Topochemical rules

The behaviour, which is not controlled by the topochemical rule but is greatly influenced by non-topochemical factors, is discussed in Section 2 in terms of molecular mobility, stabilization energy by orbital interaction and energy transfer in the 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. 

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. 

Figure 28. Topochemical rules are based on the photochemical behavior of cinnamic adds in three crystallographic modifications.

Despite the complicated excited state nature of these reactions, ground state topochemical rules for the solid state photoinduced [2+2] addition of various organic compounds were developed over 50 years ago by Cohen et al. [15,16]. The rules require that the distance, d, and the relative orientation, t], of the C=C double bonds be less than some cutoff (see Fig. 13.2). These values for different solid state structures are now relatively easy to determine using X-ray crystallography. Despite the general applicability of these topochemical rules, there are a few exceptions as noted by Ramamurthy et al. [16]. These include crystals in which [2+2] addition occurs with either the orientation or the distance between the C=C double bonds outside the accepted cutoffs. 

Molecules of amphiphilic nature which contain the diacetylene moiety and which give rise to stable solid-analogue films at the air-water interphase can generally be polymerized by UV-irradia-tion. The polymerization proceeds according to the topochemical rules. A solid monomolecular film of the corresponding polydiacetylene is thus formed (12). 

C leads to the anti-cisoid dimers (10,14,15) as major products in yields of 92,35, and 40%, respectively. Compounds 10 and 15 were hydrolyzed to 14, and final confirmation of the structure of 10 came from x-ray crystallographic analysis. The dipole moments of 10 and 11 were determined to be 5.90 and 3.65 D, respectively, in benzene at 25°C. On the other hand, irradiation of 13 in both the crystalline and the molten state gave the syn-cisoid-dimer (16) as the sole product in a yield of 65%. X-ray crystallographic analysis of the monomer 13 demonstrated stereo- and regioselective formation of 16 in the solid state irradiation of 13 according to the topochemical rules. 

Many derivatives of quinones, cinnamic acids, and mucconic acids photodimerize in solid phases to give results 16> that in many cases are not in agreement with the general PMO rule of head-to-head reaction. However, it is clear that those reactions are controlled by topochemical effects, i.e. the geometry and proximity of the reactants in the solid phase. 135> Consequently, PMO theory will not be useful for calculating reactions of that type. 

Most of the topochemical reactions, including the first finding of the topochemical polymerization of 1, were found accidentally or beyond any expectation [ 14]. During subsequent and extensive investigation, numerous studies have been carried out by trial-and-error approaches to establish any empirical rule accounting for a correlation between the geometry of functional moieties and reactions in the solid... 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

Lahav et al. applied the empirical rule of a topochemically photopolymeric structure (O to the unsymmetric diolefin crystals, thus succeeding in obtaining optically active dimers and oligomers through the crystallization of an achiral monomer into a chiral crystal, followed by the four-center type photopolymerization (12). 

X-ray structure analysis before and after irradiation revealed that the reaction proceeded via single crystal-to-single crystal transformation without decomposition of the initial crystal structure. A one-dimensional columnar structure is formed, in which two types of molecular overlaps (type 1 and type 2) are repeated alternately (Fig. 5) [116]. Two olefinic groups are aligned nearly in parallel in both types of overlap. The interatomic distances are 3.4-3.9 A, which are short enough to permit topochemical [2 + 2] cycloaddition in the solid state under the 4 A rule. Direct evidence that the cycloaddition proceeded predominantly via the type 2 overlap in 148 oDV could be obtained by comparison of the molecular arrangements. 

Intensive studies concerning topochemical reactions have been reported on the photodimerization of cinnamic acid and its derivatives. Schmidt and co-workers proposed a geometrical criterion for the photodimerization in the crystalline state that the reacting double bonds should be situated within about 4.2 A of each other and aligned parallel. In the case of the photoactive single crystal of 1-n-octylthymine obtained from acetonitrile solution (Form III), however, the distance between the reacting double bonds of the thymine bases was 4.47 A. It is difficult to apply Schmidt s rule to the photodimerization of the 1-n-octylthymine crystals. Therefore, it is necessary to determine the feature within the crystal of 1-n-octylthymine for the photodimerization reaction in the crystaUine state. 

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