A stable neutral silaaromatic compound, 2-{2,4,6- tris[bis(trimethylsilyl)methyl]phenyl}-2-silanaphthalene [17]

Full text of DOI:10.1021/ja9710924

J. Am. Chem. Soc. 1997, 119, 6951-6952
6951
bulky protecting group, 2,4,6-tris[bis(trimethylsilyl)methyl]-
phenyl (denoted as Tbt).8 Our successful application of the Tbt
group to the kinetic stabilization of silaaromatic species has now
led to the first isolation of 2-silanaphthalene 1. The 2-silanaph-
thalene 1 was synthesized as a colorless, stable crystalline
compound (mp 151-155 °C) in 80% yield by treatment of 2
with t-BuLi in hexane (Scheme 1).9
A Stable Neutral Silaaromatic Compound,
2-{2,4,6-Tris[bis(trimethylsilyl)methyl]phenyl}-
2-silanaphthalene
Norihiro Tokitoh,*,† Keiji Wakita,† Renji Okazaki,*,†
Shigeru Nagase,‡ Paul von Rague´ Schleyer,§ and
Haijun Jiao§
Department of Chemistry, Graduate School of Science
The UniVersity of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Department of Chemistry, Faculty of Science
Tokyo Metropolitan UniVersity
Minamiosawa, Hachioji, Tokyo 192-03, Japan
Computer-Chemie-Centrum, Institut fu¨r Organische Chemie
UniVersita¨t Erlangen-Nu¨rnberg
Henkestrasse 42, D-91054 Erlangen, Germany
ReceiVed April 7, 1997
1
The structure of 1 was confirmed by its H, 13C, and 29Si
Much attention has been focused on the chemistry of
silaaromatic compounds, i.e., Si-containing [4n + 2]π ring
systems, and a number of reports on the formation and reactions
of transient silaaromatics such as sila- and disilabenzenes have
appeared in the past few decades.1 Although some of them
were characterized spectroscopically in low-temperature ma-
trices,2 no isolation has been reported due to their high reactivity.
The exceptions, silole anions and dianions, reportedly have
significantly delocalized electron density in their silole rings.3,4
As for a neutral silaaromatic compound, Ma¨rkl et al. have
already reported the synthesis of a monomeric silabenzene, 2,6-
bis(trimethylsilyl)-1,4-di-tert-butylsilabenzene.5 However, it
was observed only in solution (THF/Et2O/petroleum ether, 4:1:
1) below -100 °C, due apparently to stabilization by coordina-
tion of the solvent Lewis base judging by the relatively high
field 29Si NMR chemical shift (δSi ) 26.8) (Vide infra). The
only generation of a silanaphthalene ever reported, transient
2-methyl-2-silanaphthalene, was deduced by MeOD trapping
of the reaction products in the flow pyrolysis of 2-allyl-2-methyl-
1,2-dihydro-2-silanaphthalene.6 We have recently synthesized
the first stable silanethione, Tbt(Tip)SidS (Tip ) 2,4,6-
triisopropylphenyl),7 by taking advantage of a new and effective
NMR spectral data,10 which were in quite good agreement with
chemical shifts computed for the parent and substituted 2-si-
lanaphthalenes 3-5 (GIAO-B3LYP),11 e.g., δH for H(1) [1, 7.40;
3, 7.74; 4, 6.97; 5, 7.32], H(3) [1, 7.24; 3, 7.27; 4, 7.03; 5,
7.08], and H(4) [1, 8.48; 3, 8.64; 4, 8.50; 5; 8.55], δC for C(1)
[1, 116.01; 3, 128.45; 4, 120.43; 5, 121.63], C(3) [1, 122.56; 3,
125.13; 4, 122.68; 5, 123.56], and C(4) [1, 148.95; 3, 153.38;
4, 153.26; 5, 152.45], and δSi for Si(2) [1, 87.35; 3, 67.80; 4,
100.97; 5, 94.32]. The 29Si NMR chemical shift (δSi ) 87.35)
observed for the ring silicon of 1 is comparable to those for the
previously reported sp2 silicon compounds.1c All the 1H NMR
signals of the ring protons (6.99-8.48 ppm) of 1 were observed
in the aromatic region, and the 13C NMR signals of the ring
carbons (116.01-148.95 ppm) were located in the sp2 region.
