Photoreactivity crystals

Ethyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OEt) also crystallizes with /3-centrosymmetric-type packing yielding photoreactive crystals and, upon photoirradiation (>410 nm), is converted into the /3-hetero-type dimer (7 OEt-dimer) nearly quantitatively. The 7 OEt-dimer (space group PT, triclinic) has the structure which is predicted from the crystal structure of 7 OEt (space group PT, triclinic). 

Photodimerization behavior of 4-formyl-, 3,4-dichloro-, and several other cinnamic acid derivatives is greatly influenced by other molecules outside of the crystal (9,10). For example, 4-formylcinnamic acid 1 crystallizes in two modifications, photoreactive and photostable forms. The photoreactive crystals of 1 (mp 249 °C), on photoirradiation at room temperature in the presence of even a trace of moisture, dimerize to crystalline dimer 2 containing one molecule of water. The continuous change of the x-ray diffraction pattern during the photodimerization indicates a typical crystal-to-crystal transformation process. On the other hand, the same crystal 1 photodimerizes into amorphous dimer 2 in the absence of water. The same cyclobutane derivative is produced in very high yield in both reactions. However, highly crystalline dimer 2 is obtained only by the photodimerization of 1 in the presence of water and is not regenerated by any attempted recrystallization procedures from various aqueous solutions of 2. 

These observations on the effects of the environment, especially the case with more than one crystallographically independent molecule, may serve as a useful link between the solution and solid-state photochemical behavior [91]. It would be interesting to encounter cases of photoreactive crystals containing more than two crystallographically independent molecules, which would be somewhat akin to, but by no means the same as the solution-phase reaction. 

Figure 10 reveals that molecular shape and packing of photoreactive crystals are very similar. In addition, the electron-rich nitrogen or carbonyl group approaches the electron-deficient benzene ring in all the photopolymerizable crystals41,42, u-46,49,50. These two common features may govern the formation of photopolymerizable crystals and thus allow an empirical rule of similar photoreactivity between mono- and di-olefin crystals to be established (see Sect. II.a.). 

The empirical rule16 of the extention of dimerization to polymerization in the crystalline state is useful in the search for new photoreactive crystals in the future. However, a quantitative correlation between photoreactivity and molecular alignment in the crystal... 

Such kaleidoscopic topochemical behaviour of 2 OPr is exemplified in Scheme 6 (Hasegawa, 1992). 2 OPr crystallizes with a-type packing [2 OPr(a)] from 1-propanol solution but into the jS-type [2 OPr(j3)] from a mixture of ethanol and water. On photoirradiation using a filter (>410 nm), the a-type homo-adduct dimer (2 OPr-a-dimer) and the jS-type heteroadduct dimer (2 OPr- -dimer) are produced from 2 OPr(a) and 2 OPr(j3), respectively, both in nearly quantitative yield. The dimer crystal 2 OPr-a-dimer as-prepared is entirely stable on photoirradiation whereas, on further photoirradiation without a filter, 2 OPr-)8-dimer is converted nearly quantitatively into a highly strained [2.2] paracyclophane (2 OPr-cyclo). The photostable 2 OPr- i -dimer crystal is, however, transformed into a highly photoreactive crystal complex (2 OPr-a-dimer-PrOH) if 2 OPr-a-dimer is recrystallized from 1-propanol solution. The crystal (2 OPr-a -dimer-PrOH) photopolymerizes into a crystalline a-type homo-adduct linear polymer. Judging from their X-ray diffraction patterns, recrystallized l3-type dimer (2 OPr-)8-dimer) has a different crystal structure from that of the crystal... 

Dichloro-substituted, methylenedioxy-substituted, or sulfur-containing planar aromatic molecules often crystallize in 3 structures. These effects were utilized to design [2-1-2] photoreactive molecular complexes [30]. Cocrystallization from EtOH of 6-chloro-3,4-methylenedioxycinnamic acid with 2,4-dichlorocin-namic acid and with 3,4-dichlorocinnamic acid (all three cinnamic acids are 13-type [2-1-2] photoreactive crystals) gave the 2 1 and 1 1 complexes, respectively. Irradiation of these complexes yielded mixtures of three P-truxinic dimers in a statistical ratio (Scheme 9). This result, together with the crystal structures of the component acids, indicates that each sheet that is stabilized by an interstack... 

It is known that chlorine substitution is very effective in inducing photoreactivity in the soHd state. Recently, fluorine substitution, as well, has been proven to be useful for constructing photoreactive crystals. The effect of fluorine substitution on the crystal structure and the soHd-state photochemical behavior of benzylidenepiperitone has been examined. No F - F contacts are less than the sum of the van der Waals radii (2.94 A) however, C-H F interactions are observed in the crystal structure of 1. The fact that the unsubstituted parent compound does not photodimerize in the crystalline state whereas the fluorine-substituted 1 does illustrates the importance of fluorine substitution. 

Solid-state photoreactions of heterocycles in two-component crystals 98S1. 

Solid-state photoreactions in two-component crystals with formation of heterocycles 98S1. 

