Synthesis and Conformational Analysis of 2,11-Disila[3.3]metacyclophanes

Article abstract of DOI:10.1002/ejoc.201600800

Silyl-tethered [3.3]metacyclophanes were prepared and subjected to conformational analysis. The results show that these compounds exist in unprecedented anti-rich metacyclophane forms. In the case of 2,2,11,11-tetrasubstituted 2,11-disila[3.3]metacyclopha

Full text of DOI:10.1002/ejoc.201600800

DOI: 10.1002/ejoc.201600800
Communication
Cyclophanes
Synthesis and Conformational Analysis of 2,11-
Disila[3.3]metacyclophanes
[a]
[a][‡]
[a]
[a]
Tomoo Hayamizu, Hajime Maeda,*
Kazuhiko Mizuno*[
Takashi Ouchi, Naoki Kakiuchi, and
a][‡][‡]
Abstract: Silyl-tethered [3.3]metacyclophanes were prepared graphic analyses, along with molecular orbital calculations.
and subjected to conformational analysis. The results show that However, the 2,2,11,11-tetrahydro derivative displays the oppo-
these compounds exist in unprecedented anti-rich metacyclo- site behavior, which means that the syn conformer is thermody-
phane forms. In the case of 2,2,11,11-tetrasubstituted 2,11-di- namically most stable. The syn/anti ratios of these compounds
sila[3.3]metacyclophanes, anti conformers are lower in energy in solution are governed by temperature and solvent polarity
than their syn counterparts. Evidence for this assignment comes in a manner that can be explained by considering the mixed
1
from the results of H NMR spectroscopic and X-ray crystallo- contributions of ΔΔG and dipole moments.
Introduction
Cyclophanes have been the subject of numerous studies be-
cause of their unique properties that are not only reflected in
[
1]
their structures but also in their host–guest characteristics.
Among these compounds, particular attention has been given
to [m.n]metacyclophanes, because they exist as equilibrium
mixtures of syn and anti conformers (Scheme 1). For example,
ethylene-bridged [2.2]metacyclophane 1 exists predominantly
1
in the anti conformation, as reflected in its H NMR spectrum,
which contains an unusually upfield-shifted resonance corre-
sponding to its inner hydrogen atoms (H ) as a consequence of
i
[
2]
the benzene ring current effect. In contrast, [3.3]metacyclo-
phanes in which the arene rings are bridged by three carbon
[
3]
[4]
[5]
[6]
atoms, nitrogen-, oxygen-, sulfur- (compounds 2, 3, 4,
[
7]
respectively), or selenium- containing linkages, all exist in the
Scheme 1. Conformations (anti and syn) of [m.n]metacyclophanes.
syn conformation; therefore, such upfield shifts were not ob-
served in the H NMR spectra.
1
In an earlier effort, we described the synthesis, properties, low, have demonstrated that these compounds exist in unex-
[
8]
and chemical reactivities of selected cyclic benzylic silanes.
pected anti-rich [3.3]metacyclophane conformations.
Continuing interest in this family of compounds prompted us
to undertake an investigation aimed at the synthesis and con-
formational analysis of previously unknown silyl-tethered Results and Discussion
[
9]
[
3.3]metacyclophanes. The results of this effort, described be-
The preparative method used to synthesize the silyl-tethered
3.3]metacyclophanes involves Grignard reactions between m-
[
[
a] Department of Applied Chemistry, Graduate School of Engineering, Osaka
Prefecture University,
xylylene dichloride (5) and dichlorosilanes (Scheme 2). For ex-
ample, the reaction of magnesium with a mixture of 5
1
-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
E-mail: maeda-h@se.kanazawa-u.ac.jp
http://kohka.ch.t.kanazawa-u.ac.jp/lab4/lab4.html
(50 mmol) and dichlorodimethylsilane (6a, 60 mmol) in THF
gives a product mixture containing cyclic dimer 7a, cyclic trimer
8a, and unidentifiable cyclic oligomers. HPLC (GPC) separation
of the mixture generates 7a and 8a in 36 and 25 % isolated
yields, respectively. Reaction of 5 with dichlorodiphenylsilane
[
[
‡] Present address: Division of Material Chemistry, Graduate School of
Natural Science and Technology, Kanazawa University,
Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
‡‡] Present address: Graduate School of Materials Science, Nara Institute of
Science and Technology (NAIST),
(6b), performed in a similar manner, gives cyclic dimer 7b, tri-
8916-5, Takayama-cho, Ikoma, Nara 630-0192, Japan
mer 8b, and tetramer 9b in 19, 7, and 2 % yield, respectively.
