Topochemical reaction control

Last but not least, rigid cydophanes can be used to assemble functional groups in defined orientation in space [15]. Such orientation determines in many cases the stereochemistry of their reactions, so that topochemical reaction control [16], previously restricted to the solid state, was expanded into solution. The use of functionalized cydophanes as stereochemical reaction control elements will be summarized in chapter 5 of this article. 

Keywords Crystal engineering Solid-state photoreaction Topochemical polymerization Controlled radical polymerization Dimerization Isomerization Topotactic reaction... 

In custom-designing materials with tailored properties, it is often necessary to s)m-thesize metastable phases that will be kinetically stable under the temperature and conditions of use. These phases are obtainable only through kinetic (chemical) control. In many cases, kinetic control has been achieved via the soft chemical low-temperature (e.g. electrochemical synthesis, sol-gel method) and/or topochemical routes (e.g. intercalation, ion exchange, dehydration reactions), since these routes use nuld synthetic conditions. It should be noted that not all soft chemical routes are topochemical. A reaction is said to be under topochemical control only if it follows the pathway of minimum atomic or molecular movement (Elizabe et al., 1997). Accordingly, topochemical reactions are those in which the lattice of the solid product shows one or a small number of... 

In conclusion, the four-center photopolymerization is a novel type of topochemical reaction which is crystal-lattice controlled with respect to the whole set of elementary processes44 including initiation, propagation and crystallization of polymer. 

In contrast to solutions, solid drugs have a fixed conformation resulting in topochemical reactions. The majority of photoreactions in the solid state, described in the literature, deal with lattice-controlled examples and photodimerizations. A precondition for these reactions is the parallel position of the double bond of two adjacent molecules in the crystal lattice as shown by the example of the trimorphic, frans-cinnamic acid. Irradiation of the a- and the 5-modifications causes the formation of a-truxillic acid and (i-truxinic acid, respectively, whereas the y-modification is photostable due to the distance of the double bonds fixed by the lattice (Fig. 8) (10). 

Reactions that proceed under topochemical control follow the pathway involving the minimum amount of molecular movement, and hence for a topochemical reaction between a pair of adjacent molecules within a crystal, the stereochemistry of the reaction product should be governed primarily by the relative positions and orientations of the two precursor molecules in the parent crystal. In topochemical dimerization reactions, for example, the molecular structure of the dimer should reflect the relative positioning of pairs of adjacent molecules in the parent (monomer) crystal structure. 

The preparation of new polydiacetylenes and polytriacetylenes is complicated by the fact that no one has demonstrated a direct 1,4-diacetylene or a 1,6-triacetylene polymerization in solution, 1,2-polymerizations being more favorable. However, the polymers can be prepared in the solid state as the result of a topochemical polymerization. Topochemical reactions are solid state reactions in which the product and the regio- and stereochemistry of a reaction are directly controlled by the preorganization of the reactants. 

The solid-state polymerizations of trioxane [32,33] and of diacetylenes [34], proceed as topochemical reactions - that is, the polymerization is crystal-lattice controlled and proceeds with a minimum of atomic and molecular movement. Upon ring-opening, trioxane molecules form helical polyoxyraethylene chains lying in the direction of the c-axis of the trioxane crystals. Such polymerization affords only a very small volume change. 

Equation (4.3.78) becomes identical to Eq. (4.3.73) upon replacing by Fp, by and Kg by Kp and setting 8 = 0. Furthermore, the examination of these two equations may explain why the topochemical model with chemical reaction control is approached more readily for nonporous solid reactants, and why the diffusion-controlled shrinking-core model becomes applicable for porous solids. This behavior is readily apparent on considering that, in general, > VJA ),... 

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. 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

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. 

Electronic excitation energy in a crystal is in many cases highly mobile It may diffuse very rapidly through many thousands of molecules and eventually be trapped at some appropriate defect site. If, then, photoreaction occurs at this site, the stereochemistry of the reaction pathway will be determined by the symmetry of this site, and not by the symmetry of the bulk crystal. Nevertheless, the bulk symmetry is found empirically to be the determining factor in most cases studied (topochemical control). 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

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