A study of the photorearrangements of crystalline dibenzobarrelene and cyclohexenone derivatives
Creators: name: Chen, Jianxin
Publisher: name: University of British Columbia
Year: 2011

description: Two series of compounds, 4,4-diarylcyclohexenones and 9-substituted dibenzobarrelenes, were synthesized and their photochemistry was investigated in the crystalline state and in solution medium. The differences in photoreactivity and product selectivity of these compounds between the solution and solid states have been determined, and possible structure-reactivity correlations are discussed based on X-ray crystallographic data for some of the substances. The photorearrangement of 4,4-diarylcyclohexenones was found to be affected moderately by the crystal lattice. The migratory aptitude of the aryl groups for 4-phenyl-4-arylcyclohexenones was shown to be controlled to a lesser extent by the electronic effect in the solid state compared to solution. Irradiation of 4,4-diphenyl-6-methyl-cyclohexenone in the crystalline phase gave the same photoproducts and a similar product distribution as that in the solution phase. In this case, conformational analysis reveals that both pseudo-axial and pseudo-equatorial phenyl groups of the enone moiety are capable of migrating. The di-π-methane rearrangement of a number of 9-substituted dibenzobarrelene derivatives gave two regioisomeric dibenzo-semibullvalene products upon photolysis in solution and in the solid state. In general, the effect of the crystalline environment on the reaction regioselectivity is found to be small. Resolved chiral substituents (handles) at the 9-position of the dibenzobarrelene moiety were used to force the compounds to crystallize in chiral space groups. Asymmetric inductions via photolysis of the chiral crystals varied from small to moderate depending on the nature of the chiral substituent. It was found that, in some cases, asymmetric induction in the solid state is significantly higher than in solution, but is lower in one instance. In addition, the chiral crystalline environment has different effects on the asymmetric inductions in a dual pathway di-π-methane rearrangement. The di-π-methane photorearrangement of 9,12-bridged dibenzobarrelene lactones was found to proceed very efficiently in solution and in the solid state. The unusual regioselectivity observed was rationalized in terms of an intramolecular steric effect rather than an electronic stabilizing effect. An absolute asymmetric synthesis with high optical yield was achieved by photolyses of chiral crystals of the achiral compound, methyl 3,5-dihydro-3-oxo-1H-5,9b[1',2']benzeno-naphtho[1′,2′-c]furan-4-carboxylate. Photochemical [2+2] cycloaddition of this substrate to 1,3-dienes was found to be mechanistically interesting, involving an excited triplet state of 1,3-diene which is formed by energy transfer. An unusual excited singlet state rearrangement of 9-chloromethyl substituted dibenzobarrelene diester was discovered. X-ray crystallographic analyses revealed that an intermolecular chlorine atom transfer is involved in the formation of unusual dibenzopentalene photoproducts. Finally, a novel solid state photochromism was found, and a radical species was proposed as the intermediate responsible for the observed photochromic phenomenon.