Pyrolysis of Me5Si2C=COEt (5) and (SiMe2C≡COEt)2 (6), prepared by reaction of EtOC≡CLi with Me5Si2Cl and ClMe2SiSiMe2Cl, respectively, at 180 °C gave Me5Si2CH=C=O (1), and (SiMe2CH=C=O)2 (2), respectively, which are long lived and were completely characterized by spectroscopic means. The novel 1,6-bisketene (CH2SiMe2CH=C=O)2 (3) was prepared similarly. The UV spectra of 1-3 and the known (Me3Si)2C=C=O (4) show longer wavelength absorption for the disilanylketenes, attributed to a lowering of the π* orbital by interaction with the Si-Si σ* orbital, while shifts to shorter wavelength in Me3-SiCH=C=O and 4 compared to alkylketenes are attributed to a raising of the π* level by the C-Si σ orbital. Isodesmic comparisons using ab initio molecular orbital calculated energies indicate that the Si2H5 substituent is essentially equal to the SiH3 group in ketene-stabilizing ability, while (SiH2CH=C=O)2 is only destabilized by 0.6 kcal/mol compared to SiH3CH=C=O. The rates of hydration of 1 and 2 exceed that of 3 by factors of 2-4, while 3 has a reactivity similar to that of Me3SiCH=C=O. By contrast theoretical and experimental studies of (Me3Si)2C=C=O (4) reveal that the effects of the two silyl groups on the stability and spectra are largely additive but that the second Me3Si group greatly lowers the reactivity in both neutral and acid catalyzed hydration. This ketene ranks with t-Bu2C=C=O among the least reactive known in hydration, a property attributed to steric inhibition to nucleophilic attack and ground-state stabilization of the ketene by the silyl substituents.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry