A theoretical study on the photochemical reactions of 2,3-diazabicyclo[2.1.1]hex-2-ene

The photochemical reactions of 2,3-diazabicyclo 2.1.1]-hex-2-ene (the 1,3-sigmatropic shift of a bridged carbon atom to give a nitrogen-retained product and the N₂ elimination) are investigated by ab initio MO Cl calculation. The present calculation suggests that a stable intermediate exists at the lowest triplet state in the course of the 1,3-sigmatropic shift. The methylene group of this intermediate can rotate almost freely, which results in the stereochemical randomization of the bridge carbon atom of a nitrogen-retained product. On the other hand, one of the CN of a reactant is broken at the ¹B₂ state with a bent in-plane mode. The other CN bond seission proceeds through the intersystem crossing from the ¹A″ state (which comes from the ¹B₂ state of a reactant) to the ³A′ state (which comes from the ³B₁ state of a reactant) because the ³A′ state has CN bonds with σσ* character. Once the N₂ is eliminated, a product (bicyclo1.1.0]butane of 1.3-butadiene) is formed easily via 1,3-cyclobutanediyl.