From
the previous eight initial structures, we can get eight corresponding
intermediate structures of the [3+2] cycloaddition that combine the
final CF3substituted pyrazolidine and the
Cu(OTf)2 molecule as shown in Figure 4.
Figure 4. The UB3LYP+D3 optimized eight intermediate structures of
[3+2] cycloaddition which include both the final product structures
and the Cu(OTf)2 molecule. Also shown are relevant bond
lengths (in angstrom).
As expected, the C1C3/C1C2 bond is shortened from ≤ 3.00 Å to ca. 1.55
Å, which is the typical bond length of CC single bond. At the same
time, the C2N4/C3N4 bond is shortened to ca. 1.50 Å and the C2C3 bond
is elongated to 1.55 Å. All these parameter changes indicate the
formation of the [3+2] cycloaddition products. Similarly, the
enantiomers of CR1CR8 will result eight mirror structures of M1 to M8
with exactly the same energies.
With the departure of Cu(OTf)2, we can finally obtain
the eight final products as depicted in Figure 5. The bond length
parameters are similar to above intermediate structures with the CC
bond is ca. 1.55 Å and the CN bond is 1.50 Å. A carefully analysis of
these structures indicate that P1 and P5, P2 and P6, P3 and P7, P4 and
P8 are actually four pairs of enantiomers, which have nearly the same
energy and structural parameters (see Table S2). For example, all
C1C3/C1C2, C2C3, C2N4/C3C4 bond lengths for each pair of
enantiomers are exactly the same. Apart from this, all other pairs of
products are diastereoisomers. In addition, the P1, P3, P5, P7 can be
classified into the syn structures mentioned in previous experimental
study while the remaining P2, P4, P6 and P8 can be regarded as the anti
structures. 33 However, the experimental study reveals
that the syn product is the major products when using the
Cu(OTf)2 as a catalyst under room temperature with a
ratio of 72:28. To clarify such diastereoselectivity, we need to further
investigate the detail of the reaction processes.
Figure 5. The UB3LYP+D3 optimized eight final products structures of
[3+2] cycloaddition. Also shown are the relevant bond lengths (in
angstrom).
Potential Energy Surfaces of
Cu(OTf)2 Catalyzed
Reactions
Basically, there are two possible mechanisms for the cycloaddition
reactions: the concerted mechanism and the stepwise mechanism. In the
present case, the concerted mechanism means the CC and CN bond form at
nearly the same time while the stepwise mechanism means the CC or CN
bond forms first while another bond forms from the resulted
intermediate. To elucidate the exact mechanism of the cycloaddition, we
have tried to locate all possible transition states and intermediates.
However, all our attempts that try to locate the intermediates in the
stepwise mechanism failed and only one transition state structure is
located for each initial structure. These results indicate that the
[3+2] reactions follow a concerted mechanism. All possible reaction
paths and relevant transition state structures are summarized in Figure
6.