Racemic-to-chiral transformation

The above examples clearly suggest that the reaction cavity is very effective to classify the reaction modes and to explain the rate of racemization. The next example indicates that the reaction cavity is an indispensable tool to explain the mechanism. 

3 Racemizations of Alkyl Groups in Cobaloxime Complex Crystals 

In order to make clear the mechanism of the above unusual result, the mixed crystals with different R S ratios were grown from an aqueous methanol solution. The five kinds of crystals with the R S ratios of 50 50, 62 38, 75 25, 87 13, and 100 0 were prepared, which is called pip-1, pip-2, pip-3, pip-4, and pip-5, respectively [32]. All the five crystals are isomorphous but the R S ratio of the B molecule has 0 50, 12 88, 25 25, 37 13, and 50 0, respectively, whereas the A molecule always has R configuration. The cell volume has a minimum around 62 38, not at 50 50 or 75 25. 

The crystal structures of pip-1, pip-3, and pip-5 after 40 h photo-irradiation were analyzed by X-rays. The three stractures are identical to each other within experimental error. The R S ratios of the 1-ce group of B in the three aystals became 19 31, 19 31, and 21 29 for pip-1, pip-3, and pip-5 crystals, respectively, which are 20 30 within experimental error. It must be emphasized that the converged R S ratio of the 1-ce group of B is not 25 25 but 20 30. 

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Tanaka, K., Fujimoto, D., Oeser, T., Imgartinger, H., and Toda, F. (2000) Chiral Inclusion Crystallization of Tetra(p-bromophenyl)ethylene by Exposure to the Vapor of Achiral Guest Molecules A Novel Racemic-to-Chiral Transformation Through Gas-Solid Reaction, Chem. Commun., 413-414. 

Ohashi et al. reported a fine example of racemic-to-chiral transformation in cobaloxime complex crystal by irradiation. [36] A racemic mixture of (1-... 

Racemic-to-Chiral Transformation and the Chirality Inversion Process in Cobaloxime Complex Crystals Only by Photoirradiation... 

Recently we found that the chiral alkyl group bonded to the cobalt atom in a cobaloxime complex crystal is almost (82%) inverted to the opposite configuration only by photoirradiation [12]. In this chapter we describe the processes of the racemic-to-chiral transformation and also the chirality inversion in the... 

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). ... 

From the above observation, we proposed that the reaction rate should be explained by the size of the reaction cavity for the reactive group and that the chirality of the produced group should depend on the shape of the prochiral group. Using the concept of a reaction cavity, more complicated processes such as racemic-to-chiral transformation and chirality inversion will be made clear. 

The powdered sample with the same enantiomeric structures, D or L, w irradiated with a xenon lamp for 20 h and was dissolved in a chloroform solutio The specific rotation [a]D of the chloroform solution was 4 30°. It is clear th the racemic-to-chiral transformation can be observed only by photoirradiatio Using the seed crystals, one of the enantiomeric crystals was selected in ea experiment. This means that the A molecules have R configurations in the pi 1 to pip-5 crystals. 

Both of the racemic crystals of the piperidine and pyrrolidine complexes have chiral space groups before irradiation. The most important requirement for the racemic-to-chiral transformation is that the two molecules with R and S configurations crystallize in a chiral space group. Since the racemic compounds tend to make a pair around an inversion center in the process of crystallization, the racemic crystals, in general, have a center of symmetry. Otherwise, conglomerate crystals may be deposited from a racemic solution. Therefore, only several crystals with chiral space groups have been reported so far [38]. This may be a reason that such a racemic-to-chiral transformation has not been observed till now. 

Moreover, the cell dimensions gradually changed when the crystal was exposed to a xenon lamp. The optical rotation of the chloroform solution in which the irradiated crystals were dissolved increased significantly. This may indicate that the racemic-to-chiral transformation would also occur in a diastereomeric crystal. 

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