Revisiting the Photo-Induced Paternò-Büchi Reaction Mechanism by MS-CASPT2 Method

The Paternò-Büchi (P-B) reaction can generate oxetane, which is of great value in organic synthesis and medicinal chemistry, but its mechanism remains controversial. In this work, we studied the mechanism of the P-B reaction between acetone and butene to form oxetane by using the method of multistate complete active space perturbation theory (MS-CASPT2). The calculation results establish that this reaction can occur on both singlet potential energy surfaces involving the ${\rm S_0}$ and ${\rm S_1}$ $({^1}{\rm nπ}^*)$ states, as well as on triplet potential energy surfaces containing the ${^3}{\rm nπ}^*$ and ${^3}{\rm ππ}^*$ states. The reaction pathways on both singlet and triplet states are energetically allowed. For the C-O attack pathway, there exist significant differences in structure and energy between the conical intersection point $({\rm S}_1/{\rm S}_0)_x$-1 on the singlet reaction path and the intermediate $^3{\rm I}_{CO}$ on the triplet reaction path. For the C-C attack pathway, the conical intersection point $({\rm S}_1/{\rm S}_0)_x$-2 and the triplet intermediate $^3{\rm I}_{CC}$ almost coincide, which means that the singlet and the triplet reaction paths go through a region where the energies of the four states (${\rm S}_1$, ${\rm T}_2$, ${\rm T}_1$, and ${\rm S}_0$) are approximately degenerate. Our results have provided new insights into the mechanism of the P-B reaction.