Photoisomerization of 2-Cyanoethyl Group

Ohashi, Crystalline State Photoreactions Direct Observation of Reaction Processes and Metastable Intermediates, DOI 10.1007/978-4-431-54373-2 5, Springer Japein 2014 

2-cyanoethyl group in the (2-ce)cobaloxime complex is photoisomerized to the 1-ce group only in the solid state 

Two crystal forms were obtained under the same conditions when 3-methylpyridine (3mpy) was used as an axial base ligand [2]. The crystal structures of the two forms, I and n, are shown in Fig. 5.1. There is one molecule in an asymmetric unit in the P2i/a cell in each crystal. The molecular structures of I and II are shown in 

The powdered sample of the complex with triphenylphosphine as an axial base ligand gave a strange result the sample obtained by rapid crystallization showed a significantly larger reaction rate than that of the sample obtained by slow crystallization, as shown in Fig. 5.4 [3]. Both samples, of course, contain no impurity. The 14 and 10 crystals suitable for X-ray analysis were picked up from the rapid- and slow-crystallization samples, respectively. To our surprise, there were four crystal 

The cavities for the 2-ce groups of I and II were drawn and the volumes are 10.5 and 12.3 A, for I and II, respectively. Although the cavity of I is smaller than that of II, the reaction rate of I is faster than that of II. As shown in Fig. 5.11, the cobalt atom makes a bond with the Ci atom of the 2-ce group, following the extraction of one of the two hydrogen atoms bonded to the Ci atom and transferring it to the C2 

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Figure 2 Solid state 2-1 photoisomerization of the 2-cyanoethyl group. The chiral 1-cyanoethyl group is produced after photoirradiation.

The reaction cavity can be easily defined in the crystalline-state reaction, since the reaction proceeds with retention of the single aystal form. The relation between the reaction rate and the cavity size holds well in almost all the crystalline-state reactions. Moreover, the shape of the reaction cavity well explains the asymmetric induction during the reactions. The asyrmnetric induction in the photoisomerization from 2-cyanoethyl to 1-cyanoethyl groups are well explained by the shape of the reaction cavity. Moreover, the marvelous R S ratio of 70 30 after the racemic-to-chiral transformation and the singirlar ratio of 18 82 after the chirality inversion are caused by the asyrmnetric shape of the reaction cavity before photoreaction. 

Furthermore, it was made clear that the concept of reaction cavity is applicable to the usual solid-state reactions. In almost all the solid-state photoisomerization from the 2-cyanoethyl group to 1-cyanoethyl group, the reaction cavity is a good... 

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