Synthesis of a novel silicon-bridged [2.2]metacyclophan-9-ene and its photolytic transannular reaction

Article abstract of DOI:10.1016/j.jorganchem.2005.11.054

A novel [2.2]metacyclophane in which two benzene rings are linked together with a carbon-carbon double bond and a disilane unit was prepared. Photolysis of the cyclophane in the presence of oxygen afforded the 4,5-dihydro-4,5- disilapyrene derivative via

Full text of DOI:10.1016/j.jorganchem.2005.11.054

Journal of Organometallic Chemistry 691 (2006) 1151–1153
www.elsevier.com/locate/jorganchem
Synthesis of a novel silicon-bridged [2.2]metacyclophan-9-ene
and its photolytic transannular reaction
Makoto Oba, Minoru Iida, Tomoki Nagoya, Kozaburo Nishiyama *
Department of Materials Chemistry, Tokai University, 317 Nishino, Numazu, Shizuoka 410-0395, Japan
Received 10 August 2005; received in revised form 10 November 2005; accepted 22 November 2005
Available online 27 December 2005
Abstract
A novel [2.2]metacyclophane in which two benzene rings are linked together with a carbon–carbon double bond and a disilane unit
was prepared. Photolysis of the cyclophane in the presence of oxygen afforded the 4,5-dihydro-4,5-disilapyrene derivative via a transan-
nular dehydrogenation reaction.
ꢀ
2005 Elsevier B.V. All rights reserved.
Keywords: [2.2]Metacyclophan-9-ene; Transannular reaction; Photolysis
1
. Introduction
2. Results and discussion
Over the years, much attention has been drawn to the
As shown in Scheme 1, the synthesis of 1,1,2,2-tetra-
methyl-1,2-disila[2.2]metacyclophan-9-ene (3) began with
Wittig olefination between 3-bromobenzylidene triphenyl-
phosphorane, prepared in situ from phosphonium salt 1
and potassium tert-butoxide, and 3-bromobenzaldehyde.
chemistry of [2.2]metacyclophanes due to their intriguing
physical and chemical properties such as an unusual fea-
1
ture of the H NMR spectra, the transannular interactions
and reactions, and the considerable strain exerted in the
rigid 10-membered ring, mainly based on their unique
architecture [1]. On the other hand, there have been only
a few reports concerning the [2.2]metacyclophanes involv-
ing silicon atoms in their bridge. The first synthesis of
tetrasila[2.2]metacyclophane was achieved by Sakurai and
co-workers [2]. Later, we reported the synthesis of
0
The desired 3,3 -dibromo- cis-stilbene (cis-2) was obtained
in 50% yield along with 49% yield of the corresponding
trans isomer. Ring closure of the dibromostilbene cis-2 with
1,2-dichlorodisilane in the presence of tert-butyllithium
was carried out at ꢀ55 ꢁC under high-dilution conditions
to give novel silicon-bridged [2.2]metacyclophan-9-ene (3)
in 4.9% yield based on the consumed dibromide. The struc-
[
2.2]metacyclophanes containing two or three silicon atoms
1
13
29
located at various positions of the bridges [3]. In this paper
we would like to describe the synthesis, some structural
aspects, and photolytic transannular reaction of a novel
ture of compound 3 was determined by H, C, and Si
NMR and high-resolution mass spectra. Further confirma-
tion of the structure was carried out by catalytic hydroge-
nation of the C–C double bond of 3 in the presence of 10%
palladium on carbon to give quantitative yield of 1,1,2,2-
tetramethyl-1,2-disila[2.2]metacyclophane (4) which we
previously prepared in a different way [3].
[
2.2]metacyclophane in which two benzene rings are
bridged by a carbon–carbon double bond and a disilane
unit. The observed transannular ring closure is the first to
take place in the silicon-bridged [2.2]metacyclophane
system.
It is well-known that the [2.2]metacyclophanes adopt a
step-like conformation where two benzene rings are
arranged in parallel planes. As a result, the NMR signals
of the intra-annular aryl protons, the 8,16-protons, resonate
*
Corresponding author. Tel.: +81 55 968 1111; fax: +81 55 968 1155.
E-mail address: nishiyam@wing.ncc.u-tokai.ac.jp (K. Nishiyama).
0
022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.11.054

