Experimental and DFT Computational Study of β-Me and β-H Elimination Coupled with Proton Transfer: From Amides to Enamides in Cp*2MX (M = La, Ce)

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• 作者：Sergio S. Rozenel ; Lionel Perrin ; Odile Eisenstein ; Richard A. Andersen
• 刊名：Organometallics
• 出版年：2017
• 出版时间：January 9, 2017
• 年：2017
• 卷：36
• 期：1
• 页码：97-108
• 全文大小：507K
• ISSN：1520-6041

文摘

The thermal rearrangement of the f-block metallocene amides Cp*₂MNR₁R₂, where R₁ is CHMe₂, R₂ is either CHMe₂ or CMe₃, and M is either La or Ce, to the corresponding enamides Cp*₂MNR₁[C(Me)═CH₂] and H₂ or CH₄, respectively, occurs when the solid amides are heated in sealed evacuated ampules at 160–180 °C for 1–2 weeks. The net reaction is a β-H or β-Me elimination followed by a γ-abstraction of a proton at the group from which the β-elimination occurs. When R₁ is either SiMe₃ or SiMe₂CMe₃ and R₂ is CMe₃, the enamide Cp*₂MNR₁[C(Me)═CH₂] is isolated, the result of β-Me elimination, but when R₂ is CHMe₂, the enamides Cp*₂MNR₁[C(Me)═CH₂] and Cp*₂NR₁[C(H)═CH₂] are isolated, the result of β-H and β-Me elimination. In the latter cases, both enamides are formed in similar amounts and the rates of the β-H and β-Me elimination steps must be similar. A two-step mechanism is developed from DFT calculations. The first step is migration of a hydride or a methyl anion to the Cp*₂M fragment, forming M–H or M–Me bonds as the N═C bond in the intermediate imine forms. The enamide evolves from the metal-coordinated imine by abstraction of a proton from the γ-carbon of the intermediate imine. The two elementary steps involve significant geometrical changes within the N_αC_βC_γ set of atoms during the two-step elimination process that are in large part responsible for the relatively high activation barriers for the net reaction, which may be classified as a proton-coupled hydride or methyl anion transfer reaction.