Crystallographic studies on the di-[pi]-methane rearrangement of dibenzobarrelenes
Creators: name: Wireko, Fred Christian
Publisher: name: University of British Columbia
Year: 2010

description: The molecular and crystal structures of dibenzobarrelene and a number of its diester derivatives, and three dibenzosemibullvalenes have been determined by the use of X-ray crystallography. The objectives of the study were to investigate whether or not the di-π-methane photorearrangement could be carried out in the solid state, and how such solid state results would differ from results obtained in solution. In addition, we were interested in investigating the extent to which intermolecular steric effects would modulate or change the course of the photorearrangement in the solid state as compared to its solution pathway with the view of developing a structure-reactivity correlation for the reaction in the solid state. All the dibenzobarrelenes underwent the di-π-methane photorearrangement in the solid state to give the corresponding dibenzosemibullvalene photoproduct(s). In the symmetrical 11,12-diester derivatives of dibenzobarrelene, only one di-π-methane photoproduct could be identified for each of the reactants. The ethyl/ethyl and isopropyl/iso-propyl diester derivatives displayed polymorphism. An absolute asymmetric synthesis was performed on one of the dimorphs of the iso-propyl/iso-propyl derivative which crystallized in a chiral space group, and a quantitative enantiomeric excess yield was obtained. The molecular structures of all the compounds studied showed different degrees of conjugation of the ester carbonyl groups to the central vinyl bond. The unsymmetrical 11,12-diester derivatives yielded regioisomeric dibenzosemibullvalene photoproducts. Generally, the reactions in the solid state were found to be more regioselective than the same reactions in solution. The observed differences of the relative quantities of regioisomeric photoproducts in the solid state are interpreted on the basis of intermolecular steric effects. In appropriate systems, intermolecular steric effects may be used to predict successfully not only the major regioisomeric product of a di-π-methane photorearrangement in the solid state, but also the major enantiomeric product. One regioisomeric photoproduct is obtained for each 9,11 and 10,11-diester derivative. The photoproducts obtained from these unsymmetrical 9,11 and 10,11-diester derivatives of dibenzobarrelene show that electronic effects may be the dominant factor which governs the photochemical reaction pathway of this class of compounds in the solid state. Overall, intermolecular steric hindrance and electronic factors affect the solid state photochemical pathway of each compound to different extents. There appears to be an interplay of electronic and steric factors in determining the reaction pathway which leads to the major product in the solid state. The dominance of one factor (steric versus electronic) over the other in the determination of the most favorable photochemical pathway is dependent upon the conformations of the ester groups and their relative extents of conjugation to the central vinyl bond, and the relative intermolecular steric environments of the ester groups or substituents involved in the first step (vinyl-benzo bridging) of the photochemical reaction.