The coupling constants between the ring Si atom and the two
adjacent ring carbons [92 Hz for JSisC(1) and 76 Hz for JSisC(3)
]
both exceed normal values for CsSi(sp3) (∼50 Hz)12 and are
similar to those reported for SidC systems (83-85 Hz).1c These
results clearly indicate that the 2-silanaphthalene ring has
delocalized double bonds.
The molecular structure of 1 was also established by X-ray
crystallography (Figure 1).13 The silanaphthalene ring of 1 is
almost planar and oriented perpendicular to the benzene ring
of the Tbt group, suggesting essentially no conjugative interac-
tion of the π-electrons of the Tbt group with those of the
silanaphthalene ring. The 360° bond angle sum shows the
completely planar trigonal geometry around the silicon atom.
† The University of Tokyo.
‡ Tokyo Metropolitan University.
§ Universita¨t Erlangen-Nu¨rnberg.
(1) For reviews on silaaromatic compounds see, e.g.: (a) Raabe, G.;
Michl, J. Chem. ReV. 1985, 85, 419. (b) Raabe, G.; Michl, J. In The
Chemistry of Organosilicon Compounds; Patai, S., Rappoport, Z., Eds.;
Wiley: New York, 1989; pp 1102-1108. (c) Brook, A. G.; Brook, M. A.
AdV. Organomet. Chem. 1996, 39, 71.
(2) (a) Solouki, B.; Rosmus, P.; Bock, H.; Maier, G. Angew. Chem., Int.
Ed. Engl. 1980, 19, 51. (b) Maier, G.; Mihm, G.; Reisenauer, H. P. Angew.
Chem., Int. Ed. Engl. 1980, 19, 52. (c) Kreil, C. L.; Chapman, O. L.; Burns,
G. T.; Barton, T. J. J. Am. Chem. Soc. 1980, 102, 841. (d) Maier, G.; Mihm,
G.; Reisenauer, H. P. Chem. Ber. 1982, 115, 801. (e) Maier, G. Mihm, G.;
Baumga¨rtner, R. O. W.; Reisenauer, H. P. Chem. Ber. 1984, 117, 2337. (f)
Maier, G.; Scho¨ttler, Reisenauer, H. P. Tetrahedron Lett. 1985, 26, 4079.
(g) van den Winkel, Y.; van Barr, B. L. M.; Bickelhaupt, F.; Kulik, W.;
Sierakowski, C.; Maier, G. Chem. Ber. 1991, 124, 185. (h) Hiratsuka, H.;
Tanaka, M.; Okutsu, T.; Oba, M.; Nishiyama, K. J. Chem. Soc., Chem.
Commun. 1995, 215.
(7) Suzuki, H.; Tokitoh, N.; Nagase, S.; Okazaki, R. J. Am. Chem. Soc.
1994, 116, 11578.
(8) Okazaki, R.; Tokitoh, N.; Matsumoto, T. In Synthetic Methods of
Organometallic and Inorganic Chemistry; Herrmann, W. A., Ed.; Thieme:
New York, 1996; Vol. 2 (Auner, N., Klingebiel, U., Vol. Eds.), pp 260-
269 references cited therein.
(9) The starting material, i.e., 1,2,3,4-tetrahydro-2-silanaphthalene, was
synthesized by LAH reduction of the corresponding dichlorosilane which
was prepared by the reported procedures. (a) Zhdanov, A. A.; Andrianov,
K. A.; Odinets, V. A.; Karpova, I. V. Zh. Obshch. Khim. 1966, 36, 521. (b)
Chernyshev, E. A.; Komalenkova, N. G.; Bashkirova, S. A.; Kuz'mina, T.
M.; Kisin, A. V. Zh. Obshch. Khim. 1975, 45, 2227.
(3) (a) Hong, J.-H.; Boudjouk, P.; Anwari, F. J. Am. Chem. Soc. 1993,
115, 5883. (b) Hong, J.-H.; Boudjouk, P.; Castellino, S. Organometallics
1994, 13, 3387.
(10) The assignments of the 1H signals of 1 were based on the NOE and
decoupling experiments and those of the 13C NMR signals were ac-
complished by the analysis of the CH-COSY and HMBC spectra. Full details
of the physical properties of 1 are described in the Supporting Information.
