Photodimerization mixed crystals

It may be suspected that the genuinely topotactic (as secured by the molecular precision of the AFM [18]) photodimerization of 2-benzyl-5-benzyli-denecyclopentanone [118] might be a good candidate for a quantitative preparative photo dimerization to give the head-to-tail anti-[2+2] dimer. Early quantitative solid-state [2-1-2] photodimerizations (most of the published mechanistic interpretations of which can no longer be accepted) are listed in [110]. These deal with the anti dimerization of acenaphthylene-1,2-dicarboxylic anhydride, the head-to-head syn dimerization of acenaphthylene-1-carboxylic acid, the syn dimerization of 5,6-dichloroacenaphthylene, and the thermally reversible head-to-tail anti dimerization of seven ( )-2,6-di-f-butyl-4-(2-aryl-ethenyl)pyrylium-trifluoromethanesulfonates. All of these reactions proceed fully specific. On the other hand, quantitative photoconversions of a 1 1 mixed crystal of ethyl and propyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamates gives mixtures of diesters with one (A>410 nm) or two cyclobutane rings (no cutoff filter). 

Scheme 3 Asymmetric [2-1-2] photodimerization of 1,3-butadiene derivatives in the mixed crystals [42]...

Similarly, the symmetry of the mixed crystal system composed of (E)-cin-namamide (host) and (E)-2-thienylacrylamide (guest, 8% occluded) is lower than that of the host crystal (P2j/c). The selective occlusion of the guest arises from repulsive sulfur- n interactions. The -l-b end and the -b end of the mixed crystal are enantiomorphic (space group PI) and underwent topochemical [2-1-2] photodimerization in 40-69% enantiomeric excess (Scheme 8) [29]. 

The molecular interpretation is again based on the crystal structure of the starting material (Figure 22). The molecules 13 are almost perpendicular on (100) (front), perpendicular on (010) (top), and flat on (001) (left). There is enough room on both sides to partially rotate for photodimerization in the orientations found and to move upward [010], This does form volcanoes with pillars and leaves craters (Figure 216). Those features must consist of mixed crystals out of 13 with 14, 15, and 16 admixed. From there mixed phases rich in products will be formed upon continuation of the irradiation forming the features in Figure 21c. However, the details of the secondary processes cannot yet be assessed. Local spectroscopy measurements (Sections IX and X) will have to look at the distribution of chemical compositions and nanometric Laue facilities (which are yet to be developed) at the local crystallinity. However, for the initial photochemical reaction it has... 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

---