Selectivity in the C-H activation reaction of CH _3OSO _2CH _3 with [1,2,4-(Me _3C) _3C _5H _2] _2CeH or [1,2,4-(Me _3C) _3C _5H _2][1,2-(Me _3C) _2-4-(Me _2CCH _2)C _5H _2]Ce: To choose or not to choose

摘要

The experimental reaction of [1,2,4-(Me _3C) _3C _5H _2] _2CeH, Cp′ _2CeH, and CH _3OSO _2CH _3 begins by α-C-H activation of the SCH _3 group, forming Cp′ _2CeCH _2SO _2(OCH _3), which evolves into Cp′ _2CeOCH _3 with elimination of CH _2 (and presumably SO _2). Prolonged heating of this mixture (days at 60 °C) forms Cp′ _2CeOSO _2CH _3 and CH _3OCH _3. The metallacycle [1,2,4-(Me _3C) _3C _5H _2][1,2-(Me _3C) _2-4-(Me _2CCH _2)C _5H _2]Ce, when presented with the choice of C-H bonds in CH _3S and CH _3O groups, deprotonates both with comparable rates, ultimately forming Cp′ _2CeOCH _3 and Cp′ _2CeOSO _2CH _3 at 20 °C. The experimental studies are illuminated by DFT calculations on the experimental systems, which show that the hydride selects the more acidic CH _3S bond, whereas the metallacycle reacts with C-H bonds of both the CH _3S and CH _3O groups of CH _3OSO _2CH _3. In the metallacycle reaction, the initially formed regioisomers, Cp′ _2CeCH _2SO _2(OCH _3) and Cp′ _2CeCH _2OSO _2CH _3, rearrange to the observed products, Cp′ _2CeOCH _3 and Cp′ _2CeOSO _2CH _3, respectively. Furthermore, C-H activation at the SCH _3 group forms two isomers of Cp′ _2CeCH _2SO _2(OCH _3) in the reaction of CH _3OSO _2CH _3 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.