Reactions in crystals

In order to appreciate reactions in crystals, it is of value to consider the topo-chemical postulate and the reaction cavity concepts as the starting point. Kohlshutter proposed in 1918 that the crystal lattice plays an important role on the outcome of chemical reactions as a result of its rigidity and topology, suggesting that reactions... 

As we have seen, one of the main reasons why reactions in crystals lead to high levels of asymmetric induction is that the constituent molecules can be organized in homochiral fixed conformations and intermolecular orientations that are predisposed to formation of a single product enantiomer. With this in mind, it was natural to seek other ways of preorganizing molecules in restricted environments for the purpose of asymmetric synthesis, and one approach that has shown a good deal of promise is the use of chirally modified zeohtes. The great majority of this work has been carried out by Ramamurthy and coworkers at Tulane University [23], and a brief summary is given below. 

Thompson I B. Jr. (1969). Chemical reactions in crystals. Amer. Mineral, 54 341-375. 

This is an interesting result. We cannot always neglect the space-charge width compared to the recombination length internal reactions in crystals with varying disorder types will be further discussed in Chapter 9. 

In order to analyze the temporal process of steady-state formation in a course of tunnelling reactions in crystals, Kotomin [85] solved numerically equation (4.3.28) and the relevant equation (4.1.19) for the reaction rate K(t). It is clearly seen in Fig. 4.11 that the steady-states formed after the transient... 

Reactions in crystals provide a convenient window for detailed investigation of mechanism and for discovering what factors might make chemistry in the real world, which so often occurs in rigid or structured media, different from chemistry in the laboratory. Unfortunately there are substantial limitations on the kind of studies that may be conducted. 

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). 

Many processes involve criteria other than solids suspension, for example, crystallization, precipitation, and many types of leaching and chemical reactions. In crystallization, the shear rate around the impeller and other mixing variables can affect the rate of nucleation, and can affect the ultimate particle size. In some cases, the shear rate can be such that it can break down forces within the solid particle and can affect the ultimate particle size and shape. There are some very fragile precipitate crystals that are very much affected by the mixer variables. 

While photochemical reactions in solids are somewhat counterintuitive and relatively rare in organic synthesis, recent examples have suggested that reactions in crystals maybe as reliable and efficient as their solution counterparts [57]. As it pertains to the photoelimination of small molecules and C—C bond-forming reactions, it is essential to recognize that product formation relies on the sequential cleavage of two sigma... 

With an emerging correlation between the solid-state reactivity and the radical stabilizing energy (RSE) of the a-substituents, Campos et al. suggested that solid-state reactivity may be predictable from the RSE values of the substituents [71]. Taking the reaction of acetone as a reference, and assuming that reactions in crystals must be thermoneutral or exothermic, these authors suggested that substituents with RSE 11 kcal mol 1 on both a-carbons should make the reaction thermodynamically possible. As indicated in Scheme 2.41, the proposed RSE value derives from the... 

Ng, D., Yang, Z., and Garcia-Garibay, M.A. (2002) Engineering reactions in crystals gem-dialkoxy substitution enables the photodecarbonylation of crystalline 2-indanone. Tetrahedron Letters, 43, 7063-7066. 

Mortko, C.J. and Garcia-Garibay, M.A. (2006) Engineering stereospecific reactions in crystals synthesis of compounds with adjacent stereogenic quaternary centers by photodecarbonylation of crystalline ketones, in Topics in Stereochemistry, Vol. 25 (eds S.E. Denmark and J.S. Siegel), John Wiley Sons, Hoboken, NJ, pp. 205—253. 

In a systematic study of the 2ir + 2tt photodimerization of cinnamic acids it was shown that reactions in crystals occur with molecularity, selectivity, and efficiencies that are quite different from those observed in solution. Cinnamic acids and their derivatives were shown to crystallize in three distinct packing arrangements known as a-, (3-, and 7-forms. The packing arrangement of a given crystal form determines whether or not a 2tt + 2tt photodimerization may occur in the solid state and which of the possible products will form (Scheme 1). Al-... 

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