A study on the orbital interactions of [3+2] cycloaddition by means of molecular orbital tracing method

Both synthesis and DFT calculation of [3 + 2] cycloaddition reaction (32CA) of quinolone-5,8‑dione with 1,3-dipole CF3CHN2 are studied. The focus of this work is to study the orbital interactions of this reaction by using molecular orbital tracing (MOT) method. MOT result shows that both quinolone-5,8‑dione and CF3CHN2 use three electrons occupied molecular orbitals (MOs) to interact with each other, and the evolutive courses of the six molecular orbitals of reactants (initial MOs) to six electrons occupied molecular orbitals of product (final MOs) are given in this work. Three of the six initial MOs can evolve as final MOs directly, but rest of them need to go through media orbitals to complete the evolution. Perhaps the role of the media orbitals is to enable the reaction to proceed concertedly. APT charges analysis and changes of bond lengths during the reaction are also afforded in this work, and the result shows that the reaction begins with a nucleophilic attack of CF3CHN2 on quinolone-5,8‑dione. Accompanying with the attack, negative charge transfers from CF3CHN2 to a carbon atom of quinolone-5,8‑dione, then the carbon atom attacks nitrogen atom of CF3CHN2 to complete the cycloaddition. At last, a mechanism and a diagram of the molecular orbital interactions of the reaction are given.