Finally, reaction of 5 with dichloromethylphenylsilane (6c) leads
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201600800.
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Communication
to production of two cyclic dimers, cis-7c, trans-7c, along with (compound trans-7c) derivatives have anti structures in the
two stereoisomeric trimers.
crystalline state and that the two benzene rings in each are
aligned in a parallel fashion with the inner aromatic hydrogen
atoms located on the faces of the opposing benzene rings [Fig-
ure 1(a),(b)]. To explore the reasons for these unusual conforma-
tional preferences, related metacyclophanes were subjected to
analysis. The tetraphenyl-digerma analog 12, a previously un-
known compound, was prepared by Grignard reaction of 5 with
Ph GeCl . X-ray crystallographic analysis showed that 12 also
2
2
exists in an anti conformation in the crystalline state [Fig-
ure 1(c)]. The related sulfur-tethered metacyclophane 4, which
[
6]
is known to exist in a syn-rich form, was synthesized. We have
confirmed that the syn structure of this compound exists in
both the crystalline state [Figure 1(d)] and in CDCl at room
3
temperature (inner protons at 6.83 ppm).
Scheme 2. Synthesis of 2,11-disila[3.3]metacyclophanes.
As described above, the chemical shifts of inner hydrogen
1
atoms in the H NMR spectra of metacyclophanes aid in the
determination of conformational preferences in these com-
1
pounds. Interestingly, the H NMR spectrum of 7a in CDCl con-
3
tains an unusual resonance at 6.02 ppm (s, 2 H), which is as-
signed to inner aromatic hydrogen atoms at C-9 and C-18. For
comparison, the inner aromatic protons of cyclic trimer 8a and
m-bis[(trimethylsilyl)methyl]benzene (10) resonate at more
downfield positions (6.63 and 6.69 ppm, respectively). Similarly,
resonances for two sets of aromatic protons in 7b (5.97 ppm),
cis-7c (6.22 ppm), and trans-7c (6.03 ppm) are also shifted up-
field relative to those of 8b (6.76 ppm), 9b (6.62 ppm), and m-
bis[(methyldiphenylsilyl)methyl]benzene (11, 6.51 ppm). These
observations indicate that 7a, 7b, cis-7c, and trans-7c exist ei-
ther predominantly or exclusively in anti conformations in
Figure 1. ORTEP drawings of 2,11-disila[3.3]metacyclophanes and the related
compounds. (a) 7b: C40
H
36Si
2
; monoclinic; C2/c (#15); a = 13.666(1) Å; b =
3
1
1
2.2024(9) Å; c = 19.219(1) Å; ꢀ = 94.468(5)°; V = 3184.4(4) Å ; Z = 4; Dcalcd
.195 g cm ; R = 0.056; R
=
–3
w
= 0.058. (b) trans-7c: C30
H32Si
2
; monoclinic; C2/c
(#15); a = 17.213(1) Å; b = 6.3969(6) Å; c = 24.605(2) Å; ꢀ = 109.423(5)°; V =
3
–3
2555.1(3) Å ; Z = 4; Dcalcd = 1.166 g cm ; R = 0.059; R
36Ge ; monoclinic; C2/c (#15); a = 13.7000(7) Å; b = 12.3253(5) Å; c =
9.1235(7) Å; ꢀ = 96.373(1)°; V = 3209.2(2) Å ; Z = 4; Dcalcd = 1.370 g cm
R = 0.055; R = 0.065. (d) 4: C16 ; monoclinic; P2 /n (#14); a = 9.155(2) Å;
b = 7.9497(9) Å; c = 18.883(4) Å; ꢀ = 100.055(4)°; V = 1353.2(4) Å ; Z = 4;
w
= 0.069. (c) 12:
C
1
40
H
2
3
–3
;
CDCl at room temperature. This is a surprising finding, because
3
w
H
16
S
2
1
all previously described [3.3]metacyclophanes exist as syn struc-
tures under comparable conditions.
