Photochemical and thermal reactions of a kinetically stabilized 9-silaanthracene: The first spectroscopic observation of a 9,10-Dewar-9-silaanthracene isomer

Article abstract of DOI:10.1021/ja036106m

A kinetically stabilized 9-silaanthracene (1) underwent unique photochemical and thermal reactions to afford 9,10-Dewar-9-silaanthracene 2a and the head-to-tail [4 + 4] dimer 3, respectively. The structure of 2a was confirmed by ¹H, ¹³C, and ²⁹Si NMR spectra, and the kinetic parameters for the thermal reversion of 2a to 1 were obtained by the measurement of UV/vis spectra. The dimer 3 was thermally stable, and the molecular structure of 3 was determined by X-ray crystallographic analysis. It was experimentally demonstrated for the first time that 9-silaanthracene, as well as anthracene, can afford either the Dewar isomer or [4 + 4] dimer, depending on the reaction conditions. Copyright

Full text of DOI:10.1021/ja036106m

Published on Web 08/14/2003
Photochemical and Thermal Reactions of a Kinetically Stabilized
9-Silaanthracene: The First Spectroscopic Observation of a
9,10-Dewar-9-silaanthracene Isomer
Akihiro Shinohara, Nobuhiro Takeda, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan
Received May 13, 2003; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
Scheme 1. Structures of 1-3 and Photochemical and Thermal
Photochemical reactions of anthracenes are known to result in
Reactions of 1
the formation of the dimers via [4 + 4] cycloaddition1 or the
corresponding 9,10-Dewar-anthracenes through valence isomeriza-
tion,2 depending on the substrates and/or reaction conditions. The
anthracene dimers are known to revert to the monomers thermally,
and this interconversion process has attracted much attention from
the viewpoints of not only fundamental chemistry but also applica-
tions such as molecular switching. Most of 9,10-Dewar-anthracenes
are thermally very unstable to regenerate the starting anthracenes
in solution easily, and 9-tert-butyl-9,10-Dewar-anthracene is the
only example structurally characterized so far.2d On the other hand,
much attention has been paid to the chemistry of silaaromatic
compounds in recent decades, and several 9-silaanthracenes were
prepared as transient species in low-temperature matrices.3 Although
the photochemical conversion of 9-silaanthracenes into the corre-
sponding Dewar isomers has also been examined, such a process
was confirmed only by the disappearance of the original absorptions
of 9-silaanthracene in the electronic spectra.3a Since silaaromatic
compounds are known to be thermally unstable and undergo ready
dimerization and/or polymerization under ambient conditions,4 the
details of photochemical behavior of 9-silaanthracenes have not
been fully disclosed so far.
was formed exclusively (Scheme 1). Compound 2a showed
1
characteristic peaks at 4.43 ppm (singlet) in H NMR and 48.60
ppm in 13C NMR. These peaks are most likely attributed to those
of the methine hydrogen and carbon atoms at the 10-position of
the Dewar isomer 2a,7 respectively. They are considerably upfield-
shifted compared with those of aromatic 9-silaanthracene 1 (7.72
and 116.13 ppm, respectively). In the 29Si NMR measurement of
2a using the INEPT technique, we observed a signal at 1.44 ppm
assignable to that of the ring silicon atom. This assignment was
reasonably supported by comparison of the experimentally observed
chemical shift with those obtained for some model molecules, 9,10-
Dewar-9-silaanthracene 2b (-14.04 ppm) and 9,10-Dewar-9-
phenyl-9-silaanthracene 2c (3.55 ppm), by GIAO calculations.8
Furthermore, the monitoring of the photochemical conversion of 1
to 2a in a degassed and sealed 3-methylpentane matrix at 77 K
using UV/vis spectroscopy showed complete disappearance of the
typical absorption bands of 1 at the range of 350 through 550 nm,
indicating the formation of less conjugated system 2a than 1.
