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

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 (epsilon-1)/ (2epsilon + 1) accounts adequately for the observed solvent influence. This fact implies that stabilization of the dipole moment in the diazenyl diradical (1)DZ retards the inversion process during the denitrogenation.