3.1.2 C (sp2) -H bond Cleavage of Aromatic Ring and Alkyne Insertion
Consistent with the proposal in Scheme 3, C-H bond cleavage of the 1,4-chlorobenzene was proposed to initiate the reaction in this work, followed by insertion of alkyne. The mechanisms calculated and the optimized key intermediate 7 and transition stateTS3-4 together with important structural parameters are presented in Figure 2. After the formation of the active catalystK , the C-Hσbond in substrate 1a coordinates with the active catalyst K to afford a unstable complex 3 . The coordination process is endothermic by 16.4 kcal/mol. Subsequently, the intermediate 3 undergoes the concerted metalation-deprotonation (CMD) process via TS3-4 , generating a 16e tetra-coordinate square-planar intermediate 4 . The barrier calculated is 24.8 kcal/mol, and the exergonicity of this step is -6.2 kcal/mol. Transition state TS3-4 is also the rate-determining step from the reactant to product. A subsequent dissociation of one HOAc molecule in 4affords a unstable tri-coordinated T-shape intermediate 5 over4 by 8.1 kcal/mol. As exhibited in Figure 2, the coordination of substrate 4-octyne (2a1 ) molecule to Pd generates intermediate 6 ready for subsequent alkyne insertion. Then, the alkyne inserts into the Pd–C(aryl) bond of 6 viaTS6-7 with an energy demand of 21.2 kcal/mol, leading to the formation of alkenyl palladium intermediate 7 . This process is exothermic by -7.8 kcal/mol. It is worth noting that in alkenyl palladium intermediate 7 , the aromatic-ring moiety is coordinated with Pd, and the coordination bond lengths are 2.42 and 2.46 Å, respectively.