3.1.1 Relative Stabilities of Various Pd (II) Initial Species
In the experiments reported by Duan et al., the reaction shown in Scheme 2 (entry 1) does not occur when no ligand is added, implying that ligandL1 is a key factor in the efficient conversion of the catalytic reaction. Thus we investigated the catalytic active species in this system. Firstly, we considered the reaction of the bidentate N, N ligandL1 with Pd(OAc)2 to give a compelxL1-I , the calculated results shown in Figure 1. As shown in Figure 1a, there are two similarly potential pathways for the formation of the complex L1-I . One is the monodentate-chelation route (black line), another is the bidentate-chelation channel (green line). It is found that the the formation of the complex L1-I both thermodynamically and kinetically favors the bidentate-chelation pathway with the exothermicity of -30.2 kcal/mol. Notably, the hydroxyl deprotonation of the two pathways(via TS1’-2’ andTS1-2 ) are barrierless processes. In addition, we examined various possible initial PdII species in the reaction system to establish the most stable palladium species. Apart from the catalyst Pd(OAc)2, there also exist some species such as the substrates1,4-dichlorobenzene (1a ) and 4-octane (2a1 ), the solvent 1,4-dioxane (S ) and ligand (L1 ). All four species can coordinate with Pd(OAc)2. All of the possible Pd(OAc)2adducts formed with the four species mentioned above were calculated and shown in Scheme 4. Evidently, the bidentate N, N-coordinated speciesL1-I is found to be the most stable and a square planer geometry. It can be obviously observed that the C1-N1-Pd-O1 lies on the same plane, as indicated by the D(C1N1PdO1)=179.1º(see Figure 1b). In the most stable complex L1-I , the resting acetate anion is a bidentate ligand, which is different from the most stable structure formed by the coordination of ligand (L2 ) with palladium acetate 19. Therefore, the bidentate N, N-coordinated species L1-I was chosen as the active catalyst K in this work.