Absolute Asymmetric Photochemistry Using Centrosymmetric Single Crystals. The Host/Guest System (E)-Cinnamamide/(E)-Cinnamic Acid

Article abstract of DOI:10.1021/ja00185a036

Controlled reduction in crystal symmetry has been accomplished and exploited to perform an absolute asymmetric photoreaction.The principle is based on selective introduction of a guest molecule into a centrosymmetric host structure, thus reducing the symmetry of the mixed crystal.Crystallization of (E)-cinnamamide (space group P2₁/c) in the presence of (E)-cinnamic acid results in a mixed crystal composed of two enantiomorphous halves each containing 0.5-1.0percent of the acid.The replacement of N-H...O hydrogen bonds between host molecules by O...O lone-pair repulsionbetween host and guest at the chiral surfaces of the growing crystal is responsible for the stereoselective occlusion of the guest and the reduction in symmetry.Irradiation of each half separately yielded the optically active cyclobutane dimer 1a or 1b in excess, with an enantiomeric yield in the range of 40-60percent, varying from one single crystal to another.The absolute configuration of the chiral dimer 1a formed in excess at the +b pole (1R,2R,3R,4R) was independently assigned by the Bijvoet method, NMR, and crystal-etching experiments and is in agreement with our proposed mechanism of molecular recognition.