Topochemical effects

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. 

To what aspect of the solid-state environment can the high ees be attributed In general, one can imagine two scenarios a topochemical effect, in which the course of the reaction is governed by steric effects between the reactant and its lattice neighbors, and a conformational effect, in which reactivity is controlled by the conformation of the individual reacting molecules in the crystal. Zimmerman and Flechtner showed that the photoisomerization of cyclopropyl ketone 5 most likely involves rupture of the C1-C2 (or C1-C3) bond followed by C2-C3... 

One reason that salts of general structure 7 were chosen for investigation is that molecular mechanics calculations showed the benzocyclohexadienone framework to be planar. This planarity translates into an equal opportunity of forming either enantiomer of photoproduct 8, or to put it another way, the photochemistry of these salts is conformationally unbiased with respect to enantioselec-tivity. As a result, any ee in the crystalline state would have to be due to an essentially pure topochemical effect. Only one of the salts investigated (7b) gave a respectable ee in the solid state, and unfortunately, it has not been possible to obtain an X-ray crystal structure of this material, thus precluding an analysis of the specific topochemical factors responsible for asymmetric induction in this case. 

FIGURE 10.4 Topochemical effects during kraft cooking of Douglas-fir. (Saka, S., Thomas, R.J., Gratzl, J.S., and Abson, D., Wood Sci Technol, 16, 139-153, 1982.)... 

The molecular weight of the condensed silane is also influenced by the same factors. Another factor that is important for the silane coupling effect is the molecular architecture of the siloxane networks. Caged structures and ladder structures are the extreme cases of the different siloxane structures (11,12). The solution pH, topochemical effects of the solid surface, the structure of the organofunctional group, and the concentration of the silane are all believed to influence siloxane structure, although no detailed study has yet been reported. 

Topochemical effect. This is a consequence of the organisation of surfactant molecules induced by the micellar state. Although micelles are relatively labile aggregates and less organised than vesicles, topochemical effects may nevertheless influence the polymerisation process. 

TOPOCHEMICAL EFFECTS IN CHEMICAL REACTIONS OF CRYSTALLINE ORGANIC COMPOUNDS... 

As has been previously indicated the solid state polymerization of these monomer multilayers does not fit neatly into any of the three categories of topochemical effects. In all of the multilayers, discussed above, polymerization occurs with solid solution formation(9,10, 11,12). At low conversions there exists solid solutions of polymer side chains in the monomer structure, while at higher conversions there is a phase change to a solid solution of monomer side chains in the order-disorder... 

Examples of different types of topochemical effects in solid state reactions have been discussed. In each case an attempt has been made to elucidate the molecular mechanism of the reaction. In is clear that topochemical effects can range from complete control of the reaction to more limited structural differences between the product of a solid state reaction and that which would be produced by reaction in a disordered phase. The relationship between the structure of the reactant and that of the product is of primary importance in determining the degree of topochemical control. 

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