Selectivity in the C–H\nActivation Reaction\nof CH₃OSO₂CH₃ with [1,2,4-(Me₃C)₃C₅H₂]₂CeH or\n[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\nby α-C–H\nactivation of the SCH₃ group, forming Cp′₂CeCH₂SO₂(OCH₃), which evolves into\nCp′₂CeOCH₃ with elimination of CH₂ (and presumably SO₂). Prolonged heating of this\nmixture (days at 60 °C) forms Cp′₂CeOSO₂CH₃ and CH₃OCH₃. The metallacycle\n[1,2,4-(Me₃C)₃C₅H₂]­[1,2-(Me₃C)₂-4-(Me₂CCH₂)­C₅H₂]­Ce, when presented with the choice of C–H bonds\nin CH₃S and CH₃O groups, deprotonates both with\ncomparable rates, ultimately forming Cp′₂CeOCH₃ and Cp′₂CeOSO₂CH₃ at 20 °C. The experimental studies are illuminated by DFT calculations\non the experimental systems, which show that the hydride selects the\nmore acidic CH₃S bond, whereas the metallacycle reacts\nwith C–H bonds of both the CH₃S and CH₃O groups of CH₃OSO₂CH₃. In the metallacycle\nreaction, the initially formed regioisomers, Cp′₂CeCH₂SO₂(OCH₃) and Cp′₂CeCH₂OSO₂CH₃, rearrange to\nthe observed products, Cp′₂CeOCH₃ and\nCp′₂CeOSO₂CH₃, respectively.\nFurthermore, C–H activation at the SCH₃ group forms\ntwo isomers of Cp′₂CeCH₂SO₂(OCH₃) in the reaction of CH₃OSO₂CH₃ with the metallacycle and only one in the reaction\nwith the hydride. The lack of selectivity in the reactions of the\nmetallacycle relative to the hydride is due to the metallacycle’s\ngreater thermodynamic advantage and lower energy barriers, which are\nlinked to the higher bond energy of Ce–H relative to Ce–C\nin the metallacycle.