Alkali metal trialkylborohydride-catalyzed highly regulable mono- and multi-hydroboration of 1,3-enynes

Organoboronates have a broad spectrum of applications in multi-disciplinary domains. The exuberant demand has spawned a vast number of efficient synthetic strategies. However, the highly regulable mono- and multi-borylation has remained a formidable challenge, and, particularly, such a transition-metal-free version remains elusive. Here, we report an alkali metal trialkylborohydride-catalyzed, controllable mono-, di-, and tri-hydroboration with 1,3-enynes. This method represents the first example of accurately controlling the number of incorporated boryl groups under transition-metal-free conditions. Mechanistic investigations reveal that this transformation undergoes a hydroboration of alkali metal trialkylborohydride with C–C multiple bond to form a vital nucleophilic tetracoordinate organoboron intermediate, followed by a transborylation of this species with HBpin. This mechanically innovative hydroboration approach is applicable not only to synthesizing mono- and diborylated molecules but also to the hydroboration of the sterically hindered trisubstituted alkenes to access triborylated alkanes that are otherwise difficult to access.