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)

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 NαCβCγ 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.