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.