Based
on these results, we can conclude that the Cu(OTf)2molecule plays an important role in the [3+2] reaction processes.
Firstly, the Cu(OTf)2 can form complexes with B via
hydrogen bonding and coordination bonding interactions, which can
stabilize the B molecule and benefit the following [3+2] reactions;
Secondly, the Cu(OTf)2 catalyst can decrease the energy
barrier of the [3+2] reaction, 8.0 kcal/mol vs 3.2 kcal/mol, and
facilitate the cycloaddition reactions; Finally, the [3+2]
cycloaddition mechanism is modulated from the concerted synchronous to
the concerted asynchronous mechanism with the participation of
Cu(OTf)2 molecule.
Electrostatic Potential
Surfaces
From above calculations, we can conclude that the [3+2]
cycloaddition follow a concerted mechanism. Specifically, the
Cu(OTf)2 catalyzed reaction follows a concerted
asynchronous mechanism. However, in such reactions, the CC bonds always
forms in the first place and follows by the formation of the CN bond
instead of the other way around. Even in the cases without
Cu(OTf)2, the CC bonds of the transition state
structures are much shorter than the CN bonds. How could we explain
this phenomenon? We calculate the electrostatic potential surfaces of
complex C and the isoprene as shown in Figure 11. The C2 and C3 atoms of
isoprene are in a more negative region of isoprene while in C complex,
the C1 atom is in a positive region and N4 atom is located in a negative
region. Therefore, the C3 atom always attacks the C1 atom first and then
follows by the formation of the CN bond. The electrostatic interactions
might be a driven force for the [3+2] cycloaddition.