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.