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
The reaction 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(eta2-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 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 alpha-CH activation, resulting in formation of Cp2Ce(eta2-CH2X), proceeds with a relatively low activation barrier (DeltaG++) 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 sigma bond metathesis mechanism. The reaction of Cp2CeCH2OMe and BPh3 is calculated to be a low barrier process and the ylide, CH2(+)BPh3(-), is a transition state and not an intermediate.