Crystalline-state reaction

Figure 4. GPC curves of photoproducts obtained from the crystalline state reaction of 3 (a) irradiated at room temperature for 9 h, (b) irradiated at...

II. CRYSTALLINE-STATE REACTION OF COBALOXIME COMPLEXES BY PHOTOIRRADIATION... 

Before the mechanism of the racemic-to-chiral transformation and inversion processes are examined, it may be better to explain the solid-state photoreaction with retention of the single crystal form, which is called a crystalline-state reaction. We found that the chiral 1-cyanoethyl group, bonded to the cobalt atom in 4 cobaloxime complex crystal, was racemized on exposure to x-rays or visible light [13]. Since the crystallinity was kept in the whole process of the racemization the intensity data were collected at any stage of the reaction, and the process of the structural change was observed by x-ray crystal structure analysis. The change of the unit cell dimensions with time, which was well explained by first-order kinetics, corresponded to the rate of racemization [14] (Scheme 1). ... 

For the cobaloxime complex with pyrrolidine as an axial base ligand, the crystalline state reaction was also observed when the crystal was exposed to a xenon lamp. Four kinds of crystals with different R S compositions were prepared. The crystals of pyrr-1 were obtained from the racemic solution. The pyrr-2 crystals were obtained from a solution that has the complex with an R S ratio of 75 25. The crystals of pyrr-3 and pyrr-4 were also obtained from solutions with the R S ratio of 80 20 and 90 10, respectively. The x-ray crystal analysis indicated that the four crystals are isostructural to each other. From solutions with the R S ratio greater than 9 1, pure enantiomeric crystals were obtained, which are not isostructural to the above crystals pyrr-1 to pyrr-4 [40]. 

For example, the photoreaction of cinnamic acid crystals was described by Lieber-mann in 18891. Since then, a great variety of crystalline state reactions have been reported, e. g. dimerization, cis-trans isomerization, substituent migration, and formation of cage molecules. 

In solution, as the unimolecular cis-trans isomerization of excited species seems to compete with photocycloaddition polymerization, a highly concentrated solution of the monomer is advantageous for oligomerization. Such prominent difference of reactivities suggests extremely high stereoselectivity due to the crystal lattice in the crystalline-state reaction. 

Danno, M., Uchida, A., Ohashi, Y., Sasada, Y., Ohgo, Y., and Baba, S. Crystalline-state reactions of cobaloxime complexes by X-ray exposure. XIV. Uneven racemization at the independent reaction sites. Acta Cryst. B43, 266-271 (1987). 

Dynamics of Reaction Cavities in a Crystalline-State Reaction... 

In this chapter, crystalline-state photochromic dynamics of rhodium dithionite complexes are reviewed. The chemistries described here have been achieved not only by recent developments of the analytical technique but also by discovery of a new class of transition-metal based photochromic compounds. One of the advantages of transition-metal complexes is structural diversity. In order to find the rule of an exquisite combination of metal ions and ligands, we are currently synthesizing various dithionite derivatives with other metal ions and/or modified Cp ligands. As shown in this chapter, dithionite complexes are a very useful photochromic system to investigate crystalline-state reaction dynamics. We believe that dynamics studies of newly synthesized dithionite derivatives provide useful insight into the construction of sophisticated molecular switches. A dithionite complex may appear in a practical application field in the near future. 

Among crystalline-state reactions, some have multiple steps. On photoirradiation or heating, a reactant molecule becomes excited to a transition structure and then converts to product. Analysis of the crystal structure during the crystalline-state reaction reveals the disordered structure of the reactant and product molecules. The concentration of the product increases gradually from 0 to 100% in the disordered structure when the crystal structures are analyzed at constant intervals during the... 

To observe the intermediate structures one must collect the intensity data rapidly. We designed and made a diffractometer, R-AXIS lies, for rapid collection [7] of data, which has been marketed in a revised form by Rigaku Company as R-AXIS RAPID since 1998. The two-step crystalline-state reaction was first found in the inversion of a chiral alkyl group bonded to the cobalt atom in a cobaloxime complex when the crystal on the diffractometer was irradiated with a xenon lamp [8]. Figure 5-2 shows the two-step structural change of the complex, ((R)-l-methoxy-carbonylethyl) [(S)-l-phenylethylamine] cobaloxime. After exposure for 2h, the structure (Figure 5-2b) showed the disordered structure of the chiral (R)-... 

Abstract There are two types of solid-state reactions keeping the single crystal form single crystal-to-single crystal (SCSC) transformations and crystaUine-state reactions. In the former reactions, the crystal structures before and after the reaction are very similar to each other, but the crystallinity is not kept during the reaction. In the latter reactions, the crystallinity is kept in a whole process of the reaction. The reaction cavity was defined to estimate the void space around the reactive group. For the crystalline-state reaction, it was easy to understand the way how the void space is effectively utilized in the process of the reaction, comparing the void space before and after the reaction. 

Although it appears no clear limitation exists between the crystalline-state reaction and the SCSC transformation if there are only two states, initial and final states, during the reaction, the former reaction is completely different from the latter one. As shown in Fig. 2.1, the changes of the unit-cell dimensions in the crystalline-state photo-racemization are expressed exponentially and are well explained by first-order kinetics. 

If there is an intermediate state during the reaction as shown in Fig. 2.8, the intermediate structure can be observed only in the CTystalline-state reaction. Several examples are shown in the following chapters. It may be adequate to classify the SCSC transformation into a reaction between the solid-state reaction and crystalline-state reaction. 

Fig. 2.8 Reaction intermediate observed in the crystalline-state reaction...

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