抄録
Recent advances in photochemical asymmetric syntheses through photosensitization with optically active compounds have been reviewed. In sharp contrast to the flourishing studies on thermal asymmetric synthesis, asymmetric photochmistry does not appear to have attracted much attention of synthetic organic chemists until very recently, except for the diastereodifferentiating photochemical reactions of optically active substrates' carrying chiral handles. This is simply because the other modes of asymmetric photochemistry do not provide satisfactory optical yields applicable to synthetic reactions However, recent efforts in the study of photosensitized asymmetric reactions via intimately interacting exciplexes not only materialized photochemical chirality multiplication with optical yields exceeding 90%, but also revealed that the product's chirality is totally inverted by changing the reaction temperature and consequently the optical yield increases with raising temperature beyond the equipodal temperature (T0). This unusual chirality switching by temperature has been attributed to the vital contribution of the entropy factor in the enantiodifferentiation process, which originates most probably from dynamic, synchronized conformational changes in the exciplex intermediate involving both sensitizer and substrate molecules. The present status and future perspective of photosensitized enantiodifferentiating geometrical isomerization, deracemization, [2+2] and [4+2] cycloadditions, and polar addition are discussed. Also presented are some tentative guidelines for getting higher optical yields in uni- and bimolecular asymmetric photosensitizations.
This paper is dedicated to Prof. Dr. Hans-Dieter Scharf on the occasion of his 65th birthday.
