Silabenzenes formation

The heat of formation (13.7 kcal/mol) and the first ionization potential (8.6 eV) of silabenzene have also been calculated using the MINDO/3 method (177). 

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. 

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). 

It has been a well-entrenched dogma that elements beyond argon are reluctant to form multiple covalent bonds, citing as examples the difference between C02 and Si02. One should note, however, that the formation of multiple bonds for such atoms occurs with increasing relative easiness in the order Si < P < S indeed, although silabenzene is unstable under normal conditions, phosphabenzene and thiopyrylium are normal, stable aromatic systems. 

On the other hand, the thermal decomposition of 4-diazo-1,1-dimethyl-1-silacyclohexa-2,5-diene under a variety of conditions yielded no evidence for the formation of a silabenzene derivative325. 

Along similar lines, l,4-di-tm-butyl-l-chloro-l-silacyclohexa-2,4-diene (80) reacted with lithium diisopropylamide in n-pentane and the formal dimer of a presumably initially formed transient 1,4-di-tm-butylsilabenzene was isolated (equation 153)312. The transient formation of silabenzene was supported by trapping various 1,3-dienes. An even more highly substituted silabenzene was presumably formed as a transient when the diazirine 60 was heated in solution (equation 76)128,249. The products obtained with 2,3-dimethylbutadiene and benzophenone upon thermal decomposition of the diazirine suggest strongly the intermediate formation of a 1-methyl-2,3,4,5-tetraphenylsilabenzene (81) as well as the isomeric fulvene 63. 

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. ... 

The formation of Brook-type silabenzenes via [l,3]-trimethylsilyl and [1,3]-triisopropylsilyl shifts have been examined by computational methods. ... 

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