Generation of Chirality in a Two-Component Molecular Crystal of Acridine and Diphenylacetic Acid and Its Absolute Asymmetric Photodecarboxylating Condensation

Despite the fact that acridine (1) and diphenylacetic acid (a) are achiral compounds, a chiral two-component molecular crystal (1·a) in which two molecules are self-assembled in a 1:1 molar ratio by hydrogen bonding crystallizes spontaneously from an acetonitrile solution. The space group is P212121, which is typical chiral space group. Both handed crystals (−)-1·a and (+)-1·a can be prepared as desired on a large scale by seeding. The two phenyl planes and the carboxyl plane of the diphenylacetic acid molecule in the crystal (−)-1·a have torsions in the same direction as the blades of a propeller. The oppositely handed crystal (+)-1·a has minus torsion angles. Irradiation of (−)-1·a or (+)-1·a caused stereospecific decarboxylating condensation to give an excess of the chiral compound {(−)-3} or {(+)-3} in about 35% ee, respectively, as the main product. The absolute configurations of the reactant (−)-1·a and the product (−)-3 could be determined to be (M)-(−)-1·a and (S)-(−)-3 by the Bijvoet method based on anomalous dispersion of an oxygen atom of (−)-1·a and a sulfur atom of the trifluoromethanesulfonate salt of methylated (−)-3 during X-ray crystallographic analysis. Upon irradiating (M)-(−)-1·a, the diphenylmethyl radical and the hydroacridine radical are produced via electron transfer from a to 1 and subsequent proton transfer followed by decarboxylation. The next radical coupling occurs with the shortest distance of 5.1 Å between the two preradical carbon atoms in the crystal lattice to afford (S)-(−)-3 as the major enantiomer. On the other hand (R)-(+)-3 can be produced as the minor enantiomer by coupling over a longer distance of 6.8 Å, resulting in a ca. 2:1 of S:R ratio, i.e., about 35% ee. The radical coupling is necessarily accompanied by a slight movement of the radical species in the crystal lattice, in contrast to the well-known topochemical [2+2] photocycloaddition.