Silabenzenes, additions

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... 

The silaaromatics 36 and 38 reveal in the observed UV spectra an additional bathochromic shift compared to silabenzene 24. Both, 36 and 38, disappear in an irreversible process upon irradiation with X = 290-420 nm, probably due to the formation of the corresponding Dewar silabenzenes. The kinetically stabilized silaaromatics 36 and 38 turned out to be stable up to 90 K even without an argon cage, but decomposed unspecifically at higher temperatures. 

Kinetically stabilized silabenzene 32, the crystal structure of which has been discussed in Section 7.14.3, undergoes 1,2- and 1,4-addition reactions similar to those of the boron analogs, for example, with water to afford 1-hydroxy-silacyclohexadienes 81 and 82 (Scheme 4) <20050M6141>. Reaction of 32 with phenylacetylene results in two addition products, the silabarrelene derivative 83 formed by a [4+2] cycloaddition, and the 1,2-addition product 84. 

Silaaromatics also are excellent partners in Diels-Alder cycloadditions, acting either as dienes or as dienophiles. Thus, the addition of acetylene and bis(trifluoromethyl)acetylene to silabenzenes is one of the best documented trapping procedures290,314,320,326,327. This reaction has been reported to fail only in the case of the sterically quite hindered 1,4-di-tm-butyl-1 -silabenzene312. 

Dynamic phenomena are important molecular state properties (Fig. 1), as has been demonstrated by the selected example of the radical cation (R3SiCH2)3N (Fig. 11). Along the microscopic pathways of chemical reactions, they often play a decisive role [10]. As concerns medium-sized molecules, however, their 3 -6 degrees of freedom mostly lead to nearly hopeless situations of multidimensional complexity [8,11). Even when some of them are neglected for specific molecular conformations by introducing additional assumptions, number-crunching hypersurface calculations have to be performed. Such a crude simplification of the rather complex reality will be illustrated for the pyrolysis of l-silacyclohexa-2,5-diene to silabenzene and H2 [21] (Fig. 13) in a yellow-glowing flow tube at 800 K and under 10" Pa pressure — that is under nearly unimolecular conditions. 

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