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

Abstract

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