Intramolecular vs. Intermolecular Alkyl C-H Bond Activation. Complete Thermodynamic and Kinetic Parameters for a Reversible Cyclometalation

Despite the widespread occurrence of intramolecular C-H activating^{1, 2} cyclometalation processes³(eq 1) and the possibility that they may enjoy an “intramolecular advantage”^{4, 5} over competing intermolecular processes, relevant comparative thermodynamic (ΔH, AS) and kinetic (ΔH*, ΔS*) information is surprisingly sparse. For one rhodium system, Jones and Feher⁶ showed that an intramolecular process enjoyed a slight thermodynamic (only AG° could be measured) advantage but also a slight kinetic disadvantage (only in AH*). We recently reported kinetic/mechanistic studies of intermolecular alkane activation⁷ and closely related intramolecular cyclometalation⁸ at thorium centers (eq 2–4). C-H scission was found to be rate-limiting, and thermochemical data (ΔH)⁹suggest that eq 3 and 4 are entropically driven. We now report that the reversibility of eq 4 can be rendered observable, presenting a unique opportunity to quantify, in toto, thermodymamic and kinetic aspects of the in tramolecular cyclometalation process vis-á-vis its intermolecular microscopic reverse.