Principle topochemical

For the diolefin crystals, including unsymmetrical diolefin crystals, each packing of the a- and j8-types is further classified into translation- and centrosymmetry-type packings. Of the photoproducts derived from unsymmetrical diolefins, the cyclobutane ring which has the same substituent on a ring is called a homo-adduct, and that which has different substituents is called a hetero-adduct. Corresponding to the molecular arrangements of these diolefin crystals, four types of photoproducts (a- and jS-types, and homo- and hetero-adducts) are expected to be formed based on the topochemical principle, as shown in Scheme 2. 

Polymerization of diacetylene (Fig. 8.13) is one of the most elegant examples of the topochemical principle. Wegner (1971, 1979) showed that diacetylene monomers, R—C = C—C = C— R, polymerize in the solid state by a 1,4-addition reaction at the diacetylene group to produce a polymer that can be represented by the mesomeric structures ... 

Recent studies on the photodimerization of olefinic crystals several examples, which deviate from accepted topochemical principles, have been... 

It is well established, in a qualitative sense, that chemical reactions occurring in crystals are subject to restrictive forces, not found in solution, which limit the allowable range of atomic and molecular motions along the reaction coordinate. This often leads to differences, either in the product structures or the product ratios, in going from solution to the solid state. This was first demonstrated in a systematic way by Cohen and Schmidt in 1964 in their studies on the solid state photodimerization of cinnamic acid and its derivatives (1 ). This work led to the formulation of the famous topochemical principle which states, in... 

In 1975, Cohen introduced the concept of the reaction cavity in solid state chemistry (2. The reaction cavity was defined as the space occupied by the reacting species and bounded by the surrounding, stationary molecules. Cohen viewed the topochemical principle as resulting from the preference for chemical processes to occur with minimal distortion of the reaction cavity, either in the formation of voids within it or extrusions from it (Figure 1). 

The next advance in the understanding of the forces which underlie the topochemical principle was due to McBride (3). He introduced the concept of local stress to explain the details of the mechanisms by which diacyl peroxides decompose in the solid state. McBride showed that least motion can be overcome in these cases by anisotropic stresses equivalent to many tens of kilobars of pressure exerted by the product carbon dioxide molecules trapped in unfavorable lattice poitions. 

Based on some interesting reactions in certain inorganic crystalline compounds, Kohlschutter [9,10] proposed that the nature and properties of the products obtained take place on the surface or within the solid state. Indeed, he coined the term topochemistry for such reactions in the solid state. However, systematic investigations of photoinduced reactions in crystals began from 1964 onward by Schmidt and Cohen [11], Their studies on the 2tt + 2tt photoreaction of cinnamic acid derivatives in the crystalline state and correlation with the molecular organization in these crystals led to what are now known as Topochemical Principles. The most important conclusions reached by them are as follows (1) The necessary conditions for the reactions to take place are that the reactive double bonds are parallel to one another and the center-to-center distance be within 4.1 A (2) there is one-to-one correspondence between the stereochemistry of the photoproduct and the symmetry relationship between the reactants. The centrosymmet-ric relationship (called the a-form) leads to centrosymmetric cyclobutane (anti-HT), whereas the mirror symmetric arrangements (called the (5-form) produce mirror symmetric dimer (yy -HH). 

Table 8 Examples of Photoinert Compounds Satisfying Topochemical Principles...

On first glance this seems consistent with our definition above. However, the topochemical principles, first enunciated by Cohen and Schmidt (1964a) Cohen et al. (1964c) were acmally developed from the fact that different polymorphs of a substance (trans cinnamic acid) undergo different photochemicaZ reactions, leading to different products Schmidt (1964) (Section 6.4). 

The discovery of two excimer emitting polymorphs of the stilbene derivative 6-XXXVIII (Cohen et al. 1975) provided an opportunity to determine directly the relationship between excimer structure and its emission properties. The structure of Form A was determined crystallographically and indicated a stacking arrangement, while the structure of Form B, inferred from the cell constants and the photochemistry consistent with the topochemical principles (Section 6.4), was a pairwise arrangement... 

A great deal of solid state chemistry has been carried out and understood using these topochemical principles (Schmidt 1971 Thomas et al. 1977 Lahav et al. 1979 ... 

Fig. 6.25 The topochemical principles, as demonstrated by the photochemical behaviour of polymorphic cinnamic acids. The filled rectangles in the reactant molecules represent carboxyl groups.

