We further calculated the LOL function of four typical structures in the IRC path. The LOL results indicate that in LOL­CR1, nearly no electron is localized between C1­C3 or C2­N4 while in LOL­TS1, the electron localization between C1­C3 bond increased significantly, which indicate the formation of the C1­C3 bond. However, the electron localization between C2­N4 remains to be small in TS1. In the shoulder region, it is obvious that the electron localization between C1 and C3 atom is greatly increased, which indicate the formation of the C1­C3 bond. However, the electron localization between C2­N4 remains to be small. In the product region of M1, the electron localization between C1­C3 and C2­N4 both increased significantly, implying the formation of both C­C and C­N bond. All these results imply a concerted asynchronous [3+2] cycloaddition mechanism. Our proposal is seconded by the following Mayer bond order analysis along the IRC path as shown in the right panel of Figure 7.
Along the IRC path, the bond order of the C1­C3 increases from 0 to about 0.9 first and this process is accompanied by the decrease bond order of C2­C3 from ca. 1.8 to 1.1. The bond order of C2­N4, however, changes little during this process, ca. from 0.0 to 0.1 in the case of CR1. After the formation of the C1­C3 bond, the bond order of C2­N4 increases gradually to 0.8. Therefore, we can safely concluded that the Cu(OTf)2 catalyzed [3+2]
Figure 7. The energies along the IRC path that connecting CR1 and M1 (left panel) and the Mayer bond order analysis of C1­C3, C2­N4 and C2­C3 bonds along the IRC path. Also shown is LOL analysis of four typical structures along the IRC path.
Cycloaddition of trifluoromethylated N­acylhy-drazones and isoprene follows a concerted asynchronous mechanism. Using the similar analysis procedure, we found that path1, path2, path5, path6 follow a concerted asynchronous mechanism while other paths follow a concerted synchronous mechanism (see Figure S1­S7).