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

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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. Irradiation of P212121 conglomerates leads to optically active dibenzosemibullvalene photoproduct in quantitative enantiomeric yield. This is the first example of an absolute asymmetric synthesis in a unimoelcular chemical reaction.