Arene C–H Borylation Strategy Enabled by a Non-Classical Boron Cluster-Based Electrophile

Introducing a tri-coordinate boron-based functional group (e.g., boronic ester) into an unactivated C–H bond in the absence of directing groups is an ongoing challenge in synthetic chemistry. Despite previous developments in transition metal-catalyzed and -free approaches, C–H borylation of sterically hindered arenes remains a largely unsolved problem to date. Here, we report a synthetic strategy of a precious metal-free electrophilic C–H borylation of sterically hindered alkyl- and haloarenes to generate aryl boronic esters. The reported approach consists of two simple steps that can be carried out under open flask conditions. The first step relies on electrophilic aromatic substitution (SEAr) induced by cage-opening of Cs2[closo-B10H10], forming a 6-Ar-nido-B10H13 product containing a B–C bond. This is followed by an unprecedented cage deconstruction of arylated decaboranes promoted by diols leading to selective breakage of B–B bonds in the cluster while preserving the B–C bond. The combination of these two steps allows for the preparation of aryl boronic esters that are hard-ly accessible by current direct C–H borylation approaches. Unlike the previous arene C–H borylations, this reaction does not require any precious metals, highly-engineered ligands, pre-functionalized boron reagents, or inert conditions. In addition, the unique properties of a non-classical boron cluster electrophile intermediate, B10H13+, afforded an unprecedented regi-oselectivity with unique steric and electronic control without the undesirable side reactions common in electrophilic aro-matic substitution. This work showcases how inorganic cluster chemistry can be leveraged to access new modes of reactivi-ty relevant to organic synthesis.