(11) The GIAO-B3LYP calculations were carried out with 6-311G(3d)
for Si and 6-311G(d) for C and H. The geometries of 3-5 were optimized
at the B3LYP/6-31G(d) level, where the 2-phenyl group of 5 was fixed
perpendicularly to the 2-silanaphthalene ring.
(12) Kalinowski, H.-O.; Berger, S.; Braun, S. Carbon-13 NMR Spec-
troscopy translated by J. K. Becconsall; Wiley: New York, 1986; p 600.
(13) Crystallographic data for 1: C36H66Si7, MW ) 695.52, monoclinic,
space group P21/c, a ) 12.762(6) Å, b ) 9.91(1) Å, c ) 34.67(1) Å, â )
96.58(3)°, V ) 4356(4) Å3, Z ) 4, Dc ) 1.060 g cm-3, µ ) 2.41 cm-1, R
(Rw) ) 0.071(0.069). T ) 193 K. Full details of the crystallographic analysis
of 1 are described in the Supporting Information.
(4) (a) Goldfuss, B.; Schleyer, P. v. R. Organometallics 1995, 14, 1553.
(b) Schleyer, P. v. R.; Freeman, P. K.; Jiao, H.; Goldfuss, B. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 337. (c) West, R.; Sohn, H.; Bankwitz, U.; Calabrese,
J.; Apeloig, Y.; Mueller, T. J. Am. Chem. Soc. 1995, 117, 11608. (d)
Goldfuss, B.; Schleyer, P. v. R.; Hampel, F. Organometallics 1996, 15,
1755. (e) Freeman, W. P.; Tilley, T. D.; Yap, G. A.; Rheingold, A. L. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1002. (f) Freeman, W. P.; Tilley, T. D.;
Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem. Soc. 1996, 118, 10457.
(g) Goldfuss, B.; Schleyer, P. v. R. Organometallics 1997, 16, 1543.
(5) Ma¨rkl, G.; Schlosser, W. Angew. Chem., Int. Ed. Engl. 1988, 27,
963.
(6) Kwak, Y.-W.; Lee, J.-B.; Lee, K.-K.; Kim, S.-S.; Boo, B. H. Bull.
Korean Chem. Soc. 1994, 15, 410.
S0002-7863(97)01092-5 CCC: $14.00 © 1997 American Chemical Society

6952 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Communications to the Editor
Scheme 1a
Figure 2. Calculated NICS (ppm) values (in the ring centers) for the
possible silanaphthalenes and related aromatic systems at GIAO-SCF/
6-31+G*. The optimized Si-C bond lengths (Å) are at the B3LYP/
6-311+G** level.
a Conditions: (a) TbtLi, THF, -78 °C, 34%; (b) NBS (3.7 equiv),
BPO (cat.), benzene, reflux; (c) LiAlH4 (10 equiv), THF, 0 °C, 29%
(X ) H) and 10% (X ) Br) for two steps; (d) t-BuLi (2.0 equiv),
THF, -78 °C; (e) H2O (excess), -78 °C, 94% for two steps; (f) NBS
(1.0 equiv), CCl4, 0 °C, quant; (g) t-BuLi (1.0 equiv), hexane, rt, 80%.
103) nm] most likely assignable to the E1, E2, and B bands.
These are red shifted compared to those for naphthalene [221
(ꢀ 1.33 × 105), 286 (9.3 × 103), and 312 (289) nm],16 suggesting
the aromatic character of this conjugated ring system.
2-Silanaphthalene 1 was found to be very stable thermally
even on heating in benzene at 100 °C in a sealed tube in an
inert atmosphere. No dimerization product was detected,
although 1 is air- and moisture-sensitive due to its SidC moiety.
Its thermal stability, obviously due to the steric protecting ability
of the Tbt group, is in sharp contrast to that of 1,4-di-tert-
butylsilabenzene, which cannot be isolated as a monomer and
undergoes facile dimerization even at 0 °C.17 Interestingly, 1
retains its high reactivity in spite of the thermal stability. Thus,
1 reacts with D2O, methanol, benzophenone, mesitonitrile oxide,
and 2,3-dimethyl-1,3-butadiene to give the corresponding ad-
ducts 6-10 across its SidC moiety (59, 72, 62, 77, and 72%
yields, respectively).18
The aromatic character of the 2-silanaphthalene ring system
was evaluated by computing the NICSs (nucleus independent
chemical shifts)19 of 2-silanaphthalene (3) together with the
related silaaromatic compounds and the parent hydrocarbons
(Figure 2). The large negative NICS values obtained for the
possible three silanaphthalenes, comparable to the parent
naphthalene, suggest that the aromatic character will not be
much reduced by the replacement of a ring carbon by a silicon
atom, which agrees with the experimental evidence discussed
above for 1.
