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 reactions between [1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 CeH, referred to as Cp′ 2 CeH, and CH 3 X, where X is Cl, Br, I, OMe, and NMe 2, are described. The reactions fall into three distinct classes. Class a, where X = Cl, Br, and I, rapidly form Cp′ 2 CeX and CH 4 without formation of identifiable intermediates in the 1 H NMR spectra. Class b, where X = OMe, proceeds rapidly to Cp′ 2 Ce(η 2 -CH 2 OMe) and H 2 and then to Cp′ 2 CeOMe and CH 4 . The methoxymethyl derivative is sufficiently stable to be isolated and characterized, and it is rapidly converted to Cp′ 2 CeOMe in the presence of BPh 3 . Class c, where X = NMe 2, does not result in formation of Cp′ 2 CeNMe 2, but deuterium labeling experiments show that H for D exchange occurs in NMe 3 . Density functional calculations DFT(B3PW91) on the reaction of (C 5 H 5 ) 2 CeH, referred to as Cp 2 CeH, and CH 3 X show that the barrier for α-CH activation, resulting in formation of Cp 2 Ce(η 2 -CH 2 X), proceeds with a relatively low activation barrier (Δ G ⧧ ), but the subsequent ejection of CH 2 and trapping by H 2 has a higher barrier; the height of the second barrier lies in the order F, Cl, Br, I < OMe ≪ NMe 2, 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 Cp 2 CeCH 2 OMe and BPh 3 is calculated to be a low-activation barrier process, and the ylide, CH 2 (+) BPh 3 (−), is a transition state and not an intermediate.