Silabenzene

FIGURE 2. (a) B3LYP/6-311+G optimized structure of silabenzene (8) (b-d) B3LYP/3-21G opti-mized geometries of ortho-, meta- and para- disilabenzenes, respectively values in parentheses are at MP2/6-31G. All data are from Y. Apeloig, M. Bendikov and S. Sklenak, unpublished results 

The photoelectron spectra of silabenzene was measured and assigned on the basis of a double-zeta ab initio SCF calculation44. The lowest ionization energy of 8 is 8.11 eV (2/ii state, calculated to be 7.8 eV at HF/DZ44), by ca 1 eV lower than in benzene. The authors conclude that silabenzene is best considered as a symmetry-distorted (due to the presence of the Si atom) cyclic 67r-electron system. 

Schleyer and coworkers40 studied in detail the energetic relationships between silabenzene 8 and four of its isomers, 13-16. However, as this study is quite old, the calculated relative energies of 8 and 13-16, which have been calculated only at HF/3-21G( )//HF/STO-3G, are probably only approximate and higher level calculations are required for a more reliable estimate of these relative energies. 

In summary, all the calculations find silabenzene to be highly aromatic. However, in view of the relatively basic level of theory which was used in most of the available studies, it is highly desirable to repeat the calculations for this fundamentally important molecule using more reliable levels of theory. 

The first to generate and trap a silaaromatic compound were Barton and Banasink2413, who reported in 1977 on silatoluene, 85241. The discovery was soon followed by a matrix isolation and IR, UV and PE spectral characterization of both 85241b,c and silabenzene 86242. 

These calculations revealed that the silylene isomers 88 and 89 are approximately 20-25 kcal mol l higher in energy than 86. 90 is a ground-state triplet and lies about 60 kcal mol 1 above 86. The 86-88 (or 89) energy difference is much smaller than between 

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New aspects in the chemistry of multiple bonds to heteroatoms, considered in particular for 2-silanaphthalene and silabenzene stabilized with 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl substituent at the Si atom 99PAC405. 

UV irradiation (A>320 nm) of matrix-isolated silabenzene [137a] led to the disappearance of its absorptions and the appearance of bands of other unstable molecules, which were assigned to Dewar silabenzene [138]. This assumption is reasonable because sp hybridization of the silicon atom is preferable to sp hybridization and favourable to the stability of [138]. Besides, according to calculations the energy differences between mono-cyclic [137a] and bicyclic [138] structures decrease from CeHe (314 kJ moP ) to silabenzene CsSiHe (125 kJ moP ). 

Closely related studies by Markl using a 2-diazo-l-silacyclohexadiene such as 20 led in some cases to highly substituted silabenzenes,108 109 one of which, 21, was stable at -100°C, based on its 29Si NMR spectrum (Eq. 16). 

Similar observations have been made on a few silaaromatics, such as silabenzene and 1-silatoluene, but it is not known which of the many absorption bands observed are associated with the silicon-carbon framework. The same is true for the stable silenes such as (Me3Si)2Si=C(OSi Me3)Ad. Both in solution and the solid state, a number of bands were observed in the region 1300-930 cm-1, but it is not known with certainty which are associated with the Si=C bond. 

Another thermal route which has been particularly useful for the generation of silaaromatics (silabenzene, silatoluene, 4-substituted silaaromatics, etc.) and silafulvenes is the ene reaction of allylsilanes, as illustrated [Eq. (4)]. 

The geometries of silabenzene and four of its isomers were optimized using ab initio calculations at the STO-3G level (isomers 3-5 and 1)(60,193) or using semi-empirical MNDO calculations (isomer 6)(194). 

The heat of formation (13.7 kcal/mol) and the first ionization potential (8.6 eV) of silabenzene have also been calculated using the MINDO/3 method (177). 

CuS04-catalyzed decomposition of the (l-sila-cyclopentadienyl)diazomethane 398 did not furnish defined products. The desired rearrangement reactions to a silabenzene and a l-ethylidene-l-sila-2,4-cyclopentadiene, both trapped by /-butanol, were brought about by irradiation of 398, however 388... 

The measured IEs for all the germabenzenes were compared and found to be in good agreement with those of substituted silabenzene derivatives, found in the literature81. 

Significant advances have been made in the field of sila-aromatic compounds. These have been covered in two recent reviews by Tokitoh. 8 The first stable silabenzene 336 derivative containing formally an. (//-hybrized Si atom has... 

Besides the silabenzene 336, a number of sila-aromatic naphthaline and anthracene derivatives 351-353 have been... 

This system exhibits a local minimum with attachment of the SiH moiety to the C5H5 ring however, the global minimum corresponds to the known molecule silabenzene (36) ... 

The propene split-off route to generate intermediates is well-established, cf. Brown, R.F.C. "Pyrolytic Methods in Organic Chemistry" Academic Press New York 1980. It has been applied by our group to prepare e.g. silatoluene (Bock, H. Bowling, R.A. Solouki, B. Barton, T.J. Burns, G.T. J. 7 m. Chem. Soc. 1980, 102, 429), silabenzene (Solouki, B. Rosmus, P. Bock, H. Maier, G. Angew. Chem. 1980, 2, 56 Angew. Chem. Int. Ed. 

Brown, E. C. Borden, W. T. What Accounts for the Remarkable Difference between Silabenzene and Phosphabenzene in Stability toward Dimerization Organometallics 2000, 19, 2208-2214. 

AG(E)1627, 89MI8]. The only known exception is 1,4-di-tert-buty 1-2,6-bis(trimethylsilyl)silabenzene, (258) which is stable in solution at - 100°C [88AG(E)(27)963]. For generating silabenzene photochemical reactions... 

In the lower triplet state, the silabenzene molecule retains its planar structure whose energy is higher by 107 kcal/mol than that of the structure of the singlet ground state (STO-3G) (78JA6499). Like benzene, silabenzene is more stable than its isomers (259)—(262) by, respectively, 38.4,... 

An estimate of the aromatic stabilization energy of silabenzene, based on the calculation of the ISE (STO-3G basis set) (78JA6499), concludes that its value is 2/3 of the benzene stabilization energy. Possible ap-... 

The lowering of the aromatic stabilization energy of silabenzene, compared to benzene, leads one to expect that the energy of the antiaromatic stabilization of silacyclobutadiene (253) would be lower than in the case of cyclobutadiene. 

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