Thermolysis silene preparation

While route B has been observed and utilized by Wiberg to synthesize silenes (see Section I), route A has been found to occur when less bulky substituents are used. Tamao and coworkers prepared the heterocycles (e.g. 689) by intramolecular hydrosilylation of silylated allylamines and decomposed them in toluene solution at 204 °C311. From the thermolysis of 689 they obtained the corresponding tram alkene and the dimerization product 691. In the presence of Me3SiOEt the insertion product 690 was formed at the expense of the dimer 691 (equation 230). 

Silenes (R2Si = CR2). Although a few stable molecules containing the Si = C unit have been prepared recently as crystals from which structural parameters could be obtained, the vast majority of the chemistry of silenes has been investigated by indirect means with transient molecules. Silenes can be generated from a wide variety of precursors through gas-phase thermolysis,... 

Silanones, the silicon analogs of ketones, are produced via rDA reactions. One attempt to prepare a suitable DA precursor for retrograde decomposition to a silanone met with unexpected results. The desired DA reaction between 2,2-dimethyl-l-oxa-2-silacyclohexa-3,S-diene (251) and perfluoro-2-but-yne was complete in one day at room temperature. The observed product was o-bis(trifluoromethyl)ben-zene, as the initial adduct apparently underwent retrodiene decomposition to yield the intermediate dimethylsilanone (252) (equation 111). The occurrence of this retro ene process at room temperature was not consistent with the analogous extrusions of silenes and disilenes that require elevated temperatures. However, the reaction sequence was substantiated by comparison with its carbon analog in which tetramethylpyran and dimethyl acetylenedicarboxylate react at room temperature to afford only acetone and the corresponding phthalate. Stable adducts that extrude silanones are also known. Reactions of 2-sili yrans and nudeic anhydride provide stable adducts, such as (253), that decompose upon thermolysis... 

The pyrolysis of 1,1-disubstituted silacyclobutanes has been used as a method for the preparation of 1,1,3,3-tetrasubstituted 1,3-disilacyclobutanes183. Copyrolysis of two different 1,1-disubstituted silacyclobutanes yielded mixtures containing 1,3-disilacyclobutanes arising from all possible combinations of the two silenes present (equation 51). The preparative method failed in the case of cyclopentadienyl-substituted silacyclobutanes183, 184, presumably due to competing intramolecular reactions of the silene. The thermolysis of trimethylsilylcyclopentadiene may also proceed via a silene184. 

---