Figure 4 The potential energy surface for Pd(II)-catalyzed C(sp2)-H bond cleavage of 1,4-chlorobenzene and alkyne insertion with L2 as the ligand. The free energies are given in kcal/mol.
As shown in Scheme 5, it is found that the bidentate N, N-coordinated species L22+Pd(OAc)2 (-20.9 kcal/mol) is the most stable than the other two complexes (L22+Pd(OAc)2 andL2-I ) , and it is used as the active catalyst (KK ) in this work. In four-coordinated square-planer species KK , the bidentate nitrogen-nitrogen ligand coordinates with Pd and oxygen atoms of two acetate anion form two covalent bonds with Pd, respectively, which is consistent with the structure reported by the literature19. In the following step, the Pd inKK cleaves a Pd-N coordination bond followed by undergoing CMD process together with the substrate 1a . The CMD process is 21.9 kcal/mol endothermic via TSKK-1K by overcoming a free energy barrier of 32.2 kcal/mol, indicating that CMD process is both thermodynamically and kinetically unfavorable and difficult to proceed. This fact explains why there is no reaction under experimental conditions with L2 as the ligand, as the documents recorded that 1,10-phenanthroline(1,10-phen) are seldom used for the palladium-catalysed C−H activation reactions.13b-cNote that the calculated barrier of the CMD process forKK1K with L2 as the ligand is 32.2 kcal mol-1, much higher than that for 34(24.8 kcal mol-1) with L1 as the ligand in Figure 2. The instability of TS1K-KK (11.3 kcal/mol) compared with TS3-4 (-5.4 kcal/mol) can be attributed to greater steric repulsion, instead of low coordinate ability without strong chelating groups as proposed by Duan et al13c. This is indicated by the O1…H, N…C1, and N…C2 interaction at 2.14Å, 2.93Å, and 3.10Å in TSKK , respectively, which are less than the sums of their van der Waals radii (N,1.55Å; C, 1.70Å; O, 1.52Å; H, 1.20Å). In the following step, intermediates 1K successively undergo HOAc dissociation and ligand exchange to form the 16e 3K , as shown in Figure 4a, the coordination of 2a1 molecule to Pd generates intermediate 4K ready for subsequent alkyne insertion. The calculated barrier for alkyne insertion via TS4K-5K is 28.9 kcal/mol (3K→TS4K-5K ), higher than the overall barrier of the process via TS3-4 (24.8 kcal/mol) in Figure 2. Clearly, the energy barrier is still higher and the reaction is difficult to occur.
Although the bidentate ligand L1 (2-OH-1,10-phenanthroline) developed by Duan’s group differs from L2 by one OH group, lack of OH group in L2 ligand results in the greater steric repulsion in CMD process. As a result, the reactivity is deceased to no product obtained.