3
–
3
D
w
calcd = 1.337 g cm ; R = 0.066; R = 0.082.
In order to gain information about the solid-state structures
of the new disila[3.3]metacyclophanes, single-crystal X-ray dif-
fraction analyses were performed (Figure 1). The results show silyl- and germyl-tethered metacyclophanes, has been explored
The preference for the anti conformation, observed for the
[
10]
that tetraphenyl (compound 7b) and trans-dimethyldiphenyl by using theoretical methods. Earlier, Mitchell carried out calcu-
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Communication
1
lations with metacyclophanes that led to theoretical heats of and 6.67 ppm at –90 °C [Figure 3(q)]. H NMR spectra of CD Cl
2
2
formation of syn-chair-chair, syn-chair-boat, syn-boat-boat, anti- solutions of the reference compounds, m-bis[(methyldiphenyl-
[
6v]
chair-chair, and anti-chair-boat conformations.
Referring to silyl)methyl]benzene (11) and m-bis[(dihydromethylsilyl)-
the report, we calculated heats of formation of silyl-tethered methyl]benzene (13), at room temperature are shown in Fig-
metacyclophanes and related compounds (Table 1). These re- ure 3(i) and (r). The chemical shifts of the benzylic protons re-
sults demonstrate that syn-chair-chair conformers are the most main nearly the same when solutions of these compounds are
stable when the metacyclophanes possess carbon-, oxygen-, cooled to –90 °C. The difference in chemical shift (Δδ) for Hi
and sulfur-containing bridges. However, we found that the situ- between tetraphenyl derivatives 7b at –90 °C and 11 at room
ation is different in silicon- and germanium-tethered cyclo- temperature is 0.65 ppm, whereas Δδ for the related tetrahydro
phanes, for which the anti-chair-chair forms are calculated to derivative pair at the same temperatures is 0.15 ppm. These
be thermodynamically most stable. Interestingly, theoretical observations are in accord with the assumption that at lower
methods show that the syn-chair-chair form of the dihydrosilyl- temperature the conformational equilibrium of 7b, already
tethered metacyclophane 7d is of lowest energy. Intrigued by biased toward the anti form at room temperature, shifts to favor
this observation, we prepared 7d by using HCl promoted de- the anti form even more. However, in the case of 7d, the equi-
phenylation of the tetraphenyl derivative 7b (Figure 2). X-ray librium shifts in the opposite direction to favor the syn form
analysis (Figure 2) showed that, as predicted, this compound more at lower temperature.
possesses a syn structure in the crystalline state.
Table 1. Heats of formation of [3.3]metacyclophanes.[a]
Conformer
Relative heats of formation [kcal/mol]
Z =
CMe
2
SiH
2
SiMe
2
SiPh
2
GePh
2
O
S
CH
2
syn c-c
syn c-b
syn b-b
anti c-c
anti c-b
0
0
0
1.4
1.2
2.4
0
2.0
2.2
3.3
0
2.4
2.5
3.5
0
0
0.1
0
0.3
2.6
2.0
1.3
2.3
2.7
2.4
4.4
2.8
0.4
4.0
1.5
2.9
0.7
2.5
1.8
3.6
2.5
4.0
2.8
4.4
4.3
[
a] Relative energies to thermodynamically most stable conformers, calcu-
lated by PM3 in MOPAC Ver. 94.10.
