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:I molar ratio by hydrogen bonding crystallizes spontaneously from an acetonitrile solution. The space group is P2(1)2(1)2(1), 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 Angstrom 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 Angstrom, 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.