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.