Figure 1. ORTEP drawing of Tbt-substituted 2-silanaphthalene (1)
with a thermal ellipsoid plot (30% probability).
Although we are not able to discuss the X-ray distances
because of the severely disordered 2-silanaphthalene ring of 1,14
the SiC bond lengths computed at the uniform B3LYP/6-
311+G** level for the parent 2-silanaphthalene (3) [1.747 Å
for C(1)sSi(2) and 1.790 Å for Si(2)sC(3)] compared with
silabenzene (1.771 Å), H2CdSiH2 (1.708 Å), and H3CsSiH3
(1.885 Å), suggest the delocalization of π-electrons in the
2-silanaphthalene ring system of 3 (Figure 2). These SiC bond
length relationships are like those for the corresponding CC
bonds in naphthalene, benzene, ethene, and ethane. These
calculations, along with the NMR results just discussed, are
indicative of the aromaticity of 3, which is reinforced by the
additional data below.
The Raman spectrum of 1 showed a strong line with a maxi-
mum intensity at 1368 cm-1, compared with the most intense
line of 1382 cm-1 for naphthalene. The strongest Raman shifts
observed for 1 and naphthalene are in good agreement with the
calculated vibrational frequencies (1377 cm-1 for 3, 1378 cm-1
for 5, and 1389 cm-1 for naphthalene, computed at the B3LYP/
6-31G* level and scaled by 0.98).15 Furthermore, the calculated
vibration modes of 5 showed a close resemblance to those of
naphthalene, suggesting the aromatic character of 1.
Acknowledgment. This work was partially supported by Grants-
in-Aid for Scientific Research from the Ministry of Education, Science,
Sports and Culture, Japan, and at Erlangen by the Deutsche Fors-
chungsgemeinschaft and the Fonds der Chemischen Industrie. We are
grateful to Professor Yukio Furukawa, The University of Tokyo, for
the measurement of FT-Raman spectra. We also thank Shin-etsu
Chemical Co., Ltd., and Tosoh Akzo Co., Ltd., for the generous gift
of chlorosilanes and alkyllithiums, respectively. Calculations were
carried out with the Gaussian 94 program.
Supporting Information Available: Physical properties of com-
pounds 1, 2, and 6-10, crystallographic data with complete tables of
bond lengths, bond angles, and thermal and positional parameters for
1, and a table for the observed Raman shifts for 1 and naphthalene (33
pages). See any current masthead page for ordering and Internet access
instructions.
JA9710924
(15) The geometry optimizations and vibrational frequency computations
for 3, 5, and naphthalene were performed at the B3LYP/6-31G* level; the
2-phenyl group of 5 was fixed perpendicularly (Cs symmetry) to the
2-silanaphthalene ring. For the scaling factor of 0.98, see: Bauschlicher,
Jr. C. W.; Partridge, H. J. Chem. Phys. 1995, 103, 1788.
(16) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Spectrometric
Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991.
(17) Ma¨rkl, G.; Hofmeister, P. Angew. Chem., Int. Ed. Engl. 1979, 18,
789.
(18) All the products 6-10 obtained here showed satisfactory spectral
and analytical data, which are described in the Supporting Information.
(19) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317.
The UV-vis spectrum of 1 in hexane showed three absorp-
tion maxima [267 (ꢀ 2 × 104), 327 (7 × 103), and 387 (2 ×
(14) The structural parameters for 1 reported here are still preliminary.
All the data obtained at room temperature in several runs with different
single crystals gave poorer results. Even from the data collected at low
temperature we have not yet obtained satisfactory intramolecular parameters
for the fused benzene ring moiety of 1 because of the severe disorder which
is most likely caused by the fact that the benzene ring is located on the
peripheral part of this big molecule. Although the final level has not yet
been reached, we think the molecular geometry here obtained is significant
enough to demonstrate the planarity in the silanaphthalene skeleton of 1.