Experimental and DFT Computational Study of β-Me and β-H Elimination Coupled with Proton Transfer: From Amides to Enamides in Cp*<sub>2</sub>MX (M = La, Ce)
作者:Sergio S. Rozenel、Lionel Perrin、Odile Eisenstein、Richard A. Andersen
DOI:10.1021/acs.organomet.6b00677
日期:2017.1.9
The thermal rearrangement of the f-block metallocene amides Cp-2*MNR1R2, where R-1 is CHMe2, R-2 is either CHMe2 or CMe3, and M is either La or Ce, to the corresponding enamides Cp-2*MNR1[C(Me)=CH2] and H-2 or CH4, respectively, occurs when the solid amides are heated in sealed evacuated ampules at 160-180 degrees C for 1-2 weeks. The net reaction is a beta-H or beta-Me elimination followed by a gamma-abstraction of a proton at the group from which the beta-elimination occurs. When R-1 is either SiMe3 or SiMe2CMe3 and R-2 is CMe3, the enamide Cp-2*MNR1[C(Me)=CH2] is isolated, the result of beta-Me elimination, but when R-2 is CHMe2, the enamides Cp-2*MNTR1[C(Me)=CH2] and Cp-2*NR1[C(H)=CH2] are isolated, the result of beta-H and beta-Me elimination. In the latter cases, both enamides are formed in similar amounts and the rates of the beta-H and beta-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-2*M 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 gamma-carbon of the intermediate imine. The two elementary steps involve significant geometrical changes within the N alpha C beta C gamma 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.