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

Abstract

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 $S_0$ and $S_1$ $({^1}nπ^*)$ states, as well as on triplet potential energy surfaces containing the ${^3}nπ^*$ and ${^3}ππ^*$ 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 $({S_1}/{S_0})_x$-1 on the singlet reaction path and the intermediate $^3I_{CO}$ on the triplet reaction path. For the C-C attack pathway, the conical intersection point $({S_1}/{S_0})_x$-2 and the triplet intermediate $^3I_{CC}$ almost coincide, which means that the singlet and the triplet reaction paths go through a region where the energies of the four states ($S_1$, $T_2$, $T_1$, and $S_0$) are approximately degenerate. Our results have provided new insights into the mechanism of the P-B reaction.