The topochemical principles have also been applied to the interpretation of mframolecular photochemical reactions of polymorphic materials depending on polymorphic form. 1,14-cyclohexacosanedione 6-XLII can be prepared as conformational dimorphs that undergo Norrish type II photochemistry which can be correlated with the molecular conformations in the two modifications, one leading to a cw-butanol, while the other leads to a trans product (Gudmunsdottir et al. 1996). Similarly, these principles can be invoked to understand the relative photochemical stability of two polymorphs of the active ingredient in a compound developed for the treatment of psoriasis (Lewis 2000). 

As for most solid state reactions the topochemical principle is valid in... 

In 1964, G.M.J. Schmidt and collaborators (7,8) enunciated the topochemical principle, a structural concept which notes that grouprs undergoing reaction in the solid state should be close ( 4.0 angstroms) and further emphasized reactivity in the ordered lattice rather than at defects. 

It has long been known that the photochemistry of crystalline compounds proceeds differently than upon irradiation in solution one of the best examples is that of the cinnamic acids and the research of Ciamician and Silber in 1902. Control of solid-state reactions was postidated by Cohen and Schmidt in 1964 to involve minimum molecular motion in the crystal. This approach has been termed the topochemical principle, perhaps otherwise known as least motion. Cohen and Schmidt also advanced the idea of a molecule reacting in a cavity created by surrounding neighbors. 

In spite of the topochemical principle, the details of solid-state reactions may be difficult to understand. Whai we think of chemical reactions in solution or in the gas phase, we normally focus attention on the fate of a single molecule and its interaction with one or two immediate neighbors. This kind of simplification is generally not possible when we deal with phase transitions or solid-solid chemical reactions in which phase separation occurs. Even when overall crystal orientation is maintained between initial and final... 

In a notable success, a J-shaped dicarboxylic acid was shown by Feldman to form a hydrogen-bonded molecular assembly held together by four O-H-. -O hydrogen bonds, wherein two olefins conform to the topochemical principle and are photoactive(Fig. 2b). UV irradiation produced the expected photoproduct in quantitative yield. The covalently bound naphthalene unit served to preorganize the two double bonds within the discrete complex for reaction. 

An auxiliary component that interacts with reactants by way of hydrogen bonds was also described (Fig. 3). Specifically, tetraaryl diols were shown by Toda to accommodate guests within cavities wherein the host interacted with two guests by way of two O-H- O hydrogen bonds.The guests packed within the cavities such that two olefins of two guests conformed to the topochemical principle and, upon UV irradiation, photodimerized. 

The ability of resorcinol to function as a linear template to organize reactants in the solid state for [2 + 2] photoreaction also led to a molecular solid-state synthesis by design.Specifically, cocrystallization of lA-bis[2-(4-pyridyl)ethenyl]benzene (1.4-bpeb) with 5-methoxy-resorcinol (5-OMe-res) yielded a four-component assembly. 2(5-OMe-res) 2(1,4-bpeb), wherein four olefins, as two reaction centers, conformed to the topochemical principle. UV irradiation of the solid produced, regio- and stereoselectively. a targeted [2.2]paracyclophane (yield 60%) (Fig. 4c). 

In addition to our focus here on reactions in solid organic inclusion compounds, the study of chemical reactions within other types of crystalline organic materials represents an important aspect of organic solid-state chemistry. In many cases, such reactions are governed by the topochemical principle ,which has provided a basis for rationalizing the course of chemical transformations within a wide range of different types of organic crystals. " ... 

For reactions in solid inclusion compounds, the topochemical principle may be expected to hold in most cases, provided well-defined positional and orientational relationships exist between the reactant molecules (e.g., adjacent guest molecules within the host structure). 

Although the prediction of the crystal structure from the theoretical calculation has not been solved yet, it seems possible to predict the reaction process from the crystal structure of reactant molecule, since the product molecule should be made suffering from steric repulsion in the crystaUine lattice of the reactant crystal. The term of reaction cavity was first proposed by Lee and Richards to interpret the reaction region within the enzyme molecule [15]. Cohen proposed a qualitative concept of the reaction cavity, as shown in Fig. 2.11 [16]. He proposed that since the reacting molecules occupy a space of a certain size and shape in the starting crystal, the reaction cavity is surrounded by the contact surface of the surrounding molecules. The topochemical principle can be interpreted to mean that those reactions which proceed under lattice control do so with minimum distortion of the surface of the reaction cavity. Therefore, the reaction of the route (a) is more favorable than that of (b). 

The term crystal engineering was first used by Schmidt [1] in coimection with the research on solid state photodimerization reactions on the basis of topochemical principle, which correlated the solid state reactivity of monomers with their crystal... 

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