Crystal structure of a stable silabenzene and its photochemical valence isomerization into the corresponding silabenzvalene [7]

Full text of DOI:10.1021/ja000309i

5648
J. Am. Chem. Soc. 2000, 122, 5648-5649
Crystal Structure of a Stable Silabenzene and Its
Photochemical Valence Isomerization into the
Corresponding Silabenzvalene
Keiji Wakita,†,‡ Norihiro Tokitoh,*,‡ Renji Okazaki,§
Nozomi Takagi,| and Shigeru Nagase|
Department of Chemistry, Graduate School of Science
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku
Tokyo 113-0033, Japan
Institute for Fundamental Research of Organic Chemistry
Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku
Fukuoka 812-8581, Japan
Department of Chemical and Biological Sciences
Faculty of Science, Japan Women’s UniVersity
2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan
Department of Chemistry, Graduate School of Science
Tokyo Metropolitan UniVersity, Minamiosawa, Hachioji
Tokyo 192-0397, Japan
Figure 1. ORTEP drawing of silabenzene 2 (50% thermal ellipsoids).
Selected bond lengths (Å) and angles (deg): Si1-C2 1.765(4), C2-C3
1.391(6), C3-C4 1.399(6), C4-C5 1.381(6), C5-C6 1.394(7), C6-Si1
1.770(4), Si1-C7 1.853(4), C2-Si1-C6 107.8(2), C2-Si1-C7
127.2(2), C6-Si1-C7 124.9(2), Si1-C2-C3 117.7(3), C2-C3-C4
126.3(4), C3-C4-C5 124.1(4), C4-C5-C6 126.1(4), C5-C6-Si1
118.0(3).
ReceiVed January 28, 2000
ReVised Manuscript ReceiVed April 24, 2000
In contrast to the great developments in the chemistry of doubly
bonded organosilicon compounds in the last two decades, silicon
analogues of aromatic compounds are less studied except for
charged systems.1 Previous investigations on neutral silaaromatic
compounds revealed their extremely high reactivity, although
some of them were observed spectroscopically in the low-
temperature matrices or in the gas phase.1 The most stable
silabenzene ever reported is 2,6-bis(trimethylsilyl)-1,4-di-tert-
butyl-1-silabenzene, but it reportedly survives only below -100
°C and is stabilized by the coordination of the solvent (THF/
ether/petroleum ether ) 4:1:1).2
After examination of several reaction conditions, the problem
of the formation of the byproduct 4 was finally solved by using
LDA as a base instead of t-BuLi. Reaction of chlorosilane 3 with
1 equiv of LDA in hexane at room temperature proceeded
efficiently, and silabenzene 2 was obtained in 90% yield without
any byproduct after the removal of inorganic salts.
Meanwhile, we have succeeded in the synthesis of the first
stable 2-silanaphthalene 1 by protecting its reactive silicon center
with a 2-{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl} (Tbt) group.3
More recently, a stable silabenzene 2 bearing a Tbt group was
also synthesized by the reaction of the corresponding chlorosilane
3 with t-BuLi. Although silabenzene 2 was fully characterized
by the 1H, 13C, and 29Si NMR spectra and chemical trapping with
benzophenone, the formation of an undesired byproduct 4
prevented the isolation of 2 in a pure form.4 Here, we wish to
report the effective synthesis and isolation of silabenzene 2
together with its crystallographic structural analysis and unique
photochemical behavior.
The structure of silabenzene 2 was definitely determined by
X-ray crystallographic analysis at -180 °C.5 As shown in Figure
1, the planar geometry around the central silicon atom and
silabenzene ring was clearly revealed.7 The lengths of two Si-C
bonds in the silabenzene ring were found to be essentially equal
to each other (1.765(4) and 1.770(4) Å) and in the middle be-
tween those of Si-C double and single bonds (1.70 and 1.89
Å, respectively).8 Furthermore, the C-C bond lengths
(1.381(6)∼1.399(6) Å) in the silabenzene ring are almost equal
to each other within the error of temperature factors, and also
similar to the C-C length of benzene (1.39-1.40 Å).9 Thus, it
† The University of Tokyo.
‡ Kyushu University. Present address: Institute for Chemical Research,
Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
§ Japan Women’s University.
