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A photochemical study of crystalline dibenzobarrelene diesters : the di-[pi]-methane photorearrangement in the solid state García Garibay, Miguel Angel


The di-π-methane photorearrangement has been investigated for the first time in the solid state. A series of symmetric (-CO₂R₁ = -CO₂R₂) and mixed (-CO₂R₁ ≠CO₂R₂) 11,12-dibenzobarrelene diesters, used as the model substrates, reacted in the solid state with efficiencies that are similar to those observed in solution. Photolysis of symmetric diesters in solution and in the solid state gave a single dibenzosemibullvalene photoproduct. Two diesters (R₁ = R₂ = Et and R₁ = R₂ = iPr) turned out to be dimorphic with one of their crystal modifications having the chiral space group P2₁2₁2₁. Photolysis of the chiral crystals resulted in absolute asymmetric syntheses with quantitative (iPr) and near quantitative (80%, Et) enantiomeric yields. The absolute configuration of the starting material and photoproduct was obtained in the case of the isopropyl compounds by X-ray anomalous dispersion analysis. The stereochemical correlation and the X-ray crystallographic results indicated an extremely high stereoselectivity where only one of the four degenerate solution pathways seems to be followed in the solid state. Mixed diesters gave upon photolysis two regioisomeric photoproducts that formed in nearly equal amounts in solution media. The solid state regioselectivity varied from moderate to high with no apparent correlation between the nature of the ester substituent and the photochemical results. X-ray structural information was used to analyze the effect of the ene-dioate conformation on the solid state regioselectivity. The radical delocalizing ability of the carbonyl groups, which is expected to depend on the degree of conjugation with the central vinyl bond, does not seem to play an important role in determining the solid state results. An excellent correlation was found between solid state steric factors and the photochemical results. In general, the reaction occurred at the least tightly packed vinyl carbon. Chiral dibenzobarrelene diesters containing sec-butyl groups were studied in optically active and racemic forms. Crystals of the sec-butyl/isopropyl compound were found to form a solid solution of the enantiomers that is isomorphous with the chiral crystals of the diisopropyl compound. A third example of absolute asymmetric synthesis in the solid state was discovered when photolysis of the racemate generated optically active products with very high enantiomeric yields. Photochemical and spectroscopic results from the methyl/sec-butyl compound indicated a second example of solid solution of the enantiomers, but this time with a racemic space group. Lack of diastereoslectivity in the solid state reaction of the latter compound was rationalized in terms of efficient steric control in the solid state reaction but inefficient steric control during the crystallization process. An unusual solid state luminescence of the 11,12-diesters was noted and analyzed in terms of the biradical intermediates postulated along the di-π-methane rearrangement pathway. Finally, the solution and solid state photochemical results from dibenzobarrelenes bearing an ester group at the bridgehead position are analyzed.

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