Solvent effects in the photodenitrogenation of the azoalkane diazabicyclo[2.2.1]hept-2-ene: viscosity- and polarity-controlled stereoselectivity in housane formation from the diazenyl diradical

Abstract

The stereoselectivity in the liquid-phase photolysis of the diazabicyclo[2.2.1]hept-2-ene (DBH), as expressed in terms of the k_inv/k_ret ratio for the formation of the inverted [2(inv)] versus retained [2(ret)] housanes, depends on the viscosity and bulk polarity of the medium, while hydrogen bonding has little effect on the stereodifferentiation. The similar viscosity profiles of the k_inv/k_ret ratio in alcohols and in isooctane/nujol mixtures manifest the same free-volume requirements and thereby reflect similar frictional impositions on the stereoselective inversion for the DBH photolysis in these solvents. The Onsager bulk-polarity parameter (ε − 1)/(2ε + 1) accounts adequately for the observed solvent influence. This fact implies that stabilization of the dipole moment in the diazenyl diradical ¹DZ retards the inversion process during the denitrogenation.