DISILANYLKETENES AND DISILANYL-BISKETENES

Pyrolysis of Me(5)Si(2)C=COEt (5) and (SiMe(2)C=COEt)(2) (6), prepared by reaction of EtOC=CLi with Me(5)Si(2)Cl and ClMe(2)SiSiMe(2)Cl, res pectively, at 180 degrees C gave Me(5)Si(2)CH=C=O (1), and (SiMe(2)CH= C=O)(2) (2), respectively, which are long lived and were completely ch aracterized by spectroscopic means. The novel 1,6-bisketene (CH(2)SiMe (2)CH=C=O)(2) (3) was prepared similarly. The UV spectra of 1-3 and th e known (Me(3)Si)(2)C=C=O (4) show longer wavelength absorption for th e disilanylketenes, attributed to a lowering of the pi orbital by int eraction with the Si-Si sigma orbital, while shies to shorter wavelen gth in Me(3)SiCH=C=O and 4 compared to alkylketenes are attributed to a raising of the pi level by the C-Si sigma orbital. Isodesmic compar isons using ab initio molecular orbital calculated energies indicate t hat the Si2H5 substituent is essentially equal to the SiH3 group in ke tene-stabilizing ability, while (SiH2CH=C=O)(2) is only destabilized b y 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 simila r to that of Me(3)SiCH=C=O. By contrast theoretical and experimental s tudies of (Me(3)Si)(2)C=C=O (4) reveal that the effects of the two sil yl groups on the stability and spectra are largely additive but that t he second Me(3)Si group greatly lowers the reactivity in both neutral and acid catalyzed hydration. This ketene ranks with t-Bu(2)C=C=O amon g the least reactive known in hydration, a property attributed to ster ic inhibition to nucleophilic attack and ground-state stabilization of the ketene by the silyl substituents.