All minimum­energy structures, transition state structures optimizations and internal reaction coordinates (IRC) calculations are conducted using the unrestricted DFT method with the hybrid exchange correlation functional B3LYP.47–52 However, it is known that the B3LYP method cannot properly describe the van der Waals interactions. Therefore, the van der Waals interaction is described by the empirical dispersion correction method (D3) proposed by Grimme et al.53 In all DFT calculations, the light atoms including C, H, O, N, S, F atoms are described using the 6­31G* basis set while the inner core electrons of Cu atoms are treated using the pseudopotential and the outer valence electrons are described with the LANL2DZ basis set. 54–56 All the DFT calculations are conducted using the GAUSSIAN09 software.57 To further investigate the details of the cycloaddition mechanism, we have also conducted wavefunction analysis such as the localized orbital locator (LOL), the Mayer bond order analysis and electrostatic potential surfaces (ESP).58,59 All these wavefunction analysis are conducted using the Multiwfn 3.6 software developed by Tian Lu.60

According to previous studies, the initial reactant trifluoromethylated N­acylhydrazones (A) can isomerize to B in the first step, from which the B isomer can interact with the Cu(OTf)₂ molecule and form the combined complex C via the coordination bond between Cu and the carbonyl group and the H­bonding between O atom and the N­H group. These primary steps are shown in Figure 2. The Cu­O bond is about 1.94 Å and the hydrogen bond O···H is about 1.80 Å in C complex. We need to emphasize that due to the planar structure of the B isomer, the C complex have one pair of enantiomer as shown in Figure 2.

Figure 2. The formation of Cu(OTf)₂ combined trifluoromethylated N­acylhydrazones complex (C) for the following [3+2] cycloaddition. Also shown are relevant bond lengths. It is worth to notify that one pair of enantiomers can be formed in this process.

Such enantiomers are expected to have similar reaction processes but resulted in different enantiomers, therefore, we consider only one of the enantiomer in our subsequent simulations (the left one). To evaluate the bond orders changes during this process, we have conducted Mayer bond order analysis of A, B and C and the corresponding bond orders of C­N, N­N, N­C, C­O in the red dotted bracket from left to right are listed in Table 1.