Ligand non-innocence and an unusual σ-bond metathesis step enables catalytic borylation using 9-borabicyclo-[3.3.1]-nonane

The metal-catalyzed intermolecular C-H borylation of arenes is an extremely powerful C-H functionalization methodology. However, to date it is effectively restricted to forming organo-boronate esters (Aryl-B(OR)₂) with its application to form other organoboranes rarely explored. Herein, we report a catalytic intermolecular heteroarene C-H borylation method using the commercial hydroborane 9-borabicyclo-[3.3.1]-nonane, (H-BBN)₂. This process is effective for mono- and di-borylation to form a range of heteroaryl-BBN compounds using either NacNacAl or NacNacZn (NacNac = {(2,6-ⁱPr₂C₆H₃)N(CH₃)C}₂CH) based catalysts. Notably, mechanistic studies indicated a highly unusual σ-bond metathesis process between NacNacZn-Aryl and the dimeric hydroborane, with first order kinetics in the hydroborane dimer ((H-BBN)₂). Our calculated metathesis pathway involves ligand non-innocence and addition of both H-BBN units in (H-BBN)₂ to the NacNacZn-heteroaryl complex. This is in contrast to the conventional σ-bond metathesis mechanism using other hydroboranes which invariably proceeds by reaction of one equivalent of a monomeric hydroborane (<i>e.g.</i>, H-B(OR)₂) with a M-C unit. Overall, this work demonstrates the potential of extending catalytic arene C-H borylation beyond boronate esters, while highlighting that the σ-bond metathesis reaction can be mechanistically more complex when utilizing dimeric hydroboranes such as (H-BBN)₂.