Viscosity-dependent diastereoselectivity and product selectivity in the photodenitrogenation of a spirocyclopropane-substituted, DBH-type azoalkane

Abstract

In the photodenitrogenation of the spirocyclopropane-substituted azoalkane 1 in alcoholic solvents of various viscosity (from 0.6 to 89.2 cP) to the diastereomeric housanes 2 (major product) and bicyclo[3.2.0]heptenes 3 (minor product), the same (within experimental error) viscosity dependence is observed in the diastereoselectivity of the housane 2 formation as well as in the product selectivity (2/3). These viscosity effects corroborate the intermediacy of the diazenyl diradical ¹DZ as a common branching point for the housane 2 and bicycloheptene 3 formation. In contrast, the diastereoselectivity of the bicycloheptene 3 generation is independent of viscosity, which implies that a nitrogen-free, symmetrical 1,4 diradical serves as precursor to the rearrangement products. The free-volume model of viscosity is employed to rationalize the product selectivity as well as the diastereoselectivity.