1152
M. Oba et al. / Journal of Organometallic Chemistry 691 (2006) 1151–1153
Scheme 1. Synthesis and reactions of 1,2-disila[2.2]metacyclophan-9-ene (3).
at unusually high-field due to the diamagnetic ring-current
effect of the opposite benzene ring. The H NMR spectra
ing material along with a trace amount of 4-hydro-4-silacy-
clopenta[def]phenanthrene derivative 6. Other silicon-
bridged [2.2]metacyclophanes containing various numbers
of silicon atoms at various positions, prepared previously
by us [3], were also subjected to the above photoreaction
conditions; however, all attempts resulted in no reaction
or gave a complex mixture. It can be, therefore, presumed
that the introduction of the bridging double bond facili-
tates the co-planar arrangement of the two benzene nuclei
favorable to transannular ring closure because of the pla-
1
of the silicon-bridged [2.2]metacyclophane (4) showed the
inner aryl proton resonances at 5.33 ppm as one broad trip-
let. Furthermore, the signals for the two methyl groups
attached to the silicon atom and the methylene protons of
0
0
the ethylene bridge appeared as two singlets and an AA BB
system, respectively, indicating the stepped conformation of
the cyclophane 4 is rigid at room temperature. On the other
hand, the signals for the inner aryl protons of the [2.2]meta-
cyclophan-9-ene (3) appeared at 6.83 ppm in the normal
aromatic region and the two methyl groups attached to
the silicon atom resonated at 0.18 ppm as one broad singlet.
We assumed that the introduction of the bridging double
bond made a slight change in the step-like structure of the
2
narity of the valences at sp -hybridized carbons.
The obtained fused tetracycles 5 and 6 containing silicon
atoms are new compounds and their structures are deter-
1
13
29
mined by H, C, and Si (only for 5) NMR and high-res-
olution mass spectra. The formation of compound 6 can be
explained by a photo-induced extrusion of dimethylsilylene
from 4,5-dihydro-4,5-disilapyrene (5). Similar ring reduc-
tion accompanied by the generation of divalent species
has been reported by Kira and co-workers, where the pho-
tolysis of 9,10-dihydro-9,10-disilaphenanthrene and its ger-
manium analogue afforded the corresponding fluorene
derivatives [5].
[
2.2]metacyclophane system so that the ring inversion
would become feasible even at room temperature. Using
variable temperature NMR spectroscopy, the energy bar-
rier to ring inversion for the cyclophane 3 could be esti-
mated to be 45 kJ/mol from the maximum frequency
separation (217 Hz) of the methyl proton resonances and
their coalescence point (235 K) [4]. The value is consider-
ably less than that for cyclophane 4 (66 kJ/mol).
In summary, we have demonstrated the synthesis and
some structural aspects of novel 1,2-disila[2.2]metacyclo-
phan-9-ene (3). It was found that the introduction of a
bridging double bond provided some flexibility in the 10-
membered ring to facilitate the ring inversion. Upon pho-
tolysis the unprecedented transannular ring closure of sili-
con-bridged [2.2]metacyclophane took place to afford a
4,5-dihydro-4,5-disilapyrene derivative.
The silicon-bridged [2.2]metacyclophan-9-ene (3) has an
absorption spectrum similar to that of cis-stilbene. An
absorption band around 311 nm (e
= 3616) in hexane
max
*
attributed to p–p transition is 28 nm longer in wavelength
than that of cis-stilbene, suggesting that a more co-planar
arrangement of the two benzene rings is feasible by intro-
ducing an additional linkage, the disilane bridge, into the
distorted cis-stilbene system.
One other striking feature of [2.2]metacyclophane chem-
istry is transannular reactions. When a solution of 1,2-
disila[2.2]metacyclophan-9-ene (3) in cyclohexane (6.8 ·
3. Experimental
3.1. General
ꢀ
3
1
0
M) in a quartz tube was irradiated with a 400 W
1
13
29
medium-pressure mercury lamp under an oxygen atmo-
sphere at room temperature for 1 h, an intramolecular
transannular dehydrogenation reaction between the 8 and
Melting points are uncorrected. H, C, and Si NMR
spectra were measured at 400.0, 100.6, and 79.5 MHz,
respectively. All chemical shifts are reported as d values
1
4
6-positions took place to give 69% yield of 4,5-dihydro-
,5-disilapyrene derivative 5 based on the consumed start-
(ppm) relative to residual chloroform (d 7.26), the central
H
peak of deuteriochloroform (d 77.0), or tetramethylsilane
C