Although the spectroscopic studies strongly indicate the photo-
chemical transformation of 1 to 2a, the Dewar isomer 2a prepared
here could not be isolated as a stable compound due to its low
thermal stability. 2a was found to undergo gradual thermal
tautomerization into 1 even at -80 °C in hexane. The conversion
of 2a to 1 in degassed hexane was monitored at six temperatures
between -40 and 10 °C using UV/vis spectroscopy by observation
of the increase of the absorption maximum for 9-silaanthracene 1
at 478 nm. As a result, this reaction was found to be a first-order
reaction as well as thermal isomerization of 9,10-Dewar-anthracenes
to anthracenes9 The activation energy Ea and the preexponential
factor deduced by the kinetic analysis were 4.34 kcal‚mol-1 and
1.61, respectively. The activation enthalpy (∆Hq) and entropy (∆Sq)
were 3.83 kcal‚mol-1 and -3.76 cal‚mol-1‚K-1, respectively. The
half-life of 2a in hexane at 10 °C was estimated to be about 5
min,10 the value of which is considerably shorter than that of the
anthracene/9,10-Dewar-anthracene system, suggesting much less
Meanwhile, we have already reported the syntheses of the first
stable silabenzene and 2-silanaphthalene by taking advantage of
kinetic stabilization utilizing an efficient steric protection group,
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt).5 Quite recently, we
further succeeded in the synthesis and isolation of Tbt-substituted
9-silaanthracene 1 as a stable crystalline compound and revealed
its unique molecular structure and spectroscopic properties.6 As for
the reactivity of these kinetically stabilized silaaromatic compounds,
we have already described the similarities and differences in their
addition reactions with methanol, benzophenone, mesitonitrile oxide,
and 2,3-dimethylbuta-1,3-diene.5,6 Photochemically, however, we
have only examined the irradiation of Tbt-substituted silabenzene
with light of λ ) 290-350 nm in benzene through Pyrex glass
leading to the exclusive formation of the corresponding silabenz-
valene isomer.5e Since this result is in a sharp contrast to that of
the photochemical conversion of the matrix-isolated silabenzene
into the Dewar-type isomer postulated by Maier et al., it should be
of great interest to investigate the photochemical reactions of stable
9-silaanthracene 1 for a part of systematic comparison. Here, we
delineate the first spectroscopic observation of 9,10-Dewar-9-
silaanthracene 2a generated by the photochemical valence isomer-
ization of 1 together with the kinetic study on the reverse
tautomerization from 2a to 1 and the formation of head-to-tail dimer
3 by thermolysis of 1 at high temperature.
When a degassed benzene-d6 solution of 1 in a sealed Pyrex
NMR tube was irradiated with light of λ ) 300-500 nm through
a UV cut filter at room temperature, the orange color of 1
disappeared and a new species, 9,10-Dewar-9-silaanthracene 2a,
9
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10.1021/ja036106m CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
conditions. Theoretical calculations on the molecular geometry and
the relative energy of the real molecules 1, 2a, and 3 at the B3LYP/
6-31G(d) level resulted in the conclusion that dimeric structure 3
was found to be more stable than the monomer 1 by 15.9 kcal‚mol-1
(per monomer unit) and Dewar isomer 2a was less stable than
9-silaanthracene 1 by 13.4 kcal‚mol-1,8 which is consistent with
the experimentally observed relative stability for 1, 2a, and 3.
Further investigation on the reactivies of the Dewar isomer 2a
and the [4 + 4] dimer 3 is currently in progress.
Acknowledgment. This work was partially supported by Grants-
in-Aid for COE Research on Elements Science [No. 12CE2005],
the Scientific Research (A) [Nos. 14204064 and 11304045], and
the 21COE Program on Kyoto University Alliance for Chemistry
(Novel Organic Materials Creation & Transformation Project) from
the Ministry of Education, Culture, Sports, Science and Technology,
Japan. Computation time was provided at the Supercomputer
Laboratoty, Institute for Chemical Research, Kyoto University.
