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

Abstract

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 -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 -bond metathesis mechanism using other hydroboranes which invariably proceeds by reaction of one equivalent of a monomeric hydroborane (e.g., 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.