewar benzene
approach. Here we report the rational and efficient synthesis of
a novel rectangular macrocycle 8 using acetal-bridged Dewar
benzene 1 as a building block, together with its crystallographic
characterization. Phenylene acetylene macrocycles have at-
tracted considerable recent attention in supramolecular chem-
istry and materials science.2
Dewar benzene 1 was readily prepared in four steps and on a
gram scale from DMAD and 1,2-dichloroethylene.3 Palladium-
catalyzed coupling of 1 with 2† afforded diethynylation product
3 (55% yield), which was then deprotected to 4 in 80% yield
(Scheme 1). Copper-mediated oxidative coupling of 4 with
CuCl/Cu(OAc)24 in pyridine under pseudo-high-dilution condi-
tions produced cyclic dimer 5‡ in 70% yield. Acid hydrolysis of
5 followed by the silylation of resulting tetraol 6 afforded 7§ in
73% yield. Photochemical isomerization of 7 proceeded
smoothly and cleanly by irradiation with a high-pressure
mercury lamp through Pyrex at 12 °C to afford 8§ in
quantitative yield. The less symmetrical product resulting from
the rearrangement of only one of the Dewar benzene units was
not detected by 1H NMR monitoring of the reaction.
1
It is interesting to note that, in the H NMR spectra, the
benzylic protons of 7 appear as a pair of AB doublets (d 4.10 and
4.12, J = 11 Hz) while those of 8 appear as a singlet (d 4.95),
indicating that these protons are diastereotopic in 7 and
operationally enantiotopic in 8. These observations would be
most reasonably explained by postulating fast interconversion
between the rotamers 8a and 8b, namely, fast rotation of the p-
phenylene unit(s) in 8 on the NMR timescale despite the
strained nature of the macrocycle.5
Crystals of 8 suitable for X-ray structure determination (Fig.
1) were obtained by slow diffusion of EtOH into a solution of 8
in CHCl3 at room temperature.¶ The molecule adopts a nearly
planar conformation in the crystal and the dimension of
framework is 7.32 3 11.54 Å, as defined by the four inner m-
phenylene carbon atoms. Interestingly, the framework appears
to be distorted to a parallelogram to accommodate two
SiMe2OCH2 moieties inside the cavity. The twist angles of the
benzene rings from the least-square plane formed by the
acetylenic carbon atoms are 3.7 and 0.4° in the m-phenylene
units and 3.4° in the p-phenylene units. The benzene rings are
essentially planar (deviation < 0.01 Å) while the acetylene
units show small deviation from linearity with the C·C–C
angles of 168.7(3)–179.9(3)°.
Scheme 1 Reagents and conditions: i, Pd(PPh3)4, CuI, Et3N, 45 °C, 55%; ii,
Bu4NF, THF, 0 °C, 80%; iii, CuCl, Cu(OAc)2, Py, syringe-pump addition
over 2 h, 60 °C, 70%; iv, aq. HCl, THF, room temp., 98%; v, ButMe2SiOTf,
Et3N, CH2Cl2, room temp., 75%; vi, hn, CH2Cl2, 12 °C, 100%.
Chem. Commun., 2000, 83–84
This journal is © The Royal Society of Chemistry 2000
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