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Discrimination in the Solid‐State Photodimerization of 1‐Methyl‐5,6‐diphenylpyrazin‐2‐one
Author(s) -
Kaftory Menahem,
Shteiman Vitaly,
Lavy Tali,
Scheffer John R.,
Yang Jie,
Enkelmann Volker
Publication year - 2005
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200400350
Subject(s) - chemistry , monoclinic crystal system , molecule , crystal structure , crystallography , stack (abstract data type) , enantiomer , crystal (programming language) , irradiation , hydrogen bond , solid state , photochemistry , stereochemistry , organic chemistry , physics , computer science , nuclear physics , programming language
1‐Methyl‐ and 1‐ethyl‐5,6‐diphenylpyrazin‐2‐one crystallize in two modifications, one of which is light‐stable and the other light‐sensitive. The light‐sensitive modification is known to undergo photodimerization in the solid state. This polymorph crystallizes in the monoclinic space group P 2 1 with two crystallographically independent molecules in the asymmetric unit. The molecules are packed in stacks running parallel to the unique b axis. The two independent molecules are arranged alternately along the stack. In principle, there are two different pairs of molecules within a stack that can undergo photodimerization, and each should form a different enantiomer. A large crystal was irradiated and a solution of the product was separated by HPLC. The optical purity of the (+)‐enantiomer sample was estimated to be greater than 90 %. This finding indicates that only one of the two pairs undergoes photoreaction. The structure of a single crystal of the pyrazinone was elucidated by X‐ray diffractometry before and after irradiation with a laser at a wavelength of 488 nm to 19 % conversion. The results of the crystal‐structure determinations provide additional evidence that only one of the two pairs of molecules undergoes photodimeri‐zation although there are no significant differences between the distances between the reacting centers. Furthermore, the latter results suggest that weak hydrogen bonds are a dominant factor that determines which of the two pairs is dimerized upon irradiation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)