(5) Crystal data of 2: Rigaku R-AXIS RAPID imaging plate area detector,
Mo KR radiation, graphite monochromator. The reflections were recorded at
93 K from a colorless prismatic crystal with dimensions 0.5 × 0.5 × 0.2
mm3 to 2θmax ) 55 °. The structure was solved by direct methods (SIR92)
and refined by full-matrix least-squares procedures on F2 (SHELX-97).6
Formula C32H64Si7, Mr ) 645.46, monoclinic, space group P21/a, Z ) 4, a )
18.9186(3) Å, b ) 10.9969(2) Å, c ) 20.0757(4) Å, â ) 99.2711(6)°, V )
4122.1(1) Å3, Fcalcd ) 1.040 g cm-3, µ ) 2.50 cm-1; R1 (I > 2σ(I)) ) 0.097,
wR2 (all data) ) 0.258 for 9446 reflections and 411 parameters.
(6) Sheldrick, G. M. SHELX-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(7) Sum of the bond angles around the central Si atom and that of the
interior bond angles of the silabenzene ring are 359.8(3)° and 720.0(9)°,
respectively.
| Tokyo Metropolitan University.
(1) For reviews of silaaromatic compounds, see: (a) Raabe, G.; Michl, J.
Chem. ReV. 1985, 85, 419. (b) Raabe, G.; Michl, J. In The Chemistry of
Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York,
1989; pp 1102-1108. (c) Apeloig Y. ibid., pp 151-166. (d) Brook, A. G.;
Brook, M. A. AdV. Organomet. Chem. 1996, 39, 71. (e) Apeloig, Y.; Karni,
M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig,
Y., Eds.; Wiley: New York, 1998; , Vol. 2, Chapter 1.
(2) Ma¨rkl, G.; Schlosser, W. Angew. Chem., Int. Ed. Engl. 1988, 27, 963.
(3) (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.
(8) (a) Wiberg, N.; Wagner, G.; Mu¨ller, G. Angew. Chem., Int. Ed. Engl.
1985, 24, 229. (b) Gutowsky, H. S.; Chen, J.; Hajduk, P. J.; Keen, J. D.;
Chuang, C.; Emilsson, T. J. Am. Chem. Soc. 1991, 113, 4747.
(4) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S. Angew. Chem., Int.
Ed. 2000, 39, 634.
10.1021/ja000309i CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/24/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 23, 2000 5649
Table 1. Observed and Calculated Bond Lengths (Å) of
Silabenzenes
R
Si-C2
C2-C3
C3-C4
observed
Tbt
1.765(4)
1.770(4)
1.773
1.391(6)
1.394(7)
1.400
1.399(6)
1.381(6)
1.403
Figure 2. Calculated relative energies of silabenzene isomers (kcal/mol,
calculated
H
B3LYP/6-31G(d)).
Ph
1.776
1.777
1.400
1.401
1.403
1.402
Tbtxb
field region characteristic of the three-membered ring silicon
compounds.15 Furthermore, separation of the mixture in the open
air afforded silanol 6 having a three-membered ring.16 Thus, the
product of the photoirradiation of silabenzene 2 is not a Dewar
silabenzene isomer but silabenzvalene 5. 1H and 13C NMR spectra
of 5 also supported the proposed silabenzvalene structure.17
a B3LYP/6-31G(d). b All trimethylsilyl groups of Tbt are replaced
by trihydrosilyl groups.
has experimentally been demonstrated that silabenzene has a
delocalized 6π electron ring system similar to that of benzene.
Theoretical calculations for the model compounds of 2
(RSiC5H5; R ) H, Ph, Tbtx) were also performed for comparison
(Table 1).10 Bond lengths of silabenzene rings are little affected
by substituents on the silicon, and agreement between the
experimental and theoretical values is excellent.
In the conversion from 3 to 2, downfield shifts of the ring
protons (∼1 ppm) were observed as in the case of 2-silanaph-
thalene 13b and support existence of the ring current effect of the
silabenzene ring. In the UV spectrum 2 showed absorption
maxima at ∼260 (sh), 301 (ꢀ ∼1 × 103), 323 (ꢀ ∼1 × 103), and
331 (ꢀ ∼2 × 103) nm, which are consistent with those recorded
for two silabenzenes in low-temperature Ar matrices.1a The
comparison of the UV spectra of benzene-silabenzene-1,4-
disilabenzene series were already reported.11
Among the unique reactivities of silabenzene 2, the photo-
chemical reaction of 2 is worthy of special mention. It is well-
known that the photolysis of benzene affords interesting isomers,
such as Dewar benzene and benzvalene.12 However, the photo-
chemistry of silabenzene is less-studied since no stable silabenzene
has ever been reported. Although the photochemical isomerization
of the parent silabenzene to Dewar silabenzene has already been
studied in an argon matrix,13 the characterization of the Dewar
silabenzene was only based on the shift of the Si-H stretching
frequency in IR spectrum from that characteristic of an sp2 to
that characteristic of an sp3 hybridized silicon.13
Although a stable disilabenzvalene bearing four trimethylsilyl
and two phenyl groups was recently reported by Ando et al.,18
no silabenzvalene has been reported to the best of our knowledge.
