Bridging Silyl Groups in σ-Bond Metathesis and [1,2]-Shifts. Experimental and Computational Study of the Reaction between Cerium Metallocenes and MeOSiMe3
Résumé
The reaction of Cp0 2CeH (Cp0 =1,2,4-(Me3C)3C5H2) with MeOSiMe3 gives Cp0 2CeOMe and HSiMe3, and the reaction of the metallacycle Cp0[(Me3C)2C5H2C(Me)2CH2]Ce with MeOSiMe3 yields Cp0 2CeOCH2SiMe3, formed from the hypothetical Cp0 2CeCH2OSiMe3 by a [1,2]-shift also known as a silyl-Wittig rearrangement. Although both cerium products are alkoxides, they are formed by different pathways. DFT calculations on the reaction of the model metallocene Cp2CeH and MeOSiMe3 show that the lowest energy pathway isHforOMeexchange at Ce that occurs by way of a σ-bond metathesis transition state as SiMe3 exchanges partners. The formation of Cp2CeOCH2- SiMe3 occurs by way of a low activation barrier [1,2]-shift of the SiMe3 group in Cp2CeCH2OSiMe3. Calculations on a model metallacycle, Cp[C5H4C(Me)2CH2]Ce, show that the metallacycle favors CH bond activation over σ-bond metathesis involving the transfer of the SiMe3 group in good agreement with experiment. The σ-bond metathesis involving the transfer of SiMe3 and the [1,2]-shift of SiMe3 reactions have in common a pentacoordinate silicon at the transition states. A molecular orbital analysis illustrates the connection between these two Si-Obond cleavage reactions and traces the reason why they occur for a silyl but not for an alkyl group to the difference in energy required to form a pentacoordinate silicon or carbon atom in the transition state. This difference clearly distinguishes a silyl from an alkyl group as shown in the study of “pyrolysis of tetramethylsilane yielding free d-orbitals” by Seyferth and Pudvin ( CHEMTECH 1981, 11, 230-233).