Manfred Diedering

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 kinv/kret 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 kinv/kret 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 (e − 1)/(2e + 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.

Pressure dependence of the stereoselectivity in the photodenitrogenation of diazabicyclo[2.2.1]hept-2-ene in supercritical fluids: evidence for the diazenyl diradical

Abstract The present study shows that the preference for the stereoselective inversion upon the photolytic nitrogen extrusion from the stereolabeled DBH derivative exo -d 2 -diazabicyclo[2.2.1]heptene ( 1 ) in supercritical media (sc-CO 2 , sc-C 2 H 6 ) depends on pressure. The increase of pressure up to 200 bar leads to ca. 2.3-fold decrease of the stereoselectivity ( k inv / k ret ratio) in sc-CO 2 and sc-C 2 H 6 . Analysis of the observed pressure dependence in terms of collisional (self-diffusion coefficient) and frictional (viscosity) effects discloses that frictional impositions by the fluid media account best for the experimental observations reported herein. This pressure-dependent viscosity effect is consistent with a stepwise denitrogenation mechanism for the photolysis of DBH in the supercritical phase, which proceeds through an unsymmetrical, nitrogen-containing transient, namely the singlet diazenyl diradical.

Structural dependence on the stereoselective formation of inverted housane in the viscosity-controlled photodenitrogenation of DBH-type azoalkanes

The influence of structural changes on the α parameter, which represents the effective volume involved in molecular reorganizations, has been determined through the viscosity dependence of the stereoselectivity in the photodenitrogenation of the DBH-type azoalkanes 1a–c. The structural variation reveals that the stereoselective formation of housane is not only subject to the frictional impositions exerted by the solvent molecules on the flap motion of the methylene bridge, but also on the displacement of the bridgehead substituents during the inversion process.

Enhanced formation of inverted housane through steric effects by rotationally unsymmetric bridgehead substituents in the ring closure of triplet cyclopentane-1,3-diyl diradicals, generated photolytically from 2,3-diazabicyclo[2.2.1]heptene(DBH)-type azoalkanes

The photolysis of the cyclopentene-annelated DBH-type azoalkanes 1c,d with the n-propyl and acetoxymethyl substituents at the bridgehead positions affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-2c,d, while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis) the inverted diastereomer syn-2c,d is favored. The latter novel stereochemical feature of the triplet pathway is rationalized in terms of the unsymmetric nature of the n-propyl and acetoxymethyl substituents in regard to rotation about the bridgehead position of the planar cyclopentane-1,3-diyl triplet diradicals 33DR(c,d). In their lowest-energy conformation (AM1 calculations), the rotationally unsymmetric substituents prefer to be located opposite to the annelated cyclopentene ring. The ratio of the diastereomeric syn and anti housanes is determined by the equilibrium distribution of the bridgehead-substituent conformers for the triplet diradicals on ring closure after ISC.