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 two-step, precious metal-free electrophilic C-H borylation of sterically hindered alkyl- and haloarenes to generate aryl boronic esters. The first step relies on electrophilic aromatic substitution (EAS) induced by cage-opening of Cs-2[closo-B10H10], forming a 6-Ar-nido-B10H13 product containing a B-C bond, followed by a cage deconstruction of arylated decaboranes promoted by diols. The combination of these two steps allows for the preparation of aryl boronic esters that are hardly accessible by current direct C-H borylation approaches. 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+, afford a regioselectivity with unique steric and electronic control without the undesirable side reactions. Electrophilic borylation of sterically hindered arenes is a challenging transformation. Here, authors report a metal-free electrophilic C-H borylation of hindered arenes using a boron cluster reagent producing valuable aryl boronic esters.

Automated summary

The authors report a metal-free electrophilic C-H borylation method for sterically hindered alkyl- and haloarenes, producing valuable aryl boronic esters. This reaction does not require any precious metals, highly-engineered ligands, pre-functionalized boron reagents, or inert conditions. The method is regioselective and allows for unique steric and electronic control without undesirable side reactions.