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).