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

The experimental reaction of [1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 CeH, Cp′ 2 CeH, and CH 3 OSO 2 CH 3 begins by α-C–H activation of the SCH 3 group, forming Cp′ 2 CeCH 2 SO 2 (OCH 3 ), which evolves into Cp′ 2 CeOCH 3 with elimination of CH 2 (and presumably SO 2 ). Prolonged heating of this mixture (days at 60 °C) forms Cp′ 2 CeOSO 2 CH 3 and CH 3 OCH 3 . The metallacycle [1,2,4-(Me 3 C) 3 C 5 H 2 ][1,2-(Me 3 C) 2 -4-(Me 2 CCH 2 )C 5 H 2 ]Ce, when presented with the choice of C–H bonds in CH 3 S and CH 3 O groups, deprotonates both with comparable rates, ultimately forming Cp′ 2 CeOCH 3 and Cp′ 2 CeOSO 2 CH 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 3 S bond, whereas the metallacycle reacts with C–H bonds of both the CH 3 S and CH 3 O groups of CH 3 OSO 2 CH 3 . In the metallacycle reaction, the initially formed regioisomers, Cp′ 2 CeCH 2 SO 2 (OCH 3 ) and Cp′ 2 CeCH 2 OSO 2 CH 3, rearrange to the observed products, Cp′ 2 CeOCH 3 and Cp′ 2 CeOSO 2 CH 3, respectively. Furthermore, C–H activation at the SCH 3 group forms two isomers of Cp′ 2 CeCH 2 SO 2 (OCH 3 ) in the reaction of CH 3 OSO 2 CH 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.