The Fenton reaction is a classic iron-catalyzed oxidation that employs peroxide reagents and a strong acid. Its reactivity has been elegantly exploited for aliphatic C-H functionalization to synthesize C-O 20 and C-S 21 bonds. In our investigations of the reactivity of aromatic C-H bonds under Fenton-inspired conditions, we observed that the treatment of phenol (1) with equimolar di-tert-butyl peroxide (DTBP, 2), trifluoroacetic acid, and catalytic FeCl3, led to site-selective C-C formation in 4-tertbutylphenol (3a) with 5:1 para/ortho selectivity (Scheme 2). This dual Brønsted/Lewis acid catalysis exerts considerably enhanced reactivity and selectivity with a related, but unselective, iron-mediated system where the arene reagent was employed as the solvent. Selective tert-butylation via dual Brønsted/Lewis acid conditions extend to other phenolic derivatives (Scheme 3). Substituted phenolic and anisolic substrates generally alkylate to yield one major isomeric product. Exposing 4-tertbutylphenol to DTBP (2) in the presence of iron(III) and HCl catalysts yields 73% of 2,4-di-tert-butylphenol (3ba). 4-Chloroand 4-fluorophenols require stoichiometric iron salts to proceed and are transformed into their alkylated counterparts 3ca and 3da in 41% and 64% yields, respectively. Under these reaction conditions, overoxidation to benzoquinone-type sideproducts accounts for some of the mass balance. meta-Scheme 2. Synergistic Iron/TFA-catalyzed tert-butylation of phenol using peroxide reagents with and without an acid co-catalyst.