Stereodefined polymetalloid alkenes synthesis via stereoselective boron-masking of polyborylated alkenes.

Polyborylated-alkenes are valuable polymetalloid reagents in modern organic synthesis, providing access to a wide array of transformations, including the construction of multiple C-C and C-heteroatom bonds. However, because they contain similar boryl groups, many times their transformation faces the main challenge in controlling the chemo-, regio- and stereoselectivity. One way to overcome these limitations is by installing different boron groups that can provide an opportunity to tune their reactivity toward better chemo-, regio- and stereoselectivity. Yet, the preparation of polyborylated-alkenes containing different boryl groups has been rare. Herein we report concise, highly site-selective, and stereoselective boron-masking strategies of polyborylated alkenes. This is achieved by designed stereoselective trifluorination and MIDA-ation reactions of readily available starting polyborylated alkenes. Additionally, the trifluoroborylated-alkenes undergo a stereospecific interconversion to Bdan-alkenes. These transition-metal free reactions provide a general and efficient method for the conversion of polyborylated alkenes to access 1,1-di-, 1,2-di-, 1,1,2-tris-(borylated) alkenes containing BFM, Bdan, and BMIDA, a family of compounds that currently lack efficient synthetic access. Moreover, tetraborylethene undergoes the metal-free MIDA-ation reaction to provide the mono BMIDA tetraboryl alkene selectively. The mixed polyborylalkenes are then demonstrated to be useful in selective C-C and C-heteroatom bond-forming reactions. Given its simplicity and versatility, these stereoselective boron-masking approaches hold great promise for organoboron synthesis and will result in more transformations.