Selectivity in the C-H Activation Reaction of CH3OSO2CH3 with [1,2,4-(Me3C)3C5H2]2CeH or [1,2,4-(Me3C)3C5H2][1,2-(Me3C)2-4-(Me2CCH2)C5H2]Ce: To Choose or Not To Choose
Résumé
The experimental reaction of [1,2,4-(Me3C)3C5H2]2CeH, Cp′2CeH, and CH3OSO2CH3 begins by α-C-H activation of the SCH3 group, forming Cp′2CeCH2SO2(OCH3), which evolves into Cp′2CeOCH3 with elimination of CH2 (and presumably SO2). Prolonged heating of this mixture (days at 60 °C) forms Cp′2CeOSO2CH3 and CH3OCH3. The metallacycle [1,2,4-(Me3C)3C5H2][1,2-(Me3C)2-4-(Me2CCH2)C5H2]Ce, when presented with the choice of C-H bonds in CH3S and CH3O groups, deprotonates both with comparable rates, ultimately forming Cp′2CeOCH3 and Cp′2CeOSO2CH3 at 20 °C. The experimental studies are illuminated by DFT calculations on the experimental systems, which show that the hydride selects the more acidic CH3S bond, whereas the metallacycle reacts with C-H bonds of both the CH3S and CH3O groups of CH3OSO2CH3. In the metallacycle reaction, the initially formed regioisomers, Cp′2CeCH2SO2(OCH3) and Cp′2CeCH2OSO2CH3, rearrange to the observed products, Cp′2CeOCH3 and Cp′2CeOSO2CH3, respectively. Furthermore, C-H activation at the SCH3 group forms two isomers of Cp′2CeCH2SO2(OCH3) in the reaction of CH3OSO2CH3 with the metallacycle and only one in the reaction with the hydride. The lack of selectivity in the reactions of the metallacycle relative to the hydride is due to the metallacycle's greater thermodynamic advantage and lower energy barriers, which are linked to the higher bond energy of Ce-H relative to Ce-C in the metallacycle.