Chiral sulfide and selenide catalysts for asymmetric halocyclizations and related reactions

Asymmetric organocatalysis using well-designed artificial chiral molecular catalysts is one of the most reliable methods to create important chiral compounds in a highly enantioenriched form. A wide variety of efficient asymmetric transformations have been developed by utilizing well-designed chiral organocatalysts. Among the wide variety of organocatalysts, chiral amine and phosphine catalysts that utilize the characteristics of group 15 elements are the most extensively employed for asymmetric transformations. In comparison with chiral amine and phosphine catalysts, the use of chiral sulfide catalysts has remained limited and under-developed. The catalytic abilities of chiral sulfide organocatalysts were initially investigated using Corey-Chaykovsky-type asymmetric epoxidations and related reactions via the formation of sulfonium ylide intermediates. Unfortunately, the types of asymmetric reactions with chiral sulfide catalysts are limited in comparison with chiral amine-catalyzed asymmetric reactions, and the development of other catalytic reactions using chiral sulfides is highly desired. Several research groups have recently discovered that newly designed chiral sulfide catalysts are quite effective for asymmetric halocyclizations. This review summarizes recent achievements in chiral sulfide-catalyzed enantioselective halocyclizations and halogenations. The asymmetric catalyses with related chiral selenides, which are used in enantioselective halogenations, are also introduced.