Chirality and Stereoselectivity in Photochromic Reactions

Tetsuya Nakagawa, Takashi Ubukata, Yasushi Yokoyama

Index: 10.1016/j.jphotochemrev.2017.12.004

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Abstract

In this review, the chirality and stereoselectivity in photochromic reactions are broadly discussed. Stereoselectivity is categorized into the two main areas of enantioselectivity and diastereoselectivity. Enantioselective photochromism was discussed according to the methods of generating chirality, that is: (1) optical resolution of chiral and racemic compounds, (2) spontaneous separation of racemic compounds during crystallization, (3) chiral template-assisted photochromic reactions to preferentially generate an enantiomer or cause chirality-based events, and (4) the generation of a non-racemic mixture of the photoisomers by irradiation of circularly polarized light to the racemic starting photoisomer. Diastereoselective photochromism was discussed for each family of photochromic reactions, namely, (1) fulgides, (2) diarylethenes, (3) spiro-compounds, (4) azobenzenes, and (5) organometallic complexes. Chirality-related reactions in overcrowded alkenes are discussed in a separate chapter. These include: (1) the discovery of the light-driven molecular motor, (2) the first generation of light-driven molecular motors with unidirectional rotation, which have two chiral carbon atoms in a molecule, (3) the second generation of light-driven molecular motors with unidirectional rotation, which have a single chiral carbon atom in a molecule. Finally, two distinctive novel photochromic systems are introduced in which the reactions proceed stereoselectively, although their enantio- or diastereoselectivity during photochromic reactions have yet to be examined.