Silabenzenes disilabenzenes

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... 

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 All data are from Y. Apeloig, M. Bendikov and S. Sklenak, unpublished results... 

Comparison of the energy of para-disilabcnzcne (11) with that of the isomeric Dewar disilabenzene (23) and the silylenes 24 and 25 follows the trends discussed above for the isomers of silabenzene. At IIF/3-21 G( )//I H7STO-3G the most stable isomer is the... 

After the successful preparation of silabenzene 24 by hydrogen elimination from 27, it was tempting to try to generate 1,4-disilabenzene 31 by pyrolysis of the easily accessible disilacyclohexadiene 3041 (equation 9). 

The dehydrogenation of 30 turned out to be more difficult than the analogous conversion of 27 to 24. The IR spectrum of the matrix-isolated products showed only one absorption at 1273 cm-1, which disappeared upon irradiation with X = 405 nm probably yielding Dewar disilabenzene 32. The UV spectrum proved to be of higher diagnostic value. 1,4-Disilabenzene 31 shows a typical heterobenzene electronic spectrum with absorptions at X = 408, 340 and 275 nm displaying another bathochromic shift compared to silabenzene 24. 

Early work in the area is summarized in Reference 1. A survey of successful approaches to transient neutral aromatics is given in Chart 10 most of them apply to silabenzene and its derivatives but a few of the methods apply also to 1,4-disilabenzenes (see below). 

A survey of the reactions observed for silabenzenes is given in Chart 11. Those of 1,4-disilabenzenes have been studied less but appear to be similar. 

Several studies reported on silabenzenes in which more than one carbon was snbsti-tuted by silicon. Maier and coworkers observed 1,4-disilabenzene (11) in a matrix and identified it by its IR absorbtion at 1273 cm and its electronic absorptions at 408, 340 and 275 nm . Evidence for the formation in the gas phase of 1,3,5-trisilabenzene (12) as a ligand in the dehydrogenation reaction of 1,3,5-trisilacyclohexane with [Cp,Fe]+, [Cp,Co]+ and [Cp,Ni]+ was recently reported by Bjamason and Amason. The strncture of 12 was assigned on the basis of the coUision-indnced dissociation spectrum and from deuterium-labelling experiments. ... 

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