Expanding metal catalyzed arene C–H borylation beyond boronate esters

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 arene C–H borylation method using the commercial and inexpensive hydroborane 9-borabicyclo-[3.3.1]-nonane, (H–BBN)2. This process is effective for mono- and di-borylation to form a range of Aryl–BBN compounds using either NacNacAl or NacNacZn ((NacNac={(2,6-iPr2C6H3)N(CH3)C}2CH) based catalysts. The crude Aryl–BBN products can be used in-situ for subsequent transformations, including an example that does not proceed using the analogous pinacol-boronate ester. Mechanistic studies indicated an unusual -bond metathesis process between NacNacZn–Aryl and the hydroborane, with first order kinetics in the hydroborane dimer ((H–BBN)2). Our proposed calculated metathesis pathway involves ligand non-innocence and addition of both H–BBN units in (H–BBN)2 to the NacNacZn–Aryl complex. This is in contrast to the conventional -bond metathesis mechanism using hydroboranes which invariably proceeds by reaction of one equivalent of a monomeric hydroborane with a M–C unit. Overall, this work demonstrates the utility of extending catalytic arene C–H borylation beyond the boronate esters that currently dominate this field, while highlighting that the -bond metathesis reaction can be mechanistically more complex when utilizing dimeric hydroboranes.