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.