Silaaromatics matrix isolation

The parent silabenzene 24 was first matrix-isolated by our group in 198035 by pyrolysis of precursors 25 and 26, which yield the expected silabenzene by retro-ene fragmentation. Later, it could be shown that in analogy to carbon chemistry the hydrogen elimination from silacyclohexadiene 27 also gives the silaaromatic 2436. This reaction is allowed by the Woodward-Hoffmann rules. In accordance with the Woodward-Hoffmann rules, it could be demonstrated that silabenzene 24 is not accessible by pyrolysis of the conjugated silacyclohexadiene 28 (equation 8). 

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. 

No stable neutral silaaromatic compound has yet been isolated pure or even in a fluid solution. All direct observations to date have been in the gas phase or in matrix isolation. 

The simplest and apparently also the best gas-phase pyrolytic precursors for silaaromatics are their 1,4-dihydro derivatives, which have been very successful in the production of matrix-isolated silabenzene316 and 1,4-disilabenzene317. On the other hand, pyrolysis of l-sila-2,4-cyclohexadiene under identical conditions only gave traces of silabenzene (equation 149). This result can be easily understood in terms of Woodward-Hoffmann rules. 

Irradiation of matrix-isolated silabenzene at 320 nm causes a transformation to the Dewar isomer318,324. This conversion is readily followed by the shift of the Si-H stretching frequency from that characteristic of an sp2 to that characteristic of an sp3 hybridized silicon. Irradiation of the Dewar isomer with 240 nm light318 causes partial reversion to silabenzene. A similar photostationary state can be reached in the case of 1-phenyl-1-silabenzene315. Since there are no wavelengths at which the Dewar isomer absorbs while the silaaromatic itself does not, it is not possible to achieve complete conversion of the Dewar form into the silaaromatic form. 

---