Of topochemical

Characteristic features of topochemical [2+2] photoreactions of diolefin crystals... 

As a typical example of topochemically prepared polymers, the nmr speetrum of the polymer derived from ethyl 4-[2-(2-pyrazyl)ethenyl]-cinnamate [l OEt] crystals by reaction (2), and the X-ray diffraction patterns of the same monomer and polymer are illustrated in Figs 1 and 2 (Hasegawa et al., 1989a). 

In this section, various types of topochemical behaviour such as the even-numbered degree of polymerization mechanism, topochemical induction into the syndiotactic structure, stereo- and enantio-selective reactions, and the formation of highly strained cyclophanes are described. 

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. 

The concept of topochemical induction was first assumed in the topochemical formation of a cyclic dimer from 1,4-dicinnamoylbenzene (1,4-DCB) crystal (see p. 157) (Hasegawa et al., 1985). 

Some examples ambipolar diffusion, total rate of topochemical reaction, change in the light velocity when transiting from vacuum into the given medium, resultant constant of chemical reaction rate (initial product - intermediary activated complex - final product). 

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

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. 

During work on a series of aspartyl dipeptides containing ACC 71 (vide supra, Eq. (28), Sect. 4) at the carboxyl terminus, it was reported that dispartame Asp-ACC-OMe had a distinct sweet taste [302] and that the corresponding n-propyl ester had 250-300 times the sweetness of sucrose [303]. However, replacement of phenylalanine by 2,3-methanophenylalanine gave tasteless analogues of aspartame [293, 304], and some dimethyl-ACC 214 (methanovaline) and tri-methyl-ACC 215 aspartame analogues [Asp-(Me)n-ACC-OMe] have a bitter taste. These taste properties, which depend on the number and position of the methyl substituents, have been explained on the basis of topochemical models thus, a L-shaped conformation of the dipeptide is necessary for sweet taste, Eq. (86) [3051. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Fig.1 Chemical structures of topochemically polymerizable 1,3-diene monomers...

Table 2 Monomer stacking parameters for the crystals of topochemically polymerizable 1,3-diene mono- and dicarboxylic acid derivatives [16] ...

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]. 

Thus, it has been demonstrated that the combination of weak interactions is useful for the design of topochemical polymerization to yield a new type of stereoregular polymer. The weak intermolecular interactions are tolerant of a variety of crystal structure formations and induce a different molecular stack-... 

Systematic studies of topochemical reactions of organic solids have led to the possibility of asymmetric synthesis via reactions in chiral crystals. (A chiral crystal is one whose symmetry elements do not interrelate enantiomers.) (Green et al, 1979 Addadi et al, 1980). This essentially involves two steps (i) synthesis of achiral molecules that crystallize in chiral structures with suitable packing and orientation of reactive groups and (ii) performing a topochemical reaction such that chirality of crystals is transferred to products. The first step is essentially a part of the more general problem of crystal engineering. An example of such a system where almost quantitative asymmetric induction is achieved is the family of unsymmetrically substituted dienes ... 

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]. 

The combination of the nonpalindromic nature of the peptide backbone and the chirality of the a-carbons in the amino acids comprising the oligomeric structure provides a unique opportunity for vast stereochemical and topochemical diversities. Earlier studies explored the interesting structural relationship between all-L-, retro-all-L-peptides, and their enantiomers (see reviews[1 3]). Schematically, a high degree of topochemical complementarity is obtained between parent cyclic peptides and their retro-inverso analogues.11 ... 

Champetier and Yovanovitch108 treated com starch with aqueous sodium hydroxide solutions of various concentrations, and obtained three adducts having 1 2, 1 1, and 2 1 ratios of sodium hydroxide to D-glucose residue. No 2 3 or 3 4 adducts, such as those reported for cellulose, could be obtained. As the hydroxide concentration is changed, the transition from one addition compound to another is abrupt, thus showing an absence of topochemical fixation. 

A.Ya. Rozovskii, Kinetics of Topochemical Reactions, Khimiya, Moscow, 1974 (in Russian). 

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

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. 

Figure 9 Temperature dependence of topochemical photodimerization of films of polymer, 23a. (a) HMW polymer, (b) LMW polymer, (c) Unfractionated polymer. Relative reactivities were obtained by monitoring the change in absorbance (A0 — At) of the C=C stretching band at times t relative to t = 0 (/Iq). (Reprinted with permission from Ikeda et al. [54]. Copyright 1990 American Chemical Society.)...

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