Solid topochemically controlled

Topochemical control of solid state dimerizations is well illustrated by the example of the frows-cinnamic acids.(112) The a form of ftmv-cinnamic acid is known to have a molecular separation of 3.6 A between double bonds and the molecules are arranged in a head-to-tail fashion. -Cinnamic acid has approximately the same intermolecular distance in the crystal but the molecules are arranged in a parallel head-to-head manner. a-Truxillic (101) and /3-truxinic (102) acids are the products expected and observed ... 

Topochemical control is also revealed by closely related compounds showing significant differences in chemical behavior in the solid state. For example, cinnamylidenemalonic acid (103) dimerizes in the solid state to cyclobutane (104), while cinnamylideneacetic acid (105) dimerizes to cyclobutane (106)(113) ... 

Topochemical control will also be revealed in causing a given compound to react differently in the solid phase than in solution. Crystalline 2,5-di-methylbenzoquinone yields dimers (111) and (112) upon irradiation 2,5-dimethylbenzoquinone in ethyl acetate solution yields primarily the oxetane (113)[Pg.544]

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. 

McBride and co-workers have studied extensively the reactions of such free-radical precursors as azoalkanes and diacyl peroxides (246). By employing a variety of techniques, including X-ray structure analysis, electron paramagnetic resonance (EPR), and product studies, and comparing reactions in the crystal and in fluid and rigid solvents, they have been able to obtain extremely detailed pictures of the solid-state processes. We will describe here some of the types of lattice control they have elucidated, and the mechanisms that they suggest limit the efficacy of topochemical control. 

Solvent-drop grinding was next applied as an eco-friendly modification to a previously described preparation of a crystalline organic inclusion compound. Initial work had demonstrated solution-mediated supramolecular organisation and solid-state topochemically controlled reactivity in a system involving l,2-bis(4-pyridyl)ethylene (bpe) 32 and 1,2,4,5-benzenetetracar-boxylic acid (bta) 33 [57]. A single crystal of a 2 1 bpe bta 32 33 cocrystal was... 

A further distinction has to be made between reactions taking place within a molecular crystal and reactions of a molecular crystal A with a molecular crystaT B (see Scheme 2 A and B can also be the same crystal) [7bj. Reactions of the former type, or intra-solid reactions, can be either under topochemical control, depending on the proximity of the reactants, or may imply extensive molecular motion within the crystal lattice. Reactions of the second type, or inter-solid reactions, can either take place on the crystallite surface or require molecular diffusion through the lattice. Inter-solid reactions are often accompanied by formation of eutectics. 

Lauher and Fowler et al. have proposed an elegant strategy for the control of topochemical polymerization based on the host-guest cocrystal concept. They used the ureylene and oxalamide functionality to form layered supramolecu-lar structures for the topochemically controlled polymerization of diacetylenes and 1,3-butadienes in the solid state [62,63]. 

Scheme 10.13 Topochemically controlled solid-state synthesis of bisadduct 61 and subsequent cyclopropanation to give [2 4]-hexakisadduct 62. (i) 180°C, 10 min (ii) 40 equiv. diethyl bromomalonate, 40 equiv. DBU.

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction ... 

In the absence of defects, the reactivity of organic solids is mainly determined by molecular packing. Reactions in which the crystal structure holds sway over intrinsic molecular reactivity are said to be topochemically controlled (Thomas, 1974). A classic example of a topochemically controlled organic reaction in the solid state is the photodimerization of rrans-cinnamic acids studied by Schmidt et al. (see Ginsburg,... 

Scheffer, Trotter, and co-workers have provided elegant demonstrations of the distance and angular dependence between a carbonyl oxygen atom and a gamma hydrogen atom on the ease of initial abstraction [276], Their singlecrystal X-ray structural information allows the various courses of Norrish II reactions of neat solids to be understood in the context of topochemical control [11,13]. For instance, they have noted that the cyclic diones 77 (n = 7,8,10, and 12) follow different Norrish II pathways depending upon the conformations of the individual molecules in their crystals (Eq. 9) [277]. 

Several reviews deal with the solid-state reactions of simple inorganic salts and of organic compounds.1-8 The essential differences between solid-state reactions and reactions in solution can be ascribed to the fact that solid-state reactions occur within the constraining environment of the crystal lattice. The reactant crystal lattice can control both the kinetic features of a reaction, and the nature of the products. In many solid-phase reactions the separation distances and mutual orientations of reactants in the solid determine the product. Such reactions are said to be topo-chemically controlled.9 Topochemical control of a reaction product is analogous to kinetic control in solution. The product is not necessarily the thermodynamically most stable product available to the system, but is rather the one dictated by the reaction pathway available in the constraining environment of the solid. 

Prior to the development of tether-directed functionalization methods, Krautler and co-workers developed a very elegant topochemically controlled, solid-state group-transfer synthesis [12,13] to obtain the trans-1 bisanthracene adduct 9 (Figure 2). 

On the other hand, since the concept of topochemically-controlled reactions was established, various approaches to asymmetric synthesis using solid-state photoreaction... 

Topochemically controlled solid reactions are governed by the crystal structure of the solid. Even if the intrinsic reactivity allows for a reaction, the topochemical restrictions circumvent a conversion. The photodimerization of solid anthracene was observed electron microscopically at dislocations [85], The reason for this is the possibility of molecule rotation at the dislocation, and not only the high pressure. 

Cinnamates occupy an important place in the history of photochemistry. Schmidt and his co-workers [18] used the solid state photochemistry of cinnamic acid and its derivatives to develop the idea of topochemical control of photochemistry in the crystalline state. Minsk [19] developed poly(vinyl cinnamate) as the first polymer for photoimaging. The cinnamate chromophore is still commonly incorporated in photopolymers of all types, including LC polymers, to enable them to be photochemically cross-linked [20], and a number of reports of the photochemistry of such MCLC and SCLC polymers are summarized below. 

Since the concept of topochemically controlled reactions was established, various approaches to asymmetric synthesis using a solid-state reaction have been attempted, most actively by the research group at the Weismann Institute. Their studies have been concerned with the bimolecular reactions of chiral crystals in the solid state. In these studies, successful absolute asymmetric synthesis has been performed by using topochemically controlled four-centered photocyclodimerizations of a series of unsymmetrically substituted diolefin crystals. Research on reactivity in the crystalline state has been extended in recent years to a variety of new systems, and many absolute asymmetric syntheses have been provided. Successful examples of absolute asymmetric synthesis using chiral crystals are listed in Tables 2 to 4, which are divided into three categories intermolecular photoreaction in the solid state (Table 2), intramolecular photoreaction in the solid state (Table 3, A-D), and asymmetric induction in the solid-gas and homogeneous reactions (Table 4). 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

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... 

Matsumoto, A., Tanaka, T. and Oka, K. (2005) Stereospecific radical polymerization of substituted benzyl muconates in the solid state under topochemical control. Synthesis, 1479-1489. 

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