They indicated by theoretical calculations that the energy differ-
ence between the parent disilabenzvalene and its Dewar disila-
benzene isomer is small. In contrast, previous theoretical studies
on silabenzene isomers were concentrated only on Dewar sila-
benzene and the possibility of silabenzvalene was not taken into
consideration.19 We performed, therefore, B3LYP/6-31G(d) cal-
culations on silabenzene isomers.10 It has revealed that there is a
small energy difference between Dewar silabenzene and silaben-
zvalene, while silabenzene is by far the most stable among the
isomers (Figure 2).
In summary, the structure of silabenzene 2 was unequivocally
determined by X-ray crystallography, and full delocalization of
its ring π-electrons was clearly demonstrated. Moreover, interest-
ing photochemical isomerization of 2 to the corresponding
silabenzvalene was also disclosed. Further investigation to
elucidate the characters of 2 is currently in progress.
Acknowledgment. This work was partially supported by Grants-in-
Aid for Scientific Research from the Ministry of Education, Science,
Sports and Culture, Japan (Nos. 08454197, 10440184, 11304045, and
11166250). We also thank Shin-etsu Chemical Co., Ltd. and Tosoh Akzo
Co., Ltd. and Central Glass Co., Ltd. for the generous gifts of
chlorosilanes, alkyllithiums, and tetrafluorosilane, respectively.
Photoirradiation of the stable silabenzene 2 resulted in the
formation of a new compound,14 which showed a 29Si NMR signal
of its central silicon atom at -71.6 ppm. This signal cannot be
assignable to that of Dewar silabenzene, since it appears in a high
Supporting Information Available: X-ray structural report and
spectral data of 2 and 6 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA000309I
(9) Minkin, V. J.; Glukhovtsev, M. N.; Simkin, Y. B. Aromaticity and
Antiaromaticity; Electronic and Structural Aspects; Wiley: New York, 1994.
(10) Calculations were carried out using the Gaussian 98 program. Frisch,
M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R.
E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.;
Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.;
Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson,
G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B.
B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong,
M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(14) Photoreaction of silabenzene 2: a C6D6 solution of 2 was sealed in a
Pyrex NMR tube and irradiated by a 400 W high-pressure Hg lamp using a
solution filter which is transparent at 290-350 nm (2 M NiSO4/0.8 M CoSO4/
0.1 M CuSO4 in 5% H2SO4). A 1:4 mixture of 2 and 5 was obtained after
irradiation of 15 h. Longer time of irradiation gave polymeric products.
(15) Williams, E. A. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 536.
(16) The structure of 6 was determined by spectroscopic data and X-ray
crystallographic analysis. Only this stereoisomer was obtained. See Supporting
Information for details.
(17) NMR data of the silabenzvalene ring of 5: 1H NMR (300 MHz, C6D6)
δ 2.24 (m, 1H, H5), 2.50 (d, J ) 5.1 Hz, 2H, H1,6), 5.82 (dd, J ) 9.3, 2.0
Hz, 1H, H3), 7.41 (dd, J ) 9.3, 3.5 Hz, 1H, H4). 13C{1H} NMR (126 MHz,
C6D6) δ 22.98 (C5), 39.81 (C1,6), 120.89 (C3), 161.19 (C4). 29Si NMR (79
MHz, C6D6) δ -71.60. The complete data of 5 were not obtained since a
small amount of silabenzene 2 (δSi ) 93.64) remained after the irradiation
(see ref 14).
(11) Maier, G.; Scho¨ttler, K.; Reisenauer, H. P. Tetrahedron Lett. 1985,
26, 4079.
(12) For reviews of the photochemistry of benzene, see: (a) Scott, L. T.;
Jones, M., Jr. Chem. ReV. 1972, 72, 181. (b) Bryce-Smith, D.; Gilbert, A.
Tetrahedron 1976, 32, 1309.
(13) Maier, G.; Mihm, G.; Baumga¨rtner, R. O. W.; Reisenauer, H. P. Chem.
Ber. 1984, 117, 2337.
(18) Ando, W.; Shiba, T.; Hidaka, T.; Morihashi, K.; Kikuchi, O. J. Am.
Chem. Soc. 1997, 119, 3629. Two Si atoms of the disilabenzvalene resonated
at -61.9 ppm. The disilabenzvalene was also air- and moisture-sensitive.
(19) Chandrasekhar, J.; Schleyer, P. v. R.; Baumgartner, R. O. W.; Reetz,
M. T. J. Org. Chem. 1983, 48, 3453.