Supporting Information Available: Experimental details, X-ray
structural report of 3, spectroscopic data for 2a, and kinetic analysis
of 2a. Crystallographic data for 9-silaanthracene dimer (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 1. ORTEP drawing of 9-silaanthracene dimer 3. Hydrogen atoms
and a solvent molecule were omitted for clarity. Selected bond lengths (Å)
Si1-C1 1.983(4), Si1-C2 1.874(4), Si1-C3 1.882(4).
stability of 2a than 9,10-Dewar-anthracene. Moreover, it was found
that the orange crystals of 1 in a degassed and sealed Pyrex galss
tube changed into colorless crystals by irradiation with light of λ
) 300-500 nm through a UV cut filter. The 1H NMR spectrum of
the resulting crystals was identical to that of 2a prepared in solution.
Interestingly, 2a underwent gradual tautomerization into 1 even in
the solid state at room temperature in the dark, while the solid
samples of 9,10-Dewar-anthracenes are reportedly stable at room
temperature.
References
(1) For reviews on photoreaction of anthracenes, see: (a) Becker, H.-D. Chem.
ReV. 1993, 93, 145. (b) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-
P.; Lapouyade, R. Chem. Soc. ReV. 2000, 29, 43.
(2) (a) Hart, H.; Ruge, B. Tetrahedron Lett. 1977, 36, 3143. (b) Pritschins,
W.; Grimme, W. Tetrahedron Lett. 1982, 23, 1151. (c) Gu¨sten, H.; Mintas,
M.; Klasinc, L. J. Am. Chem. Soc. 1980, 102, 7936. (d) Angermund, K.;
Clauz, K. H.; Goddard, R.; Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1985,
24, 237. (e) Hart, H.; Meador, M. A. J. Org. Chem. 1989, 54, 2336.
(3) (a) Winkel, Y. V. D.; Baar, B. L. M. v.; Bickelhaupt, F.; Kulik, W.;
Sierakowski, C.; Maier, G. Chem. Ber. 1991, 124, 185. (b) Hiratsuka, H.;
Tanaka, M.; Okutsu, T.; Oba, M.; Nishiyama, K. J. Chem. Soc., Chem.
Commun. 1995, 215. (c) Nishiyama, K.; Oba, M.; Takagi, H.; Fujii, I.;
Hirayama, N.; Narisu; Horiuchi, H.; Okutsu, T.; Hiratsuka, H. J.
Organomet. Chem. 2000, 604, 20. (d) Hiratsuka, H.; Tanaka, M.; Horiuchi,
H.; Narisu; Yoshinaga, T.; Oba, M.; Nishiyama, K. J. Organomet. Chem.
2000, 611, 71.
Next, we performed the pyrolysis of 9-silaanthracene 1 with the
hope of obtaining the corresponding [4 + 4] dimer 3, which is
another possible candidate for the photochemical reaction product
of 1 as can be seen in the cases of hydrocarbon analogues. When
a benzene-d6 solution of 1 was heated at 110 °C in a sealed tube
for 15 days, the orange color of 1 gradually diminished and the
expected [4 + 4] dimer 3 was obtained in 42% yield as colorless
crystals. Interestingly, the dimer 3 was formed more readily (58%
yield) by thermolysis of 1 in the solid state at 180 °C for 1 h
(Scheme 1). These results indicate that 1 has high reactivity for
cycloaddition at the 9- and 10-positions though it bears a bulky
Tbt group on the central silicon atom. Dimer 3 is thermally very
stable and no cycloreversion to monomer 1 was observed in the
1,2-dichlorobenzene-d4 solution even at 200 °C in a sealed tube.
(4) Jutzi, P. Chem. Ber. 1971, 104, 1455.
(5) (a) Tokitoh, N.; Wakita, K.; Okazaki, R.; Nagase, S.; Schleyer, P. v. R.;
Jiao. H. J. Am. Chem. Soc. 1997, 119, 6951. (b) Wakita, K.; Tokitoh, N.;
Okazaki, R.; Nagase, S.; Schleyer, P. v. R.; Jiao. H. J. Am. Chem. Soc.
