In order to evaluate the mechanism of Cu(OTf)2 catalyzed [3+2] cycloaddition of trifluoromethylated N­acylhydrazones and isoprene, we firstly optimized all possible initial structures for the cycloaddition. About eight possible initial structures can be found according to the different positions of isoprene relative to the complex formed by trifluoromethylated N­acylhydrazones and Cu(OTf)2, which can further be classified into two classes, namely the TOP structures (CR1 to CR4) and the BOTTOM structures (CR5 to CR8) in Figure 3.
Figure 3. The UB3LYP+D3 optimized eight initial structures for the Cu(OTf)2 catalyzed [3+2] cycloaddition. Also shown are relevant bond lengths (in angstrom).
For the TOP structures of CR1­CR4, the CR1 and CR2 differ from CR3 and CR4 mainly in the direction of the methylene group in isoprene, which results in the different cycloaddition patterns of CR1/CR2 and CR3/CR4. In CR1 and CR2, the C3 atom will attack the C1 atom while the C2 atom will attack N4 atom to form the final [3+2] products. The C1­C3 bond length in CR1 is 3.00 Å and the C2­N4 bond length is 3.49 Å. In CR2, the corresponding C1­C3 bond length is elongated to 3.71 Å and the C2­N4 bond is shortened to 3.17 Å. In CR3 and CR4, however, things are different. The C2 atom will attack C1 atom to form the C1­C2 bond while the C3 atom will attack N4 atom to form the final C3­N4 bond. The C1­C2 bond length in CR3 and CR4 are 3.14 Å and 3.50 Å and the C3­N4 in CR3 and CR4 are 3.93 Å and 3.46 Å respectively. For the BOTTOM structures, the CR5 and CR6 have similar situations as CR1 and CR2 while the CR7 and CR8 similar to CR3 and CR4. The C1­C3 bond in CR5 and CR6 are 4.06 Å and 3.58 Å and the C2­N4 bond in CR5 and CR6 are 3.45 Å and 3.76 Å. In CR7 and CR8, the C1­C2 bonds are elongated to 3.90 Å and 5.02 Å in comparison with CR3 and CR4 while the C3­N4 bond are changed to 3.56 Å and 4.95 Å. Based on these structures, we can also expect that another enantiomer of C will result another eight initial structures corresponding to the mirror of CR1 to CR8.