Cleaving bonds in CH3OSO2CF3 with [1,2,4-(Me3C)3C5H2]2CeH; an experimental and computational study

Abstract

The reaction at 20 °C of the metallocenelanthanide hydride, [1,2,4-(Me₃C)₃C₅H₂]₂CeH, , and excess methyltrifluoromethanesulfonate, CH₃OSO₂CF₃, results in formation of , , and the bimetallic complex . The metallocenes , , and react with excess CH₃OSO₂CF₃ to form , CH₃OCH₃, CH₃F, and (CH₃O)₂SO, respectively, at 20 °C. Thus, the net reaction is but the pathway is not a direct methyl transfer. Comparison of the reactivity of CH₃OSO₂CF₃ and CH₃OSO₂CH₃ (Werkema et al., Organometallics, 2012, 31, 870) is revealing since both form a similar set of products but the rates of reaction of CH₃OSO₂CF₃ are faster. The bimetallic complex, in which the SO₃²⁻ anion bridges two fragments, is unique in organometallic chemistry. The ¹H NMR spectrum is fluxional at 20 °C and the low temperature spectrum is consistent with the geometry observed in the solid state. Density Functional Theory (DFT) calculations of the Gibbs energy profiles for the reaction of CH₃OSO₂CF₃ with show that the CH-bond activation and direct CH₃ group transfer have similar activation energy barriers. This contrasts with what is observed in the reaction of with CH₃OSO₂CH₃, where CH-bond activation at the SCH₃ group is preferred. Remarkably, the activation energy barriers for C–O-bond cleavage are similar in CH₃OSO₂CH₃ and CH₃OSO₂CF₃, which is traced to the calculated small exoergicity of −1.7 kcal mol⁻¹ for the reaction of . This contrasts, perhaps, with conventional wisdom that overemphasizes the effect of the electron-withdrawing ability of the CF₃ group on the chemical and physical properties of sulfonate esters.