Figure 2. Synthesis and ORTEP drawing of tetrahydro derivative 7d: C16
2
H20Si ;
monoclinic; P2 /c (#14); a = 11.085(5) Å; b = 10.187(4) Å; c = 13.930(6) Å; ꢀ =
1
Figure 3. Variable-temperature 1H NMR spectra of 7b and 7d. Left: 1H NMR
spectra of 7b in CDCl at room temperature (a), in CD Cl at room tempera-
ture (b), 10 °C (c), –10 °C (d), –30 °C (e), –50 °C (f), –70 °C (g), –90 °C (h),
and that of m-bis[(methyldiphenylsilyl)methyl]benzene (11) in CD Cl at room
at room temperature
(j), in CD Cl at room temperature (k), 10 °C (l), –10 °C (m), –30 °C (n), –50 °C
3
–3
9
3.15(4)°; V = 1570(1) Å ; Z = 4; Dcalcd = 1.135 g cm ; R = 0.060; R1 = 0.054.
3
2
2
Variable-temperature 1H NMR spectroscopy was employed
to investigate the effect of temperature on the conformational
equilibrium of the sila-linked metacyclophanes. Inspection of
the spectra shows that the chemical shift of the inner hydrogen
2
2
1
temperature (i). Right: H NMR spectra of 7d in CDCl
3
2
2
(o), –70 °C (p), –90 °C (q), and that of m-bis[(dihydromethylsilyl)methyl]-
benzene (13) in CD Cl at room temperature (r). H indicates signals of inner
aromatic hydrogen atoms. H
,7,14,16-positions of 7b and 7d.
2
2
i
atoms (H ) of tetraphenyl derivative 7b is 5.97 ppm at room
i
o
indicates signals of hydrogen atoms at
temperature both in CDCl and CD Cl [Figure 3(a),(b)]. The sig-
3
2
2
5
nal gradually migrates upfield when the CD Cl2 solution is
2
1
cooled, reaching 5.91 ppm at –90 °C [Figure 3(h)]. On the other
The effect of solvents on the H NMR chemical shifts of the
hand, the inner hydrogen atoms of tetrahydro derivative 7d sila-linked metacyclophanes was investigated. The resonance
shift in the opposite direction upon cooling, the H signal ap- for the inner hydrogen atoms in 7d, appearing at 6.06 ppm in
i
pearing at 6.28 ppm at room temperature in CD Cl [Figure 3(k)] [D ]cyclohexane (Figure 4), shifts downfield in the following
2
2
12
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Communication
solvents: CDCl3 (6.20 ppm), CD Cl2 (6.28 ppm), and CD CN and steric repulsion between the two benzene rings in the syn
2
3
(
6.51 ppm), while the other proton resonances in this com- conformation.
pound remain nearly unchanged. Similarly, the chemical shift
The findings of the present study demonstrate that mere
of the inner hydrogen atoms of 7a experiences a solvent-de- replacement by silyl groups at C-2 and C-11 in the tether can
pendent downfield shift in the following manner: [D ]cyclo- alter the preference in favor of the anti conformation. The typi-
1
2
hexane (5.89 ppm), CDCl (6.02 ppm), CD Cl (6.06 ppm), and cal energy difference between syn and anti conformers of
3
2
2
CD CN (6.23 ppm) (see Supporting Information). The results [3.3]metacyclophanes is roughly 2–3 kcal/mol. However, the
3
suggest that, while the anti conformer is favored in less polar presence of substituted sila centers in the tether is sufficient to
solvents, the proportion of the syn conformer increases in polar reverse this energetic ordering. Two factors might be responsi-
solvents. This solvent effect is perhaps related to differences in ble for this phenomenon. One is a consequence of two axial-
the dipole moments of the conformers (Scheme 3). Because the type substituents at the silicon centers, which, in the syn-chair-
anti structure can cancel the dipole moments, it is stabilized in chair conformation, create steric repulsion with hydrogen atoms
less polar solvents. On the other hand, the syn structure ampli- at the 5, 7, 14, and 16 positions on the benzene rings
fies the dipole moments, so it will be stabilized in polar sol- (Scheme 3). The second factor is associated with the electropos-
vents.
