Silaaromatics

Very recentiy, it has been reported that silaaromatic and germaaromatic compounds, stabilized by steric protection, react with Ss to afford the corresponding trithiametallolanes and/or pentathiametallepanes via the addition across the M-C (M=Si, Ge) double bond [94-96] (Scheme 33). 

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. 

Routes for the synthesis of several of silaaromatics from silyldiazoal-kanes were shown in Eqs. (15) and (16) in Section II.B.13. 

In a nice review of the preparative methods for silaaromatics, Maier221 suggested that the flash vacuum pyrolytic techniques that had been so useful for the monosilyl compound could be utilized to prepare 1,4-disila-benzene. Thermolysis of 1,4-disila-2,5-cyclohexadiene led to formation of... 

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 experimental knowledge on silaaromatic compounds is still very limited, leaving theory as one of the main sources of reliable information about the basic properties of these compounds10. Computational methods have proved to be unusually helpful in the study of organosilicon compounds and in particular of transients and other unstable silicon compounds, for which experimental data are scarce10. 

In view of the above-mentioned controversy about the definition of aromaticity, it is obvious that there is no unique way to quantify the degree of aromaticity of a particular compound and that any quantification of aromaticity is likely to be highly controversial. Keeping in mind the above discussion on the nature and definition of aromaticity we now examine the available theoretical studies of silaaromatic and silaantiaromatic compounds. 

In this section we will discuss charged silaaromatic systems which formally possess An +2 7r-electrons, such as 45-49. These species are isoelectronic with the cyclopropenium cation, tropylium cation and cyclopentadienyl anions, respectively, which are all well... 

The first neutral silaaromatic compound stable at room temperature was recently synthesized by Okazaki and coworkers6,272. The 2-silanaphthalene 573 is characterized by... 

For several silaaromatics (573, 599-601) UV spectra are reported (Table 15). The compounds were found to behave like perturbed benzenes (or naphthalenes). 

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 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. 

Photochemical methods have been used in the past to prepare a variety of rather unstable silaaromatic compounds. The photolyses of cyclic unsaturated silyldiazo compounds, which yield silabenzenes and/or silafulvenes, are described in Section VI. 

The photolysis of unsaturated cyclic silyldiazoalkanes has also been employed as a route to several silaaromatic compounds such as silabenzenes and silafulvenes. Markl and coworkers showed that the diazo compound 285 was converted on photolysis at <385 nm to the silafulvene 286139 and that photolysis of the diazo compound 287 led to the sil-abenzene 288140. Each of the processes involved a 1,2-shift of a group from silicon to carbon. Earlier, Ando and coworkers141 had shown that photolysis of the silacyclopenta-dienyldiazomethane 289 led to the formation of both a silabenzene 290 and a silafulvene 291 (Scheme 51). 

These silaaromatics are not stable, although one example described by Markl and coworkers was sufficiently stable at — 100°C to be characterized by 29 Si NMR spectroscopy140. 

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