Topochemically controlled solid-state

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.

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). 

Scheme 11-4 Topochemically controlled solid-state polymerization pattern of 1,3-butadiynes.

We have thus far discussed in this section some of the factors associated with crystallization in chiral structures, and pointed out that the molecules in such structures must adopt conformations which are to some extent chiral, whether or not they do so in disperse phases. Because of the chirality of both the molecule and of its environment, if the molecule takes part in a topochemically controlled solid-state reaction it is conceivable that chiral non-racemic products would be produced. In fact two successful asymmetric syntheses of this sort have been carried out in this laboratory. 

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) ... 

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. 

Eq. 2-248) [Braun and Wegner, 1983 Hasegawa et al., 1988, 1998]. This polymerization is a solid-state reaction involving irradiation of crystalline monomer with ultraviolet or ionizing radiation. The reaction is a topochemical or lattice-controlled polymerization in which reaction proceeds either inside the monomer crystal or at defect sites where the product structure and symmetry are controlled by the packing of monomer in the lattice or at defect sites, respectively. 

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... 

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

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]. 

In the crystal structure of the polymer phase (Fig. 17a), the polymer chains are aligned along the c-axis and the distance (3.71 A) between the centres of adjacent cyclobutane and pyrazine rings corresponds to half the c-axis repeat of the unit cell. For comparison between the monomer and polymer structures, an overlay plot of these structures is shown in Fig. 17b. It is clear that the solid-state reaction is associated with only very small atomic displacements at the site of the [2-1-2] photocyclization reaction (the displacement of the carbon atoms of the C=C double bonds of monomer molecules on forming the cyclobutane ring of the polymer is only ca. 0.8 A for one pair of carbon atoms and ca. 1.6 A for the other pair). Such small displacements are completely in accord with the assignment of this solid-state reaction as a topochemical transformation [124—127] (in which the crystal structure of the reactant monomer phase imposes geometric control on the pathway of the... 

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,... 

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. 

The stereochemistry of photodimerization in the solid state and solution has been reported for several halogenated derivatives of t-1 (Table 2) (59-62). Solid state photodimerization of stilbenes, like other alkenes, is subject to topochem-ical control viz, the two reactive double bonds must be parallel and separated by < 4.2 A (63). The photostability of t-1 in the solid state (39b,59) is consistent with its reported crystal packing (64). The halogenated stilbenes 15-20 serve to illustrate the variety of stereochemical outcomes observed for solution and solid state dimerization (eq. 11). 

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

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