Arene C(sp2)-H bond borylation offers direct and efficient access to aryl boronic esters. Using in situ catalyst activation and photoirradiation, the iron-catalysed C(sp2)-H borylation reaction of carboarenes, pyrroles, and indoles has been developed using only bench-stable pre-catalysts and reagents. Good functional group tolerance was observed including those not reported using previous methods (ArNH2, ArOH, ArSiR3, ArP(O)(OR)2, ArC(O)NR2). Mechanistic studies revealed iron-catalysed reductive deoxygenation, C—F protodefluorination, and a demethylation of aryl methyl ethers by C—O sigma bond hydroboration