Multifaceted Hidden Catalysis Revealed by Mechanistic Analysis of FeBr₃-Catalysed Arene Borylation

Arene borylation reactions provide direct access to aryl organoboranes, including aryl boronic esters. Precious metals, namely Ir, Rh, Pt, remain the go-to for metal-catalysed borylation reactions, however, significant efforts have been expended in developing Earth-abundant metal alternatives. The iron-catalysed borylation of 2-aryl pyridine derivatives with 9-borabicyclo[3.3.1]nonane (H-B-9-BBN) offers unique reactivity by using only FeBr₃ as the catalyst, without added ligand(s). Mechanistic analysis of this borylation reaction revealed an alternative, hidden catalysis pathway whereby the iron salt acts not as a borylation catalyst, but as an initiator for the generation of a catalytically active haloborane, Br-B-9-BBN. The roles of iron salt, Br-B-9-BBN, and HBr in catalyst formation, the mode of catalytic turnover, and catalyst regeneration were all determined using single-turnover and catalytic studies. The borylation reaction was found to proceed by a catalytic electrophilic borylation using, in situ generated, Br-B-9-BBN. These studies highlight a new, orthogonal method of generating haloboranes and their previously unobserved role as hidden catalysts within arene borylation reactions.