Fred Wireko

Addition of bromine gas to crystalline dibenzobarrelene: An enantioselective carbocation rearrangement in the solid state

Abstract Treatment of chiral crystals of 2,3:5,6-dibenzobicyclo[2.2.2]octa-2,5,7-triene (dibenzobarrelene, 1) with bromine vapor results in the formation of the rearranged product syn -8-bromo- endo -4-bromo2,3:6,7-dibenzobicyclo[3.2.1]octa-2,6-diene (2) in up to 8% enantiomeric excess.

Generation of optical activity through solid state reaction of a racemic mixture that crystallizes in a chiral space group

Abstract Because of disorder in the sec-butyl group, the photoreactive isopropyl/(R,S)-sec-butyl diester 3 is able to crystallize as a racemic solid solution in the chiral space group P212121. Irradiation of single crystals of this material leads to products of high optical purity (80% ee). An accompanying process of kinetic resolution leaves the recovered starting material enriched in one of its enantiomers.

Triplet energy sensitization of a crystalline phase photorearrangement

Abstract Direct irradiation of crystals of dibenzobarrelene leads mainly to dibenzocyclooctatetraene, the singlet product, whereas photolysis of solid solutions of dibenzobarrelene with various crystalline triplet energy sensitizers leads exclusively to dibenzosemibullvalene, the triplet photoproduct. Graphical analysis of the extent of maximum conversion in the sensitized runs versus the sensitizer (xanthone) concentration indicates that one xanthone molecule can photosensitize the formation of up to 24 product molecules.

Intermolecular steric effects on unimolecular rearrangements in crystalline media

Abstract As a result of steric interactions between the reacting molecule and its nearest neighbors in the crystal lattice, the regioselectivity of the di-π-methane photorearrangement of dibenzobarrelene diesters is greatly altered in the solid state compared to isotropic liquid media.

Studies on the Di-π-Methane Photorearrangement in the Solid State

Abstract The solution and crystalline phase photochemistry of a number of dibenzobarrelene 11,12-diesters has been studied as a function of the nature of the ester alkyl substituents; results for both symmetrical (CO2R1 = CO2R2) and unsymmetrical (CO2R1 ≠ CO2R2) compounds are reported. All compounds studied underwent smooth di-π-methane photorearrangement in the solid state to give dibenzosemibullvalene diesters. In the case of reactants having non-equivalent ester groups, two possible regioisomeric dibenzosemibullvalene products are possible, and the photoproduct ratios as a function of the reaction medium (solution vs solid state) are reported. The striking differences observed are interpreted on the basis of the crystal and molecular structures of the reactants and products. One of the symmetrical diesters investigated (CO2R1 = CO2R2 = CO2iPr), which is achiral in solution, undergoes spontaneous resolution upon crystallization from the melt, affording material in the chiral space group P212121. Irradia...

Reaction pathway in the photoconversion of a tetrahydroanthracenedione (I) to its solid-state photoproduct (II)

Etude des structures cristallines du tetramethyl-2,3,4a,9a tetrahydro-4a,9,9a,10 anthracenedione-1,4 et de son photoproduit tetramethyl-1,3,4,6 benzo [8,9] tricyclo [4.4.0.0 3,10 ] decene-8 dione-2,5