Disilanylketenes and -bisketenes

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.