Planar chiral tetrasubstituted [2.2]paracyclophane: Optical resolution and functionalization

Article abstract of DOI:10.1021/ja412197j

We achieved optical resolution of 4,7,12,15-tetrasubstituted [2.2]paracyclophane and subsequent transformation to planar chiral building blocks. An optically active propeller-shaped macrocyclic compound containing a planar chiral cyclophane core was synthesized, showing excellent chiroptical properties such as high fluorescence quantum efficiency and a large circularly polarized luminescence dissymmetry factor.

Full text of DOI:10.1021/ja412197j

Communication
pubs.acs.org/JACS
Planar Chiral Tetrasubstituted [2.2]Paracyclophane: Optical
Resolution and Functionalization
Yasuhiro Morisaki,*,† Masayuki Gon,† Takahiro Sasamori,‡ Norihiro Tokitoh,‡ and Yoshiki Chujo*,†
†Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
‡Institute for Chemical Research, Kyoto University Gokasho, Uji, Kyoto 611-0011, Japan
S
* Supporting Information
ABSTRACT: We achieved optical resolution of 4,7,12,15-
tetrasubstituted [2.2]paracyclophane and subsequent
transformation to planar chiral building blocks. An
optically active propeller-shaped macrocyclic compound
containing a planar chiral cyclophane core was synthesized,
showing excellent chiroptical properties such as high
fluorescence quantum efficiency and a large circularly
polarized luminescence dissymmetry factor.
lanar chiral [2.2]paracyclophanes provide a conformation-
ally stable chiral environment due to suppression of the
P
rotation of phenylenes.1 Optical resolutions of various
[2.2]paracyclophanes have been conducted,1−3 and the
resulting optically active [2.2]paracyclophane compounds
have mainly been used as chiral auxiliaries. For example, aryl-
PHANEPHOS3a,b are well-known commercially available
compounds; they are widely used as chiral ligands for transition
metal-catalyzed asymmetric reactions.
We have previously studied the planar chirality of [2.2]-
paracyclophane and developed a practical optical resolution
method for pseudo-ortho-disubstituted [2.2]paracyclophanes to
be used as a chiral building block for through-space conjugated
compounds.3i,4 There have been several reports on optical
resolution of disubstituted [2.2]paracyclophane;3 however, only
one report on that of a tetrasubstituted [2.2]paracyclophane
compound exists.5 Considering the potential applications of
[2.2]paracyclophane skeletons in polymer and materials
chemistry,6 as well as organic and organometallic chemistry,
further development and modification of optical resolution
methods for planar chiral tetrasubstituted [2.2]paracyclophanes
would be valuable. Herein, we report optical resolution of rac-
4,7,12,15-tetrabromo[2.2]paracyclophane and subsequent
transformations to produce planar chiral building blocks for
through-space carbon-rich compounds.7 In this study, an
optically active macrocycle8 based on a tetrasubstituted
[2.2]paracyclophane was synthesized. The excellent chiroptical
properties, in particular, circularly polarized luminescence
(CPL), are also reported.
Figure 1. Optical resolution of rac-1. Crystal structures of (Sp,1S,4R)-4
and (Rp,1S,4R)-4 with ellipsoids at 30% probability. Hydrogen atoms
and solvent (CHCl3 in (Rp,1S,4R)-4) are omitted for clarity.
column chromatography and purified by recrystallization to
obtain (Sp,1S,4R)-4 and (Rp,1S,4R)-4 in 38% and 34% isolated
yield, respectively (each diastereomer ratio (dr) > 99.5%).10
The structures were confirmed by NMR spectroscopy, mass
analysis, elemental analysis, and X-ray crystallography (Figure
1).
Hydrolysis and subsequent transformation of (Sp,1S,4R)-4
are shown in Scheme 1. Treatment of (Sp,1S,4R)-4 with KOH
afforded (Sp)-2. This compound was used for the next
transformation to OTf without purification, and enantiopure
(Sp)-5 was obtained in 92% isolated yield. Sonogashira-
Hagihara coupling11 of (Sp)-5 with trimethylsilylacetylene
Optical resolution of tetrasubstituted [2.2]paracyclophane
was carried out by a diastereomer method beginning with
4,7,12,15-tetrabromo[2.2]paracyclophane9 rac-1, as shown in
Figure 1. One of bromides in rac-1 was converted to a hydroxyl
group to obtain rac-2 in 69% isolated yield,3c−e which was
reacted with (−)-(1S,4R)-camphanoyl chloride 3 to obtain a
mixture of diastereomers. These were readily separated by SiO2
Received: December 5, 2013
Published: February 17, 2014
© 2014 American Chemical Society
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crystallized into a single crystal, and the bowtie-shaped13
structure was confirmed from the top view. As shown in the
front and side views, the structure seems like a two-blade
propeller owing to the planar chiral [2.2]paracyclophane core.
This structure has previously been synthesized by Hopf, Haley,
and co-workers as a racemic compound,14 although the
positions of the tert-butyl groups were different.
Scheme 1. Synthesis of Optically Active Macrocycle
The optical properties of (Rp)- and (Sp)-10 were
investigated; the UV−vis absorption, circularly dichroism
(CD), photoluminescence (PL), and CPL spectra of (Rp)-
and (Sp)-10 in dilute CHCl3 solution (1.0 × 10−5 M) are
shown in Figure 2. The UV−vis absorption spectrum of 10
using a Pd2(dba)3/(t-Bu)3P catalysis gave only (Sp)-6 in 83%
isolated yield. Interestingly, bromide was selectively reacted,
and tetra(trimethylsilylethynyl)[2.2]paracyclophane (Sp)-7 was
not detected by thin-layer chromatography. Reacting (Sp)-6
with trimethylsilylacetylene using a PdCl2(dppf) catalysis
afforded (Sp)-7. Removal of the trimethylsilyl group was
carried out with K2CO3/MeOH afforded the corresponding
tetrayne12 (Sp)-8 in 91% isolated yield. The enantiomer (Rp)-8
was also synthesized by the same route.
Figure 2. (A) UV−vis absorption and CD spectra of (Rp)- and (Sp)-10
in CHCl3 (1.0 × 10−5 M) at room temperature. (B) PL and CPL
spectra of (Rp)- and (Sp)-10 in CHCl3 (1.0 × 10−6 M for PL and 1.0 ×
10−5 for CPL) at room temperature, excited at 314 nm.
Tetrasubstituted [2.2]paracyclophanes 5 and 8 can be
employed as conformationally stable chiral building blocks for
various optically active carbon-rich compounds. In this study,
we synthesized an optically active propeller-shaped macro-
cycle14 from 8, as shown in Scheme 1. The reaction of (Sp)-8
with 5-tert-butyl-2-[(trimethylsilyl)ethynyl]iodobenzene af-
forded the corresponding optically active compound (Sp)-9 in
89% isolated yield. Desilylation of (Sp)-9 with K2CO3/MeOH
and a subsequent oxidative coupling reaction using Cu(OAc)2
gave the target macrocycle (Sp)-10 in 40% isolated yield.
Enantiomer (Rp)-10 was also prepared, and their structures
were confirmed by NMR spectroscopy and mass analysis. A
single crystal of rac-10 was obtained by recrystallization with
CHCl3 and MeOH, and the molecular structure is shown in
Figure S20 (Supporting Information). The enantiomers co-
(Figure 2A) was identical to that of the cyclic compound
prepared by Hopf, Haley, and co-workers.14 Thus, there was no
difference in the electronic structure of the ground state
between these compounds regardless of the positions of the
tert-butyl groups.15 In the CD spectra of (Rp)- and (Sp)-10,
intense and mirror image Cotton effects were observed in the
absorption bands of the UV−vis spectra (Figure 2A). The
molar ellipticity ([θ]) was very large, with a [θ] for (Sp)-10 of
2.7 × 106 deg cm2 dmol−1. The dissymmetry factor of
absorbance, gabs = 2(Δε/ε), is another parameter indicating
chirality in the ground state; a large gabs value of 0.9 × 10−2 was
obtained. The specific rotation [α]2D3 (c 0.5, CHCl3) of (Sp)-10
was estimated to be −1494.9, whereas that of (Sp)-9 was +44.1.
