Hydrogen for X-Group Exchange in CH₃X (X = Cl, Br, I, OMe, and NMe₂) by Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH: Experimental and Computational Support for a Carbenoid Mechanism

Abstract

The reactions between [1,2,4-(Me3C)3C5H2]2CeH, referred to as Cp′2CeH, and CH3X, where X is Cl, Br, I, OMe, and NMe2, are described. The reactions fall into three distinct classes. Class a, where X = Cl, Br, and I, rapidly form Cp′2CeX and CH4 without formation of identifiable intermediates in the 1H NMR spectra. Class b, where X = OMe, proceeds rapidly to Cp′2Ce(η2-CH2OMe) and H2 and then to Cp′2CeOMe and CH4. The methoxymethyl derivative is sufficiently stable to be isolated and characterized, and it is rapidly converted to Cp′2CeOMe in the presence of BPh3. Class c, where X = NMe2, does not result in formation of Cp′2CeNMe2, but deuterium labeling experiments show that H for D exchange occurs in NMe3. Density functional calculations DFT(B3PW91) on the reaction of (C5H5)2CeH, referred to as Cp2CeH, and CH3X show that the barrier for α-CH activation, resulting in formation of Cp2Ce(η2-CH2X), proceeds with a relatively low activation barrier (ΔG⧧), but the subsequent ejection of CH2 and trapping by H2 has a higher barrier; the height of the second barrier lies in the order F, Cl, Br, I < OMe ≪ NMe2, consistent with the experimental studies. The DFT calculations also show that the two-step reaction, which proceeds through a carbenoid intermediate, has a lower barrier than a direct one-step σ-bond metathesis mechanism. The reaction of Cp2CeCH2OMe and BPh3 is calculated to be a low-activation barrier process, and the ylide, CH2(+)BPh3(−), is a transition state and not an intermediate.