SINDO1 STUDY OF THE PHOTOISOMERIZATION OF 2-CYANOPYRROLE TO 3-CYANOPYRROLE

The mechanism nf the photochemical isomerization of 2-cyanopyrrole to 3-cyanopyrrole was investigated with the semiempirical MO method SINDO1. Potential energy hypersurfaces of excited states and transition structures of the ground state were calculated with limited configuration interaction (Cl) for a qualitative explanation of the reaction pathway. The first reaction step is an internal cyclization which occurs via a minimum on the first excited singlet surface. From there a radiationless transition leads to the ground-state surface. On the ground-state surface either a back reaction to the reactant or a [1,3] sigmatropic shift of the NH group takes place. The latter forms 2-cyano-5-azabicyclo[2.1.0]pentene, which reacts by ring opening to 3-cyanopyrrole. This rearrangement mechanism is unfavorable for furan because the barrier for the 1,3-oxygen migration is much higher than for rearomatization. © 1990, American Chemical Society. All rights reserved.