Benzenes Silabenzenes

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,... 

Although the resonance energy of silabenzene is calculated to be about three-fourths of that of benzene, silabenzenes are extremely reactive and, thus, have been examined only in frozen matrices and in the gas phase. By far the simplest route to silabenzenes is pyrolysis of the 1,4-dihydro derivatives. 

Figure 2.30. Absorption spectra of benzene ( ). silabenzene (—), and 1,4-disila-benzene (—) (by permission from Maier, 1986).

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. 

The stabilization of a silicon-containing arene has not as yet been achieved. Nevertheless, silabenzene (7), 1-silatoluene (8) and hexamethyl-l,4-disilabenzene (9) have all been generated as transients.42"47 Both silabenzene and 1-silatoluene are stable when condensed in argon matrices. The UV spectrum exhibits three bands at 212, 272 and 320 nm expected for a jr-perturbed benzene. In its photoelectron spectrum, the lowest ionization energies are at 8.0, 9.3 and 11.3 eV (cfcalculated values derived by SCF methods of 8.2, 9.2 and 11.5 eV).48... 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

Silabenzene (159) results from the flash pyrolysis of the silacyclohexadiene (158) and can be trapped in the argon matrix at 10 K (80AG(E)51, 80AGphotoelectron spectrum at 8.0 and 9.3 eV corresponding closely to those found for stibabenzene and for silatoluene (Scheme 248) (80JA429). 

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. 

TABLE 2. Calculated substituent effects on the thermodynamic stability, AE (in kcalmol-1), of (pso-substituted silabenzenes and of substituted benzenes (HF/3-21G)"... 

Silabenzene 24 reveals a characteristic Si—H stretching vibration at 2217 cm-1, as expected for hydrogen attached to a sp2-hybridized silicon atom. Compound 24 shows a typical benzene-type UV spectrum with absorptions at X = 217, 272 and 320 nm, which fit into the series of the already known donor-substituted heterobenzenes30. An additional structural proof was the partially reversible photochemical conversion of 24 into Dewar... 

Absorption spectrum (cont.j silabenzene, 105, 107 substituted benzenes, 115-17 tetracene, 72, 103... 

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