After the first report on the crystal structure correlation between 2,5-DSP and poly-2,5-DSP crystals, a different crystallographic result was reported on a poly-2,5-DSP crystal (Meyer et al., 1978). It was reconfirmed, however, that the first structural analysis was correct (Nakanishi et al., 1979a). 2,5-DSP, crystallized from benzene solution, is highly photoreactive (a-form), while the same compound, sublimed at a rather high temperature... 

Of further particular interest was that the crystallographic results on 2,5-DSP and poly-2,5-DSP had pointed out a very important future possibility that an absolute asymmetric synthesis could be achieved if any prochiral molecule, e.g. an unsymmetrical diolefin derivative, could be crystallized into a chiral crystal and if the reaction of the chiral crystal proceeded in the same manner as the 2,5-DSP crystal with retention of the crystal lattice (Wegner, 1972, 1973). Such types of absolute asymmetric synthesis with a high enantiomeric yield have now been performed by topochemical [2+2] photoreaction of unsymmetric diolefin crystals (Addadi etal., 1982 Hasegawa et al., 1990 Chung et al., 1991a,b). 

In this chapter the topochemical [2+2] photoreactions of diolefin crystals are reviewed from the viewpoints of organic photochemistry, analysis of reaction mechanism, and crystallography as well as in terms of synthetic polymer chemistry and polymer physics. 

Characteristic features of topochemical [2+2] photoreactions of diolefin crystals... 

Recently several examples of diolefin crystals in which the reaction behaviour deviates from the topochemical rule have been observed. For example, in the photoreaction of methyl a-cyano-4-[2-(4-pyridyl)-ethenyljcinnamate (2 OMe), the first reaction occurs exclusively at the pyridyl side although the distance between the ethylenic double bonds on the pyridyl side is exactly the same as that between the ethylenic double bonds on the ester side (4.049 A), as shown in Fig. 5 (Maekawa et al., 1991a). A few other unsymmetrical diolefin compounds display the same regioselective behaviour (Hatada, 1989). 

Methyl 4-[2-(ethylthiocarbonyl)ethenyl]cinnamate (3 SMe) crystallizes into a typical a-translation-type packing structure in which the distances between the ethylenic double bonds are 3.988 A and 4.067 A, respectively. However, the 3 SMe crystal is entirely photostable even though it should be photoreactive based on the topochemical rule (Sukegawa, 1991). Several examples of exceptionally photostable diolefin crystals have been found in compounds having a thioester moiety. Such anomalous behaviour of crystals such as 2 OMe and 3 SMe cannot be explained simply in terms of the topochemical rule since this rule involves only the positional relationship between the reactive olefin pair. 

In contrast to the a-type crystal, the photoreaction of j8-type diolefin crystals at a very low temperature sometimes does not occur at all, or sometimes proceeds but levels off at a low conversion, suggesting that the photoreaction of /3-type diolefin crystals requires an appropriate thermal motion of the reacting molecules. 

It should be emphasized that, in all the topochemical photoreactions without exception, an apparent reaction rate at the initial stage increases with increase in the irradiation temperature, as long as the temperature is sufficiently low to maintain the molecular orientation in the crystal. 

In order to rationalize such characteristic kinetic behaviour of the topochemical photoreaction, a reaction model has been proposed for constant photoirradiation conditions (Hasegawa and Shiba, 1982). In such conditions the reaction rate is assumed to be dependent solely on the thermal motion of the molecules and to be determined by the potential deviation of two olefin bonds from the optimal positions for the reaction. The distribution of the potential deviation of two olefin bonds from the most stable positions in the crystal at OK is assumed to follow a normal distribution. The reaction probability, which is assumed to be proportional to the rate constant, of a unidimensional model is illustrated as the area under the curve for temperature Tj between 8 and S -I- W in Fig. 7. 

Similar behaviour has been observed in the photoreaction of methyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OMe) crystals in which the yield of [2.2] paracyclophane reached 65% on irradiation at — 78°C (see Scheme 10 p. 153) (Hasegawa et al., 1989b). From the crystal structure analysis of the same type of [2.2] paracyclophane, which is topochemically derived from alkyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamate crystals, a highly strained molecular shape is confirmed in which two phenylene rings are severely bent (Maekawa et al., 1991b). 

At present, based on the topochemical rule, the configuration of photoproducts, as well as photoreactivity, can be precisely predicted from the crystal structure of the starting olefin compounds, with certain exceptions. 

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

During the photoreaction of ethyl methyl 1,4-phenylenediacrylate crystals (3 OEtMe), in which the distances of intermolecular double bonds are 3.891 A between two methyl cinnamate groups and 4.917 A between two... 

In the crystal of 7 OEt, two molecules form a molecular pair as is the case in the crystal of 7 OMe. Considering the intermolecular distances between the ethylenic double bonds (3.714 and 3.833 A within the pair, and 4.734 and 4.797 A between the pairs), each molecule can react only with its partner in the molecular pair and not with any molecule of another pair. Since paired molecules are related by centrosymmetry, two pairs of facing ethylenic double bonds should be equal in photoreactivity, affording two... 

Table 4 Examples of the crystallization of 5 OEt with or without seeding and their photoreaction into the chiral dimer."...

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