A theoretical insight into the reaction mechanism of photochemical transposition from pyrazole to imidazole

The mechanisms of the photochemical isomerization reactions were investigated by using a model system of 1,3,5-trimethylpyrazole ( 1) with the CASSCF (eight-electron/six-orbital active space) and MP2-CAS methods and the 6-311G(d) basis set. Three reaction pathways were examined in the present work. They are denoted as the ring-contraction-ring-expansion path (path I), the internal-cyclization-isomerization path (path II), and the conical-intersection path (path III). Our model investigations suggest that the preferred reaction route for the pyrazoles is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the conical-intersection mechanism (path III) found in this work gives a better explanation than the previously proposed two other mechanisms (paths I and II). The theoretical findings also indicate that path III-1 should be favored over path III-2 from a kinetic point of view. This suggests that the quantum yield of 1,2,4-trimethylimidazole ( 2) should be greater than that of 1,2,5-trimethylimidazole ( 3), which supports the available experimental observations. Additionally, we propose a simple p-pi orbital topology model, which can be used as a diagnostic tool to predict the location of the conical intersections, as well as the geometries of the phototransposition products of various heterocycles.