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-(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 3Ce, 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.

Effects of cross-linking methods for polyethylene-based carbon fibers: review

“…In this region, the maximum rates of change in % mass occurred from 440 to 480 K, depending on heating rate. At these temperatures, HOS 2 decomposed and H became the radical responsible for propagation of the will therefore increase the available amount of free radical to initiate the radical chain [87][88][89][90][91][92][93][94][95].…”

Cleaving bonds in CH3OSO2CF3with [1,2,4-(Me3C)3C5H2]2CeH; an experimental and computational study

“…Furthermore, it was shown in the study of the reactivity of CH 3 OSO 2 CH 3 with the cerium hydride that the full model was able to rationalize the experimental results while the simplified model, in which C 5 H 5 replaces Cp 0 , was not. 3 The computational study evolves in the following way. (i) The excellent agreement between the calculated and experimental structures of Cp 0 2 CeOSO 2 CF 3 suggests that the calculated structure of Cp 0 2 CeOSO 2 CH 3 is close to that of the experimental complex whose crystal structure is not available.…”

“…CH-bond activation and the methyl transfer profiles. The presentation of the computational results follows the pattern used in the reaction of CH 3 OSO 2 CH 3 with [Ce] 0 H, 3 since the purpose of the present work is to compare and contrast the reactivity patterns of CH 3 OSO 2 CF 3 and CH 3 OSO 2 CH 3 with [Ce] 0 H. As was the case for CH 3 OSO 2 CH 3 , the reaction of CH 3 OSO 2 CF 3 with [Ce] 0 H begins with adduct formation using either the oxygen lone pair on one of the SO groups or the OCH 3 group, followed by a transition state for either CH-bond activation or direct methyl transfer, which lead to different products. The elementary reactions that follow these two steps have not been considered.…”

“…‡ The Gibbs energy profiles obtained for the corresponding reactions with CH 3 OSO 2 CH 3 , presented in detail in an earlier paper, are shown for a comparison. 3 Adduct formation with either the SO or OCH 3 oxygen lone pairs is endoergic by 7 kcal mol À1 . The calculated activation energy barriers for CH-bond activation and methyl transfer are also close in energy although the calculated energies of the product forming reactions are very different.…”

“…The two-step pathway begins with CH-bond activation followed by elimination and trapping of the ejected carbene fragment, eqn (1). [1][2][3] Cp 0 2 CeHþCH 3 X ! Cp 0 2 CeðZ 2 -CH 2 XÞþH 2 !…”