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Notes
In all cases, the chiroptical data for (Sp)-10 were considerably
enhanced compared with those for (Sp)-9 in the ground state.
As shown in Figure 2B, compound 10 exhibited a vibronic
emission peak at around 460 nm with an absolute PL quantum
efficiency (Φlum) of 0.45 for (Sp)-10. The PL decay curve was
fitted with a single exponential relationship (χ2 = 1.18), and the
PL lifetime (τ) was calculated to be 3.71 ns (Figure S41). This
efficient PL arose from criss-cross delocalization across the
entire molecule via the strong through-space interaction of the
[2.2]paracyclophane core.16
Intense and mirror image CPL signals for (Rp)- and (Sp)-10
were observed in the emission region (Figure 2B) with a large
CPL dissymmetry factor, glum = 2(Ileft − Iright)/(Ileft + Iright),
where Ileft and Iright are the PL intensities of left and right CPL,
respectively. The maximum |glum| value was estimated to be 1.1
× 10−2 (Figure S40). It is rare that a monodispersed chiral
hydrocarbon exhibits such a large glum on the order of
10−2.17d,f,g Recently, small molecules that exhibit CPL in dilute
solution have been extensively studied; helically and axially
chiral compounds have been known to have CPL with large glum
values on the order of 10−3−10−2.17 A conformationally stable
chiral structure of the emitting species, such as a helical
structure, in the excited state is essential to obtain CPL with a
large glum. Macrocycle 10 possesses a conformationally stable
chiral second-ordered structure (propeller-shaped structure)
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grant-in-Aid for Young Scientists
(A) (No. 24685018) from the MEXT. Financial support from
Foundation for Interaction in Science & Technology is
gratefully acknowledged. The authors are grateful to Prof.
Kazuo Akagi and Mr. Kazuyoshi Watanabe (Department of
Polymer Chemistry, Graduate School of Engineering, Kyoto
University) for CPL data analysis and valuable discussions.
REFERENCES
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There
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In conclusion, we have developed a practical method for
optical resolution of planar chiral tetrasubstituted [2.2]-
paracyclophane. The obtained enantiopure 4,7,12,15-tetrafunc-
tional cyclophane was readily modified to the corresponding
planar chiral compounds. In the present study, a propeller-
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molecules. Therefore, further investigations of [2.2]-
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ASSOCIATED CONTENT
■
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* Supporting Information
Experimental details, characterization data, and additional
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Authors
ymo@chujo.synchem.kyoto-u.ac.jp
chujo@chujo.synchem.kyoto-u.ac.jp
(14) Hinrichs, H.; Boydston, A. J.; Jones, P. G.; Hess, K.; Herges, R.;
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(15) Theoretical Cotton effect patterns were also identical, as shown
in Figure S21.
(16) Electronic delocalization between chromophores connected to
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125, 11808−11809. (b) For perylene-diimide-substituted binaphthyl
(glum = 3 × 10−3): Kawai, T.; Kawamura, K.; Tsumatori, H.; Ishikawa,
M.; Naito, M.; Fujiki, M.; Nakashima, T. ChemPhysChem 2007, 8,
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receptor−anion complex (glum = 1.3 × 10−2): Haketa, Y.; Bando, Y.;
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(18) The CPL spectra and glum charts of (Rp)- and (Sp)-9 are shown
in Figures S26 and S27, respectively. Their glum values were sufficiently
large, |glum| = 1.2−1.3 × 10−3.
(19) No thermochromism and solvatochromism were observed in
UV, CD, PL, and CPL spectra for 10 as well as 9; see Figures S29−S34
for 9 and S42−S47 for 10.
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