1999, 121, 11336. (c) Wakita, K.; Tokitoh, N.; Okazaki, R. Bull. Chem.
Soc. Jpn. 2000, 73, 2157. (d) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase,
S. Angew. Chem., Int. Ed. 2000, 39, 634. (e) Wakita, K.; Tokitoh, N.;
Okazaki, R.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 5648.
(6) Takeda, N.; Shinohara, A.; Tokitoh, N. Organometallics 2002, 21, 256.
(7) NMR data of 2a: 1H NMR (300 MHz, rt, C6D6) δ 0.16 (br s, 54H), 1.51
(s, 1H), 2.58 (br s, 1H), 2.72 (br s, 1H), 4.43 (s, 1H), 6.63 (br s, 1H),
6.71 (br s, 1H), 6.99-7.09 (m, 4H), 7.20 (d, J ) 6.7 Hz, 2H), 7.66 (d, J
) 7.8 Hz, 2H). 13C NMR (75 MHz, rt, C6D6) δ -0.71 (q), 0.90 (q), 1.01
(q), 29.75 (d), 30.01 (d), 31.33 (d), 48.60 (d), 118.52 (s), 122.48 (d), 125.52
(d), 126.27 (d), 128.29 (d), 130.77 (d), 132.44 (d), 142.21 (s), 146.67 (s),
147.09 (s), 157.02 (s × 2); 29Si NMR (60 MHz, rt, C6D6) δ 1.44, 2.17,
2.25.
1
The H and 13C NMR data for dimer 3 were completely different
from those of 2a, and the 29Si NMR signal of the central silicon
atom of 3 was observed at -21.83 ppm. The head-to-tail type
dimerization mode in the [4 + 4] dimer 3 was definitively
determined by X-ray crystallographic analysis (Figure 1), which
reveals that 3 has considerably elongated Si-C bonds [1.983(4)
Å] probably due to the sterically congested 9-silaanthracene dimer
structure.11 The synthesis and characterization of 3 should be of
great importance from the standpoints of not only the first example
of a structurally characterized 9-silaanthracene dimer but also the
requirement for the identification of 9,10-Dewar-9-silaanthracene
2a.
(8) The geometries of 9-silaanthracene derivatives and their isomers 1, 2a,
2b, 2c, and 3 were optimized using the Gaussian 98 program at the
B3LYP/6-31G(d) level. GIAO-B3LYP calculations of 2b and 2c were
carried out with 6-311G(3d) for Si and 6-311G(d) for C and H.
(9) Kinetic analysis for the first-order reaction yielded the Arrhenius equation,
ln k ) 1.61-(2.18 × 103)/T.
(10) While the half-life of 2a in hexane at 10 °C is about 5 min, that in benzene-
d6 was estimated as about 16 min by monitoring the conversion of 2a to
1 with 1H NMR spectroscopy. We observed the 13C and 29Si NMR signals
of 2a by measuring the spectra just after the photoirradiation.
(11) Dimer 3 has a poor solubility, and hence the full NMR data of 3 could
not be attributed. Spectral data for 3: white powder, mp 310 °C (dec).
29Si NMR (60 MHz, rt, CDCl3) δ -21.83, 1.50, 1.57, 1.73, 1.80, 1.86,
1.94, 2.02, 2.08, 2.23, 2.27. Crystal data of 3‚C6H14: C86H150Si14, MW )
1577.32, monoclinic, space group P21/c, Z ) 2, a ) 12.155(6), b ) 22.181
(11), c ) 18.561(10) Å, â ) 108.422(6)°, V ) 4748(4) Å3, Dcalcd ) 1.103
g‚cm-3, µ ) 0.228 mm-1; R1 (I > 2σ(I)) ) 0.081, wR2 (all data) ) 0.171,
GOF ) 1.223 for 8321 reflections, 473 parameters, and 18 restraints.
In summary, it was experimentally demonstrated for the first
time that 9-silaanthracene, as well as anthracene, can afford either
the Dewar isomer or [4 + 4] dimer, depending on the reaction
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