Part Ⅰ: Stereospecific syn-Dibromination and Regiodivergent syn-Bromochlorination of Alkenes via Vicinal Double Electrophilic Activation Strategy Part II: Selectivity Enhancement in syn-Dibromination of Alkenes through Mechanochemistry

Abstract

Part I: Whereas the conventional anti-dihalogenation of alkenes remains a valuable synthetic method with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, a conceptually distinctive strategy was developed for the simultaneous double electrophilic activation of the two alkene carbons from the same side. The subsequent iterative displacements of the vicinal leaving groups by nucleophilic halides enabled the syn-dihalogenation. For this purpose, thianthrenium dication was employed, and the syn-dibromination and the regiodivergent syn-bromochlorination were successfully archived.|Part II: Previously, highly stereosepecific syn-dihalogenations and syn-interhalogenation of alkenes were reported by our group. While these methods represented important advances in synthetic methodology, the syn-dibromination still suffered from eroded selectivity compared to syn-dichlorination. To address this limitation, various strategies were investigated. Among them, notable selectivity enhancement was detected under solvent-free conditions. Motivated by this result, mechanochemsistry was employed as a suitable method for conducting the solid phase reaction at high concentration under neat conditions. A series of alkene–thianthrenium salts were evaluated through mechanochemical methods, ultimately achieving improved selectivity in the dibromination process.