J. Am. Chem. zyxwvut SOC. zyxwvuts 1989, 111, zyxwvuts 1029-1034 1029 Absolute Asymmetric Photochemistry Using Centrosymmetric Single Crystals. The Host/Guest System (E)-Cinnamamide/(E)-Cinnamic Acid? M. Vaida, L. J. W. Shimon, J. van Mil, K. Ernst-Cabrera, L. Addadi,* L. Leiserowitz,* and M. Lahav* Contribution from the Department of Structural Chemistry, The Weizmann Institute of Science, 76100 Rehouot, Israel. Received May 26, 1988 Abstract: 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 P2,/c) in the presence of (E)-cinnamic acid results in a mixed crystal composed of two enantiomorphous halves each containing 0.5-1 .O% of the acid. The replacement of N-H-0 hydrogen bonds between host molecules by 0 - a 0 lone-pair repulsion between 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 la or lb in excess, with an enantiomeric yield in the range of 40-60%, varying from one single crystal to another. The absolute configuration of the chiral dimer la formed in excess at the zyxwvutsrqp +b pole (lR,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. Asymmetric transformations of nonchiral molecules packing in chiral space groups are well documented. These include both heterogeneous gas/solid' and topochemica124 reactions. An im- portant extension of these asymmetric transformation to centro- symmetric crystals has been p r o p o ~ e d ~ ~ * ~ ~ and recently successfully accomplished for heterogeneous reactions by Richardson et aL5 and for the resolution of racemates inside centrosymmetric crystals developing enantiotopic faces.6 zyxwvutsr In these latter works we dem- onstrated, that when solid solutions are formed within single crystals of a centrosymmetric host, the symmetry of the mixed crystal can be reduced from centrosymmetric to chiral, thus providing a new means to design chiral crystals for the perform- ance of spontaneous asymmetric transformations. This approach is illustrated here for the asymmetric photodimerization of guest (E)-cinnamic acid with host (E)-cinnamamide. Mixed Crystal (E)-Cinnamamide/(E)-Cinnamic Acid. We selected (E)-cinnamamide as a host system.' The compound crystallizes in the centrosymmetric space group P2,/c (a zyxwvuts = 9.56, b = 5.14, c = 16.01 A; = 94.1') and is known to undergo a topochemical photodimerization8 to yield the centrosymmetric dimer a-truxillamide zyxwvutsrqp (2). H5C6 H5C6 IC c - O J la L Pure (E)-cinnamamide forms prismatic crystals elongated in b (Figure la). Within the lattice the molecules are arranged in hydrogen-bonded cyclic dimers across centers of inversion. The two molecules of such a hydrogen-bonded cyclic dimer are en- antioconformers and reside at sites arbitrarily labeled d and I (Figure 2a). The dimers are interlinked by N-H.-0 hydrogen bonds along the b axis to form ribbons (Figure 2b,c). The nearest-neighbor ribbons are related to each other in the c direction by 2-fold screw symmetry and in the a direction by translation 'Dedicated to Prof. D. Y. Curtin on the occasion of his retirement from teaching. to form close-packed centrosymmetric pairs (Figure 2a,b). Photochemical dimerization takes place across these pairs, between molecules of a d and an 1 stack (Figure 2a,c). Figure 2b shows the relative orientations of the four symmetry-related molecules with respect to the unique b axis. We note that the glide and 1 symmetry elements reverse both the chirality and polarity of the molecules vis-&-visthe b axis, whereas the 2-fold screw leaves them unchanged. The four symmetry-related molecules sit at different surface sites at the four chiral {Olll faces (Figure 2b). By virtue of the 2/m point group, the (01 1) and (011) faces at the +b end of the crystal are homotopic and enantiotopic to the (01 1) and (071) pair at the -b end of the crystal. At the (01 1) and (011) faces, the N-H (anti) bonds of the zyxw d molecules emerge from the crystal surface, whereas the N-H (anti) bonds of the I molecules point into the crystal (Figure 2b). The roles of the d and 1 molecules are reversed at the opposite (Oil) and (011) faces. A cinnamic acid guest molecule may replace cinnamamide at d sites on the (01 1) and (017) faces. At such sites the N-He-0 amide bond between host molecules can be replaced by an N-He-0 bond between host and guest, even if the carbonyl oxygen atom is a somewhat weaker proton acceptor than the corresponding amide oxygen atom? (E)-Cinnamic acid is expected to less easily occupy (1) Penzien, K.; Schmidt, G. M. J. Angew. Chem., zyx Znf. Ed. Engl. 1969, 8, 608. (2) (a) Elgavi, A,; Green, B. S.; Schmidt, G. M. J. J. Am. Chem. SOC. 1973, 95, 2058. (b) Addadi, L.; van Mil, J.; Lahav, M. J. Am. Chem. Soc. 1982, 104, 3422. (3) Evans, S. V.; Garcia-Garibay, M.; Omkaram, N.; Scheffer, J. R.; Trotter, J.; Wireko, F. J. Am. Chem. SOC. 1987, 109, 5648. (4) For some leading reviews see: (a) Green, B. S.; Lahav, M.; Rabinovich, D. Acc. Chem. Res. 1979, 12, 191. (b) Green, B. S.; Arad-Yellin, R.; Cohen, M. D. Top. Stereochem. 1986, 16. 131. (c) Ramamurthy, V. Tetrahedron 1986, 42, 5753. (d) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987,87, 433. (e) Addadi, L.; van Mil, J.; Weissbuch, I.; Berkovitch-Yellin, Z.; Leiserowitz, L.; Lahav, M. Chem. Scr. 1985, 25, 91. (5) (a) Holland, H. L.; Richardson, M. F. Mol. Cryst. Liq. Cryst. 1980, 58, 31 1. (b) Chinna Chenchallah, P.; Holland, H. L.; Richardson, M. F. J. Chem. SOC., Chem. Commun. 1982, 436. (c) Chinna Chenchallah, P.; Hol- land, H. L.; Munoz, B.; Richardson, M. F. J. Chem. SOC., Perkin Trans. 2 1986, 1775. (6) (a) Weissbuch, I.; Addadi, L.; Berkovitch-Yellin, Z.; Gati, E.; Wein- stein, S.; Lahav, M.; Leiserowitz, L. J. Am. Chem. SOC. 1983, 105, 6615. (b) Weissbuch, I.; Addadi, L.; Leiserowitz, L.; Lahav, M. J. Am. Chem. SOC. 1988, 110, 561. (7) Berkovitch-Yellin, Z.; van Mil, J.; Addadi, L.; Idelson, M.; Lahav, M.; Leiserowitz, L. J. Am. Chem. SOC. 1985, 107, 3111. (8) (a) Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. SOC. 1964, 384, 2000. (b) Schmidt, G. M. J. J. Chem. SOC. 1964, 385, 2014. 0002-7863/89/ 15 11-1029$01.50/0 0 1989 American Chemical Society