Oxathiaborolium debuts as a chiral Lewis acid catalyst

Oxazaborolidines are boron-based Lewis acids with a bicyclic nitrogen-boron-oxygen core that are known for being efficient chiral catalysts. In particular, they are useful in facilitating enantioselective Diels-Alder cycloadditions and an array of other types of addition reactions—part of the standard repertoire of organic synthesis. A research team in Taiwan has now added a new twist to these borenium catalysts by replacing nitrogen with sulfur (Org. Lett. 2016, DOI: 10.1021/acs.orglett.6b03147). Inspired by earlier work on oxazaborolidines, Singam Naveen Kumar and Rong-Jie Chein of Academia Sinica and Isaac Furay Yu of National Taiwan University reasoned that sulfur, which is a weaker electron donor than nitrogen, should increase the Lewis acidity of the central boron atom and offer new synthesis opportunities. The resulting oxathiaborolium, which is no easier to say or spell than its predecessor, indeed facilitates enantioselective Diels-Alder reactions, such as the cycloaddition of cyclopentadiene with various acrylates and cyclic enones. But the new catalyst is effective with a broader range of acrylates than oxazaborolidines. The researchers believe oxathiaborolium is the first reported example of a sulfur-stabilized borenium Lewis acid and could pave the way for expanding the applications of chiral sulfur ligands in organic synthesis.