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
Results and Discussions
Primary Steps before the
Cu(OTf)2 Catalyzed [3+2]
Cycloaddition
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.