A THEORETICAL INVESTIGATION OF THE PYRAZOLE PHOTOTRANSPOSITION

The mechanism of the phototransposition of pyrazole to imidazole was investigated using the semiempirical MO method MNDO. Potential energy surfaces were calculated for S0, S1, and T1. The heights of calculated energy barriers and the experimentally observed photoproducts suggest that the disrotatory electrocyclic ring closure forming a 1,5-diazabicyclo[2.1.0]pentene intermediate and the first [1,3]-sigmatropic shift of nitrogen occur while on an excited state surface. This is followed by internal conversion to the ground state forming a 2,5-diazabicyclo[2.1.0]pentene intermediate. At this point, the intermediate either rearomatizes to an imidazole product or undergoes a second [1,3]-shift forming another 2,5-diazabicyclo[2.1.0]pentene intermediate, which rearomatizes to a different imidazole product. MNDO calculations qualitatively support the experimentally observed product ratios of the two imidazoles that are formed for variously substituted pyrazoles.