itive character of silicon. Thus, interaction between C(benzyl)–
[
11]
Si σ electron orbitals and benzene π* orbitals should lead to
an increase in electron density on the benzene rings that causes
a greater degree of electrostatic repulsion in the syn conforma-
1
tion. The observed temperature effects in the H NMR spectra
of the sila-linked metacyclophanes are consistent with the pro-
posal that the proportion of the enthalpically lower energy con-
formation increases with decreasing temperature. Therefore,
the temperature and solvent effects on the conformational
equilibria of these compounds in solution can be understood
in terms of a mixed contribution of ΔΔG and dipole moments.
Conclusion
We synthesized silicon- and germanium-tethered [3.3]meta-
cyclophanes, and we found that, in the case of 2,2,11,11-tetra-
substituted derivatives, these are anti-rich [3.3]metacyclo-
phanes. However, for the 2,2,11,11-tetrahydro derivative the syn
conformer is thermodynamically stable. The temperature and
1
1
solvent dependence of H NMR spectra, as well as molecular
Figure 4. H NMR spectra of 7d in [D12]cyclohexane (a), CDCl
and CD CN (d), at room temperature.
3 2 2
(b), CD Cl (c),
orbital calculations, indicated that the syn/anti ratios of these
compounds in solution can be explained by considering mixed
contributions of ΔΔG and dipole moments. X-ray crystallo-
graphic analyses support this behavior. This phenomenon
seems to be useful for switching devices, so further studies are
now in progress.
3
Acknowledgments
This work was supported by the Ministry of Education, Culture,
Sports, Science, and Technology of the Japanese Government
Scheme 3. Dipole moments and steric repulsion in the 2,11-disila[3.3]meta-
cyclophane conformers.
For all previously studied [3.3]metacyclophanes that are
linked by 3-atom chains containing carbon, nitrogen, oxygen,
sulfur, and selenium, syn conformers have been shown
to be thermodynamically stable. Only three exceptions to this
general rule exist. Specifically, [3.3]metacyclophane-2,11-di-
(
(
MEXT) [Grant-in-Aid for Scientific Research on Priority Areas
13029101, 14044092)]. We are also indebted to Prof. Hiroshi
Yamataka (calculations), Prof. Nobuyuki Ichinose (laser flash
photolysis and fluorescence), Assoc. Prof. Minoru Yamaji (fluo-
rescence), Prof. Kunio Oka ( Si NMR spectroscopy), and Prof.
Nobuaki Kambe (elemental analysis and HRMS).
2
9
[
3m,3n,3p,3s]
ones,
late,
[3.3]metacyclophane-2,2,11,11-tetracarboxy-
[
3a]
and a dithia[3.3]metacyclophane, which contain ex-
tremely bulky and electron-donating substituents on both
[
6p]
Keywords: Cyclophanes · Silicon · Germanium · Synthetic
methods · Conformation analysis
benzene rings,
prefer to exist in anti-favored conformations.
Analysis of these compounds suggests that the factors govern-
ing the anti preference in [3.3]metacyclophanes include (1) sub-
stitution on the 2 and 11 positions and (2) the presence of
electron-donating and bulky groups that cause electrostatic
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Received: June 30, 2016
Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Cyclophanes
T. Hayamizu, H. Maeda,* T. Ouchi,
N. Kakiuchi, K. Mizuno* .................... 1–6
Synthesis and Conformational Anal-
ysis of 2,11-Disila[3.3]metacyclo-
phanes
Until now, carbon-, nitrogen-, oxygen-, work, we first synthesized silicon- and
sulfur-, and selenium-tethered [3.3]- germanium-tethered [3.3]metacyclo-
metacyclophanes have been synthe- phanes, and we found that their con-
sized, and their conformation is known formations are unexpected anti-rich
to be biased to syn conformers. In this [3.3]metacyclophanes!
DOI: 10.1002/ejoc.201600800
Eur. J. Org. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim