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

摘要

Despite the fact that acridine (1) and diphenylaeetic acid (a) are achiral compounds, a chiral two-component molecular crystal (1 centre dot 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 P2_12_12_1, which is typical chiral space group. Both handed crystals (-)-1 centre dot a and (+)-l centre dot a can be prepared as desired on a large scale by seeding The two phenyl planes and the carbpxyl plane of the diphenylaeetic acid molecule in the crystal (-)-l centre dot a have torsions in the same direction as the blades of a propeller. The oppositely handed crystal (4 )-l centre dot a has minus torsion angles. Irradiation of (-)-1 centre dota or (+)-l centre dot a caused sterepspecific 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 centre dot a andthe product (-)-3 could be determined to be (M)-(-)-l centre dot a and (5)-(-)-3 by the Bijvoet method based on anomalous dispersion of an oxygen atom of (-)-l centre dot a and a sulfur atom of the trifluoromethanesulfonate salt of methylated (-)-3 during X-ray crystallographic analysis. Upon irradiating (M)-(- )-l centre dot 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 51 A between the two preradical carbon atoms in the crystal lattice to afford (5)-(-)-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 A, 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] photocyeloaddition.