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

Abstract 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 FeBr 3 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.