SYNTHESIS AND REACTIVITY OF SEVERAL STABLE 1-SILAALLENES

Three stable 1-silapropadienes 1a-c were prepared by the intermolecula r addition of organolithium reagents at the carbon-carbon triple bond of fluoroalkynylsilanes 3a,b followed by LiF elimination. The structur e of la and of the intermediate alpha-lithiated fluorosilane 2a were d etermined by X-ray diffraction. The chemical behavior of 1-silapropadi enes 1a-c differs greatly depending on the substituents on the silicon atom. When two triisopropylphenyl groups were present (1a), typical a ddition reactions of the Si=C double bond were observed. Thermolysis o f 1a yielded benzosilacyclobutene 7. For the more hindered 1-silapropa dienes 1b and 1c, no reaction was observed with dioxygen, benzaldehyde , 2,3-dimethylbutadiene, or trimethylchlorosilane at room temperature. However, thermolysis of Ib or treatment with excess ethanol under neu tral or acidic conditions produced 8b, the product of an intramolecula r insertion of the Si=C double bond into an adjoining primary C-H bond of one tert-butyl group in an ortho-position of the supermesityl subs tituent. In the presence of EtOD and catalytic D2SO4, this reaction oc curred very rapidly at -78 degrees C with over 95% incorporation of de uterium in the product.