M. Oba et al. / Journal of Organometallic Chemistry 691 (2006) 1151–1153
1153
(
dSi 0.00). High-resolution mass spectra (HRMS) were
compound 4 as an oil, which solidified upon standing.
determined using perfluorokerosene as an internal stan-
dard. Absorption spectra were measured using a Shimadzu
UV-1700 UV–visible spectrophotometer. Solvents and
reagents were of commercial grade and were purified if
necessary.
Recrystallization from hexane gave colorless plates, m.p.
1
50–51.5 ꢁC. H NMR (CDCl ) d 0.02 (br s, 6H), 0.45 (br
3
s, 6H), 2.24 (br d, J = 8 Hz, 2H), 3.09 (br d, J = 8 Hz,
2H), 5.33 (s, 2H), 7.18–7.22 (m, 2H), 7.28–7.37 (m, 4H).
2
9
13
Si NMR (CDCl ) d ꢀ17.9. C NMR (CDCl ) d ꢀ6.5,
3
3
ꢀ
6.2, 39.2, 128.0, 128.2, 130.7, 134.7, 138.5, 139.0. HRMS
0
+
3
.2. 3,3 -Dibromo-cis-stilbene (cis-2)
(EI, 70 eV) m/z 296.1429 (M , calcd. for C H Si
18 24 2
2
96.1419). Anal. Calc. for C H Si : C, 72.90; H, 8.16.
18 24 2
To a suspension of phosphonium salt 1 (47.7 g,
Found: C, 72.60; H, 8.12%.
9
3.1 mmol) in THF (100 mL) were added successively
potassium tert-butoxide (12.6 g, 112 mmol) and, after
3.5. Photolytic transannular reaction of 1,2-disila[2.2]meta-
cyclophan-9-ene (3)
3
9
0 min, a solution of 3-bromobenzaldehyde (17.2 g,
3.0 mmol) in THF (100 mL). After being stirred over-
night, the precipitated solids were removed by filtration
using a Celite pad. The filtrate was then concentrated in
vacuo and trituration of the residue with hexane furnished
pure 3,3 -dibromo-trans-stilbene (15.3 g) in 49% yield. The
hexane solution containing the desired cis-isomer was sub-
A solution of 1,2-disila[2.2]metacyclophan-9-ene (3)
(10.0 mg, 0.0340 mmol) in cyclohexane (5 mL) in a quartz
tube was irradiated with a 400 W medium-pressure
mercury lamp under an oxygen atmosphere at room
temperature for 1 h. After removal of the solvent, the resi-
due was chromatographed on silica gel to give 69% yield of
4,4,5,5-tetramethyl-4,5-dihydro-4,5-disilapyrene (5, 5.50 mg)
as a colorless oil based on recovered cyclophane 3 (2.00 mg,
0.00680 mmol, 20%) along with a trace amount of 4,4-
0
mitted to flash column chromatography on silica gel (hex-
ane) to give the title compound cis-2 (15.6 g) in 50% yield
1
as a colorless oil [6]. H NMR (CDCl ) d 6.55 (s, 2H),
3
7
.05–7.14 (m, 4H), 7.31–7.38 (m, 4H).
dimethyl-4-hydro-4-silacyclopenta[def]phenanthrene (6) as
1
3.3. 1,1,2,2-Tetramethyl-1,2-disila[2.2]metacyclophan-9-
ene (3)
a colorless oil. 5: H NMR (CDCl ) d 0.37 (s, 12H),
3
1
3
7.51–7.55 (m, 2H), 7.69 (s, 2H), 7.81–7.87 (m, 4H).
C
NMR (CDCl ) d ꢀ3.9, 125.4, 128.1, 130.7, 133.0, 133.4,
3
2
9
To a solution of 1.50 M tert-butyllithium in pentane
48.0 mL, 72.0 mmol) and THF (100 mL) was added drop-
136.6, 136.9. Si NMR (CDCl ) d ꢀ26.5. HRMS (EI,
3
+
(
70 eV) m/z 292.1094 (M , calcd. for C H Si 292.1104).
1
8
20
2
wise a solution of cis-2 (10.1 g, 30.0 mmol) and 1,1,2,2-tet-
ramethyl-1,2-dichlorodisilane (6.97 g, 37.2 mmol) in THF
An analytical sample of compound 6 was isolated from
a combined reaction mixture obtained by several runs. H
1
(
200 mL) over a period of 200 min under an argon atmo-
NMR (CDCl ) d 0.55 (s, 6H), 7.58–7.63 (m, 2H), 7.76 (s,
3
1
3
sphere at ꢀ55 ꢁC. After being stirred at room temperature
2H), 7.84–7.90 (m, 4H). C NMR (CDCl ) d ꢀ2.9,
3
overnight, the reaction mixture was quenched with satu-
126.0, 126.9, 127.7, 129.4, 130.2, 137.8, 141.8. HRMS
+
rated aqueous NH Cl and the organic layer was dried over
(EI, 70 eV) m/z 235.0928 [(M + H) , calcd. for C H Si
4
16 15
MgSO . After removal of the solvent, the residue was
235.0943].
4
purified by repeated column chromatography on silica gel
(
hexane) and RP-18 (MeOH/H O = 95/5) to afford 4.9%
2
References
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m.p. 90–91 ꢁC, based on recovered cis-2 (3.76 g,
[
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(
1
1
1.1 mmol). H NMR (CDCl ) d 0.18 (br s, 12H), 6.62
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1
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1
3
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3
(
2
9
31.8, 135.0, 135.3, 136.7. Si NMR (CDCl ) d ꢀ17.0.
3
[
+
HRMS (EI, 70 eV) m/z 294.1272 (M , calcd. for
C H Si 294.1260). UV (hexane) kmax/nm (emax) 311
(
1
8
22
2
[
[
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(
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18 22 2
7
3.40; H, 7.53. Found: C, 73.07; H, 7.49%.
7
3
.4. 1,1,2,2-Tetramethyl-1,2-disila[2.2]metacyclophane (4)
2
(
(
(
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A solution of 1,2-disila[2.2]metacyclophan-9-ene (3)
(
10.1 mg, 0.0343 mmol) in benzene (2 mL) was hydroge-
nated in the presence of 10% palladium on carbon
10.1 mg) for 1 h. After removal of the catalyst, the solvent
was concentrated to afford quantitative yield of the title
4
759;
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(
(